Chapter One 1 * "HURRY UP!" * ON MONDAY, January 16, 1939, I taught my morning class at Princeton University, then took a train to New York and walked across town to the Hudson River dock where the Danish physicist Niels Bohr was scheduled to arrive on the MS Drottningholm . Bohr--with whom I had worked a few years earlier--was coming to give some lectures at the Institute for Advanced Study in Princeton and spend time with his friend Albert Einstein, then a professor at the Institute, and I had decided to greet him. For a dozen years, Bohr and Einstein, probably the two most eminent physicists in the world at that time, had had a running debate on the meaning and interpretation of quantum mechanics, the subtle theory that governs motion and change in the subatomic realm. Bohr held that uncertainty and unpredictability are intrinsic features of the theory, and therefore of the world in which we live. Einstein embraced a deterministic worldview; he could not believe that God "played dice." Over the years, Einstein had proposed various thought experiments that at first appeared to expose cracks in the structure of quantum mechanics, and Bohr had been able to turn every one of them around to show more clearly than ever that his "Copenhagen interpretation" of quantum theory, with its fundamental probability, stood fast. As it turned out, however, nuclear fission, not the mysteries of the quantum, occupied most of Bohr's time during his visit. Just before embarking from Denmark, he had learned of this new phenomenon, and he had been thinking hard about it all the way across the ocean. I was not the only one who decided to welcome Bohr personally. While I was waiting on the dock, who should turn up but the Italian physicist Enrico Fermi and his wife, Laura, who, with their two children, had arrived in the United States only two weeks earlier. Enrico, short, muscular, and intense, was a man of habit and order whose mind never rested. Laura, dark and pretty, had studied engineering and science before marrying Enrico and would later establish herself as a writer. As the story jokingly puts it, Fermi, after receiving his Nobel Prize in Sweden in December 1938, became lost on his way back to Italy and ended up in New York. In fact, they wanted to get away from the Fascism of their native Italy--Laura was Jewish--and the itinerary had been carefully and quietly planned to bring them to New York, where a professorship at Columbia University awaited Enrico. Fermi came to the dock to invite Bohr to spend a day with him in New York before going to Princeton. The news of fission that Bohr had in his head would be of consuming interest to Fermi, himself a nuclear pioneer. But by chance, it would be I, not Fermi, who would become the first on these shores to hear of it. Bohr learned of fission on January 7, just as he and his son Erik were about to board the train in Copenhagen for Gothenburg, the MS Drottningholm's embarkation point. Otto Frisch, a German emigre physicist working at Bohr's University Institute for Theoretical Physics in Copenhagen, sought out Bohr to inform him of the postulate of fission that he (Frisch) and his aunt, Lise Meitner, had devised in the last week of December to explain puzzling results found by the German chemists Otto Hahn and Fritz Strassmann in their Berlin laboratory. When Hahn and Strassmann bombarded uranium with neutrons (subnuclear particles with no electric charge), they found evidence that the element barium was created. Since barium is far removed from uranium in the periodic table and has a much lighter nucleus, they could not make sense of this result. Hahn wrote to Meitner in Sweden, describing the puzzle, for she had been his longtime colleague in Berlin before leaving Germany to escape persecution, and was trained in physics. When her nephew Frisch came for a holiday visit, they took a Christmas Eve walk in the woods--he on skis and she on foot--to ponder the Berlin results. Suddenly it became clear to them. The uranium nucleus must be breaking into large fragments, resulting in the nuclei of other elements, including sometimes the nucleus of barium. When Bohr heard Frisch offer this explanation, his reaction was swift and positive. "Oh what idiots we all have been!" he said. "Oh but this is wonderful! This is just as it must be!" Bohr's was a mind prepared. He knew as much as any person alive about atomic nuclei, and could see at once that fission made sense--even though, up to that point, he and other nuclear physicists had imagined that at most only tiny fragments could break off from a nucleus. In addition to his son Erik, Bohr brought with him a young colleague, Leon Rosenfeld. Rosenfeld was to serve as Bohr's sounding board and scribe, to help Bohr formulate his ideas, and to capture for publication whatever sparks might fly when Bohr and Einstein put their heads together. Throughout the nine-day ocean crossing, fission was probably more on Bohr's mind than the upcoming meetings with Einstein. He and Rosenfeld discussed it incessantly. (Bohr had a blackboard installed in his stateroom, to facilitate their talks.) By the time Bohr shook hands with me and the Fermis on the dock, he had a pretty good idea of a direction to go in to give a theoretical account of fission. That is what would occupy us intensely for the next few months. But at that shipside greeting, he said nothing about fission. In his characteristic way, he wanted to be sure that Meitner and Frisch got the credit they deserved before the news spread widely. Not even during his day with Fermi did Bohr breathe a word of the discovery. It must have been hard to restrain himself. Fermi had just received the Nobel Prize for his studies of neutron bombardment of nuclei, and had in fact produced fission several years earlier in his laboratory in Rome without knowing it. He had interpreted the results as evidence for the creation of elements heavier than uranium rather than the splitting of uranium. Even when the German chemist Ida Noddack suggested in 1934 that Fermi had in fact split the uranium nucleus, no one paid attention. It was, at the time, too radical a thought. (One can't help wondering whether Noddack's insight would have found a more receptive audience if it had come from a man instead of a woman.) In retrospect, the blindness of physicists and chemists to fission in the mid-1930s can be regarded as a blessing. Had scientists in Germany--and elsewhere--followed up on Ida Noddack's suggestion, it might well have been the Germans, not the Allies, who got the atomic bomb first. The history of the world could have been different. After the greetings on the dock, Bohr and his son agreed to stay in New York with the Fermis for a day, while Rosenfeld would accompany me back to Princeton, where he could check into the Nassau Club and get settled, awaiting the Bohrs' arrival. Rosenfeld, unaware of Bohr's concern about priority for Meitner and Frisch, spilled the beans to me on the train. I was excited. Here was a whole new mode of nuclear behavior that we had overlooked. Monday, the day of Bohr's arrival, was the day of the Physics Department Journal Club. This was a regular weekly event during the academic year, an informal evening gathering at which faculty members, graduate students, or visitors described new results in physics--usually results that had just been published. I had been put in charge of the Journal Club that year, so the moment I heard about fission from Rosenfeld, I decided to rearrange the schedule. I asked Rosenfeld to give a report of twenty minutes or so on fission. He agreed. When Bohr learned the next day that we had "taken the cap off the bottle," he was upset. But in typical Bohr fashion, he was low-key and gentle. He chastised neither Rosenfeld nor me. Rosenfeld's report caused a stir. It was immediately clear to everyone that it was more than just an interesting new bit of nuclear behavior; it held at least the possibility of a chain reaction and large-scale release of energy. In those days, physicists could not rush to their computers to spread news by e-mail across the world in seconds, and they did not make long-distance phone calls as a matter of course. Despite the excitement in that evening's Journal Club, it took some days for the news to spread to other laboratories around the country. I. I. Rabi, the noted experimental physicist from Columbia University, was in Princeton that week and heard Rosenfeld's report. Surprisingly, he did not rush the news back to his new colleague Fermi. As it turned out, Willis Lamb, a young Columbia faculty member and, like Rabi, a future Nobelist, brought Fermi the news. Lamb came down to Princeton by train on Friday morning, January 20, in part to continue working with me on some calculations we were doing together, in part to attend the afternoon theoretical seminar. He stayed for dinner and socializing with some Princeton friends, then caught a 2:00 A.M. train that put him back in New York around 4:00 A.M. "After taking the milk train and getting very little sleep," Lamb told me later, "I went over to Pupin Lab looking for [John] Dunning [the professor in charge of the Columbia cyclotron]. I didn't find him, but I did find Eugene Booth [a postdoc] and Herb Anderson [Fermi's student], so I told them about fission. Then I found Fermi and told him. This was the first he had heard of it, and he showed great interest." No doubt an understatement. Bohr's official report on fission came on January 26, ten days after Rosenfeld's Journal Club report, at the Conference on Theoretical Physics at George Washington University. George Gamow, a Russian emigre theorist who had spent time at Bohr's institute in Denmark before coming to America and who was then a professor at George Washington, was the principal organizer of the conference. Harry Smyth, my department chair, readily agreed to Bohr's request that I be allowed to leave Princeton for a few days to attend the conference. But I decided that my obligations to my students took precedence, so I stayed in Princeton. By the time of the conference, Frisch in Denmark and several groups in the United States had confirmed the reality of fission by physical (rather than chemical) experiments--experiments in which the great energy of the fission fragments was detected directly. It may seem odd that only days were required to confirm the effect, when it had taken years of painstaking work to discover it. The very energy of the fission process is what made it easy. Neutron bombardment was already an art practiced at numerous laboratories. Once physicists knew what they were looking for, it was short work to find a uranium target, set up the right detector, and measure the characteristic large energy pulse of a fission event. At Columbia, Anderson did all of this in a single day, Sunday, January 29, 1939. A few days after Bohr's report, probably on Monday morning, January 30, the physicist Luis Alvarez--another future Nobelist--was reading the San Francisco Chronicle while getting his hair cut in a barbershop on the campus of the University of California, Berkeley. When he came across a wire story reporting Bohr's announcement of the discovery of fission, he abruptly got up without waiting for the barber to finish and literally ran to the university's Radiation Laboratory, where he brought the news to his student Phil Abelson. By the next day, Abelson had verified the fission phenomenon. When Alvarez brought his colleague Robert Oppenheimer to the lab to see the evidence, Oppenheimer switched from doubter to believer in a few minutes. Within a quarter of an hour, according to Alvarez, Oppenheimer was visualizing the whole process in his mind and imagining chain reactions. The field of fission physics was launched. But a solid theoretical underpinning for the phenomenon was still missing. Like most physicists, I was interested in nuclear fission for what it revealed about basic science, not for what it might have to do with reactors or bombs. In 1939, even after we understood fission and knew that a chain reaction might be possible, even after war broke out in Europe, I was interested only in working with students, doing my research, learning more about nature. I was slow to realize that perhaps I had a duty to apply my skills to the service of my country. Two years later, in the fall of 1941, I was involved in a particularly exciting research problem with my brilliant (and fun-loving) graduate student Dick Feynman. Smyth sat me down at a lab bench one day and said, "John, you'd better finish up your work with Feynman. You'll surely be involved in war work soon." He was right. Soon after that conversation, in December 1941, the Japanese attacked Pearl Harbor and the United States entered the war. I started at once looking for ways to contribute to the war effort. Early in 1942, I joined the exodus of physics professors and their students from the laboratories and classrooms of America's universities. Some went to the Radiation Laboratory of the Massachusetts Institute of Technology (MIT) in Cambridge, Massachusetts, to work on radar. Some went to Chicago and New York and Berkeley to work on nuclear fission. Some found war work by walking across the hall or across the campus at their home universities. Within two years there would be large concentrations of scientists on the mesas of New Mexico, among the hills of Tennessee, and in the desert of eastern Washington. After stints in Chicago and in Wilmington, Delaware, I found myself in the fall of 1944 in Richland, Washington, working on the giant reactors (atomic piles) at nearby Hanford designed to produce plutonium for atomic weapons. Many of my friends were in Los Alamos, New Mexico, and Oak Ridge, Tennessee. By October 25, 1944--a few weeks after the first Hanford reactor was powered up--the German army in Italy had been driven well north of Rome, but hills, rain, and mud had bogged down the advance of General Mark Clark's Allied forces beyond Florence toward the Po River. My brother Joe, then thirty--three years my junior--was killed in action that day. Joe, who held a Ph.D. in history from Brown University, was a private first class in the Blue Devils Unit of Clark's army. First, we learned that he was "missing in action." Much later his death was confirmed. For eighteen months, until it was discovered in April 1946, Joe's body, disintegrating to bones, lay with that of a buddy in a foxhole on the hill where he was killed. Now Joe lies buried with 4,401 other soldiers in the 70-acre Florence American Cemetery near Florence. It's a beautiful spot, with its neat mathematical rows of white crosses vividly differentiating it from the vineyards and woods nearby. Every time I visit Joe's grave, I am reminded that he is one of many--one of many millions, I calculate, both soldiers and civilians--whose lives might have been spared if the Allies had developed the atomic bomb a year sooner. On the day Joe was killed, the uranium separation plant at Tennessee's Clinton Engineer Works (the complex that included the new town of Oak Ridge) was partially operational and had produced grams of uranium 235 (U-235), but not yet the kilograms that would be needed for a bomb. A nuclear reactor at the same site had produced grams of plutonium 239 (Pu-239), but not the kilograms that would be delivered only after the Hartford plant became fully operational the next summer. At the Los Alamos laboratory in New Mexico, scientists and engineers had largely mastered the design or a gun-type weapon. They just had to wait for the uranium to make it work. Only months earlier, experiments had shown that if a plutonium bomb were going to work, it would have to be an implosion-type weapon. In October 1944, a reorganized laboratory was just getting up a full head of steam to solve the implosion problems and design a weapon that could use plutonium. In the late summer or fall of 1944, I got a card from Joe, written from the front lines in Italy. Its complete message was "Hurry up!" Enough had been written in the newspapers about uranium and nuclear fission in 1939 and 1940 that anyone who cared to think about it might conclude that the Allies, and probably the Germans and Japanese too, were making efforts to develop an atomic bomb. Joe had a little extra knowledge. He knew that in 1939 Niels Bohr and I had worked together to develop the theory of fission, a theory that predicted, among other things, that the isotope U-235 (and the not-yet-discovered isotope Pu-239) would undergo fission if bombarded with slow neutrons. He knew that I had taken a leave from my job at Princeton to go to the University of Chicago to do war work, and that from there I had moved on to E. I. du Pont de Nemours & Co. in Wilmington, Delaware, and then to a remote place in the state of Washington. It didn't take too much guesswork for him to surmise what the nature of that war work might be. Joe hoped for a miraculous means of ending a terrible war. So he told me to "hurry up." I am convinced that the United States, with the help of its British and Canadian allies, could have had an atomic bomb sooner and ended the war sooner--perhaps a year sooner than the summer of 1945--if scientific and political leaders had committed themselves to the task earlier. Between mid-1944 and mid-1945, more than 3 million lives were lost in battle and in bombings. Government-sanctioned murders accounted for at least 12 million more, including the intensified killing of Jews in the Holocaust. The total is so unimaginably great, the loss so horrible, that it staggers the mind. Yet one cannot escape the conclusion that an atomic bomb program started a year earlier and concluded a year sooner would have spared 15 million lives, my brother Joe's among them. Once General Groves took over the Manhattan Project in 1942 and scientists and industry were mobilized to give their full effort to building an atomic bomb, progress was swift. But that was three years after we understood the essential ideas of nuclear fission and three years after Albert Einstein had written a letter to President Roosevelt drawing attention to its potential military importance. It does little good to second-guess history. But I cannot avoid reflecting on my own role. I could have understood the gravity of the German threat sooner than I did. I could--probably--have influenced the decision makers if I had tried. For more than fifty years, I have lived with the fact of my brother's death. I cannot easily untangle all of the influences of that event on my life, but one is clear: my obligation to accept government service when called upon to render it. But what of my 1939 work with Bohr, when we were driven more by curiosity about the atomic nucleus than by any thought of weapons? At the time, he was fifty-three and I was twenty-seven. Bohr, a Nobel Laureate, directed the Copenhagen institute that drew physicists from all parts of the world to little Denmark. I was in my first year of an untenured assistant professorship at Princeton, where I had been hired to help move that institution into the coming world of nuclear physics (before Princeton or I had any inkling that the atomic nucleus was anything more than a fascinating little chunk of matter). In 1933, five years before going to Princeton, I had earned my Ph.D. in theoretical physics from Johns Hopkins University. Then came one-year apprenticeships with Gregory Breit at New York University and with Bohr in Copenhagen, and three years as an assistant professor at the University of North Carolina. I had married early, and had two small children when my wife, Janette, and I moved to Princeton in 1938. There we are still, after sixty years interrupted by numerous leaves of absence and by a happy decade at the University of Texas in Austin. Bohr's conversations with Rosenfeld on the ocean crossing were typical of the way he worked. He liked to be on his feet, talking, pacing, writing on a blackboard, almost always with a junior colleague at hand. He behaved no differently on shipboard than back at his institute in Copenhagen. The conviction he had reached by the time he set foot on the dock in New York was that the liquid-droplet model of the nucleus should be able to account for the fission phenomenon. That a nucleus bears some resemblance to a drop of liquid was suggested first by George Gamow. Bohr extended the idea, using it as a way to describe the behavior of a nucleus to which extra energy has been added by a bombarding particle--an example of what he called the "compound nucleus." Despite Bohr's initial hesitancy to speak of fission, little time was wasted getting early reports into print. In three successive issues in February 1939, the journal Nature carried Letters on the subject. In the first of these, appearing in the February 11 issue (having been submitted on January 16, the day of Bohr's arrival in New York), Meitner and Frisch proposed fission as the mechanism accounting for the production of barium by neutron bombardment of uranium. While Bohr was en route, Frisch had gone to his laboratory and obtained the large energy "signature" of fission. His separate report on this experiment, also submitted on January 16, appeared on February 18. Bohr himself spent his first few days in Princeton writing a short paper setting forth his general idea about fission. Wanting to be sure that he didn't accidentally upstage Meitner and Frisch, he sent his paper, dated January 20, to Frisch with the request that it be forwarded to Nature . It appeared on February 25. With these three papers, fission physics was under way. Almost at once, Bohr asked me if I would like to work with him on a more detailed theory of fission. It was a subject in which Rosenfeld had less interest and less experience. Besides, he wanted to keep Rosenfeld available to write up notes on his lectures and his discussions with Einstein (which did take place, but on a scale reduced from the original expectation). I was the logical choice, having worked in nuclear physics since 1934 and being well known to Bohr from my postdoctoral year with him in 1934-1935. I accepted readily, even though the fission work would pull me away from a subject I was deeply interested in at the time, action at a distance. For some time, the idea of particles acting at a distance on one another had seemed to me a simpler, more satisfying description of electromagnetism than the standard "field theory," which assigned "substance" to electric and magnetic fields existing in space. So Bohr and I each changed course, he away (temporarily) from pursuit of the quantum, I away (temporarily) from pursuit of electromagnetism. We worked well together. It was an exciting time. I have been told that my style of work, and even some of my mannerisms, resemble those of Niels Bohr. It is probably true. I, too, like to work in free-wheeling talk sessions with colleagues, with more questions than answers flying back and forth. I, too, try always to emphasize the positive in my junior colleagues' work, give them all credit that is due, and build their confidence. But was I drawn to Bohr (and he to me) because my approach to physics and to people was similar to his--or did I acquire my style from him? Some of both, I suspect. The word fission , borrowed from cell biology, was suggested by Frisch to describe this newly discovered nuclear process, the splitting of an atomic nucleus into two large fragments. Frisch had acquired the word by asking William Arnold, an American biologist then working in Copenhagen, what cell division is called. Bohr was not enchanted with the word. "If fission is a noun," he said to me, "what is the verb? You can't say `a nucleus fishes.'" No sooner had our collaboration begun than we raced up the stairs from our offices on the second floor of Fine Hall to the mathematics-physics library on the third floor, where we spent more than an hour looking through dictionaries and reference books in search of a term more to Bohr's liking. Our search was fruitless. After considering and rejecting several possibilities we fell back on fission , which has stuck. (For a while, Bohr called a nucleus capable of undergoing fission a "splitter." I'm glad that didn't take hold.) To me, just as to Bohr, fission seemed immediately believable. I felt stupid not to have realized, several years before, that nuclei should be able to split. My student Katharine Way at the University of North Carolina had investigated magnetic properties of nuclei using the liquid-droplet model. Her equations had no solution when the nucleus rotated too fast. This told us that rapid spin could make a nucleus become unstable and fall apart. It would have been natural to ask ourselves whether there were other ways to make a nucleus come apart. Had we followed her lead, we might have thought of fission. The carpeted office that Princeton provided to Bohr, Fine Hall Room 208, contained paneling and built-in bookshelves on one wall, a blackboard on another wall, and a bank of five windows looking out on trees on a third wall. At eighteen by eighteen feet, it was reasonably spacious although not elegant. Fine Hall, named after Dean Henry Burchard Fine, was principally a mathematics building, but it also housed some physicists and the wonderful Fine Hall Library for both mathematics and physics. My office, Fine Hall 214, a near-duplicate of Bohr's office, was a few doors away. That made it easy for us to collaborate in the face-to-face way we enjoyed. A work session might start with Bohr sitting or standing near the blackboard in my office. He would outline an idea based on his compound-nucleus model and sketch something on the board. Soon we would be trading a piece of chalk back and forth, drawing pictures and writing equations on the board. If my office began to seem confining, Bohr would lead us round and round the loop of hallway that circled the second floor of Fine Hall, continuing to talk as we walked. These circuits might end at Bohr's office, where more ideas would be exchanged at the blackboard until we decided to separate for private thinking and calculating. As Bohr became more animated, the chalk in his hand was likely to break as he stabbed at the board. On the left side of the board one thing remained always neatly in place, Bohr's list of things to do, a reminder of his outside obligations. When we finished a session or broke for tea, Bohr would lift the edge of the rug in his office and kick broken bits of chalk under it. He had learned that otherwise he would be scolded by the janitor. Bombs and reactors were only in the backs of our minds as we worked together. We were trying to understand a new nuclear phenomenon, not design anything. One thing we realized right away: for a heavy nucleus such as uranium to split into large fragments, it had to undergo considerable deformation first. (We assumed that the nucleus was spherical before it absorbed a neutron. We know now that even when unexcited, the uranium nucleus, and indeed most nuclei, are prolate spheroids--little footballs. But fission requires a temporary deformation beyond that of the normal shape.) When you cut an orange in half, the two halves fall apart. This is not true of a nucleus. Imagine a uranium nucleus hypothetically cut into two hemispheres. The powerful nuclear forces between the particles in one half and the particles in the other half will prevent the hemispheres from separating. But if a small separation is achieved in some way, to get beyond the short range of the attractive nuclear forces, the two positively charged halves will repel each other electrically and fly apart at high speed. We say that there is an energy "barrier" standing in the way of cleavage. The energy required to surmount this metaphorical barrier depends on the "route" followed by the nucleus on its way to separation, just as the altitude to which hikers must ascend in going from one place to another depends on what route they traverse between the two points. What Bohr and I showed is that the height of the energy barrier to be surmounted is lowest if the nucleus, instead of falling apart like the two halves of an orange, is deformed through a sequence of other shapes--from orange to cucumber to large peanut. This "path" is analogous to that of hikers who have found the lowest pass over a mountain range, and thus minimized their expenditure of energy to get through. Once the nucleus has acquired just enough extra energy and has deformed into just the right shape, it is perched atop the energy barrier. Then it comes "unglued" and its parts separate, blown apart by their mutual electrical repulsion. What makes the nucleus deform in the first place? Its act of absorbing a neutron gives it extra energy. Because of this extra energy, we say the nucleus is "excited." Excitation can affect the nucleus in various ways, one of which is to set it into a deforming kind of vibrational motion--much as a raindrop, with energy added, can oscillate from sphere to egg shape and back again. If the vibration of the nucleus carries it up and over an energy barrier, it doesn't pull itself back to its original shape but instead comes apart. In the act of fission, the excited nucleus wriggles through its orange-to-cucumber-to-peanut sequence of shapes in about 1 millionth of 1 billionth of a second ([10.sup.-15] s). It could lose its extra energy in other ways, such as by emitting a gamma ray (a high-energy quantum of electromagnetic energy), but that is less likely. Once excited with enough extra energy, a uranium nucleus is more likely to undergo fission than to do anything else. Eugene Wigner, a physics faculty member who was to be a key figure in the Manhattan Project and would become my lifelong friend, occupied Fine Hall Room 209, next to Bohr's office. This large corner office, complete with fireplace, had been Einstein's before he moved across town in 1938 to more modest quarters in a new building of the Institute for Advanced Study. Wigner, nine years my senior, was a Hungarian expatriate who had been trained as a chemical engineer but found his true calling in mathematical physics. He was known as much for his unfailing politeness as for the precision of his thought. Graduate students at Princeton, seeing how Wigner always held the door for others, borrowed from the Biblical story of how hard it is for a rich man to get into heaven, saying that it is harder to get through a door behind Wigner than to pass through the eye of a needle. In 1963, Wigner's achievements in physics were recognized with the Nobel Prize. When Bohr and I got going on the energy hills and valleys and mountain passes of the deforming uranium nucleus, we naturally had to address the question: What is the chance that a nucleus, having gained extra energy by absorbing a neutron, will squirm through the sequence of shapes leading to fission rather than doing something else with that extra energy? It occurred to me that this question was not unlike a question that one might ask about a complex molecule endowed with extra energy that could disintegrate into smaller fragments. I knew that Wigner had worked on such questions with the physical chemist Michael Polanyi in Berlin. So I wanted to talk to Wigner to see if he could provide any helpful leads. (Bohr and I had no hesitation in seeking advice from our colleagues on vexing questions that arose in our work.) As it happened, Wigner was at that moment in the university infirmary suffering from jaundice, which he apparently had contracted from eating contaminated oysters. Despite his yellow complexion, Wigner greeted me warmly when I showed up at his bedside, and he steered me in the right direction to figure out the answer. I was able to go back to Bohr in a day or two with a formula for the probability of fission. My office was separated from Wigner's and Bohr's by the central gathering point in Fine Hall, the second-floor lounge, or Tea Room, where faculty and graduate students of mathematics and physics gathered every afternoon for tea. As I later heard Robert Oppenheimer put it, "Tea is where we explain to each other what we do not understand." Bohr and I were regulars at the afternoon teas. At the other end of the looping hallway from the Tea Room is another large room, Fine Hall 202, then the "professors' room" and now a lounge called Jones Hall 202, serving East Asian Studies. Today, in that room, Einstein's words are chiseled in stone above the fireplace: Raffiniert ist der Herr Gott Aber Boshaft ist Er nicht [God is subtle. But He's not malicious.] In other words, there is hope of figuring things out. Adjoining Fine Hall was Palmer Physical Laboratory, which housed other physics faculty offices, lecture halls, teaching laboratories, shops, storerooms, and research laboratories. (I was later to have my office there.) In the attic of Palmer Lab was a small accelerator that could accelerate deuterons (nuclei of heavy hydrogens). These charged particles could be directed against a target, stimulating a nuclear reaction that released neutrons. By adjusting the energy of the deuterons, the energy of the neutrons could be controlled, and the neutrons, in turn, could be used to bombard other targets. Beginning in January, as Bohr and I were undertaking our theoretical work, two graduate students, Henry Barschall and Morton Kanner, with their professor, Rudolph Ladenburg, started a series of experiments in the Palmer Lab attic to find out how the probability of fission in uranium (or the nuclear target "cross section") varies as the energy of the bombarding neutrons is changed. Their results were puzzling. They found, as expected, that the cross section is large for high-energy neutrons and diminishes as the neutron energy diminishes. But, surprisingly, at very low neutron energy, the cross section becomes large again. One morning that winter, George Placzek joined Bohr and Rosenfeld for breakfast at the Nassau Club. Placzek, thirty-three, with dark, wavy hair, glasses, a large nose, intense eyes, and a quick wit, could have been an actor playing the part of a brilliant Czech scientist and master of many languages, which is exactly what he was. Born in Moravia, he earned his Ph.D. in the Netherlands, and, in the space of less than ten years, had worked with some of the world's most eminent physicists: Paul Ehrenfest in Leiden, Bohr in Copenhagen, Fermi in Rome, and Lev Landau in Kharkov, USSR. He had also taught at the Hebrew University in Jerusalem and had just finished a stint in Paris with the Austrian physicist Hans von Halban. No one was surprised to see him turn up in Princeton. I had met Placzek first in Copenhagen in 1934. He was awkward in the way that some intellectuals are, but gregarious and always amusing. He was, above all, a provocative questioner. Only a few weeks before breakfasting with Bohr in Princeton, he had been in Copenhagen and had suggested to Otto Frisch how he might most easily confirm the existence of fission--which Frisch promptly did. Now, with Bohr, he saw that the results of Barschall, Kanner, and Ladenburg created a problem for interpreting fission. "What kind of crazy thing is this big cross section for both fast and slow?" Placzek said, in effect, to Bohr. "How can you reconcile it with your view of nuclear reactions?" Walking across campus after breakfast to Fine Hall, where he would shortly be meeting with me, Bohr talked over Placzek's question with Rosenfeld. Suddenly he said, "Now I have it!" As soon as he reached my office, he told me his idea: The substantial cross section at low energy must be due to the rare isotope U-235, present to only three-quarters of 1 percent in normal uranium. At high energy, the abundant isotope U-238 can also undergo fission, and its cross section increases as the neutron energy increases (up to a certain point, where it levels off). The behavior at low energy is influenced by the wave nature of neutrons. The lower the energy of the neutron, the greater is its wavelength, and the more it can "reach out" to interact with a target nucleus. The chance of fission occurring therefore increases as the energy decreases, provided the nucleus in question--in this case, U-235--is sufficiently excited by absorbing a low-energy neutron to undergo fission at all. Bohr and I reviewed the picture of the fission process as we then saw it, and the new idea fitted in beautifully. There were subtle differences between one isotope and another, enough to determine whether the isotope would or would not undergo fission after absorbing a low-energy neutron. U-235 would; U-238 would not. This line of reasoning led us to consider what other nuclei might be subject to fission by low-energy neutrons. We could predict with some confidence which isotopes of other elements, known or unknown, would also undergo fission under low-energy neutron bombardment. It was my Princeton colleague Louis Turner who first saw the great potential significance of one such (still undiscovered) isotope, belonging to an element two places higher in the periodic table (94 instead of 92) and with mass 239 (instead of 235). That element, discovered in 1941 and named plutonium, indeed had the property we predicted. In one of the most remarkable industrial developments of all time, plutonium, unknown in nature, was manufactured in kilogram quantities at Hanford during World War II and served as the cores of the bomb tested at Alamogordo and the bomb dropped on Nagasaki. (The Hiroshima bomb used uranium.) A great deal about fission, indeed about fusion and a whole array of other nuclear properties as well, can be understood in terms of certain simple properties of the neutrons and protons of which nuclei are composed. Neutrons and protons (collectively known as nucleons) attract each other with essentially the same force. They belong to a family of particles called fermions. (Enrico Fermi delineated their properties back in the 1920s.) One important characteristic of such particles is that no two identical fermions can move in exactly the same way at the same time in the same space. It is as if they "don't like each other." Any two protons can coexist in a nucleus only if their motions are different in some way--and having their spins oppositely directed is enough of a difference. Likewise, any two neutrons must move differently, or spin differently, if they are to share the same space. However, there is no bar to a proton and a neutron moving harmoniously as a pair. It is as if ballet dancers dressed in red were required to dance around each other without touching, and ballet dancers dressed in blue had the same restriction, whereas a red dancer and blue one could embrace and dance across the stage together. The general consequence of protons and neutrons being fermions is that light nuclei contain equal, or nearly equal, numbers of neutrons and protons. The nuclei of nature's two most common isotopes of nitrogen and oxygen (the principal components of air), for example, are N-14 and O-16. The first contains seven protons and seven neutrons. The second contains eight protons and eight neutrons. But heavy nuclei break this equal-number rule. The nucleus of U-238, for instance, contains 92 protons and 146 neutrons. Why not an equal number? Because protons repel each other electrically, whereas neutrons do not. For the lighter nuclei (up to sixteen protons or so), this electrical repulsion within the nucleus is not significant enough to undermine the effects of the attractive nuclear force. But as more protons are added, their mutual repulsion does undermine the tendency of protons and neutrons to cluster in equal number. As we move to heavier and heavier nuclei, the "neutron excess" gets greater and greater. Electrical repulsion not only accounts for the neutron excess in heavy nuclei; it also explains why the periodic table of the elements comes to an end. Beyond a certain point, there are no stable nuclei at all. Nuclei heavier than U-238 have been created in the laboratory--Pu-239, for example, and other nuclei containing up to 112 protons--but they are unstable, with lifetimes much shorter than Earth's several-billion-year age. This pattern of nuclear composition has crucial consequences for nuclear energy. In brief, the rule is this: For light nuclei, fusion (combining two nuclei to make a heavier nucleus) releases energy; for heavy nuclei, fission (breaking a nucleus into pieces) releases energy. For light nuclei, the attractive nuclear force rules. Because of it, nuclei--up to a point--become more stable as more nucleons are added. Fusing two oxygen nuclei to make a nucleus of sulfur would release energy if it were practical. It isn't, but fusing two hydrogen nuclei to form helium is. That is what happens in a thermonuclear weapon and what is the focus of intense work for future practical power generation. For heavy nuclei, the same electrical repulsion that accounts for the neutron excess and that puts an end to the periodic table makes nuclei less stable as more nucleons are added. Then fission releases energy. It was clear to us and others almost immediately when fission was discovered that fission not only releases energy, it is likely to release neutrons as well. A nucleus undergoing fission has more neutrons than the resulting fission fragments need in order to be viable nuclei. There are neutrons to spare. One way or another, we assumed, some of them would be released in fission. What actually happens is that unstable, neutron-rich nuclei are formed, along with, typically, a few neutrons. Because of these extra neutrons, a chain reaction is possible. Some heavy nuclei undergo fission upon the absorption of a slow neutron, and some do not. Why? There are two factors at work. One relates to how much charge is present relative to the total mass of the nucleus. The more charge there is for a given mass (and volume), the greater is the effect of the electrical repulsion among the protons--the more delicately, we can say, is the nucleus perched on the side of stability. With each bit more of charge or bit less of mass, the nucleus is pushed toward instability; its barrier against fission is lowered. Bohr and I found that this effect depends on a single calculated quantity, the square of the number of protons divided by the total number of nucleons. For U-236, formed when U-235 absorbs a neutron, this parameter is [92.sup.2]/236, or 35.86. For u-239, formed when U-238 absorbs a neutron, it is [92.sup.2]/239, or 35.41. A small difference, but enough to have huge consequences! We estimated the barrier against fission of U-235 to be some 16 percent less than the barrier against fission of U-238. The second factor is the preference of nuclei to contain an even rather than an odd number of neutrons (or protons). This preference arises from the fact that nucleons are fermions, each with a spin that can point in either of two directions. A neutron striking U-235 is "welcomed" more strongly than one striking U-238, because in the first case, its absorption creates an even number of neutrons, whereas in the second case, its absorption creates an odd number of neutrons. The extra binding energy for U-235, Bohr and I estimated, is equal to another 16 percent of the "barrier height." These seem like subtle effects, but think of their practical consequences. Because the rare isotope U-235 is fissionable by slow neutrons and the plentiful isotope U-238 is not, it was necessary in World War II to devise ways to separate U-235 from U-238 on a large scale, a task so difficult that the Tennessee factory capable of doing it cost more than a billion dollars (more than $13 billion in today's dollars). For the nucleus that we now call plutonium 239 (still hypothetical in 1939, when we called it simply "number 94"), we could predict fissionability by slow neutrons with a very high level of confidence. Its charge-squared-over-mass parameter (after including the absorbed neutron) is [94.sup.2]/240, or 36.82, even greater than for U-236, and therefore more favorable for fission. Moreover, the preference for an even number of neutrons also works in Pu-239's favor, just as it does for U-235. In both cases, the nucleus formed when a neutron is absorbed contains an even number of neutrons (146 and 144, respectively). Neither Bohr nor I recognized at first how important was the implication of our work that Pu-239 is fissionable--although we had no reason at all to doubt the prediction. What Louis Turner emphasized was the importance of the chemical difference between number 94 and uranium. The separation of U-235 from U-238, although successful, was an almost impossible job with the technology of the 1940s. Once plutonium was created in the reactor at Hanford, on the other hand, its chemical separation from uranium and other elements became not nearly so difficult. Enormous though the Hanford operation was, its cost in World War II was only one-third the cost of the U-235 separation plant. In the postwar period, powerful centrifuges provided the means to separate U-235 at much less cost, and Pu-239 also became cheaper to produce. As a result, half a dozen smaller nations tried, and some succeeded, in making atomic weapons with one or the other of these materials. These efforts and successes remain unacknowledged. As to the five declared nuclear powers (the United States, Great Britain, France, Russia, and China), the relative ease of producing nuclear materials has enabled them to make tens of thousands of nuclear weapons. [Continues...] Copyright (c) 1998 John Archibald Wheeler and Kenneth Ford. All rights reserved.
