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Lew Kowarski’s Interview – Part 1

Manhattan Project Locations:

Lew Kowarski was a Russian-born French physicist who worked as part of the team that discovered that neutrons were emitted in the fission of uranium-235 in the 1930s, setting the groundwork for the use of nuclear chain reactions in the design of the atomic bomb. After the Second World War, Kowarski went on to supervise the first French nuclear reactors and became a staff member of the European Organization for Nuclear Research, or CERN, in 1953. In this interview Kowarski discusses his upbringing in Russia, and the beginnings of his scientific career under Frédéric Joliot-Curie. He also outlines the process through which the splitting of uranium atoms was realized.

Date of Interview:
November 29, 1964
Location of the Interview:


Kowarski: So born in Leningrad, February, 1907. Father, businessman. Mother had a little career of her own as a singer, but [inaudible]. Father, his business started to be paper, pulp and paper, and then from that he branched off to supplying paper to newspapers and magazines, and from there he branched off into participation in magazines. And so his business gradually grew less and less pulp and more and more literary, if you see what I mean.

Groueff: Yes.

Kowarski: So, there were writers around him. Some of the famous names of—famous then in the pre-first World War Russia were around him. Then Mother introduced a musical interest, so as you see, the atmosphere was pretty, well, shall we say cultural.

Groueff: Cultural, yes, and intellectual.

Kowarski: Cultural more than intellectual, I may say so. Not scientific. I began to very early be interested, because of all this reading and music, but also as an addition of my own, in mathematics. By the age of ten or so the revolution just occurred. It was 1917. Everything became very unstable and I already then was thinking of what are we going to do when I grow up. There were two lines, the musical line and the scientific line. The musical line was the influence of the whole surroundings. I already, when I was eleven, I met a professor of the Leningrad School of Music, who was considering taking me soon as one of his pupils for composition. That was the influence [inaudible].

Groueff: Did you play a musical instrument?

Kowarski: Well, I was taught the piano, as everybody those days in Russia. It was in certain social surroundings, it was—

Groueff: It was part of their education.

Kowarski: Yes. But it was recognized that I would never do a pianist. On the other hand, I seemed to be inventive, so it was rather toward composition that I was directed. On the other hand, myself, I was getting more and more influenced by English writers and especially H. G. Wells, whom I started reading at the age of eight, in Russian, of course. My English came much later. 

In fact, I have a very clear memory of having bought myself at a bookstore at a railroad station in Leningrad in May or June, 1917, a translation of Well’s book, The World Set Free, from which I learned for the first time about atomic bombs. This book, the role it played with some other people was made the subject of a communication by the well-known atomic physicist, Gale Young, of Oak Ridge, in a communication to American Nuclear Society in November 6, ’63.

Groueff: I’ll try to find this. It will be interesting.

Kowarski: There is a lot about this book, so obviously, I was not the only one. 

Groueff: But you were clearly influenced by this book tremendously.

Kowarski: Very definitely so. The whole of H. G. Wells, but this one, well it has more bearing to what happened since. I met H. G. Wells once in 1943 for a few seconds.

Groueff: And you saw him?

Kowarski: Well, we talked very briefly. Now, in English, meanwhile, Leningrad was becoming a little uncomfortable for businessmen, and my father went to his native part of what was very fast ceasing to be the Russian Empire, and to the city which today is called Vilnius. That was called Vilno.

Groueff: Vilno, ah, yes.

Kowarski: And where I resided for five years from late 1918 to late 1923, and in these five years it changed hands I don’t know how many times. There was even a short period of it being incorporated back into Russia. So that when people ask me when I left Russia, it’s more exact when Russia left me.

Groueff: There was fighting there all the time, no?

Kowarski: Not all the time, but chiefly, in 1920, during the great move of Soviet armies on Poland. I left Vilna in end of 1923 and went to study first in Belgium. I was not quite seventeen then, and I was a little young to be a student on my own. Two years later, I went to France, to Lyon, and there I did the whole course of industrial chemistry at the University of Lyon.

Groueff: At that time, you were definitely interested in sciences?

Kowarski: Well, the musical part sort of disappeared as the Leningrad environment disappeared. There was no more question of going to the musical school of Leningrad, and somehow this became quite subdued. I gradually abandoned music. When I went to this chemical industry school, by that time the family had no money. The prospects of my earning my living should have been entirely on my own. One had to learn something which gives you a job. Now, these engineering schools had an additional advantage that they did not require the French high school matriculation, baccalaureate. In my turmoil around the city of Vilna, the legal aspect of the validity of my high school studies suffered a little.

Groueff: So you had no diploma?

Kowarski: I had no diploma at that time. I acquired the French baccalaureate at the same time as my engineering diploma, but by the time I was bachelier and ready to start my university studies, they were already over. So, in 1928, at the age of twenty-one, I was at the same time a bachelier and a chemical engineer.

Groueff: In Lyon?

Kowarski: In Lyon. And, at the end of ’28, I came to Paris in search of a job. There were some unfortunate beginnings. One of them was at the Peugeot factory.

Groueff: As engineer.

Kowarski: It was called then methods engineer, ingénieur de méthode, but I didn’t quite understand what was expected from me. I think at that time, they simply hired a dozen young graduates in order, on probation, in order to keep three or four, and I didn’t make the grade. This didn’t prevent me from keeping a very good memory of Peugeot, and even now I’m a very enthusiastic Peugeot owner when I’m in Europe.

Now, in mid-’29, I got a job as what was called a technical secretary in a French firm manufacturing steel tubes for water and gas distribution. I was in the division for gas distribution, and so I became a gas engineer. To anticipate a little, in 1938, I published a book on gas distribution, which I am told is still remembered by some French gas engineers, and from time to time, they are curious whether the author of this book is any relation of the atomic scientist. 

Now, I married soon after having got this stable job.

Groueff: In Paris?

Kowarski: In Paris, to a girl from Poland, and settled in a life of a very minor technical bureaucrat.

Groueff: Not a scientist?

Kowarski: But at the same time, I was a student at the Sorbonne and at the end of each current year, I would pass examination.

Groueff: What were you studying?

Kowarski: I considered my student career in Paris strictly a matter of getting a degree, and I chose my study methods in a way as to get my university degree with the least exertion.

Groueff: But, because of the engineering, you didn’t have a degree in Lyon?

Kowarski: I did have a degree in chemical engineering, but it was not, strictly speaking, a university degree, because it was granted to no bachelier.

Groueff: I see, yeah.

Kowarski: I wanted an entirely bona fide university degree, which would open further ways with a respectable university diploma. In fact, I wanted to go to the highest doctorate in science. It seemed quite a long haul from where I was then, because I had to earn my living. And, since I started almost simultaneously with the beginning of the Depression, it was not a very easy affair. 

However, after about a year and a half of this work with gas distribution, I found the possibility of having a very modestly-paid laboratory assistantship in the Paris madhouse.

Groueff: In which one? What in a Paris—

Kowarski: Madhouse, Sainte-Anne.

Groueff: Where now? In the Sainte-Anne.

Kowarski: So I spent in Sainte-Anne, it was a half-a-half life. Half-day gas distribution in the mornings, and in the afternoons, chemical laboratory at Sainte-Anne.

Groueff: For their medical—

Kowarski: Medical analysis.

Groueff: —analysis.

Kowarski: And this gave an opportunity of doing a bit of research on some methods of analysis. In 1931, my wife got a scholarship, which enabled us to live no more on half-and-half basis, but on one and a half basis. She was having her scholarship fulltime, and I keeping my gas means for the morning.

Groueff: Was she in science?

Kowarski: She was in applied psychology.

Groueff: Psychology.

Kowarski: So, from the summer of ’31 and for the next five years, in fact all through the Depression, she had a fulltime job, very modest, and I had a half-job, halftime job in the mornings. In the afternoons, I got unpaid.

Groueff: For your laboratory work.

Kowarski: Yes. At that time, for a foreigner to get a scholarship for research work in Paris were extremely difficult. It was extremely difficult, and just as I failed to impress Peugeot, in this very highly competitive atmosphere, I didn’t do too well. The few foreigners who did get scholarships, well, I was not among them. So, I worked for free for nearly five years in the afternoons.

I passed my doctor thesis in early 1935, when I was just about twenty eight. It was on the subject of crystal growth in molecular physics, what we would call today molecular physics. Then it was still called physical chemistry. I was gradually getting away from my chemical engineering.

Groueff: I see.

Kowarski: From chemical engineering, I went to medical analysis, from medical analysis into physical chemistry, from physical chemistry to molecular physics, and from molecular physics, then I began to meet physicists.

I had difficulties with making people understand what I was interested in and what I wanted. There was one brilliant exception, and that was the son of my boss. My boss was Jean Perrin, the Nobel Prize man, a very remarkable personality, and I very largely followed his methods. But Jean Perrin, himself, was a very unorthodox scientific personality, and probably it was not a very good idea to follow his methods too closely. In fact, I think he was a little puzzled about me, as no doubt his own masters were puzzled about him.

Groueff: In what laboratory was that?

Kowarski: In his laboratory, physical chemistry of Sorbonne.

Groueff: Of Sorbonne.

Kowarski: But, his son, who already then was a brilliant physicist and in whom Jean Perrin had full confidence, his son became interested in my work. And after a while, took on himself to direct it, and it was the beginning of a sort of lifelong understanding.

Groueff: Between Perrin and you?

Kowarski: Yes.

Groueff: So, he was the first of your—the people that later you worked with, that you met.

Kowarski: Yes, yes. I think Francis Perrin took the point of view that although my methods may have appeared a little puzzling, but with due insight, one could get something out of it, and it’s always worthwhile when one gets out of any scientist, on any scientific work. It’s worth a little attention and a little patience, and that he had. He successfully brought me to the thesis, which was rather well-received. And, at the same time, recommended me to [Frédéric] Joliot, who at that time just became a dazzling glory and was rapidly expanding. He was looking for people. So, Francis Perrin recommended me to Joliot.

I started working with Joliot at the end of 1934. Joliot himself was less patient than Perrin, and it took quite a few years before we really began fully to understand each other, but then we did.

Groueff: But, at the beginning, he was difficult?

Kowarski: Well, it was always difficult.

Groueff: Yeah, or you had—

Kowarski: Yes.

Groueff: —two different characters.

Kowarski: Yes. There were some analogies between Joliot and me, this early boyish interest in science, very early. This somewhat difficult beginnings in the university. He also got his bachelier rather late in life by French standards. But in his family, there always was money, and the position was different.

In 1936, my child was born and my wife could not go on fulltime job. She had to curtail seriously her activities, and therefore, I couldn’t work anymore halftime.

Well, this coincided with Joliot’s appointment as Professor of Collège de France, so he had to build up an entirely new research center for which he had only a building and some money. And this requires hiring, purchasing, planning and so on. He developed quite a remarkable genius for that, and he needed not so much at that particular moment researchers, research workers, as this kind of scientific managers. One thing he required is a personal secretary, and I became for halftime his personal secretary, or as I immediately was named, “la petite dactylo de Monsieur Joliot.” I was always interested in typing and I could turn out quite a tolerable piece of typing.

Groueff: But, you were doing the typing yourself, yeah.

Kowarski: Oui, la petite dactylo. A very interesting memory is that the first letter I ever typed for Joliot was a letter to [John] Cockcroft. I remember, at the end of ’36, my conscientious searching the address: J.D. Cockcroft, Cavendish Laboratory, 3 School Lane, Cambridge. I was to know 3 School Lane very well.

Groueff: Yeah, very well.

Kowarski: Yes. Well, life is full of such coincidences. So, for a year, I was halftime Joliot’s secretary and halftime still involved in gas distribution.

Groueff: You still kept this job at gas distribution?

Kowarski: Half-day.

Groueff: Half-day.

Kowarski: Finally, in September 1937, when I was already past thirty and a half of age, I for the first time got a little scholarship, oh, very small one, three-eighths of a fulltime scholarship.

Groueff: Three-eighths of a full—

Kowarski: Of a fulltime scholarship, that’s what I got. 

Groueff: And, very little money, huh?

Kowarski: Well, the scholarship itself wasn’t very big, but it was three-eighths of the scholarship. And, well, this three-eighths therefore was replacing the gas mains job. But the secretary job was kept, so I began to work—as I say, I was already past thirty. For the first time in my life, I passed my whole day in a scientific establishment, paid half as a secretary and half as a research worker.

Groueff: But at this time, your work with Joliot wasn’t yet scientific corroboration?

Kowarski: No. Actually, in 1935, there was one bit of scientific corroboration. We even published a paper together. But, by that time, he was entirely immersed in this organization of his lab and at that time simply didn’t work, at that particular moment. But I helped him very much on the managerial part. This was therefore my first experience, and by far not the last. But I had to combine managerial activities with research activities, and I acquired a certain, well, working habits of combining the two things.

Joliot invited into this new establishment some very brilliant young people, in particular, [Bruno] Pontecorvo.

Groueff: Pontecorvo was there?

Kowarski: Yes. He was hired by Joliot, and [Hans von] Halban. During this first six months when I was living at last as a, more or less, as a professional scientist, I managed to invent a gadget, which was not terribly important in itself, but as least showed at least to Joliot some originality of approach, which could be used later on. He managed to get for me, for the second year, this time not three-eighths, but two-thirds.

Groueff: Of a scholarship?

Kowarski: Of a scholarship. And still kept me as a paid secretary. This, of course, was gracious.

We are now in ’38. In ’38, acting chiefly on Pontecorvo’s advice, I went to see Halban and told him, “I hear from Pontecorvo that you are now engaged on a piece of research for which experience, chemical experience of a chemist might be useful to you. Let’s combine. Of course, I know little about your experiment and what you are trying to do, but you will teach me. I will be able to provide for you the chemical aspects, which as I understand are necessary in your experiment.”

Groueff: What kind of man was Halban? First, is he a German refugee or was he from Halban? Is that his real name or his full name?

Kowarski: Well, it’s a complicated story, because the family was always a little astraddle between Austria and Germany. Halban’s father was Austrian and his mother, I understand, was of Czech origin. But the father was one of the early physical chemists, and in those days the young people followed very faithfully Professor [Wilhelm] Ostwald, who was the founder of physical chemistry more or less. When Ostwald got his chair and institute in Leipzig, quite a few of his pupils got established with him. Therefore, Halban the father also lived in Leipzig, and there my Halban was born.

My Halban was born in Germany, and also by being a professor in Germany in the peculiar citizenship laws which existed at that time in Germany, Professor Halban immediately, automatically became a German subject. The Germans never had anything against double nationality. So, until 1938, I think, or, no, until 1935, the Halban family went by their German citizenship. 

Groueff: But the young Halban lived in France and spoke French?

Kowarski: Halban came to France in 1935. He’s very gifted for languages, so I suppose he did speak French before, as an educated—

Groueff: But, you spoke French to him at that time?

Kowarski: Yes. At that time, I spoke no English. I still speak French to him.

Groueff: I see. So you found him and you offered your collaboration as a chemist.

Kowarski: Well, not as a chemist. As a physicist with a knowledge of chemical techniques. So, as we say, as an accomplished chemist and as an apprentice physicist. We did a piece of research immediately, which was fairly successful. Of course, it was planned before I arrived on the scene. I daresay I did contribute something, I did contribute something in carrying it out, the idea was not from me. But it was a successful piece of research, and therefore, by the end of ’38 when my two-thirds scholarship started, it was quite obvious that I had become a full-scale research physicist.

Kowarski: Now we come to historical times.

Groueff: Was your work with Halban, your relationship with him easier or with Joliot? Did you become personal friends, close friends, or was it professional? Joliot, anyhow was an older man, was your boss at the beginning. So, all this, when we see two or three people—

Kowarski: It’s terribly, it’s terribly difficult to describe. If I were describing to a Native American, I would be in complete despair about it. But you know Paris and you know what it is. In France, friendship is something which is based usually either on family or on fairly early stages of schooling.

Groueff: Yeah, that’s true.

Kowarski: Later on, relations which are purely professional somehow don’t mix quite. Joliot and I in working hours could talk about practically any subject and talk as very close acquaintances. Of course, there was a difference of level. Joliot was a Nobel Prize man. He was seven years older than I.

Groueff: Well-established.

Kowarski: Well-established, although I knew perfectly well that this state of being well-established was quite recent in Joliot’s case. In fact, Joliot’s difficult beginnings left him with serious resentments, and we were on different levels. His superiority as a scientist, as a person, was quite obvious and never put in question. It was more a relation from a protector to a dependent than to the relation between friends.

Halban was something else, again. He was rather rich, was sort of a society man, lived outside of the laboratory, lived on a completely different level from mine. His methods of work were such that sometimes one had suddenly to go and have a meal together, in which he would pay. In fact, I saw quite a bit of him, even outside of the lab, and I had to visit him, because there might be some sudden correction of proofs of a paper somewhere in the middle of a Sunday. So, the relation was growing quite—

Groueff: Closer than with Joliot.

Kowarski: Outside of the laboratory.

Groueff: Outside.

Kowarski: The difference between us, although there was a difference, I came to him as an assistant with a specific knowledge. Therefore, it was not quite the relation of equality either. But  he was younger than me, actually, and therefore, he was not a Nobel Prize man after all, so the gap was not so big.

Groueff: Was he a pleasant man, sort of abrupt or difficult, nervous or cold?

Kowarski: Halban is—I hope I can still say this—a very remarkable personality, and very quick personality, and I think we understood each other very well. We were very much from different origin and way of life and our views of things.

Groueff: You were still the struggling young scientist. I mean, financially.

Kowarski: Well, he is younger than me.

Groueff: But, less struggling, let’s say.

Kowarski: But less struggling. Also, Halban, by that time, was very well-established, going-up scientist with several well-known publications to his name. I was known only in a very narrow circle of crystal growers. I don’t think many men would call him pleasant. He has a charm, somewhat modern charm, and I was not among those who was most exposed to it.

Groueff: Between him and Joliot, were they friends, or it was also a professional relationship?

Kowarski: On the outside of laboratory level, they probably got closer, because Joliot, by that time, himself, became somewhat modern.

Groueff: Yes, successful.

Kowarski: Yes. He was very much lionized everywhere. And, in the laboratory, I suspect that I was closer to Joliot than Halban was.

Groueff: I see, yeah. And, that gives me a good idea of the—

Kowarski: But, of course, their levels were closer. If Joliot was on a high level and I was on a low level and Halban was an intermediate level, he was, level-wise, was closer to Joliot than I was.

Groueff: But, how can three such different men of different origins make this first world famous—

Kowarski: Well, I’m coming to that. As you know, I think you have followed already the beginnings of the story.

Groueff: Yes.

Kowarski: From first evidence for curious phenomenon happening in uranium was given by [Enrico] Fermi in ’34. During the next three or four years, there was a lot of work going about it and the complete misunderstanding of what was happening. [Otto] Hahn, [Lise] Meitner, and [Fritz] Strassmann—well, Strassmann was a young assistant. Hahn and Meitner were among those who contributed most to misunderstanding.

Groueff: To misunderstanding?

Kowarski: To misunderstanding. One fundamental clue was missing, and in the absence of this clue, false conclusions were arrived at. And, since Hahn and Meitner accumulated an enormous amount of very painstaking evidence, very correct, extremely, they were very good physical chemists. But, as long as the clue was missing, this evidence was only misleading.

Groueff: And the interpretation was false.

Kowarski: And the interpretation was false. Actually, another German lady, Mrs. [Ida] Noddack, stumbled on the clue almost immediately, ’34. Noddack. But nobody believed her. Then Joliot’s wife, Irene Curie-Joliot, produced a piece of evidence, which was completely, stubbornly impossible to fit into the interpretations proposed by Hahn and Meitner. To a certain scientific meeting in the spring of ’38, passions ran very high.

Groueff: Between the two women?

Kowarski: It was discerned in the background that Mrs. Noddack was right all the time. But nobody spoke of her then. And, Hahn said, “This is impossible, this is nonsense. I have to go and prove that it is nonsense.” He went back to his lab and spent six months in proving, and proved that it was brilliantly right. In fact, he went far deeper than that particular clue supplied by Madame Joliot. Then he arrived at his famous conclusion that as chemists, we must conclude that uranium splits in two, but as physicists, we, of course, know that it’s preposterous. We prefer to consider that there must be some other incomprehensible explanation to what we observe.

Groueff: That was when Meitner was already a refugee?

Kowarski: Yes. At Christmas ’38, Meitner discussed this with her nephew, [Otto] Frisch.

Groueff: Frisch, yeah.

Kowarski: I don’t know who first of them stumbled on the idea that what’s wrong with this as physicists. Anyhow, Frisch went back to his laboratory that was in Copenhagen and made a very simple experiment, which showed the physical nature of the, of—

Groueff: He made it or [Niels] Bohr?

Kowarski: No, he made it.

Groueff: He made it?

Kowarski: Yeah, definitely. He got his first result by the time Bohr was embarking on a ship going to New York.

Groueff: To New York, I see.

Kowarski: And, Bohr had it in his pocket. While Bohr was sailing towards New York—no jets then—in the scientific journal was the publication of Hahn and Strassmann. It was distributed in the mails. It was one of those very quick publishing journals, and Joliot got it on the 16th of January.

Groueff: This Naturwissenschaften, yeah.

Kowarski: Naturwissenschaften. He immediately said, “What’s this nonsense? Why do they say it’s physically impossible? It’s perfectly possible.” They must be two halves, splitting in two halves. The two halves must be radioactive for such and such reason. If the two radioactive bodies are very violently projected, it’s easy to observe. And also, there must be free neutrons.

Now, in order to observe this phenomenon, there must be neutrons in the system to provoke the fission of uranium. And, if there are a few neutrons from uranium, they are kind of drowned in the neutrons which already are there. In order to be distinguished between these soft neutrons, primary neutrons, and the fission neutrons, secondary neutrons, one has to be very careful about neutron measurements. Their energy, their numbers were—

Groueff: Different.

Kowarski: To find some difference, with sheer numbering it’s difficult. So, he asked Halban to help him, because Halban and I at that time were working on an experiment, or several experiments even, which involved rather similar counting of neutrons and so on. Although Joliot was himself, several years earlier, was one of the main discoverers of the neutron. But the neutron science by that time and he didn’t—he followed other techniques. So, Halban, and, possibly at that time even myself, knew more than he did about the stakes.

So he asked Halban to help him. It’s quite possible that Halban would form a team with Joliot and would say that he has no more time for collaboration with me. But there happened one of those historical accidents. Halban was, as I say, rather modern. It being the end of January, he went skiing.

Groueff: Do you know where he went? To Austria or France?

Kowarski: I’m sorry, but this I cannot remember. I suspect it was France.

Groueff: Yeah, I may find out. That’s a very interesting detail that he went skiing, end of January.

Kowarski: Now, we come to the day of the 26th of January. Joliot just before my eyes performed an experiment, which was based not on counting neutrons, but on observing radioactive fragments. Extremely elegant experiment. Joliot did not have detecting apparatus of the same strength as Frisch had, and therefore, he couldn’t conduct, but he imagined an extremely simple method without practical [inaudible] apparatus.

The phenomenon was so big. I sort of congratulated him, “Here again, you’re making a first-rate discovery, after all these years.”

Joliot saying, “Well, it’s very amusing of course, but I can tell you that I’m quite sure that this discovery has already been made in other places.”

We were 26 January in the morning. 26 January in the morning in New York, but that was, of course, five hours later, Bohr was making a communication to American Physical Society from Frisch’s notes and possibly a cable or two.

Groueff: The same day?

Kowarski: The same day. As he was talking, some of the physicists present in this New York meeting left the room. Some of them went to their laboratory in Columbia. Some of them took the train to go to Washington, and some of them sent frantic telephone communications to California. Anyhow, at the end of the same day, 26th, Frisch’s experiment essential, not Joliot’s but Frisch’s, was repeated in four or five places in America.

Groueff: And, that’s why probably [John] Dunning , who said that he was the first one—

Kowarski: I think Dunning was one of them probably.

Groueff: Yeah, he said that he was the first one in United States.

Kowarski: I’m not sure he was the first. He was one of the first, but it was repeated on the same day in several places.

Groueff: He also was in Collège de France, or where was Joliot’s experiment?

Kowarski: In Collège de France.

Groueff: Collège de France.

Kowarski: So, Joliot was then making his experiment and explaining to me that it probably had already been made elsewhere, and it was, by Frisch.

Groueff: Without knowing?

Kowarski: Without knowing. How could he know?

Groueff: Did you know at that time that communication of Hahn?

Kowarski: Yes.

Groueff: You read it?

Kowarski: Well, Joliot showed it. And, then he, Joliot told me how he first tried by seeing whether he couldn’t see by counting neutrons, but he couldn’t do it. He discussed it with Halban. Halban promised him to think about it while skiing.

Groueff: Halban wasn’t present at that?

Kowarski: No, Halban was skiing. He even told me about how Halban wanted to go about it. The idea was to make a rather fine experiment with fine counting of neutrons, so as to prove that when you add uranium there are more neutrons that are coming from somewhere. Well, I said that the idea of distinguishing them simply by numbers appears to me rather difficult. It’s better to distinguish them by some quality.

From there we are trying to propose an experiment so that when Halban came back a few days later, there were plans for two experiments, one based on numbering and one based on distinction of quality. But Joliot, who was very scrupulous, by that time was convinced that since this was born in this conversation with me, I couldn’t be kept out anymore. It was, so to speak, one Halban experiment and one Kowarski experiment.

Groueff: I see.

Kowarski: That’s how the tripartite collaboration was born. The Halban experiment gave the first result, the Kowarski experiment gave result about two weeks later. So, Halban experiment was eight days before Fermi and the Kowarski experiment was eight days after Fermi.

Groueff: But that was an experiment in measurement for, in the—

Kowarski: The Halban experiment, well, I will not go into technical details.

Groueff: Yeah, actually, for—

Kowarski: The Kowarski experiment was based on the irradiation of uranium with relatively slow neutrons, and it could be foreseen that if any neutrons are liberated, they must be pretty fast, so that one had to use a detector which can detect only fast neutrons.

Groueff: As the secondary neutrons?

Kowarski: Yes.

Groueff: Liberate?

Kowarski: Yes.

Groueff: Where was your experiment performed, in what laboratory?

Kowarski: We performed them strictly in parallel, and when I say my experiment is, it’s an exaggeration.

Groueff: Right.

Kowarski: It was [inaudible] all the time, all the time. For instance, I wouldn’t know how to dispose the best way the radiation, what vessel to take.

Groueff: So you were helping each other?

Kowarski: Yes. Also, at the same time, Halban’s experiment had chemical aspects and interpretation aspects in which I helped. So it was really a very strong—and, of course, the whole idea came from Joliot. And so I think it was really a very genuine, full collaboration.

Groueff: Where was it? In Joliot’s laboratory?

Kowarski: Yes, the Collège.

Groueff: Collège de France?

Kowarski: Yes.

Groueff: Any particular spectacular instruments or appareil? What was the source of neutron?

Kowarski: The source was always radium put in a barium block. Usually these two ingredients were mixed. It was usually radium emanation, radon, mixed with barium powder. But that gave a certain percentage of rather strong, rather fast neutrons. Whereas if you put a salt of radium in a cube, in a block of barium, not mixed them, then only gamma rays are working and they provoke only fairly slow neutrons.

So we had to use a weaker source to obtain a pure, pure phenomenon. This occurred to Fermi in his first publication so that he went through both stages. He first used the radon source, which is stronger, but then decided that it would be neater if he used barium, gamma, a gamma source, and it was very much in parallel, the same thing.

There were a few bits about interpretation where I contributed something, so I don’t think it’s fair to describe it as a Halban experiment, Kowarski experiment.

Groueff: But from that moment on, it was a more or less permanent collaboration between the three of you in this field?

Kowarski: Yes.

Groueff: You started working regularly on that?

Kowarski: Yes.

Groueff: On neutrons and uranium?

Kowarski: Yes.

Groueff: Where did you get your supply of uranium?

Kowarski: Well, Joliot was, of course, Madame [Marie] Curie’s son-in-law, and Madame Joliot was Madame Curie’s daughter. The whole radium industry from Congo grew entirely out of Madame Curie’s discoveries.

Groueff: Ah, Union Minière.

Kowarski: Yes, that’s right. So at some moment, Joliot simply went to Brussels, saw the Union Minière people.

Groueff: He was a very important man then.

Kowarski: Of course. And there were even some first agreements concluded about future participation of Union Minière and future industrial work and so on.

Francis Perrin joined us very quickly. He was never as closely to—member of this tripartite collaboration as we three, but one of the fundamental papers we published, the four of us, and also the famous patent applications, he participated in them.

Groueff: And Irene Curie?

Kowarski: Irene Curie all the time stayed away from it, stayed out of it.

Groueff: So, she doesn’t participate in this experiment?

Kowarski: No.

Groueff: And, when did you learn about Frisch and Meitner, no, Frisch and Bohr and Fermi, all this?

Kowarski: Well, Frisch, the first paper, the Frisch and Meitner on the interpretation, and Frisch on the physical proof—because they made this little distinction. They published two papers simultaneously, Frisch and Meitner on discussion of Hahn’s experiments and Frisch on his—

Groueff: Yeah, and on his experiment, yeah.

Kowarski: This was published, I think, in February. That was enough. Halban had worked with Frisch for more than a year, in ’37, ’38.

Groueff: So he knew him well?

Kowarski: He knew him quite well. In a way, one can say that Halban, as far as techniques of slow neutrons went, Halban was essentially Frisch’s pupil. I don’t know whether he would recognize it, but I think I can say that.

Groueff: But, who was the leader of the German group? Was it Hahn?

Kowarski: There was no German group.

Groueff:  He had the—

Kowarski: Hahn and Strassmann, Hahn and Strassmann made this, their historical publication, and that was that.

Groueff: That was that. After, they didn’t—

Kowarski: Well, they continued to putter for—

Groueff: Yeah, but nothing spectacular.

Kowarski: No.

Groueff: Meitner took it from there, Frisch took it from there.

Kowarski: Meitner didn’t do anything on the whole subject—

Groueff: On interpretation.

Kowarski: Well, she did a couple of experiments in Sweden over detail, good experiments of detail on some of the periods observed and so on.

Groueff: But her main contribution is interpreting with Frisch this letter that they received from Hahn.

Kowarski: Which they received, of course, before it was published in Naturwissenschaften, which was typical scientific grapevine.

Copyright 1964 Stephane Groueff. From the Stephane Groueff Collection, Howard Gotlieb Archival Research Center at Boston University. Exclusive rights granted to the Atomic Heritage Foundation.