The First War of Physics (36 page)

Fuchs would never know his contact by any name other than Raymond. He was Harry Gold, a Jewish, Swiss-born chemistry graduate who had been brought to America when he was just two years old. He had started spying for the Soviet Union in 1936. He passed on information about industrial chemical processes he obtained from Pennsylvania Sugar, where he worked, via the Soviet foreign trade agency Amtorg.

Although Gold was sympathetic to the Soviet vision of a just society (or, at least, the version of this vision promulgated in Communist propaganda), his was espionage that was not born from some deeply-held conviction about Communism or the Soviet Union. Gold never became a party member. Nor did he seek compensation for the information he acquired. Gold was a social inadequate. It seems he began his career in espionage largely because he was asked to do this by someone who had helped him get a job. He became a spy because he was grateful.

Gold would weave elaborate fantasies about himself and his life. He lived in Philadelphia, single and alone. Yet he would talk tearfully of his wife, of her betrayal and how the break-up of his marriage had denied him access to his children. He talked about his younger brother Joe, who had been killed in action in the Pacific. In truth, his younger brother – who really was called Joe – was not only alive but had been decorated for bravery.

His access to industrial secrets was rather limited, however, and he quickly exhausted his usefulness as a spy. Gold was instead put to work as a courier before being given his latest assignment. Fuchs met with Gold several times at various public places. As his work on the Manhattan Project
got under way, Fuchs once again passed on reports that he had prepared himself. These were mostly technical papers concerned with the operation of K-25, the large-scale gaseous diffusion plant at Oak Ridge, and the barrier materials that had now been adopted. Despite Groves’ obsession with secrecy, the scientists working on the project would frequently take classified documents home with them, under strict instructions not to let them out of their possession. Fuchs would pass his written reports to a secretary for typing. All typed reports and any copies were strictly controlled, but there appeared to be no controls on the handwritten originals. He would pass these to Gold, holding on to the reports until the very end of their meetings, so that if they were discovered it would be Fuchs who would be found with classified documents in his possession.

In his turn, Gold would pass the papers, together with his own reports of his conversations with Fuchs, to his Soviet contact. This was New York vice consul and NKGB
¹
agent Anatoly Yatskov.

Fuchs had warned Gold that the various projects running within the Manhattan Project were quite strictly compartmentalised, and that he anticipated that he would be transferred to work somewhere in the southwestern US later in the year or early in 1945. Fuchs believed he explicitly mentioned New Mexico, but Gold heard ‘somewhere in Mexico’. Fuchs gave him Kristel’s address in Cambridge and explained that if contact was broken for any reason, he would leave a message for Gold with his sister. It was still nevertheless quite a shock for Gold when, on 5 August, Fuchs failed to appear at a pre-arranged meeting outside the Bell movie theatre in Brooklyn. Fuchs missed the next meeting as well.

Yatskov acquired Fuchs’ New York address and advised Gold to go to the apartment and investigate. Bearing of copy of a novel by Thomas Mann in which he had inscribed Fuchs’ name and address, Gold went to the apartment with the cover story that he was returning a book that Fuchs had lent him. Getting no answer from the apartment, Gold made enquiries of the janitor, who told him that Fuchs had moved away. Yatskov advised Gold to sit tight.

Oppenheimer’s solution to Teller’s departure from the mainstream activity of the Theoretical Division was to bring Peierls to Los Alamos. Bethe and Peierls had known each other for many years (Bethe had stayed with Peierls and his wife in England in 1934). Both Bethe and Oppenheimer considered Peierls to be the best physicist available to work on the theory of implosion. By the summer of 1944 the Oak Ridge plant was up and running and there was little need for the Tube Alloys physicists to remain in New York to work on diffusion theory. It was agreed that Peierls would move to Los Alamos, and that Fuchs would join him.

Fuchs had been unable to attend his meetings with Gold because he was already at Los Alamos.

The plutonium-240 crisis

The small pilot nuclear reactor at Oak Ridge, called X-10, had gone critical for the first time in November 1943. Its purpose was to generate larger quantities of plutonium for testing ahead of regular supply of reactor-produced materials from the large-scale reactors that were being built by Du Pont at Hanford. But what the Los Alamos physicists now discovered about reactor-produced plutonium threatened to rule out completely the idea of a plutonium bomb. And, without a plutonium bomb, all the Manhattan Project could hope to produce was just one U-235 bomb.

The physicists discovered that plutonium produced in a uranium reactor behaves rather differently from the tiny quantities of plutonium produced by neutron bombardment in a cyclotron. A year before, Glenn Seaborg had warned of the risk that plutonium produced in a reactor might be contaminated with quantities of the plutonium isotope Pu-240, formed from Pu-239 by capture of another neutron. In July 1944 Segrè showed in his makeshift cabin-laboratory that Seaborg had been right. The longer the plutonium was left to accumulate in a nuclear reactor – and hence the more plutonium was produced – the higher was the proportion
of Pu-240. When it finally arrived, the plutonium from the Hanford reactors would likely contain a high proportion of this isotope.

The problem was that Pu-240 is quite unstable, emitting alpha particles and acting as a source of background neutrons. Segrè found that the rate of spontaneous fission in the X-10 plutonium was significantly higher than in the cyclotron samples, behaviour that could be ascribed to a very high spontaneous fission rate in Pu-240. With the gun method it was believed to be possible to assemble the sub-critical components within about a ten-thousandth of a second. Calculations quickly showed that the high spontaneous fission rate in Pu-240 would release a flood of neutrons into the assembling mass before it had reached its optimum configuration. Any attempt to assemble a super-critical mass of plutonium using the gun method would result only in pre-detonation. The bomb would fizzle but not explode, because the gun method simply could not create a supercritical mass fast enough. An assembly mechanism at least 100 times faster was required. Implosion was the only candidate.

This was a double blow. The gun method was ruled out because of the contamination by Pu-240. Purifying the plutonium meant separating Pu-240 from Pu-239, isotopes differing by only one neutron in their nuclei, a much harder task even than separating U-235 from U-238. The promise of plutonium – a promise of access to fissile materials without the pain of a difficult physical separation process – was cruelly snatched away.

Oppenheimer met with Conant, Compton, Fermi, Groves and Nichols to discuss the problem in Chicago on 17 July. There was no real prospect of purifying the plutonium. Without purification, the gun method could not be used for a plutonium bomb. Conant suggested an alternative approach based on mixtures of uranium and plutonium. But this would be a low-yield weapon, generating an explosive force equivalent to a little more than a few hundred tons of TNT. Conant wondered if building such a weapon would help build confidence in the push for a larger weapon, but Oppenheimer argued that taking this approach would result in unacceptable delays.

‘It appears reasonable,’ Oppenheimer wrote in conclusion the next day, ‘to discontinue the intensive effort to achieve higher purity for plutonium
and to concentrate attention on methods of assembly which do not require a low neutron background for their success. At the present time the method to which an over-riding priority must be assigned is the method of implosion.’

It was Oppenheimer’s worst nightmare. As the physicists debated what to do next, the question of German progress was inevitably raised. Had the Germans encountered, and already solved, this same problem? ‘We finally arrived at the conclusion that they could be exactly up to us, or perhaps further’, Rabi noted. ‘We were very solemn. One didn’t know what the enemy had. One didn’t want to lose a single day, a single week. And certainly, a month would be a calamity.’

Neddermeyer and his team studying implosion in the Ordnance Division had taken a rather plodding academic approach to the problem. ‘I think [Oppenheimer] felt very badly because I seemed not to push things as for war research but acted as though it were just a normal research situation’, Neddermeyer later admitted. In an attempt to get more traction, in January 1944 Oppenheimer had persuaded George Kistiakowsky to join Los Alamos full-time. Kistiakowsky had been travelling to and from the Hill in his role as consultant to the Manhattan Project. This was merely one part of a whirlwind commute between Pittsburgh, Florida, Washington and New Mexico for the NDRC’s explosives group. He had an interesting overseas appointment in the pipeline, and Oppenheimer had had to pull out all the stops and apply his legendary charm.

When Kistiakowsky arrived on the Hill for good he announced: ‘I am old, I am tired, and I am disgusted.’ Oppenheimer had made sure he got good accommodation, and sold the former Cossack one of his own saddle horses for a knock-down price. The horse was called, appropriately, ‘Crisis’.

Kistiakowsky had worked in the Ordnance Division on implosion through the spring and early summer of 1944, trying desperately to referee the interminable disputes between Neddermeyer and the division head, Captain William ‘Deke’ Parsons. It was a clash between military-style management and academic sensibility. ‘The two never agreed about anything
and they certainly didn’t want me interfering’, Kistiakowsky said. He made little progress, and threatened to leave.

Explosive lenses

It was around this time when the ultimate solution to the implosion problem was outlined by James Tuck, a Manchester-born and -educated physicist. Tuck had worked with Szilard in Oxford in 1937 and at the outbreak of the war he had been appointed as a scientific assistant to Cherwell, working on the development of armour-piercing anti-tank weapons. It was his expertise in the use of shaped charges that had led to his inclusion in the British delegation.

Neddermeyer had been wrestling with the problem of creating a perfectly spherical Shockwave by varying the shape of the explosion, the type of explosive and the number and positioning of the detonators. But the shockwave produced by a point detonator expands spherically outwards through the explosive material, just as the ripples on the surface of a pond expand outwards from the point where a pebble has been thrown. Put several detonators in close proximity and the result is an unpredictable combination of diverging and interfering Shockwaves, much like the turbulence on the surface of a pond when many pebbles are thrown in at once.

This, Tuck argued, was not a new problem. American and British efforts to develop armour-piercing shells in which all of the force of the explosive charge is directed towards the armour had resulted in the development of explosive
lenses.
These work according to the same kinds of principles by which ordinary lenses direct and focus light waves. The medium of an optical lens affects the velocity of light passing through it, exerting different effects in different places in the medium such that the light is ‘gathered’ and focused to a point. An explosive lens consists of a series of charges with different rates of detonation, such that the explosive shockwave is similarly gathered, and focused. Surround the spherical core of plutonium with explosive lenses which are then detonated simultaneously and, Tuck
suggested, the result would be a perfectly spherical shockwave directed inwards towards the core.

It was not immediately identified as
the
solution. Developing explosive lenses appeared far more complex than simply trying to get a uniform spherical shockwave from conventional explosives. However, initial experiments, run separately to Neddermeyer’s implosion work, appeared promising. Geoffrey Taylor, a leading British expert in hydrodynamics, arrived at Los Alamos in May 1944 and lent the weight of his opinions. The brute force hydrodynamic calculations seemed to tell against the simple solution and, gradually, the Los Alamos physicists began to acknowledge that explosive lenses might present the only solution. But it also became clear that success with explosive lenses would require considerable trial-and-error experimentation.

Oppenheimer now took a huge gamble. The problem of spontaneous fission in reactor-bred plutonium meant that if a plutonium bomb was going to be at all possible, it would have to be an implosion bomb. Somehow, they were going to have to make implosion work. They now had no choice but to throw the book at it.

Oppenheimer finally lost patience with Neddermeyer. ‘Oppenheimer lit into me. A lot of people looked up to him as a source of wisdom and inspiration. I respected him as a scientist, but I just didn’t look up to him that way. I didn’t look up to him. From my point of view, he was an intellectual snob. He could cut you cold and humiliate you right down to the ground.’ In desperation, Oppenheimer now moved to place his bet. He elected to reorganise the laboratory. In August 1944 he created two new divisions out of the old Ordnance Division. These were G Division (for ‘Gadget’), charged with developing the physics of implosion and the Fat Man bomb design, to be led by Robert Bacher, and X Division (for ‘eXplosives’) which would focus on the development of explosive lenses. He removed a bitterly disappointed Neddermeyer and persuaded a reluctant Kistiakowsky to take charge of X Division.

Parsons was busy with the development of the uranium gun, but was angry with the way he had been out-manoeuvred by Oppenheimer and Kistiakowsky. ‘Parsons was furious,’ said Kistiakowsky; ‘he felt that I had
by-passed him and that was outrageous. I can understand how he felt but I was a civilian, so was Oppie, and I didn’t have to go through him.’