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In footnotes or endnotes please cite AIP interviews like this:
Interview of Otto Frisch by Charles Weiner on 1967 May 3,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
For multiple citations, "AIP" is the preferred abbreviation for the location.
Career in nuclear physics, chiefly through 1939; describes differences in atmosphere among the Universities of Vienna, Berlin, London and Copenhagen; his switch from mathematics to physics at Vienna; work at University of Berlin on a grant, with Peter Pringsheim, before going to Hamburg to work with Otto Stern; with Hitler laws in effect, leaves for position with Patrick M. S. Blackett at Birkbeck College, 1933; then to Niels Bohr's Institute, until 1939; anecdotes about working on neutron experiments and nuclear models in Copenhagen; recounts how he and Lise Meitner explained fission, and memorandum with Rudolf Peierls on bomb possibilities; brief comments on postwar career.
I'd like to start by asking you to recall the early years of your life and to try to remember when you first became interested in science and how you became interested.
Well, I think it all started because I was interested in mathematics very early. I was sort of a mathematical infant prodigy, and for a long time I thought I would study mathematics. It was only in my high school days that I got interested in gadgeteering and made myself a little diffraction grating and so on. I think that interest was partly aroused because we had a great big three-volume encyclopedia of science of the middle of the 19th century, a fantastic thing.
Who was that by?
He was called Masius, I think. In this, the greatest and most recent achievement of science, was a Ruhmkorff induction coil, and that seemed to have fired my imagination. That book has completely disintegrated and doesn't exist anymore.
Was in your family library?
It belonged to my father and he gave it to me to read, and I read it insatiably for years, day and night, more or less, until it completely disintegrated. And even when I went to the University I still thought I was going to do mathematics as my main subject, but then I did realize that there were a few other geniuses around and maybe I wasn't quite so hot. So, as I had some interest in physics, and also because I began to feel that I didn't want to do something completely abstract all my life but would rather use my hands—I have quite good hands and I'm good at things—so I turned over to physics halfway through at a time when I had already been offered the chance of doing research in mathematics.
Let me go back for a minute. You said that it early became apparent that you had superior mathematical ability. In what way does one observe this?
I don't know. I mean this is family legend, and I have no real recollection, but I think I'm reputed to have multiplied fractions in my head at the age of five, something like that; it's probably exaggerated. And at the age of ten my father explained to me what a sine and a cosine is and wrote down Sin²0 + COS²0 = 1, and I said, "Obviously," which surprised him.
What about his own background?
My father was a polyhistor. He had studied law and history of art. At one time he published a kind of popular encyclopedia in the form of little pamphlets, one a week—a sort of hundred questions to test general knowledge—which he did almost entirely out of his own head, covering all fields of human knowledge except on one occasion when he wanted to do one on the atom; that he made me do.
How old were you when you got involved in doing this one on the atom?
That was when I was already at Copenhagen, and I was over 30.
Then this implies that in your home there were a great many books on a variety of subjects. Is that right? Frish: Yes, most of which I ignored, fear.
Except the encyclopedia.
Did you have formal courses in either mathematics or science in the schools in Vienna before entering the University?
Well, we had the usual sort of teaching of both mathematics and physics and chemistry. I dare say we had some biology—I have no recollection. I think we had one hour a week of each. It was all given by the same teacher who quite frankly admitted that he didn't know very much himself.
Was he relying on a text pretty much for these various courses?
I think we had regular textbooks. I don't remember.
But your interest started then pretty much on your own?
It started on my own. It was probably channeled towards physics, because of that famous aunt of mine who was already becoming known as a physicist—Lise Meitner.
And what contact did you have with her as her nephew?
She already lived in Berlin. I was only three years old when she went off to Berlin. I had hardly known her before that time. After that she only came home for occasional short visits. She did come and talk physics to me, and I asked her questions, and it certainly helped to have someone who obviously was competent and known in the subject.
How much of a difference in age is there?
So by the time you were ten or so, she was already a physicist with a reputation.
Oh, yes, indeed.
I can understand this had an important effect. You decided, though, to enter the University of Vienna as a student of mathematics and then you switched to physics. When you made the switch, what did you think doing physics meant?
What I thought I would do? Well, I thought I would get a job in industry, and in fact I did. My first job after I got my doctor's degree was to work for a man in Vienna named Sigmund Strauss, a very remarkable and brilliant inventor, who had been working for Telefunken for a time. He was the one who invented feedback and resistance-capacity coupling, and he had about a hundred ideas a day, 99% of which were nonsense. But one percent was good, and one good idea per day is really a very good score.
Was it common for a physicist to be employed in industry? Did you have any knowledge that led you to believe that this was sort of a vocational path?
It wasn't very common, but I had the impression that a certain industry was growing up making electronic tubes in Vienna and that with a good doctor's degree I might manage to get a job there. It wasn't at all clear to me what I would do in that job. Maybe it was a foolish idea. At that first job, I think for the first six months I didn't get paid anything. It was just that I was allowed to help along and to get some experience, and later on when he found that I could debunk his wrong ideas at a great rate, he paid me. But then soon after that I got my first invitation to work in Berlin and gave that up.
When you did that, apparently the thing about physics that interested you was the instrumentation aspect of it. Is that fair to say, or is it only part of the story?
I've certainly always been more of an experimenter, and whenever I have done theoretical speculation, so far nothing has really come of it. I was always very interested in inventing new methods or thinking up new gadgets and so on. Usually my attitude was that when you invent a new method, results are bound to follow; whereas a good many other physicists work the other way around. They say, "When you have got a good problem, somebody will find the method."
How about at the University? Was there much opportunity to do experimental research, as it were, once you switched to physics?
Well, how was it now? I did a total of four years and spent about the last one and a half years in experimental research. Before that I simply went to classes, and most of them were just lectures. There was no supervision, no recitation, and very little practical classes. But two and a half years after I started, when it was time for me to think about a doctor's thesis, I got an opportunity. I don't know how it happened. I dare say Lise Meitner may have made a suggestion to that effect. She knew my supervisor well. He was Professor Karl Przibram, who was not the head of the department, but I think it was the professor under him (Professor Stefan Meyer) in radioactive physics at the Radium Institute, as it was called, who suggested a problem to me. I worked on it for a year and a half and eventually wrote a doctor's dissertation of the length of about three pages of typescript, but apparently it was considered adequate. I didn't see any need for padding. I hadn't understood what was expected, but nobody seemed to object and I got my degree all right. It's of course a far shorter course of study and I dare say the Viennese Doctor of Philosophy is not really equivalent to an English or an American Ph.D.
What was the topic of the three-page dissertation?
It was "The Discoloration of Rock Salt Under the Impact of Fast Electrons." Przibram had been working on the puzzle of why certain natural crystals—in particular, rock salt—occasionally were found in a colored form and had discovered that this was due to the radioactivity of surrounding strata. And so he had done a lot of work irradiating various salts by radium gamma rays under various conditions. Then it had occurred to him that one ought to try to do that with electrons of known speed and see if one could find a threshold. Quantum ideas were still very vague, and he felt probably this was a way of testing quantum ideas. Now, it's not a very good problem on which to find the threshold, and I didn't find a very good one. The subject has later gained great importance. I think some of our radar screens are based on the behavior of luminescent crystals and things like that.
Was this dissertation published by the University?
It was published as a short note by the Vienna Academy, I believe.
You mentioned that the quantum ideas were still rather vague. During your student days the whole quantum theory was in a state of turmoil. Were you aware of the turmoil or the changes taking place?
No, we were never taught any quantum ideas at all in our regular courses. The courses were absolutely classical physics. I doubt if we as much as heard of the Franck-Hertz experiment. I don't think so. I think we were just given a solid foundation of classical physics. I do remember how one of our professors—actually not from the University but from the technical high school, a Professor Flamm—once gave the first lecture about Schrödinger's new theory and expounded it, and I remember I sort of could half follow and felt very excited about it. When discussion was asked for, there was a long pause and then Thirring said, "Of the many things I haven't understood, I wish to ask about one I haven't understood at all."
Did he get a response?
Well, I think both he and Flamm were rather uncertain of what Schrödinger meant, and of course so was Schrödinger, I'm sure. This was before the statistical interpretation was given.
Was it common to have outside lecturers visiting?
I don't recall it, no; this is the one occasion I do recall. Mind you, I have a very bad memory. There are a tremendous lot of things, quite important things, I experienced which I have totally forgotten.
Well, we'll probe and try to remind you. How would you then characterize this school of physics? I don't know whether you would even use that term. In other words, what were the major interests of the physics department at Vienna? If they were not in quantum development, what were they in?
Well, of course, I only saw the fraction at the Radium Institute. There were other departments with which I had practically no touch. I can report that one of the institutes was run by Ehrenhaft, who was at that time still trying to prove that sub-electrons existed. At the Radium Institute we also had a pair of characters who were trying to prove that all light elements could be disintegrated with alpha particles. I never got roped into that work. But apparently what they did (I think I said that in my talk in Washington) was to rope in students and make them take turns with a scintillation screen and be very pleased if one of them found many particles. So very soon they found more and more disintegrations.
Did they publish these results regularly?
A lot of that stuff was published and caused a great deal of controversy. It had a beneficial effect on the whole probably, because it compelled other people to develop the more reliable electric counting methods.
Was there any evidence that this was a factor in it? It's a good hypothesis that a large amount of dubious results makes people feel the need to develop better standards.
Yes. Well, I did have first-hand evidence of this, and in the first place in Vienna itself, the two people who were in a way largely responsible for this—Orthner and Stetter—also started a piece of research trying to develop electric detection methods. And I have been told that in England Rutherford more or less said, "We must have some independent method to check on these things because apparently the scintillation method can be misused."
Was there a name for that field of research, including the work of the Radium Institute, at the time? Did people identify it as nuclear physics or was this too early for that name?
I think the work was referred to as Atomzertrümmerung, atom splitting. I don't recall whether the term Kernphysik was used; I have no recollection of that.
According to someone in Berlin in about 1930-'31, Lise Meitner offered a seminar in Kernphysik, so the name existed then. I was trying to pinpoint it earlier and to find out if that term existed. Then after you completed your dissertation, you went on to Berlin. How did that come about?
Well, that came about because in Berlin the director of what corresponds to the Bureau of Standards was the physicist Warburg, I believe. And in those days there appeared to be a need for a better unit of illumination, a candlepower or something equivalent. I think at that time there was still a standard candle, which was really a standard lamp with a wick of so much diameter burning a specified fuel in a specified atmosphere. It wasn't very accurate, and it was felt to be irrational; so Warburg had proposed what he thought would be a better way of defining a light unit based on the radiation of a specified area of a black radiator at a specified temperature. Now that raises a question of how to find the temperature, and for that purpose Warburg had proposed a complex method using two temperatures and comparing at those two temperatures both the total radiation and the radiation at a specified wavelength. And he figured that if all this was done according to the best knowledge of the art, one should be able to define luminosity to about one percent. He had instructed one of his people, who was in charge of photometry, a man called Dr. Carl to start research on those lines, and Müller required a collaborator and had applied to a German organization which gives out grants, Notgemeinschaft der Deutschen Wissenschaft, to find someone and to offer him a grant. Apparently this grant was offered to two people in succession, one of whom refused because he wanted to do something else. The second one accepted it, and then a rich uncle died and he decided to return to a life of leisure. And then it was offered to me. That's how I got it. I went to Berlin in the summer or autumn of '27.
What would the alternatives have been?
Well, at that time I was working in Strauss's laboratory, and maybe I would have stayed and eventually perhaps become a partner or something.
You mean the industrial one.
Yes, that industrial laboratory. It was at that time manufacturing X-ray dosimeters but also was interested in the construction of radios and so on.
Let me just backtrack a moment to ask one question about your boyhood. Did you tinker at all with radios at home?
Oh, indeed. I had tinkered with radios from the day the first experimental transmission started from the Vienna station. I had one of those whisker radios, cat-whisker radios, which I made myself; and later I made myself a portable tube radio, and apparently that got me the job with Strauss. I showed this to him. It was a cigar box with various things built into it. In fact, the first job I had was that he gave me six boxes containing so-called radios which had been, I believe, built by a predecessor of mine. He said, "These don't work. Can you make them work so that we can sell them?" which I did, and after that I had to spend a fair amount of time going around to the people we had sold them to make them go again when they failed.
Then you arrived in Berlin on this grant and proceeded to work. Let me ask you a question about the difference in atmosphere between Berlin and Vienna. Of course it meant leaving home, first of all. Did you have a room?
I did have a room with a landlady, but it was in the close vicinity of Lise Meitner, so I saw her practically every day, and I didn't feel at all cut off from my family. In fact, I saw a certain amount of her work. I learned later that she had been asked by her own collaborator, Hahn, why not take me into her lab? She didn't want to do that because she didn't want to be put in a position of conflict if I should not prove satisfactory. In fact, she had always steadfastly refused to give me any kind of support or testimony. Apparently there was one occasion when a prospective boss said, "Look, but you can tell me this: Is he a disagreeable person?" She said, "No, he is not a disagreeable person." was the most she would do for me.
She was very consistent in this.
Yes, yes. And I greatly appreciate this, because at least it leaves me free of the stigma of having risen due to her support, which of course many people still suspect.
I think there is enough of a tradition of gifted families in physics to establish the point that people stand on their own two feet on the basis of their own merit. So you were drawn into the Berlin circle. What was the overall difference in the atmosphere other than the points we've just mentioned, especially at the University of Berlin—though you weren't at the University?
I was not. I was at a thing called Physikalischtechnische Reichsanstalt.
Wasn't Paschen there?
When I came Paschen had, think, succeeded Warburg as a director.
And were you in contact with the wider scientific community in Berlin, besides Lise Meitner, of course.
Not very much except that I went regularly to the colloquium at the University. That really made me feel that I was at the center of things. We used to claim that we had a front bench completely filled with Nobel Prizes, which was a bit exaggerated, but we had Hertz and Einstein and Nernst and quite a lot of people of that caliber.
Planck was still there?
Planck was there. Why didn't I run into Planck? Probably he never opened his mouth.
What were the main areas of interest during that period at the colloquia of the University of Berlin? Was it quantum theory?
I think it was ranging pretty wide, and I don't recall it. Much of it was rather theoretical attempts to extend quantum theory. This was in the years '27 to '30. I do remember when the electron dif- fraction was reported, and I do remember when the Raman effect was reported. They were discoveries at that time. But these were high points.
And at your regular employment, were there any meetings or discussions among the group of people you were working with?
At my regular appointment, no. This was a large building containing a large number of quite small groups who took very little notice of each other.
I know in that period some Americans were abroad studying there. Do you remember coming in contact with any of them?
No, our group was a very small group. There was only the Ober- Regierungsrat, Carl Müller, my boss, and myself; and we had, I think, three mechanics or assistants.
How would you characterize the development of your career in this period? Did it help you to formulate a taste for a certain problem area in physics?
Well, I would say this: I certainly learned a lot about problems. I remember that the question of recoil in beta decay was discussed a good deal, and I tried to think up experiments. I usually worked more or less during my office hours on my problem and then often stayed quite late at night to work on private problems in the lab. Müller had nothing against it. On the contrary, he was very encouraging, and he did the same, incidentally. He also had his private problems on which he worked during outside hours. He was a very remarkable gadgeteer who invented ways of making fantastically thin metal foils of enormous size. He had a gold foil through which you could read a newspaper which was, I think, eight inches in diameter stretched in a frame. And he had six little ones which were so thin that you could hardly see them and through six of which you could still read the same newspaper. He used them to develop new types of microphones, new types of electrometers. He was constantly trying to make some inventions. I don't think he ever made any money with his inventions, although they're quite good ideas.
Was it his intention, though, to patent them as he used them?
Oh, yes. In fact, we even had a patent together, and I once got a small sum of money for it—something like $100.
What was that? Do you remember the patent?
He had thought of a new way of building a monochromator for visible light which would use a quartz prism. He gave me some calculations to do, which I did with hard slogging for two weeks with logarithmic tables, which nowadays you would do by writing a computer program. But in the process I thought of a cute way of making the mechanism very simple, to make use of the particular straightness of a spectrum which he had actually suspected and which my calculations confirmed. Then he said, "Well, let's try it," and he got me the necessary prisms and lenses. After that I put in a crash program, which I'm still proud of, where in the course of three weeks I built a double monochromator and tested it from the far ultra-violet to the far infrared over the whole range of the photographic plate—all within three weeks.
He must have been pleased, sure of that.
I've never again been able to get such a speed. And so he offered to take the patent out in our joint names.
I see. That was the one.
And he very generously gave me my part of the royalty the first time he got a little payment, although it had of course never paid for his expenses. He would have been quite right in holding it back.
And you had the time to do this? I mean, in other words, the atmosphere was such that you could devote yourself to a crash program for three weeks without feeling the pressure of doing other work?
Well, that program somehow got to be regarded as part of my regular work, and the other work was dropped for three weeks.
I see. Now, you left the University of Berlin in 1930. Was this because the grant was over or because it was in general time to move on?
Well, in the first place the grant was over. The work was not completed, but by that time Warburg had died, if I recall rightly, and that gave Müller a chance to drop the whole project and to start a different one which he had thought up himself and which looked much more promising but which he had not been able to start as long as Warburg was alive and as long as my grant was running for Warburg's project.
Let me ask you another question about the grant. What types of grants were usually made from this source? I mean were they committed to the support of scientific research in general or only a certain aspect of research? And how were the granting procedures established? Would one go to them with a grant application, and if so, who would judge it?
I still don't know that. I think it was German industry who paid into a kitty sums of money which could be used to finance work for which no other financing could be very easily found, for which the laboratory funds weren't sufficient, and so on. And I presume that there were some people administering it and trying to judge the incoming applications, but I know nothing more about it. From the title it was not limited to physics because Wissenschaft quite generally means natural science.
I think that that will be enough to give us a lead and to look at some documentation of it. I hadn't heard of the name of it before. So you left Berlin because the grant had run out and the leader- ship of the institution had changed. I would like to ask whether there were other factors in terms of your own career interests that led to this transfer?
Well, it so happened that shortly before my grant ran out I had thought up a physics problem which I felt was quite exciting. Now I know it was nonsense, but at that time I even got one of the young theoreticians, Dr. Kallman, to think it was worth pursuing. The idea was to ask whether circular polarized light had an angular momentum and whether this could be confirmed in an experiment making use of mercury resonance and Zeeman effect. There it was fairly clear that a positive result ought to be found, and I did find it. But the real matter of interest was that according to my simple-minded argument, if you use a transverse Zeeman effect, you would also expect the conservation of angular momentum to impose some limitations. And that was not visible from Maxwell's theory because linear polarized light according to that theory does not have an angular momentum like a wagon wheel rolling along and having a transverse angular momentum. From my simple-minded argument, it seemed as if this ought to cause a different reabsorption of resonant light if you reversed one of the magnetic fields. Somehow or other I was advised to ask Professor Peter Pringsheim for permission to do the work in his laboratory, where the equipment was available, and this I was allowed to do in my spare time. I did quite a good experiment and found a strong positive effect for the longitudinal Zeeman effect, as expected, and no effect whatsoever for the transveral effect. I published this paper and it got me in touch with Pringsheim, who liked my approach and then offered me a job after my grant ran out. So I got a job for about six months working with Pringsheim on a problem of his own, which also was eventually published in our joint names. It was his idea in that case. But while I was still working there, I got an inquiry whether I would like to come to Hamburg and work with Otto Stern, which I did.
Do you know how that came about?
Well, I think Otto Stern was looking for another assistant and had made some inquiries and somebody suggested my name.
Were you paid as an assistant? Did you have a position at the University at Hamburg?
That was the first time I had a proper position at the University, what was called Assistant, and which really was "junior professor. I don't know what you would call it here, but it was the first step on the academic ladder. I was not supposed to give any lectures but merely to do research with Stern, helping him with his research.
And he was already pursuing the molecular beam?
Oh, yes, indeed. He had started long ago.
How did Hamburg differ from Berlin in terms of the awareness of contemporary science? Was there anything that was the equivalent of the colloquia that you had attended in Berlin?
There was a colloquium. I have very little recollection of it except that it was lively and interesting and there were a great variety of people. There was strong emphasis on astrophysics. The theoretician there was Lenz—I don't know what his own interests were apart from a constant fear of catching a cold—he was quite hysterical about it. But one of his assistants was Unsöld, who was then quite a young man and very interested and active in astrophysics. This was a time when astrophysics was very much a matter of orders of magnitude. We used to say that the plus and minus were put in the exponent. It was something like 1015173 something or other.
This was in connection with the Hamburg observatory then. Isn't there a major observatory there?
There was, but I don't think there was much connection. I think that Unsöld was just a theoretical physicist who had been following Eddington's papers and was reporting them to us. He was trying to extend that work and to compute opacities of stellar atmosphere and things like that.
So that at the colloquium, astrophysics would be discussed as a major interest. How about nuclear physics? Was this discussed in any way?
I have very little recollection. I think nuclear physics was still a very small and modest corner.
What about after 1932? It was at this time that major events happened. Do you recall hearing of the neutron, for example, in 1932?
Oh, yes. We of course heard of the neutron, of the Cockcroft- Walton accelerator, of the cyclotron; and we were particularly interested in the discovery of deuterium. I remember that I built a small separation plant to make a small amount of deuterium because Minkowski in our lab wanted to do a spectroscopic experiment. But I don't recall whether it came out. I have an idea it came off and was published, but I don't remember very well.
How did you hear of it? Do you remember how you heard of these various developments?
No. I don't know whether at that time I tended to read the foreign journals. I certainly knew very little English. I had learned English as a 12-year-old boy for a year ör two and had never acquired any kind of fluency and, as far as I remember, had never done any reading in English, until Hitler's time when I began to realize I would have to learn English.
I'd like to get into that in detail soon. But do you recall having learned of these developments somehow very shortly after they occurred? In other words, was it in 1932 that you knew of the neutron and knew of the Cockcroft-Walton work and deuterium?
Well, I'm sure it was because I think I would certainly remember the mortification if I had found it out later and realized that I had missed it at the time. That's my only argument.
Was this because of much discussion at Hamburg?
Well, I vaguely remember there was discussion of whether a neutron should be considered as a close union of an electron and a proton I think we did discuss the neutrino, because I remember in Hamburg I was still mulling over the problem of beta-ray recoil. I remember thinking of a fantastically complicated experiment, doing a molecular beam experiment with beta radioactive material and then measuring the recoil out of the beam.
Did you do this with Stern or was this something on your own?
No, that was something I merely tried to work out in my spare time. I had an odd way of thinking physics in those days. All during the day I was pretty busy looking after the apparatus, watching our mechanics and advising them, or designing new pieces, or doing calibration measurements, and so on. But I regularly came home, had dinner at seven, had a quarter of an hour's nap after dinner, and then I sat down happily with a sheet of paper and a reading lamp and worked until about one o'clock at night—until I began to have hallucinations.
Every night I worked until I began to see queer animals against the background of my room, and then I thought, "Oh, well, better go to bed."
Just an idle question: what animals? I'm not trying to do a Freudian analysis, but I'm just curious about what sorts of images.
They were not very concrete. They were just sort of vague feelings of long-necked giraffes or something like that, unpleasant feelings, and I didn't let them develop. I took it as a signal that I'd better go to bed.
Were you able to sleep then after this intense activity?
Oh, yes, perfectly. It was the ideal life. I'd never had such a pleasant life—ever: this concentrated five hours work every night.
And this continued for a three-year period?
Something like three years, yes.
When was it during that period that it became clear to you that Hitler was going to change things and that, in fact, life would be different?
Well, very late really. I didn't take Hitler at all seriously at first. I had the feeling: "Well, chancellors come and chancellors go, and he will be no worse than the rest of them." Things began to change, and of course when the racial laws were published, by which people of partly or wholly Jewish origin had to be dismissed from the universities, I realized that my days were numbered and I went to Stern to tell him so. Stern apparently had not realized that I was of Jewish origin, the reason being partly that Lise Meitner had always kept quiet about her Jewish connection. She had never felt that she was in any way related to Jewish tradition. It was an intellectual but very liberal- minded tradition in her parents' house. Although she was, racially speaking, a complete Jew, she had been baptized in her infancy and had never considered herself as anything but a Protestant who happened to have Jewish ancestors. And when all this trouble began, she felt, perhaps partly to let sleeping dogs lie and partly not to embarrass her friends, that she would keep quiet about it. It was rather an embarrassment when Hitler forced it all out into the open, so to say, and she had to go and tell Hahn: "You know, I am really Jewish and I am apt to be an embarrassment to you," and so on. In the same way when I told Stern, he took his head in his hands and said, "Oh, my God, but then my institute is even more Jewish than I had thought," he thought that he had carefully appointed two Jews and two non-Jews as his assistants so as to keep a nice balance, and not to have let his own sympathies run away, he himself, of course, being Jewish in origin. So he suddenly realized that he had only one Aryan in his crew.
Had you, in your earlier life as a child and in your family life, a consciousness of being Jewish in terms of either a cultural or religious orientation?
Well, in Vienna in the high school there was a very distinct separation between the Jews and the Protestants, practically all of whom were baptized Jews on one side, and the Catholics on the other side. They formed two separate gangs which tended to fight and which despised each other.
You mean the Jews and the Protestants on one side and the Catholics on the other side?
And the Protestants were baptized ...?
They were practically all baptized Jews. I mean they came from Jewish families but had been baptized.
So there was this consciousness because of social setting ...?
There was a strong consciousness. I mean among us, of course, it was that we, the Jews, were the élite; and the others—they were the street sweepers.
Were there cultural heroes that you could point to, to support this argument. Or was it generally felt?
I didn't think about it. It was just that I knew that all my Jewish friends were interesting individuals who usually played the piano or some other instrument well and could argue about philosophy and whatnot. It was very largely these sorts of intellectual abilities. But also we had brilliant skiers and sportsmen in our midst. I have an idea that one of Vienna's most famous football players was a Jew, and we were all proud of him.
How about at the University? Was there any feeling of Jewish consciousness or, on the other hand, reaction to anti-Semitism either in Vienna or in your work in Berlin?
In Vienna at the University, I have a feeling that I quite instinctively stuck to the Jews in my class. I made friends with several of them. It was almost, I feel, as strong as between white and colored. I mean even if I didn't have any definite objection, even if I realized that some of the non-Jews were good mathematicians or something, they were a different race. It wouldn't occur to me to talk to one of them.
And in Berlin?
In Berlin it was far less pronounced, I feel. For one thing, there a good many of the people I actually had to do with were non-Jews— I mean my boss was one. There were several young men who joined the laboratory for a year or so. I think not one of them was a Jew. I don't know whether my boss knew that I was one. The question wasn't raised at the time. That, of course, was six years before Hitler.
Then getting back to Hamburg, when it became clear to you what the situation would be and when you made this announcement to Stern, what then was implied for the future? Was it clear to you that you'd have to leave?
Well, yes. In the first place, my hope had been that I might be able to go to Rome, because some time previously I had applied to the Rockefeller Foundation, I think, which then had a scheme of offering grants to young scientists, which would enable them to get away for a year and then return to their previous employment. The grant would pay for their travels and maintenance during that year and would give people a chance internationally, which was the main purpose. Stern had applied for such a thing on my behalf, offering me the chance to go away for a year and work with Fermi in Rome, and I was very much looking forward to it. I was very disappointed and at first rather disgusted when Rockefeller told me that, the situation having changed because of the Hitler laws, they had to withdraw that offer of a grant because I no longer had a job to come back into.
Was that the first indication you had that you did not have a job to come back to?
No, no. I knew I did not have a job to come back to. I don't know whether I had the idea—probably not—of confirming that my offer of going to Rome was still good or whether spontaneously I got informed that they regretted having to tell me that in view of the changed position and so on. But of course from their point of view it was completely justified, because this was not the purpose of that sort of grant. Now, what then happened is that Stern said he would go traveling and see if he could sell his Jewish collaborators—I mean find places for them. And he said he would try to sell me to Madame Curie. So I said, "Well, do what you can. I'll be very grateful for anything you can do. Just sell me to whoever wants to have me." And when he came back he said that Madame Curie had not bought me, but Blackett had.
On what basis—do you know?
At that time the Academic Assistance Council in London, which later changed its name as you know, had been put into existence. I don't know much about that history, but I'm sure it's well documented.
I'm working on that now as a special project. Rutherford was involved.
That's right. That's all I know about it. Anyhow Miss Esther Simpson was the secretary; everybody knows that. So I got an invitation from Blackett to come and work with a grant from the Academic Assistance Council. The grant, if you work it out now, amounted to approximately $700 a year, but that was enough in those days—£250 a year.
Wasn't Stern concerned with his own future? He was worried about his Jewish collaborators. What did he think his future was?
Well, in the first place, Stern had a private fortune. I don't think he was really dependent. He also, I believe, had managed to salt away some of his money outside Germany. He came from a wealthy family. And, in addition, he felt sure—and was quite justified in thinking—he would very quickly be invited to a job somewhere else.
So it wasn't a question of having any illusions about the possibility that he would be allowed to stay?
Oh, no. He wouldn't have dreamt of staying. I mean the instant this whole anti-Semitism began, his one thought was to get out of that filthy place. I mean that was his attitude even after the war. He has steadfastly refused ever to return to Germany. He's never been back, as far as I know.
What about the reaction at the University to the racial laws? Did you feel any change on the part of the students?
I had nothing to do with students. I certainly felt no change within our own group. Our two technicians were both Aryans, and they were both just as nice as ever before. They didn't show any change in attitude. No, I really didn't suffer any kind of personal hardship or personal indignity. All my friends stayed as nice as they were. I remember one of them saying to me wistfully: "You are lucky. At least you needn't be ashamed of what's happening." I remember one occasion when I walked home through a dark street— I was living at the end of a narrow dark street—and one of my colleagues had warned me that brown shirts, S.A. boys, had occasionally beaten up Jews just for the fun of it when they could catch them in a dark corner. And there I was walking and I heard sharp and fast footsteps behind me, and I looked around and there was an S.A. man coming after me. So I decided the only thing to do was not to show any fear, because if I started running or dodging or anything, he would know that he had a likely quarry. So I just kept on walking, and he overtook me or rather pulled up at my side, politely pulled off his hat and greeted me. It turned out it was the son of my landlady.
Then there was this definite recognition on your part that your future was elsewhere. What made you and Stern think of Fermi even before this? You thought of Fermi not so much as a place to go to get away from Germany, I gather, but mostly as a place to do physics. Is that right?
I think as a place where I might learn some theory. I think Stern probably had the right idea. I had very few ideas in those days. I've always been on the whole an impractical chap who tends to stay within his four walls and think his own private thoughts. But I think Stern had this idea, and we had had a good deal of contact with Fermi actually. We had done some measurements for which Fermi had helped us with the theory.
Let me ask again the question I asked of an earlier period: was the term "nuclear physics" Was this recognized as a field of research that one could choose as his specialty?
I think by that time it was recognized. I did realize that when I went to Blackett I would switch to Kernphysik, and I think that I would have used that name. It would no longer be called radioactivity or atom splitting or something like that.
At Hamburg you did work with Stern on the magnetic moment of the proton and you published some papers on that. What was the nature of this collaboration with him?
Well, Stern was the undisputed ruler. He thought out the problems. In some cases he gave a problem to a student and told him to get on with it, looking in and giving advice when necessary. Or he would do an experiment himself with one of us as his personal assistant. When I came, Esterman was his personal assistant. Before that, I think for a while Knauer had been his assistant. And when I came, the idea was quite clearly that I should be trained, and then Esterman went off on sabbatical leave and I worked with Stern for a while. So some of our papers were published in the three names, and some by just Stern and my- self, and there was one paper which I published when Stern took a short leave and I managed to get some results in a different direction out on my own. That was nothing to do with the proton, by the way. When I first came, the work was concentrating on the wave properties of atomic beams—diffraction of beams of helium—and then Stern got a more ambitious project in mind and sketched out an apparatus for building it. But I thought I could make this apparatus more solid and more easy to adjust and so on. Stern greatly approved of that, and from then on he more or less left me to design the apparatus. Stern always kept a very strict check on what was going on and understood every detail. I would then supervise the construction of the apparatus, and then we might both to some extent work on getting it assembled and getting it tight and getting out the cigar ash which Stern kept dropping into it because he was never without his cigar. And Stern was invariably present at the measurements. There was one occasiön when I found the apparatus seemed to be ready to go, and so I worked all the night through and took some measurements, and Stern was quite pleased but he was also a little bit displeased and all the measurements had to be done a second time with him present.
This period was in a sense a period of gaining experience, in a certain sense, perhaps an apprenticeship. Would you say that's a fair characterization?
It was certainly my first introduction into the problems of modern physics and into the idea of not merely inventing gadgetry but actually planning experiments. In a way Stern and I fitted each other well because I could invent some of the gadgets and make them work where Stern might have had the idea or had the problem. But on the whole it was very much an apprenticeship. I mean Stern had nearly all the ideas, and all that I contributed was the materialization in terms of particular shapes of brass. I mean I could think out the best way of machining a piece of brass so that it would do the job.
Then, you said earlier, in addition to this, you spent your evenings working on your own ideas. What was the general trend of these ideas? Was it related to the daytime work, except in a more abstract way?
No, not as a rule. It was just almost like doodling. I would think up some queer magnetic field and compute it. Or I would see how fast I could make a shutter work and was it possible to use a beta particle to signal the decay of an atom and then open a mechanical shutter so as to catch the atom. It seemed pretty impossible, so I worked out a theory of shutters and saw how fast I could make a shutter move, and I came to the conclusion it was very marginal, if not impossible, and I gave it up again.
I see. How about reading in this period?
Practically no reading. I'm afraid I've never done much reading in my life.
About the transition, once the arrangements with Blackett had been made with the help of the Academic Assistance Council, was there any real difficulty about getting out of Germany and going over to London?
No, there was no real difficulty, and I might mention one other thing, and that was when I was still in Hamburg, Niels Bohr came visiting because he was concerned with the fate of all the displaced persons likely to come out of Germany. So he had decided to go around and talk to his various colleagues to get an idea of the problem; if possible, to meet the people who were in danger and so on. At this time I just had done an experiment in which I measured the recoil of a sodium atom when it emits a photon of yellow light. This was an experiment which in fact I had thought of before I knew of atomic beams, so I felt it was really my experiment. I thought if one could have a beam of atoms, one could do it that way. Later I found that Stern had listed it among the things to be done with the molecular beam method. Stern had written a wonderful program of what could be done which laid out the whole course of atomic beam physics for years ahead and was really to a large extent followed successfully. This was back in 1926. So when I went there, I told Stern that I would love to do that experiment, partly because I had thought of it independently. So towards the end of my stay with him, he let me go and do this experiment on my own, rather than be his assistant. And I just managed to squeeze out a few results before I had to leave. It wasn't very well done. If Hitler hadn't come, I would have worked on it for another year and done a much better job.
This was the paper that was published in Nature in 1934?
No, no. This was the recoil of a sodium atom. It was published in the Zeitschrift fur Physik as one of the regular series of communications from the Hamburg Institute.
Now, you say that Bohr came.
Yes, Bohr came and I told him about that experiment, and I remember that Bohr got hold of my waistcoat button and said: "You must come and work with us in Copenhagen. We like people who can actually perform ideal experiments," I mean thought experiments—Gedanken-Experimente. I remember I was terribly bucked up, and I remember writing to my mother that she need not worry; Hitler won't be able to do anything to me because the dear Lord Himself had taken me by the waistcoat button and invited me to work with Him in Copenhagen.
Your mother was in Vienna?
My mother was still in Vienna.
Did she stay throughout?
They stayed until '38 when my father was taken to concentration camp, and then early in 1939 we managed to get both of them out safely to Sweden.
So you had this invitation from Bohr and you were on your way to Blackett. Was this a normal trip? There was no special problem of picking up your things?
Well, I gave away a good many of my things, and I packed the rest. A friend of mine found me a place, a cabin on a very small freighter, where I was, I believe, the only passenger. We had a pretty rough crossing. I remember staying in my berth all day and watching the suitcases slide forward and backward on the floor. And then we spent a whole day sailing up the Thames very quietly and peacefully and having lunch with the captain. The landing was a little tricky because an immigration officer came on board and inquired for the passengers. I said, "I am the passenger," and so he wanted to see my papers and asked me whether I had a permit for work. I said, "No. I don't work. I have a grant." So he said: "A grant is a high-class name for a job." And he couldn't let me land. So I said: "What shall do?" So he said: "Well, you could give a half-crown to the steward and send him ashore to telephone your prospective boss and see if he can straighten it out." And that's how it was done. The steward went ashore, telephoned Blackett, Blackett telephoned the Home Office, and two hours later the immigration officer was back and let me in. Apparently they hadn't expected me to arrive in this unorthodox manner, on a small freighter.
Then you went to Blackett's lab. How would you characterize the nature of the work there? This was 1933, and you spent a year there. Is that right?
Yes, that was October 1933. It was a very strange experience for me. For one thing, I'd been brought up to think that the British were a higher breed of men. I relied largely on Goethe as my authority in that respect. And when I got to London I was so puzzled by so many things that seemed very unreasonable to me—that there were 15 different bus companies, all with different stops, and some of them with no stop signs, and the sort of general messiness and untidiness of everything. On the other hand, there were street repairmen who would work with their torsos bare in the heat and chopping the stones and so on, but they would invariably put on a jacket before they ate their sandwiches. They dressed for dinner. It all seemed so irrational and fantastic. That was one thing. The other thing was that the lab had very little provision for instrumentation and none of the sort of rather pedantic tidiness that was the usual thing in Germany. I found after a while that my imagination to think up experiments was cramped when I felt that any experiment that required a rubber tube three feet long was impossible, because the only rubber rube we had in the lab was one foot and two inches. I knew that.
There was no way of solving that problem?
Oh, one could solve it, but it all seemed so complicated. I had to go to the lab steward who considered it an extravagance to buy an extra piece of rubber tube. If there were things one could buy at Woolworth's, I just bought them myself out of my own pocket. Usually that was the quickest and the cheapest thing to do.
These were simple things, such as pieces of glass or ...
Curtain rods, I used a lot and things like that.
Or pencil caps.
Pencil caps for counters, quite right. I made my geiger counters out of pencil caps.
What was the main trend of your research at the lab during this period?
Well, Blackett gave me a problem. He said that there is a theory that positrons, which at that time had just become known, should annihilate with electrons and produce gamma radiation. And he had thought out a way of doing it, using a magnet in which the electrons would describe trochoids going one way and the positrons would go in the opposite direction. And so this was a convenient way of separating them with rather close to 100% efficiency. And of course with the weak sources, that seemed important. He even had a magnet for it, and I set it all up. I still remember my exasperation when I tried to weigh a lead block in order to compute the absorption of gamma rays in it, and I found that after I'd weighed it that it had a weight of one pound and 57 grams and three ounces and several grains and so on. Just to compute the weight with all these odd assorted weights was difficult, because there were no uniform weights, not enough of any one kind.
Did you work on your own there pretty much?
At that time I worked quite on my own, yes, although there were quite a lot of people in Blackett's work, and we called ourselves the League of Nations because there were so many different nations represented. There was a German and a Swiss and an Italian and one chap who was a Greek from France and an Indian and so on.
Were they there as visitors or permanent appointees?
I never found out. Of course the Italian was Occhialini, and he gave all his apparatus girls' names and I had to paint the name on in decorative writing with red paint. I had the best handwriting there apparently.
What sort of names on what sorts of apparatus?
Oh, I mean whatever girl friend he happened to have would have to be painted on his amplifier box or so on. On the other hand, there was Ehrenberg, who is still in London. He has become a professor and is close to retiring, who used to buy transfer pictures at Woolworth's and stick them onto his apparatus, which thereby became animals: the turtle, the elephant and so on. It's convenient if you have some way of referring to a piece of apparatus. There are probably more orthodox ways of doing it.
This never got into the scientific papers, though.
"Fig. l illustrates Mary.
At London were there discussions of new developments in physics? By this time there must have been considerable work going on in nuclear physics—if not there, elsewhere. Was this very much in mind during that year?
You know, I have no recollection at all of seminars or anything like that. Of course, Birkbeck College was primarily an institution for evening courses. It had been founded with the idea that artisans could improve their station in life by spending their evenings studying and acquiring some modest degree which would give them a better job. Frisch - 21 I have no recollection at all of formal colloquia, but rather that we would just discuss things over lunch or oyer tea, sometimes Blackett with us, sometimes just a group of the various foreigners. We used to go to an Italian restaurant some distance away and argue furiously all the way there and back.
About whatever was of interest at the time?
Yes, I remember the neutrino was much in the center of interest, and people argued whether the neutrino was real or whether it was merely imaginative. That was the time when Fermi published his first papers on the beta theory, and the more theoretically-minded felt: "Now, this is very good evidence." They meant if a particle makes a theory that works so well in degree, gives the right curves and so on, then it probably exists. And others were more skeptical, and we were still trying to think out a recoil experiment.
Did you have much to do with people from other institutions, such as the Cavendish?
No. I remember visiting Cambridge twice—once extremely briefly before I ever came to England. I came on a two-day trip, and that was very extraordinary because it was in the spring or summer of 1933 when Hitler was already in power and the racial laws had already been promulgated. Blackett had invited me to come over, if I could, on a tourist ship just so we could talk things over. And this I did. At that time it was still not necessary to have a visa. We just went as a tourist group, and the captain, when we arrived in Britain, said, "Now, listen, gentlemen. We must go back to Germany. I shall be held personally responsible if anybody is missing, so please do me a favor and all spend the night aboard. Tomorrow you have another day for looking around and then we sail." Well, I broke the first promise and didn't spend the night aboard. I spent the night in Cambridge, but I came back the next day and sailed. And six months later when I came I needed a visa and all that. By that time the British government had tumbled to it, that there was a danger of illegal immigrants.
Was this your first trip abroad?
It was my first trip to England anyway.
You mentioned in some published work about devising your instrumentation and building your own counters at Blackett's lab. Now, from what you just told me, this was the general problem at that laboratory. How widespread was this lack of detectors and the need to build your own or to actually take commercially available components and put them together?
I think that was very general. I dare say that in some laboratories they would have gotten a mechanic to turn a thin-walled brass tube from stock brass; whereas our mechanic was constantly overloaded. We only had one mechanic for the whole large group. I tended to search around for substitutes, such as pencil caps; it also usually saved time. Frisch - 22 I was rather fascinated by the facilities of Woolworth's. There was an enormous amount of primitive hardware which could be adapted to physics purposes. For instance, when I looked for this short-lived activity which does result from bombarding sodium which has only got seven seconds life, I suspected there might be some short-lived ones. So as soon as the Joliots announced their discovery of radioactivity, my idea was to look for something short-lived which other people might miss because it's not easy to find. So I built myself a kind of little railway made of a curtain rail a couple of yards long as an alpha-ray source in a lead block on one end and a geiger counter in a lead block at the other end and some simple mechanism with a spring which would shoot the glider along from one end to the other in a split second.
Does this have some resemblance to the Fermi trolley type of a gadget?
Well, it was probably a primitive precursor; I don't know. It s the same idea. It's a very obvious idea. I'm not particularly proud of it.
It's interesting to think of a primitive precursor to a primitive device.
Yes, that's right.
When you were there, had you the expectation that you would stay there or was it clear that this was just a temporary stopping point?
When I came to England, I knew I had it for one year, and I think that quite soon afterwards it was settled that I should go to Copenhagen. But I don't recall when and how that was settled.
Now, were you in England at the time of the announcement of the discovery of artificial radioactivity?
Yes. That was the great news that broke in January. As I say, I got going immediately, and within a couple of weeks I had results, and then Blackett made me write them up within the hour and take the manuscript personally over to the office of Nature. He had telephoned the editor of Nature, and my paper was published, including a diagram, I think in nine days from my handing it in, which was pretty good going, thanks to Blackett's pressure.
What was his motivation for this pressure for rapid publication?
I don't know. I think there was still just a strong feeling for priority, and he wanted a publication out of his lab. Perhaps he felt it would be good for my career to have been one of the first people to follow up the Joliot-Curie discovery.
That's what it was?
In other words, it was right on the heels of this announcement.
And do you remember how you learned of it? Was it through private communication? Was Blackett in touch with people through correspondence? Or was it a question of relying, again, on the journals?
I'm sure somebody in the lab read the Comptes Rendus. For one thing, we were very well placed because we had such an international crowd. So I'm sure that the Greek from Paris would have read them regularly. He probably came bounding in and said, "Listen, there's something really."
Do you remember his name?
While we're on that, what about the others there? Do you remember who they were? You mentioned a Swiss.
The Swiss was Herzog, who has gone into industry with an American company; Ehrenberg, who became a professor at Birkbeck College; Occhialini I mentioned; Naidu was the Indian—I don't know what became of him; Charles Webb, who was a visitor from Singapore and went back. There was one gifted Englishman who has since died, I can't think of his name.
Was the paper that Blackett encouraged you to deliver to Nature the one I mentioned before on induced radioactivity of sodium and phosphorus which was published in Nature in 1934?
So it must have been early 1934. Were you in London at the time of the international conference on physics? There was one in 1934.
Do you remember much about that conference?
I don't remember except it was a great crowd and that for one day the whole conference went to Cambridge and had dinner or lunch in Trinity, and that J. J. Thomson made a speech which to me was just a mumble and I couldn't understand a word. Other people have since con- firmed that he was in his second childhood and it was hard to understand.
I have the proceedings here. I don't see his talk as being listed—maybe because it was a mumble.
I think it was just a brief speech of welcome.
There was one from Hopkins, an address of welcome, and then Rutherford had an opening survey. Do you remember the tone of the discussion there? There were two parts to the conference. One was nuclear physics and one was solid-state physics. I don't know if they used exactly that term, but they used the word solid state. The Joliot- Curies were there, and this was probably within months after their announcement. Do you remember what the discussion was?
No, no recollection. I'm very sad. I have just a few flashes in my mind: one of a very crowded cocktail party or some sort of a reception party where I told a naughty joke to Bhabha a joke involving Irene Curie, and Bhabha gave me a poke in the ribs and pointed out that Irène Curie was standing behind me. But fortunately she was talking French, 15 to the dozen, to someone else and probably had not heard the joke that I told in German or possibly in English.
When was it that you left London for the Bohr Institute?
That was exactly one year later—I think to the day—in October '34.
Before I get to that, what would you select as most important in your own work that came out of that year in London?
First of all, I should say the experiment that Blackett gave me did no come off, and before I was able to produce any results, we were beaten to it. You know, on that occasion, Joliot in Paris and Thibaud somewhere in the South (Toulouse perhaps) had published the same result. They got into a fight, and I was glad to be out of it. Then I think it must have been just about that time that the other discovery broke, and I jumped into this radioactive physics, this artificial radioactivity. I also experimented with a new type of cloud chamber which I had hopes for, still being the gadgeteer, you know—a chamber which expands rather slowly so that it would remain sensitive for something like a whole second. That was very nice as a demonstration instrument, but it was not easy to photograph the tracks because they of course came and immediately began to fall and dissolve. I tried to put a small microphone into the cloud chamber to pick up the pressure pulses when the track formed and to cause it to trigger the camera. I got just one photograph, but it was so unreliable and difficult that I didn't pursue it. Of course the idea of triggering was rampant because the main kind of work that was going on in Blackett's lab was the use of a big triggered cloud chamber to record cosmic-ray events. Both Occhialini and Herzog worked with Blackett on that.
But you weren't involved in the cosmic-ray aspect?
No, I was left to work on my own on the two experiments I mentioned. As far as I remember, I did nothing else. I did experiment with that cloud chamber, which again, was such a primitive instrument that I could make it out of existing equipment, beakers and just simple bits of glass tube and so on.
And these were available in the laboratory, and if not, you could supplement it with something at Woolworth's. But could you get something as sophisticated as a tube, a radio tube?
Radio tubes were available. I don't think I bought those with my own money. I believe that the lab either had a small stock or ordered them when necessary. I really don't recall. Of course there was an absolute minimum. For each geiger counter we needed one tube, and I personally never used more than one geiger counter at a time, so I only needed one tube for all my experiments.
I ask this because someone has stated to us that the situation in the United States was a unique one because of the advantages of being able to walk to the corner drug store and buy a tube if you needed it and buy parts off the shelf—condensers and so forth—and I wondered if this was the case in London.
I don't recall where I got my tubes, but I remember buying curtain rails and pencil caps and a few other things in the shops—and I even once charged the lab for a pair of black female undergarments which I needed for lining my cloud chamber.
Did you give a girl's name to that cloud chamber?
But I don't remember buying tubes or condensers or anything like that, so I suspect that there was a stock. It's very likely there was because Blackett needed them to a great extent for his triggering of cloud chambers.
Do you have any idea who supported the research there? Was this from some university, from Birkbeck College?
I have no idea.
These are the questions that are most difficult to answer, it seems, but there are other ways of getting it. Then it was clear that you would be going to the Bohr Institute. How did you follow up on the original invitation, the one that he had given you at Hamburg?
There again, I fear I just don't remember. I've had a sad inclination always to wait until somebody wrote ör phoned. I may have correspondence stashed away somewhere, but I think I once lost a large tin trunk full of letters, and I don't know if I have got any of that. But I think there was some correspondence and that in some way or other Bohr informed me that the Rask-Østed Fund would give me a grant. I arrived and first I stayed in a boarding house and later I was offered a small apartment on the top of the Institute.
About how long after you arrived there did you switch to that?
I must have stayed several months in that boarding house.
I'm just checking here in the Quantum Physics Catalogue. There are letters that Bohr wrote to you, but the only ones that show up are from '39 through '58. That doesn't mean that that's complete. When you arrived there, was it with the idea that this would be a long-term, or at least for the duration of Hitler, type of stay in Copenhagen?
There again, I never really paid much attention. I sort of took it for granted that here there was another nest waiting for me, and I would try to do some work, and I never thought about the future. I somehow took it vaguely for granted that if I did well, I would probably be kept, and if I didn't, something else would turn up. I've been very careless always, with just the one exception, and that was in '39 when I realized that the situation was very threatening and I took steps to leave and go to England.
In this period from '34 to '39 in Denmark, there were many visitors there, and the Institute was a center, I imagine, for discussions. How would you characterize the atmosphere there as compared to the other circles that you had been in prior to that time?
Well, I had never come across quite that sort of brand of international, and Bohemian behavior, and teasing—so very opposite to the bourgeois attitude I was used to in Germany, where everything was tidy and orderly and polite and so on. I mean there are some little scenes that stick in my mind. For instance, on one of the seminars, Landau, presumably in order to be able to concentrate better, lay flat on the lecture desk on his back and was talking in that position, sort of arguing up into the sky, and Bohr was bending over him arguing down at him. Obviously, Bohr wasn't in the least put out by this sort of behavior. He probably hadn't even noticed it. He was having an argument with Landau on some important matter and the position of Landau wasn't important. I remember on one occasion when we were having a dinner in the boarding house; it must have been shortly after my arrival. I think it was not the boarding house where I came to stay because in that one there was no other physicist but me, but there was one which was always full of physicists. Landau raised the important point: "Was or was not Placzek a black dachshund?" This was argued with a certain amöunt of learnedness and heat for a while, and then suddenly they all turned on me and said, "Frisch is a newcomer. He's unbiased. Let him decide." And I was terrified at this. I couldn't enter into the spirit. I wasn't used to that sort of thing, and I tried to prevaricate and make some feeble jocular remark that I couldn't see any good argument for coming to a decision or something; whereupon they all fell on me and started to tear me to pieces as a person with no character, who had no convictions, and so on and so forth. In retrospect, all very funny, but terribly embarrassing at the time.
Was everyone pretty much the same age? You were about 30 at the time, weren't you?
Yes, I was just 30, and a good many of them were slightly younger; some of them were about my age. There were Felix Bloch, and George Gamow, and Placzek, as I have mentioned. Placzek went off to Israel shortly afterwards. Weizsäcker was there. I tend to mix up my first arrival in '34 with my first visit, which was nearly two years earlier, because as soon as Hitler came to power, Bohr arranged a meeting in Copenhagen—I think even before Hitler came to power. I don't know. He invited lots of people to consult and discuss, and on that occasion I was invited to one of those meetings; that can probably be verified somehow. I frankly don't remember if it was in '32 or '33. It must have been '33, but I somewhat mix up the people I met then with the ones I met when I came for good.
During the period from '34 on, what was your main work in Denmark?
When I look back on it, I have a feeling I did awfully little that was of any use. I seemed to have worked in a very leisurely way. I set up that cloud chamber again and improved it, and one student got a master's degree out of it, I believe. He later became a professor and unfortunately has since died; he had a weak heart. Then I continued the experiment on artificial radioactivity, and I found two more radioactive elements. It all took long because I had to set up new equipment, build new geiger counters. I also did, I think, a fair amount of work just helping other people. Hevesy was then starting his radioactive tracer work, and I helped to build a line of geiger counters and amplifiers for that. Then of course came Fermi's discoveries of slow neutrons and all that, and I got very much roped into it, partly because I was one of the few at that time by chance who could read Italian. And so whenever the Ricerca Scientifica arrived, they all crowded around me and I had to read it out and translate it. We repeated some of those experiments and extended them. This business of resonances was discovered more or less independently by Fermi, by us and by Szilard. I never quite figured out who did what first, but we found some results independently, which we then understood when Fermi published his and so on. Then Bohr produced his theory of the compound nucleus. That I remember very well, and that influenced our work rather.
What motivated him to reexamine the existing models of the nucleus? Was this the work on resonances that had an effect about that time?
Well, the resonances were still very badly understood because Fermi at the time still regarded them as different kinds of neutrons. He was very vague about it. In fact, I think he had given some reason why he did not think that they were just different energy groups. He simply called them A-neutrons, 8-neutrons, C-neutrons, and left their nature open. And it was not clear that they were resonances in the ordinary sense, let alone what the energies were or energy-widths were. That was pinned down, again apparently by an experiment by Szilard and by one which Placzek and I performed and where we had a bit of argument later, because he hadn't understood what Szilard said, and Szilard said: "If you had read my paper carefully, you would never have published yours."
Do you recall where he was at that time?
He was in America but I don't know where.
Quite likely, yes. This is all in Nature.
How did the resonances affect Bohr in his thinking and in his development of the compound nucleus? Do you remember anything of those circumstances?
Well, as I recall it, there simply was some rather hazy evidence, and of course Fermi's fairly strong evidence, that neutrons interacted strongly with nuclei. In the existing models, one had always assumed weak interaction simply because that was the only one that people knew how to calculate anything about. There was one colloquium which I still remember, although perhaps I remember largely the many accounts I have given of that colloquium, when either Bethe was talking or somebody was reporting a paper by Bethe in which Bethe in a somewhat artificial way had tried to account for the strong interaction without actually introducing strong interaction. And Bohr kept interrupting the speaker in a sort of absent-minded way. I remember feeling a bit impatient and wondering why Bohr didn't let the speaker at least finish his sentence before arguing back. And then suddenly Bohr broke off in the middle of one of his own sentences, and his face suddenly went completely dead and he sat down. My immediate impression was that he had suffered a stroke or he was suddenly feeling very sick. What's the matter? And Bohr sat there for a very few seconds with this completely dead face and then a smile came back on it and he got up and said: "I'm sorry. Now I understand it all." then he began to propound the theory of the compound nucleus, which had apparently been sort of working up to the surface all the time he had kept interrupting and then crystallized in those few seconds when he sat down.
Wheeler was there at the time, wasn't he?
I think very likely he was there at the time. Wheeler stayed for quite a long time. I don't remember when he went, but that can be verified.
I think it was '34, '35.
The paper was published, I believe, early in '36. Again, this is published; one can verify it.
When he developed the idea then, what was the subsequent reaction to it in discussion? Was there intensive work on his part to refine it?
I think the immediate thing was that Bohr sort of tried to give it as concrete a form as possible, and he thought up that idea of a bowl with billiard balls in it. You fire one more ball into that bowl. I remember making a very naturalistic drawing of such a bowl with billiard balls, which Bohr used as a slide when he lectured about this in England. Apparently he gave me credit for having drawn that slide, with the result that when finally I came to England again later, everybody was quite surprised that I was a physicist. They thought I was an artist. Viki Weisskopf, I believe, was there or came just about then and concentrated on the thermodynamic similarities of Bohr's new model, and everybody was busily trying to estimate level densities and sort of generally get a feeling for the new model.
Was there a feeling then of testing this model, of devising experimental tests for it?
Well, the odd thing is I don't remember ever having thought up anything sensible, but Placzek wanted to do an experiment at that time, and we started to work together. That was quite funny because Placzek had very nocturnal habits and usually came to the lab at three in the afternoon yawning and looking very tired and confused and woke up by the time it was time to go to dinner and then really got to work after dinner. Then we would work together and take readings with neutron sources and geiger counters and so on until about three in the morning, when I declared that I couldn't hold up any longer and went to bed. There are lots of little connected anecdotes which are not very important to the history of physics. Placzek obviously had this idea that he could use the absorption of neutrons in boron as a sort of rough calibration for their speed, because he—and presumably others—had grasped the 1/V law, the proportionality of cross section with the reciprocal speed. And so he used a calibration with boron to get some idea and then measured the energy of those neutrons which are stopped by gold and showed that they had got only a few times thermal energies. That was the first test to my knowledge which really measured the energy of a resonance neutron. But, as I saw, Szilard did somewhat similar measurements and in a perhaps less clear manner got the same result, it seems.
This was in the period of about 1936 after Bohr had published the compound nucleus.
Yes, it definitely grew out of Bohr's work. And this was published in Nature under Placzek's and my names.
About this time, then, Bethe had gone to the United States, and he had begun to publish his Reviews of Modern Physics articles on nuclear physics. Were you aware of this there? These articles came out in late '36 and early '37, the one by himself, the others with Bacher and Livingston.
That's right. Oh, we were very much aware of them. I mean, they immediately got nicknamed "The Red Bible."
Why, "The Red Bible"?
Because the color of The Reviews of Modern Physics was red.
And what was the reaction? What was the use that you made of them? After all, you were at the center of a good many developments, and it would be good to know whether this three-part article had any particular use.
I don't recall that I made any particular use of it, no. The only thing that I did was once again to follow up what Placzek and I had started. I had grasped that there were resonances and that here was a method for studying them. So I then set out on a piece of work on my own, extending the measurements in gold to getting higher precision and more information on the width of the level and even to a small extent on the shape of the line and extending it to a couple of other elements, which, as we now know, are not at all good examples, because there are many resonances contributing. In those days we tried to force everything into the straight jacket of a single Breit-Wigner formula, although Placzek did try to make estimates of what happened when there are more than one level contributing.
What other models of the nucleus were in the air during this period? You mentioned in Washington in our private conversation some other models—rigid structure models.
Well, these rigid structure models were regarded with considerable suspicion. Nobody quite believed it. After all, Bohr had a long time ago proposed that the nucleus was rather like a liquid drop and so it had better be ... And so we more or less automatically believed that this must be near the truth and that a rigid, structure sounded very improbable. I personally had no very strong opinions. I remember one thing which is something I'm moderately proud of, and that is I had some very faint inkling of the shell model at the time— that is, not of any model. But I was merely one of those who protested that the compound nucleus with its completely disorganized motion, with its completely thermodynamic behavior, couldn't be the whole truth. And I was very excited when Schiller published his lists of nuclear moments and showed that they seemed to be best explained in a one-particle model. In other words, they looked very much as if the magnetic and mechanical moment of any odd nucleus is very nearly determined by one single proton or neutron, depending on whether it's an odd proton or an odd neutron. And I remember presenting this in a colloquium there, and Bohr was very doubtful, but he was also thoughtful. He was apparently a little bit shaky. He sort of felt that there must be something here which eventually we will have to consider, but at the moment we can't pay attention to it. He felt it would be sidetracking him. He didn't quite believe it. He still felt some explanation must be found in terms of a more thermodynamic model.
When was this—do you recall?
That was shortly before I left, so I think it must have been in '39.
After he had returned from Princeton?
I think so. I think it was quite shortly before I left, but it may have been earlier. It was, I think, quite soon after Schüler published those things, which, again, can be verified in the literature, because I read them in the Zeitschrift für Physik or something and got excited about them and offered to speak in the colloquium.
That's interesting. There was another thing you mentioned. You mentioned the name of a man associated with one of the rigid structure models, what was he called?
Oh, yes, a man called Wefelmeyer. He published a short note in Naturwissenschaften probably at the end of '38, in which he suggested a rigid crystalline-like structure of a nucleus built out of alpha particles with neutrons squeezed in between. There's always enough space for them in such a structure. And he showed some remarkably good agreement with known nuclei of high stability—for instance, the iron nucleus is pentagondodecahedron of alpha particles with one alpha particle in the middle, which is the closest packing. An iron nucleus, as everybody knows, is particularly stable—things like that.
And was there much reactiön to this?
Not much. I think Weizsäcker presented it and said: "This is a crackpot sort of model, but it's surprising how many things can come out right." And so we started teasing him and saying: "Wefelmeyer doesn't exist. It's just a pseudonym of Weizsäcker."
Was there any feeling in Wefelmeyer's presentation of this model of direct comparison with the compound nucleus, or did he just present it as a possible model? In other words, was he deliberately posing it against the compound nucleus model?
No, as I recall, he was just one of those people working a bit by himself and playing around with numbers who had hit upon something that seemed to fit well, and had persuaded Naturwissenschaften to publish it.
I guess the other thing we haven't talked about in the time towards the end of 1938 is the fission development, unless there's something that through my ignorance I have left out.
Well, there was quite a lot of important work, I think, done in collaboration with Halban and Koch. At that time we first used the integrating tank method to do quantitative measurement on neutron cross sections. Fermi had sort of hinted in that direction, but I still think we were the first people to do accurate measurements that way and it was my idea. We were the first to show that deuterium has got a very small cross section, and carbon and oxygen similarly, that nitrogen has got five times the cross section of hydrogen and so on. All these small cross sections were very hard to measure because the absorption cross section which we wanted was much smaller than the scattering cross section, which is easy to measure in a beam, you see. But one needs an integrating tank, where the scattering cross sections have no effect. And so we developed that method and made it work and got quite good accuracy. And in some ways I think this was the basis of much subsequent work and probably the basis of the use of deuterium and carbon in reactors.
It wasn't apparent at that time?
No. It laid the groundwork for that, as it were. It was done in 1937 or '38, before fission was known.
Did Bohr get involved much in this work? Did he take much interest in this?
No, not really. I think Bohr was quite pleased, but he didn't take any great interest. These were quantitative measurements which didn't affect his deliberations very seriously. The work which I did with Placzek was quite different. Bohr took a passionate interest in that, and he pressed us to publish at once and so on. He was obviously very keen on that. It was a glorious occasion when we actually wrote that paper at about two in the morning.
Placzek, I think, assumed that I would be too sleepy to finish the work because he didn't want to write it in such a hurry, but for once I actually managed to keep awake until four; and then Placzek issued a sort of declaration that under passionate protest he would allow me to take that paper to the mails. I took the paper to the mails at four in the morning. I wanted to make sure it wouldn't get delayed again the next day. And Placzek said, "Shall we phone Bohr?" And I said, "Well, Bohr is having dinner with the King." So Placzek said, "Let's look up the King." We looked up the Royal Palace in the phone book, but there were so many phone numbers we didn't know which one to phone.
Was this also characteristic of Bohr, procrastinating in the sending out of a paper, in his own work?
Bohr was always an endless improver and perfectionist. Yes, that is well-known. I think Placzek had that same spirit of perfectionism and of putting it all in the möst concise and perfect and unassailable terms, whereas I have always been the sort of born broadcaster. My inclination is to make things look simple and readable; and if people don't understand it or don't agree, they can always write back or argue. So there were considerable discrepancies, and our paper was a compromise— the fifth draft of something which we had ultimately rewritten after tearing up each other's papers.
You mentioned in your account of meeting Lise Meitner that you had really intended to talk with her about an experiment you had planned. Was this a continuation of this work?
Well, that was a continuation of some work which we had started with Koch and Halban. This tank work was only one part. Another work we got very interested in and made some headway with was the magnetic behavior of neutrons. At that time it had been discovered—I have an idea it was Dunning, that group anyhow, who did some of the first experiments to show the magnetic scattering of neutrons. If you pass neutrons through oppositely magnetized iron plates, the transmission is different from when they are parallel magnetized. So we picked up that effect and repeated those experiments and extended them. In particular, we determined the sign of the magnetic moment by making a neutron precess in a field from one plate to the other and having the two at right angles, the idea being if it precessed that way, it would be transmitted more strongly; if it precessed the other way, it would transmit it more weakly. And we got an effect of three or four times the statistical error. It was difficult to do. The counting rates were quite small. We even did one where we tried to estimate the magnitude of a magnetic moment from a precession experiment rather crudely, and all this was published. And I'm still not sure whether we had any real effects, because we did all our experiments with iron plates which had only remnant magnetism in them. The trick of that was they were flat iron rings, and an iron ring with only remnant magnetism has no external magnetic field. So we were not troubled by what the neutrons might be doing outside. Outside they would be running in a field free space with sufficient accuracy. And I was very proud of that idea. I thought: "It's cute. You just magnetize it beforehand, but your neutron beam goes through after you've magnetized the iron. Later, Hans Staub in Stanford did work on the quantitative behavior of iron, and he found that the effect on neutrons of a remnant field is so weak that he couldn't measure it. So he kept saying that we never had an effect, and it must have been something wrong with the apparatus. He didn't say that we had cheated outright, but I think he had suspected it.
Was this work published in 1938 or '39?
That was published in the Physical Review in '37 or '38, probably '38. The magnetic study was the first time I got into the Physical Review—with Halban and Koch.
What was the reason for putting it there?
I think it was Halban, who is much more of a promotor and show- man, who said, "Physical Review is a much more important paper. It's use pu blishing things in German magazines or Danish ones or even in no Nature." we wanted a longish paper really describing the details of the work.
That's interesting because it hadn't been the case with the Physical Review about ten years before.
Well, at that time Dunning and lots of other people did publish all their work in Physical Review, and we were beginning to read it with care and finding interesting stuff there.
That takes us, I think, to the beginning of the fission story. If you think it is appropriate, perhaps we can get onto that. Had you left Copenhagen prior to the end of 1938 on trips visiting any other institutions in Europe?
Not much. I haven't much recollection. Until Austria was occupied, I used to go to Vienna regularly—I think twice a year—to visit my parents, and I visited Lise Meitner in Berlin regularly. But after '38, of course, I no longer did go. Towards the end of '38, I was still pursuing thoughts of this magnetic experiment, and I had realized that one might do a Rabi resonance type of experiment, which would give the magnetic moment of a neutron with much greater accuracy than any other method I could think of. I thought this idea was new. Of course Bloch had thought of it, too, and I think was already busy on it at the time.
With Alvarez in California?
Yes. But anyhow I had that idea, and I thought it must be done with some care to get a good uniform magnetic field, and so as I was traveling to Sweden to meet Lise Meitner for Christmas, I was sketching out that big magnet which I was going to design and have built for that purpose. When I came there, I wanted to talk about that magnet. Now, I feel this has been told so often and told in print—how Lise Meitner got that letter. As I recall it, we met the evening before, and the next morning when I came down to breakfast I found Lise Meitner poring over that letter from Hahn and wanting to talk to me about it. I was trying to wave her aside and said, "Barium, I don't believe it. There's some mistake." And she sort of had to keep heading me off my magnet and saying that Hahn was too good a chemist to make a statement like that unless he was very, very sure of it. And then we sort of kept rolling this thing around and saying, "But it's impossible. You couldn't chip a hundred particles off a nucleus in one blow. You couldn't even cut it across. If you tried to estimate the nuclear forces, all the bonds you have to cut all at once. It's fantastic. It's quite impossible a single neutron could do that." I still don't know how we got to the concept of fission, but I remember Lise Meitner drawing a dotted circle on a piece of paper and saying, "Couldn't it be this sort of thing?" Now, she always rather suffered from an inability to visualize things in three dimensions, whereas I had that ability quite well. I had, in fact, apparently come around to the same idea, and I drew a shape like a circle squashed in at two opposite points. And Lise Meitner then said, "Well, yes, that is what I mean." She had, so to say, looked at the nucleus from the poles on and with the dotted line was indicating the Equator being pushed inwards. I remember that I immediately at that instant thought of the fact that electric charges diminish surface tension. That's probably one of the problems I had once worked out in those five-hour bouts in Hamburg, when I worked through every conceivable little problem in physics that caught my interest. And so I promptly started to work out by how much the surface tension of a nucleus would be reduced. I don't know where we got all our numbers from, but I think I must have had a certain feeling for binding energies and could make an estimate of the surface tension. Of course we know the charge and the size reasonably well. And so, as an order of magnitude, the result was that at a charge of approximately 100 the surface tension of the nucleus disappears; and therefore uranium 92 must be pretty close to that instability. In fact, our calculation wasn't nearly accurate enough to tell the difference between 92 and 100. Then Lise Meitner was pursuing a separate trend of thought and was saying that if you really do form two such fragments they would be pushed apart with great energy. [pause in recording]
We're resuming after a break of more than two hours, and we were just talking of your discussion with Lise Meitner in late 1938.
I'm trying to recall who did what, and that of course isn't easy. But it really was all very much give and take. I believe that it was either Lise Meitner or me who figured out roughly the energy that would be given to two such fragments if they were pushed apart after having separated, pushed apart by that electrostatic field, and the results are well-known—something like 200 MeV. And then Lise Meitner, who had the packing fractions in her head, worked out the energy which one might expect to be liberated and came up with a very similar number. That really got us excited because now we could see that it all hung together— the process is possible in a classical framework; that it did not imply any passage through potential barriers, which of course with these enormous masses would have made it exceedingly unlikely. So then we really felt here we had an explanation and this was a totally new kind of process. I don't quite remember how we filled in the rest of our holidays. I think I stayed there for about another day or so. We had Christmas dinner with some Swedish friends, and we discussed a few details. We did not, I believe, discuss any experiments that might be done, but we agreed that we would have to write a paper about this, but we left it to be done separately somehow. It was eventually done in the way that I drafted a paper and read it off to Lise Meitner over the long-distance telephone between Copenhagen and Stockholm, and she would stop me and make comments and suggestions. So it was a slightly expensive way of writing a paper. It was also agreed that I should tell Bohr about it but otherwise keep it under my hat until we had got the publication going. I don't remember the details, but my recollection is that when I came back to Copenhagen I found Bohr just on the point of leaving for America, and I just managed to catch him for five minutes and tell him what we had done. I hadn't spoken for half a minute when he struck his head with his fist and said, "Oh, what idiots we have been that we haven't seen that before. Of course this is exactly as it must be." he added, "This is very beautiful," and asked if we had written a paper. So I said no, we were in the process of writing one. Then he asked to be kept informed. But there again my memory may have become affected by the fact that I've told that story so often in print, and I probably just keep repeating myself. I have no real first hand recollection.
And it was at that point that he left for this country.
After that I sat down to start and write the paper. As I mentioned in Washington, I think I told the story to some others on the spot—particularly to Placzek, and Placzek was very skeptical. He felt the uranium atom was sick enough to suffer from alpha instability and it was incredible that it should be suffering from a totally different form of instability as well, too much of a coincidence, he felt. And then he said, "Why don't you do some experiments?" and asked if one couldn't use a cloud chamber. Of course it was done later, but that requires rather pure neutron sources with not too much gamma rays present, which we didn't have then. But it set me thinking, and then I realized that it could be done quite easily with an ordinary ionization chamber and pulse amplifier.
And these were available?
These were more or less available. I seem to remember that I rigged up a pulse amplifier for this special purpose, and I also built a small ionization chamber; but that whole thing only took me about two days, and then I worked most of the night through to do the measurements because the counting rates were very low. And by three in the morning I had the evidence of the big pulses. I figured out it was on January 13th that I ended my measurements, because I remember thinking that once again 13 had proved to be my lucky number. I went to bed at three in the morning; and then at seven in the morning I was knocked out of bed by the postman who brought a telegram to say that my father had been released from concentration camp. So it was another bit of good news on the 13th, although I would have liked to have slept a little longer.
Where did the telegram come from?
I think it was my mother who wired. I've forgotten the details. My mother was left in Vienna whereas my father had been sent to Dachau. After that I spent several days in a state of slight confusion just repeating the experiment, with some small modifications, looking at the pulses on the oscilloscope screen. I wrote home to my mother that I felt like someone who has caught an elephant by the tail and doesn't quite know where to lead him and how. Somebody suggested that I ought to wire Bohr, and I was horrified at the expense of a trans-Atlantic wire and felt that I ought to do some more experiments and not merely tell him that this has been confirmed. The next thing I remember is that I went to a party. Now, I'm a little mystified about that. No, that's quite correct. Mrs. Bohr had not gone to America, and she gave a party for the people at the lab, even in her husband's absence, and on that occasion I spoke to one of Bohr's sons—I think it was Hans, who was a lawyer or something like that, not a physicist at all—and he asked me, "What's new in the lab?" and so I told him about this. And apparently he then put it into his next letter, a perfectly ordinary letter, air mail presumably, which he wrote to his father. After that, we suddenly got telegrams. My impression was that the telegrams from Bohr started coming when he had received that letter from his son, but other people seem to have different recollections about that period and about why Bohr started sending us wires. He made several proposals about what we should try—not all of them very practical. Some we did try, and they didn't help very much. One of his suggestions was that we should compare the decay curves of mixed fission products which were obtained with fast neutrons, with slow neutrons, with thorium, with uranium. We did some such comparisons and the curves came out pretty accurately the same. At that point I realized if you've got a very large mixture of different fission products, you can probably apply statistics of some sort to the large number of possible nuclei formed. I feel that this is, in a sense, the one theoretical paper I wrote in my life because I worked out the statistics of a random mixture of nuclei uniformly spread over the energy scale—I think I gave some grounds why that should be so—and worked out a decay curve you should get and I got perfect agreement between the calculated and the measured curve, which merely showed that the whole thing was a purely random phenomenon and nothing really could be learned from it.
This was, again, in the early part of '39?
That was all still in the early part of '39.
This was a Nature article?
That was published in Nature under my name—probably in May, '39. The other thing that happened at that time is that Lise Meitner came to Copenhagen, and we carried out an experiment which she had actually proposed in our very first paper—that is to use the recoil of the fission products out of a uranium layer, to collect them and measure their decay, measure their chemical properties and so on. By that time Copenhagen had got an accelerator going and was able to produce a considerably stronger neutron source from a d + d reaction. And so we were able to put a layer of uranium oxide or some similar compound into a strong neutron beam very close to the target and placed under- neath a shallow bowl with water. The idea was that if you collect the stuff in pure water, there could be no fallacious radioactivity, because water is not activated by neutrons, not significantly, and the fission fragments would all presumably stick to the water. And we did find radioactivity, and Lise Meitner carried out some chemical analysis on the same lines which she and Hahn had so often used to separate the so-called transuranic elements. And she found she got activity, and that again it decayed in the same manner as if the same precipitation was carried out directly on the uranium sample rather than on the recoiled products. And this we took as a confirmation, if not as the proof, that the so-called transuranium elements precipitated by Hahn-Meitner's method were in fact all fission products. It wasn't very convincing because we still had a large mixture, but not so large a mixture; and, in fact, the decay curve was significantly different from the one which was found without chemical separation. So it was probably a reasonable experiment.
How long did she stay then? Was this just a visit?
It was a fairly short visit. I think we got that experiment through in either two or three weeks. She just came to Copenhagen because she was anxious to have a hand in clearing up the mess for which she felt to some extent responsible—namely, having hoodwinked the whole world, as it were, about these transuranic elements.
Earlier, you mean?
Yes. She felt badly that she and Hahn should for so long have pursued and insisted that they had discovered a number of transuranic elements.
Was there a lot of this feeling in the period: "Oh, my God, why didn't we see it before?" Was this prevalent?
I think it was there. Lise Meitner sort of kept saying, "We couldn't have seen it. This was so totally unexpected. Hahn is a good chemist, and I trusted his chemistry to correspond to the elements he said they corresponded to. Who could have thought that it would be something so much lighter?" And, after all, they had checked right down to lead, I believe—that it was nothing of that kind. The elements that they had called transuranic had not the chemical properties of anything between uranium and lead that they had verified.
What about this reference you often hear to the Ida Noddack paper? Is this in historical retrospect, or was there a discussion of her paper at the time?
I don't recall any discussion of the paper. I know that she published that paper in the Zeitschrift für angewandte Chemie, a journal of applied chemistry, which Hahn would hardly have read and certainly not Lise Meitner—none of the physicists. I think whatever chemists read it probably thought that this was quite pointless carping criticism, and the physicists possibly even more so if they read it, because they would say, "What's the use of criticizing unless you give some reason why that criticism should be valid?" Nobody had ever found a nuclear disintegration creating far removed elements.
What about the work that had previously come from her? Was there any tradition?
That was the other thing, that she and her husband had jointly published the discovery of two elements, one of which turned out later to have been wrong. They claimed to have discovered two elements which they called rhenium and masurium. Now, rhenium stood and still exists, but masurium turned out to have been a mistake, and that's the element which later was found to have no stable isotopes at all and which is called technetium.
Do you think this was a factor in discrediting her criticism when it did appear?
It might well have been. I think the Noddacks were regarded somewhat as sort of stubborn chemists who make a discovery and then stick to it and don't listen to anybody else, and so, "Why should we listen to them?" bit of that.
When Lise Meitner visited Copenhagen in 1939, had Bohr returned at that time or was he still in the United States?
When Lise Meitner came to do that experiment with me, I'm pretty certain that Bohr was back. In fact, I believe she was living at the Bohrs' , which she always did when she was in Copenhagen.
What was the general discussion there in terms of the implications of fission? Was there any clear agenda for future research, what had to be done—in terms of physics?
I would say no. None of us dared to tackle the idea of looking for fission neutrons. They were discussed slightly. My own feeling was it would be hopeless to look for them because the experiment has to be done in such a flood of neutrons; how on earth were you going to discover if a few more neutrons came out? I'm afraid, as usual, I had a very unoriginal and entirely gadgeteer approach. I reflected whether the resolution of amplifiers could be so improved that other more radioactive elements might be studied. I worked out on paper that I could try to look for the fission of radium, for instance, using a sufficiently small specimen, and yet be able to discover the occasional fission pulse if there was one; but I never attempted the experiment. No, I'm afraid I did nothing more because, for one thing, I think that was the time of the invasion of Czechoslovakia, wasn't it? Anyhow, I was getting very word at that time what the European situation would be; and, as I said, the only time in my life that I took some initiative was when I first spoke to Blackett and then Oliphant when they passed through Copenhagen and said that I had a fear that Denmark would soon be overrun by Hitler, and if so, would there be a chance for me to go to England in time, because I'd rather work for England than do nothing or be compelled in some way or other to work for Hitler or be sent to a concentration camp or gas chamber. They were both very sympathetic and Oliphant said, "You just come over in the summer. We'll find you something to do. You can give a few lectures or something. We can discuss the matter." As a result, I went over in July. And before I had stayed two months, the war had broken out. All my belongings stayed in Denmark.
And you went to Birmingham?
I went to Birmingham first, yes, and stayed with Oliphant.
Now at this time, even in Copenhagen before you left in July, were there discussions of the use of fission for military applications? Was this in Bohr's mind, for example?
My impression is there was very little discussion. There is one piece of documentation, which I haven't seen for years but it can be found, and that was the Annual Report of the Chemical Society published in England in 1940, by which time I was in England. I had been invited to write the section on nuclear physics and nuclear chemistry for that report of the Chemical Society. I wrote it and described fission as one of the outstanding discoveries of the year of the report, and I mentioned the possibility of a chain reaction. I stated, however, Bohr has shown that there is no likelihood of basing a superweapon on that, because the only way to make a chain reaction go appears to be the use of slowed down neutrons, which at the very best would give a sort of puff like a mountain of gun powder, which can be had more cheaply. And I wrote that in perfectly good faith, but I think it was that article that set me thinking and saying: Is that really true what I have written? after I had sent it off. And I realized that if one could separate isotopes, of course it wouldn't be true. And I then went and took a formula which Peierls had published on the critical size of fissile material, and I put in a vastly optimistic figure for the fissiön cross section and came out with a critical mass of one pound. That then set me thinking. I felt one pound is, after all, not such a lot. I had always thought in terms of tons, without really thinking much, because one doesn't need a Peierls formula; one needs a bit of common sense to get a critical size, an approximate one. So, after that, I tried to make an estimate of how much equipment would be needed to generate pounds of separated 235, again making a vastly optimistic estimate of the efficiency of thermal diffusion—an estimate which fitted some of the known cases of thermal diffusion—and came out with a quite modest sum of, I think, a million pounds for a suitable plant. I used calculations by Furry. At that point I went to Peierls and said, "Look, shouldn't somebody know about that?" And then we composed that letter which Oliphant advised us to send to Tizard, and it was then I think the beginning of the organized attempt in Britain to get this research going.
Who authored this letter?
Peierls and myself. This is all part of history. You'll find it in Margaret Gowing's book and so on.
Now, since we have probably a limited amount of time today, I want to ask a general question which I think will take us to the end of the period. At that time, then, apparently nuclear physics research as it was proceeding had changed and normal nuclear physics research came to a halt since, of course, it was wartime already. Is that right? Was there normal work proceeding after this point? Or did everything then get turned towards the war?
Let me go back to Copenhagen. After our fission discovery, I think normal research went on, but I remember that I was in a state of complete doldrums. I had a feeling war was coming. What was the use of doing any research? I simply couldn't brace myself. I was in a pretty bad state, feeling, "Nothing I start now is going to be any good." Even after I went to England, I had a pretty strong presentiment that I had only got a few more months to live—so strong that for once I really believed it. I had a feeling: "This is some kind of preknowledge of disaster," that I would be hit by the bombs when the bombing started and so on. When the bombing started I felt that the time had come and so forth. I was quite surprised when a year or so later I was still alive and I began to feel that maybe the presentiment had been just a case of depression, which no doubt it was. But in Copenhagen I did no more useful work at all after the fission work until I left in July. I sort of thought of starting this and starting that, but I didn't feel like starting anything which would have taken a year to prepare. I felt it wasn't worth doing. I couldn't go on that long and in a sense I was right. Now, in Birmingham, I was teaching, for one thing; that was what I was getting paid for. Oliphant gave me a job as a junior lecturer. In addition, I went to Oliphant and said, "Could I try and separate uranium isotopes by thermal diffusion, because this seems to be one way of testing Niels Bohr's idea that 235 was responsible for slow neutron fission?" And so Oliphant gave me an old lecture room to use, because by that time all the laboratories had been taken over by people developing radar magnetrons and so on, and I wasn't allowed in. I wasn't allowed into the workshops even. It was quite difficult to work.
Was that because you weren't a citizen?
That's right, because I was an enemy alien, technically speaking. In fact, there was one time when the police invited me to an interview and asked me all kinds of questions, which really boiled down to the one question: "Is there any reason why we shouldn't intern this guy?" But I did try to set up a thermal diffusion plant and wanted to try it out first with some ordinary gas mixture. In the meantime we got in touch with Chadwick, who had already started to take some interest in the possible use of fission for military or other applications, and asked him whether he could get us some gaseous uranium compound. I knew about uranium hexafluoride somehow or other. In the end he said it would be better if I came to Liverpool to work there, and this was done. So in the summer, I think, of 1940, just at the time when Denmark was being overrun and so on, I moved to Liverpool.
And then these other problems were solved, and eventually you became involved in war work.
From then on, it was quite clear that I was doing war work. The Maud committee had been created, and I had attended several of its meetings.
Let me skip then to the general effect of the war on nuclear physics. At the end of the war, or when the end of the war was approaching, were there discussions about what was on the agenda for nuclear physics per se after the war? What were the expectations? What were the types of problems that you thought could be done, and what change, if any, had occurred as a result of the war?
At the end of the war?
Or even anticipating the end of the war, when you were starting to think about the future.
Well, as usual, I was fairly unimaginative, and my thinking was largely gadgeteering. We had developed improved detection methods— in particular, one which I even found some people called the "Frisch grid," which was the use of a grid placed into an ionization chamber filled with gas which has no electron affinity, like a noble gas or pure hydrogen, or pure nitrogen. This was actually used for a somewhat different purpose by someone in Vienna whose name I have forgotten, who never made further use of it, and I picked up the idea, and saw it could be used to get high-energy resolution combined with high time resolution. That was very useful. So for a while I rather concentrated on application of those chambers, and I tried various extensions which didn't come out too well. I had no consistent policy at all. I still had the attitude of a junior research student who picks up a problem when he sees one and thinks he has got a gadget to solve it with. Then when Los Alamos came to an end after the war, I was invited to join the British parallel group that was just being formed at Harwell. I was spending a holiday in California of a few weeks and finishing a book while staying with Hans Staub in Stanford when there came a telephone call from Cockcroft saying, "Could you be in New York tomorrow? I'd like you to meet your deputy." So I flew to New York the next day and arrived there in the evening and found a man who was listed as Dr. Cockburn, and to whom Cockcroft had referred as "Dr. Coburn." It turned out it was the same person; I mean "Coburn" is the pronunciation of Cockburn. Then I came back to England and worked for a short while in an office in London. And then they had built up enough in Harwell so it was worth moving me there. There again I was a pretty passive figure, and Cockburn very soon was really the man who ran the department. He was awfully nice about it. We were very good friends and still are. He sort of kept saying, "This is quite all right," as he ran the department and did the hiring and firing and saw to all the details while I worked out the theory of fluctuations in piles when used for measurements. His attitude was that if I had one good idea in six months, it paid for it, which was probably a bit of an exaggerated attitude. Also Cockburn kept saying that when he batted his head against some insuperable difficulty, I would come nosing around and find a hole in the brick wall somewhere a few yards away which he had overlooked.
About this period, was there some expectation of increased government support? It was the period of the building of large accelera- tors in the United States and large support for nuclear physics research, and this was also the period of the branching off of the elementary particle interest into practically a separate field. Was this reflected much in England, or were the developments in England somewhat behind?
Well, it may have been; I didn't see much of it. They built a cyclotron in Harwell at that time, which was a little beauty and worked the minute they pushed the first button—it just worked. It was a marvelous success in that respect. In fact, they got some beam and then they turned the energy up and all the alarms began to ring because they hadn't set up their shielding yet. So they hastily turned it off and ran, and they started to build up the shielding. But this was still sort of extending nuclear reactions to higher energy, maybe studying spallation and things like that. There was no very serious thought on high energy, I believe, except in Birmingham where Oliphant with his usual visionary instinct had started building a greatly enlarged cyclo- tron, during the war actually, though he had to stop building it. Then he managed to get the money and the people to finish it afterwards. In fact, he had the first BeV machine, but he was so slow in building up that the Cosmotron beat him to it.
What was his motivation in building it up, do you think? What did he want to use it for?
Well, I think Oliphant was absolutely a visionary. He said that every time a tool is raised an order of magnitude, some new results are bound to come tumbling out. He had very much the same sort of atti- tude I have: invent a new gadget and something useful will happen, only Oliphant invented gadgets on a large scale.
That brings me to another question. In reference to gadgets, what about your mechanical kick sorter or your pulse size analyzer? What year was that?
The kick sorter I started during the war in Liverpool; that means '42.
I didn't realize it was that early. When did you first publish on that?
I don't know. The very first one I published was much later. That was a sort of polished-up version.
I wanted to use that as an example. That was certainly consistent with your work and your motivation, but how does that compare with these larger-scale instruments—machines and detectors and so forth— that were being developed after the war? The question is how does that development, which is logical to you, fit into the larger development— say, the sort of thing that Oliphant and others were doing on this physically big scale?
Well, the kick sorter actually grew out of a need. We had a problem of having a quick and accurate way of analyzing uranium mixtures for isotopic composition, given very small specimens You see, I was experimenting with various ways of separating uranium isotopes and building small pilot plants. They expected very small specimens at first, particularly since they were following several lines on the laboratory scale in the hope that one might work. So after some attempts that didn't go so well, I suggested measuring the energy spectrum of the alpha rays. And at that point I realized it would be enormously speeded up if, instead of moving the bias step by step and getting an integral curve which you then had to differentiate, one could use separate simultaneous channels. It seemed an outrageously extravagant thing to ask for—about 30 radio tubes in one fell swoop for one apparatus. But I went to Chadwick with it somewhat hesitantly, and to my surprise Chadwick immediately supported me and said, "Yes, that sounds like a good scheme. Let's do it." So this was built. The very first one was built in Liverpool with the help of a research student who came to assist me.
Was this in 1942?
That must have been '42 or '43. I left Liverpool at the end of '43, and by that time we had something going; but that was purely a bank of one dozen counters, I think.
I'd like to explore this some other time at more length. Let me ask a couple of general questions. How do you account for the new or renewed—whichever way you look at it—interest in elementary particles after the end of the war? This was one of your interests.
Well, I don't know. My impression is—I haven't thought this out—that this came about somewhat accidentally, partly by the discovery of the pi meson, which was a more or less accidental discovery of Powell and Occhialini which almost synchronized with the theories of Marshak. And then very soon after that, mesons were seen to come of Lawrence's new cyclotron. An interest is always enormously stimulated if you have got an artificial source, and then of course the theories blossomed. On the other hand, the emulsion work was followed up by Leprince- Ringuet in France and with Wilson chambers. There was the work of Butler and Rochester, using Blackett's triggered cloud chamber. Simply by looking at cosmic radiation, you found those particles. This stimulated the machine builders to look for them and to make them in much larger quantities. I mean I still remember how excited I was when the Cosmotron at last came into action and the daily production of strange particles jumped from something like one particle a day to a thousand a day.
Let me ask a final question. In reflecting on your career, what is the most personally satisfying work you have done in physics?
I think it was that little piece of work I did when Stern was going on a brief holiday and I was left to play with the atomic beam apparatus. It was a quite unimportant little sideline, but it was a discovery which Stern had made previously and which we together had confirmed; that in the diffraction spectra of helium atoms, there were some unexplained dents, fairly sharp minima for which we couldn't account. We hadn't got a theory. But while Stern was away, I did a systematic pursuit of these dents through different conditions of angle and so on and applied a fair lot of spherical trigonometry, which gave me a con- siderable headache to work it out, to see if there was any regularity. And I found a regularity which I couldn't understand. It was extremely simple and quite precisely fulfilled. We left this thing standing for a year, because we hoped we might find an explanation. In the end Stern said, "Well, you better go ahead and publish this." So I published it under my own name, and it was later explained as a very pretty phenomenon— namely, that under these conditions one of the diffracted beams would become a grazing beam. And the grazing beam interacts irreversibly with the crystal surface, and this messes up the whole wave function and causes events even in those beams which were not grazing. The theory was made in England. That was the thing I enjoyed most, partly because it was careful and difficult experimental work—an application of a branch of mathematics which I had to learn for the job, and also the finding of a simple regularity which was completely mysterious.