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Interview of Luis Alvarez by Charles Weiner on 1967 February 15,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
For multiple citations, "AIP" is the preferred abbreviation for the location.
Early education in physics, University of Chicago 1930’s; high-energy particle counter; discovery of positron; discovery of neutrons; neutron experiments; reminiscences of Berkeley; Foundation support of research; 60-inch cyclotron building cloud chambers; neutron spectroscopy; neutron time-of-flight; magnetic moment of the neutron: transuraniun elements; announcement of fission; Tizard Mission; war research work; building of a betatron; effect of war techniques on post-war research; cyclotron work 1947; impressions of present day nuclear physics 1966.
Today Is February 15, 1967, and we are resuming the Interview with Professor Luis Alvarez. How about retelling the story of those cyclotrons?
I had the pleasure of contributing something to the history of physics by finding the two 6-inch diameter cyclotrons that are so well-known now. In 1936 when I first came here, I was poking around in a room in Le Conte Hall and I found these two objects way in the back end of a laboratory drawer and I naturally recognized them for their importance. I also found three of the original Lawrence and Livingston notebooks and these fine objects are now in the archives of the laboratory.
What was in those notebooks?
Just the day by day record of the experiments that Stan Livingston and Ernest Lawrence carried out. You see the resonance curves taken by Livingston with different frequencies and voltages on the cyclotron dees and beam current plotted against magnetic field—things of that sort and comments.
I’d like to ask a few questions on the helium-3 work. You tell the beginning of the story and the steps and the conclusion in your Faculty Research Lecture. There are certain things about it though that are of special interest. This was 1939, if I recall correctly.
That seems reasonable. My dates during that period are generally plus or minus one year.
The publication was ‘39.
And the interesting thing is that this was a transition period for the cyclotron. Was it when shielding was being put up?
Well, there had been no shielding procured or designed. Ernest Lawrence had a theory that when you built a new machine in a new energy range where people didn’t know what sort of radiation came out from the target, the thing to do was to turn the machine on at rather low level and measure the attenuation of the radiation in whatever material was available. Then run the beam current up to high level just for short periods of time just to see what factor of increase in beam was available, and after calculating how much shielding was required, go out and build it and install it. In the interim one would run the beam at very low level where there was no radiation danger. The same thing happened with the 184-inch cyclotron. There was no shielding arranged for that until the machine had been turned on and the attenuation was measured by taking a detecting device and building shielding around it. It’s a lot less expensive to shield a detector with 6 or 10 feet of concrete, or whatever, than it is to put layers around the cyclotron. So this is something that people who haven’t been on the forefront of this field don’t realize, that the person who is there first has to do exploratory experiments like that, and once they have been done they are available to everyone. When the cyclotron was built for example at Chicago or at Columbia, they knew what shielding to put up, and could design this from the beginning, concurrent with the design of the cyclotron. You then could design the shielding, but you never can do that if you are first in the business.
And it was during one of these periods when shielding was being developed for the 60-inch that the cyclotron became…out of action in the sense of normal use but available for special uses.
How did you get the idea to use it as a mass spectrograph?
As a matter of fact I didn’t. My original plan, as I think I said in my Faculty Lecture, was to look for helium-3 and hydrogen-3 accelerated in the 60-inch cyclotron, and these would have been manufactured in the bombardment of deuterium by deuterons in the 37-inch cyclotron. These would be introduced into the ion source and then we would look for them and presumably watch them decay with time. If they had a one-day half-life you’d see half as much each day as you re-introduced the material into the source. But the resonance curves, the current as a function of magnetic field, had only been measured with galvanometers and detectors of that sort and there was no assurance whatsoever that there would not be particles accelerated in, say, the position where helium-3 and hydrogen-3 would come in. So the experiment at the 60-inch was designed purely as a background check to see whether it was worthwhile going on and making helium-3 and hydrogen-3 by bombardment. I had observed on the 37-inch cyclotron, particles coming out under conditions where one would guess that it was quite impossible for anything to come out, and I had explained a couple of those as being due to processions of orbits and things of that sort. So one just had to go and check; put a counter there and see whether there was an appreciable number of particles coming out in the intermediate regions, where the current dropped down so it was out of sight on the galvanometer. That’s what the experiment was. It was a complete accident that helium-3 was found.
There were two parts to that accident.
That’s right. First of all doing the experiment at all was an accident, and the second was in making observations while the magnetic field was falling, which was the real key. I spoke about that in my lecture because I was real proud of having seen that and knowing what it meant.
You mentioned that this was a good example of collaboration by using someone’s detector, I think it was Professor Segre’s detector?
That was Segre’s detector for the hydrogen-3. In the helium-3 it was my own detector that I had built myself with my own hands. That was the mass-spectroscopic detector. I used Segre’s FP54 electrometer circuit to measure the ionization current of the electrons from the decay of the hydrogen-3.
These were things that people had built for their own use but which were made available?
That’s right. As I mentioned, McMillan made his electrometer available to me, and Segre made his circuit available to me, and lots of people used my linear amplifier whenever they wanted to count neutrons.
When did this use of individual equipment change to the point where the detectors became a team effort, just the way the machine itself is?
I think before the war the detectors were still pretty much built by individuals who would lend their detectors to somebody else for an experiment, but they still pretty much thought that their detector was their own detector. I would say it wasn’t until after the war that detectors were really built by teams; they got to be pretty complicated.
Did you get any strong reactions from the theorists on the discovery of the inverse situation that Rutherford had described?
No, I think everybody felt that it was a small difference between two rather large things and it was a perfectly reasonable mistake to have made.
It was nothing basic in terms of causing anyone to revise their model?
Oh no, it was 15 kilovolts out of several million volts in binding energy: it’s several million volts and you can flip it one way or the other by a very, very small perturbation. As long as we are talking about mistakes that people made, I should point out that the first determination of the half—life of hydrogen-3, tritium, that Cornog and I made was an error. We came out with a very short life — as remember, some months — and this was because the tritium was diffusing out through a rubber tube. We had this ionization chamber that we borrowed from Segre and it had a rubber tube with a clamp on it, Cornog and I had designed and were having built in the shop an ionization chamber with glass seals for the insulators because we recognized the fact that maybe the gas could leak out. But when we saw the ionization current going down at a nice rate we naturally believed it. Then later on somebody at Illinois measured the half-life and found it considerably longer than we’d said, so we had to publish a retraction.
How much of a time lag was this?
A good many months.
The retraction was several months later?
Yes. It was one of those cases where we knew this was a source of error but we didn’t think it was the world’s most important thing.
I’d like to get on to another subject: the neutron time-of-flight method. I don’t think there’s been an adequate description of it historically, so why don’t we start with why and how you got into this and how you developed it, and also what its role has been in nuclear physics.
This is an interesting example of what you can do if you take some time to think. My father was a physiologist who had missed several important discoveries because, as he told me, he was working so hard that he couldn’t take time out to sit and think. The best piece of advice in my scientific career that he ever gave me was to say: "Just sit down and clear your mind and take time to think." And so the night I thought up the neutron time-of-flight method I was following my father’s advice. I'd said to myself, "Let’s just sit down in a chair all evening and see what you can come up with." I think I wrote in my notebook, "I'm sitting down in my chair and I’m going to see if I can think up something useful tonight." And I sat there and let my mind wander around and realized that the velocity of the neutrons was such that by using reasonable distances and the kinds of turn-on times that one could probably get with the cyclotron, one could observe very slow neutrons and ignore the very fast ones. I didn’t think of it as a spectrometer at that time but really as a way of excluding the effects due to fast neutrons. Nowadays one doesn’t think about that because there are such things as thermal columns in reactors where one has a sample of pure thermal neutrons.
But in those days thermal neutrons were always made by slowing fast neutrons down—in hydrogen for example, in water, or in paraffin wax, and there the slow neutrons are eaten up faster than fast neutrons—so any time you have slow neutrons they have to be in equilibrium with a parent population of fast neutrons. It turns out that in graphite the situation is reversed—the fast neutrons are attenuated faster than the slow neutrons, so you can bring in a beam of fast neutrons, have them converted to slow neutrons; the slow neutrons then diffuse away from the region in which they were generated and then you come up with a region where there are pure thermal neutrons.
All the experiments that people did in those days with slow neutrons always had a contaminating background effect of fast neutrons. If you put in some cadmium to knock out the slow neutrons, you only knocked the effect down by maybe one-third or something like that. I don’t remember the exact number. So it occurred to me that by using time-of-flight, we could just let all the fast neutrons go whistling by and then turn on the detector and let the slow neutrons drift in, and one would then have in effect a detector that would only respond to slow neutrons. Then I realized that you could knock out some of the higher energy thermal neutrons and get the really cold neutrons coming in, and there was considerable interest in real cold neutrons.
This was all the same evening?
Yes. I plugged the numbers in and sure enough it looked okay. Then I had to figure out how to turn the cyclotron on and off. As I told you in connection with the Brobeck cloud chamber experiment, there was no way to turn the cyclotron on and off rapidly. So I built a circuit that would turn the ion source on and off with a square wave, moderately fast, but it turned out that that didn’t modulate the cyclotron beam very well. Apparently what happened was that deuterons were made by other deuterons, so if you turned off the electron current in the ion source you didn’t turn the beam off at the target, because the beam decayed exponentially—it had to do with secondary production of deuterons. Or maybe it was just that was a poor electronics maker and the ion current didn’t turn on as fast as I thought. I thought I had an oscilloscope in there measuring it, and it looked to me like it turned off rapidly but the beam current dropped down slowly. So I said, “We can’t modulate the beam this way.” That was partly because we were using an open source ion source, a bare filament shining up into the gas of the chamber. Nowadays one has ion sources that have an arc down at the bottom and one can modulate it quite well—that is the basis for all modern time-of-flight measurements, a modulated arc.
I tried the obvious thing and it didn’t work, so then I said: “the only thing I can do is to turn the voltage on and off the dees.” So I spent a lot of time building modulators that would kill the final stage in the power amplifier of the cyclotron. I didn’t have any luck. So then finally I ended up by just using the half wave modulation of the voltage on the oscillator. It was normally 3-phase rectified, or 6-phase, and I turned it over and converted it to single phase, so that there was one 60-cycle half-wave of beam and then the voltage was off on the cyclotron dees and came back on 1/60 of a second later. That was the way I modulated it and you can see that in the spaces that were available I could only concentrate on the very low energy neutrons. Modulation time was something like 1/120 of a second. Nowadays one modulates beams in tens or hundreds of nanoseconds. But the other thing was that the detectors we had in those days had a very slow response. We collected the positive ions in the detectors instead of the electrons as we do now. So the counters were very, very slow and if I had been clever enough to modulate the cyclotron rapidly I wouldn’t have been able to use it anyway.
How long after this “think” evening did it take you to get to work on it?
Oh, I imagine I was making the modulator the next day, trying it out.
And then how long before all these steps you’ve described were completed? Over how long a period of time?
Things like that went fast in those days. We worked so long and so hard that when I go through my old notebooks to see how long it took to do something, I am amazed at how fast we were able to work. So I could probably look it up. Let me go in the other room for a moment and see if I have that notebook. The first method I tried was to modulate the grids of the final stage of the power amplifier. Actually I didn’t write that down.
Now that we have the notebooks in hand, it’s clear that the day-to-day documentation of all we are talking about is available. How did the neutron time-of-flight technique then become effective; how was it first used in research?
The first real use of it was in the scattering of very cold neutrons on ortho and para-hydrogen. This was a kind of classic experiment that had been tried in a number of places and had not been successfully done. Its difficulties were just too high if you were to produce your cold neutrons by cooling down a moderator. Neutrons didn’t come into equilibrium with the cold paraffin. There were fast neutron effects that had to be eliminated. The experiment had always been done with liquid para and ortho-hydrogen, which gave problems that presumably would be eliminated if one used gaseous hydrogen. At the moment the difficulties escape me, but there were some. So everybody agreed that the experiment should be done with a source as nearly monochromatic as possible, very low-energy, slow neutrons and in gaseous ortho and para-hydrogen. This apparently couldn’t be done any place except with the modulated beam that I had available because that completely eliminated all fast neutrons. In other words, if one put a thin sheet of cadmium over the detector the counting went to zero. Nobody else had that—that’s the equivalent of a thermal column now where you have a cadmium ratio of greater than 100,000 to 1.
You started to work on the technique in 1937 according to the notebooks, and then did some runs by the beginning of the summer (June). When did this work that you just now described begin to take place?
It took place in l940, I had the thermal beam, the low-energy thermal beam, working and Felix Bloch and I thought that we would use this in the determination of the magnetic moment of the neutron. But we found out that even though the percentage polarization, or the magnitude of the Dunning effect increased as we lowered the energy of the neutrons, still the statistical errors were better if we used all of the normal thermal neutrons. In other words, we took a beating on intensity by going down to the cold neutrons, so even though the magnitude of the effect was bigger, the statistical uncertainties in the points were improved if we did not use the modulated beam. So, although we had originally planned to use it and it seemed like the right thing to use, in practice it turned out it was better not to use it. So there was a hiatus there of close to a year when we just didn’t use the modulated beam at all. It was available; had the detectors, and the ability to modulate the cyclotron, and didn’t use it.
Was Pitzer your graduate student?
Oh no, he was an assistant professor of chemistry at the time. We were personal friends and he had done experiments using liquid hydrogen and some thermodynamic experiments, so he operated the liquefier and took care of the Dewars and the conversion of ortho-hydrogen to parahydrogen. The surprising thing to me now is a remembrance of the last, the final rather, day of one run. We had a Dewar full of liquid hydrogen and we wanted to empty it and Ken Pitzer just poured it on the sidewalk right in front of the Radiation Laboratory. People don’t do things like that anymore. You are properly careful of liquid hydrogen; it’s a dangerous material.
Did you actually publish an account of the time-of-flight method as a technique?
Yes, I did, in Physical Review.
Was that in the ortho para-hydrogen work?
No, this was an article. I’ll find it here for you. Here it is, “Production of Collimated Beams of Monochromatic Neutrons in the Temperature Range 3000 - 10° K.”
It’s paper 17 in your bibliography. Did anyone pick it up after that publication and put it into use elsewhere?
After I had thought the method up and before I had actually implemented it there was an article by a fellow named Haynes, working with James Franck. I think at that time he was in the East somewhere, and he proposed the same method. I had not written anything on it. I remember writing to James Franck and telling him that I had this idea and that I was well along in the method and hoped that he didn’t think when I came out with some results that I had gotten the idea from Haynes. He wrote back a very nice letter saying no that he understood these things. (I am sure that I have that letter.) As far as I know Haynes never did anything with it.
He just published the idea of it?
Yes. People did that very rarely in those days—it was considered bad form. I remember an experiment that I was working very hard on in ‘39 to look for the recoil of an electron-capturing radioisotope under the neutrino emission. An electron is captured by the nucleus, a neutrino is emitted, and therefore the nucleus ought to jump off the surface. Before that time all attempts to look for neutrino recoil had been done in beta decay, where the recoil spectrum goes from zero up to the maximum depending upon how the energy and momentum are shared between the neutrino and the electron in beta decay. It occurred to me that in the case of the electron capture all of the recoil energies would be identical, because the neutrinos all had the same energy and there was no beta ray to share in the decay momentum. And so I said, “Here is a really good way to look for neutrino recoils.” Nobody had done a neutrino recoil experiment adequately; there was no experiment that you could point to and say: “Look, this proves that there are neutrinos because the distribution of recoil momenta does not follow the distribution you would expect from the emission of the beta decay.” In other words, the two things were mixed together and it was hard to observe the recoil at all, let alone try and measure the recoil spectrum. Again we didn’t have time-of-flight methods that would handle those velocities in those days. So it occurred to me that one could do it using an electron capturing isotope, and then one could just apply retarding potentials and all of the things should cut off at the same energy—in other words they all should have the same velocity.
So I didn’t write up anything on this; I just started building the equipment. I built it in the old Radiation Laboratory building with a lot of other equipment for other experiments until finally a graduate student, Carl Helmholz, came along and took the the thing over. But I had actually built a lot of the original equipment myself, blown the glass, and put the thing together. This experiment was actually completed after I left for MIT and was published by Carl Helmholz, Byron Wright, and my name was put on in absentia. But the thing that disturbed us was that a theoretical physicist—I think his name was Wang—published a suggestion that this experiment could be done, and we thought that this was kind of dirty pool. Nowadays this is a standard thing; somebody has an idea for an experiment, and he writes up a letter to some journal called, “A Proposed Test for Such and Such a Theory.” It was interesting how upset we were that somebody would sort of violate the social customs of the day and come out and propose an experiment that he had no intention of performing. He was a theorist, so that was a little more reasonable, but I had had this idea for well over a year and by the time this man proposed it we were well into the building stage of the apparatus. It was a pretty good idea.
When was this?
The experiment was published in ‘42 or thereabouts, probably. I remember reading it or having somebody tell me about it.
It would show up with Helmholz’s name on it?
That would be Paper 30.
Yes, “Recoil from K Capture,” Helmholz and Wright.
Carl Helmholz was your student?
Yes, a graduate student. That was in ‘41 you see, and I started building the apparatus for that in ‘39 with my own hands.
Now, getting back to the neutron time-of-flight method, you mentioned the one other instance of someone coming up with the idea, but after your publication, where it had been demonstrated and you’d said, “We’ve done this,” then did people start using it?
No, no one else used it as far as I know until Havens and Rainwater in the early days of the war developed it to a very high degree. They essentially made the thing a paying proposition—in other words they had the advantage of the fast electronics which had been developed at Los Alamos and MIT by physicists, so they knew about electron collection. They didn’t have to use ion collection that slowed all of our circuits way down. They found out also how to modulate the cyclotron, they had a hooded ion source where they could modulate the beam, and they used longer times-of—flight. They really made the neutron time-of-flight technique a going thing. They didn’t know it at the time but they really shook up the whole Manhattan District because they found two important things with their time-of-flight experiments: they found the scattering edges for low-energy neutrons in carbon, which Fermi explained; though that just showed that they were really doing a good job and that their apparatus had good resolution.
The tremendous thing they did was to find the resonances in neutron absorption in uranium-235. You will remember that Bohr said that the reason he thought that there was a big slow neutron fission cross section in 235 was that there were resonances which everyone agreed must be in the capture of slow neutrons in uranium 238, (Uranium 239 was formed by neutron capture.) And that had to be a slow process; it took about 15 seconds; the neutrons had to rattle around inside the nucleus in order to make the sharp lines. And Bohr argued that fission had to be a very fast process, the neutron went in and the thing came apart right away. Well, when Havens and Rainwater found the sharp lines in uranium-235, there was a panic at the atomic bomb project because if fission took that long, then it was clear that there could also be some capture competing with the fission. Nobody had considered this; they had just measured total cross sections for U-235 and assumed that these were equal to the fission cross sections and had calculated the critical mass of the bomb on that basis. It turned out, in fact, that there was a substantial amount of capture leading to uranium-236 and this meant a complete redesign of the bomb, because the critical mass was bigger than had been anticipated. This all came from the time-of-flight method. It may well be that Havens and Rainwater had never heard of my work. I had never talked to either of them about it—you might ask Bill some day. They could perfectly well have started out from scratch without knowing about my work. The two things are almost unrelated. I’d be interested to learn when they first heard what a shock wave they sent into the Manhattan District.
They were doing this work at Columbia?
Yes, and I don’t even know who was their professor because most of the professors had gone away to fight the war. I think they just had the cyclotron pretty much to themselves.
They were students at the time?
Yes. I was at Chicago working with Fermi when the first time-of-flight curves came in showing the resonances in uranium and the scattering edges in light crystals.
How else has the method been used since then?
You’ve seen the “Barn” books, haven’t you, with all the cross sections? It’s a standard technique: there are lots of installations around the world and one couldn’t build reactors or bombs or do many experiments in engineering nuclear physics without the numbers that come from that technique. I really don’t feel that I had a terrible lot to do with that. I think Bill Havens and Jim Rainwater should have the credit for putting the neutron time-of-flight business on a real paying basis. If I hadn’t done an experiment using the time-of-flight method earlier, I would feel that I hadn’t had essentially anything to do with it; it had been an idea and I tried it out as a trick, but actually I did do an experiment with it so I feel I nailed down my claim to it, but not to the kind of work that’s in the “Barn” book—I couldn’t even dream of that because we were limited both by our ability to modulate and our ability to detect.
When did it really start to be used in this widespread way?
Shortly after the war.
Then it became widely known.
Yes, it became the standard technique.
It was used in the anti-proton discovery?
Well, people have been using time-of-flight in everything now because we have such fast circuits and we have counters that measure to a fraction of a nanosecond; that’s a time that high-speed particles [take to] go a few inches.
We ought to take up the fast electronics and short time measurements a little later.
I’ll tell you how we’ll get into it: by getting you into the war. But first I’d like to know the reaction of Berkeley to the announcement of fission—how you first heard of it and then what was done subsequently.
I remember exactly how I heard about it. I was sitting in the barber chair in Stevens Union having my hair cut, reading the Chronicle. I didn’t subscribe to the Chronicle, I just happened to be reading it, and in the second section, buried away some place, was an announcement that some German chemists had found that the uranium atom split into two pieces when it was bombarded with neutrons—that’s all there was to it. So I remember telling the barber to stop cutting my hair and I got right out of that barber chair and ran as fast as I could to the Radiation Laboratory where my student Phil Abelson, who is now Editor of Science, had been working very hard to try and find out what transuranium elements were produced when neutrons hit uranium; he was so close to discovering fission that it was almost pitiful. He would have been there, guaranteed, in another few weeks.
Let me digress for a moment to say what he was doing. I think it was at my suggestion, or at least he was using apparatus that I had built with my own hands and he rebuilt it—it was a bent crystal spectrograph for measuring the wavelengths of X-rays, a focusing X-ray spectrograph so that you could get a sort of Bragg X-ray spectrum using rather weak sources. This came about from my interest in electron capture—in getting the spectra of the X-rays from electron-capturing isotopes rather than using the critical absorption method that only works on K X-rays; you can’t do it on the L X-rays because the L X-rays are much too complicated. They extend over a rather wide band of wavelengths in contrast to the narrow lines in the K X-ray regions. So Phil Abelson had been repeating the experiment that had been done by Fermi and others, in which one bombarded uranium with neutrons and generated what were called the transuranium elements; we now of course know that these were the fission products. Phil had first of all found that some of these isotopes gave off X-rays, which he believed were L X-rays of the transuranium elements, so he wanted to take a photograph of these and see whether they were element 94, 95, 96 or what, which he could identify by means of their L X-rays. He had absorbed the X-rays in aluminum and in various other absorbers and had found that they had the right hardness to be L X-rays of transuranium elements.
But he didn’t think of using critical absorption because he thought they were L X-rays and there was no point in that, as I’ve just said. Sometimes it doesn’t pay to be too smart. Also everybody had said these were transuraniums. So he had set up a source of something that later turned out to be iodine-l29, which he separated out of the fission products using very strange chemistry; in other words he did it purely by looking to see where the radioactivity went when he made some chemical operation. In other words, he’d say, “I am going to add some chemical and I’ll see whether more activity goes into the filtrate or into the precipitate.” He found after a while that he could separate out this particular radioactivity, which again turned out to be iodine-129, by a very long complicated process, and he could get it fairly pure. He would put it in the X-ray spectrograph, and he didn’t see any lines in the band where he was looking; let’s assume for a moment that he was looking between 93 and 96, and he had a finite “window.” He then showed that he had enough X-ray photons to have produced a series of lines on the plate had they been in the right wavelength region. The only indication he had of the wavelength was the absorption in aluminum, and he checked this by taking sources that were in this wavelength region, measuring their ionization or strength and seeing that they produced photographically observable lines. The next thing that he was going to do was shift and look for elements on both sides of this “window” that he was set for, either higher atomic numbers or lower atomic numbers. He had just gotten to that point; he just realized that he wasn’t seeing the lines because they were shifted slightly from where he thought they’d be, but he knew he had enough intensity; he knew that they would appear if he adjusted the spectrograph properly.
So he was in that position when I arrived panting from the Students Union with my news about the fission. I played it kind of dramatically when I saw Phil. I said: “Phil, I’ve got something to tell you but want you to lie down first.” So being a good graduate student he lay down on the table right along side the control room of the cyclotron. “Phil, what you are looking at are not transuranium elements, they are elements in the middle of the periodic table, and you can use K absorption right away; you don’t have to worry about the complex structure of the lines.” I showed him what was in the Chronicle, and of course he was terribly depressed. That afternoon, he got out the critical absorbers and sure enough, identified what he’d been looking at as the eight-day iodine 129. He was a very good chemist, but naturally it never occurred to him that what he was separating out was a halogen, and that it had the same period as the well known iodine, so it was a very depressing day. I called up Gamow. No, I guess I sent him a telegram. We didn’t make long-distance calls in those days.
Where was he?
At George Washington University in Washington. The paper the next day talked about the conference of physicists at which this had been announced, so just assumed it was held in Washington. So I called or telegraphed Gamow to find out about it, and got a reply back saying the the large fission pulses had been seen in ionization chambers by Frisch in Cambridge. That afternoon Ken Green who now operates the AGS at Brookhaven and was then a National Research Fellow at Berkeley—Ken and I looked for the ionization pulses and found them. I remember telling Robert Oppenheimer that we were going to look for them and he said, “That’s impossible,” and gave a lot of theoretical reasons why fission couldn’t really happen. When I invited him over to look at the oscilloscope later, when we saw the big pulses, I would say that in less than 15 minutes Robert had decided that this was indeed a real effect and, more importantly, he had decided that some neutrons would probably boil off in the reaction, and that you could make bombs and generate power, all inside of a few minutes. He just had a block on the thing because he was so sure that Coulomb barriers wouldn’t permit the nucleus to undergo fission. But it was amazing to see how rapidly his mind worked, and he came to the right conclusions. There is an interesting thing about this experiment that we did. We published this as a confirmation; as a matter of fact in my letter to the editor—which I wrote and Ken and I signed—we said that we had called Gamow and he’d told us about these large pulses and we had looked and we had found them, and we said something about what fraction was caused by fast and slow neutrons and a few things like that. But it was essentially just to confirm that this was really so. In that same issue of the Physical Review there were, as I remember, three other letters: one by Dunning and some of his collaborators, one by some people at the Carnegie Institution, Tuve’s group and somebody else—I believe there were three. And none of these people mentioned that they had heard about this observation of the fission pulses. We felt sort of stupid, because it looked as though everybody else had heard about fission from Hahn and Meitner, or from the newspaper, and immediately decided there should be large pulses and went to look for them.
But a few years after the war, I had a letter from K.K. Darrow inquiring into this particular affair and he said, “I hope you will be able to tell me that you thought of the existence of the large energy release in fission by yourself because I’ve talked with the other three people who had their papers in that same issue of the Physical Review and they’ve all told me that they heard about it from somebody who was at the Washington conference, and they had heard that Frisch had observed these pulses, but they just neglected to mention it in their letters.” So I wrote back and said, “Karl, if you will look at our letter a little more carefully, you’ll see that we start out in the first three sentences and say that we had heard about this and were just confirming it.” I felt considerably better about it. I was a little surprised to learn that all the other fellows had heard about it and hadn’t mentioned it. It was strictly a confirming experiment as far as they were concerned but they let it appear in print as though they had heard about the chemical identification of the fission products and had done the fine thinking that Frisch was apparently the only one to do.
I see. Was anyone from Berkeley at that meeting?
All this within a few days?
And all this without seeing any scientific…
No. It was the first time it had happened. Before we had always gotten our information from the Physical Review. Physics just wasn’t newsworthy enough in those days. I guess I did say that I first learned of the positron from the paper.
What about the mesotron as it was called?
Oh, that didn’t make the papers as far as I know.
In Pasadena it did.
It did? I don’t think it appeared up here.
I saw a clipping on it. Was there any seminar or were there special meetings of the Journal Club or anything on that?
No, I don’t think so.
No real internal discussion on it?
Oh, there was a very great deal of informal discussion, yes. Everybody was terribly excited about Phil Abelson’s paper. As a matter of fact I wrote the paper for that and we sent it in by telegram because Phil made the final identification that this was indeed iodine; these were the appropriate X-rays. I don’t remember the details. Anyway I do remember writing that telegram; it was the first time I’d ever sent a telegram off as a letter to the editor and it did get published.
How long a time lag was that? Do you have references to that?
It’s in the same issue, or maybe the next one—probably the same issue which contained the letter by Ken Green and me.
[Consulting bibliography] It would be on the first page.
Yes, here it is, “Heavily Ionizing Particles from Uranium, Phys. Rev. 55, 417(1939), with Ken Green. Some months earlier, I had actually looked for ionizing particles coming out from the bombardment of uranium with neutrons. I thought there would be long-range alpha particles. The thought of fission products, heavy fragments, never occurred to me, and so I put my ionization chamber right up in front of the cyclotron neutron beam and put some uranium in it. I covered the uranium with 2 centimeters air equivalent of foil to knock out the ordinary alpha particles from the uranium. They wouldn’t have bothered me anyway, but I did it just because it was good experimental technique. And of course everybody knows the range of fission products is two centimeters of air equivalent so I put myself out of business. And I understand that a half dozen other people did the same experiment, and put themselves out of business the same way! I guess I was in good company.
And everybody seemed to do it with aluminum too.
Well, that was the standard thing; we had little packages of aluminum foil of various thicknesses. The aluminum company put out little samples of wrapping material, different numbers of thousandths of an inch thick, so everybody had his little sample packages of aluminum wrapping foil.
In your Faculty Lecture—I believe it was in there that you mentioned, after the announcement, the search for fission neutrons…
Yes, and as I say, if I had taken another hour to move my counter closer to the cyclotron I couldn’t have missed finding the emission of “prompt neutrons.”
This was within the days after the initial announcement?
I don’t really remember when it was, and I don’t believe I wrote anything in my book. It was just an experiment, an exploratory type of experiment, that you do; you have all the equipment already there. As I said, I was the only person in the world then who had the equivalent of a thermal column, where you can observe the fission neutrons in a few seconds.
The discussions taking place about fission had to do with the physics of it, or the social implications as well?
Nobody talked about social implications, that I remember.
On the bomb for example?
I don’t recall any discussions about bombs.
I don’t recall anyone talking about bombs except the original suggestion of Robert Oppenheimer who said, “Look, huge amounts of energy will become available in very short periods of time.” I don’t know whether he said bombs or not, but I just remember thinking about the possibility.
Then how did things proceed at Berkeley from that time? I mean from the time of the discovery of fission through the time when you left for M.I.T.
Well, Ed McMillan did the beautiful experiment, looking for the radioactivity of the recoiling fission products. He went down to Telegraph Avenue, and bought some cigarette paper, a package of old fashioned roll-your-own-cigarettes cigarette papers and put a stack of these with uranium on the top of a piece of paper. He bombarded this with slow neutrons and fast neutrons as well, and then took the individual pieces of paper and separated out the radioactivities, and he found out that the uranium itself had an excess amount of the radioactivity, which was known to be uranium-239, and also of another radioactivity, which later turned out to be neptunium. One of these is two hours and another one is two days (I never remember which is which). But at any rate it was clear that there were two non-recoiling radioactivities. He used this not so much as a way to find out what the radioactivities of the fission fragments were—although he did do this—but the exciting thing was that there were two radioactive species which didn’t recoil out of the uranium, because they were just due to neutron capture in the uranium; they didn’t have anything driving them out by recoil.
Then he tried to find out what were the chemical properties of the thing that later turned out be neptunium, and he was unable to identify it. He confirmed the fact that one of these things was uranium, I should really know which is two days and which is two hours, but anyway one of them was uranium-239 as everybody knew, and the other one was a non-recoiling radioactivity. As I say, McMillan did not make a determination of the chemical properties of this activity. Segre then took the matter up, and Segre concluded that this other unknown non-recoiling isotope was a rare-earth. It seemed very difficult for people to understand why when there were a lot of rare-earths that did recoil. After all, one of Hahn’s original activities was lanthanum. I guess it was barium and then krypton, but lanthanum is right next to barium. So Segre put out a letter to the editor saying that he had investigated the chemical properties of the second non-recoiling neutron induced radioactivity in uranium and for reasons that he could not explain, it apparently was a very heavy rare-earth. If it were very heavy, it wouldn’t recoil so far. That is the way the matter rested until the next summer, when Phil Abelson came out from Washington, where he had gone after receiving his Doctor’s degree, to work with Merle Tuve on the cyclotron there. He came out in the summer with the express purpose of proving that this second non-recoiling activity was element 93, that it was a daughter of the uranium-238.
The difficulty in this was that the daughter was much longer lived than the parent, and so the amount of radioactivity was less. There were the same number of atoms of each, so if the daughter had a longer lifetime, then its specific activity would be less, That makes it difficult to establish the growth of the daughter; it’s much easier if the lifetime is shorter because then there is the same amount of radioactivity. (This also tells me that the two hour activity was U239, and the two day activity was Np239.) So when Phil got out to Berkeley, he found out that Ed McMillan had independently just started back on the same subject. He had also decided that this thing probably was neptunium, element 93, and that the chemical properties were probably different than had been predicted for element 93. Well, Segre had proved that chemically it was a rare-earth. You will remember that at that time, one did not know about the actinide series of elements and so one naturally assumed that element-93 would be ekarhenium; it would be the lower homologue of rhenium, it should have rhenium chemistry. So people looked for it separating out rhenium; they used rhenium as a carrier, and nothing came down, and they found out that it did come down with the rare-earths. So McMillan was presented with the problem of showing that this thing, which had rare-earth properties, as shown by Segre, and which he showed didn’t recoil, was in fact 93, and this had to be shown by the decay scheme, that uranium 239, which was known to exist, actually decayed into this new thing. Phil Abelson had exactly the same thought in mind when he came out here, so he and Ed joined forces and they did the work together and published a paper together. Poor Phil got the short end of the stick, when Nobel Prize award time came around and the Prize was given to Ed McMillan and Glenn Seaborg. That was fine, but I think Phil should have been in the circuit some place, because he certainly played a key role; after all, he was co-discoverer of neptunium; no question about that.
This was the summer of 1940?
I’d guess so, yes.
He must have gotten his degree in ‘39 and then come out in the following summer.
Is there anything else significant to say about the period prior to your going to the radar laboratory at M.I.T.?
Well, certainly not in the area of fission.
About nuclear physics in general? What did Ernest Lawrence feel about this? How did he react to these developments?
Well, he was always interested in everything that was going on.
He was a man with prophetic vision and I just wonder if he saw what the role of this laboratory would be.
No, not at all. It was just a tremendous awakening for Professor Lawrence when he met Cockcroft and Tizard when they came over here with the magnetron. Ernest just hadn’t given any thought to things of this sort at all. I should mention one thing that I don’t think is down in the records any place, and that is when, I would guess in the spring of 1940, Ernest decided that maybe there might be something we could do to help the armed services in some way. He didn’t have any ideas, and he said, “Look, we can make several curies of radio-sodium; maybe somebody in the Navy might like to have some of this for gamma-ray radiography. In those days we thought of radium as being so exorbitantly expensive that nobody had a gram of radium except an occasional rich hospital. So I remember driving up with Ernest Lawrence and I forget who else; we drove up to Mare Island to talk with some naval officer. Ernest had called and found there was a commander so-and-so, who might possibly be interested in such things. We talked with this man, and it turned out that he was in charge of radiographing big castings and things like that in the Mare Island Naval repair yards. Ernest told him how he could make curie-lots of radio-sodium.
The difficulty was that it only has a 15-hour half-life and we would have had to truck it up there in lead boxes if they wanted to use it. It turned out that this naval officer said, “Well, we wouldn’t really need it; you see, we have these ten grams of radium over here in this lead box that we use every now and then.” So that was a real eye opener. It was also interesting that we were dealing at the “commander level in those days, and this particular commander had never heard of Ernest Lawrence. We got a kind of brush off. That was our first military experience, and apparently that convinced Ernest that the military was in pretty good shape and didn’t need our help. It was only when the Tizard Mission came over and told what a great job the nuclear physicists had done in developing radar and other things in England that Ernest got interested in these problems.
That’s exactly what I’d like to talk about now, the Tizard Mission. I am not clear on the circumstances of it, and the argument that they advanced, specifically on the role of nuclear physicists. I’d like to retrace that and see how it relates to your own work later at the M.I.T. laboratory.
The Tizard Mission was the result of the fact that Randall and Boot in England had invented and built the first cavity 10-centimeter magnetron. Before that time the highest powers available in microwaves were microwatts, not useful for anything, and all of a sudden in almost two or three jumps Randall and Boot had produced 10 kilowatts of radio-frequency power in the 10-centimeter wavelength range. It was obvious that this would be of tremendous importance in radar, because for the first time, it gave you the ability to make moderately narrow beams. This is because diffraction theory says that the angular width of the beam is the wavelength divided by the aperture of the radiating system. At that time, all airborne radar was at a wavelength of one and a half meters and no one could make a radiator big enough, that could be put in an airplane, to give a narrow beam. Unfortunately, the British scientific effort was completely tied up in things of more immediate concern—there were submarines to be found and radar sets had to be put in fighters, to find night bombers. The British had simply run out of scientific talent, and it was clear that here was something that was very important and if they wanted to get something done in this area, they would have to bring some new people into the system. Since they had no more of their own, they said: We’ll have to compromise security (which at that time had kept all of this information tightly guarded in Great Britain) and we’ll go to the Americans and ask them if they’ll help.” That was the genesis of this Tizard Mission. Tizard and Cockcroft and Bowen and a few others came over here and brought with them a magnetron as well as the stories of what their physicists had accomplished and said, “Please can you help us.”
Where else did they go? They went to other American institutions, didn’t they?
They essentially got in touch with Alfred Loomis, who had built the only American 10-centimeter radar set then in operation. Loomis was “The anonymous friend of the Physical Society.” Are you familiar with Alfred Loomis, an extraordinary man?
[pointing to a photograph on his desk] There’s his picture. Why don’t I digress on Alfred just for a moment, because he should be somehow or other in the history of American physics where he has played an enormous role. Alfred Loomis came from a wealthy family, and went to Harvard and majored in mathematics and then went to Law School and got his law degree, just before World War I. His hobby was artillery, and he had apparently collected more information about European artillery than anyone in the Ordinance Department of our army had. When the war came along, he went into the army and came out as major or lieutenant colonel, and he was put in charge of the experimental program at the Aberdeen Proving Grounds, even though he was just a young lawyer. With his mathematical background, and his wonderful intellectual talents, it turned out that he really did a good job and invented a number of things that were used for 25 years in measuring external ballistics. When the war was over, he had had a wonderful taste of experimental physics, and so he set up his own laboratory at Tuxedo Park where he lived.
He used to drive into Wall Street every day and do his business, and then come back to Tuxedo Park where he had this marvelous laboratory. In fact he had a much better laboratory than any university laboratory at that time—better equipment, more expensive equipment. He hired R.W. Wood as his private tutor, and R. W. Wood came up and spent every summer at Tuxedo Park doing the experiments that he couldn’t do at Johns Hopkins because they didn’t have enough money. R.W. Wood taught Alfred Loomis physics, and Alfred would invite all the distinguished foreign physics visitors to this country up to Tuxedo Park where they had seminars. He would go touring around Europe in the summer with R.W. Wood as his guide, and meet all the important physicists. In Wood’s biography, there is a story about the time he and Alfred went to see the manufacturer of the Short clock—Mr. Short, who made astronomical clocks.
Their pendulums were in a vacuum; they had a slave pendulum and a master pendulum; they were the latest thing in clocks and there were only maybe ten of them in existence, all in big observatories. Alfred said he’d like to see these clocks, and Mr. Short showed him two of them and then Alfred asked, “How much are these clocks?” Mr. Short apparently said, “$15,000 apiece,” which was a lot of money in those days. Alfred said, “That’s fine; I’ll take three of them.” And he did take three. Mr. Short almost fell over. Alfred set them up in his laboratory and used them to find the effect of the moon on the pendulum clock—it had never been seen before. He had these three clocks running in his laboratory, and he had the best crystal clocks that the Bell Laboratories could make, and he collected a lot of data and analyzed it to show the effect of the moon on the pendulum clocks. He still is a kind of a “time nut;” if you see him today, he always wears two Accutrons, one on his right wrist and one on his left wrist, and he checks them every day against WWV, and if one is gaining a half second on the other he will wear it on the outside of his wrist instead of the inside so that gravity changes the rate of the tuning fork, and the two watches track each other, and WWV, to within less than a second a day.
He must be up in the 80s now.
His 80th birthday is this fall.
He still lives in Tuxedo Park?
No, he lives on Park Avenue. I believe I mentioned in my book that he was one of Ernest Lawrence’s closest friends, and of mine; he’s sort of a father to me, a wonderful person. I could go on telling stories about Alfred Loomis all day. But as I say, during the thirties, when physics departments were hard pressed for money, there was a custom that if you couldn’t pay the page charge, or your institution couldn’t pay the page charge, there was an “anonymous friend of the Physical Society” who would kick in the money, and that was Alfred Loomis. Then in his laboratory he built the first microwave radar set—I imagine the first that anyone built. He got one of the first klystrons that was hand-made out at Stanford from Hansen and his co-workers, and he and his friends at Tuxedo put this into a Doppler radar set working in the 10-centimeter range. I saw that thing working incidentally, shortly after the M.I.T. laboratory came into being, but when the Tizard Mission came over and told about the magnetron and the virtues of the pulsed system over the continuous wave system that the klystrons used, Alfred Loomis abandoned the cw radar sets and went bull bore on the pulse sets. He was chairman of the Microwave Committee of NDRC. He had charge of the laboratory; Lee DuBridge worked for him, and he reported in turn to Conant and Bush. So he played a very, very important role in the development of microwave radar. And as I said in the biography of Ernest Lawrence, he also took Ernest around and introduced him to the financiers in Wall Street to get money for the 184-inch cyclotron. I remember Ernest telling how he was introduced to the chairman of U. S. Steel, and managed to get steel for the 184-inch cyclotron at a very reduced price. He got some copper from the Guggenheims at reduced price. He went all around and introduced Ernest to a group of people that he wasn’t familiar with at that time, that he got to be good friends with after the war.
Sounds like a George Ellery Hale type. By the way, I note than the 184-inch cyclotron the authorization for this was in 1940 and that about $1,250,000 had been given. Was it Carnegie?
No, the Rockefeller Foundation gave it.
Now, just to digress. By that time the plans had been laid for a 184-inch conventional cyclotron. What arguments were advanced that would be convincing to the Rockefeller Foundation?
Just that every cyclotron that had been built had brought in something new and exciting. That’s the only argument that I know that would have been brought in. I don’t think that Ernest came in with any particularly strong medical arguments; perhaps that if you make more neutrons for treatment, certainly you’d make more radioactivity, and you would just find out interesting things.
Even though this was after the discovery of fission, no connection was made?
I never heard any of the discussions. I am sure Alfred Loomis would love to tell you about these things, because he sat in on them. He’ll be back in this country towards the end of March. He’s with a group of friends on the Queen Elizabeth cruising around through the Mediterranean right now. He takes a bunch of friends with him. My wife and I have been down to Jamaica with him on two occasions. He just rents some cabins on the Queen Elizabeth, and takes his friends along.
I think interviews go much better on the Mediterranean.
Alfred knows more about what got physics going in those days, because he dealt at a level where people were getting money and where people were thinking about the future. The rest of us were sort of workers in the fields, and we didn’t come into contact with the kinds of things that Ernest Lawrence and Alfred Loomis and other people were thinking about in those days.
Now, the Tizard Mission came over and talked with him; then they were recommended to visit the…
Yes. As a matter of fact they met up at his home in Tuxedo Park. There was a week-long meeting at Tuxedo Park and Van Bush and A.T. Compton, Ernest Lawrence and a lot of influential scientific-statesman type people were there.
Was this an official type of Government mission?
No. Let me say it was this: it was official insofar as the British were concerned. I believe the charter of the Mission was that they were to contact influential American members of the scientific community. There was no organized scientific effort as far as I know in those days. I think the NDRC was a glimmer in Van Bush’s eyes at that time, but I really am not sure whether it had been set up or not. In other words, it had either been set up just a month or two earlier, or it was set up as a result of this meeting. I really don’t know which came first, but they were almost simultaneous.
What about this argument on the role of nuclear physicists?
It was simply a fact that the British nuclear physicists had gone into war work largely in the area of radar. Blackett, of course, had invented the art of operational analysis and had a very successful group going in that area. There were, of course, a number of physicists working on magnetic mines, who had done a good job. But the really big effort was in radar.
This was thought to be the special domain of nuclear physicists then?
Well, the nuclear physicists who worked in cyclotrons had a lot of experience with high-frequency radio generators and knew electronics, from their counter work, and this electronics was useful in radar receivers—after all, it involved pulses, there weren’t very many people in the radio frequency fraternity, the electrical engineering types, who thought much about amplifying pulses. That was the domain that had been pioneered by physicists for counting techniques—high-speed counters and gates and pulse amplifiers and things of that sort. So that was a natural area in which a nuclear physicist could make a contribution.
In England then, did the nuclear physicists come from any particular center of research?
They came from everywhere. From all the universities.
They came from all the universities that were doing nuclear physics. When I went over to England in ‘43, all the people whose names I knew in connection with Cavendish Laboratory physics, for example, were in rather high positions in the radar establishment. I remember sitting in on a discussion between Peter Dee who was, I guess, the first one to see the high-speed Helium-3 and hydrogen—3 particles coming out of the d-d reaction—Dee and Oliphant. I listened to a conversation between Dee and Skinner, who was at the Cavendish, and was a well-known nuclear physicist. And they were talking about a conversation that Dee had had with Churchill a couple of days earlier, in which Churchill was giving him a hard time because some particular radar set wasn’t working the way it should. I was impressed by the fact that these people, who were only my senior in nuclear physics by a few years, were talking with the Prime Minister of Great Britain about technical matters; in our country that never came about as far as I know. President Roosevelt never got into any technical conversations with anybody about things of that nature. He talked policy with Van Bush and maybe occasionally with Conant, but he certainly never talked with any of the working types, or with people who shortly before had been working types.
Would you speculate whether the reason for that was because of Roosevelt or because of the American physics community?
I think it probably had to do more with Roosevelt’s disinterest or inability to understand technical matters. Churchill, on the other hand, had Lord Cherwell as his right hand man. Lord Cherwell has taken a terrible beating in the press, but he did have the ability to communicate with Churchill and convince him of the importance of science, and tell him of the exciting things that the scientific group was doing. I never understood how Churchill and Cherwell had anything in common, because I had dinner with Cherwell on several occasions. He was a vegetarian, didn’t smoke or drink, was a bachelor; he had no vices at all as far as one could see, and he was very, very stuffy as far as I was concerned. He used to tell jokes that I thought were so boring as to be unbelievable. And yet he and Churchill apparently had a tremendous rapport.
The only thing I can think of in Cherwell’s background that would bring this about was the fact that he was apparently the first man to put an airplane deliberately into spin and bring it out. Before that time anybody who had ever been in a spin—which happened after you stalled an airplane in those days—and you were dead, because they didn’t have parachutes. Cherwell figured out (he was Frederic Lindemann before he was elevated to the peerage) from aerodynamic principles what you should do to get an airplane out of a spin. When he was at Farnborough, the main RAF experimental station, he took an airplane up—so the story goes—he stalled it, let it go into a spin, let it spin a few times and then did the right things to get it out. Everybody before that time had been trying to lift the nose of the plane, because here was the thing going down at a steep angle, but Cherwell realized that what you should do was get the thing going faster and build up some air flow over the control surfaces and then you could get it out. Instead of trying to pull back on the stick, and get it out, which is the natural thing—if you find an airplane in a steep dive you try to pull back on the stick and get it out—he put the nose down as steeply as he could, got some control again, and then pulled it out. So any man who would do that must have something more than appears on the surface of the stuffy old gentleman that I knew during and after the war. But as I said, he took a terrible beating from Snow. I am not sure how justified it was; I read a number of things that Cherwell did wrong, but after all, nobody is perfect. But he certainly played a very important role, and he shouldn’t be brushed off the way he was. He did have more power than he should have had, and he used it improperly on occasion, but I think that probably on balance he did a good job for Churchill and for his country.
This all occurred in the days when it was not clear what science-government relations should be? There was no real experience to go on.
Now, after the conference in New York, Ernest Lawrence returned here. Did he recruit you for the M.I.T. radar lab?
How about others?
He recruited McMillan; we were the only two. The interesting thing about that was that the radar laboratory was supposed to be at M.I.T., a place where people would come to get refreshed on the latest techniques and then go back to their own laboratories and carry out the work. In other words, this was not supposed to be a full-time endeavor for McMillan and me, but rather something that we would do with part of our time. And we’d continue to teach and do research on the cyclotron, and set up a radar laboratory and work on receivers or pulse modulators—something like that. But it wasn’t more than a week or two after we’d been back at M.I.T. that we realized that we just had to stay there or we’d be hopelessly out of date, if we ever came home for a few weeks and went back. We would be completely out of touch with what was going on; things were moving so rapidly.
You mentioned that this provided a large gathering place for nuclear physicists, and in fact that was all right as far as the public was concerned because they would think that this laboratory was working on nuclear physics with no war implications. This is true even in 1940?
Even though there were some people already urging the President…
Well, what they were urging the President to do was to make reactors and there was no reason to have a reactor, since all a reactor can do is make heat, and there were plenty of sources of heat; lots of coal around, and gasoline and what not. I might just tell you why it was that people thought that nuclear fission had no impact on the war. Everyone believed that in order to make a chain reaction, you had to use slow neutrons. After all, the cross sections were fantastically large in the slow neutron range, and as you speeded up the neutrons, the cross sections went way, way down and as far as we could tell, disappeared. So everyone—I use everyone in quotes—that I was aware of, thought that a chain reaction had to involve slow neutrons. But in order to have an explosion, you have to have a detonation wave going through the material, and this has to go with sound velocity in the material, and it’s hard to see how you could propagate a detonation wave with something that involved slow neutrons diffusing around, because the velocity of the slow neutrons is not too different from the detonation velocity in an explosive; maybe the detonation speed is a little higher.
So it seemed that you would not be able to make an explosion using thermal neutrons, and since that was the only way that anyone had dreamed of making a chain reaction, it seemed to everyone—and again I use everyone in quotes—that you could not make an explosion. As a matter of fact, long after I knew that you could make fast chain reaction—that is, chain reactions set off by fast neutrons without any moderators—I used this argument to convince people that what they had heard, or what they thought, about the military applications of chain reactions, was not true. I gave them a snow job for security reasons. I said, “Look, people went through that years ago and here is the reason why you can’t do it.” This always satisfied them; I never had a dissatisfied customer. The person, as far as I know, who first realized that you could make a chain reaction using fast neutrons—which would therefore make it possible to make a bomb—was Peierls. Peierls is the name that comes to my mind, in Great Britain.
No, he was in this country and was thinking about beta decay and not about this; if he did I never heard of it.
I don’t know why I associated the name, but there was a meeting at Berkeley later.
Oh, this was long after. As I say, nuclear physicists just about stopped thinking about nuclear physics in this country, except for the theorists. The experimentalists, most of the ones who would have been able to think of such things, had gone into radar.
At the radar laboratory, there are two types of things that I think are important for our purposes: one is the specific techniques that were developed and later became effective in postwar work in nuclear physics; and the other is the effect of bringing together so many nuclear physicists on a problem which is not really nuclear physics. Was there any effect? Did you meet new people? Were you able to exchange information on nuclear physics and on techniques even though your day to day work was not involved?
No, we all forgot about nuclear physics almost immediately. I might have said in my talk—although I don’t believe I did—that when I went to M.I.T., one of the first things I did was to find out when the weekly seminar on nuclear physics was. Probably Robley Evans had such a seminar, and said: “I certainly want to keep up with what’s going on by going to the seminar.” I went to the M.I.T. Library, and found out where they had their journals. I remember going to the library one time to read The Physical Review, and went to one nuclear physics seminar, and never showed up again, because the exciting things for me then were what we were doing in the radar laboratory. The same thing was true of all the other nuclear physicists; we just completely forgot nuclear physics. It’s amazing; I’ve used this example many times lately when my young colleagues come around and say, “Wouldn’t it be terrible if we don’t get a big accelerator, because there is nothing else to do.” Nuclear physics, particle physics at the moment, is exciting because the best theorists are working in it and the people that you admire most in experimental physics are working on it, but if the theorists go away and leave it because there is no more experimental data coming out that they can analyze, they can quickly go to some other fields, and that will suddenly be the most interesting field, and you will go work in that. This is a completely new concept to the young physicists, because they’ve spent all their lives so far in particle physics and they think that’s the only important thing in physics. I’ve been in a number of fields and I’ve found out that when the best people you know have moved suddenly into a new field, that’s the most exciting field, because if you are working in the old field you are left behind; you are not good enough to be working with the first team over in this other field. That happened to me at M.I.T. and it happened again at Los Alamos.
That’s an interesting reaction. Do you think it’s a conscious attempt to be in a field where something is happening?
I think that if you’ve been on the first team, you don’t want to be playing with the scrubs, you want to be in the first team, you want to be working with the people that you admire most and that you think have the greatest skills.
Do you think there is any logic about the way that theorists choose new fields?
I really couldn’t say. I think it would be improper for me to suggest how theorists get interested in fields. If I were to guess I would say they get into fields where there are interesting and unexplained data coming out, and which seem to be on the forefront of their particular area of interest.
I ask the question because someone has proposed to us that there is a certain philosophical motivation, namely the study of the structure of matter and that in fact this particular predisposition can account in part for the division between nuclear physics and high energy physics, that the people who are interested in studying the ultimate structure of matter will take the branch that will lead them toward that. But perhaps you don’t feel that way.
Oh no, I think it’s kind of an autocatalytic thing, that the best physicists tend to move out in that direction and they draw the next best and they draw the third best and so on. This phenomenon that I am talking about here—people like to go and try to compete with the best competitors, and if you can play in a major league you don’t want to be playing in the 3-I league, you want to go out and demonstrate that you are as good as the people in the major leagues. I think that’s what brings them in. You have the ability to talk with people who are interested in your problems; that’s the important thing in science as far as I know the fact that you have people to talk with. There is nothing more discouraging than to go some place where nobody understands your particular lingo.
It’s true of any field; not only science.
I am sure it is.
I just wanted to know if it’s possible to trace the kind of physics that you were doing to develop the Ground Control Approach, the Microwave Early Warning System, and perhaps the Eagle Radar System into the postwar period. In other words, can you identify techniques you used at that point, trace them into the postwar period and see how they became operative in experimental physics?
Well, a lot of these things in the G.C.A. area have been written up, I believe, I think a lot of people are aware of the fact that I thought of G.C.A. when I was watching the first microwave anti-aircraft fire-control set operating up on the roof of M.I.T. Louis Ridenour had build this and I watched it tracking a plane, and it occurred to me that if you could track a plane well enough to shoot it down—know its coordinates well enough to shoot it down—you ought to be able to find its coordinates well enough to use them in a landing procedure. And that was the basis for the whole G.C.A. system. I went off and built a system to implement this idea, and it was a horrible failure. We borrowed Ridenour’s gun-laying radar for a week, after it had completed some coast artillery firing tests, and it turned out that it wouldn’t work. The radar beam was too wide, and some of the energy hit the ground and reflected up and so we’d see the reflection of the plane from the ground, as well as directly and the radar tracker would oscillate back and forth between the real airplane and the image in the ground.
It couldn’t tell you where the airplane really was, so we had to develop an antenna with more resolution. That’s how I invented the big arrays that we used in both Ground Control Approach and Eagle. Before that time, all radar antennas—all microwave radar antennas—had consisted of a single dipole at the focus of a paraboloid of revolution. But inside of a very short period of time, my colleagues and I were making antennas that had 200 little dipoles on a big long linear feed in the focus of the paraboloid, a cylindrical paraboloid rather than the paraboloid of revolution. And this actually came from the interest in, and a feeling for, physical optics that had picked up at Chicago. I had worked on diffraction gratings for my thesis, and had always had an interest in them just because that was the thing that one talked about at Chicago. This was an area where the radio engineers said: “You can’t make an antenna with so many elements in it, because if you try to put two together it’s okay; if you put three or four together it gets very difficult, and you try to put ten together like we have done in some lower frequency arrays, it gets terribly difficult to tune them all up; and if you go beyond that, it’s impossible.” I reasoned differently, saying this is like the difference between two-body mechanics and three-body mechanics.
Two-body mechanics is easy to solve and three-body mechanics is impossible; but if you start at the other end and start with thermodynamics, again it becomes easy because you don’t worry about all the little details. So it occurred to me that if we went to very large numbers of radiating elements, each one would be so lightly coupled to the transmission line or waveguide or whatever, that there would no interaction between them, and you’d be back again to an easy situation. So we just jumped over this difficult area beyond, say, 5 to 10 elements, which had put the radio engineers out of business; they just couldn’t handle this. They couldn’t tune the thing up because there were too many knobs to twirl. I just predicted that if you go to very large numbers then there are no problems at all and it turned out to be true. Apparently this had not occurred to the radio engineers. But it occurred to someone who had worked with diffraction gratings where you have indeed hundreds of thousands of lines on a grating. So there is a connection between my early physics and three of my wartime radar sets.
Now can you carry the connection forward?
Well, you asked me yesterday why I thought that there would be lots of surplus radar equipment. I knew that the army had the so-called SCR 268 radar set that worked at a meter and a half wavelength. The microwave radar had pretty well put it out of business; nobody wanted a 268 if they could get a microwave radar set, and so I knew there were 1000 or more of these 200-megacycle, 15-meter wavelength, radar sets. I knew they would be a drug on the market and thought that if we could make an accelerator using these things that it had a good chance of being built. It didn’t occur to me that we would get the huge post-war support for physics that later turned out to be true. I thought we would go back from the wartime era when we had unlimited amounts of money to the system we had before the war when we didn’t get money at all. So it occurred to me that if we had large quantities of essentially free surplus radar sets, that one could build an accelerator using them.
When did this occur to you?
When I was at Los Alamos.
Towards the end of the war?
No, actually I started thinking about it when I was at Chicago for six months between M.I.T. and Los Alamos.
Yes. I started thinking about it there because I remember talking with Ernest Lawrence about it. Ernest came through Chicago one time. He was very depressed and really quite sick because the first magnets that had been wound and turned on at Oak Ridge had all shorted out; apparently Westinghouse had not done a good job of cleaning the tanks out and there were lots of iron chips in the tanks. When the transformer oil went in to cool the magnet coils for the big mass spectrometers, these metallic chips shorted out lots of turns in the magnets, and they were burning up all over the place. Oak Ridge was on a terribly tight time schedule and this was just more than Ernest’s constitution could stand; he came up and holed up in Michael Reese Hospital in Chicago for a week to recover his equilibrium. He had a sore back and a stomach ache; he was just bushed. He called me up and I went to see him, brought him books and talked about various things. I remember telling him at that time that I thought I had a use for the 200 megacycle, meter and a half wavelength radar sets that were going to become obsolete at the end of the war, and I think I hurt his feelings when I said: “I assume that your isotope separation plant will also be surplus at the end of the war” because the diffusion process would come in and it would be cheaper. He didn’t like that a bit so I dropped that subject. At least I thought there would be large numbers of vacuum pumps and vacuum vessels available, which I needed to make my “poor man’s accelerator.” So you see, I was looking around for lots of vacuum pumps and lots of radar sets.
You thought of using this on the proposed 184-inch?
No, on the linear accelerator.
When did that idea come about?
This was in ‘43. I had been talking about this. I really didn’t have any ideas about the 184-inch in this period. My ideas had to do with linear accelerators for light particles, originally electrons and then I gave that up the day I heard that McMillan had invented the synchrotron. When he told me about that I said, “that puts me out of the linear accelerator business.” Then I began thinking about protons, accelerating protons with obsolete radar sets, and this I did carry on after I got back from the war. We started with the radar sets, and then quickly found out that we weren’t saving any money, and built our own oscillators, which were more efficient. We just had lots of money which I hadn’t expected that we would have. I had expected that we would be poor again and I would have to use excess vacuum pumps from Oak Ridge and excess radar sets.
I want to pick up the specifics of the technical developments again a little later. But what about this period now, near the end of the war, at Los Alamos, when people were speculating and planning for the future, what types of discussions went on? Did people make up any sort of agenda of what they wanted to do?
No, I don’t think there were very widespread discussions of this kind. It was certainly true that at M.I.T. Ed Purcell was thinking about looking for magnetic resonances as soon as the war was over, and at the Harvard Radio Research Laboratory Felix Bloch was doing the same thing. After all, he had gone to the Radio Research Laboratory and had learned a lot about radio-frequency techniques; he got interested in it during our experiment—he’d never seen an oscillator before in his life or any electronic equipment; he was strictly a theoretical physicist. So he thought he would look for it and Ed McMillan was thinking about accelerators; I was thinking about accelerators. I was not aware of any other thoughts in this area. It was not widely discussed.
What did you have in mind as a type of experiment you would conduct with the proposed linear accelerator?
Well, the first thing that everybody had in mind in those days was to do proton-proton scattering. That was the “in” thing to do. Any time anyone got higher energies from any accelerator, he did proton-proton scattering, because the theorists were terribly excited about this. That was the critical experiment insofar as gaining an understanding of nuclear forces was concerned. Breit, in particular, at Wisconsin, would analyze every fresh bit of data that came in from p-p scattering, and naturally the first experiment one had in mind when one had high-energy protons was the repetition at higher energies of p-p scattering. As a matter of fact, that was almost the first thing that was done with the linear accelerator at Berkeley when it was completed. We did it by two separate methods. Panofsky and his coworkers did it with nuclear emulsions and Bruce Cerk and his coworkers did it with proportional counters.
Was this interest in nuclear forces pretty much the thing on the agenda—for the theoreticians, anyway?
How about your discussions with McMillan. You said he had invented it, and you further stated of course that he’d worked on the ideas at Los Alamos.
What he started thinking about was building a large betatron, an air core (non-iron core) betatron. When you say “air core” a lot of people think it’s air C-O-R-P—S, remember that he had designed this air core betatron to be installed at Boulder Dam, and to use the capacity of Boulder Dam in the period when there wasn’t a big load, in the midnight to 5 a.m. period, the same way Carl Anderson got free power from the Cal Tech wind tunnel generator. And I remember Ed had calculated the design to show that he could build a betatron using conductors to define the field, rather than iron pole pieces. And then suddenly—I don’t know how suddenly—it occurred to him that he could get to very high energy by using the phase stability principle which he discovered in thinking about accelerators. Exactly how he came to this I don’t know, you’d have to ask him. I’ve never heard him say what line of thought brought him to this, but I know that he had been thinking for a good many weeks or maybe months before that about building a betatron to take most of the power out of Boulder Dam. He didn’t like that because that meant he had to go and live at Boulder Dam to use it, so he was very happy when he invented the synchrotron.
Who would run this machine at Boulder Dam?
I don’t know that he ever got that far.
It was just a source?
Yes, he was just starting to think again about physics and how to get high-energy electrons.
Then, he was discussing the final stages of his thinking with you?
He told me that he had just invented the synchrotron and he told me how it worked.
And then this influenced your thoughts?
Oh, I just said: “I can’t compete with that; it’s such a good machine, there is no point in building a linear accelerator for electrons if you can do it this way.”
What happened at Los Alamos as far as the attempt to recruit people for work at Berkeley and at other institutions? Was there a lot of scrambling going on of people trading jobs and so forth?
I wasn’t aware of it. I came back to Los Alamos and stayed there only a week or so after coming back from Tinian. I just wanted to get back to Berkeley as soon as I could. I found that I didn’t like the atmosphere at Los Alamos; I found I didn’t know anybody anymore. When I went away to Tinian, everybody was gung-ho about winning the war. I came back, and everybody was crying around about what a terrible thing we’d done to drop bombs. I thought it was great because I’d been out there and seen the preparations for the invasion and cemeteries full of dead Americans, and I knew how many ordinary bombs, and fire bombs were being dropped, and how many Japanese were being killed every day, and I had a pretty good idea how many Americans would be killed in an invasion, and I just thought the atomic bombs were great. I came back and found I couldn’t communicate with the people. They were all moaning, “Isn’t it awful that we’ve killed some people.” I had been in the European theater and lived with people whose job was to go out and kill people every night; I mean RAF bombers. I had gotten used to the fact that in wars people get killed. I was more concerned with the fact that we had played a key role in stopping all this killing, which I don’t regret. So I just found I couldn’t communicate with my Los Alamos friends anymore. I found it distasteful, so I got out of there as fast as I could and came back to Berkeley. So I am not aware of these recruiting problems. I know that a lot of people were scrambling around for jobs; one of my very close friends who had done a good job at Los Alamos naturally assumed that he would have a good job waiting for him. He found that he was interviewed by 10 or 15 research laboratories and universities and didn’t get a single offer—it really shook him up.
Did Berkeley pick up new people as a result?
Not a terrible lot. Ernest Lawrence kept on a few of the people who had come here during the war and done outstanding jobs—Burton Moyer for example, who is now head of our Physics Department here, was a young physicist who worked on the calutron project, and he stayed and went on the faculty. Wilson Powell who had done most of the magnet design work stayed on in the faculty. Wilson had come from a small school in Ohio. A lot of people had found themselves stuck in small places like that because of the depression. One of the best people in the radar business was Andy Longacre, who had been teaching physics at Exeter, which is a prep school; he was teaching at the high school level, and he taught there just because they paid good salaries and gave good living quarters, and it was a nice life. So hardly anybody knew that Andy was very good as a physicist, except a few people who’d been in graduate school with him. He was just lost; he was off the main stream. I got Andy out of Exeter, because I had known him in Chicago. I got several of my friends who’d been at Chicago in the depression years. One of them was teaching physics at The Citadel, not a very high grade place. And other people who were working down in the lower rungs of research establishments in various companies doing routine things. These people came into the laboratory and a lot of them did marvelously well, and have gone on to very successful careers either in industry or in universities. A lot of them used the war as a short of a way to jump up a good many levels—they came in from a small school, went through the radiation laboratory and went out as full professors in first class institutions; they would have been lost otherwise.
That’s interesting. You certainly took advantage of the microwave work and applied it to nuclear physics. How about other people? How widespread was this? In other words, take the group at the radar laboratory at M.I.T. who later diffused to various institutions. As you indicated, they were nuclear physicists before they went in, so we assume they are still nuclear physicists. Did they also use these techniques?
A particularly good example—one of the two or three best examples—is Willis Lamb who was working in the microwave laboratory down at Columbia, Professor Rabi, who was in charge of the advanced development work at M.I.T., making shorter and shorter wavelength magnetrons, had led the Radiation Laboratory from 10 centimeters to 3 centimeters and had sponsored the work at 1.25 centimeters, which turned out to be a horribly bad guess; it landed right smack in the middle of the absorption band in the atmosphere, so that wasn’t a very good guess. But anyway, in the Columbia University Physics Department, he set up an advanced magnetron development group, and this group was making 5-millimeter magnetrons and later shorter wave magnetrons, klystrons and things of that sort. This was what I guess in the baseball business you would call a farm club. They were sort of subsidiary to the Radiation Laboratory, but they worked by themselves and had very little contact with the M.I.T. laboratory. Willis Lamb, who had until that time been a theoretical physicist, learned how to make magnetrons, got interested in experimental physics, and went on to use the techniques he’d learned in that laboratory to observe the Lamb Shift. Purcell and Bloch used the things they had learned during the war to develop nuclear resonance spectroscopy.
Bob Dicke had been an expert on microwave receivers; he worked, I believe, in the receiver division, and he used his knowledge to make very, very sensitive radiometers in which he used lock-in amplifier techniques to measure very small differences in Temperature. He used these techniques after the war to measure the temperature of the sun and the moon, and paved the way for some important advances in radio astronomy. Recently he used similar techniques to observe the oblateness of the sun. Radio astronomy was another field into which people who had learned radar techniques went directly after the war. Taffy Bowen (E.G. Bowen) who was our liaison man between the British and the Americans—he brought the first magnetron over—was our source of all information about radar for a long time at M.I.T. He came in as a practicing radar technician, who had helped Watson-Watt build the first big radar stations and who later installed the first airborne radar set in Great Britain. He came over and lived with us at M.I.T. for the first two years. For a long time, if we wanted to know anything about radar, we’d ask Taffy, but later on we learned enough so that we knew more than he did. He went back to Australia. I shouldn’t say he went back to Australia, he emigrated to Australia at the end of the war and immediately started doing radio astronomy; in fact he was the first one to do high—resolution radar; he showed that the sunspots gave out meter-wave radiation. He is sort of the father of Australian radio astronomy, which for a long time was the best radio astronomy in the world. I visited him in Australia a few years ago and he has had an awful lot to do with the renaissance of physics and astronomy in Australia.
What about people who applied the radar techniques to accelerator work and the accompanying detectors?
That was a sort of a two-stage process. Los Alamos was the first laboratory as far as I know that routinely collected electrons in ionization chambers, and I think that was probably due to Bruno Rossi and Hans Staub with Willy Higinbotham making the amplifiers. Willy Higinbotham had been in the receiver division at M.I.T., had built the indicators for my Eagle blind bombing set, and he went out to Los Alamos and was the leader in the electronics counting area there and is now in charge of that department at Brookhaven. He’s been at Brookhaven for the last 20 years I guess. So Willy brought the modern electronics techniques that he’d learned and helped develop at M.I.T.—the microsecond pulse techniques, which as physicists, we had never needed to know because we were always limited by the collection time of ions, which was like milliseconds. All of a sudden to work radar, we had to make microsecond pulses and here we borrowed from television. Television requires fractional microsecond pulses. It requires that the cathode ray beam be turned on and off several times per microsecond, which required brand new pentodes, vacuum tubes, to be designed and built by companies like RCA.
The earlier tubes didn’t have such a fast response, they just lost all their amplification if you took them up close to a megacycle bandwidth. So in order to make television, the vacuum tube engineers just before the war had developed this new line of vacuum tubes that had a very wide frequency bandwidth at useful amplifications. They were put into radar sets and were very useful there, and then they appeared in the pulse amplifiers that nuclear physicists used at Los Alamos, because nuclear physicists had learned these techniques in radar. So then they appeared at Los Alamos and eventually everybody was using Los Alamos model-200 pulse amplifiers after the war. That was the standard amplifier for a long time until we started using scintillation counters, which were introduced by Kallman at Columbia. He is a great man in instrumentation, whose contributions to physics have not been adequately recognized. He was the first one to show that one could make scintillation counters of appreciable volume. Before that time, the only scintillators we had were zinc sulphide, where the fluorescent light from the zinc sulphide was rapidly absorbed in the zinc sulphide crystals, so you couldn’t get the light out from the interior of anything but minute crystals. Kallman was the first to show that organic crystals like naphthalene—that was the first one that he used—or anthracene or stilbene or things of that sort, were transparent to their own fluorescent radiation, so that you could use a very large volume of fluorescent material and get the light out. This was the great advance in counting techniques that came after the war.
Up to that time all counters involved ionization effects, in which you made particles go through a gas, produce ions, and you collected the ions. There was a certain maximum speed at which you could collect the ions, which was limited by diffusion and voltage breakdown and stuff like that, to about a microsecond. And Kallman came along and showed that the light would come out very rapidly, so then we had to find ways to improve the response time of the photomultipliers, and it has been sort of a seesaw back and forth. First of all scintillators get faster, then the photomultipliers catch up; sometimes the photomultipliers get a little ahead and then someone finds a faster scintillating material. So at the present time the standard resolution time one gets is one nanosecond. Anybody can buy nanosecond circuitry and if you want 1/10 of a nanosecond you can do it without too much difficulty. That’s the time that it takes light to go about an inch. So from before the war, where we were at about a millisecond—maybe a 5th of a millisecond—the time scale at the end of the war was microseconds; right now everybody is using nanoseconds, and there are always a few people who are pushing the state of the art by a factor of ten. At the present time if you have a particular need for it, you can push the resolution to a 10th of a nanosecond. But again, we are beginning to get limited by the spread in transit times through a photo-multiplier, so they had to redesign photomultipliers to knock this spread down. As I say, the ball went back and forth between electronics people and scintillator people. Right now if you use Cerenkov light, which is essentially instantaneous, you can get as short a pulse of light as you please. And now the ball has been tossed back to the photomultiplier people, but then the amplifier people are also in the circuit, because you have to have coincidence circuits and things of that sort to go on after the photomultiplier; so at the moment we are kind of stuck at a 10th of a nanosecond.
People tend to specialize as photomultiplier people, and so on?
Well, everybody counts with photomultipliers now. I doubt that you could find an ionization chamber any place around the high-energy physics end of the Radiation Laboratory.
I didn’t mean it in that sense but in terms of the people who develop the instrumentation to the next higher stage. Are there detector experts?
Yes, there are detector experts, like Willy Higinbotham for example. I should say that in the last few years new counting technique has come in which doesn’t involve photomultipliers and these are the solid-state counters of lithium-drifted silicon. They are wonderful for low-energy physics; they are not too good for high-energy physics because the thicknesses, the active layers, are rather thin and you just don’t make very many electron-hole pairs in these thin layers with relativistic particles that don’t ionize very much. So they are not very much used in high-energy physics, but they are just marvelous for the kind of nuclear physics I was doing before the war. You can run X-rays into one of these things and get a pulseheight spectrum and you can see all of the lines and distinguish copper from nickel; you just look at the X-ray spectrum and you see all the lines and you see them shift over from one element into the next.
You mentioned that in the first proton-proton scattering that was done after the war the two kinds of detectors used were nuclear emulsions and the proportional counters. Did the war, or this period at any rate, have any effect on the nuclear emulsions?
No, the nuclear emulsions had been developed at a very low key by someone from the Kodak Company working with a physicist whose name I don’t remember at the moment, who was at the University of Rochester. He was about the only person who used nuclear emulsions. It was a very tedious technique and the big impetus to the nuclear emulsion technique came with the development in Powell’s laboratories—Powell in Bristol. That was sort of anti-correlated with the war: Powell was not permitted access to any classified material in Great Britain. He was one of the very, very few physicists who was not in the war effort. I think they were worried that he was a communist, or so I’ve been told. I am sure he didn’t work in any war projects, so he used the wartime to develop this technique, which very quickly after the war was over, blossomed out and gave rise to the discovery of the pi-meson. So it was very fortunate for physics that Powell was restricted. I base this on a number of conversation with British physicists. I think it’s true; I hope I am not libel in Dr. Powell, but I am quite sure that he wasn’t working in the war effort, whereas every other British physicist was in the war effort, as far as I know.
You made the point about people at Los Alamos having picked up the radar experience. This implies a transfer of the people from the radar laboratory at M.I.T.?
Yes, that actually took place.
Was it a large group that was first working in the radar and then went to the Manhattan Project?
I don’t know; I think actually the experience from the Radiation Laboratory and the other radar laboratories filtered into the pre-Los Alamos nuclear physics. There was work going on with cyclotrons at various universities where they were trying to measure the fission constants, cross sections, things of that sort. As far as I can tell, people who had worked at the Radiation Laboratory knew what was going on and would go back and see their old friends trying to use their beat-up old prewar equipment and would say, “Look, why don’t you make a circuit like this?” And they would probably draw the latest video amplifier from M.I.T. out for them and their friends would build it and it would work well.
This was in the early 40s?
Yes. I think there was a general filtering over from the radar into the Manhattan District or pre-Manhattan District of the latest techniques. Everybody felt that the fellows working on fission were cleared and they ought to know these things; no reason why they should have to invent them themselves. And then of course there were a number of people who did leave the Radiation Laboratory and go to Los Alamos.
And some went back to their own universities for some time?
I think some of them did, I don’t know. But I know for example at Los Alamos, in addition to Willy Higinbotham, Bob Bacher was the head of the instrumentation division for a while, and later became head of the implosion bomb division. Bob Bacher had been a division leader at the Radiation Laboratory at M.I.T. Ken Bainbridge had been a division head at M.I.T.; he went to Los Alamos. I’d been a division head at M.I.T. and I went to Los Alamos. So all of us knew what sorts of techniques were available at M.I.T., and who to go and see and who to call and what blueprints to ask for.
Your home institutions were scattered, so these were all centers of work in the fields.
Yes. There was a lot of cross fertilization.
[After lunch] We are resuming after a lunch break. I’d like to explore just a bit the trends in nuclear physics in the postwar period which demonstrated a branching off into the higher energy type of work. I’d like to know how this transition came about.
I think it came about largely by the introduction of the tools that gave higher energy particles, particularly the 184-inch cyclotron which originally gave 200 million volt deuterons and 400 million volt alpha particles. The protons didn’t come until the machine was converted so it would operate on higher frequencies. The proton linear accelerator would be forgotten by anybody but me in this connection, but it did have the highest energy protons available in the world for a period of about a year—32 million volts, and as I said, the cyclotron didn’t get its proton beam until it had been converted some time later. And then, of course, the 300 million volt synchrotron that McMillan built came in about nine months after the proton linear accelerator did. After the war, the cyclotron came on first and then the linear accelerator and then the synchrotron.
What was the timing? When did the cyclotron come on?
The cyclotron came on at the end of ‘46.
Did you work on it as soon as you were back?
No, I was spending most of my time on the linear accelerator. I did do some experiments on the 184-inch cyclotron after it was completed. I worked on nitrogen-l7 and on a number of short-lived radioactive elements, I measured the lifetime of the mu meson, and that’s about all I did on the 184-inch.
But this was different from the earlier work. This was using it as an experimental device rather than being involved in the construction and the perfecting of it.
I was not involved in the construction at all, no. I followed it closely; was in the building several times a day and knew exactly what was going on and who was doing it, but I didn’t have any responsibility myself. I had all the responsibility for the linear accelerator and the 4 million volt Van de Graff that injected into it. That was the highest energy Van de Graff generator that had ever been built.
This took how long a period of time?
It was almost exactly two years from the time we started to make the linear accelerator to the time we had the first beam. It was probably November 1947 when the first beam came on.
And you got back then in the fall of ‘45.
So we can say that the trend then was nucleon-nucleon scattering and that kind of experiment started just at that point?
No, it re-started, it had been going on before the war, and the Van de Graff generators had done p-p scattering in a number of energies increasing with time. Bob Wilson had done some p-p scattering on the cyclotrons, both 37-inch and 60-inch. I guess he did the 60—inch right after the war, as a matter of fact. It was just something that was obvious to do.
How large a group worked with you on this?
On the linear accelerator? Well, you saw that picture of the tank with all the people sitting on the top; that was just about everybody. There were probably a few people there who were working on electronics or something from a laboratory pool.
About a dozen people?
Oh, much more than that, about 40 people.
That was the new approach—you never worked on that scale before?
Well, I had in radar at Los Alamos, but I hadn’t in physics, no.
And I imagine the other thing that was different was the fact that money was available, and you hadn’t had this expectation. Didn’t you at the end of the war at Los Alamos even anticipate that government support for physics research would be greater?
Well, I thought that there would certainly be more than there was before the war, but I didn’t anticipate anything like what turned out to be the case.
What were the first significant results of the proton linear accelerator?
The first thing of course was the p-p scattering; then we also did a lot of work on short-lived radioactivity which couldn’t be reached with the accelerators that were then available. I think that it was not really terribly significant. The greatest significance of the proton linear accelerator is that it just turned out to be the standard injector for all the higher energy circular machines. I started it because there seemed to be a limit on the energy attainable with cyclotrons, and with betatrons. I thought here was a way to circumvent the two energy limits; with the betatron the energy limit is the radiation and with the cyclotron, the limit was the particles falling out of phase, because of magnetic focusing and the relativistic mass increase. People knew about the Thomas cyclotron, but nobody really believed in it; nobody had spent any time verifying it. The big cyclotron had been built as a brute force standard cyclotron where the deuterons would gradually fall out of phase as they got heavier and as the magnetic field fell off. There appeared to be a clear limit on the energy of the cyclotron, and the linear accelerator would circumvent that. The other thing was that machines appeared to be getting more and more expensive at a rate between the square and the cube of the energy. In other words, if you take a cyclotron and double its radius, then you multiply the cost of the magnet and most of the equipment by 8, because the gap gets twice as big and the area of the pole gets four times as big. So it seemed that as the energy of the machines got bigger and bigger, the cost would go up and some day it would be so high that one couldn’t ... [Interruption for telephone]
I’d like to ask, in the nature of a summary question, for your own interpretation of the history of the field of nuclear physics, from the experimental viewpoint, from the time when you began to know something about it to the present, emphasizing the differences between the earlier period and the present one. Nuclear physics is a field that developed in time. How can you identify the major trends in the field and how they changed?
I am not very much up on nuclear physics now. I occasionally look at an article in The Physical Review on some isotope that either discovered or worked on to some extent, and find out that where I identified one gamma ray or one beta ray end point, they have analyzed the beta ray spectrum into 5 components and have 17 gamma ray lines and have the polarization and the angular correlations and the strength of each line, the internal conversion coefficients and a lot of things like that. It’s just a completely different ball game. I find very little to tie what we were doing in the late ‘3Os to what is now going on. The instrumentation now is so much more sophisticated, and I hardly recognize that it’s the same business.
One difference is the instrumentation. What about the types of problems that one adjusts himself to?
I think now when so many of the isotopes have been found—and I should say that we considered it good fun, and you got brownie points for discovering new isotopes. But now that the isotope table is pretty well blocked out, what you get your points for is not even finding the levels—that was where one got points, say, 15 years ago. Tommy Lauritsen used to keep track of all the levels on his chart down at Cal Tech. Now you get your points for identifying spin and parity and the relationships between the levels. The kind of thing that we are doing in particle physics now is to find the groups to which the levels belong.
Why was the identification of nuclear levels so important at that particular point?
Well, it’s important now in constructing theories of nuclear structure, in the same way that people had to identify optical spectral lines as belonging to a particular series before they could unscramble optical spectroscopy. You had to go and find out that if you put a magnetic field on the source certain lines would split up one way and other lines would split up another way, and this helped you identify, give the names to the lines. There is the famous story of the lady who heard an astronomer give a talk and afterward she said, “Professor, I understand everything about what you do except how you find the names of the stars.” That’s what the game is now in nuclear physics, getting the names of the levels.
That’s an interesting difference, I think, and it brings to mind another question. How has the changing theoretical work on models of nuclear structures affected the types of work that experimentalists have done?
It’s pretty far out of my business now. I don’t really keep up on nuclear theories; there really weren’t any nuclear theories when I was in the business. There was the liquid drop model of Bohr’s which played a significant role in certain identifications. He happened to be lucky in the case of uranium isotopes, but as I say we would have been in very severe difficulties if we had taken it at face value, and if it hadn’t been for these young chaps—Havens and Rainwater—working completely outside the fence, we would have been in terrible trouble.
And as far as the present state of nuclear physics, I understand your disclaimer here of direct recent knowledge, but what is your impression of it as a field in terms of its development? Do you think it has reached a plateau, or is it at the end of a certain stage where it now is sort of inactive, although there are many good people doing a lot of good work? Do you think it’s ready for a new takeoff?
I don’t know; to me it’s just a little dull, so much of the work can be done by technicians. It’s the kind of work that before the war I used to observe going on at Shell Development Laboratory and places like that, where people were measuring the infrared spectra of organic compounds. What you do is go out and buy an infrared spectrometer from some commercial company and you hire a couple of girls in white suits to put various samples in and plot curves and measure them up. It’s completely programmatic. And the same sort of thing goes on in this field now. You have technicians who run alpha particle spectrometers and beta ray spectrometers and gamma ray coincidence circuits. And the people working in the field are doing very much what our graduate students are doing, they are putting things into computers and analyzing the print-out, and they are pretty well disconnected from the experimental side of it, in the same way that we are. I can’t complain because our people don’t go down and look at the bubble chamber very often or at the bevatron. They ask the bubble chamber operators to expose a certain number of millions of frames of film, and then they ask somebody else to measure them, and then run them through computer programs, and then they start with computer program output and process this data. I notice as I walk past the chemistry building that people are loaded down with print-out, just like we are. I personally find this very dull, but have to bear some of the responsibility for its existence because people didn’t do this before we made the bubble chamber programs. But we are trying to get away from that now in the balloon program.
Do you feel that this pattern of beam to bubble chamber to computer, which characterizes so much of the work in high-energy physics here and elsewhere, imposes a certain pattern on the choice of experiments that one has?
Certainly it does.
And the balloon is an attempt to get away from this?
Yes. The interesting thing to me is that if I were a graduate student coming into physics now, I wouldn’t work in my own group because I would find it too dull, and I am very pleased that young graduate students that I think are the best available, tell me the same thing—that they wouldn’t work in my group if it weren’t for the balloons. They find this very exciting, to get their hands on some equipment. They don’t spend all their time looking at print-out. So I get a lot of personal satisfaction out of perhaps being able to start turning the wheel around in the other direction, because I brought the computers in. I didn’t do it personally, but if it hadn’t been for the big bubble chambers they wouldn’t have come—the big bubble chambers and the data reduction techniques that my group and I pioneered. So now perhaps we are reversing this a bit.
It’s interesting. One of the things that makes the work done in nuclear physics per se dull now is not only the stage…
No, it’s dull to me; it’s terribly exciting to the people that are in it.
I understand that qualification. It is not only the subject matter but even the technique that you find dull in this field of high-energy physics?
You refer here to this work as nuclear chemistry. Disregarding the difference in techniques today, is nuclear chemistry what all of us would have called nuclear physics in the 30s?
Yes. The only reason we call it chemistry here is that the Radiation Laboratory was started by physicists, Ernest Lawrence and all his young henchmen, though a few chemists came in and joined forces with us, like Glenn Seaborg and Phil Abelson and others. Then during the war, Glenn went away and became a very distinguished leader in his own right; he was the leader of the plutonium chemistry project at Chicago. When he came back to Berkeley, Ernest Lawrence invited him to set up his own group, and to distinguish it from the physicists who were also working in radioactivity, working with machines; Glenn’s group was called the Chemistry Group. Chemistry is by definition what chemists do.
L. W. Alvarez, A. C. Helmholz, and B. T. Wright, "Recoil from K Capture," Phys. Rev. 60, 160 (1941), Abstract.
National Defense Research Council