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In footnotes or endnotes please cite AIP interviews like this:
Interview of Edoardo Amaldi by Charles Weiner on 1969 April 9,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
For multiple citations, "AIP" is the preferred abbreviation for the location.
Family background; early interest in physics; chance meeting with Enrico Fermi in youth and early friendship with Emilio Segrè; enrolling in physics at University of Rome; recollections of Orso M. Corbino; 1931 Rome Conference on Nuclear Physics; 1934 visit to Cambridge with Segrè; transition from spectroscopy to nuclear physics work at Rome; reaction to discovery of neutron; Ettore Majorana's work; slow neutron experiments; Fermi's approach toward theory and experiment; failure to discover fission; break-up of Rome group; 1936 trip to America; construction of two accelerators at Rome; 1939 trip to America; decision to discontinue fission experiments at Rome; usefulness of Hans A. Bethe's review articles; style of Rome group; physics elsewhere in Italy during 1930s; contacts with physicists outside Rome during 1930s; Italian physics during the war; postwar concern with elementary particles; recollections of Fermi in postwar period; work considered personally satisfying. Also prominently mentioned are: Herbert Anderson, Gilberto Bernardini, Torkild Bjerge, Patrick Maynard Stuart Blackett, Niels Henrik David Bohr, James Chadwick, Conversi, Otto Robert Frisch, George Gamow, Ettore Majorana, Pancini, Oreste Piccioni, George Placzek, Franco D. Rasetti, Westcott; Accademia Nazionale (Italy), Cavendish Laboratory, Columbia University, Conference on Nuclear Physics (1931 : Rome, Italy), Istituto superiore di sanità, and University of California at Berkeley, CA.
This is an interview with Professor Edoardo Amaldi, and this is Charles Weiner asking some questions. We are sitting in a hotel room in Florence, and the date is April 9th, 1969. started in our conversation this morning, when the machine wasn't working, to talk about the sorts of questions that we thought might be of interest. The idea is not to duplicate what Professor Kuhn discussed but to supplement and then to extend it to a later period. But I would like to go back to the beginning in this case, because we know very little — about your early family background. I know that your father was a mathematician. He was professor of mathematics at Padua from 1918 to 1924.
Yes and later in Rome.
I want to know a little bit about your early family life and how you got interested in scientific things — where you went to school prior to your study of engineering at Rome.
Well, I was very much interested in physics when I was a high school student, and I was always interested, as you say, in mathematics and physics; and I wanted to study physics. And when I finished my high school, my family went for a summer to a village in the Dolomites; and I was with my family for vacation. This village was San Vito d'Catlore, and in this village Fermi was also on vacation. This was the summer of 1924, I believe. Fermi was very young; he was 23 or 24 years old. A friend of his had come to the same place. That was R. De L. Kronig who is now professor in Groningen. Kronig was a great friend of Fermi. They met when Fermi had been in the Netherlands to work with Ehrenfest. You know that he had been in Leyden with Ehrenfest for some time. He had met Kronig there, and so Kronig went to San Vito d'Catlore to spend his summer. They were quite young, but they were both quite good physicists already. I knew almost nothing. But in that summer I was extremely impressed by these two yo - people, and when they were hiking in the mountains, I started to follow them trying to learn something about what they were saying. I did not learn very much, because the type of conversation was too difficult for me. But certainly I was very much impressed by the personalities of Fermi and Kronig and the type of conversation they were doing.
You had no contact with them before. This was just coincidence that they were there?
Well, it was partly coincidence, because my father went there because there were other mathematicians going to the same place. There was Guido Castelnuovo, who was later one of my professors, a well-known mathematician; and there was Enrico Bombiani, another mathematician; and, if I remember correctly, there was also Levi-Civita. My father was a great friend of Levi-Civita and they had written together a treatise on mechanics. Fermi also came there for completely different reasons, but in a certain sense there was also some connection. I had never seen Fermi before, and one could also say that it was an accident. It could have been that I did not come with my family, since usually I went for vacation by myself. I was very much interested in climbing mountains and at that time I created troubles for my family by climbing too difficult mountains. Usually I wanted to go by myself in order to be able to do what I wanted. But that summer I went with my family to San Vito.
Was it then that you made the decision to study physics?
I was already interested, but that certainly made a tremendous impression to me—I should say that I met Segrè because I was very interested in climbing mountains. Segrè was already a student of engineering; he was a few years older than me. With him and with Ciaranti, who is now a well-known professor of medicine, we went to Grakasso d'Italia in Central Italy to climb the Corno Piccolo We were living for a few days in a mountain hut and there we were talking and discussing a lot because both Segrè and I were interested in physics. He had decided to start engineering, because it was not clear if it was possible to do a career in physics in Italy at that time. For similar reasons I also enrolled in engineering. I remember quite clearly that I said to myself, "Well, it does not make any harm," because in Italy at that time the first two years were exactly the same. There were only very minor differences. There was one extra laboratory for physicists, and there was a design course for engineers. So I had to do this engineering drawing course, and I followed the laboratory that was made for physicists since it was allowed to choose also this course. But I was not happy to be an engineer. Later it was very simple for me to change from engineering to physics.
You mentioned that you met Segrè. Under what circumstances did you meet him?
Oh, he was a friend of Giovanni Enriques, who was the son of a mathematician and was a great friend of mine. So we had a common friend.
So it had to do again with your father's connections in a sense.
In a certain sense. Giovanni Enriques and I had also been climbing mountains together.
I see. Now getting back to what you were saying, you didn't lose any time because of studying engineering.
Yes. Well, Segrè lost some time because he went on studying engineering up to the fourth year. And the third and the fourth years in engineering are completely different from physics. So he had actually lost some time. I had not.
When did you make the switch? Was it after the second year?
At the end of the second year. I used to go to the lectures of Corbino. The course which I give now at the University of Rome was given at that time by Corbino Corbino was extremely brilliant. He gave beautiful lectures—very brilliant, very attractive, and making nice little jokes to explain things in a very clever way. One day towards the end of the second year, he gave a rather famous little speech saying, "Well, I think that if there are people that are ready to do a sufficient effort, this is the right moment to change from engineering to physics. We've arranged now to have Fermi come to Rome. Maybe you don't know who Fermi is, but I'm absolutely sure we have never had in Italy for many, many years a physicist of this class. He's very young and he knows what is modern physics. We old people don't know, but he knows what it is; and if you switch over, there'll be a lot of opportunity for very interesting work." This was a very simple invitation. I had been for two years so interested, but in a certain sense had not had the courage to do it. This was the occasion for deciding to make the change. So the next fall I enrolled in physics. It was very simple. had to go to the Secretariat of the university and simply say that I wanted to change from engineering to physics. At the same time Segrè made the same change. The only difference was that he had lost at least one year. How he changed is explained very well in the biography of Fermi that he has written. This biography I think is a very good one. All old friends read it before publication and gave suggestions for improvements.
We read it in manuscript a few times.
talking about a new one that Segrè is doing.
I read also that. I've written a number of comments contained in many pages that I sent to him.
He's pretty near the end, I think. It's done, but...
He told me that after I had written these comments he has practically written again almost everything. I've not seen the final version, but I received it in manuscript some time ago. I read it very carefully, and I commented. It was rather well done. I added some details that he did not know, and he told me he has incorporated this into the final ver- sion. I've done the same for this Majorana biography. I've written the biography in a preliminary form. I've sent it to all people who knew Majorana, including the members of his family, and I have collected information from all of them and comments and have added all that they knew, what they could recollect. So I've tried to do the best that can be done.
We were talking this morning about Corbino, and it might be a good time now to interrupt the continuity of your story and talk about him. You were telling me some interesting things about his family background in Sicily at...
Augusta. He was born in Augusta. That is a little town not very far from Syracuse. His father had a small macaroni factory where the macaroni was homemade by Corbino and his family. There were seven children in the family, four boys and three girls. Two of the brothers, Mario and Epicarmo, both became professors at the university. Epicarmo, the younger brother, is still alive. He's a professor of economics at the University of Naples. When they were very young they were helped a lot by an older brother Leone who was a noncommissioned officer in the carabinieri. This man was apparently extremely intelligent, and he felt that the younger brothers were very gifted, and he helped them, and they had to study and try to learn, and that's what happened. Mario, too, was a very remarkable person, extremely intelligent and very much interested in science. Corbino was an assistant in Palermo and produced a lot of very good work.
The professor there at the time was Professor Damiano Maculuso, and an effect was discovered by Maculuso and Corbino It was at that time known as the Maculuso-Corbino effect. It was after the discovery by Zeeman of the Zeeman effect. And these in some way are related. It's a variant. Now it is quite clear that it's not a completely different phenomenon, but is something related to the Zeeman effect. About two years after Zeeman had discovered that the effect of the magnetic field—the split of the lines, the spectroscopic lines in the magnetic field—they measured the rotational power of a gas, for example, sodium, near a resonant line. And they found that the rotational power becomes extremely high—infinity—near an absorption line. And they measured this effect, and they discovered it and explained it very well; and this is related to the Zeeman effect. They are different aspects of the same phenomenon.
But he did this as a student?
Well, of that I'm not so sure. It would be better for some of these details to look into the biographies. There are biographies of Corbino I will try to find some good biography of Corbino and send it to you. In that you would find more details about that. I think there is also a biography by Fermi of Corbino, but I should find out.
I just wanted to talk about that for a minute. But I'd like to get back now to your story. There's a lot of the story that I already know. But there are a few missing connections. For example, you were Fermi's assistant, but it's not clear how you became Fermi's assistant.
No, I was never an assistant of Fermi. I was an assistant of Corbino actually, as also was Segrè. We were both assistants of Corbino. But when Corbino took us as assistants, he said, "The best thing that you can do is work with Fermi." But the situation was such that he [Fermi] couldn't take assistants. Fermi was much younger; it was not so simple; and Fermi had the chair of theoretical physics. So Wick was assistant of Fermi, not Segrè and myself. We were experimentalists. And also Rasetti was an assistant of Corbino. So we were all assistants of Corbino, but then we were working actually with Fermi, and Corbino was essentially taking care that we could do our work—that Fermi could do his work and that we could do our work under him.
So you went under his umbrella, so to speak.
Oh, yes, certainly. All of us recognize that the action of Corbino has been essential to all the development of physics in Italy. There's no question.
This raises a question about Corbino's speech of declaration that there are two new fields of physics to go into—that spectroscopy is no longer going to be the field, and you have to think of the study of the nucleus and the study of large cells, living systems. This was the 1929 speech. And he even said in this speech that to do the study of the nucleus we need expensive equipment and big energies; and if this is not practical, then perhaps the living system and the cell itself. What interests me about this transition period is the fact that it was before the neutron, and it's not at all clear to me why Corbino and others felt that nuclear physics was the field where you would get results or where it would be fruitful. Do you have any feeling about that?
Oh, yes. I remember very well that this was very much discussed at the Institute. And there is one page on that in my biography of Majorana, because really this idea of Corbino was also an idea of Fermi and certainly of all of us in the department of physics in Rome. I'm not able to say who started the first idea—whether it was Corbino or Fermi. But certainly this point—that it was very important to move to a new field, that spectroscopy was a very nice field but the exploratory work was in some way finished, that you had to move to something where the phenomena, the facts, were still unknown—it was very clear that this was urgent.
It hadn't anything to do with theory? In other words, the theory of spectra was known. So in a sense you were getting new facts, but they were predictable.
Yes, yes. The idea was to look for new facts that were not predictable, and there was also need for new facts and new theories and so on.
I can understand how this would be the case with artificial radioactivity when you wanted to do a systematic study of it because there was no theory involved in it, and there was an opportunity to do experiments. You had the sources; you were tooled up; you had the special techniques that you had developed, all of you on various trips. And then with the discovery of artificial radioactivity, there was an opportunity to do systematic fact-gathering where there was no real theory accompanying it, where you would feel that it's very promising. I understand what one means by that, but I'm not clear how in the years prior to this anyone saw such an opportunity-in the years between 1929 when Corbino spoke and, let's say, even before the neutron, even before 1932. What facts can you gather?
We actually started in 1931 to work in this direction and to shift our interest from atomic and molecular physics to nuclear physics. It's not clear to me what is your question.
In other words, what was so compelling?
We started first with spectroscopy. The basic parts of spectroscopy were clear. There was the idea that with quantum mechanics, spectroscopy was a very important field but where you needed development of computational methods to compute the wave function and levels, but there was nothing basically new to be found. And the idea was too look for a field where there was something basically new.
So it was a combination of a negative feeling about spectroscopy...
Well, negative in the sense that the exploratory work was finished. There was need for precision work and precision computation. That is beautiful but is not really new exploration.
You've discussed elsewhere the discussions that took place about the switch, but one of the things that interest me is the 1931 Rome meeting. That was a meeting on nuclear physics specifically. I gather that the idea was that to learn as much as you can about the subject, you call a meeting and bring in all the experts.
Yes, a meeting was called, and I remember that Bohr was there, and Bohr was giving speeches explaining that probably it was necessary to renounce the principle of conservation of energy of the beta decay. He suggested that there was no conservation of energy and momentum. There was a number of discussions on this point. I remember that Millikan was there. I remember that Blackett was there and quite young, and Mott was there, slightly older but still very young.
The Curies were there.
Many, many more people were there.
Goudsmit was there.
Well, Goudsmit used to come to Rome almost every year. Goudsmit and Uhlenbeck were both great friends of Fermi, and they used to come to Rome almost every year for one month or so And so we used to see them quite often. They came also on their vacation. But the old Madame Curie was there, and Sommerfeld was there. There are a number of very nice pictures of this.
I have them.
There are a number of very nice pictures of the people there.
Segrè gave us a set of pictures. You gave it to him, think.
I gave them to him last October, guess.
That's right, and he let us copy some for our archives. This was earlier. Maybe there are more that you gave him.
I gave him some more I think last September or October. We looked through them, and he said, "I'm interested in this and this and this, and then we had them copied.
The conference itself—let's get back to that. Was it mostly formal papers or were there a lot of informal discussions at the meeting? I think I've seen the formal proceedings.
There was also a lot of discussion, of course. I was too young to take part in many of the discussions, so I was listening essentially. But I remember there was a lot of discussion. You have seen the people who were there?
Yes. I'm just curious: was it on the experimental side? I get the feeling that it must have been; in other words, that the emphasis...
Was mainly on the experimental side. At that time there was also the theory of Gamow on the alpha decay, and that was the only sound piece of theory existing at that time on nuclei.
Gamow, as a matter of fact, was supposed to come to that meeting but couldn't, wasn't allowed to; and the people at the meeting sent him a greeting—a card—which he showed me.
The theory of Gamow was one of the things that was discussed. It was generally accepted. It was considered a great success of quantum mechanics. And then Bohr was discussing very much the beta decay and the possibility to have to renounce the conservation of energy.
He apparently had been talking about that even earlier at the Faraday lecture that he gave in 1930. And in his correspondence it turns out that even after the neutron...The proceedings weren't published until after the neutron was discovered and when he corrected the proofs, he was still talking about the same thing. It's something that I'm trying to understand—why he didn't change his mind on that after the neutron. But let me go on to some of these trips that you took in the period, in particular in 1934 when you traveled to Cambridge. You went with Segrè, and your wife was pregnant at the time.
Yes, our first child.
I'm just curious about your observations there—what you saw. There was Chadwick; there was Goldhaber.
Oh, it was very nice. We went to the Cavendish. There was Cockcroft; there was Ellis; there was Oliphant, Goldhaber and Dee. And on that occasion in London we went to see Blackett at Birbeck College, and we met Szilard. Szilard was in London. And I remember that while we were there, the Solphus affair happened in Vienna. Solphus's murder was the first attempt of Germany to incorporate Austria. And I remember that we had a lot of conversation on that with Szilard. Then at Cavendish we met also some other people, like Bjerge, who was a Dane, and Westcott, who is now in Canada. He's mainly involved in nuclear energy, in reactor physics. And we were very much impressed by Rutherford, of course. He was an extremely impressive personality. But we were discussing a lot with Bjerge and Westcott because they were the only people at that moment working in Cambridge on neutrons.
This was just before the discovery of slow neutrons, but there was a very difficult problem. That is described in the book of Segrè, about the so called radiative capture of neutrons. It was very difficult to understand at that time how a neutron could be captured by a nucleus. Any estimate of the probability of such a process turned out at that time to be very small, much smaller than what was observed. When by bombardment of a nucleus with a neutron we had observed an isotope of the initial nucleus, it was the question to know—if it was a process of capture of the neutron, or a process of emission of a second neutron, a second neutron knocked out. And the proof that it was a capture of a neutron—at least with the neutron that we used at that time which was not of very high energy—came from a paper of Bjerge and Westcott and another paper of Segrè D'Agostino and myself. This was done in September. We had done this work when we came back from Cambridge.
How about Chadwick and Goldhaber? Were they doing anything on this?
Oh, yes, they were doing this at that time, and we had some conversations with them, but they were very busy at that moment doing their experiments on the deuteron. And so we had some exchange but not so much as with Bjerge and Westcott. Of course, we met them, we had a conversation, and it was very interesting. u u
You had been thinking about this before you went. What was your reaction in coming from Italy? You had been in Leipzig before and then you were coming to the Cavendish for the first time. What was your reaction to the style of the research group? Did this in any way contrast with the sort of thing that you had been developing with Fermi and others in Rome?
Well, I was very much impressed. But in a certain sense we felt much better...Well, I had been for one year in Leipzig working with Debye before, and Segrè had been in Hamburg working with Stern. And I don't know what was the comparison of Segrè, but I think it was the same as mine. We felt at Cavendish much more at home than at Leipzig. In Leipzig it was very happy. Debye was a very nice man, and I liked him very much, but the Cavendish was really a very, very attractive place. And we became great friends with Bainbridge from the United States. He was there working with Aston. Aston was rather old at the time, and Bainbridge was doing very good work with him. I remember that we became very good friends with him and his wife and enjoyed very much the company of the Bainbridges.
You spent the summer there. Your wife was pregnant...
We had to come back, because my wife had to have the baby, and she wanted to have the baby in Italy, so we came back just before the baby was born.
There's a story that Segrè mentioned, that you knew it was time to go back when your wife could no longer fit between the posts on a certain street. That's how I knew about that. When you did this earlier traveling—you worked on X-rays, for example, for a while with Debye...
I was working with the scattering of X-rays by liquids with Debye.
And then other people in the Rome group became involved with the Raman effect, for example. Now, how did these two things—this background— influence the transition to nuclear physics? Was it work that was really used—for example, in the interpretation of the neutron, in the reaction to the neutron? How did the Raman effect work on the nitrogen 14 thing? Did this figure in it? Did it prepare you, or the group?
Well, the work that you mention about the Raman effect of nitrogen was done by Rasetti. Well, Segre and I both learned to do experimental work from Rasetti. Rasetti was a very good experimentalist, and he taught us how to work. He was much better than Fermi as an experimentalist. And so we started all to work in spectroscopy, and later Rasetti went to Berlin, and he was working with Meitner. It was part of this idea to learn nuclear physics techniques—"the radioactivity technique," we said at that time. So we had prepared ourselves. But in the meantime we knew rather well spectroscopy and molecular physics and so on, and so we were still devoting a lot of work to spectroscopy. Actually, we worked with spectroscopy all during 1932, and then we changed from spectroscopy to radioactivity in a couple of months.
Was there a time when you were working on hyperfine structure?
I was never working with . Segrè was working on it.
I'm talking about in general. Was this a kind of transition?
It may be.
In his work anyway it would have been.
In Segrè's work it was a transition, not in mine. In the work of Segrè it was, in a certain sense, a transition. But this was all still seen from the point of view of spectroscopy, to go deeper into the under- standing of spectroscopy. I was more involved at that time trying to compute, as we had done with Fermi—to prepare tables of wave functions of all atoms. We had the idea to compute what we called at the time the x-arum" a joking Latin expression, i.e. prepare a book with all wave functions of all atomic levels. And we started, and in 1932 a paper by me published by Fermi and myself on the s-wave functions of atoms. It's in the collected papers of Fermi.
I have that, yes. Well, in 1932 when the neutron was discovered, one of the interesting things is that Fermi in his talk in July of that year at the international conference on electricity made this statement it's in the collected papers): "We see the uncertainty still prevailing on the subject of the neutron." This makes me feel that there was perhaps still some skepticism about it when it was announced. What I'd like to explore with you is what the reaction was, if you can think back, when the neutron was announced. One possibility was that it was immediately evident. You told me about Majorana, for example.
Majorana foresaw its existence before the discovery of the neutron, and this story is told in detail in my biography of Majorana.
You covered it in the interview with Kuhn, so I know that part of it.
But that was written, and I have thought very carefully what have written, and it has been read by Segrè and by Rasetti and by all people that were there; and everybody thinks that it is the best that you can recollect.
So we won't try to improve on this.
Well, you can always improve. But certainly when the paper of the Joliots came out, Majorana said, "Well, the Joliots have not understood anything." He was very critical. He said, "Oh, these Joliots, they don't understand what they do. It's so clear. They talk of gamma rays. That's clear. There is a neutral proton." Hecalled it a neutral proton. He did not call it the neutron. He said, "It's so clear. These papers of the Joliots are merely saying there is a neutral proton."
Had he read the Rutherford references to this?
I'll tell you. The reason was this. He was a great friend of another man that was in the department. That was Gentile. Gentile had done some theoretical work on an idea of Rutherford. Rutherford, in order to explain the anomalous scattering of alpha particles at a certain moment, suggested the idea that there could be neutral particles of a mass of the order of that of the proton going around the nucleus. And he had done a fantastic sketch...He had sketched a certain scheme that did not work. And Gentile took this idea put forward by Rutherford in a rather qualitative way, and he wrote a paper showing that the idea was inconsistent.
So there was a paper done in our institute by Gentile which showed that this idea, put forward in a very fantastic way by Rutherford, who was a genius, was not correct. It did not work from a quantitative point of view. And Gentile was a great friend of Majorana, and so he knew of this old idea. That is what we know. That we are sure of. But what is also sure: that when the paper of the Joliots came out, he said to me, "That's quite clear. The Joliots have not understood. These experiments are just the proof of the existence of a neutral proton." Then after one or two weeks (I don't remember, but very, very shortly) arrived an issue of Nature with a letter to the editor of Nature by Chadwick which discussed some experiments he had done on the same line as the Joliots but better in that he could really give the proof.
The reply of the Joliots were sufficient to suggest that the existence of the neutron was a possible interpretation, but it was not a proof, while Chadwick proved its existence. So Chadwick had done a very essential step, but Majorana had done this, too. So we were prepared in a certain sense, and nobody was astonished at it, and everybody accepted immediately in Rome that the neutron existed and this was essential.
Then what is the uncertainty that Fermi mentions in July? Was it a question of the interpretation of it?
I don't remember. I'm sorry I don't remember this point in this paper of Fermi's. I'm not able to tell you because I don't remember.
Right. Well, that's where we always draw the line.
Well, I could try, but it would be a present reconstruction that would not make any sense. If I can imagine, Fermi was extremely cautious before saying anything of that type. I remember on later occasions when something for everybody was proved, he was always very careful not to say, "This is the only explanation." He would rather say, "You have to accept a certain fact as proof only when you have really explored all other possibilities." A rather amusing thing. Later in 1936 when I was working with Fermi, and we found out the neutron resonances, and it was rather obvious that the differences between the neutrons that showed the resonance were only differences in velocity.
But Fermi did not want to talk of velocity. He would say, "That is the most obvious explanation, but we should not adopt an obvious explanation as long as it's not proved." That's the reason why when we found all these resonances, we called these resonances with alphabetic letters. We called them Group A, Group B, Group C and so on—in order to avoid getting into the habit of thinking in terms of velocity. So we classified all these resonances, because many people ask me, "Why did you classify resonances with letters, calling them A, B, C, D?" I remember that if I tried to talk of velocity, Fermi would say, "Please don't talk of velocity. That's the obvious explanation. We should avoid to get used to it, because later we will believe, even if it is not true."
That's very interesting, the fact that he wanted you to discipline your thoughts to allow other possibilities.
You had to discipline not to get used to it before it was proved, because when you get affectionate to an idea, then you are not any more free to be critical. That was the attitude of Fermi. That could be the explanation, but I don't remember this specific fact.
You know, talking about words and names, this idea of the neutrino At the Rome conference in 1931—you said you weren't involved in all of the discussions, but...
Well, I was listening, but I was young.
Well, anyway, the word neutron was used in the other sense. This was before the discovery of the neutron. But apparently the word neutron was beginning to be used in that period more and more.
Well, there is that paper by Chadwick, a very good one; you know that. And this was given at the meeting at Ithaca. I would take this paper of Chadwick as probably the best.
There are a few things on that I will probe with him.
In these short three pages—I have a photographic copy of this paper—he really explains what was the thinking in Cambridge and how the idea of a neutron was older when it was discovered. That does not reduce the merit of the discovery of the neutron, but it is interesting.
Apparently nobody else was looking for it, you see. They were the only group looking for it, although the idea was common. That's the very interesting thing. In one of your papers, the one on highly excited alkali atoms...
The Excited Atoms.
Yes, the "Effect of Pressure on the High Terms of the Spectra of the Alkalis," with Segrè The experimental effects observed were explained in a theoretical paper by Fermi.
By Fermi, yes. We had discovered this effect, and we could not understand what it was. We asked Fermi, "What do you think?" and then he made the theory.
In this paper he talks of slow neutrons.
We were working with electrons in such excited states, they were bound with ~1/100 of electron volts, extremely weakly bound, and moving 100 almost free, bound so weakly they were almost free and with very long wavelengths. And in that paper, in order to explain the effect that we had found, he made the theory of a collision of a very slow electron against an atom. And this is exactly the same theory that was used one and a half years later for slow neutrons against nuclei. This is very interesting. People don't usually know that.
This is the question that I have written down here. In 1934, the idea of the slow neutrons came out. Now, this paper was earlier than that. It was the same year but it was earlier. So was there a connection in his mind? Was it the theory of slow electrons that suggested the theory of the slow neutrons? Is there any way you have of knowing?
I don't think so. This effect in atomic physics was found by Segrè and myself. I don't know if you know how we found it. We were studying excited atoms in an electric field—what is called the Stark-Lo-Surdo effect, similar to the Zeeman effect but with an electric field.
Now, we were studying this effect in absorption. That means we had a long tube containing the vapor to be studied and closed with two quartz win- dows, and we had an ultraviolet source, and we have sent the beam of this light through the tube. The windows were cold while the vapor was at 500-600 degrees, so there was the problem to avoid, that the vapor (of sodium or potassium) would distillate on the windows. To reduce this effect, we introduced into the tube a very low pressure of noble gas (argon or neon). We started to put this noble gas at a very low pressure, and we said we should not put more pressure because at that time everybody knew that if you have a foreign gas, then by collision you get the lines wider because you shorten the mean life of the levels.
So we were very cautious to use a very low pressure. Then at a certain moment we saw that the lines were very thin, exactly the same. So then we said, "Why don't we increase the pressure?" and we started to increase the pressure, and we saw that the lines were not becoming wider—they were the same. Then we said, "We should try to increase the pressure again." So from a pressure of a few centimeters of mercury, we went to one-atmosphere, and the lines were still quite thin. And then we tried two-atmosphere. We changed the tube in order to be able to support the higher pressure, and we found that the lines were still the same but they had shifted a bit. And then really we were astonished, because this was against all common ideas at the time.
Then we asked Fermi what was his opinion. He immediately pointed out that when the electrons are bound so weakly, the orbits are so big that at high pressure of the noble gas between the electrons and the rest of the atoms, there are of the order of 10,000 foreign atoms. So you have atoms with a dielectric medium inside. So we had to calculate the levels of the atom by taking into account the dielectric constant. The shift of the lines due to this effect has, however, the wrong sign. Then Fermi found that one had also to take into account the collision of the electrons against the atoms of the foreign gas; so that the electrons don't go around simply, but in some way they do a complicated orbit, because they are scattered. They are kept weakly on their own original orbit, but in some way they are scattered around it.
And then he made the theory of the collision of a very slow particle against an obstacle. But this was later the same as the collision of a slow neutron against a nucleus. The ratio between the wave length and the dimension of the obstacle was accidentally the same in the two cases.
Did the idea—not the specifics but the idea—of slow electrons suggest a model for the slow neutrons?
I don't know, because when the discovery of slow neutrons was made, it was also a purely experimental discovery. At that time we were planning our work in a very effective way. So we were making a meeting every second day and saying, "Emilio does that, and Edoardo does that," and "Fermi does that." Everybody was supposed to do a well–defined part of the work.
We were all very precise people; if we had to start at 8:30 or any other time, we started exactly at that time. We were working as many hours as necessary. We planned the use of our time in a very precise way. After Segrè and I came back from Cambridge, I was asked with the help of Pontecorvo who had been one of our students and who had finished at that time, to try to prepare a kind of absolute scale of the activations of the different elements.
That means to find a standard condition of irradiation and to see how the different elements become active when irradiated all in the same conditions. This was the idea, because in the first paper that was published in the Proceedings of the Royal Society, we had just given a qualitative scale, by saying "very strong activity,", "weak activity" and so on. We felt it was unsatisfactory. Then we felt that we had to take, for instance, silver (I remember we thought of silver), to call 100 the activity of silver, irradiated under certain conditions, and then to irradiate all other elements in the same number of nuclei, in the same flux of neutrons, and to give to each element a different number corresponding to an arbitrary scale of activity.
Pontecorvo and myself started to do this in a very precise way, but we found out irregularities. We were not able to obtain all the same results irradiating under the same conditions—at least apparently the same conditions—the same piece of silver. First of all, we had to choose a standard, and the standard had to be reproducible. So we started to look if our standard was reproducible.
You calibrate your standards.
Yes, we were trying to calibrate the standard. And at that time Rasetti was saying, "Well, you do mistakes I'm quite sure. Is it possible that you find different results? Well, you find different results. In the same flux of neutrons, the same amount of silver is irradiated, and you get different results." We protested and said that we were not doing mistakes but that the results depended on the presence of some material around. We used some lead to protect ourselves from the gamma rays of Rn present in the neutron source and we had always thought that its presence was not important for the activation due to neutrons. But by changing this lead, we had differences.
You put it in wood?
We had made already before the irradiation on a table of wood sometimes, on a table of marble sometimes, and we had observed differences. And also the presence of lead makes a difference, because lead has a slowing down effect, not by elastic collision but by inelastic collision— much smaller than hydrogen, but also lead has a slowing down effect, owing to a type of collision in which the neutron loses energy by exciting the nucleus of lead to a higher level, and then the nucleus goes back emitting gamma rays. But we did not observe the gamma rays. We observed only a certain slowing down.
When you put the paraffin...
We had prepared a cleaner experiment to see this effect in lead. But the morning it was decided to do the experiment with lead, Fermi said, "Let us try paraffin, not lead." And with paraffin, the effect was much more evident.
Now, when he saw that, later that same day he theorized that this was a slowing down?
Yes, that was in the morning, and in the early afternoon he said, "This is just an effect of slowing down." I don't think this was so directly connected with the slow electrons—I don't think so But I'm not sure.
I wasn't pursuing it for that. I was just trying to get to a different point on this. What was the reaction of the rest of you to Fermi's hypothesis later that day about the filtering effect of the paraffin?
Oh, we understood that immediately. What we could not understand was when Fermi came and he said a very strange thing. He said, "In each collision on the average the energy of the neutron is reduced by 1/e, 'e' the basis of the natural logarithm." And that was difficult for us to understand. What was clear to us was that the energy in each collision was reduced in average to one half. This point—if the neutron energy reduced in average to one half or 1/e—gave rise to a lot of discussion in scientific magazines, because Fermi for a long time did not explain why he was claiming l/e, and any elementary calculation showed it was reduced to 1/2. But when Fermi said it, it was rather clear that it was a slowing down, but he never explained for a long time why he was claiming the lie factor. And I remember Goudsmit wrote a paper on that, and other people wrote papers, because many people thought that Fermi was wrong.
Did the difference...
The difference was that in order to calculate correctly the effect of slowing down, the best way is to use instead of the energy, the logarithm of the energy. And if you calculate in a logarithmic scale, you find out the factor of 1/e. But Fermi did not explain even to us why he claimed 1/e.
And yet you were willing to accept
He said, "I will explain some time, but we have now to work, and it's much more important to go on working than to explain l/e or 1/2."
Well, he had a practical result. Let me ask you. One of the practical results was that you could explain the difference between the slow and the fast neutrons, which would explain the case of aluminum, for example—the anomalies you were having with irradiated aluminum.
Yes. But if it was a factor of 2-1 or a factor of e-1 coming in at each collision did not affect this as well as other results at this stage of our experiments.
But that gave you confidence in his idea.
We were convinced of the qualitative idea. The discussion was about the numerical factor. This led to a rather long discussion in the journals.
But you were willing, forgetting about the numerical factor, all of you, to accept immediately the idea of the slowing down of neutrons although you had been convinced earlier—and everyone had accepted the idea—that neutrons had to be fast.
Well, there were two things. One was that the neutrons were slowed down, and the second that the cross-section should go up by slowing down the neutron. So actually the assumption was a two-fold one. There were two facts that you had to assume to explain the observed effect. But we started immediately to do experiments and to check the various aspects. And in four or five months devoted to do experiments on that we had continuous confirmations. Thus at the beginning we were not sure 100%, but Fermi's explanation was a reasonable one. And immediately a number of experiments were planned to check it.
Do you have any idea how others reacted elsewhere?
No, there was some discussion, but it was accepted. It was accepted because it was the most reasonable interpretation, I should say. One could explain so many things that had been observed by ourselves or by other groups, that the assumption appeared pretty soon really a very good one.
That leads to another question about someone else's theory. This is the compound nucleus—the Bohr theory.
This came in 1936. Breit and Wigner and Bohr's papers were both published at the beginning of 1936.
Yes, I'm just using that as an example to get to another category here about how Fermi reacted to the compound nucleus theory. Did he think that he could come up with a similar approach, and what was his attitude toward models of this type in general? Did he have any feeling for that?
Well, I think he accepted it; he found it was a very good idea.
You see, you mentioned earlier that he didn't want to have preconceptions. For example, he didn't want to think about velocities in one case, and this "hypothesis non fingo" approach. This apparently was useful, but was this his general feeling? Was there a turning away from theory for a while in the Rome group?
Well, in that period he was essentially doing experiments. He was devoting most of his time to doing experiments. I'm not able to say though I feel he did accept it. His attitude was the following, he would say, "I should avoid preconceived ideas—as an experimentalist. A theoretician is different."
That's interesting to separate them.
That's a very, very sound position. "As an experimentalist I should get proofs and limit myself to saying I have observed this, this, and this, and from this I can conclude this. Then I can do assumptions. But I should do a very clear distinction between assumptions and what is really proved as an experimentalist." For a theoretician it's an entirely different proposition.
A lot of experimentalists feel the other way around. They feel that the theory has to be very tight, but with the experiments one can...
I should say that on many, many occasions I have the impression that he would not be very happy now about the attitudes of many theoreticians and also of many experimentalists. He would not be very happy, in my opinion. Because there is today the idea in many places, also by very good people, that you have to do a theory and then do experiments only to check the theory.
In 1936 you go to the United States to look around. It seems to me that this ties into the next series of questions on the development of higher-energy machines, accelerators and so forth, and this is a nice way to break it off. It leads up to the early war years, for example, and then the differences in the type of research that you were doing. So this is the logical point, I think, to stop.
I don't know if I could add maybe a short comment about the fact that we did not find fission in the winter of '34-'35.
Oh, yes, we started to talk about that.
think that that maybe is interesting.
Do you want to talk about it now?
Just to add a bit. When I was in Cambridge, one of the points that I tried to learn was to build a linear amplifier. Wynn-Williams, who was a British scientist, had built a very good one, and I tried to learn from him how to build another one, and then I went back to Rome and in the fall I built one of these amplifiers. Then in the winter we started to use this amplifier to study boron reaction. Actually, I worked very much on the boron reaction and also on lithium disintegration by slow neutrons.
Then, since uranium and deuterium both had shown so many activities, we thought that there could be a kind of new family of radioactive bodies in which there are beta decays alternating with alpha decays. So we decided looking for alpha decays, with this amplifier that had been connected with a little ionization chamber. And in order to see also short lives, I put a piece of uranium in front of the ionization chamber. I irradiated the uranium there and then I took away the source of neutrons and looked for radioactive bodies emitting alpha particles.
Since I did not observe any alpha activity, I thought to look for extremely short lives and I looked for the emission of alpha particles when the source was kept nearly the uranium. But, of course, the amplifier was disturbed by the alpha particles of uranium. Before doing that I discussed the arrangement with Fermi. Fermi calculated with the Geiger Nuttall law that the mean lives that we could observe were certainly very short; then according to this law, the range of the alpha particles should be long enough to put a foil of aluminum in front of the uranium so as to eliminate the alpha particles from the natural decay of uranium. And so a foil of aluminum was put there, but the foil of aluminum that was stopping the alpha particles of uranium also stopped the fragments of fission.
So we went on doing these measurements for quite a while, and we did not observe any artificial alpha emitters, because it was rather natural to look also for alpha emitters, but we did not find any fission. Some time later, I found out that a similar experiment was done in Berlin-Dahlem at about the same time by Von Droste.
At that time?
More or less. This was told to me once by Otto Hahn, and Lise Meitner knew about that, and maybe Otto Frisch knows. I never talked about this with Otto Frisch. We talked on many occasions of many things, but not of this. So maybe he knows, but I'm not sure.
While we're talking about fission, this paper by Ida Noddack had predicted fission in '34, and it was read by your group—at least by some of the people. Do you recall reading it? Amaldi: Yes. But on this point I'm sure that Segrè knows more than I. About resonances, or things like that, I know more than Emilio, but on this, Emilio knows more than I. I remember, but I feel that the best information you can get is from Segrè.
You see, he doesn't know the answers to this either. Nobody knows.
He knows more than I.
But I mean on this particular point he still wonders and all of us do, you know, just why at that time their paper didn't make sense.
Yes, that was what we thought, but we were wrong.
Well, then we'll pick it up from 1936.
Yes, tomorrow night.