Oral History Transcript — Sir James Chadwick
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James Chadwick; April 17, 1969
ABSTRACT: Chadwick describes his life and work from his boyhood years to his retirement. He details his discovery of the neutron in 1932 and nuclear physics work at the Cavendish Laboratory from 1919-1936, when he left Cambridge for Liverpool. Among the important events in this narrative are the following: Chadwickís first meeting with Rutherford at the University of Manchester; his graduate work with Hans Geiger (1913-14) in Berlin, where he was interned for four years as an enemy alien; his return, upon release to Manchester, where he teamed up with Rutherford when the latter moved to direct the Cavendish Laboratory in 1919; his functions as Assistant Director of Research at the Cavendish from 1923-36; his role in building a working cyclotron at Liverpool. More briefly, he describes his relation to the US atomic bomb project, his admiration for General Groves; and his final return to Cambridge where, after the war, he served as Master of his College until retirement in 1959.
Weiner:This is Side 2, of Tape 2, and weíre resuming on the morning of Thursday, April 17th. There were a number of things that we covered yesterday and I indicated when we were through yesterday evening that I would like to go back to a few questions that were of interest but that we only touched on, and maybe there are some things that you have in mind, too, in thinking since then.
Chadwick:Yes, there is one thing I ought to have said. I told you of Rutherfordís consideration for people working with him in reference to myself when I had given a very bad lecture to the Cavendish Physical Society, and he arranged for me to give another one to rehabilitate myself. Well, I ought to have added to that that I had no teaching duties in the Cavendish. I gave no series of lectures. My duties were to look after the research students. Of course I did have college teaching duties of a somewhat different kind. But what I wanted to add was that I did give occasional lectures in the Cavendish as Rutherfordís deputy. You see, it was a tradition at the Cavendish that the Cavendish Professor gave a course of lectures on more or less general physics. They werenít advanced; they werenít elementary; they were of a very general kind. And anybody who happened to be in Cambridge had the right to come and listen. Generally the lecture room would be pretty full. Well, there were times when for one reason or another he could lecture on an afternoon. Then I replaced him. He would tell me what he had intended to talk about, and so I substituted. But that did not happen very often. I should say not more than twice a term. When I say not more, I mean it was really less than twice a term. That was one thing.
Weiner:When you gave these talks and when Rutherford did, was there a sequence, or was each lecture on a different subject, something of interest to Rutherford at the time or something that he thought would be appropriate?
Chadwick:Well, there were things always of interest to him. But they were quite general — that is to say, not necessarily on nuclear physics at all but maybe on atomic physics. In fact, I should say that very seldom indeed on nuclear physics as such, but on quite general topics. They werenít in a sense part of a regular course, but the bulk of the audience would be students reading for the natural sciences. But there were others. There would be visitors. When I say ďvisitorsĒ I mean people without any particular interest in physics. I remember very well one occasion when I lectured, an American woman sat in the front row and read the newspaper. I had no objection to that so long as she didnít make any noise turning over the pages and so disturbing me or disturbing somebody else, but she didnít — she was very quiet. She had no particular interest in what was happening. She just wanted to be there.
Weiner:That can be disconcerting, though.
Chadwick:Oh, no. That kind of thing happened to others as well as myself. In the old days, for example, when Quiller-Couch was professor of English, I suppose, his lectures were attended by hosts of people who had no professional interest but merely wanted to listen to him talking about something that interested him. That was the usual thing. It was a kind of duty of the professors to give such a course. Whether it is continued now or not, I donít know. It was in those days.
Weiner:I know you have a listÖ
Chadwick:But you ask me, and perhaps it will come out.
Weiner:These two questions I have are related to the points youíve brought up. One specific piece of information that I didnít get yesterday on the attic course that you taught was this: You mentioned that the duration of it was over a few months, and it was an orientation course.
Chadwick:Well, not a few months — a month perhaps or two months, that kind of thing. It depended partly on the man, partly on whether we were ready to put him onto something.
Weiner:I wasnít clear, though, about the daily schedule. Was it full time every day for the month, say?
Weiner:This is what you did every day for that period.
Chadwick:Well, it was only part of my time. It was just to get them to do something, lead them to it.
Weiner:But the students would be involved all day, although you might be off.
Weiner:That was just a minor point. The other one in a sense relates to the Cavendish Physical Society. You mentioned a few occasions of the meetings. I wasnít clear about the function of it and the organization of it. To whom was it open? Was its purpose to report on latest developments in the journals?
Chadwick:Very seldom. It was generally a man reporting on his own work, but occasionally it would involve work that others had done on the same subject, and the meetings were open to anybody who wished to attend. There would be times when, for example, Eddington might be interested, not very often, but not infrequently. Mostly they were men working in the laboratory.
Weiner:How about the theorists, people who were interested more in the theoretical aspects Ö?
Chadwick:Well, at the beginning, you see (when I say the beginning, I mean 1919 and so on), there was not much contact, because there were very few mathematicians interested in what you want to call theoretical physics. There was Charles Darwin, and Ralph Fowler became interested. It was through Ray Fowler that the interest grew, very largely through him. He was a mathematician of a very broad ability, very wide. There was almost no branch of mathematics in which he couldnít do something useful. And, of course, he became Rutherfordís son-in-law, though that is, in a sense, incidental. But he gradually built up a small school of theoretical physics. The Cambridge mathematical courses at that time were not adapted to produce men interested in theoretical physics. I would like to say any more about that, but that was so. But very gradually Fowler built up this small school, and there were some very able people amongst them. There were some visitors who would come In a sense one could say that Dirac came not as a physicist, but as a research student. There were various visitors. Later on — I canít remember the exact year — Ralph Fowler and I ran a colloquium rather on the style of Rubensí colloquium, and I remembered it in the old days. Well, the purpose of running that colloquium was really to bring together the theoretical physicists and the experimental physicists. I think we met every fortnight in a relatively small room. It wasnít very small, but it was small enough to be full, because it was my experience that you got a freer discussion when people were close together and even overcrowded — much better than you would in a large room with only a few people in it. And he would choose Ö There was no set rule about it, but on the whole we alternated between some paper more on the theoretical side and some paper on the experimental side, not very much on our own work but on work done elsewhere — just to bring it to the notice of people; also to give people an opportunity of putting their views before other people and an opportunity to discuss the work from either the theoretical side or the experimental or both. That worked really well, I think. I canít say how many years it went on.
Weiner:Do you remember when it started?
Chadwick:No, thatís why I say I donít know how many years it went on. It went on until I left Cambridge, but when we started it, I donít know; I donít remember. I should say probably not much earlier than 1930 anyhow. It might have been a year or two earlier. It must have been around 1930.
Weiner:Was one of the motivations in starting the colloquium a feeling of lack of communication?
Chadwick:Oh, yes. There hadnít been any, you see.
Weiner:Was this due to the absence of individuals with those interests or because physics had developed to the point where this was more necessary than before?
Chadwick:Lack of interest of the mathematicians, largely due to the mathematical course which had developed in Cambridge. It was quite different from anything you would find on the Continent. It was largely due to the type of mathematical course in Cambridge that there was so little theoretical physics in this country until Ralph Fowler developed a small school.
Weiner:It would be more of the abstract mathematics rather than the applied.
Chadwick:Well, there would be some applied mathematics, but they would have very little contact with the physical laboratory or even anybody else. You see, there was Larmor here in the early days, and I canít remember Larmor ever coming to the Cavendish Physical Society. But there was also the Cambridge Philosophical Society, which had been relatively inactive until Rutherfordís days. It more or less sprang to life then. Most of us joined it. I remember very well at least one lecture which Larmor gave there. That was rather exceptional, you see. And they might publish a paper — but even that was very rare — I mean in the Proceedings of the Philosophical Society. But that was very rare. I donít know what it is like now. I believe it is half inactive at the present time. I donít know. Iím only saying what I gathered from J. A. Ratcliffe, whom I saw a couple of months ago and who was trying to collect evidence about the activities of the Society. Somebody is writing a history and he was collecting some evidence and was asking me about the part that the Cavendish played in reviving the Philosophical Society.
Weiner:Do you remember who participated in the colloquium? Fowler and you, of course, but Ö?
Chadwick:I expect we did once or twice give a paper, but that wasnít our business. Our business was to get the young men to give a paper. Itís difficult to remember names. The first one that comes to my mind is A. H. Wilson, who wrote a book which I believe is still almost the standard work. It was called The Theory of Metals or something of that kind.
Weiner:This wasnít H. A. Wilson?
Chadwick:A.H. H.A. Wilson was a different generation almost by two. But A.H. Wilson published a number of papers of great interest — a very competent man. He went later on to industry, to Courtauldís research department. He became a director of Courtauldís and a few years ago he was the chairman designate of Courtauldís. Thatís a long story. There was talk between him and the chairman of ICI of a possible merger between these two very big companies. And that was opposed very strongly by two men at Courtauldís, two of the directors, one of whom was Frank Kearton, whom Iíve known for quite a long time, who, incidentally, when I first knew him was in ICI — and he opposed this very strongly. And Allen Wilson, left Courtauldís, and the chairman of Courtauldís is now Frank Kearton and Sir Alan Wilson is chairman of Glaxo.
Weiner:Thatís interesting. Well, getting back to the colloquium, there was Allen Wilson, Fowler and yourself. How about Rutherford? Did he come into this?
Chadwick:No. I donít know. He may have turned up once. No, he left it to us.
Weiner:And there was Dirac.
Chadwick:Oh, yes, Dirac would be there.
Weiner:And then visitors who would be present as either research Ö One thing Iím not clear on is the designation for a visitor who was not a student — I mean people who were beyond their degree but who came to work in the laboratory for a while. What do you call them — visitors or guests?
Chadwick:Visitors or guests, just as you say. Itís difficult for me to remember names. There was, for example, Kovarik for a time. There were quite a number, but few of them did actual experimental work — Oh, they might do a little. But their main purpose was that they were taking a sabbatical year, I suppose, and they were just talking to people in the laboratory and taking part in the general life, but only a few would do actual experimental work. Oh, they might do something for a few months, you know. But they were always welcome, and, of course, contributed quite a lot.
Weiner:One other question on the colloquium. Did this exchange, including theorists and experimentalists, influence the work of the laboratory? Can you think of instances where some theoretical ideas influenced the experimental work?
Chadwick:I have no doubt whatever that it did influence some of the work in the laboratory. But I can remember only one instance, and I remember it because it affected me. That arose probably not out of the colloquium but out of a paper published by Mott on the scattering of alpha particles on the basis of quantum mechanics. I donít remember. I think it was a general paper on that topic. But the particular point emerged, which was that you couldnít tell the difference between one alpha particle and another, which meant that under the conditions in which the inverse square law of force applied between an alpha particle and the helium nucleus, the observed scattering would be twice as much on the quantum theory as it would be on the classical theory. Well, that was an interesting point which was just possible to observe, because the inverse square law force would only apply to the scattering of rather low-velocity alpha particles, which didnít come too close to the helium nucleus; so alpha particles of relatively small range. So I immediately sat down and observed it. It was a bit troublesome, but it came out quite nicely. We had never in the experiments with Rutherford gone quite so far as that. There didnít seem very much point. We just didnít do it. That was one instance, but I have no doubt there were others. But this was a long time ago.
Weiner:I just thought as a general point it would be of interest, because the main focus of the laboratory was, of course, experimental; and itís good to probe how these theoretical components played a role. For example, this colloquium is of much interest. Was the colloquium in any way related to the Kaptiza Club?
Chadwick:No, that was purely Peter Kapitza. He ran that entirely himself. It was a small private club.
Weiner:I see. Its function was also a discussion group, though.
Chadwick:Yes. But it was run in the evening in his own rooms, not in the Cavendish — either his own rooms or somebody elseís.
Weiner:What was his general relationship in the laboratory? I know, of course, that the Mond Laboratory essentially was built for him and for his line of research. Did he have much interaction with the people in the Cavendish proper, the people who were working on nuclear physics, would you say?
Chadwick:No, a little, not very much. The interaction had more influence on him than it had on anyone else. I forget exactly how this arose, but he had been making experiments on the ranges in air gases of alpha particles of relatively low velocity. Iím getting this a bit muddled. But there was a difference of opinion between Peter Kapitza and Blackett on this topic. And in order to separate, Kapitza turned his mind to the production of the magnetic fields. I think in the first instance he used banks of accumulators and discharged them suddenly through a magnet in the magnetic field of which was an expansion chamber which the alpha particles were traversing. And those magnetic fields were not sufficient, and that led him on to the idea of producing big magnetic fields. Well, I wonít say anything about the Mond Laboratory or the way it started. Most of what I know about that is information given to me by other people. You see, the first thing that he did was to build this great big generator and then kind of short-circuit it to produce a high magnetic field and observe various phenomena in it. To me that was the wrong way of going about the business of observing the particular phenomena in which he was interested. The right way, in my opinion, was to observe them at very low temperatures — observe these phenomena at very low temperatures. Certainly in a temperature of liquid hydrogen, and if possible, a temperature of liquid helium. And, of course, it came to that in the end. But I was not in sympathy with that. I was not in sympathy with this big generator business, which was an expenditure of money in the wrong direction, in my opinion; and what concerned me much more was that if such money had been available, we would have done a great deal more in the Cavendish than we had done, because we were always so limited by lack of money, lack of apparatus. No, I donít think I would like to say any more about that.
Weiner:On another subject, I gathered that the only outside grant that was made to the Cavendish was the Austin grant; that otherwise the funds came from the University budget. Is that correct? Up until the time you left, certainly.
Chadwick:Yes. There may have been quite small grants, a man leaving £100 or two. I donít remember. It was very small. And one thing I should have mentioned at the beginning was that in the early days the University had no grant from the government, which the newer universities had and on which they largely ran. I donít remember exactly when the grant was first given. I should say somewhere between 1923 and Ď26. And if I remember rightly, the grant was £30,000 a year. Now, when I came back here in 1948 or shortly after that, the grant to Cambridge was £3,000,000 a year.
Weiner:From the same source?
Chadwick:From the government. Until this recent cut, I should imagine that itís probably about 5,000,000. And, of course, a great deal of money has come for building. The University couldnít have put up these buildings out of its own money.
Weiner:Those figures that you mentioned — are those for the Cavendish or for science at the University in general?
Chadwick:Oh, thatís for the whole University.
Weiner:Oh. I wondered, because that would have been a lot for the Cavendish.
Chadwick:Oh, I donít know what the grant was to the Cavendish.
Weiner:Anyway, that wouldnít have been made to the Cavendish. It would have been from the general University funds. In other words, there was no direct grant made to the Cavendish, was there, during this early period?
Chadwick:From the University, yes.
Weiner:But the government would grant to the University and thenÖ
Chadwick:Oh, there was no grant from the government to any laboratory or department.
Weiner:It wasnít earmarked for that purpose.
Chadwick:No. One must distinguish between a government grant, which came through the University Grants Committee, and grants which might come from the Department of Scientific and Industrial Research, also a government department, that made grants to students, which enabled them to stay for two or three years and do research. There may have been occasional grants of a small amount for specific purposes. I can only remember one grant. It didnít come from DSIR, a grant of a couple of hundred pounds which was made to me from some fund or other — I canít remember its name.
Weiner:For a specific purpose?
Chadwick:Yes, it was merely to build a microscope with the biggest aperture objective that I could manage — too big actually, but I could cut it down. That was in connection with another matter, which perhaps I ought to mention. I put it down.
Weiner:Now weíre ready for your list.
Oh, well, I did mention that matter of lecturing for two reasons — one, that I did not have lecturing duties and the other that was really slightly egotistical. I merely wanted to say that Rutherford did have some confidence in my ability to lecture after my first distressing experience. That was all. Well, there was another matter which I wanted to mention because of the light it throws on Rutherfordís character. It involves me, but the reason I tell it is to explain his attitude toward public controversy. He would never indulge in it. He thought it was wrong, If you disagreed with a man on a scientific topic, have it out with him in private, but donít start quarreling in public. The incident which I was going to mention was in connection with experiments made in the Radium Institute in Vienna, Kirsch and Pettersson. Well, they got quite different results and we had no doubt whatever about ours. I didnít pretend to be able to explain why they were getting such different results, but Rutherford and I had no doubt whatever they were wrong. I add this because this is in the end all private.
He got a number of communications from Vienna, chiefly from Pettersson about the matter and other matters touching on it which Pettersson wanted Rutherford to communicate to Nature. We knew they were wrong, and Rutherford dissuaded Pettersson from publishing them. Now, this is no criticism of Pettersson. Iíll come to that. Stefan Meyer was rather disturbed about this discrepancy and he wanted us to go over and discuss it. Well, Rutherford felt that it wasnít quite proper really for him to go over. Also, he didnít want to, much as he would have liked to see Stefan Meyer. So it was arranged that I should go over during the Christmas vacation. Things quieted down in December, and I could find time to go. I took my wife with me. Itís difficult for me to remember the exact year, but I think it was in December 1927. I saw Kirsch a few minutes only. All my contacts were with Hans Pettersson, who was obviously the leader, and an able man. I saw the experiments. I didnít like the way they were being done.
The observers, the counters of the scintillations, were three youngish women. There were at least two, if not three, of what Pettersson called of Slavic descent, because he believed that (Iím only repeating what he said to me) first of all, Slays had better eyes; secondly and mainly, that woman would be more reliable than men as counters of scintillations because they wouldnít be thinking while they were observing them. Well, what I noticed was that all three of them (there were generally two at a time taking part in the experiments; there were three altogether; I donít remember their names except one: Kara - Michai — very nice and friendly young women) knew exactly what was happening all the time. And so I said to Pettersson that I would like to conduct an experiment myself, and we arranged to do it. And I did not tell them what was happening. They knew what element was being bombarded, I think — I wouldnít be sure of that — but Iíd never said how much absorbing material was in the powder. The results were exactly what I expected, quite different from what Kirsch and Pettersson had reported. Well, that was very awkward. Various excuses were made, the chief one being that it wasnít a very suitable evening. The counting was done latish, after 5 oíclock, and probably their eyes were tired — something had gone wrong. So we arranged to do another one. I changed the conditions slightly. The results were exactly the same, exactly what we had found in Cambridge, and quite different than what they had observed before. That was extremely awkward, because oh, Pettersson was very angry indeed. I mustnít say any more than that. But I must say that these young women were perfectly honest.
There was no question of cheating or anything of that kind. It was simply they were deluding themselves. They were seeing what they were expected to see. The next morning Pettersson and I met Stefan Meyer in his room. I think he had expected something of this kind, but he was very upset indeed, and he said he would do anything I thought was necessary to put things right — make a public retraction and so on and so on. And I said no; I thought the best thing to do was to say nothing more about it. They could drop the experiments and say nothing. Now, I took it on myself to act in that way, because I knew that that was what Rutherford would have wanted. I didnít need to consult him. I knew that that was what he would do, because he would never have done anything which would have caused pain to Stefan Meyer. This was a personal matter, but at the same time he would never have indulged in this kind of public argument to settle a matter of that kind. And so it was left like that. It had one result — that Pettersson and I became quite good friends, which was very pleasant because he was a very able man, Heís really an oceanographer. His father was a very well known oceanographer, and later on Hans Pettersson returned to oceanography — he was a Swede — and took up what I think must have been his fatherís chair in Goteborg on the west coast of Sweden. We used to go to Sweden practically every summer later on — starting in 1939. A friend of ours had a house there and the fishing rights on a big lake, and he spent his summers fishing for trout, and we would spent a fortnight there. Well, if we were passing through Goteborg, weíd call on Hans Pettersson. The last occasion on which I saw him was after his expedition in a Swedish vessel built for the purpose. I think he was taking samples of the ocean floor. He was invited to come to give an address to the Royal Society about his results, which he did. I didnít go to it — I was busy — but he came and spent the weekend with us. So what was a bitter quarrel at the time completely disappeared. As I say, we became very good friends, and I enjoyed his company. Iím telling you because I was acting for Rutherford as much as for myself, and I was doing what I knew he would do. He would often say that kind of thing — that not under any circumstances would he start a fight in the press or in the scientific journals.
Weiner:I understand that the results of the Vienna group fell by the side and that generally they werenít accepted. So in a sense it took care of itself without a public retraction.
Chadwick:Well, the whole thing changed, too. You see, what brought it to my mind recently was an account written by Otto Frisch in Physics Today. He came from Vienna. I donít know whether he had worked in the Radium Institute. I donít suppose he did. But the other departments were in the same building, and he knew about it. And in his account he said something to the effect that he had never understood how the differences had been reconciled. Well, if he had been in the Radium Institute, he would have been told. But it was just dropped. There were some other things there that I saw that werenít very good.
Weiner:What about the relationship of the Cavendish with other laboratories — for example, Paris and Rome? For example, with the Joliots — were communications with them on a regular basis?
Chadwick:Very little. Itís difficult for me to remember, but there must have been some, because I think it was in the summer of 1927, but it might have been 1928, we held a conference in Cambridge to which we invited various people from abroad who were interested in the same things. There were Geiger, Lise Meitner, Bothe — I canít remember. There were several others. Amongst them was Irene Curie. I donít think she had married Joliot at that time. I really donít remember. I donít think that Joliot himself was there, but I wouldnít be certain about that. It was a very interesting conference in many ways. It didnít last very long. We were able to exchange views on a number of matters. I donít think that we ourselves gained very much from it except personal contacts. I really canít remember anything about Joliot on that occasion.
Weiner:They were following similar lines of work. Thatís one of the reasons they had a representative there. In the course of later development was there much communication — on the artificial radioactivity business, for example?
Chadwick:No much. We had met in Brussels at a Solvay Conference, October 1933, I should say — Iím not quite sure. Madame Curie was there — a great many other people. There were a great many people there, but the chief ones I remember are Madame Curie, whom I hadnít known at all; Ernest Lawrence, whom I met for the first time; and Joliot; Heisenberg (I had little to do with him). But it was a very interesting occasion in its way. Joliot told us about some of his observations, but of course at that time there was no suggestion that they were due to the formation of artificial radio-elements. It was perhaps rather stupid of us. I think Fermi was there, but Iím not sure about that.
Weiner:I think he was. I think Gamow was there, too.
Chadwick:Oh, Gamow was there, yes, certainly. But, of course, I had known Gamow before. But that was, shall I say, the opportunity for him to leave Russia — to put it the right way. I heard about the experiments of Joliotís from Gamow before they were published, but I was not surprised really. I rather kicked myself violently, because I missed it and should have foreseen it. (Brief Interruption)
Weiner:You were on the subject of kicking yourself rather violently.
Chadwick:Yes. There were a great number of occasions on which I did that. That was one on which my failure, our failure, to see the point was not really due to the lack of equipment. We had at that time these Geiger-Muller counters, and were using them for various purposes. We didnít have available the apparatus already set up. You see, Iím really coming back to a point on which I had some argument with Rutherford. I mentioned yesterday that I had wanted to have two or three rooms set apart in which certain pieces of apparatus would always be ready and in working order. This was not quite the case altogether, but if we had had such facilities available, I think things would have been very different.
Weiner:Was that the reason in this case, though?
Chadwick:Not really, no. Not really. Only to a slight degree. I was really thinking of some other matters, some of which would be tedious to mention. But I must mention one. It came first. It started at this conference that I mentioned — I did the arrangements and so forth — in the summer of 1927 or possibly Ď28. There were reports by C. D. Ellis, who was working in the laboratory, and possibly Lise Meitner, about the gamma radiations from radioactive bodies; and I remember very well Ellis reporting on the radiations from a nucleus he called thrium C2ó, or whatever it was.
Weiner:C double prime (CĒ)
Chadwick:Yes. Well, no doubt I had known that a week or two before. It was quite new. I think it was confirmed by Lise Meitner, but Iím not sure — she may have done it later. But, you see, this was the first time we had available a source of homogeneous gamma rays. There were, in fact, at least two groups of gamma rays, but the others were very easily absorbed, and the main and intense group had an energy of about 2.8 million volts. Now, I gave the job of investigating the absorption of these hard gamma rays to two men — Gray and (T.G.P.) Tarrant. When I say ďIĒ gave it, I mean with Rutherfordís consent. I think that if we hadnít been training so many research students, I might have done it myself. It was a thing that we had wanted to do for years, but did not know until about that time that there was a source of very energetic gamma rays available. Well, they found that in addition to the absorption due to the photoelectric effect and the Compton effect there was something else which increased with the atomic number of the elements and we called it, if I remember rightly, nuclear absorption. We didnít know what it was due to. It was there and unexplained. A little later I put a man on the business of trying to see what happened. The energy had to go somewhere. Obviously, it didnít come out as Ö I mean one simple explanation would be perhaps excitation of a nucleus and later on (and it must be very quickly afterward) the emission of some sort of radiation. But that wasnít there. That would have been seen immediately; it would have interfered with the experiments. They couldnít possibly have missed it. So I put a man onto the job, again with Rutherfordís consent, of hanging a thin foil of lead in an expansion chamber, which was placed in a strong magnetic field — well, strong: I mean a thousand or two thousand gauss, that kind of thing — and passing the beam of this hard gamma radiation through the lead plate in the expectation that we might see something. I didnít know what. I certainly at that time (I donít remember what time it was, and Iím not going to mention the manís name) was not thinking about the positive electron, because that of course is what we ought to have found. He didnít find it. Now, part of the reason was certainly that the equipment was not quite adequate. Oh, we had everything that was necessary to take the photographs, but not to take such clear and beautiful photographs as one ought to have been able to take; and to some degree that was the fault of the equipment. That was a very stupid business.
Weiner:It had been observed, that is, if you saw this track that would indicate something — it is not necessarily predictable that you would have identified it as a positive electron. It would have been an unusual phenomenon to be explained.
Chadwick:Oh, no, there couldnít have been any doubt about it.
Weiner:Under those circumstances.
Chadwick:There would be two tracks coming from the same point, one being bent one way and one the other.
Weiner:Then itís a question of interpretation.
Chadwick:No other interpretation would have been possible.
Weiner:This was before Diracís ideas on the subject?
Chadwick:I donít know. I donít know, because, you see, not a great deal of notice was taken at once about this hole or positive electron — not a great deal, not until a year or two later. But it was very sad.
Weiner:In 1932 Millikan came to visit the Cavendish, and at that time he showed the Anderson photographs, and thatís when I guess you and Blackett and Occhialini then did some work on electron pairs. Is that right? Was it as a result of Millikanís visit?
No. I donít think he said it was a positive electron. He said it was something he couldnít explain. I donít think those were were used for quite a time. But I may be mistaken. But it was about that time that I really got very angry that these experiments hadnít been done properly, and I wanted to do them again. Well, I donít want to say very much about that, because it involves people, some of whom may be alive. But I couldnít get some people to take any interest in it. And I hadnít the facilities available for doing it myself. I did go to a Chinaman who was working in the laboratory on a kind of temporary basis, who was using an expansion chamber, but he was not able to — again the equipment wasnít very good. And the photographs werenít good enough. So I went to Occhialini, and with Blackettís agreement, used his set-up. But that was already too late. Oh, it was the old old story of not having the facilities available. I must say one thing, which I passed over yesterday. I was thinking about it last night. I avoided it, but even now Iím very doubtful indeed whether I should say anything. But at one point — I suppose about 1924 — Rutherford and I parted company for a time. He was not at all well. Iíve forgotten what was the matter with him, but there was something quite definitely wrong with his health. It was mainly due to that that we decided to go our different ways. I wasnít the only one affected by it. For a time Charles Ellis and I and one or two others did not attend the meetings of the Cavendish Physical Society. But the effect on me was that I made some abortive attempts.
First, I started trying to get high voltages for use of high-frequency circuits. I think I said before — I may have said it when we did not have the recorder on — that I failed completely to get anything satisfactory because there were no facilities available. Rutherford was not interested. I couldnít spend money, laboratory money, on such things which werenít for the general benefit of the laboratory; so I gave them up. And about that time or immediately following I began to think about the collisions of quanta with electrons. That is to say, I did work it out. It was the Compton effect. Well, as you must know, Debye published something on this, quite definitely describing the Compton effect; and he didnít do any experiments. And I wanted to observe these collisions in an expansion chamber. None was available. I spoke to Rutherford. He was completely uncooperative. It was very difficult, but I realized that he was really ill. I did build up an expansion chamber, for which I paid myself. I didnít have much money at that time. But I paid for it, bought my own lenses. There were some things in the laboratory I could use, but I couldnít do it properly. And just as I was about to start Ö Iíd enlisted the help of a young student, K. Emeleus, not the chemist, and told him what I was going to do — when one day C.T.R. Wilson came around. I told him what we were about to do. ďAh, thatís just what Iím doingĒ. Well, it was, and it wasnít, because he was doing it with X-rays at quite low levels — I considered it to be of low energy. I was interested in the higher energy things. And I knew that he could take far better photographs that I could.
In any case, he was doing it. So we dropped it. And we then did some very trivial experiments on the delta rays, as they were called, produced by alpha particles in their passage through gasses — that is, the collisions of alpha particles with electrons. But I wasnít interested. It was perfectly obvious what was going to happen. There was no difficulty about it. But I had to give Emeleus something to do. This was a kind of disagreement (well, it wasnít exactly a disagreement) between us, and certainly an estrangement which didnít last very long. He got better, and we began to work together again. Iím very reluctant to speak about this, but I was thinking last night that I really had to explain why I did practically nothing for about a year. It wasnít much more; it could have been two years.
Weiner:That was during the period of Ď24 to Ď25, somewhere in there?
Chadwick:Somewhere around there, somewhere around Ď24.
Weiner:What results did C.T.R. Wilson get? Anything?
Chadwick:Yes, but not the explanation.
Weiner:By that time, of course, Compton had Ö
Chadwick:No, it was before Comptonís experiments.
Weiner:Ross published something, Debye, and Compton all around that period.
Chadwick:Did he? I donít remember that.
Weiner:We have Comptonís notebooks, and thereís a letter — I canít think of the date — but Iím trying to think of that because it might help us to date the incident youíre talking about.
Chadwick:Yes, well, I donít remember the exact date.
Weiner:But it was before. It was after Debye and before Compton published?
Weiner:I see, because both were very close.
Chadwick:Yes, it was before either. But it was an unhappy time for me.
Weiner:That must have been just at the time when you became director of the research Ö
Chadwick:Oh, it was after that. It was about that time, yes.
Weiner:I have a feeling 1923 is when these papers began to come out. It may be 1923 that youíre talking about.
Chadwick:I can look it up.
Weiner:Well, Iím glad you clarified that.
Chadwick:Yes, well, then we joined up again and did some more experiments on artificial disintegration and scattering and tried to do anything we could to find out something about the nucleus. Now I donít know where to go on.
Weiner:Well, I was going to ask you a question on that. You mentioned in your Rutherford Memorial Lecture that that period from the mid-Ď20ís was a period of consolidation rather than a period of discovery. Was there a feeling at the Cavendish or elsewhere that the new work on quantum theory overshadowed the work on radioactivity, as if the work on radioactivity was not really in the forefront of physics anymore?
Chadwick:Not on the experimental side, but from some aspects, yes. But, you see, it took quite a time to absorb the meaning of the new quantum mechanics. It was rather slow. At least it was with most people. And you couldnít see much application to the problem of the structure of the nucleus. I mean it explained the escape of alpha particles through the potential barrier, which had been a great puzzle. But there was no immediate application to the structure of the nucleus, which was what we were interested in. All the work in the Cavendish was not directly on nuclear physics. There was also some more general physics going on. Oh, dear, I canít remember the exact topics now.
Weiner:There was some work in solid state?
Chadwick:Very little. You donít mean the experiments of G.I. Taylor?
Weiner:No, I was just thinking that the London conference in 1934 had two sections — one on nuclear physics and one on solid state — and I was assuming just from that that perhaps the Cavendish had been working on something.
Chadwick:There might have been one or two, but I donít remember. There were people like Harry Massey, Boulard, and others who were working on atomic physics — the behavior of ions and so forth, not nuclear physics. But that kind of thing had been going on most of the time.
Weiner:When did things change? When did the applications of quantum mechanics to the study of the structure of the nucleus become apparent?
Well, I donít know if it was very apparent, but as far as I was concerned, it started with the neutron. My interest was the structure of the neutron, which was not immediately apparent at all. One naturally assumed that, because of the closeness of the mass, of the mass of the proton, proton and electron. Then of course one saw that one had to bring in the neutrino. I think I saw quite early that the forces between the proton and the neutron must be exchange forces. I remember talking about it to Massey quite early on. I couldnít work it out myself. Massey did a few calculations. It was certainly in Ray Fowlerís mind. He could have sat down and produced a paper on the subject at any moment, but he didnít think it went far enough, so he didnít do it. The first publication that I remember was either Majorana or Heisenberg. The one I remember first was Heisenberg, and I remember Ralph Fowlerís annoyance with himself when he saw this paper. He said: ďWell, I could have published that months ago.Ē So could I — at least Massey and I could. But it was only a small beginning to the real problem. And, you see, we were still stuck with the neutrino. Now, I did experiments with a man, Lea, now dead, who went into medical physics shortly afterwards; and we tried to detect the neutrino by measuring the ionization produced in an ionization vessel at quite high pressure — Iíve forgotten now, very high pressure — so that a good deal of absorption could take place inside the ionization vessel. And we used as a source some pure radium E which I prepared, which does not emit gamma radiation, but is a beta ray body. And of course we stopped the beta rays and any gamma rays that could be produced by the beta rays in the material with absorbing streams of lead.
Then thinking and almost knowing, that the neutrino must be able to pass through large quantities of material. But we couldnít find anything. We could only set a limit to the number of ions produced by a neutrino in its passage through matter. But I think from that we deduced that its mass must be very small and perhaps zero, and also that it had no magnetic moment; because I remember talking to Dirac about that. I did some calculations, and I consulted Dirac to see that they were on the right lines — not necessarily correct in every detail but on the right lines. And that was all we could say about it. It might appear from that paper — it was a very short one — that we didnít believe in it, but that is not true. I had neither belief nor disbelief really, but in a sense some belief, because you could not get round the difficulties of, say, the even structure of the neutron, or the continuous spectrum of the beta rays from the radioactive bodies without introducing the neutrino as Pauli had done. But, as far as I know, nobody had done any experiments. Thatís why we did them. But later on — and not much later on — we had a visitor from Russia, a very able man and an attractive man, I found, called Roginski. And he was interested in the problem, and he devised a method based on the recoil of the body which emitted beta rays. But we did not have the facilities to give him an opportunity to carry out experiments. He would not have observed anything actually under those conditions. And he didnít stay long enough. He was only a visitor for something less than a year. I should mention that one of our earliest visitors was Arthur Compton, quite in the early days.
Weiner:It was about 1918 — no, he was there once earlier.
Chadwick:It was not in 1919.
Weiner:In the Ď20ís you mean again?
Chadwick:Oh, it must have been, I should think, in the quite early Ď20ís, quite early. It was in the days when he thought that an electron was as big as a balloon — for some reason or other; Iíve forgotten his arguments. I donít remember the conversations but I remember that we did have conversations about it. When I say ďweĒ I mean particularly Rutherford with Compton when I was present.
Weiner:You mentioned the fact that the neutron still had to be explained, although this was the thing that you were searching for for a long time. Once it was found, and even with the papers of Heisenberg and Majorana and the understanding by Fowler and others, it was just the beginning of a long journey. But let me go back to the neutron itself and to your reaction to the Joliot-Curie work. In your paper you talk about the gamma ray hypothesis, and you demonstrate how thatís not affected. Did this argument against that hypothesis come to you once you recognized that the neutron was involved, or was it obvious from the start that this was an inadequate explanation?
Chadwick:Itís very difficult to put my mind back. I should say obvious from the start, because I had Ö That explanation was really a kind of Compton effect on the proton, wasnít it? Iíd looked at that and never said anything about it, but of course, did a lot of experiments about which I never said anything. Some of them were quite stupid. I suppose I got that habit or impulse, or whatever youíd like to call it, from Rutherford. He would do some damn silly experiments at times, and we did some together. They were really damned silly. But if weíd gotten a positive result, they wouldnít have been silly. But he never hesitated. At times he would talk in what seemed to be a rather stupid way. He would say things which put down on paper were stupid or would have been stupid. But when one thought about them, you began to see that those words were inadequate to express what was in his mind, that there was something in the back which was worth thinking about. I think the same thing would apply to some of these experiments that I have said were silly. There was always just the possibility of something turning up, and one shouldnít neglect doing, say, a few hoursí work or even a few daysí work to make quite sure. Oh, no, there were many things. But until 1932, you see, it was in many ways a frustrating time. You see, (in) the work on the observation on gamma rays produced by alpha particles, I had done something in Manchester which I think must have been wrong. I donít know the explanation of it, but I wanted to do it in Cambridge. I needed a source of polonium. We didnít have it. I could have separated it. I could have separated the radium D from our radium sources. Rutherford wouldnít let me do it. Perhaps he was right. I did later on get some that way. But it would have interfered with the work of the laboratory, so I think he was right. But I talked about these matters and others particularly with Lise Meitner and Bothe, and Lise Meitner said, oh, she had some polonium — she would send me a source. She did. But it wasnít big enough, using the methods of detection we had at that time. And Bothe went on to do it. I donít know how he got his sources, but he did.
Weiner:You mentioned that after reading the paper by Joliot and Curie on their observations that you engaged, I think you said, in a few days of strenuous work and that during these days you showed that the strange effects were due to a neutral particle and you measured its mass. Iím just curious about the circumstances of those few days. What happened? What did you do? (What do you mean) when you say strenuous work?
Chadwick:Well, I made as many observations as I could of the collisions of these particles with various atoms. The point of that, of course, was to get some estimates of the mass. I had to eliminate any other effects. It was a strenuous time because I was just ready to do that, you see. I had prepared — this work of H. C. Websterís was done under my direction. And I thought then: ďNow, this is it. Weíve got it.Ē But so it was there obviously. He didnít quite believe it. He was a little bit too definite in the paper he wrote. I donít know why he wrote it quite like that, because he was slightly inclined at any rate. I was quite excited about it. I thought we now really had found the neutron, but again, as I said before, I had not thought deeply enough about the properties that such a particle would have. And that came to me, you see, with these observations of Joliot and Curie. It was perfectly obvious. (Interruption)
Weiner:I wanted to cover a specific point that we had left out, and from there I want to take it up further in time. I remember what we were talking about. I had mentioned that in your 1932 paper on the existence of the neutron, you referred in a footnote to unpublished discussions with Bohr. This regarded the number of collisions between neutrons and electrons as being less or very few compared to nuclear collisions. And my general question on that is: what was the nature of these discussions? Was it on one of Bohrís visits?
Chadwick:Well, it must have been. But, you see, I had never properly considered this point. I had thought that there would be some rather sparse ionization but a little. And we had looked for it. I mentioned earlier on that H. C. Webster had been examining the gamma radiation produced by the bombardment of beryllium by the alpha particles of polonium, and he found that the radiation emitted in the same direction as the alpha particles was much harder than the radiation that came backwards. And that, of course, was a point which excited me very much indeed, because I thought, ďHereís the neutron.Ē I discussed it with him, and I wanted to try to see if one could see any sign of a track produced by these radiations passing through an expansion chamber. Well, just at that point I was due to go with my family for a holiday. In any case, we didnít have an expansion chamber in working order — I mean available to us — and there was a man in the laboratory who had, and I asked them to try this, and write to me. They saw nothing at all. Of course, they should not have seen anything, any evidence of collisions with electrons. But if it had been the usual kind of expansion chamber, which had gelatin around the cylinder, around the walls of the expansion chamber, they would have seen the protons ejected from the gelatin. But they hadnít. It wasnít so arranged that there would have been protons. They wrote and told me what had happened, that they hadnít found anything, which disappointed me very much, but I never saw the photographs. I never understood it. What they had done with them, I donít know, by the time I came back the photographs had disappeared, so I couldnít see with my own eyes if there had been something. And though Webster had been very interested in the possibility of the radiation being neutrons, not gamma radiation, he changed his mind when he wrote his paper. And then he went to Bristol. At that time, you see, I didnít have a big source of polonium. I had a fair amount. It was large compared to anything I had before or Webster wouldnít have been able to do the experiments. But, if I remember rightly, I had separated this polonium from one of the radium solutions. I separated the radium D by electrolysis, I think, but I canít quite be certain how I did it; but I did separate the radium D from one of the radium solutions, and I got about 10 millicuries of polonium, something of that kind; it might have been a bit less. It was sufficient for Webster to do the experiments. But then, as Iíve said in this note that I wrote for the meeting at Cornell, I obtained through Feather, who was then in Baltimore, some old emanation tubes which had large quantities of emanation in them and got a very very fine source of polonium, which I had just prepared, when this note of Joliot-Curieís appeared. I was going to take up the thing again. I was all ready. Well, is there any other point?
Weiner:Well, on that, in February 1932, you wrote to Bohr. We have the letter from the Bohr correspondence. And you used a phrase there that Iíd like to ask about. You said, ďAs you will see, I have put this forward rather cautiouslyĒ Ö
Chadwick:Yes, well, I think it was a letter to Nature. I have a copy somewhere. I think it was headed, ďThe possible existence of the neutronĒ, rather than ďthe existence of the neutronĒ. But there was no doubt whatever in my mind or I should not have written the letter. Iíd done most of the experiments. Iíd made rough measurements of the mass, you see, already.
Weiner:So anyway the feeling was — ďcautiouslyĒ was just the word you used here — but you were firmly convinced.
Chadwick:Oh, yes, quite. There was no doubt at all. Going back to the point about the electron collisions of the neutron: you see, one of the first experiments was done by Dee, I gave a source of neutrons — what I considered to be the source of neutrons, that is, beryllium bombarded by this large quantity of polonium to Dee, who was working in the observatory of C.T.R. Wilson and was able to take the most beautiful photographs in expansion chambers, as only C.T.R. Wilson, indeed, could do at that time. And he looked for the ionization along the tracks of the neutrons and, of course, didnít find anything at all. And he estimated an upper limit to the ionization produced by neutrons. The paper was published at the same time as mine in the Royal Society Proceedings. And at the same time Feather had a small expansion chamber working for some reason Iíve forgotten, and we asked him to look for disintegrations produced by the neutrons in the gas in his chamber, whatever it was, and of course he did observe some. That paper was published at the same time. No doubt itís here somewhere.
Weiner:This was in the same issue of the Proceedings of the Royal Society?
Chadwick:Yes. They were all published together.
Weiner:I have here a copy of yours.
Chadwick:Well, thatís it, you see.
Weiner:So is this it — ďThe Existence of A NeutronĒ by Chadwick; ďThe Collisions of Neutrons with Nitrogen NucleiĒ by Feather; and ďAttempts to Detect the Interactions of Neutrons with ElectronsĒ, by Dee.
Chadwick:Yes, well, he didnít find anything, you see. Feather, of course, found the disintegrations.
Weiner:This must have been a very active several months for you, for all of you.
Chadwick:It was only a few months. This is May the 10th, you see.
Weiner:Yes, received May the 10th, and you wrote to Bohr on February 24th. The letter to Nature was sent off about that time because you refer to it in your letter to Bohr.
Chadwick:Yes, it is ďThe Possible Existence of the NeutronĒ — itís February 27th.
Weiner:Thatís very interesting, because the next one, of course, is ďExistence of the NeutronĒ.
Chadwick:Yes. But when I wrote that I mean I was absolutely certain. There was no question whatever about it. But I worded it carefully, because I wasnít ready to produce a full account of the experiments. And I wanted to do them properly.
Weiner:Did you have the opportunity to work on them full time or pretty close to it during those intervening months? Did you drop other things and concentrate on that?
Chadwick:Well, as far as my own work was concerned, certainly. But I performed my usual duties in the laboratory. But it was quite a strenuous time because I wanted to know as much as I could about the properties of the neutron before publishing. I was anxious, if I could, to find out or to get some indication about its structure; but, of course, I didnít.
Weiner:What was the reaction in the laboratory during that period of time, those intervening months? Was there excitement?
Chadwick:Well, I suppose most people were interested. I was too busy to notice very much. I canít even tell you about my own feelings properly. At one time I had in mind another explanation that was just possible, and I had to clear my mind of that, you see; but that didnít take very long — it was a matter of two or three days.
Weiner:Letís see. We were describing those intervening months and the difficulty of describing your own feelings — certainly having no doubts, even though for two or three days there was some other possible explanation that you had considered that you rejected after a very short period.
Chadwick:Yes, that didnít last very long, but I had to get rid of it before saying definitely that here was the neutron that we had been looking for for so long. And certainly Rutherford and I had believed for years in it as one of the main constituents of the nucleus. Itís true that he never mentioned it, except once,  I think, in public after his Bakerian Lecture: and itís also true that to a certain degree he had lost interest. Well, he had many other things to do. Only part of his time, as he got older, was given to the laboratory. He had University duties and duties outside. He was Chairman of the Committee of the Department of Scientific and Industrial Research and had been President of the Royal Society. Outside duties were beginning to fall on him, and he was beginning to take much more interest in them than he had in his earlier days, when practically his only interest was in his laboratory work. But I just kept on pegging away. I didnít see any other way of building up nuclei. So I did quite a number of quite silly experiments, when it comes to that. I must say the silliest were done by Rutherford.
Weiner:Just plain speculative experiments?
Weiner:Can you give me an example?
Chadwick:Oh, well, I mentioned some in that little note.
Weiner:When did Rutherford become convinced that it was the neutron?
Chadwick:Oh, immediately I told him.
Weiner:After the Joliot-Curie paper
Chadwick:Well, after Iíd done the experiments.
Weiner:At that point.
Chadwick:Oh, yes. That letter would not have appeared in Nature if he hadnít been convinced. We would never have published Ö I donít think ever in his life had he published anything he wasnít convinced about. He may have made mistakes once or twice but only once or twice.
Weiner:How about people from other laboratories and other centers of research? For example, did you hear from the Joliots in this period — in this intervening period?
Chadwick:No. You mention other centers of research. You mean nuclear research.
Chadwick:But, you see, the real center was the Cavendish. Nuclear physics began in Manchester with Rutherfordís statement of the structure of the atom, and it was there that he did the first experiment on the artificial disintegration of the nucleus. It was in Manchester, not in Cambridge. But there were no other laboratories which had a deliberate policy of pursuing work in nuclear physics, except perhaps — I would except Lise Meitner. But the others didnít begin till later.
Weiner:Till sometime in the 1930ís.
Chadwick:Yes. You see, that may be a slight exaggeration, but let us say in the late Ď20ís. Iím forgetting about the conference we had in Cambridge, which I think would be 1927 or Ď28. It was a small conference, but there were, I suppose, about a dozen visitors from other countries. Iím not sure whether Skobelzyn was there or not. He is Russian.
Weiner:He was a cosmic ray man, wasnít he? Didnít he do cosmic rays?
Chadwick:No. Itís stupid of me that I canít remember whether Skobelzyn was there at that time or not, but he did come to Cambridge, and I canít see any other occasion on which he should have been there. And what Skobelzyn had done was to observe the interaction of gamma rays with electrons in an expansion chamber, and he had some very nice photographs of — well, they were Compton electrons, of course. I mean electrons produced by the Compton effect. And he had one which interested me very much indeed, because I didnít know then, but I knew later; it was the first picture of a pair — a negative electron and a positive electron, And what I couldnít understand was how the negative electron had picked up energy from the nucleus in a collision. And it was, in any case, going the wrong way. But he gave me a picture of that — that photograph.
Weiner:That didnít puzzle you sufficiently, though, for you to do any work on it at the time?
Chadwick:No, I couldnít understand it. Oh, I was busy on other matters altogether. And, in fact, it would have been difficult. But a good deal of the interest of that conference was on the interaction, on the gamma rays from radioactive nuclei and the relation between — well, I donít remember. I do remember talking to Lise Meitner and to Bothe about production of gamma rays in some elements by bombarding them with alpha rays, and I remember saying that the reason why we hadnít done anything Well, I think we had. I think a student had tried to do something with the best source of polonium I could provide at that time, which was very small, and, of course, he had not been able to observe anything. He used an ordinary electroscope as his means of detection. The Geiger-Muller counter had not been invented at that time. And I remember talking to them about the cosmic radiation as well because some of us at the Cavendish had talked a good deal about it. Iíd talked particularly to C.T.R. Wilson about it and the possibility of the acceleration of electrons in the electric fields in the atmosphere and that kind of thing. But to come back to what I was saying: then Lise Meitner offered to send me a source of polonium to do this work on the production of gamma rays, and she did send me a source, but it wasnít big enough. It was only about two millicuries.
Weiner:You mentioned that the other day.
Weiner:Yes, part of that, but in less detail.
Chadwick:Well, I mentioned Lise Meitner again because she was, I should say, the most active person in the nuclear field outside the Cavendish at that time, and, of course, did some very beautiful work, And I am quite convinced that she would have discovered the neutron if it had been firmly in her mind, if she had had the advantage of, say, living in the Cavendish for years, as I had done. I do feel fairly certain that she would have.
Weiner:In other words, she was capable of doing the experiments and capable of recognizing what she saw. But she wasnít at all, in any time that you know of, searching for a neutron.
Chadwick:No, because I donít think it was ever mentioned. I canít remember anybody but Rutherford talking to me about the neutron.
Weiner:And you didnít talk with anybody else about it?
Chadwick:No. When I did the experiments, as I did off and on, they were done at odd moments and sometimes when nobody was about. As I say, some of them were quite stupid — appeared to be quite stupid. And they would only perhaps have made people laugh. I donít think youíll ever see any mention of the neutron in the literature.
Weiner:I donít know. Were you at the 1931 meeting in Rome? Did you go to that?
Weiner:Rutherford went to that, I guess. Thatís where people were discussing the neutrino, although they didnít use that same name. But it seems to me implicit in those discussions was an understanding of another kind of particle, but itís not clear that they were thinking of the neutron as such. In other words, there was a big dilemma, of course, prior to Ö
Chadwick:In 1931 in Rome Ö Was Rutherford at that meeting?
Weiner:I have a list at home, not with me.  It was called by the Rome group. This ties in with my question, because it was then that they were beginning to get interested in nuclear physics, and they were calling this together to give themselves a briefing in a sense — bringing together people from other areas who were doing work.
Chadwick:Yes. I have some dim recollection of it. But I was not fond of going to big meetings. I didnít mind traveling. I didnít like it very much. But I just didnít have much time for things of that kind. I had a family, and we had a regular habit of taking our holidays from the middle of June to the middle of July, and that was all fixed up long beforehand. I couldnít break that kind of thing. And then my duties in the Cavendish were really rather absorbing. Besides, you see, I did like little meetings, small discussion groups, when you could talk about things — not merely sitting on a seat in a large lecture room. And, also, discussion groups, but you canít think in groups. At least I canít. Everybody needs a quiet time in which he can think. And very often discussion can lead one astray rather than put one on the right lines. But I was never very fond of those discussions. I didnít go to very many meetings. I only once or twice in my life went to meetings of the British Association, For one thing, they were very big meetings indeed. It was quite nice to meet a lot of people, but it was of no particular scientific interest. And they came at a time when I should naturally be rather busy preparing either with students in the laboratory or preparing for things in the next year, you see.
Weiner:At the beginning of September, you mean. The mention of the conference in Rome was just an example of what I meant by other research groups. But the point that you made is that there were no groups established other than Lise Meitnerís work — established sufficiently by 1932 Ö
Chadwick:Well, not in the way the Cavendish was. You see, from the beginning, as I said before, we had to decide the policy of the Cavendish and decided in that way, and we pursued it that way. In the end it was successful, but, of course, for many years it was not very happy.
Weiner:Not very happy in terms of the points you mentioned before about criticism Ö
Chadwick:Oh, no, I donít mean that. When I say not very happy, I mean it wasnít very production and throwing any real light on the structure of the nucleus, which was what our business was. And it was so difficult to get any line on it, however hard we tried. That, of course, was what the neutron did for us, because it began to explain the trouble about the nitrogen nucleus.
Weiner:Was this clear to other people, immediately clear? That the neutron could solve many of these problems which were troubling physicists?
Weiner:Did you hear from people to this effect — I mean during those early months?
Chadwick:Well, I didnít need to hear from people. They were there, such as Ralph Fowler and Dirac and so forth.
Weiner:How about Bohr? The letters show that Bohr invited you to come to Copenhagen to discuss the neutron with the people there, but you were unable to go.
Chadwick:I donít remember. Sorry, but Iíd completely forgotten about that. I mean Iíd completely forgotten that Iíd written a letter to him.
Weiner:Well, you see, what Iím trying to get at here youíve answered — whether anyone, you or anyone, was cautious and slow about accepting the neutron. And your answer is that you certainly saw it clearly and that you knew of no other reactions that were negative or more skeptical.
Chadwick:Oh, no. Oh, no. It was the obvious answer to the problem of nuclear structure. And, you see, it removed that difficulty about the nitrogen nucleus.
Weiner:It was clear to you and clear to others, that you could solve these difficulties.
Chadwick:Yes. Oh, yes. There was no question whatever of not believing it, as I say, once the evidence could be produced.
Weiner:Well, there was subsequent work you did in the months following — not only the months but over a period of time — regarding the mass determination of the neutron, because Joliot then got one mass, Lawrence got another and yours was somewhere between the two. Now, did this work connect with the work that was undertaken with Goldhaber on the photodisintegration of the deuteron? Wasnít this related to a way of determining the mass of the neutron?
Chadwick:That was the best way of determining the mass. But in between times it was a question of relating the masses in a disintegration experiment, and one didnít know the masses of the nuclei to a high degree of accuracy. There was some inconsistency which Oliphant, or rather Rutherford and Oliphant, I think, pointed out a few years later. Then in addition one had to measure the kinetic energy of the neutrons which were emitted. That was not so easy to do either. And so you only got a rather approximate mass that way. But the best way was the way in which Goldhaber and I did it. But we didnít do the experiment merely to determine the mass of the neutron. It was done to see if photodisintegration could be observed. In fact, I had looked for photodisintegration — oh, some years before — unsuccessfully, because I had at that time to use a scintillation method as a method of detection, because I would have found protons, but I couldnít have found neutrons; and I did in fact try deuterium in a very rough and ready way. I was able to get hold of some paraffin wax, a very small quantity, which contained a fair amount of deuterium, and I tried to observe the photodisintegration. That was the target, but I didnít find anything. But we should have gone on to try other things. It happened that those experiments were done just before Easter, and I went off for a few daysí holiday. The object of most of the experiments I did around that time was to get some clue to the nature of the neutron, to find out whether it was an elementary particle or whether it was made up of the proton and the electron. And it wasnít easy. We neglected the various other experiments that could have been done, such as the disintegrations. I didnít do them. I did very little. We did later do a few. But not at that time. I wanted to know what the neutron was, but it was very difficult. I had to decide — and I did decide — that it was a proton-electron neutrino. You had to bring in the neutrino to get the Ö
Weiner:The mass Ö?
Chadwick:There was a difficulty about the mass. It always kept coming too high. No, the difficulty was the spin.
Weiner:This is a question that, of course, would involve you. How did you get into the slow neutron work in the laboratory? How was this taken up — the work that led to the understanding of the slow neutron?
Chadwick:Oh, that was done in Rome.
Weiner:Yes, but there was no work done in the laboratory Ö? I guess it was a question of them picking up then — Bjerge and Westcott, and I think Goldhaber was involved, too, on slow neutron work or disintegration.
Chadwick:Yes, he did a little. But that was after Fermiís work.
Weiner:Yes. You worked on lithium and boron with the slow neutron disintegration. Nutt was the assistant, was that right? He was the technical assistant during all of this period?
Weiner:And was his primary responsibility to serve all of the research groups?
Chadwick:Oh, no. Well, but that time I had an assistant whose main duty was to assist me. I must have had him for a year or so. I canít remember exactly, but I do remember he helped in the counting of scintillations. Thatís one period. And what I was doing then, I just donít remember anymore.
Weiner:Did you find that you had a decreasing amount of time available for research as the years went on and as Rutherford took less of a role in managing the laboratory?
Chadwick:No, I donít think so. I had no other duties than to act as Rutherfordís deputy on the research side of the Cavendish and certain duties in College, which only amounted to a few hours a week, perhaps three or four hours a week — no more than that, So that I did have a fair amount of time for my own work.
Weiner:Iíd like to get on another subject. We were talking before about the Cockcroft and Walton accelerator and the artificial disintegrations which they achieved. Now, this was going on during the same year — I guess it was a question of a few months. They got their first positive results a few months after the neutron was announced. And I think we discussed the fact that they had been working towards this for a number of years. What was the reaction in the laboratory with this second major development, after this period when not much seemed to be happening, and then all of a sudden Ö
Chadwick:Well, of course, that made things very exciting indeed, and particularly so perhaps to Rutherford, both things coming more or less together. And as his papers show, he began to work on that subject, because it amused him; it interested him; and he had Oliphant, who was a very good experimenter and who built up the apparatus, in which Rutherford was not at all interested — so long as it worked; but he enjoyed very much taking part in those experiments. Iím not saying that he didnít take any interest in the neutron experiments but there was, in fact, no way in which he could have done more than take an interest. He couldnít have started experimenting on that way. There werenít the sources, you see. I was still dependent on the polonium I could get from these radon tubes which came from Baltimore, and I was feeding two or three research students from this material, and they were doing some experiments of one kind or another — Iíve forgotten now what. Iím sorry my memory is so bad.
Weiner:Itís remarkably good. Weíve covered many things in great detail.
That was Ď32. There were a lot of people in the laboratory doing various things. I had to take an interest in them. Then my real interest, as I said before, was to see if I could find out what the neutron was. I knew it could do all kinds of things. I didnít think of the slow neutron business. If I had, I should naturally have tried the experiment — just to think of it, that the fast neutrons could produce disintegrations. But, as a matter of fact, Feather and I did some experiments — nothing of any great interest but enough to show us that practically everything probably would be disintegrated by and affected by the absorption of the neutron. But the effects were small, you see. It was the slow neutron which produced the big effects. And while there were one or two small experiments we did, I donít think we ever should have done very much, because that was Fermiís business. I would have tried to interfere with what he was developing. In those days that was common practice. One didnít barge into another manís subject unless by arrangement with him, and one didnít go round saying what other people thought. You might say what other people were doing before they had published it.
Well, thatís putting it rather baldly, but it was rather different. How shall I put it? In these days there is a great deal of competition between people, between different workers, even, you may say, between different laboratories. In those days there were many fewer workers. One, you might say, knew them all. And one wasnít competing with them. You were competing with nature. And in a sense experimental physics particularly I regarded as a kind of sport. It was contending with nature. There were other things in life besides physics. I did try to develop the photographic plate method, about which I had known for a long time. A woman at the Radium Institute had done a good deal of work on tracks in photographic plates, Iíve forgotten her name, Bertha something. And I wanted to use that method for observing the collisions between neutrons and protons. That was what I hoped would give me some line on the nature of the neutron. But our sources of neutrons were rather weak, you see, and that was the reason for using a photographic plate, because that would integrate the effects over a long time. But I couldnít get anybody to take an interest in making suitable photographic plates. That, again, is not quite true. One man in Ilfordís, whose name Iíve forgotten, did take some interest; but, of course, we were a bit of a nuisance to him, and also we didnít get the right idea of making these thick emulsions, which was first done in Russia by two men whose names Iíve forgotten. It was too late for me to do anything by that time. I had moved to Liverpool and did not have a source of neutrons.
Weiner:Thatís another thing I would like to pursue when we get to Liverpool. This courtesy of not embarking on a line of research that you know is the main interest of another laboratory — would you think that that had something to do with Rutherfordís attitude toward the cyclotron?
Chadwick:Oh, no. No, because he knew that Ernest Lawrence would have been delighted if weíd started to make a cyclotron in Cambridge. You see, we met Ernest Lawrence at the Solvay Conference in Brussels in October 1933, I think. And, of course, we heard about the cyclotron before that from published papers, but we really got to know a bit more about it from Ernest Lawrence, who was bubbling over with enthusiasm and who we liked very much. Rutherford was quite impressed by him. I remember his saying to me: ďLawrence reminds me of my young days. Heís just like I was when I was his age.Ē Of course he had some bee in his bonnet about some experiments heíd been doing. Heíd got the wrong explanation, but that was not a matter of any consequence. Oh, Lawrence would have been delighted. He would have been consulted beforehand, of course, but he was always ready to provide all the information that he had.
Weiner:He did for Cockcroft in 1933, I think, when Cockcroft visited him the first time.
Chadwick:Ernest Lawrence would have had no hesitation whatever in giving anybody the fullest information.
Weiner:Then it was more of Rutherfordís attitude toward this. There were no other restraints than his own feeling.
No. Well, now, weíre coming to a very difficult point. You see, it was becoming very difficult to push on without some new equipment. I couldnít get any further with what I had. I was at an end really with the equipment which I had or could see myself getting, and it was quite clear to me, as it was no doubt to others, that we needed a means of accelerating protons or other particles, particularly protons, at high energies. But that meant more space, particularly more money, and particularly engineering. It meant complicated equipment, and Rutherford had a horror of complicated equipment. Quite naturally. Heíd done everything with very simple means, and he didnít want big equipment in his laboratory. That was the chief reason why I left the Cavendish. It got to that point. I couldnít get on any further. I knew Rutherfordís attitude, I understood it. I was not prepared to quarrel with him. It might have come to that. I wasnít going to do it. Itís very difficult for me to talk about this business at all, but that was the main reason. I couldnít get on any longer without something of the nature of the cyclotron, and he said he would have nothing to do with the matter.
There were perhaps other reasons. One was that I found academic life somewhat circumscribed. I had lived 40 odd years, most of it in universities, and this is a difficult thing to say, difficult to use the right words: I wanted more contact with other people with different interests. And I was to some extent getting them through my marriage. My wife came from Liverpool, and her family was very well known there. We paid visits there. I had come across — itís quite true — only a few of the business people. I liked them. That was a reason for going to Liverpool rather than elsewhere. Itís true I had had one or two offers before. Then this one came at a time when I was faced with the certainty of a very big argument with Rutherford, and what I felt was also the certainly of a quarrel. And, you see, I owed too much to him. I donít say we hadnít disagreed from time to time — we had — but very seldom. But we had never really quarreled. So I made up my mind to go. I didnít consult Rutherford particularly about it. I did consult one man. But it was almost inevitable. Some parting of the ways I felt was inevitable. There were one or two other aspects which weighed a little with me. I had met the vice-chancellor of Liverpool, Hector Heatherington, whom I liked. I went to Liverpool to see the University, which I had never really seen. I had seen it a good many years before, but only during a meeting of the British Association there. In my visits to Liverpool in the ten years since my marriage, I had never visited the University, because I had no reason to do so. When I went there I found a laboratory which was much behind the times. This is between ourselves. I was quite shocked. Little oil lamps were used to illuminate the galvanometer mirrors. You can imagine the state of the laboratory.
Now, that didnít put me off, because it meant that even if I couldnít do very much, at least I could put the laboratory straight, and I knew that I would get as much help from the University authorities as they could possibly give me. That Hector Heatherington made quite clear. But, you see, some of the chief people of the University Council, the business people, were friends of my wifeís family, perhaps even connected by marriage. So I went, and to my pleasure, JJ — you know who J. J. is, he was master of Trinity, but he was always J.J. — he wrote to me and said he was sorry that I was leaving the Cavendish, but he thought I was quite right to do so. He had friends in Liverpool. I knew very well he had one particular friend whom I also knew slightly but got to know much better. He seemed quite pleased about it. (Pause)
Weiner:You were talking about your plans for Liverpool and your expectations of what could be done and what you found there.
Chadwick:Well, I found very little in the laboratory, a very small staff; it was understaffed, both in lecturers and in technicians. The apparatus was antiquated. When I say the apparatus, Iím really thinking of the apparatus for the teaching laboratories. Everything wanted renewing, practically everything. But, you see, I was assured that I would be given every possible help to do what I wanted to do. That was promised me. I knew there would be a limit to the amount of money that could be produced, naturally, because they didnít have any big funds. But I didnít anticipate much difficulty. I was, of course, from the beginning determined to build a cyclotron. Now, that was October Ď35 when I started there. I suppose very soon indeed I must have written to Ernest Lawrence and told him what I had in mind. There was a man there, a former member of the Cavendish — I think heíd only been at the Cavendish one year as a research student or perhaps two before he went to work for Ernest Lawrence. He had not worked with the cyclotron directly. He had, I think, been accelerating particles in some other way. His name was Bernard Kinsey I was able to offer him a fellowship in quite a suitable emolument, which he accepted. So I was going to have somebody who knew something about the cyclotron firsthand. Then, as you can see, later on from Rutherfordís letter that Walke had been out there, too. He was a very nice fellow. He came. What I should have said at the beginning that helped me with the teaching, I was able to offer a lectureship to Norman Feather, and he came with me to Liverpool from Cambridge. That was of course a very great help. But he only stayed one year. He was then offered an official fellowship at Trinity and a college lectureship, really a certainty also of a university post, and so he was practically bound to accept it. So I only had him for one year, but he did help to get things started. Then I got E. J. Williams, a very able man, a Welshman, who had been at the Cavendish and Manchester. I suppose he was in Manchester at that time. Of course he was.
Weiner:He was concerned with cosmic rays, wasnít he? Iíve come across some letters that indicated that he was making some cosmic ray measurements in a later period.
He did a number of things. The order-disorder idea in crystals was his idea. He and Bragg worked it out. Oh, he was a man of very wide abilities indeed. He did some cosmic ray work in Liverpool. Then about the question of finding the money to build the cyclotron, I must say that I realized that the University would not be able to find it. I had to get help from somewhere else. Now, I might have got help from some of the business people in the town if it had been necessary. But there was a fund in the Royal Society. Iím not sure whether the money was given to the Royal Society or administered by them. I think it was given and administered partly by the Royal Society and partly by a bank in Manchester or Liverpool, representatives of the bank. But the money was given by a man who had made his money in Liverpool. He wasnít well known there. And so I made an application to the Royal Society for a grant, and the vice-chancellor and I were asked to go down to London and meet the officers or some committee to discuss the business. They made a grant of I think £2000, but I wonít be sure about it. I think it was £2000 toward the capital expense. And either then or a little later — I donít remember — gave a grant for running expenses. I knew that somehow or other it would be possible to do the thing. So I got into correspondence with John Cockcroft. I knew he wanted to build a cyclotron, and I saw I was going to build one and was all ready to start and if we built them together or built them at the same time we could save a little money. And he, in fact, designed the magnet. It didnít need very much, but that he did. That had been done before I saw Rutherford in the end of May or beginning of June 1936. That was the last time I saw him. I was an examiner for the tripos and I had to spend two or three days in Cambridge. Of course Rutherford asked me to stay with them. I didnít see much of him because I was busy during the day. The only definite memory I now have is of my last morning there. I was leaving that day, and after breakfast we sat out in the garden in the sun to have a talk about things. He was rather annoyed with me about a certain matter which I have never mentioned, I couldnít explain everything to him for various reasons, and we had to leave it with a very definite disagreement. Iíve been sorry for it ever since.
But then he attacked me about the cyclotron, the implication being that I was pushing John Cockcroft toward the cyclotron and so forth. And I said, ďWell, I am going to build a cyclotron in Liverpool. I think Iíve got enough money. Cockcroft has designed the magnet. I am ready to begin. Iím waiting. Iíll begin any moment. But if you build a cyclotron at the same time of the same size we shall be able to do it more cheaply. We shall each of us save some money.Ē And he said, ďWell, Iím not going to have a cyclotron in the Cavendish.Ē And a few more things to this effect. I said, ďI think youíll have to have a cyclotron. Otherwise the people will not be able to get on with the job, and I will wait until the end of June before giving my order.Ē It was going to be to Metro Vic for some things, I didnít know who was going to build the magnet, but Metro Vic would machine it. We left it at that. It was a sad ending in a way to a very long friendship and cooperation. But by the end of June we were both ordering the magnet. He had to come round. But he just hated large equipment which had to be looked after practically by engineers and which he would not at his age take the trouble to understand. I can quite realize that, but the damned thing had to be done. And we managed. It was unfortunate for both of us that at that time — that was the middle of Ď36 — this country was beginning to look after its rearmament. And firms like Metropolitan Vickers became very busy on defense contracts. And so it took much longer to build than we expected. The cyclotrons were only ready to start working just before the war broke out.
Weiner:Did you ever get it into operation before the war?
Chadwick:Yes. We had unfortunately made what we considered to be improvements. They werenít. They turned out to be a great obstacle to getting the thing to work, so we had to go back more or less, not quite, to Lawrenceís design. But, oh, yes. It was on that cyclotron that I made — shall I say we made — all the experimental work which showed that a nuclear bomb was feasible. It was fortunate that it was ready.
Weiner:Would you say that was the first use of it, the first real use, for that work?
Yes, it was the first real use, because the beginning of term in October 1939 — Ď38 or Ď39? It was Ď39 that the war started. Yes, I was late getting back because weíd been out in Sweden fishing with a Liverpool friend who had a house there and had fishing rights on a very big lake with the best trout in Sweden, and we were out of touch with the news. A farmer would tell us what he heard on the wireless. And there we were. We got back to Stockholm to find no way of getting back to this country immediately. In the end we managed to get a plane to Amsterdam and had to stay there a few days, because there was no transport between Holland and this country. We were always told that our plane was about to leave. ďAt any moment you may hear that a plane will leave for London, and you mustnít leave the hotel.Ē So we took turns to take a walk around the hotel while the others stayed inside. Not in our party, but also caught in the same predicament was H. G. Wells.
Rutherford, "Structure of the Radioactive Atom and Origin of the X-Rays", Phil. Mag., 4 (Sept., 1927). Collected Papers of Lord Rutherford, Vol. III, 200.
Rutherford is not listed among the participants at the Rome Conference in 1931.