Notice: We are in the process of migrating Oral History Interview metadata to this new version of our website.
During this migration, the following fields associated with interviews may be incomplete: Institutions, Additional Persons, and Subjects. Our Browse Subjects feature is also affected by this migration.
Please contact [email protected] with any feedback.
This transcript may not be quoted, reproduced or redistributed in whole or in part by any means except with the written permission of the American Institute of Physics.
This transcript is based on a tape-recorded interview deposited at the Center for History of Physics of the American Institute of Physics. The AIP's interviews have generally been transcribed from tape, edited by the interviewer for clarity, and then further edited by the interviewee. If this interview is important to you, you should consult earlier versions of the transcript or listen to the original tape. For many interviews, the AIP retains substantial files with further information about the interviewee and the interview itself. Please contact us for information about accessing these materials.
Please bear in mind that: 1) This material is a transcript of the spoken word rather than a literary product; 2) An interview must be read with the awareness that different people's memories about an event will often differ, and that memories can change with time for many reasons including subsequent experiences, interactions with others, and one's feelings about an event. Disclaimer: This transcript was scanned from a typescript, introducing occasional spelling errors. The original typescript is available.
In footnotes or endnotes please cite AIP interviews like this:
Interview of Maurice Goldhaber by Charles Weiner and Gloria Lubkin on 1967 January 10,Niels Bohr Library & Archives, American Institute of Physics,College Park, MD USA,www.aip.org/history-programs/niels-bohr-library/oral-histories/4632
For multiple citations, "AIP" is the preferred abbreviation for the location.
Early education, Real-gymnasium; Universität Berlin, 1930; early interest in physics; courses, books studied, method of noting original ideas; University of Cambridge, 1933; first formal paper on nuclear physics; reaction in Berlin to discovery of neutron, colloquium of Lise Meitner; beta decay and the neutrino hypothesis; working habits at Cavendish Laboratory; collaboration with James Chadwick; photodisintegration of the deuteron; work with slow neutrons; circumstances of move to U.S., 1938; consequences of death of Ernest Rutherford on research at Cavendish Laboratory; use of proportional counters, oscilloscopes, nuclear emulsions in mid-1930s; important centers of research, publications; early failures to recognize fission; ways of determining nuclear spin; comparison of available equipment, technology in England and U.S.; comparison of motivations for doing experiments in 1930s and at present; nuclear models, conditions for acceptance, usefulness; distinctions between nuclear structure and nuclear forces as areas of study; money as a determinant of possible experiments; World War II as a determinant of work in nuclear physics; postwar work in nuclear physics; improvements in detectors and techniques ca. 1950; origin of high-energy physics; mobility of physicists among fields of study; postwar conferences, Shelter Island, Rochester; separation of belief from established results in pedagogy; current capabilities of theory in nuclear physics. Also includes an 8-page bibliography. Also prominently mentioned are: Niels Henrik David Bohr, Chang, John Cockcroft, Critchfield, Sydney Michael Dancoff, P. I. Dee, P. A. M. Dirac, Enrico Fermi, George Gamow, Gertrude Goldhaber, Gordy, Frédéric Joliot-Curie, I. V. Kurchatov, Ernest Orlando Lawrence, Douglas Lea, Alfred Loomis, Lothar Nordheim, Nutt, Wolfgang Pauli, Rudolf Ernst Peierls, Isidor Isaac Rabi, Rosenblum, Robert Green Sachs, Max Schiffer, Erwin Schrödinger, Emilio Gino Segrè, David Shoenberg, Esther Simpson, Leo Szilard; American Physical Society, Columbia University, Magdalen College (University of Oxford), Manhattan Project, Trinity College (University of Cambridge), University of Illinois, and University of Rochester.
We would like to know the origin of your interest in science and where you studied.
Well, I was not born in Germany, but I came to Germaiy when I was about ten years old I was born an Lemberg, which at that time was a part of Austria, then Poland and now Russia Then I lived in Egypt for a while, and finally after the First World War ended up in Germany There I attended the so-called Real-Gymnasium, which is an institution where science as emphasized to a considerable extent I think the mathematics there and the physics was quite exciting You did learn, of course, nine years of Latin and seven years of French and five of English all in the wrong order for me, but there was quite a bit of emphasis on science as compared with the pure Gymnasium which has more the classical emphasis—Greek even.
The time I got interested in mathematics was when I was about 13. Soon after that I got interested in engineering, but I quickly switched to physics. By the time I was 17 I was quite sure I wanted to be a physicist. There was a popular book on atomic physics, which in retrospect I think influenced me a lot. I think it was by a chap called Kirchberger. I know it exists in the Library of Congress. I have a friend in the Library of Congress who found it is there. And I thought, just for fun, I want to get it out some time because if I have the time one day it might be worth writing an up-to-date version of that sort of book. It was the kind of book that was probably just right for young people. I met the author later. He used to come to colloquia in Berlin. Anyhow I went to this Real-Gymnasium in Chemnitz in Saxony, which is now Karlmarxstadt. And from there after graduation, which was called the “Abitur,” I went to the University in Berlin in 1930.
That was of course at that time a very exciting place. The old generation was still alive—Planck and Einstein. You could see them in colloquia, although I didn’t have direct relations with them. I could hear them talk occasionally. But the main physics was run by Laue, Nernst and Schroédinger. Nernst gave the freshman lectures and he was a very dramatic sort of lecturer. The colloquia were very exciting because there were always very interesting visitors like James Franck from G3ttingen, and Lise Meitner was there. I got the first course of nuclear physics in her lectures. That was one hour a week in those days for one semester. That was all the nuclear physics about ‘31.
Was it called nuclear physics at that time?
I think it was called Kernphysik. I could probably trace that. I’ve kept lecture notes. It would take a long time to find it all, but if one day it gets important—if you want to wait 20 years and I’m still around and have more time, I ‘11 perhaps find some of these things. But I think it was called Kernphysik. It could be easily found out. Then when the neutron was discovered in 1932 Lise Meitner gave a colloquium which was extremely well attended; it was exciting news, and she was very excited about it and she talked about the isotopes of the “brass nucleus” in her excitement; you know, the ”Messing=Kern” in German. You understand the unplanned joke—because brass is a mixture of copper and zinc, but nevertheless it has less isotopes than tin, so it isn’t such a bad mistake. Anyhow she was so excited that she said this. Lise Meitner, by the way, if she is still willing to be interviewed, might be of some interest to you; but she is of course now very feeble. We saw her about two years ago in Cambridge. We are probably going back to Cambridge this spring. We hope to see her, so maybe I could ask her a few questions.
I tried to reach her when she was in this country about a year ago, but she was ill at the time.
But of course Otto Frisch, her nephew, could tell you some things.
Other than this book that influenced you, were there specific courses?
I had already read some more technical books while I was in the Real-Gymnasium, though I didn’t completely understand them, like Sommerfeld’s book. We had a pretty good town library where I could pick up those books. One amusing memory I have is that I got out Sommerfeld’s book and I didn’t understand very much, and then I saw it years later, and I was surprised to see how much smaller the book looked. In my memory it loomed very large.
When you thought of physics, what did you think was of special interest?
I was pretty much interested right away in the more fundamental things, though I was always very interested in phenomena—-any complicated phenomena, whether they were solid-state or plasma. I was trying all the time to invent new ways of doing something, at least on paper. I wouldn’t pursue them all. I used to jot down my ideas since the age of 17, and I’ve done it more or less continuously.
Have you saved those jottings?
Yes. I saved all. Maybe one or two little notebooks have been lost, I haven’t been as consistent at all periods in writing down. Sometimes I jotted it down and couldn’t read it again. But most of the time I have been fairly consistent. I wish I had the time to go through these notes, but I estimate it would take me five years to really go through this.
We have devised a technique to cut it down to a few weeks-seriously—by not analyzing every page, but by generally deciding that something is worth preserving.
I didn’t mean in the sense of preserving, but there are some ideas there which I’m sure are still worth following up—at least I have this ambition And I know I have jotted down ideas which later on, though I did nothing about them, have become very important things So I know that there are still some nuggets in there, and in that sense going through would mean judging again “Is this still worthwhile? Is it foolish now? Has it been completely overtaken?” It may be of historical amusement I mean I have all sorts of ideas there which were very close to fission or very close to this or that. That’s only amusing, but are there still some things which are worth doing?
This is from the period prior to 1930?
It goes from when I was 17 in 1928. This is when I started my notebooks, I remember.
And you came to Berlin when—in 1930?
In ‘30. I was a student. I was still in the Real-Gymnasium, as it was called, high school you would say, when I started this habit. And then I kept it up. I’m afraid I was not a very systematic fellow because during a lecture if I would have an idea I would write it down on the margin. I used to know stenography in those days, which I don’t know anymore. But I know the kind of German stenography. I could learn it again because it’s a standard book. It was one of two standard methods I believe. I would recognize it right away—which of the two it was.
I’ve forgotten their names. I probably still have somewhere the instructions, so I could learn to read my old stuff. That’s why I say it would take me five years as a minimum to really do it justice, and I still have always kept up the hope that I would take the time—perhaps by doing it more leisurely at a slow rate ... But, as you know, all these good intentions ... So I won’t make any predictions, but I hope that I’ll still have a little time for it. And I’m still keeping up my notes, and each New Year’s I make a resolution to be more systematic. See, sometimes I jot it down quickly because I’m in a hurry or in a meeting and then I don’t always enter it systematically. And it makes such a difference. If I start systematically, I right away have much clearer ideas, I find, and one idea develops naturally into another. And sometimes when I’m in the mood I have to write many pages to catch up with my ideas. Sometimes it’s only two lines. So I always have this small notebook in my pocket.
Is this your characteristic way of working creatively—to write out the ideas?
I find that it is very rewarding if I write them out. If I don’t write them out, it seems to be just a little idea. If I do, it usually looks right away either nonsense or very good. I think I’m a fair judge of it once I’ve written it out, but I’m not a fair judge if I’ve just a line. I can be self-critical. But of course some of them are ideas which I know I couldn’t carry out by myself, but I might suggest them to somebody. It may be a theoretical idea. It may involve too highbrow a theory for me. Or it may be an idea for an experiment, some of them relatively simple experiments which you could do in a few days-some are more complicated. And so over the years—this should be perhaps off the record—I have had a lot of these great ideas, like the second neutrino and so on. You cannot talk about it later when somebody else did it because I did nothing about it. And sometimes I did nothing about it—inertia I suppose—you can always find a different excuse. But some of the things I have done, so I don’t feel I have let much go unnecessarily. But I haven’t been very professional about pursuing my ideas.
You can’t pursue all of them.
I’ve had too many deflections perhaps. I’ve allowed myself too many deflections, shall we say?
Perhaps too many ideas.
Yes, that’s true too, but I haven’t been disciplined. Now, one also has to worry about talking too early…For instance, I’ve noticed if you had a bright idea, say, in the evening, you have a tendency when you come in in the morning to tell it to your colleagues in the neighboring office. I find you mustn’t do that because unless you’ve thought enough about it, it can be killed because the other guy usually sees the negative part. You are a little blind to that part yourself if you pursue it a little while. Then if you don’t see the negative, it’s very good to expose it to criticism. But if I’ve thought a while, I find that if I have not talked about it right away, I’ve usually been able to answer the criticism. If I talk about it right away, it has this negative effect. It’s very interesting. I’ve learned this through experience and then often forced myself not to talk for a few days.
It’s sort of a greenhouse effect. You can’t set the plant out until it’s survived the initial period.
That’s it exactly. I think it’s very important advice to young people—to sit just a little while on the idea, not just to rush in like Archimedes. Someone is sure to say, “Oh, nonsense,” especially some clever critical people. Well, there are some people who are more critical than creative and very good at it, and they can see something negative. Maybe a day or two later they pick it up themselves, but the first reaction is: “Nonsense.” You know, the famous Pauli reaction. There are some people whose first word that comes to mind is: “Nonsense.” That can be quite destructive to a worthwhile new idea, but not when you’ve thought a while about it.
Getting back to this period, prior to 1930, what specific courses did you have in physics before coming to Berlin?
I don’t know whether in this country high school physics is as detailed because it took more than a year or two. It was a fairly detailed physics course by a fairly enthusiastic, though not too clever, teacher ,so you can see already that here is something that you understand a little better, which is probably quite healthy. And I had very good mathematics—geometry especially. I enjoyed that. I liked to prove theorems, and I once worked very hard to prove a theorem on ellipses and wrote it up in great detail. You could write a sort of micro-thesis in high school, so I worked very hard. I still have that. It’s probably 20 pages. And then when I came to Berlin I met Bargmann. I told him what I had proved and he did it in two lines by an algebraic method. I had done it geometrically. This was quite a lesson.
In the physics courses, did the new quantum mechanics get into the teaching or the textbooks?
I don’t think into the courses or high school texts. I believe I had that more from my own reading. I was very well aware of all the new quantum mechanics, though in a somewhat superficial sense. I could think about these orbits and so on—you know, everything which is more classical—and I knew about the wave picture, but I couldn’t really do a calculation. But I was aware of that all during my high school days.
Was there anybody who was a special inspiration at that time?
The only thing is, as I say, that I had good teachers in mathematics and physics, but I don’t think I talked with them about ideas. That was not customary. I did this more or less on my own. I don’t think I talked to anyone about physics ideas till I became a student in Berlin.
When you went to Berlin it was with the intent of studying physics and becoming a physicist. What did becoming a physicist mean to you at the time?
Well, in 1930 I would say that it meant, though I was probably vague about it, that I would end up in a university and try to make discoveries and try to understand the particles. As I say, when the neutron came along—that was ‘32 and a little later I heard about heavy hydrogen—I had my first important idea. I still remember it exactly. I have somewhere the little piece of paper where it’s jotted down, but I haven’t looked at it since ‘33. That was in March, ‘33.
I was still a student, and the Nazi students were rioting on that day and classes were just stopped. So I went to the library to read, and there was a German journal, something like the Scientific American. Its correct name escapes me. I don’t know whether it still exists. Anyhow that had a little paragraph there saying that G. N. Lewis had made a cc or a gram of heavy water. So I jotted down: “What can you do with heavy water?” I had five or six ideas but one of them was: “The photon could disintegrate the deuteron.” It shows you the speed with which things moved in those days. This was March, ‘33. I left Germany about May, ‘33, and I had applied in various places and I was accepted in Cambridge at the Cavendish. Laue and Schroedinger wrote me recommendations and Rutherford wrote me that I could come as a student. When I arrived in Cambridge—it must have been early sumer of ‘33—I ran into Shoenberg. I don’t know whether you know him. He is a professor there now. He’s a low temperature expert.
Yes. See, in England you did not use first names so much. I still have a hard time remembering first names. So I told him I want to study there and I asked, “What’s the cheapest way?” No, in fact it was Chadwick I asked. I had found out the cheapest way is to join something called the Fitzwilliam House. It was for all people who were not in a college. It’s quite an amusing story, but let me just remember. I think what happened was I ran into Chadwick very early. That was the right order. It was within a day after arriving-just to look around. And I told him I wanted to study and I said I’d found out the cheapest way is at the Fitzwilliam House.
And he said, “Oh, don’t do that. Join a college. They do things for you.” I didn’t know what it meant. So then I ran into Schoenberg and said, “What are the good colleges?” He said, “Trinity, St. Johns, Magdalene.” So I went off to Trinity, and they said, “Sorry.” It was perhaps by that time August. You know, in Germany you just go straight to the university and you just sign your name and you are a student. In Cambridge you had to be accepted like a modern student at Harvard and so on. So I. went up to Trinity and they said, “Sorry, we are full,” because the term started soon, in October. So I went on to St. John’s. They said, “Well, we’ll let you know in six weeks,” which seemed a bit late. So I went to Magdalene. You go there to the senior tutor. I walked into his office.
He said, “Ah, you are a refugee. I suppose we ought to have one.” [Laughter] So I was in. Next he said, “I suppose you have no money. We better give you lOO £,” which was exactly half of what I needed. He was a very nice fellow. I got to know him quite well. So I got the other half from refugee organizations and the Cavendish and so on. Thus I managed to be a student. I left during the seventh semester in Berlin, and in Cambridge they allowed that as the equivalent of a bachelor’s degree, so I started straight as a research student. I first did theoretical work with Ralph Fowler, who was the son— in-law of Rutherford. He was a theoretician interested in thermodynamics, but I had an idea which I think led to my first paper, maybe my second paper.
We have your bibl iography.
By the way, my secretary has a vita. Maybe you have that, too. We can give you that, and maybe that will be helpful for me, too. Let me ask her for that
We have a rather incomplete one.
She has one sort of written up for journalists. I started with Fowler, and I had this idea on the probability of artificial nuclear transformations and its connections with the vector model of the nucleus. And though he was not interested in that, he said, “Fine. Go ahead and do it.” I mean it was not in his line of research. So I talked with Mott a little about it. Maybe you’d like to know a little what’s in this paper because at least Wigner paid it a compliment. It was the first sort of single— particle model of a nuclear reaction. It was very simple. Cockcroft and Walton had found disintegration of lithium-6 and 7 by protons, (first 7, then 6) and I noticed they got a much more copious yield from 6 than from 7, though the energy release was much higher in the case of 7. And this seemed somewhat strange.
So I had this very simple idea that the spin of the nucleus plays a role. Lithium-7 was known to have a spin of three-halves, so if a proton comes along, you can form one or two. If you formed one, then you couldn’t get two alpha’s because they ace symmetrical. If you formed two, you could get two alpha’s, but if the three-halves was really due to a P-proton, then you would have, so to say, what you now would call the wrong parity. My knowledge wasn’t good enough to say the wrong parity, but in a more complicated way I said it couldn’t happen with an S-proton. Anyhow I developed the idea that the yield is so small, because the reaction could not proceed with S-protons. But for the lithium.6 there is nothing which stops you. And so that yield can be high.
That actually turned out to be the correct thing. It was developed further I think by Konopinski and Critchfield and Teller. Wigner in a little book with one of his former students on nuclear physics credits this idea with beginning the single particle picture. Anyhow, not enough theoreticians perhaps took it seriously right away. It got sort of a little bit of credit later. The other thing which may interest you was that Cockcroft and Walton found that with deuterons on lithium-6 they got a very good yield, and in those days lithium-6 was “known” to have spin zero. I said the good yield could not be understood with spin zero. You see, you get zero plus one, from the deuteron, but you need zero or two for the total angular momentum. Lithium-6 must have spin one. Then Rabi and Fox measured the spin and confirmed it to be one. The previous belief was due to its small magnetic moment. I’ve always discovered methods of measuring spin in the craziest ways. This was the beginning of that. I’m still at it.
By that time in ‘31 when you were completing your degree.
I was a student.
You were already a nuclear physicist.
Let me backtrack just a bit because we don’t really have enough information on the University of Berlin, from the time you were there in 1930 to ‘33. think it would be most interesting to go into a little more detail on the events you mentioned—on Lise Meitner coming and the news of the discovery of the neutron.
I did take, I believe in ‘31, her course in nuclear physics, which gave me a sort of systematic idea of what was known. She was more interested in phenomena and so was I at the time. She did not probe too deeply theoretically. But for her every line was greatly exciting—you know, e.g. a new alpha particle line. She knew about the alpha fine structure in those days. About this same time also, though it doesn’t in my memory play a big role, but I must have been exposed to this, these ideas on the neutrino of Pauli came along, because Lise Meitner and Ortman, who was a younger man there, were measuring the heat developed in beta decay and noticed something is missing. Actually they were confirming Ellis and Wooster who had done this at the Cavendish. And when I came to the Cavendish, Ellis was still there doing beta-ray work with his students, and Ellis and Mott I believe in a review of the situation, Ellis and one of his students in experiments for the first time proved quite definitely that in beta rays what matters is the upper limit, that there is a well- defined upper limit of the spectrum.
And did they do that before Pauli made his hypothesis?
No, but it was not closely related to Pauli’s. They did it after Pauli. But, you know, in those days ideas just didn’t get taken seriously so fast. But anyhow what was taken seriously was that there was a well-defined upper limit. Before that Bohr had had the hypothesis that maybe these electrons go up to infinity because it seems very strange that electrons would come out of a nucleus, first of all, and everything seemed to go wrong at about that time. The spin was wrong. So Pauli’s idea was a very necessary idea. When I give a talk on neutrinos, I like to joke that Pauli, who was not a very timid physicist, suggested this idea very tentatively only because in those days there were only these two particles— the proton and the electron&mdashmdash;and to imagine a new particle seemed to be terribly radical. But he was provoked by the fact that energy would not be conserved, and then it turned out also momentum and spin. I usually add: “And today it needs much less provocation to invent a new particle.” This is progress, you see.
But when did people first start taking it seriously?
The neutrino? Well, I would say ... For instance, in our paper—I’ll come to this under photodisintegration—we come to the conclusion that a half integer spin is also needed for the neutrino, and I think by that time I certainly took it very seriously. I just cannot remember what I would have said about it in ‘33, but by ‘34 I was quite convinced of it. Even before the war I remember thinking how you could discover it, and that’s actually one of the ways by which it was discovered. Only before the war you couldn’t think of using a reactor, so I thought perhaps with a cyclotron, but it would have been a very borderline experiment. People did look for it before the war, but it’s clear now with the cross sections expected that they didn’t have a chance with the sources they used. There was a physicist, Nahmias, who looked for it.
This course on nuclear physics with Lise Meitner: did you save your lecture notes on that course?
I think so. They are in terrible shape. I’ll look at them some time.
It would give some clues to the state of knowledge.
I don’t think it had a tremendous effect on me. Then I went to the Cavendish and there I took Rutherford’s course in nuclear physics. He was a very dramatic lecturer and full of anecdotes. He made it come alive. So this was very impressive—also very phenomenological, everything he did; very simple derivations. I think that’s very important for the first learning and this is perhaps something students now miss. They get the theory of nuclear physics thrown at them; sometimes before they ever know there is a phenomenon they have the complete theory of it. The phenomena are not sufficiently emphasized, I think, in teaching today.
In Berlin again, was there a student discussion group of any type? Did you have a particular group in which to kick around these ideas?
No. I just developed alone, though I think the modern idea works very well for some people.
There was no other student that you could discuss problems with either?
I don’t want to say there wasn’t. I did, for instance, have a close friend who’s now professor of mathematics at Stanford, Max Schiffer, but we didn’t discuss physics much. He was more impressed with mathematics; I was more with physics. I was teasing him, saying the difference is that in mathematics the professor gives the problems and in physics nature does. I did certainly have discussions with him, but I don’t remember probing deeply. It must have been my private way only. I don’t think it was in those days so common. Maybe the pure theoreticians had more of these bull sessions. I’m sure they had them at Copenhagen, but I didn’t go to Copenhagen until I was invited after the photodisintegration work to come to one of their meetings—I think in ‘35—when I was a student and then again in ‘36. But that was one big group, largely theoreticians—they would allow a few experimenters.
What about the change that took place after the announcement of the discovery of the neutron? Was there a change in the atmosphere in Berlin and in the atmosphere of discussion of nuclear physics?
There must have been. I can’t remember much of a change because I just didn’t have the close contacts. I wasn’t well known to the professors, though I knew Schroedinger, and certainly there was some discussion. You see, it took a little while before it entered people’s minds how important the neutron was for understanding the whole nucleus, though there is this interesting, prophetic statement of Rutherford’s in ‘22 I believe where he predicts the neutron.
In the Bakerian lecture, I think, in 1920.
And this came about by one of his very few experimental mistakes. He thought he had seen a helium 3 come out among his long-range alpha particles by a deflection, and so he said, “Helium 3—well, what does that mean? That’s three protons and one electron. So what would happen if you had two protons and one electron? That would be a heavy hydrogen.” That was ten years before the discovery. “And what if you had one proton and one electron? That would be a neutral particle.” And then he said, “Come to think of it: how could the elements have ever been made unless you had a neutral particle?” The Coulomb barrier would not allow it, if only charged particles existed. It was a very clever remark, coming out of a mistake. Then he found his mistake later, but he kept looking for the neutron, and he and Chadwick looked for it already in ‘27.
I don’t know whether you know that. Earlier one of his students looked from a discharge for neutrons—just put scintillators outside the discharge and of course he saw nothing. But with Chadwick he looked in a way where he might have found it, only they didn’t have sensitive enough detection. They just again used scintillators with which they discovered protons and alphas, and from aluminum with aiphas they had very long-range protons—about a meter in air. And then they were looking whether there was anything beyond, which would be neutral; but the scintillator being a very poor discoverer of neutrons, they didn’t see any. But there must have seen some neutrons. This was ‘27. So Chadwick was well primed for neutrons when Curie——Joliot and his wife—(Joliot and Curie, they were called at that time) came out with the discovery that beryllium plus alphas yields gammas which knock on protons in a cloud chamber. I once heard that when Chadwick saw that report in Comptes Rendus, his pipe dropped out of his mouth. He “knew” these were neutrons.
So all he had to do was go back to the lab and prove it, and he did really very simple experiments. When you read it again you can see how simple they are. But he was so convinced. That shows you your mental picture is very important. And he got a rough neutron mass. It was all very rough. But the main point was the change in thinking; that the Compton effect for protons should have had a much smaller cross section. Also, he showed that if you use helium instead of hydrogen, the Compton effect would give you a different energy; whereas with neutrons you get a consistent energy. Crude as it was, it showed there was a massive particle. So he had this little ionization chamber which he could fill with hydrogen or with helium, and he worked very hard for two weeks— nights also, something very unusual in the Cavendish; you couldn’t get in after six or so.
It was locked up and he had to climb through windows to get in to discover the neutron. I still remember later when I had to get special permission to stay till ten—I’ll come back to that—to make a discovery, because you were not supposed to work at that time, which on the whole was probably very good because you stopped at six and you thought in the evening and you didn’t start until ten in the morning. On the whole it was a very productive period because the apparatus wasn’t so complicated that you had to work at it all the time—although there were some Englishmen who had very different habits and worked really around the clock. But in the Cavendish in the days when I was there they had these leisurely hours and produced the most marvelous work because the bottleneck is still usually up here in ones own head, I think.
This can be attributed somewhat to Rutherford as the overseer in the sense that you felt that when you did work it had better be productive. Is this a true picture of his attitude?
It’s true. He is supposed to have been very impatient when you were not productive and he appreciated ideas and he once even asked me whether I would write down a research program for the next year. I don’t know how many people he asked to do that. He didn’t do these researcheA-’ then, but he asked me to write it down. I ma have a copy-I don’t know. I gave it to him, but I may have kept a copy. It would be amusing to see what I wrote down.
Or what you carried out.
No, this was not for me.
Oh, the lab.
Yes, I was expected to write down a general research program. This was probably around ‘35 or ‘36.
That would be interesting because it would tell a little something about you and would also tell about the problems that were important.
It probably is very trivial stuff.
But it would reflect what you •felt was important.
If I can find it, but I cannot promise that I am going to look tomorrow because, as I said, my papers are in a mess and I know I have to plow through literally nearly 40 years of notes that are not systematically kept. 1 have the dates. I was always very careful to write down the dates, and nearly always it’s really the date. Sometimes I say, “less than ...“ because it was a week earlier and I didn’t get around to it, but nearly always it’s the day of the idea. But these little books are not all in order ,so I have to first put them in order. Then there are these many marginal things, and some of it will be unfortunately unreadable. Some of it I’ll have to decipher. So I’m afraid, unless I become a historian myself, probably nobody else will want to go to that trouble.
Well, we’ve been helping people.
Well, if I ever feel I can get help
But the main investment of time has to be your own.
Not in the immediate future. But if your project continues and I am here ... When I asked for this vita and this bibliography, I was trying to tell you my very first paper was called “Spontaneous Emission of Neutrons by Artifi4ally Produced Radioactive Bodies.” Now that sounds interesting today. In those days it seemed like a crazy idea. What happened was that Joliot, who was a very imaginative physicist, had thought he had seen neutrons emitted, from aluminum-28, which has a 23-minute activity. You make it with neutrons—aluminum-27 captures the neutron. He made it in another way. I think he made it from silicon with fast neutrons. Slow neutrons were not yet known.
What was he using as a neutron source then?
Fast neutrons, everybody was using polonium-beryllium or radium- beryllium He had the world’s largest source of polonium inherited—not yet inherited, she was still alive—from the old Madame Curie I unfortunately never met her She died about a year or two later They had this large polonium- beryllium source, so they had these fast neutrons.
He made the first artificial radioactivities with polonium aiphas on nitrogen and other light elements Well, he made aluminum-28, with fast neutrons or maybe with aiphas You can make it from magnesium with alphas It’s not relevant how he made it He said, “It’s aluminum-28 and it emits neutrons,” because he saw proton recoils in a cloud chamber. All I did in this “letter” is to point out this doesn’t work. You cannot have aluminum-28 emitting either a beta ray or a neutron because the neutron has to be bound. If it isn’t bound, it will come out right away. If it comes out later (I didn’t deny his experiment), then I said it must follow the beta. That’s delay. neutron activity. So when fission was discovered, the first thing I said “There must be delayed neutron activity.”
Of course other people found it faster than I did. I actually looked for it in Illinois, but we didn’t have the best means. Anyhow I right away said that there must be what I called “spontaneous emission,” following Joliot. By the way, if you ever want them, I still have a lot of the reprints for my papers. I used to keep reprints. I can tell you an amusing story on that, too. So this was my first paper. I remember some people took it just as a criticism of Joliot. They didn’t realize that I tried to make a suggestion that really he may not be wrong experimentally, but the neutron, logically, must come out after the beta-ray. Actually, he was wrong experimentally.
But how was the delayed neutron idea accepted—not until fission?
Then it was revived, but people didn’t seem to realize that I had said it so much earlier. It was revived and the people who found delayed neutrons first—-I think someone in this country
But this must have been after
It was very important for the fission. I don’t know who found it first in this country. But I remember saying it as soon as fission was discovered, just reminding myself of my old paper.
Did Rutherford or anyone encourage you preparing in this?
Well, this paper had an interesting effect on my career. This shows you how things go because what happened was No, I keep on mixing it up. I was actually working on this first paper—the one which I said when I was Fowler’s student on the probability of artificial nuclear transformation and its connection with the vector model of the nucleus—and when I was working on that I had to know the lithium masses. So someone said, “Chadwick is the one who knows the masses best. Go to see him.” So I went to see Chadwick asking him about the lithium masses, and when I was talking to him I told him about my year-old idea of the photodisintegration of the deuteron, and he first didn’t catch fire. I said, “Look, here’s a deuteron.
Couldn’t we disintegrate it with gamma rays?” He didn’t catch fire at first. But then I said, “If you can do it, you get the neutron mass out of it.” And then he caught fire because he had a big fit with the Joliots who had a different massan he. Lawrence had the much lighter mass, and the Joliots had the heavier one, and he was in between. Let me get it right: the heavier it is, the more bound the neutron is. And the lighter the neutron is, the less bound it is. And Lawrence thought actually the deuteron was unstable because Lawrence had shot deuterons at brass targets and found neutrons come out at very low deuteron energy. So he thought he just tickled them and they disintegrated. It was really unstable, he thought, but what Rutherford and Oliphant showed later was that Lawrence had been misled by the deuterium being absorbed on his brass target, and so he had had the d-d reaction, which works at very low energy, and produced neutrons. But at that time Lawrence kept maintaining, in ‘31k, that the deuteron is really unstable; the neutron mass is quite wrong.
The Joliots had a heavy one and the neutron mass, I said, “You can see you get the binding energy.” Then he caught fire. Well, I left him then and about six weeks later I came to ask him about this other paper. “Spontaneous Emission of Neutrons by Artificially Produced Radiactive Bodies.” I wanted him to read it and to find out what he thought of it. As I talked to him he said, “Were you the man who suggested to me this photodisintegration? We didn’t call it deuteron; “diplon” it was called then. “This nuclear photoeffect?” or whatever he said. I said, “Yes.” He said, “Well, it works. It worked last night.” And then he said, “Would you like to work with me on it?” And I said, “Fine.”
So I started working with him and from then on we did everything together. Chadwick was a busy man, and he gave me a lot of leeway in doing the experiments. So this paper, which we then wrote, which is a very detailed paper, had a big number of experiments all following from that one. Chadwick was a very modest man. In those days there were no secretaries. When he saw me write the paper, he said, “They’ll never be able to read your handwriting; I better write it.” I have the paper in Chadwick’s long-hand—the whole paper: 20-odd pages. And then he went on vacation when the proof came, and with the proof came a note: “How many reprints?”
I had never ordered reprints before. I didn’t know. So I just thought 250. Then a trememdous bill came. So Chadwick said, “Are you crazy? Two hundred and fifty. You pay for it.” So that taught me a lesson. Still most of those reprints are gone. Anyhow he was a very nice fellow and a true gentleman as you can see from this story because he hadn’t had the Nobel Prize yet. Who would tell you that it works when he doesn’t even remember your name and not run away with the show? He took me in and I considered this as normal. It took me many years to realize that he was a special gentleman to do that. He really was. So I’ve been telling you about these first papers in a sort of roundabout way.
Well, this give a feeling of the atmosphere.
Well, it’s a little bit of egotistical, and tell me if you want more of the general things because some of this is perhaps not so nice.
You are the only one who can provide this information, so it is not egotistical. We must go to you for this. We can’t go to other people.
Well, we had many amusing things. Now, there was one interesting thing. When we wrote our first letter to Nature on the nuclear photoeffect, which appears here as number two in my list ... You see, in real order the first paper I started on was this number three. The second one was this number two. But in the meantime I finished what is now number one. I still remember: that was the only one I typed myself, and I never worked so hard in my life. I hope I have that manuscript. Of that I was really proud. Terrible one—finger typing. From then on I learned it’s not worthwhile. I’d rather dictate to Chadwick. (laughter)
Or some other scribe.
Anyhow, I was trying to say: there was one interesting thing. When we wrote this letter on the photodisintegration of the deuteron to Nature, I had the last paragraph there where I said the following in the manuscript (it is not now in this paper, but it’s of historical interest to you- perhaps) ... There was a fellow called Lea, an Englishman, who worked I suppose originally under Chadwick but by that time had his Ph.D. Lea had found an interesting effect. When he shot neutrons at paraffin, he saw gammas come out. Now, we had done the photodisintegration and I knew how to calculate from one cross section the inverse, and I found from our cross section that Lea should have seen a cross section a thousand times smaller than he did. So I said, “Well, he couldn’t have seen just the inverse.
Probably what he saw was that the neutron slowed down in the paraffin and then the slow neutrons were captured.” This was my last paragraph. So Chadwick said, “This is too speculative,” and I agreed with his taste. We cut it out. This was a few months before slow neutrons. But Lea knew about this, and the only paper where I am credited with discovering slow neutrons is in Lea’s paper, which you can find in the Royal Society. He credited in this order: Goldhaber and Fermi. Now, of course, Rutherford also knew about it. During the summer Amaldi and Segr were visiting the Cavendish and I suggested to them. . They had the big problem whether in sodium where they got the neutron activity it was fast neutrons making it—they knew it was fast neutrons but whether it was a neutron knocked out, like (n, 2n) you would call it now, or whether it was (n,), they couldn’t distinguish that. So I said, “Look, why don’t you first slow them down in paraffin and see whether you get much less?” Then it would be (n, 2n) I didn’t say you would get much more. I don’t want to claim that.
So Segre threw up his arms. “There’s so much to do— why should we do that?” Bjerge and Westcott who were there may remember that. Anyhow Rutherford thought I had suggested slow neutrons. When Fermi’s paper came out he said, “He’ll give you credit, won’t he?” I said, “Why should he?” because he didn’t know about it. He actually found it by chance. He put a neutron source on woodand noticed the increased effect and was clever enough to realize it had to do with the hydrogen. So the systematic way in which I could have found it if I didn’t have my own counters. I suggested to Bjerqe and Westcott, who could have done it in an afternoon, but brushed it off because it was pooh-poohed. There were so many things in the air. I’m giving you this as background. I hope this you will keep secret for at least 50 years or so because there are too many people involved.
So I said this effect of Lea’s could only be explained by slow neutrons. Now, while we were doing this paper on the photodisintegration, Bethe and Peierls did the theory of it. And so I wrote to Peierls or both of them—-I think I wrote to both, they were in Manchester at the time—saying, “Look, this inverse cross section should be much smaller and maybe what Lea is seeing is that the slow neutron is captured in hydrogen.” So Peierls wrote back saying (I think I have this correspondence), “No, this could not be because the theoretical photo-electric cross section goes to zero, near zero neutron energy,” which is true. See, what wasn’t known at the time was that there is a photo-magnetic cross section which does not go to zero. So I wrote back then that maybe it was not the hydrogen but the carbon or the iron of the ion chamber which captured these neutrons, and this he did not answer anymore.
This is probably the right explanation. It was the iron which captured the slow neutrons—chiefly. I have copies in my own handwriting. That is, I wrote one copy and it was too poor and so I wrote it again and sent him the clean one. This I have kept just for fun and I think Peierls’ answer. This I think should be kept for 20 years. Some people might feel awkward about it. But it is of interest perhaps to you. It just shows you in those days I was really swimming with the thing, and I felt these things didn’t hit me as a surprise. When Fermi’s paper came out I had a double preparation which nobody else in the Cavendish had. First of all the word “slow neutrons” even wasn’t new to me, so I could understand right away. Secondly, it was in Italian and since I told you I lived in Egypt and in Egypt a lot of people speak Italian, so my mother learned Italian and my father always spoke Italian well. And whenever they talked about something which we children were not supposed to understand, they talked Italian. So when Fermi’s papers came out I understood them.
You were one of the few I imagine.
Because all this Italian background which I was not supposed to understand had worked very well. Of course a little Latin and French helped too.
You mentioned your parents. What was your father’s occupation?
He was a businessman at the time, but he always has been very interested in Egyptology. •That was his real love. So when he lived in Egypt he learned all about it and he’s still very interested in it. So it wasn’t a scientific bacround. I did have an uncle who studied mathematics, but he was killed in the First World War, so I never knew him, though I heard about him often. So there was no other direct scientific influence, but there was this usual sort of background that learning was accepted as normal.
That was something that we left out before and I thought we’d go back to it. This is a most interesting period. Let me go back and ask one question: How did Lise Meitner know of the neutron discovery and what time lapse was there before she introduced it into her class? Was it a class lecture?
No, she gave a colloquium. I only remember it from the colloquium. I think I took the class before the discovery.
You said ‘31.
I think so. I’m not sure of that. Anyhow, I don’t remember it much in the class. I do remember this colloquium because this was a very well-attended and exciting colloquium. Remember, in those days everybody was brought up with only two particles—proton and electron—and some people talked of the photon. It was such a radical thing to say, “There is one more particle.” She learned it I think from Nature. Preprints came only into this field I think with Fermi’s school. They had a very good arrangement with their printer that the day essentially they sent it in they could get a hundred reprints. They sent out the reprints before the journals arrived. When they knew me better they sent me something which you might call a “preprint.” Incidentally, I should say I knew Szilard well.
I first met him in Berlin, but then I got to know him much better in England. He had left for England also in ‘33 very early. And through me, he said, he got interested in nuclear physics through all these discussions. And he thought, “If this young chap can do it, why can’t I do it?” So he started with a fellow in a hospital in London, Chalmers, who is still around there. He visited me here the other day. So as soon as we did the photo effect, Szilard knew enough ... We actually mentioned in the first paper that you might disintegrate beryllium. He right away got himself some beryllium, which was not easy to get, and he showed the neutrons from beryllium. I wasn’t very jealous. Some other people thought he had butted in. I didn’t feel so.
What was he hitting the beryllium with?
With photons, with radium gamma rays. We had done the deuterium in great detail, showing the protons and the neutrons and everything. He showed first the beryllium photo-neutrons. In fact, before we saw the neutrons from deuterium—we had seen the protons first—he did the neutrons from beryllium. But it’s all contained in our paper as a sort of a program obviously. You might like to read it just for fun. It’s this reference—you can easily get it—in Nature. But I still have some reprints.
At this exciting colloquium in Berlin.
I suppose Szilard heard that, too, though I cannot be sure. He was at that time at Berlin. And he got very interested. In neutrons. You know, he had the idea of a chain reaction from beryllium—that’s why I mention it—about in ‘34. He told me once he was waiting for a traffic light to change (in London) and suddenly it occurred to him because, you see, the binding energy of beryllium was also wrong, and so it looked as though the beryllium needed only tickling and the neutron would come out a la Lawrence. So he thought, “If you knock out the neutron so easily with photons, then this neutron could knock out another one and you’d have your chain reaction.” Because the binding energy was wrong, this idea turned out to be wrong. It does work actually with faster neutrons, that you get neutrons from beryllium; and in some reactors it’s a little extra contribution but not enough to keep a chain reaction going in pure beryllium.
But it gave him the idea, and so when fission was discovered, he only had to transform it and right away he thought of the chain reaction. So all these things are so interconnected, and I only know a few strands, you know, It’s like seeing a beautiful Persian rug and all you know is you worked on those strands and your friends on those and at the end there’s a nice pattern. Now, I was in very close contact with Szilard when he was in England until he left England in ‘37—Christmas—because he saw the handwriting on the wall. I remember just before he left he was up at Magdalene and had dinner with me. There was this famous Englishman, who was also a fellow at Magdalene and then became a professor at Harvard, A.I. Richards.
He was just telling Szilard that he was going to visit America and Szilard said, “Buy a single ticket.” Actually he did end up here. I don’t know whether that was the, trip. He was a fellow who had lived in China a long time and had a beautiful Chinese furnished room, which I got later when he left. I worked with a Chinese, who is now in China, of some importance—Chang; you may have heard of him. He was in this country a long time. He worked with me after I got my Ph.D. Chang was once in my rooms, and I said, “How old are these things?” and he said, “Not very old—500 years”
Szilard saw the handwriting on the wall, you mentioned. Wasn’t he involved in a rescue operation, in trying to help people get out?
Yes. The first years he did a very vital and important job. I don’t know whether he started it or took it over or was the important cog in it, but I can tell you exactly from whom you can find out. He, had this society for rescuing scientists. My wife will also remember. She got once a small grant from that society—it was called the Academic Assistance Council. Anyhow, the English girl who was really running the routine and always had pleasant fights with Szi lard because he had so many ideas was Esther Simpson, who still runs the Society for Visiting Scientists, or it may just about be gone too.
Yes. She still lives in London. Whenever we go there we see her. She’s a great friend of my wife’s and she also knows me well. She knew us before we were married. I still remember: when Rutherford died and the new professor was appointed—Bragg—I felt that the Cavendish will get out of nuclear physics, that my interests are not in the x-rays where Bragg’s are. Nearly everybody left. So I felt: where could I go? Elsewhere in England physics seemed so provincial compared with Cambridge that I couldn’t think of going anywhere in England, so I thought I had better make a visit to the States. In ‘38 I came to the Washington meeting. But when I said goodbye to Miss Simpson, she said, “If they offer you a job, take it.” It was quite a surprise to me because I didn’t think of a job. I just wanted to see this country. You know, in those days one wasn’t as job conscious as now. You know, when one is young and unmarried
Even with the threat of Naziism?
Well, I tell you what happened Just before I left, Dee, who was one of the physicists at the Cavendish, came back from a meeting in Berlin He spoke with such enthusiasm about all these Nazi crowds— how patriotic they were I mean he was naive He’s a fine fellow He was just naive He was so taken in by this great patriotism, and that really practically changed my mind about staying in Europe when I heard him talk. Of course before that one thought of England as so strong compared with Germany that one wasn’t worried yet in ‘37 unless one was Szilard who always saw it a little earlier. So I was not yet worried. I was more worried about Dee’s reaction, that he was so misunderstanding the sign. That was actually an interesting warning about how the English thought. They did misunderstand it. So I went to the Washington meeting in ‘38 largely to see America once for myself because I had a very wrong idea from the movies. I was very prejudiced.
What about from physics journals?
I was ,also prejudiced because there were so many half-baked papers, and that7in England the great sin.
In the field of nuclear physics?
Yes. Of course Ernest Lawrence with his great enthusiasm, you know, and the radioactive papers, you know—we criticized them. But when I visited here I realized the people are much better than their papers—they just write them in a hurry, they are not critical when they write them. I learned from people you work six weeks but then you write it all in one hour. With me it might sometimes be the opposite. I wrote carefully. And so I think one got a much worse impression from a distance than when I visited. So when I visited I was quite impressed, and in the course of the visit, Loomis, who was then chairman of physics in Illinois, offered me a job. I think what had happened was that he had offered jobs to Bethe, Franck, Rabi—those people—and they didn’t come.
And then he asked them, “Who next?” And Rabi had met me on this visit and seems to have suggested to Loomis that he offer me a job. So Loomis at the end of the Washington meeting said, “Would you like to have a job in Illinois?” And I said, “Oh, I cannot just leave the Cavendish like this.” This was in the evening. Then at breakfast somehow Szilard had had breakfast with someone or other and heard that I had just said this, and he said, “Oh, you should at least look at it before you say no.” So I said to Loomis: “All right, I am willing to look at it.” And then on this long trip on the train out, I thought, “It won’t do me any harm to accept for a year. I’ll let the Cavendish know—let me go for a year.” And so on that basis I accepted the job. I did go out because I had just been in the Midwest but missed Illinois because it looked so uninteresting from the papers I had seen. I had seen Chicago——I had met Sam Allison. And Sam Allison and the Graves. (You know, Diz Graves and her husband, who just died recently) gave rig, a car trip from Chicago to the Washington meeting. Thus I learned about the landscape.
So I had just missed Illinois and had to go all the way back on this little train which stopped everywhere. I once told this story to one of the professors who had come from Poland, a classics professor who was a very great wit in Illinois, and he said yes, the first time he came he had stopped in Arcola, then in Tuscola and he thought the next would be Pepsi Cola but no, it was Champaign. [Laughter} Anyhow there I was. Loomis was very good in persuading people. He met me at the station, gave me the red carpet treatment, put me up in his house, warned me not to marry any of his daughters because in Illinois they had a strong nepotism rule. He said, “If you marry one of my daughters, you have to go or I have to go.” Anyhow they were little girls then and now they’re all mothers. He offered me this job as assistant professor and then he told me what the salary was. It seemed to me fine. I had no feeling for salaries. It was a lot more than I had gotten in England. Of course the value of the dollar was already less but not much less in ‘38, so I felt, “This is fine.” So I went back, and when I arrived I said, “I have this job.”
You went back to the Cavendish?
Yes. I had been on just a six-week visit. This was for the Washington meeting, starting some time in March and returning in May. I had a lot of friends in many places. I met Bainbridge in Cambridge and so I went to Harvard. I gave talks everywhere, and they paid my trip around the country. I remember I had to declare all my income before I left. It came only to about $75 I guess. The man said, “What did you talk about?” I said, “Nuclear physics, atomic physics.” He said, “Well, the newt time you come talk about the love life of the atoms. Then you’d real ly make money.”
That’s a beautiful story. This was the customs man?
Before you leave this country as a noncitizen, you must always get an exit permit and you have to declare that you have paid up on your income tax. I don’t think I had to pay on this $75, but you still had to declare all the income you made while in the United States.
So you went back to the Cavendish.
Yes, and at that time the new professor had not yet arrived. There was an acting professor, Appleton. You must have heard of him-the man who discovered the Flayer. When I told Appleton that I had this job in Illinois, he said, “Fine,” as though that was a worry off him because on the one hand they didn’t feel they wanted to throw me out and on the other hand, they had no positions. I really sympathized with that.
You had just received your Ph.D.?
I had two years of fellowship. I was the Charles Kingsley Bye Fellow of Magdalene College for two years after my Ph.D The college gave me this fellowship, which was a temporary fellowship, not a permanent one. At the same time Cavendish gave me some kind of a D.S.I.R. grant, so I was living quite well. But the next would have, had to be a real job-—either a lecturer or demonstrator, as they called it—and they just didn’t have these positions Or you could have had a fellowship The English also have just fellowships on which you live modestly and do nothing else You’re forever a research fellow Now, I quite appreciated that they didn’t have enough room. Maybe they were a little blind. I mean everybody disappeared—Bethe, Peierls, Teller, Rabinovitch, all these people left; Szilard had gone earlier. The only ones who stayed were in the low-temperature group where Lindemann made room for them.
Do you feel they left in response to Rutherford’s death?
No. I did, but the others left earlier. They left because they didn’t see a permanent position and they perhaps also saw a better opportunity for research. This is what finally convinced me. I felt in Illinois I could really build up my own and not depend on some prof, you know, that there is this independence in this country. Unless you are the professor, you don’t have it in Europe. On the other hand, I didn’t know enough about it. Had I been offered a nice fellowship, I perhaps would not have looked except that I felt the Cavendish was going in a different direction from my interests.
Was there a strong nuclear physics department to attract you at Illinois at that time?
No. There was a small one. They had a little cyclotron. Kruger had built a little cyclotron. They wanted to build up nuclear physics. They gave me a magnificent grant of $3000 for research from the graduate school, which was really a lot of money in those days.
Was this university money or was it outside money?
This was money from the state to the graduate school. So I felt I really could do what I wanted, and I got students right away.
What about the difference in the economic situation in England at the time? Wasn’t this a factor? The ‘30s still was a period of depress ion.
I don’t know. This may have been a factor—that they couldn’t create positions.
That’s what I mean.
I did not feel the depression in Cambridge of course. I was well insulated. I mean the things which excited England at the time were the Spanish Civil War and the Nazis coming up. Only some people saw that. Most of the English still had a bad conscience from the First World War and thought the Germans were only getting their own back. So they completely misjudged the Nazis. Then I left in ‘38, as I say, for Illinois, and I went back in ‘39 to get married. My wife was at Imperial College. I came back in May and the weather was beautiful, but there was this uncanny feeling that the English were asleep. I talked to Cockcroft. 1 said I had seen only one anti-aircraft gun and one airplane. He said, “You should go north. You would see much more,” that the English industrial heart is north. He came from Manchester and he was in close touch. So he was very confident they were preparing well enough. But of course you know how well prepared they were in ‘39. Had it been another year or two they might have finally been prepared. This was probably one reason that Hitler started so early.
It seems that Rutherford was conscious of the problem, at least the problem of the refugee scholars, because he wrote some articles.
Yes, he did a lot. I remember a very interesting, huge meeting at Albert Hall where Rutherford gave a talk to collect money for these refugee scientists and Einstein talked and Rutherford introduced him as his great friend. This was probably ‘33.
Just after you had arrived really.
Yes, it was a huge meeting—probably when I was still in London because I don’t think I would have gone from Cambridge to a London meeting just to go for an evening for the meeting. Having just arrived, I don’t think I would have been in the habit of doing it, so it was probably still before I had gone in October to Cambridge. My sister at that time was also in London. She was staying with a family where she was watching little children, so I was not far from her and stayed in London until the school started in October. I was of course always reading. I started reading Nature very early and other journals. So I don’t think I was wasting my time that summer. I must once find out what I put down in my notes if I can find them.
Before you get to the transition to the States and get back, can you tell us a little more about the atmosphere at the Cavendish-about Chadwick, about Rutherford?
One was very well-informed there because a lot of visitors came through—Gamow, for instance. Gamow was the one, I think, who told us as soon as it was discovered of the Joliots’ discovery of artifi1 radioactivity, which was at the end of ‘33. I think it really did get published in January of ‘314, but we knew it by Christmas, ‘33. He had heard the news. So this also was very exciting. After that, I found I could predict more or less 9O of what was going to happen. I always found that from then on. After artificial radioactivity had come, you could play around with these things. I made probably one of the first Segr charts, as it’s now called. I had my own little book. I showed it freely to everybody. I didn’t make it systematically. Maybe it was quite good that I didn’t pursue all of my ideas because I would have been making Segre charts perhaps, but I had my own; and I had a good memory. I remembered all the isotopes, and whenever I worked with them I didn’t have to look them up.
What sorts of experiments could one do in that early ‘30s period?
Well, one had the neutrons by the time I was there. Then when the slow neutrons were discovered, that’s what Chadwick and I then started pursuing
Was that immediately?
Immediately. In fact, we were the first to see the lithium and boron disintegrations, which are pretty important. You know, the lithium reaction now is used for hydrogen bombs, but it’s also used to make H3, which is one peaceful application. Now, that came out of my first paper there, which is number three here. You see, what happened was after we did the photo-effect, that when slow neutrons came along, I just jotted down right away that lithium-6 and a zero-energy neutron could disintegrate, getting a particle out. Usually it had been only capture with gamma emission. I could see that the particles, the alpha and the H3 hold come out. This is really a prototype of fission.
This is why that was also not so new to me. Lithium-6 with a slow neutron I could see would release a lot of energy, but the masses were wrong, so I predicted too much energy—or this was rather for Boron-lO. But nevertheless when we did it we found it. So I convinced Chadwick that we should put lithium in the ion chamber. He had this nice ion chamber where we had done the photodisintegration, and he was an assistant called Nutt. And there is a little joke in the Cavendish. They have one of these post—prandial songs—N is for Nutt who discovered the neutron, because he was “the hands” in the experiment. He was later a second hand car dealer. I’ve lost touch with him. He tried to sell me a car when I couldn’t afford it. Anyhow Mr. Nutt would always be our assistant, and so I got Chadwick’s permission to put lithium fluoride in.
I had calculated that lithium should disintegrate with a certain energy release. As soon as we putheutron source there we saw the H3’s coming out. But it wasn’t good. I said to Chadwick, “Look, if we put lithium metal in, that would be much better because it would give a better yield.” However, Chadwick was afraid this would ruin the ion chamber. So one day when he was in London at the Royal Society we put in the metal and it worked beautifully. Then we could tell him. So we got very nice “kicks.” And in those days you photographed each kick. The kicks would appear on an oscilloscope. This was a small mirror oscilloscope, and you would photograph it and then Nutt would develop the film, and I also learned to develop it. I still have some of these originals that I kept of the first lithium reaction. But unfortunately these oscilloscopes were very noise sensitive, so when there was any noise around it would also look like kicks.
Only when you photographed and looked carefully, you would see the distinction. The next thing I calculated should work was boron. So I put in boron, but the noise kept us from doing it, so I asked special permission to stay in after ten at night. It was a tremendous story. We worked after ten at night. You can see how different the times were. I just had one minute’s run and I had thousands of disintegrations because boron has a very large cross section, you know. It’s used as an absorber for neutrons. Actually, Fermi had found it absorbed neutrons, but he had not found any reaction yet. So it was easy to see that it was related. I cannot in each case without careful thought say who was ahead on what, but we were always neck to neck after this on slow neutrons with Fermi.
But then he went with tremendous, peed through all the activities. We did these careful investigations’ disintegrations by slow neutrons. And I right away went up to uranium, but there I was too clever. I covered it with aluminum to stop the natural alphas looking for long range aiphas and that stopped the fission products. Anyhow we missed it half a dozen times (we cannot be too proud), and each time in a very interesting way, but that is purely prehistorical, not even historical.
But it shows consistency.
We missed it first by covering the uranium with aluminum. Then I looked for it in a photographic emulsion. I discovered this emulsion method with Taylor; it is a later paper—number six here. You see, Fermi and we had a little discussion in the literature because he then also did the boron reaction, and he wrote down the correct reaction: lithium-7 plus alpha; but I being in the Cavendish where Aston had measured the masses “knew” that the reaction energy was quite wrong for a two—body reaction. So I invented a complicated reaction—two alphas and H3, which would fit the masses. So the question was: who was right? Is it two particles or three particles? I was having lunch with Taylor, an Englishman who had worked in India for a long time, and had come back for a while to do a Ph.D. thesis. He was much older than I, and he had done work with nuclear emulsions, shooting in protons and fast neutrons, and had come to the conclusion that emulsions are useless for nuclear research. He was just finishing his thesis at the Cavendish. And so at luh with him I said, “Look, we have this fight. Is it two particles or three particles? Why don’t we put boron inside and see?”
Boron in the emulsion?
Yes. So we went back from lunch and soaked an emulsion in boric acid, dried it overnight and bombarded it for a few weeks with neutrons. When we developed it it had thousands of tracks and each one was a single track. So I was right away convinced it’s two particles, but it took me another six weeks to convince Rutherford and Chadwick because you couldn’t send in a paper on your own—it had to come from the prof. This is one reason why I think I went to Chadwick to have my first paper published. Someone had to send it in to Nature.
With your name of course.
My name, but someone had to write a recommending letter— otherwise they would not publish. They would publish any nonsense if a professor would send it in from somewhere. But Rutherford of course was very careful.
So this was the refereeing system as it existed then. It was refereed at the parent institution.
Yes. There was, for instance, an awful lot of nonsense from one little university, but it was sent in by their professor. A young man did it and the professor sent it in. He perhaps couldn’t criticize it. Anyhow our “letter” was sitting on Rutherford’s desk for six weeks. By that time we had also done lithium and we still could add a footnote that lithium disintegrates as we had concluded before into H3 and alpha. But then we thought we’d better try an ordinary emulsion with nothing in it. You see Chadwick and I had already found that nitrogen could be disintegrated and you make the famous carbon—14. That’s the reaction in nature which makes carbon-l4.
And so Burcham and I, the next year used just ordinary emulsions and proved that in nitrogen-l4 you don’t get an alpha out but a proton, you make carbon-14. And I tried to find the activity of carbon-l4. I still have an old filter paper marked carbon-l4, with my poor chemistry, probably there’s none on it. Anyhow, in those days it was supposed to live three months; but it actually, as you know, lives 5000 years. It’s still not completely understood, why it lives so long. So while I thought it was going to live three months, which I had predicted from the existing data, I thought I could find it with a few months irradiation, but I missed it because of weak sources and probably my poor chemistry. I still have that filter paper labeled carbon-l4, which is probably the first filter paper labeled carbon-l4. I should once bring it in and our chemists could look at it. My chemistry was really lousy. I used just a big nitrogen solution, and tried to get it out with a colloid. I had to do it myself. It was not customary to collaborate with someone. I finally collaborated with one chemist on some work later. I’ll mention it if you want, but this one I tried to do alone.
Did Rutherford come by and check the apparatus?
He would not check the apparatus. His habit was to come by once a day or once a week and sit down on a high stool—he liked to sit on a high stool—and just talk for an hour or two, see what you were thinking about—”What are you doing? Show me your results.” He wasn’t interested in apparatus. He couldn’t build with his own hands. He always had somebody who would do it for him. I had later the idea to put boin trifluoride, the gas, into an ion chamber, which is now the way of using it because you get a hundred per cent yield in detection. Now this needed some dangerous chemistry; I wasn’t aware how dangerous it was. Aston gave me advice on how to make the boron trifluoride. I read up a little on the history. The first man who made it died from the fluoride. Anyhow we made it and then we ordered it from the United States I think in a cylinder, and then it turned out it couldn’t be sent because it was a well-known poison. But here in this country as soon as I arrived we built boron trifluoride chambers from the commercial gas. In fact, the Manhattan district took over my design quietly. They just said “Thank you.”
You were the first to actually build a bon trifluoride chamber?
What other kinds of detectors were in general use in the early ‘30s?
Early or through the ‘30’s.
There were the Geiger counters where you could discover radioactivity We used those of course, too—like, for instance, the famous things which Fermi and his collaborators did you bombard silver with slow neutrons, it gets radioactive, you run fast away from the source because you couldn’t keep your counter near that strong neutron source. You put the target on a counter and then you measure its lifetime You can measure the beta ranges. This was one of the important techniques. The other one was the cloud chamber, which I used only indirectly. I made some suggestions to others I didn’t work with the cloud chamber myself.
And there was this ion chamber, and there we really produced all the techniques of putting targets inside. One always used to shoot charged particles from a target into ion chambers before, you see, and when fission came along our technique of inside targets was the one by which it was found. People take this for granted—-that you take this from somebody else, but I think it has had its effect (if you keep it secret for another 50 years I can say so)—-on the whole fission development. Now, I thought of fission in many ways. I was very close to it because I knew that alphas were emitted from uranium and jotted down once in my noteboook: “What if five alphas came out together bound as neon?” Then you get the extra binding energy. I calculated it would be 80 million volts. I jotted down a note to look for that. That was without neutrons. I would have found spontaneous fission if I had done the search right. And then when Meitner and Hahn reported all these complicated elements, I said to Bretscher once: “Look, they ought to get a new emanation because they thought they had radium. I know how to get a gas out. Let’s look for that.” We never did. Of course all the Xenons and kryptons would have been there. I was just leaving the Cavendish in ‘38 before fission was discovered.
But you said earlier you had bombarded uranium.
Yes, we had bombarded uranium, but we had the aluminum foil on top of the uranium target so we couldn’t see the fission particles come out. We also put uranium into an emulsion. We were looking for long- range alphas, not for fission. You see, uranium emits alphas naturally. So [calculated: you add the 6 million from the neutron, you ought to get an alpha of high energy which should come through the foil. Actually this was seen much later. It does exist as a rare phenomenon. The fission is a much more common phenomenon but that was nicely covered. Well, it certainly would have changed my history if I had done this at the Cavendish while Rutherford was still alive. I think it would have changed the whole world’s history because obviously while Rutherford was alive he could have convinced the English government to move very fast. It might still have had an effect on the Nazis—who knows? Szilard always thought maybe the Nazis would have done it faster if it had been known and I should be glad I missed it. This was ‘35. I can tell you exactly when we could have perhaps seen it. Yes, it was early ‘35 when we did the experiment with uranium and the aluminum foil. And with Taylor I tried uranium in the emulsion to look for long-range alphas, but we apparently didn’t have enough sensitivity. We didn’t find fission. We should have found it that way.
What decided you to look at uranium right after treating the light elements?
Because from my little calculations I knew ... I was looking for elements where slow neutrons would release energy, exoenergetic reactions. So I calculated lithium should be one, boron should be one, nitrogen we found just empirically to give something. But uranium I did know would be exoenergetic because it emits alphas on its own. You add an extra neutron—-that’s an extra 6 million volts. So it should have emitted some 11 million eV or so alphas. But that’s a very rare thing. It has recently been found with some of the rare earths—these long- range alphas, which I was looking for, turn out to be very rare. You know, to some extent these things are rarer than they should have been, and this was only understood later with more complicated nuclear models. On a single-particle model, these alphas should have come out profusely. It isn’t a good model there.
I notice you were thinking in terms of a single-particle model. Was that a generally accepted concept in, say, ‘35?
No. At least Wigner thinks I was first for reactions. I don’t know. To me it seemed very natural. Then Bartlett had some early ideas. There was a so-called orbital model. He had said there was a p neutron. And Elsasser had done some things which were not taken sufficiently seriously. But in these reactions I always thought of the spin. I always tried to see the complete picture, all the properties together. You should not see just one. On the other hand, when Bohr came along with the liquid drop I certainly appreciated that it was a good idea. However I remember the first time Bohr talked about it in Cambridge, I pointed out in the discussion that there seems to be a contradiction because bismuth and lead don’t absorb neutrons much, and he sort of brushed it off. There the single particle works; there’s a closed shell, as we now know.
When was that, do you remember?
‘36 when he came with that model.
Were you thinking in terms of shell behavior?
No, I don’t want to claim that. Empirically I knew these facts well because one could see large cross-sections for thermal neutrons in the rare earths, but that has turned out to be the nicely deformed region, though one didn’t use these words in those days. But then when you get again to a closed shell, you talk of single particle behavior. But this is modern language. All I knew is: why isn’t something much heavier than a rare earth absorbing neutrons well, e.g. bismuth and lead? I saw a paradox there, and he brushed it off. He was probably wise for that time. You can’t explain everything at once. But it was really a paradox which should not have ever been forgotten, and it led in a sense to the shell model of Maria Mayer. Well, I always knew these facts well, so Maria Mayer talked to me a lot. I knew where the energies were wrong, where things were apparently too high or too low. I was always very sensitive to these things from the very beginning. When something doesn’t fit an idea, I would make a mental note without having to make an effort. This was always very useful. Of course nowadays things move much faster. There are many many people who watch these things, but I think in those days it was still just a handful of individuals.
There was also less data. You could keep track of it.
Yes, I could read everything which was worth reading. I would soon learn who was worth reading. You had a definite feeling whose papers were worth reading. This is very hard today, for me at least. Perhaps for other people, it is still easy.
How big would you say the relevant community of nuclear physicists was at the time? By relevant community, I mean the people whose papers it was worth reading.
I would say there were a number of important centers. There was the Cavendish, which I think was foremost, and then the Rome school when Fermi was there was top-notch; and the Joliot—Curie crowd in Paris. Then there was Kurchatov and his people. They did good work. I always guessed it was Kurchatov who was the important man in atomic energy in Russia because I had read his papers. He was not far behind us considering the time difference in receiving journals. There were always interesting papers coming out of Kurchatov’s school.
How did you hear about their results? Where were they published?
The Russians published in the French Comptes Rendus and also in their own Soviet Physics, which was in German and English sometimes, but more in Comptes Rendus that was for fast publication. The Italians published all their fast things in Ricerca Scientificia, some in Nature, some in the [Proceedings of the] Royal Society [London]— their summary papers.
Did the Soviets try the experiments with the neutrons right away and the French as well?
After slow neutrons everybody got into the act, but the good centers still remained those few. The Germans did lousy work. That’s why I was always sure they would never build the bomb in time. They didn’t know how to measure cross sections. And the American effort was sporadic but was just getting into high gear. There was Dunning at Columbia and so on when fission came. So by that time very many people were tooled up. The Americans until that time had concentrated more on developing techniques.
Are you speaking now about neutron experiments in particular?
Yes, I thought that was your question because there was all sorts of other work. There was this nice work of Tuve, with the early Varde Graaffs or whatever instruments he had at the time. The cyclotron was being developed. Technology was moving here; later the situation was reversed. After the war Europeans developed techniques for a while and they are now catching up and doing good work.
Would you say that there were any centers of experiments in the ‘30s in the United States or just techniques? Goldhaber Yes, there was certainly Columbia You see, they had a lot of influence Fermi, Rasetti and Segr were visiting That was a pretty good school where they did some important work, and then of course there was also a school developing around the cyclotron of Lawrence Seaborg did some early work and Libby It was fairly good work I don’t think it stood up to the European’s at that time But it reversed very fast.
How important were accelerators in the ‘30s in terms of providing experiments?
They became soon very important because the neutron sources were so intense and you could make also many more radioactive isotopes. Very soon they pulled ahead of these little radium sources.
When do you think that began to really become effective?
Just about when the war started.
It really took that long then?
One could still make nice discoveries. During the war I was at Illinois confined to a small source and we still made interesting discoveries with neutrons. We still measured smaller cross sections than some people had done on the project, who used our cross sections. We would send them in and they were used.
How did you feel about accelerators? Were you at all enthusiastic about using them yourself?
No, I was not. You know, when you still have ideas with small equipment—one counter or one little source—it’s so much more fun because everything is at your disposal, so as long as I didn’t run out of ideas, I stuck to it. But then of course with time you lose out.
Was this the general attitude at the Cavendish?
Yes. They finally did build a cyclotron about ‘36 or ‘7.
Was the Cockcroft-Walton apparatus very important in the results coming out, would you say?
Yes, it was important. It was of course the first work of this type. And Rutherford had put them to it. I suppose Gamow had a big influence, that he realized that because of the penetration of the barrier one didn’t need as high energies as people thought at first; and so they built this 600 keV apparatus, which would have seemed otherwise to have too low an energy. And Rutherford got impatient with them. He said to them one day, “If it doesn’t work next week, I’ll kick you out,” or something like that. That’s the story. That’s why they bombarded lithium and it worked. I haven’t heard this from Cockcroft. I’m supposed to visit him this spring. Maybe he’ll tell me a little more about it.
How did you feel when you first heard of their proposal and that of Van de Graaff? Did you know about those things when they were first introduced?
No. Remember, when I came to the Cavendish they had already done their first disintegration with protons. The great year was ‘32 when the neutron was also discovered and Anderson in America discovered the positron. Heavy hydrogen had come in early in the year, and the first disintegration by protons had been done by Cockcroft and Walton. So of these discoveries, there was the neutron and the first artificial disintegration at the Cavendish the deuterium and the positron in America. People still say the more important discoveries were the neutron and the proton disintegrations. But this is where the lines of progress approximately started crossing, and then roughly by the beginning of the war they crossed and have stayed ahead in this country.
Were you hearing about the early accelerator work much in Berlin?
Yes. In Berlin there were Lange and Brasch who built one of the very early accelerators. They wanted to use lightning first, and then they wanted to use high-frequency coils. I think I learned that Brasch died recently in this country. I don’t know what happened to Lange. I think he got to Russia. He was an anti-Nazi and went to Russia. How he fared there, I don’t know. Lange and Brasch were among the first accelerator builders and they in fact repeated the Cockcroft-Walton work the same day they heard about it. So a lot of people are always close. There’s sometimes just a tissue paper in between success and failure. If one hadn’t found it, another would very soon by a different route. By some routes you find it very easily. By another route, it’s a little harder. The way Hahn found fission was perhaps a hard way—his chemistry— but he was so good at his chemistry. He’s written his story recently.
Yes, I’ve been looking through it.
How about energy-level diagrams? When would you say it was possible to first draw these?
These came very early. I think the first ones I saw were probably Gamow’s for natural radioactivity. You see, he predicted alpha fine structure. Or he explained it—I’m sorry. Alpha fine structure was found and he explained it. It was found by Rosenblum in Paris, and at that time Rosenblum thought this was such a tremendous discovery he was expecting the Nobel Prize and he thought Madame Curie was not quite fair, not pushing him. You see, I find people are always thinking they have found something very great when it differs from general beliefs. Alphas were until then thought to be most homogenous in energy. Suddenly there is a fine structure, so the discoverer thinks this is tremendous. When finally it fits into the picture it’s such a small part. Gamow explained the fine structure by saying, “You just excite these levels.” He was drawing diagrams. I heard this story that in ‘30 or ‘31 he was giving a talk at Lake Como at a conference and Miss Meitner was there in the audience. He would say, “Here is the alpha fine structure and it excites these levels and from these levels you get the gammas and Miss Meitner has found them.” She said , “I’ve never found them,” but she went back to the lab and found them. He had such imagination, he would even add a little fact ahead of time. Gamow perhaps you should talk to. It might be amusing.
When were ready. I think we ought to be pretty well prepared.
He’ll give you a lot of amusing background.
This was natural radioactivity that one could then actually observe.
That’s right. So you had these nice level schemes of natural radioactivity. Then it wasn’t hard from that to write them for ordinary nuclear levels.
In other words, the detection techniques were already good enough in the early ‘30s
Well, you could measure energies with fair accuracy, not the accuracy of today, not to a kilovolt but maybe to tens of kilovolts.
What about spins?
Now, spins of course there were many ways of measuring. There were these few spectroscopic methods, and then Rabi developed these very successful methods and measured a lot of them. Now, I got into the business of guessing spins, and another one I guessed was boron-lO. That was in Brookhaven, the first summer I was in Brookhaven in ‘48. You see, lithium-6 was first zero and then it changed to one. Deuterium was known to be one. Nitrogen-l4 was known to be one. So by interpolation, everybody guessed boron-lO was one. Now, I remembered still that in Rabi’s work they had not measured the spin. They had only measured the gyromagnetic ratio, the U/i. So I knew that the spin was really not explicitly measured, though the table said one.
I’ll tell you something amusing on that. But beryllium—lO was found to live a million years instead of living a few months, and one couldn’t find an explanation, and suddenly it dawned on me that everything would fit if the spin were three. And I was visiting Brookhaven. I was just walking up and down the street with Feshbach when this suddenly dawned on me. He was encouraging. It was one of those rare ideas I just blurted out and he was encouraging. He was not discouraging. But then Bob Sachs was here and Rabi was here that summer. Rabi was very careful. He said, “Maybe you’re right; maybe you’re wrong.” But he realized it wouldn’t contradict any of his statements. Bob Sachs had a theory explaining all these nice gyromagnetic ratios if every spin was one, and it fitted smoothly. So he didn’t like it. Then a day before we went back to Illinois—it was the end of the summer—I wrote up a letter to the Physical Review. At that time it was sent to Tate.
I sent off the letter. We drove back to Illinois, and when we arrived in Illinois I got a telegram from Art Roberts: “Congratulations. Spin of boron-lO is three.” No further explanation. Then I learned what happened. Nordheim was coming through Brookhaven with his little son. He had spent the summer somewhere near here. He asked me what I was doing. I said, “I’ve just been thinking about the boron-lO spin. I think it’s three.” So he went off to Duke. And when he arrived at Duke, he asked Gordy, “What are you doing?” He said, “I’ve been studying this boron spectrum all summer but it makes no sense.” Nordheim said, “Goldhaber says the spin is three.” Gordy said, “With three it makes sense.” Off he sends a letter with spin three without mentioning me. Anyhow these two letters arrived two days apart. So Critchfield told me later, “Look, it couldn’t have been stolen by the experimenter because he must have worked for weeks It must be the theoretician who has stolen it.” Anyhow they published both letters but they put Gordy’s first. But later Gordy published a detailed paper where he gave me very handsome credit. I’ve never had as handsome a credit. He had just sent off his letter in the excitement, forgetting these rumors. So I’ve been always in the spin guessing game. That’s been one of my old foibles.
What about isobaric triplets?
That I think started with Wigner. He started thinking about it very early.
But you couldn’t guess the spins this way?
Well, if you knew one, you would know the others, but usually there wasn’t one known.
I see. So it was only later that you could fill in gaps like that.
Yes, then of course the great game started of guessing spins of excited states, and now you see tables with maybe ten or 12 spins known in succession—not just the ground state. You see, all this tremendous field of spins and magnetic moments of ground states has become a small subfield of the spins and magnetic moments of all states; very many are now known. Of course one still measures every day new magnetic moments. You know how we do it. We made a contribution to that. I sound like bragging; the things just always happen.
This is history. What are you going to do?
You’d have to ask some others. Not everybody has the same opinion because I find that one usually gives credit where one has heard it first. It doesn’t matter who was first, but where you’ve heard it first. And since you’ve heard your insight first, it’s not too selfish to give one’s self credit if you’ve really heard it first from yourself. But the other may not agree because he heard it first from somebody else.
Or from himself.
I’m a tolerant person. If someone wants to give credit one way or the other, it doesn’t really matter too much. It’s more just a matter of amusement and historical interest.
Were magnetic moments easy to find already in the ‘30s?
Well, Rabi ‘S methods were very powerful for ground states. Excited states came much later.
Would you say that didn’t happen until after the war?
Largely, yes. That’s true after the war the first excited states. In fact, we have a paper here done at Brookhaven where we first proposed a method, but the method was used successfully first by Frauenfelder. That’s about paper 70: “Isomerism in Lead 204 and ‘Memory’ in Angular Correlation.” What we found for the first time, I believe, was an angular correlation where there was some memory some time after the first radiation. Once we realized there was a memory we said, “In a magnetic field this angular correlation should rotate and you can measure magnetic moments.” And this has become a classic method now. But our own experiment did not succeed.
What year was that?
That’s ‘50. The first ones then came that year or later-—I think the next year—the first successful ones. Now Sunyar, who is the first author on this paper, the senior author, is doing this all the time. You can see a real up-to-date thing if you want for amusement to walk through his lab, how it’s done with great accuracy.
Is this Andy Sunyar?
Yes. He was an early student of mine in Illinois, and then he came here when I came and he is now a senior physicist here. You might find it amusing to see his set-up, how techniques have changed.
I’m just wondering about procedure now. We have so much ground to cover, and I’m wondering what the best timing of it would be. Maybe we can continue to the end of this tape, a few more minutes, if that’s all right and I’ll ask Gloria [Lubkin] to get back into the ‘30s with some general questions.
Yes, I’m just going through my bibliography, and I think the first page roughly finishes my English period. So maybe you’d still like to do it. There is one more paper worth mentioning.
I meant to get into that.
Well, two papers are worth mentioning. Paper 12 was a paper with Briggs where we made the first systematic study of the scattering of slow neutrons which were the first scattering cross sections which were very important later for the Project in a way, and they have stood up quite well. We developed a new method for measuring scattering cross sections relative to carbon, which we could measure as a total cross section. And for the first time we went through 70 elements or so and I had a lot of fun getting the most valuable elements out of the International Nickel Company. Their samples were mostly very valuable noble metals. They had to be locked up every night because I had the world’s biggest supply of osmium and iridium. We measured all these cross sections. It’s all in that paper there. Now, I think this is a paper which has stood up quite well, but of course now it’s completely forgotten because all of these things have been improved. If it’s a new phenomenon, it’s something; but if you only measure something, that gets always superceded. Then the next paper I want to mention is: while Szilard was still in England he had found an interesting new activity in indium with neutrons which seemed to be one more activity than there were isotopes for. Indium has two isotopes and yet he found a third activity, so (n, gamma) could not explain it. And he and I and a student of mine, Hill, who was later professor at Illinois for a while but now is in Santa Barbara, started investigating this indium activity. We finished it in the Cavendish, but we wrote it up in this country, so it appears in The Physical Review because by that time I had gone to the States and Hill joined me. He had a fellowship where he could go where he wanted, so he just came along.
He hadn’t finished up yet.
He hadn’t finished up yet. And it cost him dearly because hen the war started and it took a long time for him to get his Ph.D. because of this little lark. He went to Australia—he was an Australian— during the war. He came back to Illinois and remained there when I left for here and now he has gone to Santa Barbara. He and Szilard and I wrote this paper then in America. It’s published in ‘39 in Physical Review. It’s “Radioactivity Induced by Nuclear Excitation.” What we realized was that this activity which Szilard had found was the first time an activity was just induced by knocking a nucleus-(n, n) we call it ... You make an isomer of this nucleus. It wasn’t the first isomer. The first isomer was found by Hahn in natural radioactivity, but it was the first time one was produced in such a simple way. You just added a little energy, which proves that isomers are just nuclei with added energy. Once I r.ealized that, I said, “Well, you can add this energy in many ways. Let’s do it with alphas, with protons, with X-Rays.” So there are more papers. The next one is with Collins, Waldman, Stubblefield. I started this business of just visiting the guy who has the tool, which Brookhaven is now famous for Collins is now here. Waldman is at Notre Dame still. I don’t know where Stubblefield is. So we excited indium with X-Rays at their X-Ray machine.
At Notre Dame. I went from Illinois to Notre Dame to do this experiment. We excited it with X-Rays, which was great fun—to get a radioactive isotope produced just with low-energy X-Rays—l,000,000 volts. And this was the first of its kind. Then I went to Rochester and we did it with protons. I didn’t add my name to this. They published that on their own. Barnes did it there with protons. And then others did it with alphas, and one knows now that one can do it with anything. I want to do it once with neutrinos, but I haven’t done it—very difficult but of interest, though it doesn’t look like a hopeful experiment. So this ends the English period in a way and then starts my Illinois period which goes through ... It gets a little complicated. After the war I was allowed as a consultant into Argonne so I went from the little neutron source to the big reactor.
During the war my wife and I were not citizens till towards the end of the war and we were not used by the Manhattan project, though they used our papers. We sent in a lot of papers which were kept secret during the war and then published after the war. With Coitman we measured the first beryllium cross section for slow neutrons showing that it is a useful neutron moderator. I believe we did do the first work on that-—some 10-20 square centimeters. But this was all kept secret until the war was over. So there is a period: Illinois plus Argonne mixed. And then comes my period at Brookhaven, which starts on page five I think. Several of the people from Argonne and from Illinois went with me here, so some of the same names keep cropping up. For a while I worked together with my wife.
Then we decided it was too much—day and night to work on the same—so we’d work on different things, from page six and a half on, though some time we still work together. Then I got a few theoretical ideas occasionally. The amusing thing is that people remember theoretical ideas but not experimental ones, so when I come to a place—like Cornell recently—to give a talk, the young students think I’m a theoretician, though I’ve published perhaps six theoretical papers and 120 experimental ones. But it’s interesting, because theoretical papers live on. Experimental ones as a rule do not. This bibliography is not quite up-to-date. There is still another page I could give you. But you are interested in the ‘30s, so this is really sufficient.
No, we want to take it to the present.
She can give you another page [his secretary].
There are questions that I think you [Lubkin] have (in the ‘30s) that relate to the general situation. Maybe we can keep those up until we run out of tape.
Yes, let’s go on.
You mentioned that you were using emulsions already in 1935 in paper number six. Wasn’t this unusual for that period?
Yes, it was. I can tell you the whole history of emulsions. It was found very early—I think about 1911 or ‘12 already—by a Japanese, that emulsions are sensitive to single tracks. But people used to just shoot particles into emulsions. And then when neutrons were discovered, I believe it was Blau and Wambacher who first saw protons knocked on by neutrons. But we were the first to see specific disintegrations of specific elements, and it had a great historical influence in two ways. First of all, somebody at some meeting said, “Why couldn’t we use it now for cosmic rays?” And this started the whole star work. And then Powell took these emulsions and developed them further and found the pi-mesons because he deliberately took one of our boron emulsions. He wanted an emulsion where he would know in a nucleus definitely what’s going to happen, and in those emulsions—they happened to be very sensitive—-he found the pi-mesons. In the very first papers and in his book he credits us with that. And so I think it’s had great historical influence. Again, if you promise to keep it for 50 years I can say it. You see, these are all little seedlings out of which big trees have grown. You now see the big tree, but you don’t remember. I hope I’m not exaggerating. Powell, if you talked to him, might confirm this for you.
That was ‘47 already.
You see, what happened was that Ilford was developing these special emulsions and they were very good about it; whenever we asked for something special, they would make it. We got most of our emulsions free because it was always something new. We said, for instance, finally, “Can you put in bon?” “Yes.” “Can you put in Lithium?” So these emulsions are now commercially available. And they tried then many other elements. They put in everything up to manganese I think at one time.
When did you start your cooperation with Ilford? This was after ‘35.
Wait a minute. I must tell you the whole story. This fellow Taylor, who was doing his thesis, had already gotten Ilford emulsions for testing, but he had sort of come to the conclusion that emulsions are not especially valuable. And then there was this twist when we found that by putting in something, we can do specific reactions; and emulsions were revived, and it was this shot in the arm which made Ilford go on developing better and better ones. I left England, as I said in ‘38. By that time they were just getting to these more sensitive emulsions. Powell took over at this point, and had the great success. I don’t want to say that I had more to do with it than in this indirect way.
Were you using them for other than those early neutron experiments with the boron and with him?
No, lithium and boin, and then we searched for uranium and protons from nitrogen. There are several papers. We proved that carbon-14 is made. This was a paper with Burcham who is now a professor in Birmingham. We proved that carbon-l4 is made in slow-neutron disintegration of nitrogen; with the ion chamber we didn’t know whether it was a proton or an alpha and the emulsion proved it.
What would you say decided you to concentrate more on ionization techniques?
It was perhaps partly historical; that Chadwick had these ion chambers. His were probably among the foremost techniques in this respect and so that was very easily accessible to me. Emulsions anyhow are very laborious things to scan. Ion chambers, when they can be used, give much quicker results.
Would you say that you felt you should save the emulsions for neutrons only at that time?
No, I certainly followed this cosmic ray work. That was very interesting. No, I didn’t have this prejudice. Only I always liked— maybe this is one of my pet things—-I like neutral radiations: gammas, neutrons, neutrinos. They are very clean and they come in and they don’t ionize until something happens. I don’t like these messy things where you get ionization all along, like alpha reactions, so I’ve done very little with those. You’ll find this is a sort of leit motif that I have used the neutral radiations preferably It’s a much cleaner sort of work Nothing happens until something happens You see, a gamma goes through when it disintegrates something you notice it; an alpha goes through, all the way you notice it And so, it’s actually perhaps that I’m lazy I’ve escaped the harder work. It’s harder to work with something which disturbs you all the time, as these charged particles do.
Would it be possible to identify what was the best technique in the ‘30s for each kind of particle: charged, neutral, gama rays?
It would depend also on the problem. It’s not easy. You see, it started out when Rutherford first used the scintillatioris. That was extremely laborious. You had to observe by eye. It was fairly efficient and simple. You could set up in a hurry. Then the next things were these ion chambers, and there was a fellow in the Cavendish, Wynn-Williams, who made the important first contributions to amplifiers. He’s now at Imperial College, I believe. And so when I arrived at the Cavendish, the ion chambers already existed. They had been proven out for detecting protons and alphas, and Chadwick had used them in discovering the neutron by knocking out protons. In fact, I still have that paraffin from which he first knocked out protons. When he was leaving the Cavendish, he said, “This was the one I used for the first neutron observation. Do you want it?” And I said, “Fine.” He gave it to me. He was not a sentimental fellow. When he was packing up to leave for Liverpool, he would look at things and say, “Well, do I need it in the next two weeks? If not, out it goes.” Not a nice way for historians. So I don’t know how much he has kept. He has written some historical notes to his books on Rutherford. You have seen that perhaps. Rutherford’s Collected Papers have historical notes by Chadwick and by others.
Right. Someone was over there this summer going through Rutherford’s papers—someone who is at Santa Barbara now, as a matter of fact.
I have some of Rutherford’s original reprints. When he died, they just let everybody help themselves. Also, Lady Rutherford sold all his library. I bought for one shilling the book from which he probably learned the mechanics he needed for the Rutherford model because it has the same symbols and it has his handwriting.
Do you know the title of that book? Do you remember?
I can easily look it up. It either was the first or second book from which he learned it. Feather has the other book, and it looks to me like mine was closer, though I don’t want to make a hard and fast statement. You can once perhaps get both titles and compare. They exhibited Feather’s book during the Rutherford memorial conference, 1961, for the 50th anniversary of the nucleus. They had a memorial conference in Manchester which I attended and they had a few exhibits, and I think Feather’s book was exhibited there. It looked to me like mine was closer, but I may be prejudiced.
What about the neutron time-of-flight technique? When do you think that that became useful?
I think the first one was probably the use of rotating wheels. I think that Rasetti and Dunning had something to do with that at Columbia, and then Alvarez developed the time-of-flight electronically. You see, that first was just mechanical wheels, and then I think it was Alvarez who did it electronically.
It was 1938.
About ‘37, ‘38, yes There were earlier ways of showing that slow neutrons moved There were people who moved neutron sources rapidly through water and things like that This was done by Moon and Ttllman, I believe, and it was at Imperial College when my wife was there at the time. So one knew that slow neutrons really have about thermal velocities. Once you know that . . . I don’t have too great admiration for just techniques by themselves. Techniques are always very obvious once you understand the fundamentals and once you also know what is the best electronics and mechanical techniques. If you know the engineering side and the phenomena side, to be then inventive I find not very hard, you see. I mean somebody had to notice that sodium iodide is a good detector; this was a very important discovery, but once you notice that, a tremendous technique develops. So everybody develops more or less his own tailor-made techniques for the particular problem. He cannot just take over a very general technique.
Were coincidence techniques of much use by then?
Yes, I think they were started by Bothe in the early ‘30s, and then Rossi developed the circuit for it. Before that, they had to use other tricks. Early coincidences were done just by eye, with two observers crying out when they saw things simultaneously. So it went from this way to the more or less oscilloscope way, where you saw coincidences in kicks, and then finally the electronic way, through the Rossi circuit. This has developed very far. Of course timing went on and on from milliseconds now to lO or 10-10 seconds, so the techniques move every year. And you might say there is a technique revolution nearly once a year or nearly once every two years—-the germanium counters now. If you look since the war how many technical revolutions there have been ... There’s been the bubble chamber, the solid-state counters, the first sodium iodide counters, the anthracene counters: these are all really very different; the whole photomultiplier development, and of course the whole solid-state computer development, which is part of technique now.
This is all because of a pool of existing technology which allows you to piece together different things. Is this the point you were making?
Yes, and some bright ideas are certainly necessary: miniaturization …
Was this general feeling of the availability of technology or technological capability—did you see many differences in it when you came to this country as compared to England?
Yes, there was a tremendous difference. I mean in England if you wanted a tube, that was big business. Here you bought it in a drug store. I remember that impressed me on my very first trip. Allison used to always repeat this as a joke telling my reaction to that when I saw the first tube in a drug store. And in England of course money was on a much smaller scale. You had to get things from an old-fashioned storeroom, and often when I was impatient I would use my own pocket money to buy something—something you don’t c1.o in this country, as a rule. You were much more the individualist there. Now, this works as long as the competition doesn’t overwhelm you. It was clear even if Rutherford had stayed alive and there had been no war, a few years later—with the tremendous technology here, with cyclotrons, with the counter techniques, amplifiers and so on—America would have overwhelmed the British if they had not learned to use them, too.
Were there such developments in England at the time you left? In other words, with Rutherford’s passing, did this signal a change in app roach?
Well, I’m afraid one thing is per:haps true: When such a strong man passes he doesn’t leave an obvious successor. The very strong men around him did not stay—-people like Blackett, who was very individualistic. They left early. Blackett got interested in cosmic rays, and Cock- croft had gone. Everybody had gone somewhere. And so there was no obvious successor, and this was perhaps why they went to Bragg, though it would have been better for continuity perhaps to go over toa nuclear physicist. Bragg did some great things in keeping the Cavendish in the forefront in molecular biology. You see, the Cavendish in those days attracted the best from a population of 500 million-—from the whole empire. The best would be squeezed out and come there—from everywhere: India, Australia—so it was very polyglot, including a few continentals. And because they had always the best people, they always did good work. They’re still doing very good work. But in nuclear physics they have lost out and in elementary particles too. They are now doing their good work in other directions—in radio astronomy, for instance, they are first class. So the spotlight in any one field moves usually. Then it moved to Bristol during the emulsion heyday when they found all these new particles with emulsions; and then after the war it moved here with bubble chambers and accelerators. And now of course Europe has comparable technology to ours. The people are in such close touch that you cannot be far out of tune.
Right. That’s a vast difference.
This is it. Now it spreads very fast. Nowadays the ideas seem to mean less because they are sort of in the air. People work on other people’s ideas. I was too much of a snob to, say, take the suggestions of Lee and Yang, to work on their idea. I was willing to go to Oak Ridge and tell them, “Do what Lee and Yang have suggested.” But that’s not enough compared with saying, “Let’s work on it together.” Then they would have done it. I haven’t quite reached that stage yet, but the modern experimenter finds it safer as a rule to work on a theoreticians’s idea. This is a real switch compared with my young days. And this, to me, is in a way a very dangerous switch for physics because now it depends often on whether you are faster. At least you have to be competent and accurate and fast. These are still necessary qualities. But you don’t have to have ideas.
In other words, you think that the experimenter has abdicated his responsibility.
I’m afraid so to an important extent and it’s not healthy because if he goes too far in this respect, he may be the unnecessary middle man between the theoretician and the engineer This has perhaps already been happening in space. I don’t know whether you need the experimental physicist always If someone makes a suggestion, the engineer can carry it out But the experimental physicist has given us something which we should not lose And if the experimenter always were to work on the theoreticians’s ideas, this might be lost. I cannot understand the people who come in in a rush to do something which somebody else has suggested, only to be the first.
This is perhaps something we can get into later because it represents the change in the size of the community itself and the reward system within this community, and we can’t expect a community of 22,000, let’s say (just checking the membership figure of the Physical Society today) to set the same values and standards as
No, but we are certainly in a transition period. It can’t go on long like it does, that one guy suggests something, and the other does it as fast as he can. That is not a very stable situation. I think you’ll have to find a new way. I have some ideas on that. We can talk about it some time. But I don’t think that this is a very healthy way.
Do you think you could trace the influence of models of nuclei in the development of nuclear physics from, say, the first concept of tunneling of Gurney and Condon and Gamow?
Well, I think every experimenter probably had a little picture of his own for a while, and Rutherford already in the first work realized that there is this hard core. You must have heard this story about the first time his student, Marsden, came back with the result that alpha particles are being back-scattered from platinum, he felt as if somebody had told him he had shot a bullet at a window and it had come back at him, because he had this picture of a jelly-like atom at the time—J. J. Thomson’s picture. So here was the first model of a hard point nucleus. But they soon realized in alpha scattering that it wasn’t simply a point because there were deviations from the Coulomb Law. And then Gamow and Condon and Gurney made this a more definite statement. So there was some work on nuclear radii I think from scattering, but I don’t know how much until somewhat later in the ‘30s. And then the models started, first a single—particle model, then it was forgotten again; and then there came Bohr’s liquid—drop model.
Would you distinguish that from the compound-nucleus idea?
No, that was very closely related to the compound nucleus. He produced it as one idea. It wasn’t terribly surprising at the time; maybe to some people.
Do you feel that the evidence had been crying out for some kind of new explanation?
There had been empirical evidence only for a short time. There were the neutron resonances and you had to explain why they are so sharp because when a neutron goes in, it should be able to get out quickly and why does it stay so long? So this is what Bohr explained, by sharing its energy with so many other particles, it couldn’t get out right away. I mentioned before that there was this little paradox. Why could it get out of lead so easily? It couldn’t share because the excitation energies were so high in the closed-shell nuclei, and this answer became clear much later, but at that time it was in a way a contradiction. So you needed something more. You needed the shell model, which came then as a revival of the single-particle model of Elsasser and Guggenheim, by Maria Mayer and Jensen and co-workers. And then when it was revived, there was much more data and so it looked much more convincing—all the evidence for magic numbers and neutron cross sections varying and excitation energies. Then we also started a systematic search to see how electromagnetic transitions fit in with the single-particle model. Weisskopf developed a formula for the radiative transition probabilities. Sunyar and I went through all the data and systematized it for the isomers, and it was a great success for the model, but there were also deviations.
And what year was that?
‘51. That’s a paper that you should have here. It appears in so many different stages. Here it is: paper 81, “The Classification of Nuclear Isomers,” with Sunyar. There we found that you could assign a definite multiple number to each eltromagnetic transition, and you had mixed ones like Ml, E2’s, and you had pure E 2’s, which were much faster than the single particle model predicts. We found those. Just the systematics showed that there were E2 transitions which were faster. And then my wife showed that the E2 transitions have a very regularly varying energy. Now, all these things were also partly done by others; this was our trend of interest at the time. So we found a lot of the empirical regularities which have guided especially the new theories of the so-called unified model of Bohr and Motte4son. They could explain many of these regularities and have predicted some actually.
Do you think anybody else was recognizing the rotational behavior at the same time as Bohr and Mottelson?
Well, I think Rainwater made an important contribution. He recognized this collective behavior, the deformed nuclei, but Bohr-Mottelson—so to say—pushed it most systematically and developed the formulas in many directions and compared it more with data. It seemed at that time fairly straightforward—not revolutionary but evolutionary, shall we say.
What about the optical model?
The optical model came out of the work of Barschall largely, on neutron scattering. He had found empirical regularities, how neutron cross sections varied with energy. Then Weisskopf, Porter and Feshbach come in. The optical model had existed before, but they took it more seriously and put in the right constants, and could get very good agreement with experiment.
Would you say that each time one of the new models evolved, it was generally accepted right away?
It depended a little bit on the successful comparison with experiment when there was not enough data, it was not accepted When there was overwhelming agreement then it was accepted. People are conservative. They don’t like a new idea unless they are forced to accept it, and they are only forced to when there is really good agreement. And so all these models have a prehistory. Nothing just comes out of the blue like this: It has its older history. And then finally after a long time of stewing around, it crystallizes. That can come rather suddenly, but in some cases there were these long-time developments.
Is the general reaction of an experimenter to rush to verify or disprove the model?
I think certainly today there is a strong tendency. It was less when I was young. One just had the attitude: “Okay. Here is this model. Let’s see how it will look in a year or two.” Now people right away like to test an idea, which is perhaps the only way to do physics in the long run—to test each idea to see howell it holdout. Perhaps it’s more democratic. It doesn’t matter who had the idea. Everybody wants to test it.
Could you characterize periods in the 1930s when a particular model held sway and in fact was accepted by at least the experimental group that you were associated with?
There was this, you might say, sort of one-particle model until Bohr came along, but because of Bohr’s great prestige, it went completely overboard the other way. So the single-particle model was just dead. Though there were contradictions, they were brushed aside.
Would you say that you yourself generally ignored the single- particle behavior after that?
No, I did not. I was ohe of those who did not. I could work with both. And that was very useful to me in my thinking. So then when the single-particle model came along again it soon became generally clear that you need some of each aspect.
When did the evidence for that start to accumulate?
Well, it really started early, but it became overwhelming after the war when there were a lot of data from fission, from neutron cross sections and beta decay energy data and spins and neutron binding energy data. So Mrs. Mayer put all this together. I think she first of all did it perhaps at the suggestion of Teller, just to put it together; and then Fermi asked the bright question, whether perhaps the spin-orbit force is important. She worked it all out and it was very successful. There was a spin-orbit force which gave the prediction of which spin is more bound’—up or down—relative to the orbit. She found spin up relative to the orbit is more bound. The higher total angular momentum is lower down in energy. And this gave this tremendous clarification. I remember saying to Dancoff long ago—poor Dancoff, who died so young, he pooh-poohed this because it didn’t fit anything—that the isomers are much too systematically occurring to be random. People thought of them at that time: “When by chance there are two levels near to each other which differ in spin by a large amount, then you have an isomer—say nine-halves and one—half.” But I said it happens too often to be chance.
And of course when the single-particle model with spin-orbit coupling came along, the so-called islands of Isomerism came out as a consequence. This was again one of these ideas I didn’t pursue at first, but this was early in the war—maybe ‘1i2 or. so, and then it didn’t become really clear until ‘50. No, it must have, been early after the war. But anyhow it was four or five years before it all became clear. I already said, “This is not a chance distribution,” and of course if you. look at a modern chart, you see this right away. You might, by the way, like from the point of view of models to walk into my wife’s office. She has a beautiful wooden model of all the excited states of even-even nuclei. You might like a photograph of that. It’s really very pretty. We can do that, if you like, on our way over to tea—just pass by and then we can pass Goudsmit and Sunyar or catch them at tea—just pass her office first and then go to tea, and whoever doesn’t turn up at tea, we can look them up.
That would be interesting; good.
What about the concurrent work in nuclear forces? Do you think this has much influence on experiments?
Well, there were very interesting papers on scattering of nucleons by nucleons, protons by protons, and so on, and a few people pursued this—especially Professor Breit at Yale. At that time he was in Wisconsin, I think. He pursued it to the finest refinement. He made very early suggestions, I think to Tuve, to investigate proton scattering. Over the years he has developed more and more accurate descriptions of proton-proton scattering in terms of potentials and of phase shifts and there are of course still some discrepancies. I’m afraid I haven’t followed it too closely lately, and I know that the different groups do not agree a hundred per cent, but I think there’s more or less an understanding in a phenomenonological sense of what happens when nucleons collide. Bethe is one who uses these forces in complex nuclei. His latest work, he may have told you, is to try to understand the nucleus from these fundamental forces. Maybe you need nothing more. There may be no mysterious three-body forces, but probably there’s a little bit, but it may be relatively unimportant or it may be just a higher order correction.
Do you feel that the attempt to understand nucleon-nucleon scattering had any effect on the general trend of nuclear physics until recently? Was it peripheral?
A little bit. Brueckner of course started using these forces. It has had a little bit of effect. But if Bethe is now successful in this latest approach, that’s perhaps the first really convincing combination of the two fields. Brueckner started it for infinite nuclei, and now Bethe supposedly can do finite nuclei. I have not seen all the details yet.
What two fields are you distinguishing here?
I mean the field of just nuclear structure and the field of nucleon-nucleon scattering. This is what Bethe tries to bring together
How would you distinguish between nuclear structure and nuclear forces, say?
Well, nuclear structure deals with energy-level distribution and the neutron cross sections, etc. in complex nuclei. You can describe these things without knowing the individual forces. But then it’s an ad hoc description. Or you can use the optical potential. But Bethe would like to get all this from the more fundamental forces. After all, a nucleus may have a hundred or so nucleons. Between each pair of nucleons you have forces which we know from these other experiments. So one tries to put this in to explain the optical potential and the energy levels and the binding energies and the masses from this. Now, I cannot give you an up-to-date account of how successful it is, but perhaps Bethe told you.
Yes, he did discuss it; but I gather you feel these two fields developed independently.
They developed pretty independently for a while. I think since Brueckrier there is thisttrnpt to combine theme, You can do nuclear structure physics without doing any nucleon- nucleon scattering. But of course the nuclear structure being the more complex situation must somehow contain the simpler situation. And the difficulties arise because of the great mathematical difficulty and also because of questions, what are the three-body forces like, if any? So people have been hesitating. But it seems that there is now quite a bit of success inpriori deduction of the nuclear structure properties from the empirically known proton-proton forces. You would also like to make anpiIori theory: why are the proton-proton forces what they are? And then you have to go into meson physics and complicated vector mesons and all. Now this is another attempt which people are making.
Would you say that Yukawa’s idea had any influence on nuclear physics as an experimental science in the ‘3Os?
Yes, it had an influence because one started thinking about beta decay and of the way the nucleons are held together, the “glue” which holds them together, the pi-meson exchange. It had certainly an influence on the thinking.
Did anybody actually set out to look for the meson?
Well, they thought at first that the mu meson which was discovered in ‘37 was Yukaw&s meson; they called it even the “Yukon” once. And it turned out that it was not. The mu meson, as you know, is a weakly interacting particle. Then only in ‘46 or ‘7 when Powell found the pi-mu decay, and at the same time Bethe and Marshak realized there ought to be two particles, then onlyws it realized that the Yukawa meson was much nearer to the pi
meson and the mu meson was just an interloper which happens to bea daughter product of the pi, having nothing to do with the strong pi interactions.
But were these essentially accidents rather than things which were looked for?
The mu was not directly looked for except that one found in cosmic radiations some anomalies. It didn’t behave like an electron. So the mu was discovered, you might say, by just looking at cosmic radiations, like many other particles were found.
Like the positron.
Like the positron. The positron, of course, Anderson might have known of from the work of Dirac, but he didn’t. In fact, it’s very amusing— when there were historical talks at the Physical Society everybody who did some great work mainly said, “But I didn’t know about this.” “I didn’t know about Dirac, etc.”
Or if they did know, it didn’t have any effect on. Their work because they either didn’t see the connection at the time or they didn’t take it seriously.
Today this would be much faster. Today if a new idea like this comes along, experimenters would be running to the lab to deliberately test it. In those days there was somehow this naive approach. I won’t say it’s always true. It could still be so mysterious that people don’t feel inclined to look for it, but today if anyone invents a particle, like the W meson or the magnetic mono- pole or the quarks or the alphas, right away many look for them—even if you don’t think they have any chance to exist. But just in case they exist, they know here is something worthwhile doing. This is what has changed. There are many particles which may not exist, but because they are talked about, people look for them. I get a lot of requests for time at the big machine, the Brookhaven Alternating Gradient Synchrotron to look for these priorities.
Also, there is a great deal of mobility. By this I don’t mean that people can get around physically, but they can switch from one experiment to another, from one field of interest to another because the big machines are so versatile.
That’s right. And also the thing which we somehow got deflected from: during the Los Alamos days—or maybe you are coming to this later-—the technique made a tremendous jump forward, and a lot of people learned very difficult tech— niques, and they have no fear of using difficult, complicated techniques—building them up and up. This is what has changed from the pre-war days. It might have been a lifetime job to develop an amplifier, so now you just rush into a new technique quickly and it spreads very fast. So when there is an idea, you know how to best do it very quickly.
This reminds me of a question which we had listed earlier: that the electronics technology for building pulse height analyzers was available in the ‘30s, but physicists didn’t adopt that technique for about ten years.
Well, by the middle ‘30s there were the first electronics devices. Experimenters would just have little telephone meters where the numbers of counts would appear. They were a little slow in adopting them in the Cavendish, but then once the technique was developed it spread fairly fast. Of course it was still limited. It wasn’t very fast counting and it broke down easily and so on and so on. But physicists are usually pretty fast in exploiting a technique There may be sometimes a little lag, though today there is very little lag
In the ‘30s one didn’t normally bring in an engineer to help design equipment
That’s true Physicists built their own cyclotrons still Just occasionally one odd engineer who might be sort of a physicist at heart but didn’t formally become a physicist would work with him. Some of the physicists became good engineers also in a way because physics is good training for an engineer. So these are the people who then helped develop radar. They were so good at it because of the training they had had. They couldn’t leave that completely to the engineers. But today I understand engineers have caught up again. In NASA many things are done by engineers which during the war would have been done by physicists— developing satellite electronics, etc.
How would you say the war influenced technique?
I think during the war the scale changed completely. People first of all noticed that one can spend a lot of money. If one wants to know the answer to something and it’s worth spending money for, one can find it. And so they had courage to ask much more expensive questions, you might say—like the neutrino search after the war and things like that, which before the war looked simply out of reach. If I had wanted to do that, before the war I would have had to spend a lot of time just raising the money. This was relatively easy after the war. Another thing of course is that a lot of people had learned very first class electronic technique and brought this to their work.
Are you saying that it was a question of education of the physicist in existing techniques?
Partly, and also they had learned that when you need something special, you develop it fast. So they developed a lot of techniques during the war. In the war years I think the number of fundamental ideas was very small, but the number of technical developments was large. I think this is perhaps not atypical of a war—that fundamental ideas essentially stand still. Maybe the mind is too absorbed. And so the whole Manhattan Project made very few fundamental contributions, but it made great technical contributions. Then after the war, you could use the reactors and the counters, and then there came great fundamental contributions. To do good research you probably need some peace of mind, and during a war you are just sufficiently deflected. If you look back, the ideas nearly stood still.
Let me ask a bit more on that. Do you mean that, in fact, after the war, given all these new factors that you mentioned, that people picked up where they were before but with new techniques? In other words, did they tackle the same types of problems?
Well, they tackled the hard problems which they might have just brushed aside before. Take the Lamb shift. It was already known before the war that there was something wrong with the line spectra, but one just thought: “Oh well, you can’t do much about that. But Lamb, with his modern techniques, developed for radar, found he could c something. That’s what changed. And without the war I think it would have been still another five or ten years off. But for fundamental ideas the war is a big drawback. Let no one tell you that science really progresses during a war. What progresses is technology. Then, when there’s peace, you can use it. There are, of course, exceptions.
Well, what technology did come out of the war? Let’s say after the war Illinois, for example, was one of the first to be right on the scene with large machines. Now, wasn’t this started before the war?
Started before the war, but one didn’t dare to go into such a scale until after the war.
But that’s—I’m engaging you in debate here to get more information—but that’s not so much technology but motivation, self—confidence, and knowing that you can get the support.
Yes, but also, of course, what was developed during the war was the fast electronics, and I’m not even sure enough of my knowledge of electronics to say it was developed, but it certainly was spread to a lot of people who then ended up in universities and the national labs and they were now able to do things on a scale which very few would do before. I think that was perhaps the important contribution. A lot of the good physicists now were young people who during the war were at Los Alamos or the Rad Lab and picked up this first class technique. And I think they would have been different physicists if they’d had a different education.
That would make the difference. Even if we extended your idea of fundamental contributions, or the lack of them, to technology, it would be the diffusion of techniques and the development of a style of research and a style of
Well, there certainly was a lot of inventiveness during the war. There’s a difference between inventiveness and recognizing deep new concepts.
Are there any things that can be pointed to as inventions coming out of the war that had an important effect on the technology for subsequent work?
The radar techniques, the high-frequency techniques, the things which made these high-energy machines go round. The synchrotron was invented at the end of the war. As soon as the war was over, you see, practically all the people had time to think and so McMillan and Veksler invented that at that time. But the technology they needed for that was part of radar.
What was the influence, would you say, of the invention of the synchrotron? Was this of great importance?
It was very great because out of the synchrotron arose all these modern machines—the Cosmotron and the AGS, which is an alternating gradient synchrotron. It’s a version which uses a different magnetic field, but the idea of the synchrotron is that the rf frequency changes, and this sort of technique Would have been very hard before the war.
Well, getting back to something related to the war, the idea of fission: what role did this play in the development of the concepts of the nucleus?
I think from the fundamental point of view, only Aperipheral role; but it played an indirect role in leading to the reactor with it intense neutron beams, intense neutrino beams, and intense radioactive sources; you could do new kinds of physics. So suddenly everything was changed by factors of thousands to millions, depending on the experiment. And so you could do quite new things which you couldn’t do before; and at the same time after the war there was the development of the sodium- iodide counter, and the other scintillation counters. And so you could get quite a new kind of spectroscopy, and you could do lots of things which you couldn’t have done before—e.g. resonance scattering of gamma rays. Like this work we did on the neutrino helicity; that was possible because we could get the europium sources we used so easily in the reactor. And that would have been quite a job before the war. So once some things are easy, you can let your imagination play on other things. Otherwise, you are fighting the hard part. Research, is, after all, a very marginal activity and it’s easily cut off. But if you have all this background, which helps you, then you can work at peace and do something. So the fact that neutron sources are easy is very vital. And today even it’s not generally enough recognized that you must make neutron sources easily accessible or certain things will not be done. You just go to so much trouble for each experiment, though the younger generation always seem to be able to go to a little more trouble than the older one.
There’s also a lot of the past knowledge and past technique that is packed down and easily available for…
That’s right. I mean you can now run a gamma ray spectrum on a germanium counter, which if you go back decades, for each decade it might have taken you a hundred times longer or ten thousand times longer or a million times longer, and you might never have gotten it even, if you go back far enough. First you did it only by absorption and then by beta spectroscopy, and then by sodium iodide counters. Now you can run in minutes an accurate spectrum, which is a fantastic development, and the techniques have improved so fast, one hasn’t completely exploited them yet. No one has rerun all the gamma rays with the new technique, though you could do that and get more accuracy. It would be a worthwhile project.
We were talking about the improvements in detectors, and I think we fairly well explored the ‘30s, but would you go into more detail, say, ct the first five years after the war? What kinds of detection were now available?
Up to the war years there were the Geiger counters,, the ionization chambers, the cloud chambers, proportional counted4o—called. This was all. Then after the war there was Kallmann’s discovery of the scintillating crystals. The first ones were these anthracene or napthalene counters, and then sodium iodide discovered by Hofstadter.
Which was Kallmann’s contribution then?
I think anthracene was—the organic crystal. So there were these different detectors. Then came these fast counting techniques for scintillation. These scintillators are also very fast, faster than Geiger counters, so you could jump a factor of a hundred in time to shorter times. And then came improved photomultipliers and improved general solid state electronics so you can speed up things more.
You’re jumping ahead now, am I right? When did solid state detectors come in?
Solid state detectors were there, but I mean solid—state electronics came earlier—transistors and semi-conductors.
But still we’re in the middle ‘50s then probably.
Yes. The solid-state detectors came in the middle ‘60s or first part of the ‘60s about three or four years ago. And they are the latest in accuracy. Then of course there came also spark chambers, which allow you huge detectors, weighing now 40 tons in one of our neutrino experiments here.
When did that become important?
Well, the spark chamber is actually an old technique, but the great new idea, which occurred to two Japanese physicists, was to combine it with a coincidence so you get a spark only when a certain particle goes through. So you can now choose the particular particle you see. This was a new idea which is about six or seven years old. This was used here for the neutrino detection by the Columbia-Brookhaven group. And there you can make use of the fact that the neutrino beam comes at just a certain time. You know the time when the target is hit by the protons and it produces pi-mesons which produce neutrinos, and so you make your spark chamber sensitive at just that time, and that allows you to get rid of cosmic-ray background because you are only sensitive for a fraction of a second. So this was very great progress. And then there are of course Cerenkov counters. There are new counters all the time. They were developed largely in this country soon after the war.
What about improvements in targets? Did this have much effect?
Well, especially hydrogen target technique is something— this liquid-hydrogen technique which has developed since the war very largely. And then one can use very long liquid-hydrogen targets. The most popular target is still the proton, so you like to get a lot of that. And then deuterium became very available after the war. It became cheap. And of course we must not forget that the bubble chambers came along about a dozen years ago.
Was that important for nuclear physics as well?
For nuclear physics not as much as for high-energy physics For nuclear physics only peripherally That’s mainly for high-energy physics.
That brings me to the next question, which is, when does high energy physics become a separate discipline?
I think this happened after the pi-meson was discovered in the accelerators in Berkeley first. You see, first you had cosmic—ray physics, and that was pretty separate from low-energy physics. And in cosmic rays a lot of particles were discovered—pi-mesons, mu-mesons, strange particles. Then the accelerators started making some of the same particles, and it took quite a while before the accelerators started finding new particles. Most of the important particles were discovered in cosmic days—the positron, the lambda, the theta, as K° now, the mu-meson, the pi-meson. And the neutron was discovered with little polonium—beryllium sources. The accelerators did not make contributions to new particles until fairly late, in the ‘50s.
But the intention was to duplicate the older high-energy particles.
The Cosmotron was built to try to make pi-mesons. But before it was finished, they had been found in Berkeley because one didn’t need as high an energy as was thought. Then the Cosmotron was used to find associated production, which was a concept developed from cosmic ray work but never this well proven; that you make always two of these strange particles at the same time. And then, you know, by this time the accelerators have found many new particles, like the second neutrino, the omega minus, and by the time you find so many particles, of course, you realize many are just excited states—we are often just finding excited states of particles. At least that’s one way of looking at them instead of having “137” particles.
When do you feel that people who were working at high energies felt they were not working with nuclear physics?
Usually either people made a deliberate switch to high energy from low energy—say, starting to investigate pi-mesons ... It’s somewhat arbitrary thing to say. I don’t like to be too specialized. I, personally, don’t like to call myself a high-energy man or a low-energy man.
I think you have worked in both all along.
I have worked in both, and I like to work where the interest lies; and if it happens to be even on the structure of the universe or cosmology, then I’ll find work there if I have a bright idea. But the high-energy field is in a sense specialized. The techniques are so special to it that the young people who learn it stay with it usually unless they go into industry. They don’t come back to low energy. Some people have made a successful transition from low-to high-energy, like Adair, but very few have done this. Most of the low energy people have stayed with low energy. Some few cosmic-ray people have made a successful transition to accelerators—like, for instance, Piccioni, but most cosmic-ray people have stayed in cosmic rays.
Are you saying then that the high-energy physics attracted only the young people?
On the whole this is true.
I mean only some very versatile people like Fermi changed into high-energy, and some cosmic-ray people have gone into high-energy but not terribly many. A few low-energy people have gone into high energy, like Alvarez; and most of the others were trained on these machines because, after all, there is this explosive number of physicists. Most of them are young. You can’t find many of my age because there were so few at the time. This is why people like me have to be director. They can’t find them old enough and they don’t like to give it to too young people. It’s a mistake.
What date would you put on this beginning of high energy as being really clearly defined as a separate branch, when you had to be either full time in that or full time in something else?
I tell you, it is more the practicalities of life because there would be a Rochester meeting and the low-energy people just would not be invited. I was not invited to the early Rochester meetings. It’s only after people noticed a paper of mine on high energy that they thought maybe I should be invited.
Now, the first Rochester meeting was in 1950?
Yes. I was perhaps invited in ‘53—I don’t know.
But would you say that the very calling of that meeting confirmed the fact that the small conference of theoretical physicists originally held at Shelter Island, had already outgrown the room and a much larger group was now involved? Was this a symptom that high energy had already come into the picture?
Actually, after a while, one realized it was more elementary particle physics. It didn’t matter where you got your knowledge. If you got an interesting low-energy experiment, you would still talk about it—like neutrino helicity or Miss Wu’s work. You would still talk about it at the Rochester conference. So it is really elementary particle physics and never mind what technique you used—whether you used low-energy, high-energy, cosmic rays. But from a practical point of view, you know, the funding agencies like to distinguish. Now they’ve added medium energy between low and high.
That would mean that Breit’s work, even in the ‘30s, could be called elementary particle physics.
Sure. When we measured the neutron mass, we did elementary particle physics. Had I done it in the ‘50s I would have been invited to Rochester to report the value. And when they measured the magnetic moment of the proton and the neutron—as Rabi did for the proton—that was also elementary particle physics, so Rabi got invited. So you see it really is more who is interested in the fundamental question, experimental or theoretical. So the word, “high-energy physicist,” is really too specialized a word. It’s unfortunate But you can be interested in elementary particles; you can be interested in nuclear structure; you can be interested in cosmology and you can be interested in all of them, and in theory and experiments—so there are many variations.
But it’s hard to be interested in several fields simultaneously when in fact it requires a full time involvement to keep up with one.
This is the point—to have some technical competence in any one of these fields, you have to spend so much time being up-to-date on all the data. So you either have to cooperate with others, which I have often done, with experts in the field, or you have to take the slow route of learning a new field. It depends on your temperament-—whether you can sit on an idea for a year or whether you want to right away do something with somebody who happens to be an expert and who must first catch fire; you can’t just sell him anything—you must have a good enough idea. I worked with Lee and Yang once because I just at lunch threw out an idea, and they are so fast in working it out; and before we knew it, we were writing a paper together—paper no. 110. This has many interesting applications. I have worked with Gary Feinberg, whom you may know. I’ve worked on some theoretical ideas with him.
I’d like to take the remaining few minutes to ask this general question: If you can, in looking over some of the things we’ve said— and I admit it’s fragmentary because we left you off in the middle of a very exciting period to go to some general questions, and we hope to resume at another date—if you were asked to characterize periods in the development of nuclear physics that are historically logical periods, and to identify transitions between one period of time and another, where in fact the f]d changed qualitatively or quantitatively or split or fragmented or whatever, where would do it, where would you start?
Well, for instance, in beta rays, you know that the very first things in radioactivity were beta rays; and it took about 60 years to really understand them well. Now you can say we understand them pretty well. There are certainly always some fine points left. But as a big field it’s probably finished. It may start again. As a refined, new, second order question, it may continue. And so fields rise. There are first very obscure data obtained—contradictory data—and finally reproducible data, and finally somebody sees the systematics and then someone sees the fundamental way of putting it and then someone sees that you need more than one constant.
So it takes a long time for a field to go through all these stages. Then when you finally get to the stage where the theory is good enough that it can predict the next experiment, it gets boring. As soon as it gets to that stage that you can predict correctly, then each experiment becomes a demonstration experiment. Now, it’s useful not to be too snobbish and do a few of those because you might get a surprise. Einstein said they were just demonstrating his theory. He was so sure that it was only a demonstration as far as he was concerned. Some others might call it a test. That depends on your psychology—whether you call it demonstration or test. But a lot of people do not realize how often they do demonstration experiments where everybody is convinced of the answer beforehand, but physics is an empirical science and you can be wrong and you must do even demonstration experiments and test things. Whenever I haven’t done one, I’ve usually had reasons to regret it because some interesting things can come out. So I feel when a science becomes predictable, like electro-dynamics, you get occasional flurries of excitement and they turn out to be wrong, like you have recently heard. So these very very settled fields you shouldn’t revolutionize so easily. If you have a contradiction, you should be very careful.
Then if this contradiction gets repeated, there’s something missing in this field. On the other hand, electrodynamics has only been tested to a certain accuracy. If anyone knows a better way to test it, to push it a little further, the more strength to him. And I’ve always tried to do such experiments. You may find here some on the conservation laws. People always said, “The proton is stable,” and I said, “How do you know? All you know is from experiment. You feel it in your bones it is stable because if it lived less than 10^16 years it would kill you. So you have a certain measure right here, but you can do better.” We’ve pushed these lifetimes—and other people have pushed them—from 10^23 years, to 10^24 years, much more sensitive than your bones. Now, no one has a theory right now which says the proton must be stable, so it’s only a belief. It’s neither a theoretical statement, nor an experimental statement. Experimentally, you can only say, “It lives longer than .. .“ Theoretically you say you do not know but you believe it is stable. So it’s good to teach it correctly.
That’s one of my thoughts which I’ve often brought up: Say what you know and say what you believe, but don’t put the belief as a fact. And with many of the conservation laws it would be very useful to know their limits of accuracy, and this is one thing which we emphasize in this paper with Feinberg. I feel people who have gone in that direction have sometimes found something very exciting, like the Cronin-Fitch experiment, testing something which was only believed to be true for a few years. They pushed it a little further and found it didn’t hold. And so this way of thinking is now not very new, but my attitude on this has been much older than even the weak interaction revolution. Here also was something that didn’t hold when you tested it. So it’s always worthwhile to bring out explicitly in an experiment something which you believe is true, so if you are aware that it’s only a belief, you are always willing to test it when you see an easy way to do it. It’s hard to go out of one’s way and make a tremendous effort to test something which everybody believes, but when it’s easy, at least, do it. And many of these were quite easy.
How would you apply this to the development of nuclear physics? This is a very good model that you’ve given us. Now, how would you fit in the stages, which appear to be now wrapped up, looking back from the early ‘30s to the present? That’s a tall order.
Well, I’m not quite sure what you mean. For instance, nuclear structure?
Yes, let’s trace that
Nuclear structure. Well, there one had this unfortunate situation that one knew always the calculations are too difficult, so one could never take a model too literally If there was a little contradiction you didn’t feel that a revolutionary statement was made. It’s only as the models get more and more refined and better and better calculations are made that you start taking contradictions seriously That’s why I said before in one example: Bohr could brush it off— that, well, we cannot make such exact statements. But as models are refined, these contradictions become important and because this field had always the difficulty of exact calculations, it hasn’t been so sensitive to the shock of the contradiction as other fields would be. But finally this will always be very important—to see whether it all fits or whether there are some exceptions.
Do you feel that nuclear physics is now at a stage where that will happen? Do you think that concepts of the structure of the nucleus are sufficiently refined now?
To see such fine contradictions? I think not really fine ones, but some more obvious ones. You see, the models are still terribly versatile because they are approximations. I call it the battle of the approximations. You are not fighting for fundamentals but fighting for what is a good approximation, and everybody has his own feel what is a good approximation, and if it agrees with the experiment, he leaves it at that. If it doesn’t, he’s willing to change it. But you don’t call it a revolution.
So it is just getting out of this stage perhaps, but it hasn’t quite reached the stage where if a level is found in a wrong position, you will say, “Oh, everything is wrong in our understanding of nuclei.” You will just try to refine your ideas. Now, it may well be very soon that a level in the wrong position will excite you terribly, like it would in hydrogen, in an atom. If you suddenly found a level where it shouldn’t be, you would be terribly excited. In the nuclei, we haven’t quite reached that stage. They still discovered levels in helium just only recently. Helium 4 was supposed to have no levels and recently it got four levels. Well, people didn’t get too excited: “Oh, yes, we had forgotten this and this and that term” They used the empirical data to guide them in their approximations, and this is of course sometimes the self-fooling which enters. If there was an a priori reason for an approximation, then you get the result and then you get a contradiction and then you have something to correct. But there is this big to and fro, between model and experiment and nuclear physics as it is now could not yet be described independent of the data. Once a theory is really finished, you can throw away the data and then lecture about it to students and they think you’re predicting it all. They think you’re predicting nature. This you can do now with electrodynamics, with Maxwell’s theory, with Newton’s theory; you can throw away all the reasons how they came to the laws, but you cannot do it in a field while it is still evolving; and I think some people forget this in modern education especially.
Would you say then, that in the 35 years from 1932 to the present, that this is of a piece as a historical period in the field of nuclear structure; that although there are highlights that we have discussed and there are little turns here and there...
Well, there are big changes in slope because of the war where ideas were standing still and technology went up so that you could suddenly do things which you could only dream of before the war but couldn’t yet do.
Any other changes of slope? That’s a good way of putting it.
Changes of slope. Well with each new invention, like scintillation counters and solid-state counters, there’s again a change in slope. You have a new degree of accuracy, a new speed with which results are obtained, and fantastic detail. Now, there is the new technology of putting computers on line so you get fantastic detail which would have taken too much time before, so you look at complicated reactions, three-body reactions, and detailed nuclear reactions. So I think we are right now again collecting data at a great speed, but unfortunately there are not enough people who can sit back and. integrate these data. It’s not enough to produce a table. You cannot talk to a table. You must have an interpreter, and there is not enough of that. That’s really the biggest lack right now.
[Atombau und Spektrallinien]
 Note added April 1967: You might call this “missing out” the “Gorter effect” [see C.J. Gorter, “Bad Luck in Attempts to Make Scientific Discoveries,” Physics Today 20 (1), 76-81 (January 1967)].
 Bibliography follows transcript.
Chadwick said his part later