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Lew Kowarski’s Interview – Part 2

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 late 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 in the European Organization for Nuclear Research, or CERN, in 1953. In this interview Kowarski recounts his experience secretly transporting the French supply of heavy water to England to keep it out of Nazi hands. He also discusses his time working in the Cavendish Laboratory at Cambridge University with James Chadwick and other esteemed physicist. He also explains the Manhattan Project from a European perspective, including the increasing secrecy of the project.

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


Stephane Groueff: One thing I don’t understand, and it’s a very ignorant question, but what was actually the difference between [Enrico] Fermi’s experiment in ’34 and [Otto] Hahn’s? Because, why do we say that Hahn was the first one, while Fermi also bombarded uranium?

Lew Kowarski: I don’t it’s true to say that Hahn was the first one.

Groueff: It’s not true.

Kowarski: I think it’s one of those simplifications—there are people who find them all right. I don’t.

Groueff: Actually, Fermi was the first.

Kowarski: Fermi was the first.

Groueff: But, couldn’t interpret it?

Kowarski: He didn’t interpret it. He was not interested. He observed a tremendous number of chemical properties in irradiated uranium and it was not his business to—he was irradiating all the elements.

Groueff: But, did he realize that there was a splitting of the atom?

Kowarski: No.

Groueff: No.

Kowarski: Hahn, as I said earlier, Hahn and [Lise] Meitner worked several years and provided a full interpretation without introducing the splitting hypothesis.

Groueff: I see. The splitting came for the first time after the Frisch and Meitner interpretation, right?

Kowarski: No. After Hahn/Meitner interpretation showed impossible from the metal experiment of Madame Joliot.

Groueff: Yes.

Kowarski: Madame Joliot, it’s almost worth going into a bit of detail about it. Madame Joliot observed one radioactive period which had the chemical properties of the element lanthanum.

Groueff: Lanthanum.

Kowarski: Yes. We know now that it’s one of the perhaps fifty or so different chemical bodies. Well, there are not so many elements with different period, there are about fifty of them. And, we know now that there was another body of almost exactly the same period, which had properties very close to lanthanum.

Now, if you study this body by its radioactivity, you study the mixture of lanthanum and the other body. And, the mixture of lanthanum with something else has not the same chemical properties as lanthanum.

Groueff: I see.

Kowarski: Therefore, Madame Joliot, with this very beautiful and very precise experiments, said that her observed body was not lanthanum. It had different properties.

Groueff: Which contradicted Hahn and Meitner?

Kowarski: Yes. Now, Hahn, starting from that, which he decided that he had to prove that it wasn’t true, made the same experiments in a far more, in a far deeper way. He spent a lot of time on it and suddenly had more help and in a way, he was one of the very leading radio chemists of his generation. And, he pushed the degree of purification a little further.

So we are in the position that Madame [Irène] Joliot says, “I observe something which is very, very much like lanthanum, but not quite.” And [Otto] Hahn, pushing the degree of purity a little further says, “Now, I’m observing something a little purer than what you have, and it is not very much like lanthanum, it is lanthanum.” And then, by this extraordinary temerity, or I don’t know how come. And, then Hahn not only says, but puts it in print, “It is lanthanum, but I don’t believe it.”

Groueff: And it was lanthanum?

Kowarski: It was.

Groueff: That’s extraordinary. Between the two of them, the interpretations that the two of them—

Kowarski: Both were wrong, in a way.

Groueff: Yeah.

Kowarski: But Madame Joliot’s elegance was perfectly exact. In fact, I think the fact that she noticed that there was a slight difference between what she observed in lanthanum, I think it’s a tribute to her experimental skill. I think a more sloppy experimenter would probably not notice the difference. But Hahn suppressed the difference, not because he was sloppy, but because he was then further, you know that.

Now, I think you were asking about where the Germans were at that time. Hahn and [Fritz] Strassmann were working just on other aspects of the same fundamental paper. [Lise] Meitner was doing a few experiments on her own, not very fundamental. Frisch for some reason never were caught on the idea of chain reaction. Frisch and I became very close friends later on, and he has a historical bent. So, we went quite a lot into the early history of—

Groueff: During the war?

Kowarski: Yes.

Groueff: Is he alive, Frisch?

Kowarski: Yes.

Groueff: And he lives here in America?

Kowarski: Frisch lives in Cambridge, England. He became completely Anglicized and he took the chair at the Cambridge University, which was occupied by [John] Cockcroft before the war.

Groueff: Meitner lives in—

Kowarski: Meitner, I think she’s sort of, yes, she—

Groueff: She lives in Cambridge. She must be very old now.

Kowarski: Yes. I saw her last—when was it? About a year and a half ago.

Groueff: What kind of a woman was she?

Kowarski: Meitner is a very typical educated German spinster. Very, very nice, very cultured, very frail sort of physically. [00:03:00]

Groueff: Culture then, interesting?

Kowarski: Yes, by cultural interests. Frisch himself had a life story rather similar to mine, with the father in publishing and the mother a singer.

Groueff: Ah, that’s interesting.

Kowarski: Yes. Although they are different in many ways. Frisch is not at all a managerial personality.

Groueff: He’s purely—

Kowarski: Very purely scientific intellectual. But our forms of mind somehow seem to be very similar. During the war, we would exchange ideas and get ideas for experiments.

Groueff: He was a refugee, you know. They escaped Germany because of the anti-Semitic—

Kowarski: Yes. Frisch’s story is told, I think, in this book.

Groueff: Yeah, and I have something on Meitner’s story.

Kowarski: Yes. I consider that if one can speak of anyone discovering fission, it was Frisch.

Groueff: Frisch.

Kowarski: Yes.

Groueff: More than Hahn and Meitner?

Kowarski: Well, I don’t know exactly what happened between Frisch and Meitner on that Christmas. Maybe Meitner was talking, was having all the ideas, and Frisch only say, “I think of a piece of apparatus wherever I can check it.” But I don’t believe it. I know Frisch. I think it was a very genuine collaboration, and since, after all, Frisch performed the experiment alone, and the experiment is what it counts. So, I would, without hesitation, attribute the discovery to Frisch. But, it’s not a matter of fact, it’s a matter of also for what I consider as being the discovery. I say nothing against Hahn. In fact, the way in which Hahn, during the year ’38, amassed extremely difficult and extremely painstaking and by a series of quite brilliant work amassed arguments against his own findings of a year before, it speaks very highly for him.

Groueff: It’s quite his honor, yeah.

Kowarski: Yes. So, I wouldn’t like to be construed as saying anything against Hahn.

Groueff: Now, we left your experiments with this, March ’39, I have.

Kowarski: Yes, in March ’39, then we arrived at evidence that fission neutrons were there. We are now credited with it generally in historical books.

Groueff: Yes.

Kowarski: And it was also fairly obvious that there were more than one, and therefore the chain reaction was possible.

Groueff: So, that day, you didn’t experiment with chain reactions, but you proved the possibility?

Kowarski: As a matter fact, what do you call chain reaction? There is this considerable difference between the convergent chain and divergent chain. Convergent chain is when each fission neutron in its destiny generates less than one neutron. You can arrange the generation so that it will be not much less than one. For instance, one generates 100, generates, shall we say eighty, then the eighty in the next generation will generate sixty-four.

Groueff: I see.

Kowarski: Sixty four in the next generation will generate 51.2, and so on. It will decrease, and it will die down like population of a city in which each generation contains less than—finally dies down. If, on the other hand, you have more than one, then it grows exponentially, indefinitely. Now, whether, in a given medium, this fatal number is less than one or more than one can depend on entirely trivial circumstances, just as it is with population questions. In France, before the war, the population was decreasing. Quite a small change in economics, in fashions, was enough to put it again above one and the population start increasing again.

We had this idea very clearly by, how we say, late March or April, and we began first to see in what way the chances for a fission neutron to cause further fissions would be increased. That is, how to transform a convergent chain reaction to a divergent one, and second, what would be the consequences of that once made. By that time, I became quite a frequent guest in [Frédéric] Joliot’s house, and we would have weekend sessions with Francis Perrin.

Groueff: Where, in Paris?

Kowarski: Yes. And, my status has changed completely.

Groueff: It became the equal?

Kowarski: I became, well, let’s say a little junior partner or a bit, but I had another factor which was in my favor, a quite unexpected one. The physicists around me were accustomed to deal with radiations. You know, source or propagation in a straight line, going through a screen, impinging on another screen. Now, neutrons in a big material mass behave far more like a diffusing gas. Well, I was a gas engineer, wasn’t I?

Groueff: Ah, that helped, I see.

Kowarski: All my imagination went on different lines. In fact, in several months, it became quite clear that we assumed our functions, the three of us. Joliot was, of course, it was he who conceived the whole problem, and who started by simply outlining for [Hans von] Halban and to me, “This is the physical phenomenon. I want to know how to detect it. You are the people who know the technicalities for detection.” Of course, as soon as we would come with results of detection, he would go again in big lines, “Where do we go from there?”

Halban was essentially a man with considerable knowledge of the detection techniques and of what happens to neutrons. His work previously, before Frisch and with Frisch and after Frisch, prepared him for that. I was far weaker on that, because I began to share Halban’s knowledge only less than a year before. On the other hand, when it came to entirely new situations with these big masses and this diffusing neutron gas in it, I find myself more at home than the other two.

Groueff: I see. Was it difficult for you to judge your own talents? Your contributions in this case, or even later in your career and your success in this particular research, to what do you attribute it mostly? Is it your extraordinary talent for abstract conception of idea? Or is it some ingenuity in the practical way of doing it? Are you a very inventive man? Or is it as a result of some painstaking—I don’t know. It’s very difficult for me to say.

Kowarski: I’m definitely a lazy man. I’m a lazy man.

Groueff: Well, you’re not the kind of terrible worker. You’re intuitive? Would you say that you’re intuitive?

Kowarski: Yes. Do you know this book, Little Science, Big Science?

Groueff: No.

Kowarski: [Derek J.] de Solla Price, he’s an Englishman, I think, and he’s a professor of history of science at Yale.

Groueff: Does he answer my question here?

Kowarski: Yes, I will read it out to you:

“From other studies of creative ability in the scientific fields, it appears the general and specific types of intelligence have surprisingly little to do with the incidents of high achievement. At best, a certain rather high minimum is needed. But, once over that hump—”

Well, let’s say that I, with modesty, I will attribute that I achieve at least this high minimum. After all, I’m a docteur de science-physique; it must prove something.

“But, once over that hump, the chance of becoming a scientist of high achievement seems almost random. One noted quality is a certain gift that we shall term “mavericity,” the property of making unusual associations and ideas of doing the unexpected. The scientist tends to be the man who in doing the verb association tests responds to black, not with white, but with caviar.”

Groueff: It’s a good description.

Kowarski: Well, I certainly have a certain degree of mavericity, as my whole career seems to be showing.

Groueff: Are you like that in your personal life?

Kowarski: I think so.

Groueff: You’re not overly sort of pedantic, orderly? Or you’re more a man of intuition, impulses?

Kowarski: I think so.

Groueff: So you don’t correspond to the image of the professor with a laboratory, very methodical, pedantic?

Kowarski: Oh, you know, I became a professor only here at Purdue [University].

Groueff: Not in France?

Kowarski: Oh, in France, I was a kind of junior professor in Conservatoire des Arts et Métiers for a while in the evenings, so I think—

Groueff: But you’re not the type of scholastic, absentminded, methodical professor, rather ascetic in life? What is your attitude to life, for instance? Do you enjoy life?

Kowarski: Well, I hope I shall give you an evening meal before you leave us. It will not be the meal we planned, because we had to eat most of it ourselves, so we will give you, well, some leftover. There will be dessert. There will be also a bottle of Ferdinand du Pape.

Groueff: So, you like the good things, and you appreciate the [inaudible] terrestre.

Kowarski: Yes, there is also this piano. It’s a Steinway, belongs to us. It’s the only part of furniture that we—

Groueff: You still play piano?

Kowarski: My wife does.

Mrs. Kowarski: You do too!

Groueff: I see. She says that you do, too. Because it’s very difficult to talk about yourself, to describe yourself, but—

Kowarski: In 1938, when I did this little piece of work with Halban, the first one, a rather neat piece of work, I would say, he wrote the paper about it, published it in Nature in London. I read that paper and I said, “Never again.” All the next papers were written by me.

Groueff: Why?

Kowarski: Well, I probably am a better writer. I told you that I grew up in a rather literary atmosphere.

Groueff: And after that, you were the writer of—?

Kowarski: Oh, definitely.

Groueff: So you were, of this group, you were the one who wrote the papers after.

Kowarski: Yes. Almost all of it, not entirely. From time to time, sometimes Halban would be so impatient that he would act as writer. But I was the main recorder and also very often, I was the main typist.

Groueff: When you started to be involved completely in science, did you continue your interest in literature, for instance, continued, or are you a man who reads books, literature?

Kowarski: Yes.

Groueff: Not only technical and scientific?

Kowarski: Oh, no. I should say I read far more non-technical books than technical ones.

Groueff: For instance, in your youth, of the great Russian authors, who was your favorite?

Kowarski: Do you know them?

Groueff: Some of them, yes.

Kowarski: Well, here I can give a fairly exact answer. Until the later lessons, my number one was [Nikolai] Gogol.

Groueff: Gogol, yes.

Kowarski: The fantastic part of it, and so on. I was always high on H.G. Wells, too. I was told by a man whose opinion I respected that I should not start [Fyodor] Dostoyevsky until I’m seventeen. At seventeen, I duly started. I was very much impressed. The appreciation for [Leo] Tolstoy came a bit later.

Groueff: But Dostoyevsky left some imprint in your life or? You were shaped by him, or impressed?

Kowarski: I think the main impact of Dostoyevsky was to understand that the rather shallow progressivism of the H.G. Wells type was not enough. And later on, I was curiously enough and rather seriously influenced by the English author, [G.K.] Chesterton.

Groueff: Chesterton.

Kowarski: Again, since I grew up on a diet of H.G. Wells and later on [George] Bernard Shaw. Bernard Shaw played probably the biggest role in the elaboration of my style in English. I considered myself as a bit of a writer and I do care about question of style. I think on the whole, Bernard Shaw probably, in his prefaces rather than his plays, was the one who influenced my style quite a bit. But then it was Chesterton. Chesterton was the corrective, because Chesterton held views almost on everything exactly opposite.

Groueff: It gave you some [French].

Kowarski: Yes. And, on the whole, as I usually say, I am a sucker for a good style. My favorite statesmen were [Winston] Churchill. Later on I began to pay very much increasing attention to [Charles] de Gaulle, as De Gaulle reached through his—

Groueff: Yeah, he writes beautifully.

Kowarski: Yes, very beautiful style and beautiful speeches. In the same way, I was always interested in [J. Robert] Oppenheimer, because Oppenheimer writes rather interestingly, and so on.

Groueff: So, this part, the form interests you?

Kowarski: So, one of the anecdotes I wanted to say that we were, when we were working, the three of us, we were saying, “Well, which Nobel Prize shall we get first, Physics or Chemistry?”

And then somebody would say, “Well, I think it will be Peace, because war will become impossible.”

And, that, everybody would finish, “But, Kowarski, of course, will first get the Pulitzer Prize for Literature.”

Groueff: Yeah, that gives me an idea. One story I would be very interested to hear is about your departure for England, the 17th of June, 1940.

Kowarski: Well, this story was told so many times that, incidentally, among your sources, you might try to procure the French literary periodical, La NEF [La Nouvelle équipe française].

Groueff: La NEF. Ah, yes, I have—

Kowarski: September ’55.

Groueff: I saw that it has an article by you.

Kowarski: Yes, and I would recommend this article.

Groueff: I have it.

Kowarski: Just because I think it’s my best piece of French writing.

Groueff: It was Gestopalevsky, editor of the collection.

Kowarski: Yes, yes, yes.

Groueff: Yeah, I received that.

Kowarski: It’s a somewhat typical piece of mine from— 

Groueff: For your style.

Kowarski: —style, yeah, and so on. It’s not very long.

Groueff: Yeah, I received it and I’m going to—

Kowarski: There is an amusing mistake in the beginning. I say something, that something started in early ’39 and then the bomb exploded in August ’45, and it’s less than six years later. Of course, it’s from early ’35 to—

Groueff: After, yeah, it’s more—

Kowarski: I was misled by figures, ’39 and ’45.

Groueff: I have this article, but you don’t tell there, I think, the details of your—

Kowarski: No, but in the same number, there is the whole “Bataille de la Lotte,” told I think by Jean Moran.

Groueff: Jean Moran, yes.

Kowarski: Yes. And [Jacques] Allier also tells it, I think, pretty in detail.

Groueff: Who?

Kowarski: Allier.

Groueff: Allier. Yes, but first some elements, do you remember what boat it was? Did you travel with your wife or your child?

Kowarski: I can give you this, it’s very definitely—

Groueff: A journalistic—

Kowarski: —an imitation of journalistic style.

Groueff: I would like very much to have it for the details. For instance, the name of the S.S. Broompark, and Dr. Halban’s family, wife and small daughter, wife and four years old, these kind of details.

Kowarski: I can send it to you in a few days, a photocopy of this.

Groueff: I would appreciate it very much. So that will answer my question. Does it say here how physically you transported the heavy water? In what containers? Where it was hidden?

Kowarski: No, here it doesn’t say, but I can tell you that. The total quantity was 165 kilograms, no, 180—

Groueff: Eighty-five, let’s say.

Kowarski: 180—it was 165 liters, so it’s 182 kilograms, 182 kilograms. Most of it, I would say something like eighty or ninety percent, was in about two-gallon cans, like small gasoline cans, which were made at that factory especially for the transport and with welded aluminum.

Groueff: In Norway, made in Norway?

Kowarski: Oh, yes. But, as I say, it was at that time one of the first— 

Groueff: One of the first times that you saw the welded—? 

Kowarski: Welded aluminum.

Groueff: —welded aluminum. Before that, it was impossible to weld aluminum?

Kowarski: It was very little known. I’m sure that there were other examples, but it was very little known as general technique.

So, most of it was there, was in these cans, and the remainder was in glass, little glass vials or bottles, ready already to dispatch the small quantities to customers. They sold small quantities and these vials were wrapped in cotton and kept in big tin buckets. There were maybe three or four of them, of these buckets. So the total number of containers was approximately of the order of twenty or twenty-five, something like that. 

Now, the caravan left, there was two cars left Clermont-Ferrand at dawn on the 17th. Halban was driving a car with his family and I think with all the documents, a smaller car. I was, at the time didn’t drive at all. Now, I don’t drive again. I was in the car was driven by that mechanic. It was a sort of bigger car. It was a kind of station wagon.

Groueff: And the heavy water was there?

Kowarski: It was a Peugeot, by the way.

Groueff: And you were in this car, with the heavy water?

Kowarski: Halban’s car was also a Peugeot. Both the cars were Peugeot, and the mechanic drove and my wife with the four-year-old daughter sat inside, and I was probably in the back on the load part, sort of reclining, reclining on the cans hastily covered with various dresses and a few towels taken together and so on. It was a very typical French exodus car, because it was in the middle of exodus of course, and 17th of June.

Groueff: How long did it take you?

Kowarski: We arrived in Bordeaux shortly before midnight on the same day.

Groueff: And you took the boats in Bordeaux?

Kowarski: We were led on the boat a few hours after that, that is, the middle of the night. As I like to remind us, staff officers, colonels and possibly the generals carried our suitcases, because it was at this moment of despair. They had the dim impression that we were carrying some kind of hope.

Groueff: Were they French?

Kowarski: Yes, French.

Groueff: French officers?

Kowarski: Yes. We very, very definitely, went not as refugees, but as a member of a wartime mission, which was very duly countersigned by all sort of authorities, and that’s how we both considered ourselves all through the war. That’s why I immediately came back to France after that, because I considered myself—

Groueff: On a mission.

Kowarski: —on a mission, and therefore I was not saved by this extraordinary deployment of circumstances. I was not saved for my own skin, I was saved because I was on a mission. Being on a mission, of course, means that there’s something expected from me. That’s why I came back to France.

Groueff: Was the luggage checked by somebody?

Kowarski: Oh, no.

Groueff: It was just official?

Kowarski: No. I had no passport, for instance. No, I arrived in England without a passport.

Groueff: But the ship was what, English?

Kowarski: Scottish.

Groueff: Scottish. Nobody knew exactly what was it hiding, secrets?

Kowarski: Well, everything was secret in those days.

Groueff: Heavy water, so nobody knew what exactly it contained.

Kowarski: Of course, the man who commandeered the ship, Lord Suffolk, he was the agent between the French military supply and the English military supply. He knew it.

Groueff: But not the crew of the ship?

Kowarski: The crew probably hardly spoke English at all. I think Suffolk just commandeered the ship and the captain obeyed the orders, and that was that. He said, “These people have peculiar but particularly precious cargo, it’s all hush-hush.”

Groueff: Where did you put the cargo? In the cabins with you?

Kowarski: That I cannot remember. I think it was in the cabins.

Groueff: In the cabins.

Kowarski: No, sorry. This was a little bit told in this story. It was very soon, a raft was built and it was wrapped and put on the raft and the raft was on the deck.

Groueff: I see. In case you hit a mine or something that?

Kowarski: Yes. Actually, that was the week of the heaviest losses, shipping losses of all the war.

Groueff: So you were very lucky.

Kowarski: A German bomber flew overhead, but it already had no bombs.

Groueff: You were not attacked then. That was very lucky.

Kowarski: One ship, as told here, sank before our eyes on a mine. It sank very slowly, so there was no loss of life. Generally speaking, it was considered [inaudible].

Groueff: Mine, yes.

Kowarski: But this was useful, because for several weeks the people who sent us off were able to claim that we were on that ship and therefore, it’s over.

Groueff: I see. Of that period, the heavy water considered so highly important, that, in Normandy, was there any talk about making the bomb? And the Germans, as we saw later, were not very advanced in bomb-making. So why were Germans so much after heavy water?

Kowarski: Well, this is pure technique now. You can go by fast neutrons or you can go by slow neutrons. For fast neutrons, you need to have pure uranium-235. On the European continent, we knew that uranium-235 at that time wasn’t separated even in milligram quantities. Niels [Bohr] was to do it later on, as you know. The idea, too, that there could be several kilograms of uranium-235, pure, separated, appeared quite preposterous. So, we said, of course, there is such theoretical possibilities but—[John R.] Dunning, who knew better the possibilities of American industry, never considered this impossible, and he started propaganda for this from the start. But on the whole, the neutron physicists in America were rather against this.

Groueff: Against, yeah, I heard that.

Kowarski: In England, as this book says, tells you the story, the situation was reversed. The neutron physicist par excellence was Frisch. Frisch was in England. The British at that time were quite desperate, and the idea of having this explosive appealed to them. It was really a very desperate position in England. One has to understand that.

Groueff: Oh, yeah, with the blitz.

Kowarski: Yes, even before it.

Groueff: Before losing the whole continent, and the generals getting stronger and stronger.

Kowarski: I still remember this, all these maps which were published in England with German submarines from the North Cape to the Spanish border, and England like a little something surrounded by a big encircling hand, totally dependent for all supplies on shipping and all the submarines converging to sink the shipping. It was a desperate situation. It was radar of course, that saved them.

Groueff: Radar?

Kowarski: Only because of radar, they were able first to fend off the bombing of the cities, and second and more important, to save the convoys.

Groueff: Yeah, against the submarines.

Kowarski: Yes. England in those days was an extraordinary site and an extraordinary place to live. That’s why it probably was the strongest influence on me. But this story is told in the book.

Groueff: So you said that they were desperate for this explosive.

Kowarski: The first preparations were made by [Rudolf] Peierls, a German refugee in England, and Frisch and Peierls combined their—Peierls, his general mathematical knowledge, and Frisch more his knowledge of neutron experimentation. They made very detailed proposals, which the British took quite seriously. So by 1941, when Dunning was still struggling here in England, there was complete unanimity. But again, why should I anticipate this book? Now, what was I going to tell you?

Groueff: About the role of the heavy water, the importance of heavy water.

Kowarski: Oh, yes. So, from the continent, and for a considerable part of American physicists, the fast neutron way did not exist. There was only the slow neutron. Slow neutron way had to have substance to slow down the neutrons. At first, in a kind of race, we in Paris, and [Enrico] Fermi and [Leo] Szilard in New York, conducted experiments to show that water wouldn’t work. Water absorbs too many neutrons. From then, with the lightest element, the hydrogen, the lightest element not being suitable, well, the second lightest would be looked at next, and so on. As you go to heavier and heavier, the whole task becomes more and more difficult. This moderation is not so efficient.

So, on one hand, you have deuterium, which is very light, it’s the next lightest after, and which doesn’t absorb neutrons at all. Then next common element suitable is carbon, graphite, which absorbs neutrons rather little, so it’s far better than hydrogen, but on the other hand, it’s not so efficient. Whether carbon could work or not, whether carbon with its less absorption, but also less moderation, would prove better than hydrogen was a thing which required a lot of very painstaking scientific work, and which Fermi did finally successfully. But, with heavy water, we knew that it would work, because it was far more efficient in moderation than carbon, and absorbed neutrons even less. That, everybody knew. We didn’t have any idea of genius. The Russians were writing papers about it at about the same time.

Groueff: The heavy water, everybody knew about the qualities of it.

Kowarski: Yes. Not everybody could stage the necessary experiments, which would convince the governments that heavy water should be manufactured. Well, our experiment, we did convince the British. The British were convinced. I wouldn’t say that our experiment was scientifically completely convincing. It was more about how, what an English physicist said, “It wasn’t more over ninety-five percent certain, rather than 100 percent.” Just, [inaudible] we had to work fast. So we worked in a bit of slapdash way, in some ways, by working really fast.

But our ninety-five percent certainty was quite serious. It was enough for the English to take the thing very seriously. But by that time, the English couldn’t consider manufacturing it in England. England became an advanced fortress. Whereas in America, it took so much time for the scientists to get first money and means and support, that Fermi couldn’t think of using heavy water. He had to work with carbon, and by the time the support came, he was sufficiently on the way to carbon to be able to go to success on that way.

Groueff: But in your thinking at the beginning and the English thinking, was there a deal of making a reactor, which will produce plutonium and the bomb will be made of plutonium? Or, you saw the reactor as a sort of explosive?

Kowarski: No, I never thought of a reactor as explosive, that is a silly idea, always was silly. In my article in La NEF, I had to be diplomatic, especially at the time when I was already inserted in Geneva and therefore, I had to observe some rules of European diplomacy. But there is a sentence, that in the German work, there is a curious lack of insistence on plutonium. But I think about it myself, they simply didn’t have the idea.

Groueff: They even didn’t suspect that.

Kowarski: I think they had a vague idea that there might be something like that, and looked for it with somewhat elementary means, didn’t find anything, and let it rest at that.

Well, the ideas in England were far clearer, more definite, and therefore were more within sight to American scientists. It was perhaps an American scientist who had the first clearest idea, a man called [Louis] Turner.

Groueff: Turner.

Kowarski: Turner, as early as late, end of ’39.

Groueff: The clearest idea about—?

Kowarski: Plutonium.

Groueff: —plutonium.

Kowarski: Yes.

Groueff: Before [Glenn] Seaborg produced it?

Kowarski: And then that incited Seaborg to produce it.

Groueff: I see.

Kowarski: And Seaborg, who is, well, he probably was the greatest radiochemist of his generation.

Groueff: Seaborg.

Kowarski: Yes, a very great man. For him, with his experience and with cyclotrons, even there, there was some remarkable ingenuity displayed in this business. Well, he arrived at it pretty quickly.

Groueff: But your idea was to detonate it to produce with the heavy water reactor, to produce plutonium, which will be the fission of—

Kowarski: For the war.

Groueff: For the war.

Kowarski: I must say, we had this idea in France in a very vague way, sort of saying, “Well, when neutrons are observed, something must produce and this something may be fissile.”

Groueff: I see. But why were then the Germans interested in heavy water, and did all this thing in Norway?

Kowarski: It’s difficult to say. Germany, somebody had some ideas about explosive reactors.

Groueff: That explosive reactors, yeah.

Kowarski: Definitely, yes, which I think is nonsense. Also, there was the question of—it’s a very delicate question and even now, I think, it’s very delicate to talk about it. I suppose in Germany, there were quite a few people who were thinking of producing radioactive poisons in considerable quantities. This idea was known in England and America.

Groueff: But definitely not to produce plutonium?

Kowarski: Such evidence as I have of work with plutonium in Germany is scanty, and just says that “We tried to look for alpha emitters and we didn’t find any,” or something like that.

Groueff: But, you definitely knew what you were doing?

Kowarski: Oh, yes. Especially, after the conversation with Britain, with the British.

Groueff: When you arrived in England, your work continued in the Cavendish Laboratory?

Kowarski: Yes.

Groueff: What was the team there and the physical surroundings and life at Cavendish? Is it in Cambridge?

Kowarski: Do you know Cambridge?

Groueff: No.

Kowarski: You should try it, it’s one of the most beautiful spots on earth. It’s some extraordinary combination of parc à l’anglaise with extraordinary buildings, and it has this scholarly, leisurely look about it. It’s a very beautiful place.

Groueff: And your lab was in the town of—?

Kowarski: 3 School Lane. On 3 School Lane, as I told you earlier. Inside, yes.

Groueff: Was it, by the standards of that period, a very beautifully organized laboratory?

Kowarski: Oh, no.

Groueff: Advanced?

Kowarski: No. Cavendish was very sort of old world laboratory with dusty rooms, traces of enormous ingenuity. It was still the physics place, number one in the world for a couple of generations. For instance, this kind of apparatus, which Frisch had and Joliot didn’t, was invented in the Cavendish.

Groueff: In Cavendish.

Kowarski: Yes. Lots of remarkable ideas came from it, and still are coming. Now it’s microbiology, mostly.

Groueff: Who are the people working with you and Frisch?

Kowarski: Frisch didn’t. Frisch was in Liverpool.

Groueff: I see.

Kowarski: There were two. Most of the British physicists by that time were in various radar establishments. The leading physicists in the Cavendish were left, Professor [Norman] Feather—

Groueff: Feather?

Kowarski: F-e-a-t—

Groueff: —h-e-r. I see. Feather.

Kowarski: Dr. Feather, who is now a professor in Edinburgh, and Dr. [Egon] Bretscher from Switzerland.

Groueff: Bretscher, B-r-e—

Kowarski: t-s-c-h-e-r.

Groueff: Bretscher.

Kowarski: Yes. You will find all these names—

Groueff: I see. And, then from the French side, you and Halban.

Kowarski: We recruited very soon a few young Englishmen. We were gradually, gradually joined by a group of slightly younger, mostly refugees, but not all of them. In fact, at a certain stage, we got a relatively senior British physicist, a very good physicist, who was sent there as a kind of, well, England’s eye at this foreigners’ team, yes. It was Alan Nunn May.

Groueff: Alan Nunn May, the spy? 

Kowarski: Yes.

Groueff: So he was sort of supervising?

Kowarski: He was sort of—not supervising, but he was a sort of man on the spot, a British on the spot. He was in senior physics.

Groueff: So you were in contact with him?

Kowarski: Oh, I was quite friendly with him. He was a very nice man and a very good physicist. Only as like many other people during the war, he somehow got his ideas wrong. The whole thing was too big, and he began to have the ideas of playing a role in history.

Groueff: And was Peierls with you?

Kowarski: Peierls was in Birmingham.

Groueff: Who else was there?

Kowarski: Frisch was at Liverpool.

Groueff: [Wallace] Akers?

Kowarski: Akers was essentially a manager, chemical industrial manager, industrial research. But he came later on, in late ’41.

Groueff: In your laboratory, you worked only on the plutonium side?

Kowarski: Well, it’s the reactor side, because we sort of knew, thought not only of plutonium, we thought very much of uranium as a source of power after the war.

Groueff: But not separation?

Kowarski: No, not at all.

Groueff: And you lived there?

Kowarski: Yes.

Groueff: Was Cavendish bombed or attacked? Cambridge wasn’t, I think.

Kowarski: Cavendish—Cambridge was attacked once as a retaliation for a British raid on Heidelberg.

Groueff: I see.

Kowarski: I mean, seriously attacked once. I think there were minor skirmishes.

Groueff: Your lab was not destroyed?

Kowarski: On that night, Cavendish received an incendiary bomb, which landed in a sink full of water. And yet in prior raids, earlier raids, before that big one, there was enough of them so that one physicist was killed, actually, but not in this raid, but not in the lab. He was killed at his home at night. But after that only raid, only I managed to get a bomb in the garden where I lived.

Groueff: So, it wasn’t, it was not like—?

Kowarski: The difficulty was with my daughter who at that time, she was four and a half. There were sirens, of course, every night. And, we would go down, but I would explain to her that it’s pure bureaucracy and that it would never happen in Cambridge, and we all thought it would not happen. And in fact, this raid was quite an exception. It was in February of ’41.

Groueff: ’41.

Kowarski: Yes, and when there was a raid, she, in spite of being four and a half, knew quite well that it was the real thing. She sort of asked me [inaudible]. I said, “Well, that was a mistake.”

The next night, there were again sirens and we became somewhat shaken by the [inaudible] and we had to go back to basement and she positively stared at me, “Do you think there will be more mistakes?”

Groueff: You stayed at Cavendish until ’43, ’44?

Kowarski: July ’44.

Groueff: I mean, Halban left before?

Kowarski: Halban left—well, Halban was hardly at Cavendish at all, already in ’42. I was carrying on in his absence as sort of deputy leader.

Groueff: And you came in ’44, not to the United States, but to Canada?

Kowarski: To Canada. Halban went in late—he spent quite a lot of time in the States, then he went to Canada to organize the Montreal lab. I didn’t go there. This story is told in the book. Then in late ’43, there was a complete change of leadership. The English participation was taken over by [James] Chadwick. Cockcroft was appointed to lead Halban’s lab, and in early ’44, I received an order to go to Montreal.

Groueff: Early ’44. At that time, did you know very much about the American bomb? Or everything was very secret?

Kowarski: Well, the answer is neither yes nor no. There was, of course, an enormous amount of secrecy. On the other hand, we knew far, far more than the man on the street.

Groueff: I see. But did you know, for instance about Oak Ridge or about Los Alamos?

Kowarski: We knew that there was a place called Site X in the south, and that there was a place called Site Y in the southwest, and a place called W in the Pacific Northwest. That much we knew.

Groueff: But no details? Who was there, what was done, whether they were successful or not?

Kowarski: We knew, for instance, that Oppenheimer was at Los Alamos or was going to Los Alamos, that we knew. We knew that the isotope separation would be outside Y—outside X.

Groueff: X, yeah.

Kowarski: About the Chicago reactor, we knew, I think a couple of weeks after it started.

Groueff: So that was the part that you knew the most?

Kowarski: Yes.

Groueff: Because you were involved. Did you work directly with [Arthur] Compton and his team?

Kowarski: Well, I never did.

Groueff: You arrived too—?

Kowarski: In Cavendish, I worked essentially with Halban, with Halban in a somewhat senior position and I in sort of deputy position. Then when Halban left, I worked under Chadwick, and Chadwick being in Liverpool, but I went to Liverpool fairly often. Then when I arrived in Montreal, Cockcroft was already there, so I worked under Cockcroft.

Groueff: Cockcroft. But did you come to the United States during the war?

Kowarski: Well, I first, in order to go to Canada, had to land in New York to begin with. That was my first arrival.

Groueff: But did you stay here?

Kowarski: Well, I stayed three days.

Groueff: Three days. But you didn’t work in the Manhattan Project? Not directly.

Kowarski: Not at all.

Groueff: Not at all.

Kowarski: But already then, the collaboration with the British was subjected to rather serious rules and they tried to limit it to the Americans not to disclose too much to too many people. The position of the French citizens was very uneasy. Again, this story is told in this book in detail. In my case, there was an aggravating circumstance that I was of a Russian birth, and the Americans, by that time, already had quite a few ideas about the Russians.

Groueff: Were you a citizen, a French citizen?

Kowarski: I was naturalized French shortly, well, after already, after the outbreak of the war, I was naturalized in November, ’39. So, it was a bit difficult for me to get access to American laboratories. I insisted on [inaudible] very much and Cockcroft got it for me, and I paid a visit to Chicago, Metallurgical Lab in Argonne, which existed then not at the present site. I met quite a few people. In particular, I met [Walter] Zinn. It sort of was the start of our friendship. By talking to Zinn and looking at his heavy water reactor, which worked already for a few months, I got the ideas for the first—

Groueff: For your reactor?

Kowarski: Yes, yes.

Groueff: But it was a different design, your design?

Kowarski: No, no, it was a less ambitious. It was practically zero power. It could at most go to one or two kilowatts. Whereas Zinn’s could go to 300.

Groueff: So you didn’t participate directly in the—

Kowarski: Manhattan Project.

Groueff: —Manhattan Project?

Kowarski: Never did.

Groueff: Yeah, so your contribution what you did in ’39 and then ’40, and the theoretical side.

Kowarski: Well, scientists use the word “theoretical” in a different sense. You oppose theoretic applications, but the scientist opposes theoretic experiment.

Groueff: I see. So you were also in the experimental?

Kowarski: Yeah, I was on the experimental side, and, of course, in the developing of leading ideas of, they are still being called.

Groueff: Yeah, about the chain reaction.

Kowarski: Chain reaction and reactors and how to—

Groueff: And heavy water and reactors.

Kowarski: Yes, reactors in general and plutonium, what to do with it, and so on. There was quite a lot of thinking and calculating and writing.

The work we did until the outbreak of the war was very definitely of, I would say, of definite importance for the work they were doing in America. We simply were working in parallel with Fermi and we quoted each other and it was a kind of collaboration by publication. We took each other’s results, and so on. That phase ended with the last publication, which was in September ’39.

After that began the secret phase, in which we influenced very much first the French government and then the British government. The British government did not very much influence the American scientists, but influenced very much the American government. When in summer ’41, the British government transmitted its conclusions, governmental conclusions, to America, it found in a stage when in America the government was very little interested. It was under impact of these British conviction that the American government began to pay greater attention to this business and to their scientists. Of course, by that time, we again began to receive American reports and they began to receive ours. For instance, our results with heavy water were known in America almost immediately.

Groueff: Yes. They were doing some work here, I think. The British asked Hugh Taylor, I think, to—

Kowarski: Yes.

Groueff: —from Princeton—

Kowarski: Yes.

Groueff: —to represent them here, I think, in heavy water.

Kowarski: Yes.

Groueff: And then they switched him to diffusion.

Kowarski: Yes. Taylor is [inaudible] British [inaudible].

Groueff: I met him last week.

Kowarski: I never met him, I think.

Groueff: Very nice old gentleman.

Kowarski: By the way, have you met [Samuel] Glasstone?

Groueff: No.

Kowarski: Glasstone became first known by producing a very big book of physical chemistry together with Taylor. And, after that, he became more involved in the nuclear business, and wrote some very fundamental books, textbooks. Among them, there is a book called The Sourcebook on Atomic Energy, sourcebook, which contains a lot of history, a lot of fundamental papers.

Groueff: It’s probably a good source for me.

Kowarski: Yes, it is a good source.

Groueff: But Taylor who was involved after that in diffusion—

Kowarski: Yes, I know it.

Groueff: —and the barrier problem and all this. At the Cavendish laboratory in December ’40, you and Halban proved for the first time the possibility of obtaining—

Kowarski: Well, shall we say ninety-five percent proved.

Groueff: That was the ninety-five percent proved. What kind of experiment was that?

Kowarski: Well, there was a big sphere of aluminum containing about 100 liters or so of heavy water. Uranium oxide was present in the form of very fine suspension in heavy water. This had two remarkable consequences. First of all, in order to keep it in suspension, not allow the suspension to settle, we keep the sphere rotating all the time during the experiment. And second, since it was foreseen that there might be some leak of something, the outside medium in which escaping neutrons had to be measured, which normally would be water. But of course, if heavy water escapes into water, it’s lost. Therefore instead of water, we used mineral oil, so that if heavy water escapes, it would rise to the top and would be skimmed, very easy to skim off. There would be some contamination, but very little.

The second interesting consequence was that in order to prepare the suspension, one had to learn how the finely divided powder behaves in bucketfuls of water. So for a couple of months in the growing English fall weather, I could be seen—it was me, of course—I could be seen walking around the courtyard shaking huge buckets. As I say, it was getting cold already and I had to wear a very nice, navy blue sweater. It was the same that I wore on [inaudible]. I still have it.

Groueff: That was the experiment when you proved that ninety percent, ninety-five percent reaction chain was possible.

Kowarski: We proved that more neutrons are measurable outside when there is uranium in, than when there is no uranium in. This can be proved very easily. It’s a bit of mathematical juggling, that if the chain is convergent, then in these conditions the addition of uranium can only decrease the number of neutrons which escape outside. If the neutrons, if more of them, if the number of escaping outside is increasing, that means that the chain is divergent.

We had already observed convergent chain in ordinary water. But then it’s convergent because hydrogen, ordinary hydrogen, eats up too many neutrons. But when the ordinary hydrogen is replaced by heavy hydrogen, then this is this little change which flips the number over one. The fact that there were more of them outside—so that is a very simple experiment and a very simple proof. The remaining uncertainty comes from various secondary phenomena such as absorption, the walls of the vessel, and I will not go into that.

Groueff: This sphere was in a container full of oil, mineral oil?

Kowarski: Yes.

Groueff: I see. The measurements were made in the oil?

Kowarski: Yes.

Groueff: To whom did you communicate the results? To Joliot?

Kowarski: No. Joliot, we had—

Groueff: No contact.

Kowarski: No contact. Contact was established only in 1944.

Groueff: Between when you left in ’41 and—

Kowarski: We left in 1940.

Groueff: ’40 and ’44—

Kowarski: Well, you know, the French Resistance was always in communication with England.

Groueff: Yes, but not direct.

Kowarski: Not direct, no, far from it.

Groueff: So you communicated it to the British side?

Kowarski: Well, there existed a kind of directed committee of this effort, on which we were not members, but the usual British way, there is a policy committee and then under the policy committee there is a technical committee. On the technical committee, we were members. 

[End of audio]

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.