by Peter Watson
But in Berlin Otto Hahn spotted something available to any physicist but missed. The more common isotope of uranium, U238, is made up of 92 protons and 146 neutrons in its nucleus. If neutron bombardment were to create new, transuranic elements, they would have not only different weights but different chemical properties.11 He therefore set out to look for these new properties, always keeping in mind that if the neutrons were not being captured, but were chipping particles out of the nucleus, he ought to find radium. A uranium atom that lost two alpha particles (helium nuclei, atomic weight four for each) would become radium, R230. He didn’t find radium, and he didn’t find any new elements, either. What he did find, time and again when he repeated the experiments, was barium. Barium was much lighter: 56 protons and 82 neutrons, giving a total of 138, well below uranium’s 238. It made no sense. Puzzled, Hahn shared his results with Lise Meitner. Hahn and Meitner had always been very close, and he had helped protect her throughout the 1930s, because she was Jewish. She was kept employed because, technically speaking, she was Austrian, and therefore, technically speaking, the racial laws didn’t apply to her. After the Anschluss, however, in March 1938, when Austria became part of Germany, Meitner could no longer be protected, and she was forced to escape to Göteborg in Sweden. Hahn wrote to her just before Christmas 1938 describing his unusual results.12
As luck would have it, Meitner was visited that Christmas by her nephew Otto Frisch, then with Bohr in Copenhagen. The pair were very pleased to see each other – both were in exile – and they went lang-laufing in the nearby woods, which were covered in snow. Meitner told her nephew about Hahn’s letter, and they turned the barium problem over in their minds as they walked between the trees.13 They began to consider radical explanations for Hahn’s puzzling observation, in particular a theory of Bohr’s that the nucleus of an atom was like a drop of water, which is held together by the attraction that the molecules have for each other, just as the nucleus is held together by the nuclear force of its constituents. Until then, as mentioned earlier, physicists had considered that when the nucleus was bombarded, it was so stable that at most the odd particle could be chipped off.14 Now, huddled on a fallen tree in the Göteborg woods, Meitner and Frisch began to wonder whether the nucleus of uranium was like a drop of water in other ways, too.15 In particular they allowed the possibility that instead of being chipped away at by neutrons, a nucleus could in certain circumstances be cleaved in two. They had been in the woods, skiing and talking, for three hours. They were cold. Nonetheless, they did the calculations there and then before turning for home. What the arithmetic showed was that if the uranium atom did split, as they thought it might, it could produce barium (56 protons) and krypton (36) – 56+36=92. They were right, and when Frisch told Bohr, he saw it straight away. ‘Oh, what idiots we have all been,’ he cried. ‘This is just as it must be.’16 But that wasn’t all. As the news sank in around the world, people realised that, as the nucleus split apart, it released energy, as heat. If that energy was in the form of neutrons, and in sufficient quantity, then a chain reaction, and a bomb, might indeed be possible. Possible, but not easy. Uranium is very stable, with a half-life of 4.5 billion years; as Richard Rorty dryly remarks, if it was apt to give off energy that sparked chain reactions, few physics labs would have been around to tell the tale. It was Bohr who grasped the essential truth – that U238, the common isotope, was stable, but U235, the much less common form, was susceptible to nuclear fission (the brand-new term for what Hahn had observed and Meitner and Frisch had been the first to understand). Bring two quantities of U235 together to form a critical mass, and you had a bomb. But how much U235 was needed?
The pitiful irony of this predicament was that it was still only early 1939. Hitler’s aggression was growing, sensible people could see war coming, but the world was, technically, still at peace. The Hahn/Meitner/Frisch results were published openly in Nature, and thus read by physicists in Nazi Germany, in Soviet Russia, and in Japan, as well as in Britain, France, Italy, and the United States.17 Three problems now faced the physicists. How likely was a chain reaction? This could be judged only by finding out what energy was given off when fission occurred. How could U235 be separated from U238? And how long would it take? This third question involved the biggest drama. For even after war broke out in Europe, in September 1939, and the race for the bomb took on a sharper urgency, America, with the greatest resources, and now the home of many of the exiles, was a nonbelligerent. How could she be persuaded to act? In the summer of 1939 a handful of British physicists recommended that the government acquire the uranium in the Belgian Congo, if only to stop others.18 In America the three Hungarian refugees Leo Szilard, Eugene Wigner, and Edward Teller had the same idea and went to see Einstein, who knew the queen of Belgium, to ask her to set the ball rolling.19 In the end they decided to approach Roosevelt instead, judging that Einstein was so famous, he would be listened to.20 However, an intermediary was used, who took six weeks to get in to see the president. Even then, nothing happened. It was only after Frisch and Peierl’s calculations, and the three-page paper they wrote as a result, that movement began. By that stage the Joliot-Curies had produced another vital paper – showing that each bombardment of a U235 atom released, on average, 3.5 neutrons. That was nearly twice what Peierls had originally thought.21
The Frisch-Peierls memorandum was considered by a small subcommittee brought into being by Henry Tizard, which met for the first time in the offices of the Royal Society in April 1940. This committee came to the conclusion that the chances of making a bomb in time to have an impact on the war were good, and from then on the development of an atomic bomb became British policy. The job of persuading the Americans to join in fell to Mark Oliphant, Frisch and Peierls’s professor at Birmingham. Strapped by war, Britain did not have the funds for such a project, and any location, however secret, might be bombed.22 In America, a ‘Uranium Committee’ had been established, whose chairman was Vannevar Bush, a dual-doctorate engineer from MIT. Oliphant and John Cockroft travelled to America and persuaded Bush to convey some of the urgency they felt to Roosevelt. Roosevelt would not commit the United States to build a bomb, but he did agree to explore whether a bomb could be built. Without informing Congress, he found the necessary money ‘from a special source available for such an unusual purpose.’23
*
While Bush set to work to check on the British findings, Niels Bohr in Copenhagen received a visit from his former pupil, the creator of the Uncertainty Principle, Werner Heisenberg. Denmark had been invaded in April 1940. Bohr had refused a guarantee by the American embassy of safe passage to the United States and instead did what he could to protect more junior scholars who were Jewish. After much talk, Bohr and Heisenberg went for a walk through the brewery district of Copenhagen, near the Carlsberg factories. Heisenberg was one of those in charge of the German bomb project in Leipzig, and on that walk he raised the prospect of the military applications of atomic energy.24 He knew that Bohr had just been in America, and Bohr knew that he knew. At the meeting Heisenberg also passed to Bohr a diagram of the reactor he was planning to build – and this is what makes the meeting so puzzling and dramatic in retrospect. Was Heisenberg letting Bohr know how far the Germans had got, because he hated the Nazis? Or was he, as Bohr subsequently felt, using the diagram as a lure, to get Bohr to talk, so he would tell Heisenberg how far America and Britain had progressed? The real reason for this encounter has never been established, though its drama has not diminished as the years have passed.25
The National Academy of Sciences report, produced as a result of Bush’s October conversation with the president, was ready in a matter of weeks and was considered at a meeting chaired by Bush in Washington on Saturday, 6 December 1941. The report concluded that a bomb was possible and should be pursued. By this stage, American scientists had managed to produce two ‘transuranic’ elements, called neptunium and plutonium (because they were the next heavenly bodies beyond Uranus in the night sky), and which
were by definition unstable. Plutonium in particular looked promising as an alternative source of chain-reaction neutrons to U235. Bush’s committee also decided which outfits in America would pursue the different methods of isotope separation – electromagnetic or by centrifuge. Once that was settled, the meeting broke up around lunchtime, the various participants agreeing to meet again in two weeks. The very next morning the Japanese attacked Pearl Harbor, and America, like Britain, was now at war. As Richard Rhodes put it, the lack of urgency in the United States was no longer a problem.26
The early months of 1942 were spent trying to calculate which method of U235 separation would work best, and in the summer a special study session of theoretical physicists, now known as the Manhattan Project, was called at Berkeley. The results of the deliberations showed that much more uranium would be needed than previous calculations had suggested, but that the bomb would also be far more powerful. Bush realised that university physics departments in big cities were no longer enough. A secret, isolated location, dedicated to the manufacture of an actual bomb, was needed.
When Colonel Leslie Groves, commander of the Corps of Engineers, was offered the job of finding the site, he was standing in a corridor of the House of Representatives Office Building in Washington, D.C. He exploded. The job offer meant staying in Washington, there was a war on, he’d only ever had ‘desk’ commands, and he wanted some foreign travel.27 When he found that as part of the package he was to be promoted to brigadier, his attitude started to change. He quickly saw that if a bomb was produced, and it did decide the war, here was a chance for him to play a far more important role than in any assignment overseas. Accepting the challenge, he immediately went off on a tour of the project’s laboratories. When he returned to Washington, he singled out Major John Dudley as the man to find what was at first called Site Y. Dudley’s instructions were very specific: the site had to accommodate 265 people; it should be west of the Mississippi, and at least 200 miles from the Mexican or Canadian border; it should have some buildings already, and be in a natural bowl. Dudley came up with, first, Oak City, Utah. Too many people needed evicting. Then he produced Jemez Spring, New Mexico, but its canyon was too confining. Farther up the canyon, however, on the top of the mesa, was a boys’ school on a piece of land that looked ideal. It was called Los Alamos.28
As the first moves to convert Los Alamos were being made, Enrico Fermi was taking the initial step toward the nuclear age in a disused squash court in Chicago (he had emigrated in 1938). By now, no one had any doubt that a bomb could be made, but it was still necessary to confirm Leo Szilard’s original idea of a nuclear chain reaction. Throughout November 1942, therefore, Fermi assembled what he called a ‘pile’ in the squash court. This consisted of six tons of uranium, fifty tons of uranium oxide, and four hundred tons of graphite blocks. The material was built up in an approximate sphere shape in fifty-seven layers and in all was about twenty-four feet wide and nearly as high. This virtually filled the squash court, and Fermi and his colleagues had to use the viewing gallery as their office.
The day of the experiment, 2 December, was bitterly cold, below zero.29 That morning the first news had been received about 2 million Jews who had perished in Europe, with millions more in danger. Fermi and his colleagues gathered in the gallery of the squash court, wearing their grey lab coats, ‘now black with graphite.’30 The gallery was filled with machines to measure the neutron emission and devices to drop safety rods into the pile in case of emergency (these rods would rapidly absorb neutrons and kill the reactions). The crucial part of the experiment began around ten as, one by one, the cadmium absorption rods were pulled out, six inches at a time. With each movement, the clicking of the neutron records increased and then levelled off, in sync and exactly on cue. This went on all through the morning and early afternoon, with a short break for lunch. Just after a quarter to four Fermi ordered the rods pulled out enough for the pile to go critical. This time the clicks on the neutron counter did not level off but rose in pitch to a roar, at which point Fermi switched to a chart recorder. Even then they had to keep changing the scale of the recorder, to accommodate the increasing intensity of the neutrons. At 3:53 P.M., Fermi ordered the rods put back in: the pile had been self-sustaining for more than four minutes. He raised his hand and said, ‘The pile has gone critical.’31
Intellectually, the central job of Los Alamos was to work on three processes designed to produce enough fissile material for a bomb.32 Two of these concerned uranium, one plutonium. The first uranium method was known as gaseous diffusion. Metal uranium reacts with fluorine to produce a gas, uranium hexafluoride. This is composed of two kinds of molecule, one with U238 and another with U235. The heavier molecule, U238, is slightly slower than its half-sister, so when it is passed through a filter, U235 tends to go first, and gas on the far side of the filter is richer in that isotope. When the process is repeated (several thousand times), the mixture is even richer; repeat it often enough, and the 90 percent level the Los Alamos people needed is obtained. It was an arduous process, but it worked. The other method involved stripping uranium atoms of their electrons in a vacuum and then giving them an electrical charge that made them susceptible to outside fields. These were then passed in a beam that curved within an electrical field so that the heavy isotope would take a wider course than the lighter form, and become separated. In plutonium production, the more common isotope, U238, was bombarded with neutrons, to create a new, transuranic element, plutonium-239, which did indeed prove fissile, as the theoreticians had predicted.33
At its height, 50,000 people were employed at Los Alamos on the Manhattan Project, and it was costing $2 billion a year, the largest research project in history.34 The aim was to produce one uranium and one plutonium bomb by late summer 1945.
In early 1943 Niels Bohr received a visit from a captain in the Danish army. They took tea and then retired to Bohr’s greenhouse, which they thought more secure. The captain said he had a message from the British via the underground, to say that Bohr would shortly receive some keys. Minute holes had been drilled in these keys, in which had been hidden a microdot, and the holes then filled in with fresh metal. He could find the microdot by slowly filing the keys at a certain point: ‘The message can then be extracted or floated out on to a microslide.’35 The captain offered the army’s help with the technical parts, and when the keys arrived, the message was from James Chadwick, inviting Bohr to England to work ‘on scientific matters.’ Bohr guessed what that meant, but as a patriot he didn’t immediately take up the offer. The Danes had managed to do a deal with the Nazis, so that in return for providing the Reich with food, Danish Jews would go unmolested. Though the arrangement worked for a while, strikes and sabotage were growing, especially after the German surrender at Stalingrad, when many people sensed that the course of the war was decisively changing. Finally, sabotage became so bad in Denmark that on 29 August 1943 the Nazis reoccupied the country, immediately arresting certain prominent Jews. Bohr was warned that he was on the list of those to be arrested, and at the end of September, with the help of the underground, he escaped, taking a small boat through the minefields of the Öresund and flying from Sweden to Scotland. He soon moved on from Britain to Los Alamos. There, although he took an interest in technical matters and made suggestions, his very presence was what mattered, giving the younger scientists a boost: he was a symbol for those scientists who felt that the weapon they were building was so terrible that all attempts should be made to avoid using it; that the enemy should be shown what it was capable of and given the chance to surrender. There were those who went further, who said that the technical information should be shared, that the moral authority this would bring would ensure there would never be an arms race. A plan was therefore mounted for Bohr to see Roosevelt to put forward this view. Bohr got as far as Felix Frankfurter, the president’s aide, who spent an hour and a half discussing the matter with Roosevelt. Bohr was told that the president was sympathetic but wanted the Dane to see Ch
urchill first. So Bohr recrossed the Atlantic, where the British prime minister kept him waiting for several weeks. When they finally did meet, it was a disaster. Churchill cut short the meeting and in effect told Bohr to stop meddling in politics. Bohr said later that Churchill treated him like a schoolboy.36
Churchill was understandably worried (and he was frantically planning the Normandy invasions at the time). How could they know that the Germans, or the Japanese, or the Russians were not ahead of them? With the benefit of hindsight, no one was anywhere near the Allies on this matter.37 In Germany, Fritz Houtermans had concentrated since about 1939 on making element 94, and the Germans – in the ‘U-PROJECT,’ as it was called – had thus neglected isotope separation. Bohr had been given that diagram of a heavy-water reactor and, drawing their own conclusions, the British had bombed the Vemork factory in Norway, the only establishment that manufactured such a product.38 But that had been rebuilt. Fresh attempts to blow it up were unsuccessful, and so a different plan was called for when, via the underground, it was learned that the heavy water was to be transferred to Germany in late February 1944. According to intelligence, the water was to be taken by train to Tinnsjö, then across the sea by ferry. On the twentieth of the month, a team of Norwegian commandos blew up the ferry, the Hydro, with the loss of twenty-six of the fifty-three people on board. At the same time, thirty-nine drums containing 162 gallons of heavy water went to the bottom of the sea. The Germans later conceded that ‘the main factor in our failure to achieve a self-sustaining atomic reactor before the war ended’ was due to their inability to increase their stocks of heavy water, thanks to the attacks on Vemork and the Hydro.39 This was almost certainly the most significant of the many underground acts of sabotage during the war.