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Oral History Transcript — Sir John Cockcroft

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Interview with Sir John Cockcroft
By Charles Weiner
At Washington, D.C.
March 28, 1967

Listen to Cockcroft talk about all the new discoveries around 1932 and how they found out about them.

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Sir John Cockcroft; March 28, 1967

ABSTRACT: Three years of preparation which led up to achievement, with Ernest T. S. Walton in 1932, of the first artificial transmutation of elements by accelerated protons, and the joyous reactions of his colleagues at the Cavendish Laboratory. With a three month grant from the Rockefeller Foundation, in 1933 visits with Robert Van de Graaff in Boston, Merle Tuve in Washington, Charles Lauritsen in Pasadena and Ernest 0. Lawrence in Berkeley. In 1937, on his second American trip, noticed that the “sealing wax and string” at University of California at Berkeley had been replaced by engineering. Effect of influx of German refugee physicists. Rutherford’s attitude toward a cyclotron at Cavendish because of Marcus Oliphant’s low voltage ion source. Need for higher voltages and benefaction of a quarter million pounds from Lord Austin. Rutherford’s complete control of Laboratory, the changing role of Cavendish over time; impact of the discovery of fission in England; effects of the war on nuclear physics and the differences in postwar planning and funding of research. Also prominently mentioned are: Niels Henrik David Bohr, James Chadwick, Ralph Howard Fowler, Petr Kapitsa; Cavendish Laboratory, European Council of Nuclear Research, Dept. of Physics at University of California Berkeley, and University of Oxford.

Transcript

Weiner:

In the interview that was done for the quantum physics project you covered in great detail your education, your first interest in physics, and the general atmosphere in the 1920’s. I would like to jump to 1932. In late April 1932, you achieved the first artificial transmutation of elements by accelerated protons. You described the work leading to this in your Nobel Prize address. I would like to know about the reaction to this work at the Cavendish Laboratory at that time.

Cockcroft:

Well, the reaction at the Cavendish Laboratory at the time, I think, was not unexpected. After all, we’d had the discovery of the neutron around about the same time. We had the work of Blackett and Occhialini on positive electrons, and the Cavendish Laboratory was very ripe for new discoveries. It was prepared by very frequent and thorough discussion of all the latest theories of quantum mechanics, penetration of nuclei by alpha particles on wave theory, and so on. So there was nothing, I would say, unexpected in the reaction of the Cavendish except probably very great satisfaction at having pulled it off.

Weiner:

You had been working for this goal for about a three-year period.

Cockcroft:

We’d been working on it for about three years, from the beginning of 1929. However, it’s not an unusually long time for a project of this kind.

Weiner:

When you did achieve the results, though, was there immediate discussion? Was there a feeling of elation?

Cockcroft:

Oh, yes, a feeling of great elation, certainly. Oh, yes, tremendous. It was extremely exciting to see the alpha particles in this transmutation. The first thing we did was to call up Rutherford on the laboratory exchange and invite him to come down and have a look at the scintillations, which he did. He, of course, was very excited about it.

Weiner:

How much time elapsed between the discovery of the neutron and your work?

Cockcroft:

It was within two weeks. It was very close together. I’ve forgotten the exact dates, but very, very close. Chadwick was working just around the corner within 20 yards of where we were working.

Weiner:

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One of the things that’s particularly interesting about that year is that so many things happened together. Did you have a feeling when all of these things were beginning to break that this was the opening up of a new field? Or did you feel that it was just the fruition of work that was a continuing tradition?

Cockcroft:

Well, I think it was a general acceleration of progress which was really very largely due to the introduction of new techniques: new electronic counting methods and methods of applying high voltages to acceleration of particles, Geiger counter controlled expansion chambers — in the case of Blackett and Occhialini. There were all these new techniques which had been developing for three or four years in the labs and suddenly blossomed and bore fruit.

Weiner:

How do you account for these developing in this short period? You mentioned that there was a great deal of discussion, and I assumed this was either at the colloquia or at the Kapitsa Club.

Cockcroft:

Yes, as a result of these very frequent discussions, very intense discussions, we were very fully informed of what was going on in other countries — in Copenhagen, in Germany and France. So we would discuss the latest papers in the various journals within one or two weeks of their publication. There was no time lost in taking advantage of the latest developments overseas. For example, the work of Curie and Joliot was known in the lab within a few days.

Weiner:

In 1934 you mean.

Cockcroft:

No, not 1934, ‘32, just before the discovery of the neutron. The fact that this was immediately appreciated in the lab led, of course, at once to Chadwick getting on to the neutron. There was no time lag at all.

Weiner:

This was through reading journal articles. Was this also through visits?

Cockcroft:

Well, it was primarily through reading journal articles and discussing them as they appeared and not waiting for several months before you got round to appreciating their significance.

Weiner:

These discussions generally took place at the Cavendish colloquia.

Cockcroft:

At the Cavendish colloquia, yes, or at the Kapitza Club. It was more informal there.

Weiner:

Was there a feeling of a difference between the theoretical and experimental approach? Was somebody generally the theoretical interpreter at these meetings? Did Fowler play this sort of a role?

Cockcroft:

Well, I think Fowler interpreted the theoretical papers for Rutherford. I don’t think that he was interpreter for the general body of the research group. We were quite capable of understanding the significance of Gamow’s articles in the Zeitschrift fur Physik without any interpreter. And we would discuss these papers directly, you see.

Weiner:

Gamow had been a visitor in an earlier period.

Cockcroft:

Yes, Gamow was working in Copenhagen in 1928 and he came early in 1929 or late ‘28 -- I’ve forgotten which -- and he came several times later on.

Weiner:

After your work with Walton in April 1932, how soon was it before you received reaction from other institutions, from people in other countries and other places?

Cockcroft:

Well, the reactions were almost immediate, I think. The most immediate reaction, I think, came from Berkeley and from Tuve’s lab in Washington, where they had all been working on development of high-voltage equipment, such as the cyclotron or the Van de Graaff machine, toward just this kind of experiment. So they were the very first reactions.

Weiner:

How did they learn of it? Did Tuve and the others learn of it through the journal literature or were you in correspondence with them?

Cockcroft:

We had correspondence with Lawrence, with Tuve, with Van de Graaff.

Weiner:

I see. You remember keeping up with Lawrence’s work at the time. So when you say you were generally aware of what was going on, it was not only through the journal literature, but also through correspondence with these groups.

Cockcroft:

Other groups, that’s right.

Weiner:

Where did you first hear of Lauritsen’s work, which followed your work? When he learned of what you had achieved, he used his high voltage tube as an accelerator. Did he write to you about that or did you learn of it through the literature?

Cockcroft:

No, I think I learned of it through the literature. Then I went out to see Lauritsen in 1933, of course, and went to see Lawrence at the same time and saw Van de Graaff. That was my first personal acquaintance with him.

Weiner:

I see. What were the circumstances of the trip? Were you dispatched as a representative from the Cavendish?

Cockcroft:

No, I got a grant, I think from the Rockefeller Foundation, to go on this three month trip. I took my wife across for the first time, and I went first to Boston to see Van de Graaff, went down to Round Hill where he had the very big 2-million-volt generator; and then on to Tuve in Washington, where he had this smaller Van de Graaff generator; and then on to see Lauritsen at Pasadena; and then up to Berkeley to see Lawrence. It was a wonderful round trip.

Weiner:

All of this in a three-month period.

Cockcroft:

That’s right, yes.

Weiner:

What was your impression of Berkeley when you came there? You’d heard of it through the literature and through correspondence, but what feeling did you get?

Cockcroft:

Well, of course, it was really interesting to see it actually in operation after having read so much about it in the journals. I was very much impressed by the way of working; to see the sealing-wax and string way of working on the cyclotron, which functioned for very short periods of time. They had a two-shift system, one shift doing the experiments, the other shift keeping the cyclotron going. And as soon as a leak developed, the maintenance shift would dash in and the experiment shift would retire backwards. A highly organized system.

Weiner:

Was Lawrence on the scene?

Cockcroft:

He was on the scene. He was in Berkeley. He looked after it. Of course, he was a tremendous enthusiast. We decided right away that we would like to build a cyclotron in Cambridge, but we didn’t make a start until four years later. Rutherford wasn’t agreeable to our making a start on the cyclotron at that time.

Weiner:

What was his feeling on it? Why did he resist?

Cockcroft:

The reason was that Oliphant had built for him a low- voltage, 200 kilovolts accelerator, and they started work on the d-d reactions and were able to get transmutations down to very low voltages, about 20, 30 keV. They thought that they were going to do everything with low voltages, which of course turned out to be wrong. And so he wouldn’t agree to our building a cyclotron.

Weiner:

Was it a question of the style of physics as well? In other words, was the idea of the cyclotron somewhat different than the tradition of physics as it existed at the Cavendish?

Cockcroft:

I don’t think it was that so much. Rutherford would never have objected to trying something new in the way of innovations, but he just felt that he could get all the results he wanted at low voltages and with increasing the intensity of the ion source. And that’s what they did. Oliphant produced an ion source of a hundred times our proton intensity. As long as they worked with light elements it was all right, but once you got past sodium, you couldn’t do much with these low voltages.

Weiner:

And during this four-year period what had transpired to change the situation so that you finally got a start on it? I know you went to Berkeley again in 1931.

Cockcroft:

First of all, it turned out that Oliphant and Rutherford were wrong. They realized that they couldn’t do everything with low voltages. We never were able to work very much above fluorine with any accuracy -- that was the first factor. The second factor was that the Cavendish had been able to get a benefaction of a quarter of a million pounds, which was quite a lot for those days. So we had some money to build a cyclotron with.

Weiner:

What was the source of this gift?

Cockcroft:

Well, it came from the great motor magnate, Lord Austin. We set out about 1934 to get money for the Cavendish, and Baldwin, who was Prime Minister, managed to persuade Austin to provide this money for the Cavendish. That gave us some spare cash to build a new wing of the Cavendish.

Weiner:

What arguments did you use in order to get this support?

Cockcroft:

Just that we needed higher-energy particles. We wanted a high-voltage lab with up to two million volts; we wanted a cyclotron; and we wanted more space for the Cavendish.

Weiner:

But why is it that a high government official would feel the need for this type of support?

Cockcroft:

We didn’t go to government officials.

Weiner:

You mentioned the Prime Minister, I’m sorry.

Cockcroft:

No, this was a private benefactor, Lord Austin.

Weiner:

I thought you had mentioned, though, that you had gone to someone who was the Prime Minister and he had gone

Cockcroft:

We went to the Prime Minister because he was chancellor of the University, to get him to find a benefactor or to sign an appeal for money. And Mr. Baldwin was so lazy; that he decided he’d do it in one go by going to Lord Austin, who in turn got a peerage for the benefaction. So he produced £ 250,000 and in return he was made a Lord.

Weiner:

Where had the support come from prior to that time?

Cockcroft:

Well, it had come mainly from the Royal Society. We got a grant of a thousand pounds from the Royal Society, which enabled us to build our first accelerator. We spent £ 500 on the transformer and with the remaining £500 built the rest.

Weiner:

When did government support for the laboratory begin?

Cockcroft:

Well, there was a limited amount of government support from the Department of Scientific and Industrial Research. For example, Kapitza got the money for his machine from the Department of Scientific and Industrial Research, and then he got £ l5,000 from the Royal Society to build the Mond Laboratory for his work. So there were various sources of funds, but it was very unusual to get anything very large. The Kapitza grant from the Royal Society was very unusual.

Weiner:

Were any special results implied or promised from this work? Do you think that this may have been a factor?

Cockcroft:

No, it was probably just that Kapitza had the strong backing of Rutherford to get the money. He must have exerted a fair amount of persuasion to extract this from the Royal Society. It was an almost unheard of benefaction.

Weiner:

How were those things done in those days? Was it by a formal written proposal, or was it done through private conversation?

Cockcroft:

Well, it would be done in the first place in private conversations with officials of the Royal Society and then of course it would have to go to the Council of the Royal Society and get their approval. But Rutherford was a very powerful man in this respect.

Weiner:

I asked that because I wonder if in the archives of the Royal Society one may be able to find the records of this proposal for support. It would be interesting to see what reasons were advanced at the time.

Cockcroft:

It would be in the archives of the Royal Society or it might even be in the Cambridge University Library. All the Rutherford letters are in the Cambridge University Library, so it’s almost certainly there.

Weiner:

One gets the impression that during this period of 1932-1934, many things were happening in the field and that it began to establish its identity as a very popular field of physics. What I’d like to know is whether the term “nuclear physics” was articulated as such, and whether people described themselves as nuclear physicists in that period.

Cockcroft:

Yes, I think we would all have described ourselves as working in the field of nuclear physics. I don’t think we would have called ourselves nuclear physicists, a generic title of that kind, but nuclear physics was certainly our field.

Weiner:

Was there a time when you didn’t feel this. In other words, can we put a date on when you definitely said that your field of research was nuclear physics?

Cockcroft:

Well, I suppose the numbers started to become appreciable around about 1925 or ‘6 or thereabouts.

Weiner:

You mean number of people in the field.

Cockcroft:

Yes.

Weiner:

Was there any upturn later, in the early ‘30’s? Was there a movement of people from other fields into nuclear physics?

Cockcroft:

Not so much in the Cavendish. There were of course some significant moves. Oliphant was moved from-his work on the impact of positive ions on surfaces over to nuclear physics. Certainly a number of the good people moved over at the Cavendish, but the big move over was in the United States where there was a wholesale conversion of people to nuclear physics.

Weiner:

When do you think that occurred? In what period of time did this conversion occur in the United States?

Cockcroft:

I think this was after 1932. For example, the Chicago group -- the Allison people in Chicago -- moved over in 1932.

Weiner:

And yet you feel that in England this wasn’t as noticeable, that there was a core group in the field?

Cockcroft:

Yes, there was a very strong group in the Cavendish. There were probably about 50 people working in the field, people who worked on the more classical branch of nuclear physics -- Alpha-particle scattering, gamma-ray spectroscopy, beta-ray spectroscopy and all these things that were going on in our times.

Weiner:

Would you characterize the Cavendish at that time as a nuclear physics laboratory? In other words, was the overwhelming majority of work definitely being done in nuclear physics?

Cockcroft:

Oh, yes -- 80% at least. The other activities were only a sideline: the Appleton group was one and there was a group at the Observatory under C.T.R. Wilson working on cloud phenomena and thunderstorms. And there was G.I. Taylor working on classical physics. Very few.

Weiner:

In the group working on nuclear physics, was there a division of labor? Not in today’s sense of a team, but a conscious division of the work.

Cockcroft:

Oh, yes, there was, very much so. Blackett had the Geiger controlled Wilson chamber group (?); Dee was out working with C.T.R. Wilson on Wilson chamber photographs of nuclear events. Ellis, by and large, was the head of a small group on gamma-ray and beta-ray spectroscopy. And so on. So the field really was divided up in a quite informal kind of way.

Weiner:

Although informal, was this division by design?

Cockcroft:

Yes, I think so. It was Rutherford really who, I suppose, had the over-all decision about who should look after what in the Cavendish and had to approve what (projects) they carried out even though they were suggested by other people.

Weiner:

And yet the small group or the individual working on a specific aspect of nuclear physics would still be responsible for building its own equipment.

Cockcroft:

Entirely, absolutely. Oh, yes. It was all “do it yourself” in the Cavendish.

Weiner:

What if you needed funds for the equipment? How would one obtain these funds?

Cockcroft:

Well, you had to go to Rutherford and get the money. That was the only way. If it was a small amount, you’d ask Chadwick. Chadwick was the assistant director of research in nuclear physics, and he looked after equipment. He’d provide high vac pumps or diffusion pumps or whatever the laboratory could afford. But if you wanted a thousand pounds, you’d have to go and talk to Rutherford.

Weiner:

What if you were in the 20% group, the group not in the mainstream of nuclear physics? Was it more difficult for this group to get funds, given the fact that resources were limited?

Cockcroft:

It was much more difficult, yes. I think they were rather tolerated by Rutherford. So long as they didn’t ask for too much it was all right; on a small scale, it was all right. But I think there would have been difficulty if they’d tried to get large sums of money. They wouldn’t have had the support.

Weiner:

Did the laboratory serve as a stopping off place for visitors, as a focal point for visitors from other countries, especially during this period of the early ‘30’s?

Cockcroft:

Well, there were a very large number of short-term visitors from the continent, people like Heisenberg and Gamow and Schrödinger, Einstein, Bohr. A continual stream of visitors who would come for two or three days and give a talk, a colloquium; and then of course, there were the overseas research people who would come for two or three years to take their Ph.D. degree, people like Oliphant. So half the laboratory groups were from overseas, roughly I’d say.

Weiner:

Does “overseas” include Americans in this case?

Cockcroft:

Including Americans. There were quite a lot of Americans. Smyth was there, I remember, at one time. Bainbridge was there. In fact, I borrowed a Geiger counter from Bainbridge to see whether we could produce artificial radioactivity with protons. Fortunately, he’d brought one over from Harvard.

Weiner:

What about the impact on the laboratory when refugees started coming from Germany? I imagine they were not just a casual visitor or two, but rather a significant group.

Cockcroft:

Yes, they started to come over a bit later -- I suppose about 1934. Peierls, for example, was one of those who came and worked in the laboratory for a time. But this wave of people from Germany was about two years later than the 1932 discoveries.

Weiner:

You mean about ‘34, from ‘34 on.

Cockcroft:

From ’34 onwards, yes.

Weiner:

Yes, that would correspond to the events in Germany.

Cockcroft:

They had a much bigger impact at Oxford because Simon went there, from Germany. He really revived physics at Oxford, which had almost been dead, and so he had a tremendous impact.

Weiner:

You characterized this impact as having primarily theoretical influence.

Cockcroft:

And also new experimental techniques. Simon started the low-temperature school in Oxford.

Weiner:

But I mean in general the impact of the group on nuclear physics. Do you feel that the impact of this group was theoretical?

Cockcroft:

Yes.

Weiner:

In what sense? In the sense of theories of nuclear forces or in nuclear models?

Cockcroft:

Our main contact with theoretical physics was through our contemporaries -- Dirac, L.H. Thomas, Mott, Wilson, Lenard-Jones, Hartree, Gamow and later Peierls. The contact with the mathematical faculty was less effective because theoretical physics was taught in the mathematical faculty.

Weiner:

And this was primarily Rutherford’s influence?

Cockcroft:

Yes, that’s right.

Weiner:

He apparently played a very important role in taking up the cause of the refugee physicists.

Cockcroft:

He did, yes. He played a very important role, yes. You see, he was a very liberal-minded person, of course, with great prestige and great influence.

Weiner:

I know that Goldhaber was there, and there were a number of other people we know of -- Peierls, and Bethe, who didn’t go to the Cavendish. Why was it, though that such a large group moved on, that they didn’t stay in England but moved on to the United States?

Cockcroft:

That they didn’t stay in England? Well, because I suppose there wasn’t really money to find the jobs. I would think that that was probably the major reason.

Weiner:

Were they received well?

Cockcroft:

Oh, yes. I think there was no feeling of opposition at all. I think they were welcomed. We liked to have Peierls there; we liked to have Goldhaber. There was absolutely no prejudice against them at all.

Weiner:

Was the economic situation difficult at that time? Did you feel the pinch in general in the academic world in the ‘30’s?

Cockcroft:

Well, you see, there was a very limited number of jobs in the Cavendish. At that time the staff was limited. Whenever positions became vacant, there were a lot of young people about who’d be appointed. Local people, like Oliphant or me or Feather or Dee were in the running for jobs as they came along. So you see, there were certainly no spare jobs available. You couldn’t go to any other source and get money for a new post. So it wasn’t very easy to fit a large number of refugees into the Cavendish organization at that time. I think that was part of the trouble.

Weiner:

The impression that I get in talking to some men who passed through is that there was no permanency about it; they had temporary fellowships of one sort or another.

Cockcroft:

That’s right.

Weiner:

What about new people from England being trained and receiving their degrees in the laboratory? Was there a considerable increase in Ph.D.’s during this period?

Cockcroft:

Yes, because I think the population of the lab was growing. That was the principal reason.

Weiner:

One other question I didn’t mention relates to the contact with the Rome school of physics: Did you keep in touch with them? Were you aware of their work?

Cockcroft:

Well, we had all their papers, of course. We received all the journals and received all the reprints of the papers. I have a complete collection of reprints of the Fermi school still in my possession. So we were in very close touch with what was going on.

Weiner:

Some of the people around Fermi visited England. I guess Rasetti visited, or perhaps Amaldi and Segre? Certainly not all of them, but a few of them visited in the early ‘30’s.

Cockcroft:

I think they came over to the international conferences. There was a big one in ‘33. But I don’t remember them coming to the lab. Maybe my memory isn’t very good, but there’s no record of them in the Kapitza Club proceedings, as far as I remember. Perhaps Rossi, but I don’t remember Fermi coming.

Weiner:

Did the Kapitza Club meet in the summertime? I think some of them came in the summer. I don’t remember just who, but I think it was a tradition because Rome was so hot, to travel to various parts of the universe during the summer.

Cockcroft:

Yes.

Weiner:

Some went to Columbia later on. In the earlier times some went to the Cavendish.

Cockcroft:

Yes. I don’t know. We’d have the record of that, of course, in the Kapitza Club minutes book. You could easily check on that, but I just don’t remember Fermi coming. I remember going to visit Fermi but not the reverse visit.

Weiner:

Well, maybe it was some of the other people. Maybe it was Segre or Rasetti -- I’m not sure . You mentioned an international conference. There was an international conference on theoretical physics in 1934 in London.

Cockcroft:

On nuclear physics.

Weiner:

Oh, that’s right. What was the main thing of interest at that meeting in your recollection?

Cockcroft:

Well, I think it was the reporting on all these results on transmutation, including the first results from the United States and results from Germany and results from France on artificial radioactivity. It was a very exciting kind of conference. Prior to that there was the Solvay Conference in 1933 which was a much smaller more intimate conference.

Weiner:

At this 1934 -- conference, did you have the feeling that this field was really in full bloom?

Cockcroft:

Oh, yes. It was growing very fast. It was quite obvious that it was developing in a very big way.

Weiner:

Was it apparent at that time that the United States was going to play such a strong role in this field?

Cockcroft:

I think that the big development of cyclotrons had hardly begun in l934. It was just about getting underway, I think. By the time I went again in ‘37, there were dozens of cyclotrons about. But it was just beginning. Of course, there’s always a lag before you start to build when you get into a new field.

Weiner:

On the conference itself, do you know anything about its origins, who called it, and who organized it, and what the motivation was?

Cockcroft:

Well, as far as I remember, it was the Physical Society. There are of course all the records of the conferences.

Weiner:

Are the informal records preserved? I know the Proceedings were published, but are there any informal records that you know of that do exist now?

Cockcroft:

No, not that I know of. There probably are, of course, but I just don’t know where they are.

Weiner:

For example, it would be of interest to find out where the money came from, because I think it represents a departure for nuclear physics.

Cockcroft:

Yes, for that side of it we could probably find the records. There would certainly be information. I might easily have some in my own files.

Weiner:

I’d like to remind you of that someday and write to you. Perhaps we can follow it up. You mentioned that on your second trip to Berkeley, in 1937, you noticed that the cyclotron was being more widely diffused in other American institutions. Were there other aspects of the situation that you noticed had changed? In other words, did you see much of a change at Berkeley itself?

Cockcroft:

Well, of course, there was a terrific change in Berkeley. Yes, a tremendous change because they’d moved from the primitive cyclotron of 1933 all stuck up with wax to the first engineered cyclotron of 1936 and ‘7. That was a tremendous change, and all this business of shift work, rushing in and out to repair the cyclotron, had all gone. It was nicely engineered, completely different.

Weiner:

Still working around the clock though?

Cockcroft:

Yes, still on the shift basis, oh yes, but with a different organization.

Weiner:

Were they doing many experiments at the time? Did you have the impression that it was still in the development stage, where the intent was to get higher and higher energies? Or that, in fact, it was being used more and more for experiments?

Cockcroft:

Oh, no, by that time there were some good experimental groups there. Certainly there were quite a number of good experimental groups. I think the whole set-up in Berkeley had matured by then.

Weiner:

Where else did you go on that 1937 trip? By the way, how long was that for?

Cockcroft:

Well, it was about the same time, just about three months, a little shorter. Of course I went to Caltech again and saw the Lauritsen group there. Then I went up to Berkeley. Then I went to Chicago and saw the group in Chicago, Allison and people like that. And I went to Boston (Cambridge, Mass.) and met Van de Graaff and those people. At Cambridge there was a mass spectroscopy group.

Weiner:

Was this trip also supported by Rockefeller or other foundations?

Cockcroft:

This was a supported trip. Now, where did I get the money for this trip? It was certainly supported, but I don’t remember. You wouldn’t get it from the lab funds.

Weiner:

Did you in any sense go as an emissary of Rutherford on these trips -- you know, as a representative of the laboratory?

Cockcroft:

Well, yes, I suppose in a way, because by that time it had become clear that we ought to build a cyclotron, and one of the objectives in the work was to go to Berkeley to pick up the latest information -- the drawings and the cyclotron magnet and the dee chamber and all that kind of thing. So that was one of the main objectives of the trip.

Weiner:

Then by this time the cyclotron had Rutherford’s blessings?

Cockcroft:

Oh, yes. It was well-established, and it was clear enough that we also ought to get into the high-energy business.

Weiner:

How did you feel at the time about the American journals? Did you regard the American journals as being the hot ones in the field? Or did you regard other journals as being of more significance? Which journals would you go to first for information?

Cockcroft:

Well, up till 1932, we would go to Zeitschrift fur Physik and the Journal de physique in France as the major journals in the field of nuclear physics outside our own, and we’d hardly ever read the Physical Review. As far as I was concerned, it wasn’t until people like Lawrence and Tuve started publishing in about 1932 or perhaps two years before that, when they were publishing articles on high-voltage machines, that we started to read the Physical Review.

Weiner:

And then from that time on...

Cockcroft:

Then from that time on, it became relatively more important than the German journals, although we did, as a matter of fact, get a rather complete collection of all the reprints from the Zeitschrift fur Physik in the field of nuclear physics -- the Bothe school., radio chemists and so on.

Weiner:

And these are the articles that would be discussed?

Cockcroft:

Yes. I kept all my reprints of that time, so it does give a cross section of what we were reading.

Weiner:

What do you think was the effect of Rutherford’s death on nuclear physics at the Cavendish?

Cockcroft:

Well, I think it had a rather profound effect because the school was just beginning to break up, Chadwick had gone, Blackett had gone, and Oliphant had gone. This was really just the beginning of the break-up. Now, if they’d brought Chadwick back, this would have had a big effect in stabilizing the situation, but then the next thing was that in 1938-39, we were beginning to be absorbed by the war work. The whole school practically disappeared the first day of the war. Most of the senior people never came back. They went to other jobs, went to other chairs, set up their own schools.

Weiner:

There was a very short time lag between Rutherford’s death and the beginning of the war really, wasn’t there?

Cockcroft:

That’s right. We really started getting involved in the war preparations towards the end of ‘38. It was about a year after Rutherford’s death.

Weiner:

One other question on Rutherford before we get into the war period: Did he lecture much on nuclear physics?

Cockcroft:

Oh, yes. He gave a regular course of lectures, three days a week, yes.

Weiner:

Do you know if his own lecture notes for those were preserved? It might be interesting to compare the differences over the years.

Cockcroft:

Yes, I think they’re almost certain to be preserved. They have the whole collection of the Rutherford documents and notebooks in the Cavendish.

Weiner:

And this would likely be in the collection.

Cockcroft:

It would very likely be there, yes.

Weiner:

Did you lecture at the time?

Cockcroft:

I lectured in the Cavendish just as everybody else did. Everybody who was on the staff had to give some lectures, but I was lecturing more on classical physics -- electromagnetism and that kind of thing.

Weiner:

What was the effect in England of the announcement of the discovery of fission? What was the reaction to this announcement?

Cockcroft:

Well, I would say that the reaction was one of very great surprise -- a thing that we kicked ourselves for not having found before. I think to everybody at the time it seemed so simple. There were plenty of neutrons about. And of course there was a tremendous interest in the consequences. At once everybody began to write down in their notebooks requirements for a chain reaction and so on. Peierls did some theoretical work on the subject. It was all very much a hot subject at the time.

Weiner:

When you did calculations for a chain reaction, did you have in mind the military applications of it?

Cockcroft:

No, we didn’t have any military applications in mind. It was just quite general calculations on the conditions required to produce a chain reaction. We hadn’t thought of bombs at all at that time. It was about two years later before that came along.

Weiner:

You mean the early ‘40’s.

Cockcroft:

About February -- March, 1940, yes.

Weiner:

And you could date the beginning of war work and the ending of nuclear physics as it had existed late in 1938?

Cockcroft:

Yes, yes. We had expected that we might have war towards the end of ‘38, and the radar involvement started during the course of 1938.

Weiner:

Let me ask a broad general question here, which is: What do you think the effect of the war was on nuclear physics? In other words, was it just a question of standing still and then picking ‘up where one left off? Or was it a, question of perhaps taking off in new directions as a result of the war? Did it have an accelerating or decelerating effect?

Cockcroft:

I think it had a big effect on the technology of nuclear physics. Many of us became very expert in microwave technology at that time, got used to big projects and so on. So from a purely technical point of view, I think we came back much better equipped.

Weiner:

Specifically with the use of fast electronics and radio frequency techniques.

Cockcroft:

Yes.

Weiner:

Someone has told us that in this country there was a great feeling that nuclear physicists -- that is, people who had done such work in the ‘30’s -- would be very good people for the radar work because their experience would be very adaptable to this work. This had been the experience in England and that’s why the idea was brought over here: get the nuclear physicists into this and you’ve got something. Now, is there any basis for that in the English experience?

Cockcroft:

Well, it turned out that the nuclear physicists were very adaptable. You see the primary group that went into radar, apart from those that had been working on it for some time, was a new group entirely of nuclear physicists from places like the Cavendish and Bristol and Birmingham. And they made a very big impact because they were good experimenters, they had no inhibitions, they could go into microwaves without any background whatsoever except general knowledge. So I think it turned out to be very good that the nuclear physicists went into this field, probably much better than any other group.

Weiner:

You mentioned that one of the things that came out of the war was the new technology, and as an example you cited large project work. What do you mean by that? I think I know what you mean, but I’d like you to explain it.

Cockcroft:

Well, I think that none of us would be afraid of building large machines, proton cyclotrons, and things like that. We got used to engineering in a major way. People went into radio astronomy with a background of experience, and so on. They were able to develop linear accelerators on the basis of that and microwave techniques. All those things were quite natural to them.

Weiner:

Was this apparent at the end of the war? Was it characteristic of groups of physicists in England to think a bit about plans for the future, about their expectations for the postwar period? Do you recall such discussions?

Cockcroft:

Certainly during the latter years of the war, there was a good deal of thought on the question of new accelerators, such as linear accelerators, and so on, in the groups working on radar -- yes, very much so.

Weiner:

Where would these discussions take place, for example?

Cockcroft:

In the radar establishment.

Weiner:

I see. In an informal sense?

Cockcroft:

Oh, yes, entirely informally, yes: the application of high-powered magnetrons and all these things.

Weiner:

Other than the feeling that these things can be done and that now a new technology of group work and radio-frequency techniques is available, what do you think motivated people to want to build these machines? Was there anything special they wanted to study with them that was different than the sort of things that were studied in nuclear physics before the war?

Cockcroft:

Well, first of all, the groups were interested in techniques, in building linear accelerators just for the fun of it. They were very nice machines to build really, and the professional electronics people were involved. And then the nuclear physicists who had been out of nuclear physics for five or six years wanted to get back and use this new variety of machines. So it is really a combination of both -- the engineering physicists and the pre-war nuclear physicists mixed up together in the radar establishments.

Weiner:

Did the theoreticians have any role in this? Did they make any demands because of problems that they were interested in?

Cockcroft:

No, not as far as I remember.

Weiner:

Was there any talk of nuclear forces and proton-proton scattering in that period?

Cockcroft:

No. I think the next important thing was the discovery of the pi-mesons and the mu-mesons and their connection with nuclear forces. At the same time we had the first generation post-war machines and were able to start these nucleon-nucleon experiments and pi-meson experiments and so on. This was in the 1948-49 period onwards. But the pi-meson was the starting point discovered by Powell.

Weiner:

That was about ‘47, I guess. Was there an increased expectation of financial support for research that might not have been forthcoming before the war? Did you anticipate this new situation?

Cockcroft:

Oh, well I think the war had completely changed the attitude of governments toward supporting science. They realized that it had been crucial in the war. It was very much easier to get money afterwards.

Weiner:

At the time, though, as the war was ending, did people consciously think that one of the changed things in the post-war world would be the increased availability of research support?

Cockcroft:

Well, I think they got used to spending money in quite a big way. In fact, we never thought of money at all. In the war we got what we wanted for the war research without any trouble at all. So money was not a limitation. It was a completely different situation from that before the war.

Weiner:

There are some general questions I’d like to ask. One of them is about this question of the rise of high-energy physics or of elementary-particle physics as almost a distinct field -- a field that apparently branched off, some people feel, from nuclear physics. When do you feel that occurred and what do you think were the factors involved?

Cockcroft:

Well, I think this occurred round about 1950 when the cosmotron came into operation and we got the production of the strange particles. I think this was the dividing point. Prior to that the cyclotrons had been useful in the study of nuclear forces and the production of mesons and so on, but it wasn’t until you go to the next step, the strange particles, that it started to branch.

Weiner:

Where did that leave the traditional field of nuclear physics, or, as we now call it, nuclear structure? Did it have an adverse effect because people left nuclear physics and then went into this new branch?

Cockcroft:

I don’t think it really had an adverse effect. I think you got many more people moving into the field, but you still had strong groups in the nuclear structure field. Cyclotrons of a hundred megavolts and up to 200 megavolts were working finally, and the nucleon-scattering field, and all that kind of thing. And high- energy physics proper really was composed of cosmic-ray physicists and the people who had the much higher-energy machines and the theoretical groups. But we had some people who continued to work in both. We had Dennis Wilkinson, for example, who worked in both.

Weiner:

Talking about the whole period starting with the ‘30’s, did the models of the nucleus guide experimental work? When you did experiments did you have a particular model in mind? Or did you invoke a model for interpretation of data?

Cockcroft:

Well, of course, in the late ‘20’s the model of the nucleus was very important in the thinking about penetration of nuclear barriers by protons and different particles. And then I think the Bohr liquid-drop model of the nucleus was a very important factor. It’s very important in explaining neutron interactions and fission processes. So I think models did have a very important effect.

Weiner:

You’re implying that these models were consciously in mind in your own work and the work of other people close to you?

Cockcroft:

Oh, very much so, yes, particularly in the probabilities of transmutation and explaining the results and forces between different kinds of particles. I think it had a very important effect.

Weiner:

Were experiments done specifically to test a model in some cases?

Cockcroft:

Well, our experiments were done specifically to test a model. Yes, very much so. We worked out probabilities of transmutation of particles by protons of different energies up to a million volts with a whole range of nuclei and calculated what the probabilities were that we would be able to observe these. This was done in early 1929. It had a very big effect on our work. I don’t think that we would have had the courage to build a 300 kilovolt accelerator at the Cavendish at that time if we hadn’t thought it was necessary; but it was quite clear from the theory that we should be able to transmute nuclei with about 300 keV protons.

Weiner:

Do you feel that the situation in the post-war period was significantly different as far as the usefulness of models was concerned? For example, take the shell model: Do you think the influence that it exerted on experimental work was of a different degree or nature than the influence of previous models on experimental work?

Cockcroft:

I don’t think it was as important. I think it was useful in interpreting the results, but perhaps not so important in suggesting new experiments.

Weiner:

On suggesting new experiments: It’s been said by some people we’ve talked with, although there’s been some dispute on this, that the available techniques very often determine the choice of experiment. Do you think this has any validity?

Cockcroft:

Well, certainly, in the 1930’s, ‘32 onwards, the fact that we didn’t have Geiger counters about in large numbers in the Cavendish, and such as there were tied to cloud chambers, probably stopped us from getting onto artificial radioactivity. If we’d had Geiger’s lying about the Cavendish at that time, we probably would have tried them and tried to see what was coming out of our targets after bombardment. In fact, the first thing we did after hearing the news from Curie-Joliot was to look around and see where we could find one, and there was only one available in the lab -- quite by accident.

Weiner:

Has that changed in this period, though, when you find very complex, expensive plants and equipment? Does it increase the flexibility of experiment or does it make it more rigid to have a fixed investment in a certain type of instrumentation?

Cockcroft:

I think it’s easier today to set up experiments. Technique is much more developed nowadays, and you don’t have to make everything yourself as you did in the Cavendish in the ‘30’s. You can buy so much more off the shelf; you can buy cards for circuits and so on. Of course it’s very much more complicated, but on the other hand you have greater facilities. It doesn’t take any longer to set up an experiment in the Rutherford lab at Harwell now than it would take us to set up an experiment in the Cavendish in the ‘30’s. There is much more assistance, many more technicians available to help you do it. So although the experiments are more complex and more difficult to do, they can be turned out rapidly.

Weiner:

There’s just one final question I’d like to ask and it’s a personal one. What do you feel has been the most important research work that you have done, in your own estimation?

Cockcroft:

I think the 1932 experiments.

Weiner:

Is this also the most satisfying?

Cockcroft:

I think so, yes.

Weiner:

There’s another question, which will be the final one. It is on the changed position in nuclear physics as far as a focus of research is concerned. In the late 1930’s the center shifted to the United States. Has this begun to reverse itself in any way in the post-war period?

Cockcroft:

I think it has to some extent primarily due to the fact that we have both the Rutherford laboratory, and we have CERN available for nuclear physics work in Europe -- I think these laboratories made a tremendous difference. We do have the facilities now. We have the money for one thing, which is very important. To build a bubble chamber costs nearly half a million pounds. This enables us to compete with Brookhaven, which we couldn’t do before. So I think the situation has changed very considerably.

Weiner:

You previously mentioned off-the-shelf components. Was part of the change due to the varieties of supporting technology in Europe that may not have been the case before the war, whereas in this country some people felt that it was characteristic of nuclear physics research that one could walk down the street and go to a drug store and buy a tube.

Cockcroft:

Yes, well, I think it’s due to the fact that we had the technology available after the war and we were able to build high-energy cyclotrons; we were able to build a proton synchrotron at CERN -- primarily by these physicists who had worked on radar in the war. If you look around and see who designed these machines, you’ll see it was these radar technologists -- like John Adams from TEE and so on. They were able to do these jobs after the war. Before the war we probably wouldn’t have been properly equipped for them, technically equipped.

Weiner:

And yet it was the people in nuclear physics who were able to make the radar activity a successful one, and in turn, once they developed that, they gained a new experience that could then be re-applied to nuclear physics.

Cockcroft:

Yes, that’s right.

Weiner:

I’m sure there are other questions I could think of, but we’ve gone overtime. Thank you very much.