The First War of Physics (29 page)

But the critical mass is the minimum mass required to support a chain reaction, not the mass required to make an effective bomb. It was already clear that a bomb would need an amount of active material greater than the critical mass, called a ‘super-critical’ mass.
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The first step was therefore to assemble a super-critical mass from several sub-critical components. The components would come together, creating a ‘divergent’ chain reaction, meaning that more neutrons are produced during the reaction than are consumed.

Timing is critical. It was estimated that a kilo of U-235 would fission in about a millionth of a second, producing an explosive force equivalent to 20,000 tons of TNT and generating initial temperatures that can be thought to be equivalent to about a thousand suns. At these temperatures, the uranium is rapidly vaporised and the vapour expands, making it more and more difficult to keep the chain reaction going. At some point, the vapour reaches ‘second criticality’, where neutrons produced by fission balance those escaping to the surrounding environment. Beyond this point, there is no further explosive release of energy.

Assemble the components too slowly and the mass would pre-detonate – blow itself apart prematurely – yielding much less than the potential explosive force. One proposed solution was to shoot a cylindrical plug of active material, called a ‘shy’, into a corresponding hole in a sub-critical sphere, thereby producing a combined mass in excess of the critical mass. Following the work of the Tube Alloys physicists, this method of assembling a super-critical mass became known as the gun method.

This was also the one aspect of early bomb design that was least understood. The pieces of active material had to be brought together fast enough to prevent pre-detonation, which meant a relative velocity of about 100,000 centimetres per second or more. The highest muzzle velocity available from conventional armaments was 3,150 feet per second (about 96,000 centimetres per second) with a projectile weighing 50 pounds. This was a US Army gun with a bore of 4.7 inches and a barrel 21 feet in length, weighing five tons. If, as expected, the shy in a U-235 bomb needed to be twice as heavy, then a correspondingly heavier gun would be needed. A U-235 bomb would therefore need to incorporate a gun weighing ten tons.

Then there was the question of initiating or triggering the bomb. Despite all the concern about pre-detonation, or ‘fizzles’, caused by stray neutrons coming from spontaneous fission in uranium or dislodged by cosmic rays (charged particles that shower the earth’s atmosphere from space), it could not be assumed that simply assembling a super-critical mass would in itself be sufficient to initiate a chain reaction. The right kinds of neutrons had to be available at just the right time to get the initial chain reaction going. The core of the bomb could be shielded from cosmic rays and the problem of pre-detonation caused by neutrons from spontaneous fission could be resolved by using high muzzle velocities.

Serber suggested using small amounts of polonium and beryllium to trigger the chain reaction. Like radium, polonium is radioactive, producing alpha particles which can then go on to liberate neutrons from beryllium, the method that had been used to produce neutrons long before the first cyclotron.
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The idea was to shield the polonium and beryllium from one another until the shy was fired. The blast from the gun would expose the two components of the initiator, producing a burst of neutrons just as the super-critical mass was assembled.

Although the gun method was undoubtedly the simplest, Serber also included alternative, rather more esoteric arrangements for assembling a super-critical mass. These had yet to be properly analysed. ‘For example,’ Serber said, ‘it has been suggested that the pieces might be mounted on a ring … If explosive material were distributed around the ring and fired the pieces would be blown inward to form a sphere.’ Serber’s sketch showed wedges of active material and tamper mounted on a ring. Forcing the ring in on itself would bring the wedges together to form a super-critical mass.

It was during a subsequent lecture by an expert on ordnance that Seth Neddermeyer, a young physicist from the US National Bureau of Standards, made the connection. He raised his hand. He was somewhat vague on the details because this was an area outside his expertise, but he thought that one way of assembling an explosive mass of active material without the need for a gun would be to make use of an
implosion.
His idea was basically to construct a hollow sphere consisting of separate plugs of active material which is then forced together at its centre by conventional explosives packed around the outside. Driving the sphere in on itself in this way would assemble an explosive mass of active material very quickly indeed.

His proposal met with objections from all sides. The most significant of these concerned the need for a near-perfect spherical Shockwave generated by the conventional explosives if the explosive mass of active material was to be assembled properly. Oppenheimer himself was quite critical of the idea. However, he had also been wrong before, about several things. In conversation with Neddermeyer after the lecture, he agreed that this was something that should at least be investigated further. He promptly set up an implosion experimentation group in the Ordnance Division, and appointed Neddermeyer as group leader.

Serber concluded his lectures by outlining the challenge ahead:

From the preceding outline we see that the immediate program is largely concerned with measuring the neutron properties of various materials, and with the ordnance problem. It is also necessary to start now on techniques for direct experimental determination of critical size and time scale, working with large but sub-critical amounts of active material.

Any thoughts about what might happen if this weapon was actually used were pushed to the backs of the physicists’ minds. They focused instead on the immediate challenges, which concerned neutron backgrounds, pre-detonation, critical masses, gun designs and implosive Shockwaves. Many embraced these challenges with great eagerness.

Fermi, for one, was puzzled. The Italian Navigator saw his work on the bomb as a duty born of the necessities of wartime. He told Oppenheimer,
with a note of some surprise in his voice: ‘I believe your people actually
want
to make a bomb.’

Threats to security

The bugged conversation that had taken place in Steve Nelson’s office on 10 October 1942 had stirred the FBI’s interest in Oppenheimer. There followed an inevitable tussle between the FBI and the army’s military intelligence organisation, G-2, over which of them had authority over counter-intelligence related to Oppenheimer and the Rad Lab physicists. The further bugged conversation between Nelson and ‘Joe’ on 29 March 1943, subsequent surveillance of Nelson and the confirmed link to Zarubin promptly ended the tussle. This was clear evidence of Soviet espionage against a secret military weapons programme.

The FBI was advised in outline about the military project now under way. It was agreed that G-2 would focus its attention on Rad Lab staff under contract to the bomb programme, while the FBI would focus on suspected Communists with links to the laboratory. J. Edgar Hoover, director of the FBI, sent a memo on 7 May to Roosevelt aide Harry Hopkins concerning the taped conversation between Nelson and Zarubin.

John Lansdale, who had been appointed head of security for the atomic programme, had by now prepared a counter-intelligence strategy. A graduate of the Virginia Military Institute and Harvard Law School, Lansdale had joined a law firm first in Cleveland, then in Washington, and was a lieutenant colonel in G-2 based in Washington. He had been called in by Conant in February 1942 to investigate potential security lapses at the Rad Lab and was very familiar with the territory.

Lansdale had installed Lieutenant Lyall Johnson in an office on the Berkeley campus. Johnson, a former FBI agent now working in G-2, had proceeded to recruit informants and place agents on the research staff. Lansdale also set up an office across the bay in San Francisco, supervised by Colonel Boris Pash. Pash was a native of San Francisco and a Russian expert. He was the son of Theodore Pashkovsky, a Russian Orthodox priest sent to California by his Church in 1894. The family had been recalled to
Russia in 1912, and Boris had fought in the White Russian navy during the October Revolution. He had returned to America in 1921.

Pash had already seen enough to be convinced that Oppenheimer was a real security threat. On his own initiative, he asked Peer de Silva, a young West Point graduate and Lieutenant in G-2, to begin his own investigation.

Despite the growth in surveillance by both G-2 and the FBI, the identity of ‘Joe’ was uncovered by accident. ‘Joe’ had been glimpsed leaving Nelson’s home by an FBI agent, but this glimpse was insufficient to provide an identification. When in June a commercial photographer by chance took a photograph of four Rad Lab physicists walking arm-in-arm at Sather Gate, one of the entrances to the Berkeley campus, an undercover agent observing them purchased the negative. The ‘Joe’ glimpsed leaving Nelson’s home was recognised in this photograph. He was Joseph Weinberg, hired by Oppenheimer to work on theoretical aspects of calutron design and operation.

The four in the photograph were Weinberg, Giovanni Rossi Lomanitz, David Bohm and Max Friedman. All were students of Oppenheimer. They had different backgrounds but were united by their firm friendship, their work on aspects of the electromagnetic separation method, which had begun the previous summer, and their political activism through organisations such as the Young Communist League. Weinberg had been a member of the American Communist Party since 1938. Bohm had joined the party in November 1942. Lomanitz had established a local chapter of the radical FAECT union at the Rad Lab, and Friedman was its organiser. All four were immediately put under surveillance.

To Pash, Oppenheimer’s links with this group of young radical physicists and his recruitment of Weinberg to the project was yet more evidence of Oppenheimer’s guilt. Pash was preparing to confront him with his allegations when he received news that Oppenheimer was planning an unexpected visit to Berkeley.

As he had been preparing to leave Berkeley for Los Alamos, Oppenheimer had said his farewells to Nelson over a quiet lunch. He had also received
an urgent request for a farewell visit from Jean Tatlock, to which he had chosen not to respond.

Oppenheimer had first met Tatlock at a fund-raiser for Spanish Loyalists in the spring of 1936. She was young, just 22 years old, with an attractive figure, long dark hair, red lips, and hazel-blue eyes beneath heavy lashes. She had studied English literature at Vassar, before turning to psychology at Stanford University in California. Theirs had been an intense, and tempestuous, relationship. They shared interests in literature and psychology, and Tatlock’s heightened social conscience, which had led her to join the American Communist Party in 1933–34, had encouraged similar leanings in Oppenheimer. It was Tatlock who had introduced Oppenheimer to Haakon Chevalier.

Their relationship had collapsed towards the end of 1939. They had come close to marriage several times, but Tatlock could be moody and introspective, and suffered bouts of severe manic depression. She would disappear for weeks and months at a time, only to return with taunts about who she had been with, and what they had done. It was Acock who ended the relationship. Oppenheimer met 29-year-old Katherine (Kitty) Puening Harrison just a few months later.

Tatlock went on to secure her medical degree from Stanford in June 1940. She had then worked as an intern in a psychiatric hospital before becoming a resident physician at San Francisco’s Mount Zion Hospital. She had continued to see Oppenheimer after he had married Kitty, and it is clear that their feelings for each other had remained strong. By the time Oppenheimer left for Los Alamos, Tatlock was a qualified doctor, working as a paediatric psychiatrist at Mount Zion.

Oppenheimer may have felt guilty about not saying goodbye. In June 1943, on the pretext of recruiting a personal assistant, he arranged instead to meet Acock in Berkeley. Pash, his suspicions already firmly raised, arranged for military intelligence agents to follow Oppenheimer’s every move. What they observed would only deepen suspicion.

After dinner at the Xochimilco Café on Broadway, Tatlock drove them to her apartment in San Francisco. The surveillance team noted the intimacy between them. Sitting in a parked car on the street below, they observed
the lights in Tatlock’s apartment go out at 11:30pm. They saw nothing further until both Acock and Oppenheimer emerged from the apartment at 8:30 the next morning. They met for dinner again the following evening. After dinner, Acock drove them to the airport so that Oppenheimer could catch a flight back to New Mexico. Groves had insisted that all laboratory directors travel by means other than air, for fear that an accident could set the programme back many months. Oppenheimer’s dalliance meant that he would now have to break Groves’ rule.

As far as Pash could tell, Acock was undoubtedly suspect, a prime candidate for a Soviet spy with access to the head of the Los Alamos laboratory, whose very existence was a state secret. Two weeks later, Pash drafted a memo to the Pentagon recommending that Oppenheimer be denied security clearance and be removed from the programme. He also wrote to Lansdale, suggesting that if Oppenheimer could not be removed, he should be threatened with the legal consequences of his actions.

But Lansdale’s own appraisal of Oppenheimer was rather less hysterical. He judged that Oppenheimer’s own ambition, driven behind the scenes by Kitty, would guarantee his loyalty to the programme. Lansdale believed that Oppenheimer was sincere. He suggested that Oppenheimer be apprised of the evidence of Soviet espionage against the programme and that he be encouraged to supply names. Groves, who already considered Oppenheimer to be irreplaceable, agreed. He pushed through Oppenheimer’s security clearance on 20 July.

Lomanitz was promoted to the position of group leader in Lawrence’s team on 27 July, with the intention to relocate him to Oak Ridge to supervise the work on electromagnetic separation. Three days later he was told that he was to be drafted instead. He had been fired from the programme. Oppenheimer sprang to his defence, but was advised by Lansdale that Lomanitz was a lost cause. In discussion, Oppenheimer mentioned his anger at Lomanitz’s activism at the Rad Lab. He insisted that there was a direct conflict between loyalty to the Communist cause and loyalty to the atomic bomb programme, and America. He therefore wanted to ensure that there were no Communist Party members working on the
programme. Lansdale judged that whatever connections Oppenheimer had had with the party were now firmly in the past.