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In footnotes or endnotes please cite AIP interviews like this:
Interview of Carl Anderson by Charles Weiner on 1966 June 30,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
For multiple citations, "AIP" is the preferred abbreviation for the location.
Anderson talks almost exclusively about his work during the thirties with particles of high energy involved in nuclear reactions. He covers in detail his discovery of the positive electron, his pair production work with gamma rays, his expedition to Pike’s Peak with Neddermeyer and their discovery of the mesotron. He mentions that it was in his speech accepting the Nobel Prize in 1936 that he first mentioned the possibility of negative and positive particles of intermediate mass. After noting the absence of any cosmic ray work during the war years, he mentions the postwar development of cosmic ray work into high energy physics.
I’d like to start by asking you to reflect on the beginning of your interest in physics or in science in general if that’s more appropriate.
When I was a youngster I was interested in electricity -- from as early times as I can remember. I used to go to the garages in those days and get used discarded dry cells because many automobiles had to replenish these batteries occasionally and do experiments with them. I decided to become an electrical engineer, and this was clear ail through my high school days, and I enrolled at Caltech with the idea in mind of majoring in electrical engineering, which I did for almost two years.
How do you account for this interest in electricity? Was there anything in the family or in the home -- for example, books that interested you or friends?
Not in the family. My father was not an engineer or a technical person. I don’t know. It may have been certain books that I happened to find when I was very young. I just don’t know. It goes back as far as I can remember.
What was your father’s occupation?
He was a chef.
Then in high school did you have science courses?
Yes. I think I was fortunate in having a very good training in high school as a background for technical work in college. In fact, we had four years of electricity, including automobile generators and batteries and relays. One term we wired a house. Some of the courses were quite good -- AC laboratories, synchronous motors, and an unusually full course in electricity for a high school.
And this was in Los Angeles. Was this the Polytechnic High School?
This was what used to be LA Polytechnic High School at Hope and Washington Boulevard.
Why would the other students who went there choose that school? Had they in mind careers in science or engineering?
Well, I chose it completely by accident, or at least it wasn’t really my choice. The family had moved out to a region in Los Angeles that was within 200 yards or so of the city limits of Glendale, and I had planned to go to Glendale High School. It was much closer. But I was told later on that their enrollment had exceeded a certain number of people and that they therefore could not take anybody from outside the district, so I then chose Polytechnic High School because it was accessible by streetcar.
You made up your mind in high school to go into electrical engineering?
Oh, yes. There was no doubt about that.
How about the other students? Why were they in that school? Do you have a general impression of their interests?
I think perhaps a higher percentage of students than in a normal high school were interested in engineering or technically oriented things. Maybe the fact that the high school, as its name implies, LA Polytechnic High, may have influenced them. I just don’t know.
It’s a side question, but it’s of interest. And you selected Caltech. Was this a question of convenience again or had you considered other schools as well?
Well, it was partly a question of convenience. It was nearby, not very far from Glendale. I couldn’t afford to board at a college. I had to live at home. I had heard about Caltech. Millikan had I guess on a couple of occasions come over to LA Poly High and had given talks in the auditorium to the students, and this impressed me. I didn’t know what he was talking about, but it was pretty good stuff, I thought. I can’t remember the details. I don’t believe I considered any other institution seriously. There were four of us in my senior class who came to Caltech. All four of us were very strongly advised not to come to Caltech for a variety of reasons by our counselors and our high school teachers. There was one exception to this, a physics teacher who said that he thought it might be all right. The reasons they gave were that it was extremely difficult to get in; it was too hard a place; the effort required was too much for the results, and we’d have to work too hard and probably wouldn’t do well, and even if we did, it wouldn’t be worthwhile. But all four of us came here. All four of us were in what’s called Section A, which is the upper five percent scholastically of the people who were picked out and put in a separate section. All four of us stayed with Section A. All four of us got scholarships throughout our four years of work. All four of us remained at Caltech as graduate students and got our Ph.D.’s on the same day.
And did they get theirs in physics as well?
One was in electrical engineering. One was in chemistry. The third one was a very close friend of mine, had been all through my high school days, who started out in Caltech as an electrical engineer, as I did, but switched to geology and got his Ph.D. in geology.
It might be of interest to know who they were.
The electrical engineer’s name was Goattir. We called him “Goats.” I can’t remember his first name. The second one was Bernard Moore. He’s the chemist. The third one is Louis Gazin, the one who started out as an electrical engineer and majored in paleontology and now is at the National Museum in Washington and has been ever since he got his degree at Caltech in 1930.
You said that you stayed for two years in electrical engineering and then what happened?
I changed at the end of my sophomore year. In those days the Section A people were given all of the normal sophomore physics course in two terms instead of three. And then the third term was devoted to an introductory course in atomic physics, which Ike Bowen gave -- he was here at the time -- and this opened my eyes very, very much. I didn’t know there were such things in the world, and during that term decided to major in physics.
What year is this?
This is in the academic year ‘23 - ‘24, and Ike Bowen might have been assistant professor or maybe a research fellow -- I don’t know. This we could look up.
I see. And then was there any problem in switching?
No, there were no problems in switching at all because even in those days, the first year, the freshman year, was the same for all people at the Institute. There was very little difference in the second year. In fact, I got a degree -- I think it was in both physics and engineering. There was no problem at all in switching.
Were there any other courses or any other contacts especially significant as far as having an effect on your interests and later work during your undergraduate days?
No, I don’t think so. There were not nearly as many courses required in those days as there are now. And by the time of the beginning of my senior year, I then had hopes and plans to stay on at Caltech as a graduate student. I started doing my thesis research and worked … I can’t remember the exact date, but I think it was fairly early in the senior year that I started doing research for a Ph.D. thesis.
You were evidently rather an advanced student as an undergraduate. Was this early start on the thesis unusual?
I think unusual in the sense that most people did not do it. I don’t know the percentage of students of the senior graduating class at Caltech in those days who went on to graduate work. Now it’s about 85, but I guess it was much less then. Country-wide, a much smaller percentage of students went into graduate work, I guess -- I don’t know the statistics.
Then you got your degree in ‘27, was it?
What was the thesis work and how did you get interested in the problem?
Well, I didn’t have enough to do and wanted to do some research, so I saw Professor Watson, who was in charge then of helping the graduate students and helping them get started on research, and he suggested that I start out by working for DuBridge, who had just come here then as a National Research Council Fellow. So I did, and DuBridge assigned me to the job of building a spectrometer. He was setting up, getting together equipment for determining the work function of either platinum or tungsten -- I can’t remember which it was. No, it was a monochronometer, a device for giving him a beam of light of a certain frequency. So I worked on that I think about three weeks or a month, and then Millikan came back from a trip abroad, I guess; and he called me into his office and said I should not be working for DuBridge, I should be working for Don Lockridge because it was a more interesting field. So I then began working with Don Lockridge, who was a graduate student just finishing up his thesis work, and it was shortly after I started working with him -- I think about a week or so -- he accepted a position and left Caltech. But I did inherit his equipment, and the problem then was to study the space distribution of photo-electrons produced by X-Rays in a gas. And I worked on that and did my thesis on that subject, using a cloud chamber. The central part of the apparatus was a Wilson cloud chamber.
Who had built the one you used?
This was built by Don Lockridge.
This was your first experience with a cloud chamber?
Yes, that’s the first time I had seen one.
And then it was three years before you got your degree. When was the research itself finished, though? Did it take that period?
Well, I kept working on that problem until perhaps six months before I got my degree or maybe more than that -- I can’t remember -- but I wrote a thesis and one paper on that subject.
There is a paper published in the Physical Review in 1930, which I guess is your thesis: “Space Distribution of X-Ray Photo-Electric Cells.”
So the awarding of the degree coincides pretty much with the completion of the dissertation.
I don’t know the exact date on this publication.
1930, and of course my information gives the volume number. More significant, it doesn’t give the date received.
I don’t know. I got my degree in June, 1930.
And then during the period when you were completing your degree, did you have in mind what you were going to do once you had finished?
Yes. One thing that I was trying to do as a graduate student was to learn something about quantum mechanics. It was a very young subject in those days -- from ‘27 to ‘30 -- and I felt that it would be very worthwhile for me if I could spend one more year at Caltech as a postdoctoral fellow with the idea in mind of devoting at least a good part of the time to studying quantum mechanics. I asked Millikan if this would be possible, and he said, “Well, it’s a very poor idea. You’ve been here four years as an undergraduate and three years as a graduate student. You’re getting very provincial. What you should do is apply for a National Research Council fellowship and go somewhere else.” So I then did apply for a National Research Council fellowship and wrote to Arthur Compton in Chicago and he said it would be fine; he would welcome me there. So I made those plans. Incidentally, the problem that I would have worked on had I gone on to Chicago was to study the Klein-Nishina formula, the production of electrons by high energy gamma rays, the Thorium C double prime gamma rays, with a cloud chamber in a magnetic field. I made experiments with the cloud chamber that I used for the X-Ray photo-electrons to learn the technique of working with the higher energy electrons, which was considerably more difficult, because there the ionization was at the minimum and the X-Ray photo-electrons were of much lower velocity and gave much higher tracks.
It just required a little more precision in operating the cloud chamber, a little attention to temperature control; and I found one day that alcohol worked very much better for these minimum ionizing tracks than water vapor. Up until this time I had used water vapor, as I think that was the standard substance in those days to use in cloud chambers, but the alcohol gave substantially larger drops and they were therefore much easier to photograph. It was a problem in those days to photograph the very thin tracks produced by high energy electrons. Now, if I’d gone to Chicago, I don’t know what would have come out of this work. It’s work that Skobelzyn was maybe doing at that time -- I don’t know -- but I think shortly after that he published his results and made no mention of positive electrons, although when he used the lead plates something like 15 or 20 of his particles must have been positive electrons.
When you said that had you gone to Chicago you would have worked on the problem, does this mean that in your correspondence with Compton, he indicated that this was a problem that he wanted you to work on with him?
I don’t know if this was mentioned in any correspondence. I just don’t remember. I might have said that in a letter. I might just have said would like to work in his laboratory. I don’t know if this problem occurred to me after I had asked him if I could come to Chicago or not.
What I’m getting at is that you seemed reasonably sure that you would be working on that problem, whether it was your inclination or not, so you prepared for it. I wondered how you knew you would be working on the problem.
I think it’s sort of a natural thing. For one thing, the cloud chamber technique would have been a very good technique for working with gamma rays. There was at that time considerable interest in the processes of absorption by high energy, in those days: 2.2 million volts gamma rays. I don’t know just when the Klein-Nishina formula was first published.
But it was available.
I don’t know if that particular formula was in my thinking or not. There was a Mr. Chao who was here in Norman Bridge lab and who was working on the absorption of high energy gamma rays and the scattering of high energy gamma rays by using ionization chambers to measure the rate of absorption and the scattering. And there were these suggestive results which suggested an anomalous scattering, as it was called in those days. And was familiar with what he was doing and talked to him about his work, and it just seemed like a good problem. I mean there were interesting things in the absorption of gamma rays in this energy range.
Before we go to the next step: If you had stuck with your original plan to stay here for a year and studied quantum mechanics, what did that mean to you? How could you have studied it here?
Well, Epstein was the theoretical physicist so far as most of the graduate courses were concerned in the days I was a graduate student. He gave courses in electricity and magnetism and kinetic theory, thermodynamics and the older quantum theory. But as quantum mechanics was being developed, he would put that into his courses.
Did you have any courses from him as an undergraduate?
No, this was all as a graduate student. Also, Robert Oppenheimer was spending one term a year at the Institute. Now, I don’t remember when this started -- whether it was during the time I was a graduate student, which I think it was, but I don’t remember in what year. In any case, he was giving a course in quantum mechanics that I signed up for. And after two or three weeks I realized that I did not have the preparation to take this course. As I recall now, he spent the first day reviewing the Hamiltonian formulation of classical mechanics, which was something that I had just heard about. I had heard about a Hamiltonian but really didn’t know about contact transformations and all that sort of thing. So I saw him in his office and told him that I wasn’t prepared to take this course and that I would have to drop it because I couldn’t handle it. He pleaded with me for quite a while not to drop the course, and then it finally turned out that I was the only student still registered in the course and he wanted to have an official course at the Institute. So he gave me a kind of assurance that everything would be all right at the end of the term. So I did stay registered in that course, although there were a great many people in the room -- I guess 30 or 40 people -- who were auditing the course. I’m pretty sure that I was the only one still registered. I can’t be positive there. That might have been the last year I was a graduate student at the Institute. It might have been the middle year -- I don’t remember.
I think it’s easy to trace because I have other information which tells me when he was in Europe during that period. There’s a letter in the Millikan papers from Zurich right in that period with a date on it. So your desire to stay meant that you would have done work in quantum mechanics perhaps with Oppenheimer if he was there.
No, I had no hopes of going into theoretical physics in any serious way, such as working with Oppenheimer. It was just to learn basic ideas of quantum mechanics and to try to understand what the fundamentals are, not with the idea ever of trying to do work in quantum mechanics or to do work in theoretical physics.
I understand that. But in your considerations of the advantages of staying for a year was there the fact that Oppenheimer could give you some background on that as well as Epstein?
Yes. I thought there was a good opportunity here to learn something about quantum mechanics.
Then you had plans on Millikan’s recommendation to work at Chicago with Compton. What happened?
Millikan called me into his office one day and asked if I wouldn’t like to stay on at Caltech for another year and work on a problem in cosmic rays. This was his chief research interest at that time. With a variety of students, he had built a series of ionization chambers and had made many measurements of the strength of the radiation as a function of depth in the atmosphere. And I guess Knopf (?) in those days had made tests over the surface of the earth -- that is, to study the rays as a function of magnetic latitude. I’m not sure about these dates. But his belief, and I think the universal belief in those days, was that cosmic rays were electro-magnetic radiation from space. I had not at this time really done much thinking about cosmic radiation. But he wanted me to use the cloud chamber magnetic field technique to measure the energies of the Compton electrons produced by this high energy electromagnetic radiation and thus to get much better information on the photon energy in this radiation than one could get from absorption measurements, which is ail that he had done up until this time -- either absorption in layers of lead around an electroscope or absorption in the atmosphere or in high altitude mountain lakes.
This would involve a different type of detection.
Yes. Now, it’s interesting that in this paper of Skobelzyn that I mentioned on the secondary electrons produced by thorium C double prime gamma radiation, that he found a few tracks -- maybe three or eight or something like that -- in his cloud chamber which were essentially straight and therefore must have a minimum energy of 15 (I think it was) million electron volts which he attributed to cosmic rays. Now, this experiment was the idea that gave Millikan the idea of using a cloud chamber for studying cosmic rays. It’s one of those things. At least that’s what gave him the idea that this would be a good thing to do. And he at the time was struggling very hard trying to fit the Klein-Nishina formula -- to use it to infer the quantum photon energy of the gamma radiation, as it was thought to be in those days, that he was measuring the absorption coefficient of.
This is the thing that he wanted you do do at this stage.
Yes. And this was not completely unrelated to the thorium C double prime radiation that I had been thinking about. And also he told me that after I had had another year here my chances of getting a National Research Council fellowship would improve tremendously.
So he wasn’t contradicting himself in a sense then.
By that time I think I had decided what I really wanted to do was to go to Chicago, so I used all the arguments that he had used on me. And he said, “I know that’s all true, but you’ll have a better chance a year from now to get a National Research Council fellowship.”
You stayed on here in what capacity?
Caltech postdoctoral research fellow.
And so the funds were Caltech funds.
They were Caltech funds. There was a rule in those days that ail National Research Council fellows had to go to another research institution for their fellowship tenure, not where they had done their graduate work.
Then this was in 1930 when you started to work on the cloud chamber. What was the first thing you had to do -- build one?
The first thing was to start from scratch and build a magnet with a cloud chamber built into it and to arrange it with its large dimension vertically so that it would be suitable for particles that were moving vertically. And the design was based on the premise that the experiment would not be a very long one; so that the weight and cost of the magnet was kept to as low a figure as possible. Funds were very important. They were extremely limited. There was available at the Institute a large motor generator set in the Guggenheim Aeronautical Laboratory that was rated, I think, at 480 kilowatts and it could put out 600 kilowatts for an indefinite period. So the magnet was designed with a minimum amount of iron and a maximum amount of coils to produce as high a field as one could get. It would handle, so far as its cooling was concerned, the full 600 kilowatts of the output, which was the output of this motor generator set, and it was set up in the Guggenheim Aeronautics Laboratory where the m.g. set was.
What field did you get it at?
About 25,000 gauss on 600 kilowatts, or something like that. However, as everyone knows, the field goes up as the square root of the power if iron is not playing an important role, as it wasn’t with these fields. So most of the work was done at lower fields. There were very short runs at 25,000.
Did you work on the construction of the magnet and the chamber itself on your own. Was this your project?
The design of the cloud chamber and the magnet and so on was entirely my own. It was built in the Institute shops, but I made all of the drawings for all the parts of the apparatus -- the cloud chamber and the cameras and the valve mechanism and so on.
How long did it take before it was completed and in operating order?
I can’t remember. I can’t remember the time it was decided that I should remain at the Institute and work on this project. It was some time before I got my degree. I don’t know if it was weeks before that or months before that, presumably in the early months of 1930; and I don’t remember just the first day. In fact, I guess there wasn’t any first day. It sort of worked and it took a little while to get the bugs out of it.
But it was working before you got your degree.
It was some time in 1930 that it started to work?
It might have been in late 1930 or early 1931 -- I don’t know. I can’t remember that.
Now, of course, the question is what happened when it was completed?
Well, I took photographs. They were not elegant ones at all because of scattered light and poor background. The photographic technique had not been worked out -- all the little details about depth of focus. I had an F-2 lens which wasn’t very good when it was wide open, and there was a shortage of light and all this sort of thing. Light sources were perhaps the major experimental problem in the whole thing. It was some time during ‘31 that I got the first pictures, and I think that the first really meaningful pictures came while Millikan was in Europe, and I made prints and sent him maybe a dozen or so of these prints, which he then reported at a meeting I think in England.
Do you recall what that meeting was?
I recall that in later years there was an international conference on nuclear physics.
There was one in 1934, an international conference on theoretical physics.
And one on nuclear physics because Neddermeyer and I sent in a paper to that 1934 meeting -- I remember that.
Oh, you didn’t go to that 1934 meeting.
No. Millikan went to it. He reported that paper.
But this was 1931?
As I recall, it was fairly late in 1931. I remember the date October of ‘31, but it may have been November.
But you did get results in the summer of ‘31?
Yes. I can’t remember in detail just what came when in this experiment. And, of course, it was immediately obvious that the plan that we had to measure the energy distribution of the Compton recoil electrons from the gamma radiation was irrelevant essentially; that what we observed were positively charged particles in abundance, which had no place at ail in the known absorption processes of gamma rays in those days. The thought was that it was Compton recoil electrons that were responsible for essentially all of the absorption but with these indications of a small nuclear process. Some work in England had been done on this, too, and I’m thinking more of the work that Chao was doing here at Caltech at that time.
In the Becker and Bothe experiment they had bombarded beryllium with alpha particles. There were all sorts of very interesting nuclear reactions then. Did this influence your thought in terms of getting interested in the nucleus as something to study?
I don’t know. I remember very well of course the Bothe-Becker experiment in which they used alpha particles from polonium to bombard beryllium. I don’t know if the first results of those experiments were known to me when I was getting these first cloud chamber pictures, but in any case the pictures themselves clearly showed that the nucleus was the important element in the absorption of cosmic radiation -- whatever they were. We still thought, of course, that they were electro-magnetic radiation of an energy range of a few hundred million electron volts. But the frequent occurrence of positive particles showed that it was not the Compton electron process in any case that was the major mode of absorption, as we had thought.
This was in ‘31. And Millikan during this period was in Europe, so you communicated the results to him. Was he around when you got your first results in the summer?
This is what has been going on in my mind. I would guess that he was, but I can’t remember showing him any pictures. I would guess that he was because I don’t think I could have collected as many as I did in that short time he was away. I just don’t know.
Is this letter that we found in the Millikan papers here at Caltech -- it was dated November 3rd and it was found between papers that indicated it was November 3rd, 1931 …
It must have been 1931, yes.
And it’s a rather complete account of a series of pictures, and from the tone of it, it appears that you’re telling him not for the first time necessarily, but at least making a first presentation of this. It’s not clear from the letter that you had agreed with Millikan that this is the field that you should now follow up. You make a suggestion in the letter that this is of fundamental interest and that you believe it should be pursued.
Yes, my recollection is vague on this. I believe it’s the first time that he really had a chance to look at clear-cut results about the existence of positive particles and so on. My memory is vague. I have no memory of having discussed any of our photographs with him before. I just don’t know.
I found no reply to this letter, but you were suggesting that it was worth plunging ahead on the follow-up studies. Now, that apparently was what you did, and did he go along with that?
Oh, yes. Oh, by all means. He was very much interested in cosmic rays. That was his main research activity in those days, and he of course was extremely interested in these results.
What about the reaction of other people here? Did you discuss it with others and what did they have to say?
Well, here again, my memory is pretty vague. I’m sure I gave a colloquium report on these results. I think there was great interest in them. I can’t remember trying to discuss in detail what the mechanism for the production of the positive particles might be. My thinking was that they were ejected from the nucleus –- protons and alpha particles and I think there is a slight hinting at the possibility of electrons. I mean it’s just completely not clear. There was certainly no thought in my mind when I wrote this letter that they were positive electrons. But as I read it now … Anyhow there was no thought in my mind when this letter was written that there might be positive electrons.
When you do refer to the possibility of it being a proton, you put proton in quotes. That’s very interesting.
That means, I think, that it is not definitely known not to be an alpha particle. My thought then was that it was a proton or perhaps an alpha particle. One statistical quirk is that in these very early photographs I happened to get a fairly substantial fraction of particles which showed an ionization above minimum. This I think was a statistical fluctuation because the next thousand exposures and so on gave fewer of these clearly heavily ionizing particles which must be protons or alpha particles, conceivably higher mass particles, but there was no evidence that there was anything greater than either protons or alpha particles. So that sort of set us thinking along the nucleus proton alpha particle thing -- perhaps more than we would have been thinking had it not been that out of the first few pictures there was this fluctuation in showing more heavily ionizing particles than the normal photographs showed. I remember this point very distinctly from those days.
There’s an interesting statement you make in the letter. You’re sort of summarizing. You say, HA word or two now about some new facts which stand out quite definitely: (1) The presence of positive particles as well as electrons indicating nuclear disintegrations by cosmic rays.” Perhaps this simple statement is the beginning of the whole field of inquiry into the nucleus for you in a sense.
And the next one: “The very frequent occurrence of simultaneous ejection of electron and positive particle.” The germs of the thing which followed are all in here.
I say this may well be true in nearly all cases, and 3) simultaneous ejection of three particles in at least one instance.
You mention in the letter: “These data have all been obtained in the last very few days” -- that is, the study of the photographs. The photographs themselves were taken earlier?
This was done here?
Oh, yes. The photographs were actually taken in the last few days because the measuring technique was very, very primitive. It was merely to scratch on some pieces of plastic, I guess, arcs of circles of different radii and putting them down over the photographs and in that way measuring the radius of curvature, and then it was trivial computation to get the momentum of the particle. For any particle of charge one, the radius curvature in the known magnetic field was a measure of the momentum of the particle. Then on the assumption that it’s an electron or a proton, one could calculate the energy of the particle.
Then had you been at Pike’s Peak earlier in the summer?
Oh, that was years later. We didn’t go to Pike’s Peak until 1935. That was four years later.
Then with encouragement and your own confidence that you were after something of great interest, or, as you put it, it promises to be a fruitful field and no doubt much information of a very fundamental character will come out of it.” And then you indicate: “In hope in the near future to get pictures of better resolution and clearness.”
I don’t know what field strength was here, but I say, “It’s a simple matter to work at higher fields.” Oh, 12,500 gauss.
Yes, you say that in the first paragraph. Then you continued this work from late 1931. I imagine you continued to get results culminating in what has become the famous photograph in the summer of 1932. Is there much to say about this in-between period?
Well, I thought I saw a mention in this letter. Yes. Picture #1. It says, “One track of slight curvature [which would mean high energy] -- direction of curvature that of a positive particle moving downward or an electron moving upward. The latter seems unlikely (in view particularly of pictures 2 and 3) so that a reasonable interpretation is that this track represents a nucleus, perhaps an alpha particle or a proton ejected from an atom of the atmosphere or from the metal parts of the apparatus moving downward.” Now, it would have to be a positive particle if it were moving downward. This was known from the direction of the field. But it is interesting that I did mention even in these early days the possibility of an electron moving upward. Now, in the next series of photographs, as I previously mentioned, there were very few particles showing an ionization greater than the minimum, as one would have to expect in a certain energy range if they were protons or alpha particles. So the question then occurred … Well, there seemed to be too many upward moving electrons, and I would tell Millikan, “They can’t be protons. They’re minimum ionization, and a proton should have one and a half or two times the ionization or something like that that would be clearly seen on the photograph.” And he said, “Everybody knows that cosmic ray particles go down. They don’t go up except in very rare instances.” So then I got the idea of putting in a lead plate across the center of the chamber to distinguish between upward moving and downward moving particles, whatever they might be.
Do you remember how that idea came about? Was it just a logical next step in your mind or had it been suggested by something?
No, I think it was just an obvious thing to do. Some of the particles showed enough curvature so that a reasonable thickness of lead, a small thickness like a quarter of an inch, should cause it to lose enough energy so that one could unambiguously tell which way it was going. Now, this was not true of many of the almost straight particles. First of all, it was very difficult to measure the curvature at all, and clearly impossible for the high energy particles to measure a difference in curvature, radius of curvature below the plate. But there were many of the intermediate range where this was a factor. I mentioned one particle in this letter whose energy was -- what was it? -- 5 1/2 million and then there were many, at 75 million and 50 million and so on. A quarter of an inch of lead would easily allow one to determine unambiguously the direction of motion of the particle and therefore the sign of the charge.
And so then you did this with the lead plate in. Do you remember when you first introduced this? I don’t mean the very moment, but the month.
No, I can’t remember the month. Even before the lead plate was put in, there were cases that indicated the existence of positive electrons, where one had a group of, say, three particles clearly coming from an origin and therefore where the direction of motion was known, because one couldn’t have particles that agreed to have an appointment at some place in the future, and so the direction was clearly known, which showed particles of positive and negative curvature great enough so that if they were protons -- that is, the positive ones would have shown ionization clearly, observably above minimum.
Now, in this period you’re gathering more and more evidence by eliminating other possibilities.
Yes. The main point was the existence of particles of a high enough curvature so that they clearly could not have the mass of a proton, and the only other mass that anyone could possibly think about in those days was the mass of an electron. In other words, they didn’t ionize enough to be protons, and therefore it looked like positive electrons.
Now, had you known of any theoretical predictions?
Yes, I knew about the Dirac theory. I didn’t know it in its details, but I knew that there were in this relativistic equation these negative energy states, which he interpreted … Or the absence of a particle in the negative energy state he interpreted as a proton, but with difficulties in the theory that apparently one couldn’t have a different mass for a whole particle than the particle that was in the positive energy states. But I was not familiar in detail with Dirac’s work. I was too busy operating this piece of equipment to have much time to read his papers.
Did it occur to you while you were doing this work … Of course I’m asking you in retrospect: at the same time you were doing the work did you know about Dirac’s paper? That’s one question. The other question is: Did you think about this paper while you were doing the work?
Not to any serious extent, no. I don’t know whether the existence of Dirac’s work had any effect at ail on the work I was doing. I was looking at the cloud chamber data and going by that. I don’t know the first instant when the idea of a positive electron was thought of, and I don’t know when the Dirac relationship to the experimental work was first thought of. The Dirac work was not an important ingredient in deciding which way the experiments should be carried out or what should be done experimentally.
The purpose of the experiment in the first place was completely independent. I was just asking whether once you had got results which sounded as if they might fit in, whether there was a connection? Your answer is clear enough -- that you can’t think of an obvious connection. Then you mentioned in your paper published in 1932 the definitive results based primarily on the very solid interpretation of …
Yes, then that one picture that you refer to came along, and this just said, “It’s got to be a positive electron.” I worried a great deal about the simultaneous occurrence of independent tracks, which is always a possibility -- two different electrons which happen to have this orientation -- and felt that it was so extremely unlikely because we had stereoscopic cameras and could make fairly precise measurements of the position in the chamber in all three dimensions, and the lining up was just fantastically accurate. So that caused the publication.
You mentioned in the publication that this picture has been discussed with “a whole group of men” at the Bridge Laboratory. They studied the photos. I’d like to know a bit about, that.
Well, there was no secret that it existed. I showed it to many people. The only individual I can really recall discussing it with was Charlie Lauritsen. I must have discussed it with other people, certainly with Millikan -- I guess with Neddermeyer. He was a graduate student. I don’t know just when he started working with me on research, but I guess it was in ‘32 sometime. But outside of that, I don’t remember many other individuals, although I’m sure there were many -- I guess Oppenheimer was one of them, although I can’t remember the exact afternoon or day or whenever I showed it to him or discussed it with him.
Well, we can try by talking with others to piece together that atmosphere. That’s the advantage of this approach. The editor of Physical Review wrote the abstract of this paper. Was this usual?
The first publication was a note in Science -- I guess with no photographs. And the Physical Review publication was a few months later. You say the editor wrote the abstract.
Yes, I was wondering why that was. It had his name on it. There was the abstract and then it said “Editor.” I think it was unusual. What was the response then at that time? You discussed it with individuals. And once you had made up your mind that this was it…
I think that there was very little reaction to the article in Science. Again, I have trouble recalling details. I’ve heard it said that Joliot, for example, didn’t read Science or didn’t see the paper. He had an experiment going, maybe the repetition of the Bothe-Becker experiment at about the same time, where there were many positive electrons produced, primarily through the radioactivity, induced radioactivity which had not as yet been discovered at that time. Also, there was some disbelief I think. People just thought the magnetic field was reversed or something was wrong. This is hard for me to assess. I don’t know what each individual really thought. I think it’s safe to say there was not a very substantial reaction. I don’t think people were sold on the existence of positive electrons in any widespread way. But again this is difficult to say.
I notice that the article in Science was preceded by an article in the Physical Review on energies of cosmic ray particles.
There was no mention of positive electrons.
Although you were working on it at the time?
Well, obviously you were working on it. Then the Science article and then the Physical Review article following -- which was what date? Oh, you didn’t publish in the Physical Review until 1933.
I see. You had the results in the summer and you had this photograph that you had discussed with a whole group of men. You were convinced and it was the culmination of an attempt to clarify the results that you had achieved as much as a year earlier. Why didn’t you publish it in the primary journal of physics?
I don’t know. I think I would have. It was Millikan who suggested it be sent to Science because one could get much quicker publication. I just don’t know. I hadn’t published very many papers by then. I must admit that I was not a hundred per cent sold on the validity of the data when the article was written, but a month later or maybe two months later I’m sure that there were days when I was worried about the thing because there was a period there when not very much new really came in or any other cases that were as convincing as this. Now, the details of this I don’t know.
Then when you did publish in Science, evidently you continued to work. It was reinforced in the Physical Review when it was published with the photograph. Meanwhile, you were on to more work that Neddermeyer assisted on -- in this case, the direct proof that gamma rays from thorium C double prime produce positrons.
That was published in 1933. I wondered when that started.
What we did was to get some thorium C double prime and put that above the cloud chamber and reduce the magnetic field down to a value appropriate for working with the thorium C double prime secondary electrons and put in a plate of aluminum to prove that positive electrons were produced by gamma rays. This was scrambled up -- this whole business -- with the follow-up of the Bothe-Becker experiment. You remember the Joliots got into this work -- I can’t remember just when -- and they thought that they had photon energies of something like 50 gamma rays of 50 mev energy produced by the impact of alpha particles on beryllium, which Chadwick showed were neutrons rather than gamma rays and the energy was not 50 million but was of the appropriate amount that one would expect from a nuclear interaction like this of the disintegration of beryllium of the order of 5 or 6 million volts. They were also present from the radio-activity gamma rays, but in small intensity. But then also in that experiment the induced radioactivity was being produced, so positrons were being produced. And I believe the Joliots found positive electrons produced as a result of the radiation -- gamma or neutrons-produced in some way from the Bothe-Becker experiment that they were doing.
You published in the spring of 1933, and Meitner and Philipp had worked on this, too, I guess. All of this was in the same period.
Yes. They produced positive electrons in the summer or late spring of ‘33.
I think the date of publication is the spring of ‘33.
We must remember that Blackett and Occhialini were in here, and they published a paper also in the spring of ‘33.
Showing pair production.
In which for the first time, as far as I know, the idea of pair production was clearly brought out. I very well remember reading that paper and being wholly convinced on the first reading that this was the proper explanation.
That would link in your mind then with the Dirac theory. But of course you had already published, but this would make you feel better about it. Who thought of the name positron? It’s an obvious contraction of course.
I think that’s a very poor name. It was first thought of by a man whose name I can’t think of that was editor of Science Service I think.
Watson Davis. He sent me a telegram suggesting positron and negatron and wanted to know what I thought of it. I should have thought about this longer than I did, but I remember the occasion very well. I was playing bridge, and I don’t play contract bridge except on two or three occasions when I’ve gotten involved, and I’m not at all experienced. And it was during that bridge game that I was trying to think how to reply to Watson Davis’s telegram. I don’t remember whether it was done the next day. It probably was. I wired back saying, “Okay, this may be a good idea.” But this is a long subject in itself.
I think it’s quite interesting. I’d like to take the time if you’d like to talk about it.
Well, the ambiguity or the vagueness comes from the fact that the word “electron” was such an old well-established word and was clearly the name of that particular particle — the negative electron. Nobody could change that. On the other hand, since positive electrons exist, then if they're to be called positive electrons, then the others are obviously negative electrons; but maybe if you don't put in an adjective there, the word “electron,” as is the usage today, implies a negative electron. On the other hand, if you are doing experiments in pair production or working with pair production, you like to say “pair of electrons.” This could go on and on.
You feel that it's sort of asymmetrical.
Yes, it's part Latin and part Greek — the roots of positron.
That's a good answer to that question, but I had another answer in mind. That is, that since pair production shows a certain type of symmetry in nature, the word itself should reflect this.
And the very idea of particle and anti-particle. Maybe a better word would have been “anti-electron” — electron and anti-electron. And that would have been quite consistent with present usage of these — like proton, anti-proton.
I think of the title of Millikan’s book, Electrons, Plus and Minus, as a more symmetrical way to handle it. I just wanted to clarify that. I think it’s of historical interest — how names get established. Going back to thorium C double prime, who in your recollection was the first to get the gamma rays from it because there were so many people working on it?
Well, my recollection is that we were the first ones to use a clean source of gamma rays, like from thorium C double prime, where there were no neutrons involved or other things. And I don’t remember what Meitner’s and Philipp’s source of radiation was now. I think it was alpha particles or beryllium, but I don’t remember.
That’s very similar to the Bothe-Becker …
Yes. I think people would expect gamma rays to do this and not expect neutrons to do it.
On the pair production, did you start looking for pair production or was that, too …?
This letter [you mentioned earlier] mentions cases of the production of pairs and even in one case simultaneous occurrence of three particles.
So it was a question of them presenting themselves to you and your pursuing it, but it wasn’t a question that you were looking for pairs anymore than it was a question that you were looking for positive electrons.
No, the idea of pair production didn’t occur to me at all.
Now, of course, everything becomes a pattern. But at the time were you aware of Blackett and Occhialini’s work? Were you in communication with any of these people?
No, not in connection with positive electrons. We knew that they were building an apparatus to study cosmic ray particles, cloud chamber apparatus in a magnetic field. We did not know that they were using the geiger counter triggering mechanism, which was a big step forward technically. I mean it multiplied the amount of data gathered per hour by a factor of 50; I don’t know exactly but a very large factor. We did not know they were doing that until publication in the spring, although again (I think this is an interesting historical note) Mr. Mott-Smith, before Blackett) and Occhialini used the geiger counter technique, had rigged up a pair of geiger counters and put a cloud chamber in between to trigger their light source. But they did not get the idea of triggering the cloud chamber itself, so that particular thing of triggering the light source didn’t add anything to the apparatus. So then the minute we learned that Blackett and Occhialini had this counter control, we immediately set about building one. In fact, Bill Pickering built the electronics for the first geiger counter control of our cloud chamber.
You said when you “first learned.” How did you first learn? Through a publication or other communication?
Through a publication.
Had you had any private communication with any of these people, other than Millikan, of course?
I don’t think so.
Did it go through Millikan perhaps? Was there any tradition of Millikan in his travels picking up news and passing it on through letters or …?
I suspect he would report anything he saw. My memory now is that the first time I learned about the geiger counter control in the cloud chamber was from Blackett and Occhialini’s publication. I don’t think that we knew about it from any other source, although I could be mistaken, but I don’t think so.
I notice that the Carnegie Institution of Washington supplied a good deal of the financial support for a lot of the cosmic ray work. From my study of the Millikan correspondence, it was through his direct reporting to them of the results here that he kept contact with them and got more money. Now, did you have any role in this in obtaining the funds?
No, Millikan handled that himself. But we must remember that Millikan was working right along with the electroscope technique in continuing observations on cosmic rays with a number of students, and then Victor Neher got into it and worked with Millikan for many years, and is still doing it now.
There’s a good deal of correspondence signed “Victor” over this early period in the ‘30s in various parts of the world reporting on progress of things. Well, let me ask you about the in-between period. By saying “in-between,” it’s obvious that I’m thinking of the mesotron work and its final publication. What happened after you had published this article and then did pair production work in ‘33? Apparently in ‘34 you started related work.
Well, there was no hiatus. We kept collecting more pictures and trying to improve the energy measuring capability by cutting down distortions in the gas and sharpening up the tracks and improving the light source -- higher intensity, shorter time after the passage of the particle and so on. And then did make energy measurements of the spectrum as we saw it in our chamber of the cosmic ray particles and did get interested -- and just how or why is vague at the moment -- in the penetrability of these particles, and we did put in (maybe it was the same lead plate in the cloud chamber) and we got some statistics together on the energy loss of particles in this lead plate. Now, at about this time -- I think ‘33 or ‘34 -- Bethe and Heitler were studying the penetrability of electrons from the theoretical point of view and calculated that Bremsstrahlung should be a process that made these particles highly absorptive. Bremsstrahlung would make these particles lose energy at a very, very large rate compared with what we were observing in the cloud chamber.
Then there was the question of how valid their calculations were for the energy loss by Bremsstrahlung of an electron at the higher energy range. And one way out was to say, “Well, this process didn’t occur or tapered off and that very high energy electrons were highly penetrating.” Street and I guess Stevenson also got a meter of lead or something like that and put geiger counters above and below it and showed that particles went through that meter of lead. Well, that was absolutely impossible. We had statistics that we reported in the 1934 meeting of so many thousands of traversals of particles through this lead plate, with apparently nothing happening except we lost by ionization, which was very much less than the theoretically predicted loss by Bremsstrahlung. So there was a period then when it was thought that maybe the theory wasn’t any good -- at least these particles, whatever they are (and they were positively and negatively charged particles with minimum ionization, and ail that we knew of were electrons)… So there was a paradox. We talked for a while of red electrons and green electrons. The green ones were penetrative ones and the red ones were stopped. Also, there was clearly the existence of these high energy electron showers, which could be interpreted as successive pair production events in a cascade and so on, which indicated that at these high energies the electrons did radiate to a large degree. Then there was a period of checking experimentally to see whether the Bethe-Heitler formula was valid to as high an energy as we could go. And clearly if that were valid to indefinitely high energies or to energies of the order of several hundred million volts, then these penetrating particles could not be positive and negative electrons. They had to be something else. So that the idea of the meson penetrating particles was sort of present for a long time. If you read the footnotes, I think, in this 1934 paper, you can see that this was the struggle -- the big problem that Neddermeyer and I had to try to understand what was going on.
By the way 1934 seems to have been a particularly productive year. You had nine papers in that year.
I didn’t know I’d ever published nine papers.
I’m just trying to think which paper of the nine you’re referring to.
The one I’m thinking of was to this international conference in London.
“Fundamental Processes in the Absorption of Cosmic Ray Electrons and Photons.”
That was soul-searching around this paradox of the existence of penetrating particles and the existence of showers and the theoretical work of Bethe and Heitler, which indicated a very high absorption on the part of high energy electrons because of the Bremsstrahlung process. So then our thinking was that there seemed to be here particles which were not positive-negative electrons, but which nevertheless had a unit charge, and that the key to the whole thing was to find out whether highly energetic electrons were in fact highly absorbable in a heavy but high Z material like lead, or were highly penetrating like the penetrating group of cosmic ray particles. And one of the things that we tried to do was to find out the mass of these penetrating particles by studying the distribution of the knock-on electrons that were produced, and this would again give … In principle you can do this, you see, but experimentally it’s very hard because it has to do with the difference between the energy and momentum of the particle -- that is, the rest mass. But these are so energetic that the rest mass is only a small fraction of the total mass or the total energy of the particle. So it was in that ‘34 paper we have some … No, that’s another later paper. I guess this experiment was not done at that time.
This is quite different from the idea that there’s something that behaves like an electron or like a proton in your earlier work.
Well, we did a lot of work to try to sec if they were protons, too. (I think this was in the ‘36 paper) by letting them go slant-wise through the atmosphere and trying to look at the tail-ends of these particles. You see, a high energy proton and electron would look alike so far as ionization and curvature are concerned. But we put the plates slant-wise and looked at the particles coming in those that had passed through a larger thickness of atmosphere and hence would lose more energy by the time they got down to sea level.
What I’m getting at here is the difference in this approach, where you suspect from the first that this may be a particle that is different from an electron and a proton, and so you pursue some kind of a new …
There was a real paradox.
But in the positron work there seemed to be a difference. You were not looking for a new particle.
I think one difference is that with the positron work there was much less data available. Now with the meson work, there was a great deal of information. There was no question at all but that there were highly penetrating particles, both positive and negative, of unit charge. There was no question about that. There was no question about the existence of these bursts or electron showers, although the theory of the production of electron showers had not been worked out in ‘34. That didn’t come until ‘36 or ‘37. But there was also the existence of that highly absorbable component of cosmic rays that Millikan and Neher were working with and going up in the atmosphere. So there were highly absorbable particles that couldn’t be due to a difference in energy. The only way you could get out of the paradox was to say that electrons were highly absorbable up to an energy of about 2- or 300 million electron volts, and then this absorbing process died out and disappeared or it became much smaller at high energies. And that energy range had to be in the 3- or 400 million volt range. There was that possibility. That was the only way out of saying that these penetrating particles were different kinds of particles. So we tried to follow this up, to work with higher and higher electron energies. We then set out to find experimentally how electrons lost energy, whether they radiated or not, to as high an energy as could go.
So another factor is introduced here -- that the needs for higher energies become apparent in your experimental program.
Well, we could only measure energy loss up to a few hundred million volts. Technically, to measure an energy loss, you have to measure the difference between two curvatures; and if that difference is small, you have to know the curvatures to some precision. So the main reason for taking the equipment to Pike’s Peak was to get up into this softer component of the radiation where there should be present many more of these electrons in the proper energy range to work with. And of course it’s true. I mean you get up into an altitude of 14,000 feet and the soft component is very, very much stronger there than it is at sea level. At sea level it’s mostly what we now know to be mesons that one observes in an apparatus like this. Have you seen a copy of a letter I wrote to Millikan from Pike’s Peak?
No, I haven’t come across it. In ‘35, would it be?
I have a distinct memory of writing a fairly detailed letter in which I point out the strong evidence for the existence of intermediate mass particles.
I’m going to work on the papers some more later this afternoon and tonight.
I’m quite sure I don’t have a copy of that letter. In fact, I don’t think any exists. I just sat down and wrote it.
Probably under those circumstances. What about the circumstances at Pike’s Peak? Can you tell a little about that? There were you and Neddermeyer. Were there other students up there?
No, he and I went up. We were short of funds. I can tell you how we got together our traveling equipment. We bought a used trailer from a used trailer lot. It was a flat bed trailer, and I happened to remember that an alumnus of Caltech was with the Beacon Storage Company, so I called him up and said we wanted an enclosed van type trailer, so he gave us some big packing boxes, and Neddermeyer and I built the enclosure for the trailer. We designed the hitch. We got a ‘33 or ‘34 used Chevy one and a half ton truck, and on that one and a half ton truck, we put a Cadillac engine that was rigged up to drive a generator, to get power; and we got together some great big heavy steel tanks to store water in for cooling the magnet -- borrowed those from Jesse DuMond, and then installed a magnet in the trailer and set out for Pike’s Peak. We had -- I can’t remember whether it was over ten thousand pounds or over ten tons gross with a one and a half ton truck. I don’t know if you know Lake Avenue from California to Colorado. There’s a slight grade there, very slight. That was a very stiff second gear pull for our equipment.
That was just starting out.
Well, there were some interesting experiences, but it finally arrived at the top of Pike’s Peak -- not under its own power. There was a private toll road in those days. The company that operated the toll road had trucks and they pulled us up to the top.
You camped out there.
No. We lived with the road gang at the half-way point. Echo Springs? Well, there’s a clearly defined half-way point on the mountain road and there’s a bunkhouse there, and we lived in the bunkhouse.
What was the physical layout and what was your routine?
On the summit was a restaurant for the tourists. There were these motor tours. Quite a lot of tourists up there went up the mountain from Colorado Springs. There was this cogwheel railroad that was operating, and our equipment was parked up there, and there was a large water tank. We got a certain amount of water from them which we recirculated through the equipment; had large tanks with enough capacity so that we could run several hours. We used the water. But we spent our nights at this halfway point whose name I can’t remember.
But you’d be up there during the day.
Oh, yes. We worked long hours and well into the night often.
The equipment itself is described in detail in the papers?
Not in any great detail, no.
Is there anything that you can perhaps add?
I don’t think so, of any significance. It was essentially what we had used here except that we had to supply our own power. There were many troubles, about operating at 14,000 feet with this Cadillac engine but those are details.
How long were you up there?
I think we arrived in late June and spent July, August and most of September -- I can’t remember the exact dates.
Were you married then?
So there was no problem of taking your family. Did you interpret anything until you got back?
Yes, we could to some extent. We developed pictures up there. We had to as a control to see that things were working all right. And we did find some cases of heavily ionizing particles that seemed to have an intermediate mass by our rough measurements. It’s those that I mentioned in this letter to Millikan. We were interested in getting as many photographs of electron showers as we could in order to learn about the energy mev loss of electrons in this range of a few hundred mev. Now, those measurements -- the energy loss measurements -- were not made up there. But I think we developed and scanned practically all of the photographs that were taken, maybe not every roll.
And then you came back …
Late in September, I believe, of ‘35.
There are no publications in ‘35. You had a very active period in ‘34 and then you were away a good deal of ‘35. The first paper is 1936 and that is on cloud chamber observations of cosmic rays at 4300 meter elevation and near sea level. Is there anything in ‘36 that was published? It may have been submitted in ‘35.
I doubt if it was. The interesting thing about that paper is: We present ah sorts of evidence to show that these penetrating particles are not electrons -- by the energy loss measurements of electrons. I think it’s in that paper. And we presented other evidence to show that they’re not protons. Then we leave it there. Those are sort of two conclusions but they’re not clearly spelled out.
What did you feel they were?
Oh, that they were intermediate mass particles.
The next question is: Did you know about Yukawa’s predictions?
You didn’t know about them. In the other case, you knew about the Dirac thing but that didn’t affect your work. In this case, you didn’t know about Yukawa’s.
I didn’t know about his predictions until after we published the paper in the spring of ‘37, I guess, where we came out flatfootedly for the existence of intermediate mass particles based on some further work on the energy loss of electrons. That was still the key to the whole matter.
On your bibliography that paper would appear to have been published in 1938.
It must be a mistake then on your bibliography. “Cosmic Ray Particles of Intermediate Mass” was published as a letter to the editor -- Neddermeyer and Anderson, June 16, ‘38. Here we are. You refer to an earlier paper, which is not on your bibliography, but a paper published in 1937, and you say, “A discussion of certain fundamental difficulties with identifying the penetrating component with either electrons or protons was given in the former paper” also by Anderson and Neddermeyer. The former paper refers to the 1936 paper. But anyway you’re talking definitely of a 1937 paper.
The first discussion of new particles outside of a seminar which I gave in ‘36 before going to Sweden …
This is November of ‘36 somewhere.
Yes, it was not published, but I gave a seminar on intermediate mass particles. It got in the newspapers, but there was no publication.
The seminar was to people at Caltech?
There was something in the newspaper that I’ll ask you about later from April …
Okay. Well, I put in a sentence in my Nobel address about this thing.
About the colloquium?
No. It was about the positive electron, but I put in a sentence about the highly penetrating positive and negative particles of unit mass that were not electrons and that would be an interesting subject for future study or something like that. Nobody reacted.
It would be interesting to find out while we’re at it your reactions to winning the prize. This was in the period when you’re doing exciting new work, and when did you first realize you were being considered for the Prize?
When I was told of the award.
You didn’t know that in previous years you had been considered or that Millikan had recommended you for the Prize earlier.
No, I did not know that. He didn’t say anything about it.
This comes from the correspondence. And so you were notified. It’s just nice to get on the record how an individual would react to that.
Well, I was very surprised naturally. I don’t know what else to say about it. I think this was a situation in which an accidental discovery was recognized rather than a long period working on a problem over many, many years, such as is the case of many other years when the Award was given. I think the neutron was again a case of working for a very short period of time on a problem. My own guess is that Chadwick, when he read Curie-Joliot’s paper, was 99% sure those were neutrons. But he set up his own experiments, and they were beautiful, very convincing experiments. I think that paper on the neutron is a real classic in physics. But it was work that was done in a relatively short length of time on this particular problem. I’m sure that Chadwick perhaps in years past had thought about why aren’t there neutrons, so he was fully primed and ready to go on this sort of thing.
But on your own reaction to it -- you certainly knew the significance of the work by 1936.
You were fully aware that it was a major thing, and of course it affected all of physics. And so, had it occurred to you that this is of Nobel Prize stature?
I can’t swear that for a short period of time this possibility may not have crossed my mind -- but it certainly was a bolt out of the blue when it happened.
And you went to Sweden. You interrupted your research for a good reason. But before you went you had this colloquium on positive and negative particles of intermediate mass. You continued work on that and you published for the first time on that really in your Nobel address in a sense.
The real first publication was in the spring of ‘37. We hemmed and hawed about it in the ‘36 publication, and then I gave a colloquium in which this was brought out. But the first real publication was …
I found in the Millikan papers some clippings -- one from the L.A. Times on April 25, 1937; the other one is the Pasadena Post on April 26th. The headline on one is: “Cosmic Ray Secret Believed Found Caltech Scientists report discovery of a new form of matter vital to physics.” I’m trying to determine: Did they get this from Caltech people? Had it been published by April?
I don’t think this precedes the publication date if that’s what you have in mind.
This one is clearer perhaps. The Pasadena paper begins: “Eyes of the scientific world were fastened on Pasadena’s California Institute of Technology last night as discovery of a new atomic particle by youthful Nobel-winner Dr. Carl D. Anderson and Dr. Seth H. Neddermeyer was held by colleagues as one of the most important findings of the country” -- what they meant was “century”, I guess –- “in the field of physics.”
That was Caltech hailing it.
It says here, “The discovery was announced yesterday at Caltech.” And the news that you and Dr. Neddermeyer were on the trail of an unknown particle became public some months ago, but at that time the exact nature of the particle had not been discovered.
Well, we were both very hesitant to talk about it. We couldn’t talk to reporters.
Then of course once it was announced for scientific publication, then the public announcement was made. In that period it was called “particle X.” I was surprised. Did you call it that? I noticed that in some of Millikan’s letters he …
People called it by various names. “Heavy electron,” which is the most accurate description of it in the light of present day knowledge because the Mu-meson is just a heavy electron. It was called “heavy electron.” It was called “X particle.” It was called beritron. There was a meeting in Chicago. No, this precedes that.
The Chicago meeting was ‘39.
Neddermeyer and I wrote a letter to Nature where we suggested that the name be mesoton -- meso for intermediate. And that was published in Nature, with a revision. Millikan was away and I told him we had sent off this letter to Nature suggesting a name for this particle: mesoton. He hit the ceiling. He didn’t like this name. He said, “There should be an “r” in it. It should be mesotron. Look at electron and look at neutron.” I said, “Look at proton.” Well, so we cabled the “r.” I think the word “mesoton” might have stuck. Nobody liked mesotron. It’s as bad as positron, I guess.
But it was used, I guess, for a long while.
It was used for awhile and then somebody shortened it to meson. And then later there was the pi and the mu.
In 1936 there were private communications that you have referred to in print to Oppenheimer, Carlson, Bhabha and Heitler in May, 1936, on the results of the energy loss measurements. Do you recall that and what type of replies you received and what type of discussion went on?
They were not detailed letters. We were interested in the energy loss of electrons, and they were working on the cascade shower production theory. Now, that theory assumed, of course, that Bremsstrahlung was a process that continued to take place at indefinitely high energies. And we were not really convinced ourselves that this was in fact true. We were overly cautious, I guess, because large electron showers were known to exist. At that time it was not known just how large they were, although there were bursts in electroscopes which clearly we know now were electron showers; and our Pike’s Peak pictures showed electron showers by the thousands because we were at this high altitude. Blackett and Occhialini had many electron showers. But it was just that one remote possibility that made us hesitate on the new meson. I guess, in looking back, we were much too cautious and conservative in this thing, but that’s the work that this has to do with. Carlson and Oppenheimer and Plesset and Oppenheimer I think worked on it, and Bhabha worked on it.
You were in touch with Heitler. That’s understandable.
Bethe and Heitler did the really early pioneering work on the Bremsstrahlung production by electrons. The shower work was just to use that fundamental idea.
When you communicated, did you write to them -- you refer to private communication? Or did you see them at a meeting?
I may have seen them at a meeting. I just don’t know. I have a vague memory at some time or another of writing to Bethe. I don’t know in what detail this was, but it had to do with this paradox of penetrating particles versus shower-producing particles. But I can’t recall specifically what that was about.
It’s interesting that at the Chicago cosmic ray symposium of June, 1939, you presented a paper with Neddermeyer that was ultimately published in Reviews of Modem Physics.
Oh, that was a sort of semi-review thing.
But there’s an interesting statement. You said, “Further studies are needed to determine whether mesotrons can be identified with the particles postulated by Yukawa in 1935.” And this is 1939. My question is: Didn’t people think they were the same as early as you announced them?
The first reaction was: Of course these are the same particles.
Was that your first reaction?
When I heard of Yukawa’s work, which was after our ‘37 publication on mesotrons, yes, obviously. But there were troubles that gradually became apparent -- namely, that the mesotrons here were highly penetrating. They would go through large quantities of lead. They would go by the thousands through our plates without doing anything. And Yukawa’s particle had to do something if it was to explain nuclear forces. It had to be a strongly interacting particle and not these things that didn’t do anything. Now, as I remember, there was no real quantitative study of this. There may have been. I’m sure there were numbers around, but it was sort of, “Well, new mesons don’t interact strongly.” And the key experiment that proved this and at least sold this idea to the world was this experiment during the war, I think, in Italy by Conversi and Pontecorvo –- I can’t remember them. Anyhow they had a magnet and collected mesons and showed that the new mesons and cosmic rays did not interact strongly. Therefore, they could not be the Yukawa particle. And then it wasn’t until the pi was discovered in ‘47.
But I saw some reference to some work that Tomonaga had done in Japan during the war, too, which began to postulate that this was not really what Yukawa had talked about. Of course that didn’t come to light until later.
Yes. Now, when the war carne in ‘42, I guess we quit everything. The war years so far as cosmic rays were concerned were a complete blank to us. I didn’t read any papers or think about mu mesons.
Let me ask about an important thing that I sort of skipped over, and that is the final verification to satisfy you and your colleagues that this in fact was a particle of intermediate mass? We got almost to that point and then we began to talk about the culmination of it.
I think by the spring of ‘37 Neddermeyer and I were completely convinced of the existence of intermediate mass particles, and there was a question as to their mass -- what the mass might be? If they were of intermediate mass, did they have a distribution in masses or was there a unique mass, and if so, what was the value of that unique mass? So we set about to try to measure the mass of the mesotrons, as they were called in those days. And this reprint that you have, the ‘38 thing, has to do without putting a geiger counter inside a cloud chamber. That’s an ideal case for measuring the mass because this is low enough energy so you have a good measure of the energy here, or the momentum, and a very good measurement of the range. And from those two you can calculate what the mass is. I forgot what we got: 240 electron masses. We later broke this geiger counter -- we had estimated the thickness of the glass. We later broke it and measured the thickness of the glass and revised that to 240 as against 209, the right value. Now, there were many attempts to measure the mass, mostly by curvature and ionization in the cloud chamber, and there were figures published from 30 or 40 electron masses all the way up to a thousand and all kinds of figures. I think this is the first real measurement of the mass.
When you published the ‘37 article, did that begin to excite attention …?
Yes. Street and Stevenson about the same time published a paper … I don’t know if you want to talk about the post-war stuff.
Yes, I think we’ll go into that, too. We were talking about the work that was done about that time by Street and Stevenson.
About the same time as this spring of ‘37 article of ours, this letter to the editor, Street and Stevenson published a letter to the editor in which they said that they had found particles of unit electric charge that were more highly penetrating than electrons should be if the Bethe-Heitler theory were valid. Now, this to my mind is not a new experimental discovery, because it was known for several years before that there were particles of unit electric charge that were much more penetrating than the Bethe-Heitler theory would permit, but the key point was: Is this theory valid at the energy range that we were working in? Now, I suppose that Street and Stevenson believed the theory, and if they believed the theory then they had evidence for intermediate mass particles. They also said they could not be protons; they were lighter than protons.
They made mass measurements, too.
Yes, they ruled out protons. The whole point was they were not electrons; they were lighter than protons; they could not be electrons because the Bethe-Heitler theory would not permit them to go through six or eight inches of lead or whatever they had. Now, our contention was that this fact alone doesn’t prove that they are particles until you prove that electrons can’t do this. And this is the point we had been struggling with for two or three years, I guess. So there is that difference in the publications. But these two papers were published closely.
Apparently, with the paper in the spring of ‘37, you knew you had it and it was cause for a public announcement. I notice that the public announcement doesn’t talk about Yukawa’s prediction; it just talks about a particle X.
I had never heard of Yukawa when this was done.
But it was sufficient cause for excitement because there was a new particle in nature established.
Sure. There were only three and these were numbers four and five. Oh, yes, there was also the neutron. I don’t know about the photon, the neutrino. Anyhow, it was exciting even though so far as I was concerned at this stage there was no theory involved in the thing.
Then subsequently, as soon as you heard about the Yukawa prediction, you associated it with this. But in this paper in 1939 at the Chicago cosmic ray symposium you raised the point that I mentioned before; and that is that further studies are needed to determine whether mesotrons can be identified with the particles postulated by Yukawa in 1935. Now, this suggests that there is some doubt. Was the doubt of the nature you expressed before?
Yes. They apparently did not interact with nuclei with the cross-section that one would expect if they were so-called strongly interacting particles, as they would have to if they were to be the carriers of nuclear force, a la Yukawa’s idea. But quantitatively, I don’t think we tried to work this out numerically; and if you ask me just exactly the instant when it was proved that these particles are not the Yukawa particles I would have to refer to these experiments during the war by the Italian group, which was somewhat later than this. And then you mentioned Tomonaga. Now, he must have had some experimental work in mind or he wouldn’t have written that …
Yes, I don’t know if it was necessarily new work. It was probably an interpretation of existing work. I haven’t seen the paper.
But the thing was sort of understood. I don’t know in ‘39 how general the feeling was one way or the other about whether they are the same particles or not.
Let me ask a general question: You started with particles of high energy involved in nuclear reactions from the early ‘30s. What did you consider yourself? If someone had asked you what was your field in physics in the ‘30s, what did you say? How did you regard yourself during that period?
Oh, cosmic rays and particles.
Did you use the term “particles” then?
Oh, I think so.
Would you have called yourself a nuclear physicist?
Yes, I would say nuclear physics. We weren’t really trying to study cosmic rays as such. We had started out to do that, but it soon became a study of nuclear physics, high energy physics using cosmic ray particles, although that was not the idea at ail in the beginning of the experiments.
Were you aware of and in personal contact with the work that was beginning to be done at Berkeley and of course here too with Lauritsen -- in higher and higher energy?
I was thoroughly familiar with what Charlie Lauritsen and his people were doing from the old treating of cancer with the X-Ray tube and the work that they were doing with the X-Rays until after the Cockcroft and Walton experiments. Then they accelerated positive ions very quickly in a few days. They should have accelerated positive ions a year before they did. And I was thoroughly familiar with what was going on at Berkeley in the cyclotron developments. I made many trips up there and was very familiar with the progress of early cyclotrons and the Sloan accelerator, the beginnings of the linear accelerator and various models of the cyclotron -- oh, yes.
And did you feel that their field of inquiry was the same as yours? I’m not trying to put words in your mouth.
Well, clearly so, except we were working at much higher energies. I don’t think I recognized clearly that they would go up and up and up in energy and that the time would come when you could so much more in high energy physics with an accelerator than we were doing in high energy physics with cosmic rays. It never occurred to me to guess what the future of the cyclotrons might be.
Did the results they were getting and the theoretical interpretations of the work and of the general developments in nuclear physics-for example, the Bethe articles he did with Bacher in Reviews of Modem Physics indicated growing interest and things falling into place in the field of nuclear physics -- did that have an effect on your work?
Well, yes. I think at that time I was teaching a course at Caltech and had to keep up with the developments in low energy nuclear physics and classical nuclear physics as well as high energy things. Is this the point that you wanted?
Yes. I think we’re on now the broader subject of the development of nuclear physics, and there were many things feeding into it, and one was the cosmic ray work and the high energy work. The other was the accelerator work, and the third was the whole new concept of the nucleus …
The accelerator work was more what we would call now nuclear structure, and cosmic ray work was what we would now call particle physics, but I think we used the term particle physics in those days, although again I’m not sure.
This is something we’re trying to dig into by looking at the publications and seeing what terms people use in the private correspondence. Then you were certainly in touch with them. What about the effect of the war? You mentioned that the cosmic ray stopped completely except for some isolated examples. What happened after the war? Was it a question of just picking up where one had left off or was it a whole new spirit, a whole new approach?
Well, I think there was in a sense a new approach. I had worked very closely with people in the Navy during the war and knew the people in ORI, the Office of Research and Invention, that later became ONR, the Office of Naval Research, and asked them if there was any way we could get an airplane to continue our work at still higher altitudes. They were receptive to the idea; and after a few weeks or so I learned that they would give us space aboard a PV-2, but which was not the kind of airplane that would be useful to carry the same old magnet cloud chamber that was built in 1930. It was ideally suited for an airplane, but we needed a B-29. So then the Navy made arrangements with the Army, and we were told that three B-29s were to be made available for scientific work at Iniokern up here, and then we did modify our apparatus and did install it in a B-29. So we did install a cloud chamber in a B-29 and did fly -- I don’t know the total number of hours, but it was probably over a hundred hours -- at as high an altitude as the B-29 would operate at, and we did take many pictures and did analyze some results and wrote an article or more in the Physical Review. But we certainly missed an opportunity here. It was a case of not thinking about physics, but thinking about how to install the equipment in a B-29 and how to get proper cooling and proper safety and handle the fairly high currents in a safe way and temperature control the equipment, and so on, rather than thinking of the physics as to what we should be doing.
Obviously what we should have done was to trigger that apparatus, trigger it by nuclear events. It was Janossy who shortly before the war had arranged a geiger counter set-up with thick pieces of lead to study nucleon-nucleon collisions or nucleon-nucleus collisions. We did not do that. If we had done that, we would have discovered strange particles probably on the first flight because the production rate of strange particles at that altitude -- around 30,000 feet where we were flying -- is around 350 or maybe 400 (I forget the number) times the sea level production rate; and we would have had several cases on each flight, probably two or three dozen. Well, this is the thing we didn’t do but the thing we should have done. Out of this B-29 work came the first measurement of the energy of a decay electron from the mu meson. I think this is the first time where one could measure the energy of the decay electron of a mu meson, and we got statistics on shower production and on energy distribution of protons and certain characteristics of the cosmic rays, but it was not what it would have been if we’d taken a half hour out to make the proper change in the arrangement of the equipment.
That’s in retrospect.
Yes, that’s in retrospect.
In ‘47 I believe was the discovery of the pi meson and the realization that in fact the pions were Yukawa’s middle particles. And what was your reaction to this?
Oh, I thought it was wonderful work. It resolved this very important paradox of course about the Yukawa particle and the mu meson, that they were so different in characteristics with respect to interacting with nuclei, but so similar in mass and in that property of them. No, that was very exciting work, the pi meson.
Was there a general feeling that way about it?
0h, I think so -- yes.
Then what did you think was the feeling just after the war about where these fields within nuclear physics were going -- where the different approaches to them would go? There were several conferences around and meetings and people were beginning to look at the future. I was just curious if you remember what the feeling was about the new lines of research.
Well, of course, nuclear physics really came of age during the war, with nuclear energy, fission, uranium fission, and so on. But our interest -- or at least my interest -- was in the particle business. There were questions about what are the decay products of the mu meson and what do they decay into, and one belief was they might decay into an electron and a neutrino. And it turns out that the first two measurements that were made of the energy of a decay electron of a mu meson were identical within the experimental errors. I remember one was 25 and one was 26 mev. Well, on the basis of these, you would conclude that decay into two particles is possible experimentally. And then it was not until ‘49 or so when it was proved for the first time that mu mesons did decay into three particles because the energy of the electron had a spectrum from very low energies up to a maximum of about 55 mev. And I think that was independently shown by Steinberger at Columbia and by Bob Leighton and me here at Caltech.
You did work in this later pod with Leighton, I notice. In some of the publications Leighton shows up as a collaborator.
Yes, he joined our group right after the war. And then about the same time Rochester and Butler published their two cases of what we now call “strange particles,” one charged one and one neutral one. And then nothing happened I think for about three years in that field until we got about 30 or 40 or 50 cases on White Mountain in California. I say “we.” This was with Bob Leighton and me and some students. If we had used our heads, when we had the B-29 this could have been strange particle research. It could have started three or four years earlier. But again it’s a curious thing: There were two cases observed and then not any more for a period of about three years, I guess.
When do you feel that high energy physics, as we know it now, became a separate discipline distinct from nuclear physics?
It always has been in my experience.
That’s very interesting.
I think the minute that there was a positive electron, for example, that it then was quite different from the ordinary low energy nuclear physics. Up until the machines got to higher and higher energies, I don’t know exactly what you’d call high energy physics, where the dividing line is, maybe at meson production.
And the deliberate attempt to build machines that will create mesons.
Right. We were not involved in any sense of the word, I think, in what you might call nuclear structure. I mean we used lead or platinum or carbon not because the nuclei were different but simply to get a high “z” and a low “z,” a high atomic number and low atomic number, and we were not interested in the details of nuclear structure.
At the same time Lauritsen was working on that, and, as a matter of fact, he used your Wilson cloud chamber in some of his early work, he told me, because he didn’t have one, and only later did he build one.
Well, he gave us some induced radioactivity sources, and we ran with those from hi high voltage tube up to the third floor of the aeronautics building and put them in the cloud chamber and measured in it in a rough way the Beta spectrum of various isotopes, like carbon 11, I guess, and nitrogen 13 and so on. Then they did that work themselves. A little later on they built their own cloud chamber.
So it was helping them out in a sense during this period when they didn’t have the equipment.
I guess so.