by Ray Monk
By the autumn of 1944, when Churchill and Roosevelt were agreeing to dismiss any notion of sharing the ‘secret’ of the atomic bomb, it was becoming increasingly clear to the Allies that the Nazis, though fully aware of the potential military use of nuclear fission, had achieved only very limited progress towards building a bomb. In February 1944, the Alsos mission had returned to Washington from Italy, where they had been able to do little but wait for the Allies to break through the German lines. After the landings at Anzio in January, the Allied forces had met with determined resistance at Monte Cassino, preventing them from advancing into Rome. When the Germans were finally defeated at Monte Cassino in May, however, the Alsos mission returned to Italy, and Colonel Pash was able to enter Rome with the victorious Allied forces on 5 June.
After questioning the leading physicists left in Italy and finding that they knew next to nothing about the German atomic-bomb programme, Pash and his team switched to France, where they followed the advance of the massive force that had landed on D-Day. In August 1944, following the liberation of Paris, Alsos was able to interrogate Frédéric Joliot-Curie, who told them at least something they did not already know, namely that the German programme was probably led by Kurt Diebner. Then, finally, in November 1944, after Strasbourg was taken by the Allies, Pash and Goudsmit, after reading through files taken from Weizäcker’s office, had pretty conclusive proof that the Germans had not so far managed to construct a working nuclear reactor, and that they had no serious programme to build an atomic bomb.
The knowledge that there was no danger at all of the Nazis building an atomic bomb before the Allies did not have the effect that one might have expected. Most of the scientists who had been recruited to Los Alamos had been persuaded to work on the project because of the awful possibility of losing the race against the Nazis. Now that it was clear there was no such possibility, did that not call into question the whole rationale of the Allied bomb project? In fact, only one person left the project after the discovery of the rudimentary state of the Nazi bomb effort. That man was Joseph Rotblat, a Polish Jew who had done pioneering work on nuclear fission at the University of Warsaw, after which he was offered a fellowship at Liverpool to work with Chadwick. He arrived in Liverpool in the summer of 1939, having left his wife in Poland because she was too ill to travel. The intention was that she would follow him to England, but after the Nazi invasion of Poland she was unable to leave the country and he was unable to return. He never saw her again.
Feeling deeply the anxiety aroused by the prospect of the Nazis being first to develop the atomic bomb, Rotblat was an enthusiastic participant in the British Tube Alloys project and was happy to go with the British mission to Los Alamos. In March 1944, however, when he had been at Los Alamos for just two months, he received what he later described as a ‘disagreeable shock’, when, at a dinner party given by the Chadwicks, he heard Groves say: ‘You realise of course that the main purpose of this project is to subdue the Russkies.’ ‘Until then,’ Rotblat said, ‘I had thought that our work was to prevent a Nazi victory, and now I was told that the weapon we were preparing was intended for use against the people who were making extreme sacrifices for that very aim.’ On 8 December 1944, very soon after it had been established beyond all reasonable doubt that there was no danger either of the Nazis winning the war or of them developing the bomb, Rotblat left the Manhattan Project. Despite efforts by the FBI to show that he had been a Soviet spy, he went on to have an outstanding career as a physicist. Feeling betrayed by the use of the atomic bomb against the Japanese, Rotblat devoted himself for the rest of his life to the cause of nuclear disarmament, his contribution to which was recognised by the award of the Nobel Peace Prize in 1995.
Astonishingly, Rotblat was the only person ever to leave the Manhattan Project on grounds of conscience. Why? A clue, perhaps, is contained in Fermi’s remark, when, during a visit to Los Alamos, he exclaimed to Oppenheimer: ‘I believe your people actually want to make a bomb.’ Though most of them had originally been motivated by the thought of the Nazis getting there first, after a while at Los Alamos they simply wanted to see the project through to a successful conclusion. This, I think, cannot be understood without taking into account just how successful Oppenheimer was as the director of the Los Alamos laboratory.
When scientists were asked to recall their time at Los Alamos, one thing that is repeated over and over again is how inspirational Oppenheimer was. His influence went beyond that of a laboratory director; he was seen as the leader of an entire community – a community that was somehow purer, more noble, better than the world from which it was so conspicuously and effectively cut off. For his book on Oppenheimer and Lawrence, Nuel Pharr Davis collected a series of eulogies of Oppenheimer from those who had worked with him on the bomb. Among them was the British scientist James Tuck, who captured the prevailing mood of the place when he described Los Alamos as ‘the most exclusive club in the world’, where ‘I found a spirit of Athens, of Plato, of an ideal republic’:
By the grace of God the American government got the right man. His function here was not to do penetrating original research but to inspire it. It required a surpassing knowledge of science and of scientists to sit above warring groups and unify them. A lesser man could not have done it. Scientists are not necessarily cultured, especially in America. Oppenheimer had to be. The people who had been gathered here from so many parts of the world needed a great gentleman to serve under. I think that’s why they remember that golden time with enormous emotion.
True, Oppenheimer had never managed a laboratory (or anything) before, and as a physicist he was as purely theoretical as it is possible to be. And yet, in a way that amazed and impressed everybody who knew him, his entire life up to that point – his early interest in minerals, his determinedly wide-ranging education at Harvard, his absorption in the literature and art of America, France, England, Germany, Italy and Holland, his mastery of several European languages, his omnivorous devouring of all aspects of theoretical physics and his close following of major developments in experimental physics – turned out to be the perfect preparation for the task he had been set. He was the ideal man to lead Los Alamos, considered not just as a laboratory, but also as a new kind of city, one with far more than the normal proportion of extremely clever people and one, moreover, devoted to the accomplishing of a single, extraordinarily demanding task.
Hans Bethe memorably remarked to Pharr Davis that Oppenheimer ‘worked at physics mainly because he found physics the best way to do philosophy’, adding: ‘This undoubtedly had something to do with the magnificent way he led Los Alamos.’ Bethe is surely right. Oppenheimer could bring to the task the intellectual detachment of a man who could see the bigger picture and therefore not get bogged down in detail. However, though this is true and important, what sticks in many people’s minds is the remarkable way in which he could grasp the details in every aspect of the laboratory’s work. Norris Bradbury, who was to replace Oppenheimer as the director of Los Alamos, recalls: ‘Oppenheimer could understand everything, and there were some hard physics problems here to understand.’
I’ve seen him deal incredibly well with what looked like dead-end situations technically speaking. It was not that his decisions were always correct. But they always opened up a course of action where none had been apparent. They were made with a sense of dedication that moved the whole laboratory. Don’t forget what an extravagant collection of prima donnas we had here. By his own knowledge and personality he kept them inspired and going forward.
‘He could understand anything,’ echoed Robert Serber. ‘One thing I noticed: he would show up at innumerable different meetings at Los Alamos, listen and summarize in such a way as to make amazing sense. Nobody else I ever knew could comprehend so quickly.’
And along with this, he developed tremendous tact. There was a big advisory council that gave Los Alamos the appearance of a democracy just because he handled it so well. Everybody was convinced that his problems
were the urgent and important ones, because Oppenheimer thought so.
Oppenheimer had arrived on ‘the Hill’ (as people who lived there called Los Alamos) determined to use all his persuasiveness, all the power of his many and varied intellectual gifts and the best physicists in the country (and beyond) to solve the very difficult problem he had been set: to design and build a type of bomb that no one had ever seen, which could be manufactured with either of two metals, one of which was a rare isotope of uranium that was extremely difficult to separate and the other a metal that did not exist in nature, and which, up to that point, existed only in microscopic amounts. The design of this bomb was dependent upon a number of facts about these metals that so far remained unknown: what were the critical masses of U-235 and plutonium? What were their densities? When they fissioned, how many neutrons were released per fission? How fast did the emitted neutrons travel? And, given the time restraints placed upon the completion of this project – the target set was two bombs, usable for military purposes, to be produced by the summer of 1945 – the design of these bombs had to proceed alongside (not after) the scientific discovery of these facts. In other words, the bomb had to be engineered in the dark, with the expectation that it would be re-engineered when light dawned. This was extremely wasteful, but the US government was apparently prepared to give Groves an unlimited budget to see this project through.
From the very beginning it had been decided that both uranium and plutonium bombs would be built. Each had its advantages and disadvantages. The advantage of uranium was that, thanks to the early theoretical work done by Bohr and Wheeler in 1939 and the intensive experimental work subsequently carried out in both Britain and America, the basic science of the fission process for U-235 was pretty well known and understood. It is true, as David Hawkins points out in his official history of Los Alamos, that in April 1943, when the scientists started to gather on the Hill, there were still two possible reasons for doubting that an atomic bomb using U-235 could be made. The first was that ‘the neutron number had not been measured for fission induced by fast neutrons, but only for “slow” fission’. The second was that ‘the time between fissions in a fast chain might be longer than had been assumed’. However, even Hawkins concedes it was ‘extremely unlikely’ that either of these questions, once settled, would turn out to provide a serious barrier in the way of building a uranium bomb. And so, rather quickly, it was proved. By the end of 1943, both these questions had been answered: the neutron number for fast fission was greater than two, and therefore an explosive chain reaction using fast neutrons could be produced just as surely as Fermi in Chicago had produced a controlled, non-explosive chain reaction using slow neutrons. And, as Robert Wilson established, the time between fissions in U-235 was not long enough to prevent an explosion from occurring.
After the first nine months of the laboratory’s work, then, the science of a uranium bomb was, as Teller had announced it as being a year earlier, a solved problem. The problem was, as Bohr had seen in 1939 and as the Germans had discovered for themselves, that the effort involved in separating enough U-235 to make a bomb was almost unimaginably huge. When Bohr arrived in Los Alamos, having been brought up to date on the Manhattan Project by Chadwick, Groves and Oppenheimer, he said to Teller: ‘You see, I told you it couldn’t be done without turning the whole country into a factory. You have done just that.’
In fact, at the end of 1943, it was beginning to look as if even turning the whole country into a factory might not be enough; the construction of the enormous electromagnetic and gaseous-diffusion plants at Oak Ridge, occupying several square miles and employing tens of thousands of people, did not look likely to produce what was required to make one bomb, let alone two. The Y-12 (electromagnetic) site, in the words of the historian of the atomic bomb, Richard Rhodes, was by that time ‘dead in the water with hardly a gram of U-235 to show for all its enormous expense’. Neither had gaseous diffusion – though it was looking a more promising method than electromagnetic separation – yet produced any significant amounts of enriched uranium. In January 1944, the navy began work on a plant in Philadelphia that used a different method of isotope separation: thermal diffusion. As this looked promising, a thermal-diffusion plant, S-50, was added to the existing plants at Oak Ridge. In the meantime, Lawrence and the Rad Lab team at Berkeley worked round the clock to get the Calutrons at Y-12 working, while the physicists at Columbia, supported by Fuchs and Peierls, worked equally hard trying to perfect the gaseous-diffusion plants at K-25; but it was clear to Groves and Oppenheimer that, even with this truly colossal effort, there was no possibility whatsoever of having enough U-235 to make two bombs by the summer of 1945. If they were going to achieve this target, they would have to produce at least one plutonium bomb.
But, of course, plutonium too had its problems. Just as the severe difficulties in separating uranium-235 had convinced the Germans that the only practical route to the atomic bomb lay in producing plutonium, so the British Tube Alloys project had considered only the uranium bomb, for reasons equally compelling: plutonium does not exist in nature and nobody knew very much about it. The idea that one could build a bomb using a metal, the basic science of which had yet to be done, seemed fanciful. At Los Alamos, Oppenheimer set about coordinating that basic science, while at the same time, designing a bomb that would make use of its results. Inevitably, therefore, there was a lot of guesswork and many false starts.
Given that the physics of uranium fission was relatively well advanced and the task of making a bomb out of uranium (assuming enough U-235 could eventually be produced) relatively straightforward, the Los Alamos laboratory concentrated its considerable financial and intellectual resources on the plutonium bomb. When the scientists at Los Alamos talked of the ‘gadget’, what they were referring to more often than not was the plutonium bomb. And, in particular, during the first year of the laboratory’s work, they were referring to a plutonium bomb using what Serber in his introductory lectures had called the ‘gun assembly method’. This is the basic bomb design originally envisaged by Frisch and Peierls in their memorandum, in which the fissionable material – uranium-235 or plutonium – is split into two subcritical parts, one larger than the other. The smaller part is then fired into the larger part, thus assembling a supercritical mass of the fissionable substance.
Though much about the chemistry and metallurgy of plutonium remained to be discovered, two extremely important things about it were already known. The first was that its critical mass is smaller than that of U-235, though exactly how much smaller had yet to be determined. The second was something brought to Oppenheimer’s attention by Glenn Seaborg, the discoverer of plutonium, just before work at Los Alamos began, the full significance of which would not be appreciated until the summer of 1944, when the realisation dawned that, in fact, it threatened to undermine the entire bomb project.
What Seaborg pointed out was that plutonium, despite its many advantages as a fissionable bomb material, had a potential disadvantage, which has to do with what is called ‘spontaneous fission’. Unlike ordinary nuclear fission, spontaneous fission does not require the nucleus of an atom to be hit by a neutron; it is, rather, a kind of radioactive decay, like the alpha emissions of substances such as radium (or, indeed, uranium and plutonium) – something that occurs without anything being done to the material. When spontaneous fission takes place, the result is the same as ordinary nuclear fission: the nucleus splits, neutrons are emitted and energy is released. Spontaneous fission created a problem for gun-assembly atomic bombs because the neutrons emitted by it might set off a chain reaction before the two pieces of the fissionable material could be brought together. This chain reaction, though it would produce a great deal of heat and energy, would not be explosive, and therefore the bomb would ‘fizzle’.
Just as it was known that heavy nuclei with odd mass numbers – U-235 and Pu-239 – are more liable to undergo ordinary nuclear fission, so it was known that those with even mass numbers, such as U-23
8, are more likely to undergo spontaneous fission. This meant, Seaborg explained to Oppenheimer, that Pu-240, an isotope of plutonium, would be likely to have a high rate of spontaneous fission. In the spring of 1943, this was a merely theoretical worry, since no Pu-240 had yet been created, but, Seaborg warned, it was likely that the plutonium produced in a nuclear reactor would not be pure Pu-239, but rather a mixture of Pu-239 and Pu-240. This is because in a reactor there are far more free neutrons flying around than in a laboratory accelerator such as a cyclotron (until the nuclear reactors at Oak Ridge and Hanford started to go critical, the only plutonium anyone had ever seen had been produced by cyclotrons), and it is therefore more likely that some Pu-239 nuclei would absorb a neutron and become Pu-240.
To begin with, this warning of spontaneous fission, though taken seriously, was not treated as potentially fatal to the entire project, largely because it was assumed that the differences between accelerator-produced plutonium and reactor-produced plutonium would not be so very great. Soon after work got under way at Los Alamos, Emilio Segrè was put in charge of experiments designed to measure the rate of spontaneous fission in both uranium and plutonium, using material obtained from cyclotrons, and his initial results were very encouraging. The rate was, he discovered, not large enough to make the gun method impossible. True, the gun in the plutonium bomb would have to fire its ‘bullet’ pretty fast, and the gun barrel would have to be pretty long, but there seemed to be no reason, in principle, why such a gun could not be designed and built. One thing making it easier, ballistics experts were quick to point out, was that, unlike almost every other gun ever made, it would be fired just once, so durability was not an issue.
When the figures were established, Deak Parsons and his rapidly growing ordnance team were set the task of designing a gun capable of firing a piece of plutonium a distance of seventeen feet into another larger piece of plutonium at a speed of 3,000 feet per second. Making this task much more demanding was the fact that they were to do so in advance of any hard information about the relevant chemical and metallurgical properties of plutonium. Dealing with such uncertainties might be what theoretical – and, to a lesser extent, experimental – physicists did for a living, but it was not what engineers were used to. The first three men chosen to head the Engineering Group in Parsons’s division left after a short time in the job, because, as Parsons put it, of the ‘frustrations which these people experienced when one week they thought they had a problem in mind, and had evolved a solution, only to find, when they proposed it, that the concept of the problem had changed in the meantime and their solution was irrelevant’.