Inside the Centre: The Life of J. Robert Oppenheimer (76 page)

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.’

‘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.’

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-238, 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’.

Despite the many difficulties and uncertainties, by January 1944 the ‘gadget’ had been designed and a suitable name, the ‘Thin Man’, had been chosen for it. All that remained, so it was believed, was for Parsons and his Ordnance Division to test the dropping of it and work out the details of its internal ballistics. A few months later, however, in April 1944, Segrè finally received some samples of reactor-produced plutonium and, to everybody’s horror, discovered that the rate of spontaneous fission was
five times
that of the cyclotron-produced samples he had measured earlier. Just as Seaborg had warned, the plutonium had far more Pu-240 in it than that produced by a cyclotron. The alarming but inescapable conclusion was that the ‘Thin Man’ was a non-starter. The whole idea of a gun-assembly plutonium bomb – the idea that up until then had formed the central focus for almost all the work done at Los Alamos – would have to be abandoned.

This was devastating news, but in Segrè’s earlier measurements of spontaneous fission in uranium there was a silver lining: a gun-assembly bomb made with uranium
would
work and, in fact, was even more straightforward than they had thought. The uranium bullet could be fired at a mere 1,000 feet per second and the length of the gun could be reduced from seventeen feet to six. Thus, in place of the plutonium ‘Thin Man’ bomb, there emerged the uranium ‘Little Boy’, the bomb that would be dropped on Hiroshima. So confident were Oppenheimer and his colleagues that ‘Little Boy’ would work that they did not see any need to test it. The bomb was designed and built and then left to one side, waiting for the U-235 that would form both the bullet and its target.