The First War of Physics (8 page)

Despite the paltry nature of the sum that had been mentioned, Adamson bridled. ‘Gentlemen,’ he berated them, ‘armaments are not what decides war and makes history. Wars are won through the morale of the civilian population.’ Wigner, normally polite and formal in his dealings with colleagues, became angry and spoke up for the first time in the meeting. ‘If that is true,’ he declared in his high-pitched voice, ‘then perhaps we should cut the Army budget thirty per cent and spread that wonderful morale through the civilian population.’

Adamson visibly flushed, and muttered that the physicists would get their money.

Szilard drafted a blueprint for the American uranium research project and mailed this to Briggs five days after the committee’s first meeting. In it he suggested which experiments should be conducted and identified the American laboratories that should be involved. He also urged that all future research reports be subject to the strictest secrecy and withheld from publication in the open scientific literature.

But the Advisory Committee lacked resolve. It reported back to Roosevelt on 1 November a commitment to explore controlled chain reactions in uranium as a potential power source for submarines which, if the reaction turned out to be explosive, could be further explored as a source of highly destructive bombs. It agreed to supply four tons of purified graphite to support Fermi and Szilard’s experiments, to be followed by 50 tons of uranium oxide, if this could be subsequently justified.

Briggs was respected but obsessed with secrecy and dogged by poor health. He was unable to imbue the committee or its sponsors with any real sense of urgency. The war in Europe was, after all, a long way away. What’s more, he was reluctant to commit large sums of money to the project. The money that had been promised at the 21 October meeting was not quickly forthcoming.

Szilard might have been elated by the fact that the importance of uranium fission had now been recognised, but this gave way to more
frustration as the first months of 1940 unfolded. He was still without formal employment, and uncertain how long his loose affiliation with Columbia University could be maintained. He was not in a position to repay the $2,000 he had borrowed to carry out experiments to verify the production of secondary neutrons and was obliged to go back to his sponsor to declare this a bad debt.

He heard nothing from Briggs.

Zeal for secrecy

News that a secret German research project on nuclear fission had begun at the Kaiser Wilhelm Institute in Berlin reached America in January 1940 through Pieter Debye, recently expelled from his position at the Institute and now on extended ‘leave of absence’. Debye played down the significance of the project. The Uranverein physicists were very well aware of the German army’s objectives but considered success ‘improbable’, he claimed. In the meantime the German physicists had a splendid opportunity to carry out fundamental research at the army’s expense. On the whole, Debye was inclined to consider the situation a good joke on the German army.

Debye visited Fermi at Columbia University shortly after arriving in America. Fermi too, it seemed, was unconcerned by Debye’s news. The Uranverein physicists were working at laboratories all over Germany, he observed, and would not be able to make any kind of concerted effort towards a bomb.

But the news had precisely the opposite effect on Szilard. He had spent the previous weeks working on a couple of theoretical papers on self-sustaining nuclear chain reactions,
¹
work which no doubt convinced him that a nuclear explosive of some kind was now inevitable. The existence of a German fission project greatly alarmed him. He discussed the matter with Einstein at Princeton, and together they decided to draft another letter, this time to Sachs.

In this letter they emphasised that interest in uranium had intensified in Germany since the outbreak of war, that nuclear research had been taken over by the German government and was being conducted in great secrecy. The implications were reasonably clear: whether they liked it or not, they were now locked in a race with the Nazis to build an atomic bomb. The letter also contained a threat: unless there was a change of policy, Szilard would publish his latest research on nuclear chain reactions in the open literature.

The letter was sent to Sachs on 7 March 1940. A week later, Sachs wrote of these new developments to Roosevelt, who called for a further meeting of the Advisory Committee. Progress was still painfully slow: the meeting was not scheduled until 27 April. Einstein was again invited, but again declined. At least the further letter to Roosevelt prompted the release of the $6,000 that had been promised.

By the time the meeting was held, Alfred Nier at the University of Minnesota and John Dunning at Columbia had gathered experimental evidence confirming that U-235 is indeed responsible for slow-neutron fission in uranium, vindicating Bohr and Wheeler’s original hypothesis. They had used tiny quantities of U-235 and U-238 obtained from uranium compounds of chlorine and bromine. They went on to conclude that a fission chain-reaction would not be possible without separation of U-235.

The opinion of the Advisory Committee was split. Briggs expressed doubts that a chain reaction would be possible in natural uranium. Sachs urged that they should nevertheless move ahead with experiments on the uranium–graphite reactor that Szilard had proposed. All agreed that they should wait for the results of measurements on neutron absorption by graphite.

The funds were transferred to Columbia University and used to purchase a quantity of purified graphite. Szilard had been careful to specify high levels of purity. At lunch with representatives of the National Carbon Company, Szilard had probed for details about likely impurities in commercially-available graphite. He specifically mentioned potential contaminants that would absorb neutrons and render meaningless any
attempts to measure neutron absorption by graphite itself. Half-jokingly, he said: ‘You wouldn’t put boron into your graphite, or would you?’

His visitors looked at each other in embarrassed silence. One of the principal uses of graphite is in the manufacture of electrodes for electric arcs, and boron is typically a component in the manufacturing process. Any graphite they supplied would therefore likely be contaminated. They agreed to supply a quantity of graphite manufactured using different methods, without the use of boron.

Four tons of graphite duly arrived at the Columbia laboratory in the form of carefully-wrapped bricks. The simple process of unwrapping and stacking the bricks in a neat pile was enough to give the researchers the appearance of coal miners. The results of the neutron absorption measurements were, however, strongly positive: graphite could indeed be used satisfactorily as a moderator. The idea of a nuclear reactor in the form of a uranium–graphite ‘pile’ took a critically important step towards becoming a reality.
²

Szilard urged Fermi not to publish the results of these experiments. The relationship between the two had to this point been quite tense, but now it reached breaking point. They were two quite different personalities. Szilard was a loner, always ready to challenge conventional wisdom and norms of behaviour, sometimes outrageously. Fermi was an out-and-out scientist, much more collaborative and polite, caring little for the world outside the domain of science. Szilard’s experiences had led him to be extremely wary of the world outside science, and he fervently believed that scientists had a duty to behave responsibly in matters likely to have a significant impact on this world. ‘Fermi and I had disagreed from the very start of our collaboration about every issue that involved not science but principles of action in the face of the approaching war’, he later wrote.

Szilard could also be intensely irritating, and Fermi now lost his temper. He thought Szilard’s zeal for secrecy absurd, but eventually relented under pressure. The results were not published.

Super-cyclotron

Ernest Lawrence was a visionary. The inventor of the cyclotron was a rather atypical physicist. A blond, blue-eyed Midwesterner of Norwegian parentage, he carried the values of his Lutheran upbringing into adulthood, and into his science. His preference for smart suits and his magisterial manner lent him an appearance that was more businessman than scientist. And, in truth, managing the kind of scientific enterprise that he was keen to establish at Berkeley’s Radiation Laboratory – or ‘Rad Lab’ – demanded a much more businesslike approach. His teenage experiences as a kitchenware salesman had given him the necessary selling skills, and had taught him the rudiments of fund-raising.

Lawrence had invented the cyclotron in 1929. Use a magnet to confine a stream of protons to move in a circle while accelerating them to higher and higher speeds using an alternating electric field and, Lawrence had figured, you had a machine for penetrating the secrets of the atomic nucleus. He built a small demonstration model for just $25. It was four inches in diameter and covered in red sealing wax. Although it didn’t quite deliver the proton energies that Lawrence claimed it should, it was enough to impress his scientific colleagues and prove the principle. However, the machine’s scientific name, the
magnetic resonance accelerator
, was too abstract and clumsy.
Cyclotron
sounded much more futuristic, and therefore much more appealing to potential sponsors.

He was already thinking on a larger scale, and there quickly followed a succession of such machines. A cyclotron containing a magnet with an eleven-inch diameter pole face delivered proton energies of over a million electron volts. This was followed by a 27-inch machine, which then quickly became a 37-inch cyclotron. When news of the discovery of nuclear fission in uranium reached Berkeley in January 1939, Lawrence was planning
a 60-inch cyclotron that would deliver proton energies of the order of 20 million electron volts. It would need a magnet weighing 200 tons.

The 60-inch machine was barely operational at the Rad Lab’s Crocker Laboratory before Lawrence was busy designing the next one. This was to be a gargantuan 120-inch super-cyclotron with a magnet weighing 2,000 tons. Lawrence estimated that this would deliver proton energies of 100 million electron volts, on the threshold of nuclear-scale energies. Lawrence approached the Rockefeller Foundation with requests for support. His pitch was greatly strengthened when, in the middle of a game at the Berkeley Tennis Club on 9 November, he was informed that he had just won the 1939 Nobel prize for physics.

Suitably emboldened, as Christmas approached Lawrence escalated the scale of the super-cyclotron even further, to include a magnet with 184inch pole faces (the largest diameter of commercially-available steel plate), weighing 5,000 tons. It would cost an estimated $1.5 million to build.

The outbreak of war in Europe in September had an immediate personal impact on Lawrence – after several days of anxious waiting he heard that his brother John had survived the sinking of the
Athenia
by a German submarine on 2 September. But life at the Rad Lab continued pretty much as normal. There were interesting experiments to be performed on uranium using the 60-inch cyclotron, but this was work that would have been carried out irrespective of the war. There was no sense yet that the Rad Lab was in any way involved in ‘war work’.

A photograph from around this time shows the Rad Lab faculty, gathered in three rows beneath the magnet of the 60-inch cyclotron. Lawrence is sitting in the centre of the front row. Oppenheimer is standing in the centre of the back row. At the extreme right of the first and second rows are two Rad Lab physicists who were now busy at work on the uranium problem – Edwin McMillan and Philip Abelson.

McMillan, a native Californian, had worked on Lawrence’s cyclotrons for many years, and when the discovery of fission had been announced he had devised some simple experiments to confirm the phenomenon. He had now become intrigued by some of the discovery’s more subtle aspects. Bombarding uranium with neutrons produced a radioactive substance
which decayed in a characteristic time of about 23 minutes. Like Hahn, Strassman and Meitner, McMillan surmised that this substance was U-239, formed by the resonant capture of a neutron by the predominant isotope, U-238. But there was another radioactive substance produced, which had a characteristic decay time of about two days.

He believed this second substance to be a new element, formed by emission of a beta particle from U-239, in the process turning a neutron into a proton. Just as Weizsäcker had done, sitting on the Berlin underground railway, so McMillan had reasoned that this was element 93, perhaps the first in a series of transuranic elements. And, just as Hahn had done, McMillan further surmised that element 93 might behave somewhat like the element rhenium.

With the help of a Berkeley research associate, Emilio Segrè, who had worked previously with Fermi in Rome, McMillan tried to obtain evidence of rhenium-like chemical properties. But they could find nothing of the sort. It seemed that, after all, the transuranics would continue to remain elusive. Segrè published the results in
Physical Review
, as an ‘unsuccessful search for transuranic elements’.

McMillan had now refined the measurement of the decay time of this second mysterious substance to 2.3 days and became determined to identify precisely what it was. In the spring of 1940 he used the 60-inch cyclotron to investigate it further, and was joined in the quest by Abelson, who had by this time moved to the Carnegie Institution in Washington but had returned to Berkeley in April for a working vacation. Abelson had studied chemistry as well as physics and turned his attention to the chemical identification of the mysterious substance.

It turned out to have properties not so very different from uranium itself. Bohr had in fact already suggested some time before that the transuranics – if they existed – might behave chemically more like uranium. Further work demonstrated unambiguously that the substance with the 2.3-day decay time was formed directly from U-239, with its characteristic 23-minute decay time. There was only one conclusion: the second substance was element 93.