The First War of Physics (7 page)

With the north of the country secure, German forces launched south into France on 5 June. Italy declared war on Britain and France on 10 June, Paris fell on 14 June and the French government fled to Bordeaux. French resistance quickly collapsed, and the government signed an armistice with Germany on 22 June. This was signed at Compiègne in the same railway car, and in the same forest, as the armistice of 1918.

The Soviet Union had signed a non-aggression pact with the Nazis in August 1939, and had invaded Finland in November that year. With the fall of France, only Britain, Greece, the Commonwealth and the exiled forces of European Allies stood between Germany and the conquest of all of Europe.

Union Minière in Belgium had thus far fulfilled orders received from Germany for about a ton of refined uranium compounds a month. Now under German occupation it received an order from the Auer company for 60 tons.

Uranverein physicists hastened to Joliot-Curie’s laboratory in occupied Paris towards the end of June. Bothe was first to visit, followed by Schumann and Diebner. All but Joliot-Curie himself had fled. With his co-operation, Diebner assimilated the results of the work of the French nuclear physicists and arranged for the completion of the assembly of the cyclotron that they had begun.

Joliot-Curie could not hide the fact that he had accepted deliveries of uranium ore from Belgium and heavy water from the Vemork plant in Norway. When the Uranverein physicists demanded to know where these materials were, he simply stated that the uranium ore had disappeared ‘south’ along with the French government (it had in fact gone to Algeria) and that the heavy water had been loaded onto a ship known to have been sunk (it had actually gone to Britain, along with Joliot-Curie’s colleagues Halban and Kowarski).

Element 93

In his second report to the German War Office Heisenberg had been reticent on the subject of a bomb. His reasons are unclear. It may be that, although Harteck had begun construction in Hamburg of a large-scale Clusius-Dickel apparatus to separate U-235, and had reasons to be optimistic, separation on the scale required for a bomb still appeared incredibly daunting.

The key question was one of scale: precisely how much U-235 would actually be required? There is no evidence in the historical record for this period (to spring 1940) of any formal calculation to determine the quantity of U-235 that would be required for a bomb. If Heisenberg or any other Uranverein physicist had made such a calculation at this time, it did not survive. It is possible that no such calculation had been carried out. For whatever reason, the possibility of a bomb based on ‘almost pure’ U-235 was not pursued.

A second route to an explosive device was potentially available in the form of an unstable reactor based on uranium enriched with U-235, a reactor on the edge of a runaway chain reaction. Calculations by one of Heisenberg’s Uranverein co-workers suggested that such a ‘reactor-bomb’ would need to contain 70 per cent more U-235 than U-238. It was, of course, very difficult to imagine how such a reactor-bomb might be delivered to its target. And enrichment on the scale required for a reactor-bomb still appeared beyond the bounds of possibility in any timeframe likely to affect the course of the war.

But then a completely unanticipated third avenue appeared. Heisenberg’s close friend and Uranverein colleague Weizsäcker would pass time on Berlin’s underground railway reading papers on nuclear fission which were still being published in American scientific journals, oblivious to the suspicious glances of his fellow commuters.

Hahn’s group in Berlin had found that U-239, formed from U-238 by the capture of a neutron, is unstable and undergoes radioactive decay
within about 23 minutes. It was believed that emission of a beta particle
³
from U-239, which turns a neutron into a positively-charged proton, would transmute the uranium nucleus, characterised by its 92 protons, into a new element with 93 protons. Hahn thought this element might be chemically similar to the element rhenium and had called it eka-rhenium, or
eka re.
Weizsäcker suspected that this new element might be fissionable, just like U-235.

On the surface this proposal seems innocent enough. But it is far from innocent. Unlike U-235, element 93 does not occur in nature and is
chemically
distinct from uranium. Weizsäcker realised that it would be possible to separate element 93 from uranium by chemical means. In essence, he was suggesting that if element 93 could be produced in a uranium reactor in significant quantities, it could be separated relatively easily and used to make a fission bomb.

That element 93 could indeed be produced by bombarding U-238 with neutrons was demonstrated by American physicists Edwin McMillan and Philip Abelson at the Radiation Laboratory in Berkeley. But they also noted that this element was relatively unstable, decaying within a matter of days. Astonishingly, they published their results in the open scientific literature in June 1940. Here was concrete proof of the practical feasibility of using a uranium reactor to produce fissionable material for a bomb. In July 1940 Weizsäcker wrote a paper for the Army Weapons Research Bureau in which he enthusiastically recommended that this possibility be pursued.

It was now clear to the Uranverein that the construction of a bomb depended on first solving the problems related to the construction of a reactor. There remained the question of the most appropriate moderator. Some initial measurements of the rate of absorption of neutrons by graphite were reported in a confidential paper first issued by the Heidelberg group in June 1940. The results were rather inconclusive. It seemed that the rate was too high for graphite to be used successfully as a moderator, although it was conceded that part of the problem lay in the homogeneity and purity of the graphite used. At this stage Bothe was reasonably confident that further tests with purer samples would demonstrate the potential of graphite as a moderator.

The Virus House

The War Office takeover of the Kaiser Wilhelm Institute for Physics created great difficulties for its director, the esteemed Dutch physicist Pieter Debye. The German authorities presented Debye with an ultimatum: take German nationality and continue as director or take a temporary leave of absence. Debye refused to cede his Dutch nationality. He left Germany in January 1940 and embarked on a lecture tour of America. He never returned.

Debye’s departure left the directorship open. Schumann favoured Diebner, but Diebner’s appointment was resisted by the Kaiser Wilhelm Foundation. Weizsäcker and fellow Uranverein physicist Karl Wirtz, who expressed concern that they now ‘had Nazis in the institute’, conspired to bring Heisenberg to Berlin. Diebner was appointed as interim director, and Heisenberg agreed to travel once a week from Leipzig.

Heisenberg now had considerable influence over the work of the theoretical group, the reactor experiments that were being established in Berlin, and the reactor experiments that he himself was setting up in collaboration with his colleague Robert Döpel in Leipzig. Heisenberg was not director of the uranium research project, but he was running a substantial part of the show.

The German atomic programme was not a coherent research effort driven relentlessly by the demands of war. Rather, it was a loose association of rival research teams that would sometimes squabble over supplies of uranium and heavy water.

But for those able to read them, the signs were ominous.

The Uranverein physicists now had access to thousands of tons of refined uranium. They were building their first cyclotron in Joliot-Curie’s captured Paris laboratory. They had the promise of substantial quantities of heavy water. Separation of U-235 was proving to be as difficult as had
been anticipated, but some of the greatest minds in chemical and physical science were being applied to the search for a solution.

In July 1940 work was begun on a new building to house an experimental nuclear reactor at the Kaiser Wilhelm Institute for Biology and Virus Research, next door to the Institute of Physics in Berlin. To limit unwanted attention, the building was called the
Virus House.

¹
Grandson of the composer Robert Schumann.

²
A hydrogen nucleus consists of a single proton. The heavier deuterium nucleus consists of one proton and one neutron.

³
A beta particle is a fast-moving electron ejected directly from a neutron inside the nucleus during beta radioactive decay. During this process the neutron is transformed into a proton.

Chapter 2

ELEMENT 94

September 1939–September 1940

M
uch to Leo Szilard’s frustration, Einstein’s letter to Roosevelt was slow to have any kind of impact. The letter had been drafted in early August 1939 but as the days and weeks passed he heard nothing from Sachs. In the meantime, war in Europe had begun.

When Szilard and Wigner visited Sachs towards the end of September, they discovered to their dismay that Sachs still had the letter in his possession. He had tried repeatedly to gain an audience with Roosevelt to discuss the matter, but had not so far managed to get past Roosevelt’s secretary.

Sachs finally gained access to Roosevelt in the Oval Office on 11 October. He prepared the ground with a parable about Napoleon, and this prompted Roosevelt to ask for a carafe of Napoleon brandy and a couple of glasses. As Sachs sipped brandy with Roosevelt he tried to present a verbal summary of the content of Einstein’s letter. But Roosevelt appeared distracted and inattentive, and asked if Sachs could return the next day. Fearing he had blown his chance, he returned next morning with some trepidation. But this time Roosevelt was ready and willing to listen.

Using his own 800-word précis of Einstein’s letter, Sachs chose to emphasise the peaceful uses of nuclear power, mentioning last of all the threat of ‘bombs of hitherto unenvisaged potency and scope’. He concluded
with the observation that we ‘can only hope that [man] will not use [subatomic energy] exclusively in blowing up his next door neighbour’.

Roosevelt got the message. ‘Alex,’ he said, ‘what you are after is to see that the Nazis don’t blow us up.’ He called for immediate action, and responded to Einstein’s letter a week later.

It was quickly agreed that the administration would establish an Advisory Committee on Uranium to be headed by Lyman J. Briggs, director of the US National Bureau of Standards. The committee consisted of nuclear physicists and ordnance experts from the US Army and Navy. To Szilard and his fellow Hungarian conspirators, it looked as though something was finally going to happen.

The first meeting of the Advisory Committee took place on 21 October in Washington. Szilard and Wigner held a pre-meeting with Sachs at the Carlton Hotel to discuss tactics, before joining Teller and other members of the Advisory Committee at the Bureau of Standards offices in the Department of Commerce. Einstein had been invited to attend, but declined.

Szilard explained the scientific background and the importance of putting the theory of nuclear chain reactions to the test in large-scale reactor experiments, which he proposed should be constructed from uranium oxide and graphite. These were experiments he had been trying, but failing, to set up with Fermi at Columbia University since July. The ordnance experts were openly sceptical of the physicists’ claims. The destructive potential of an atomic bomb was simply way beyond their reckoning. It takes two wars, Lieutenant Colonel Keith Adamson declared, before one can know if a new weapon is any good or not.

The physicists themselves were relatively ill-prepared. When asked directly how much was needed from Treasury funds to start work on Szilard’s proposed experiments, they were at a loss for a reasoned answer. Teller leapt forward with a request. He asked for just $6,000. ‘My friends blamed me,’ he later said, ‘because the great enterprise of nuclear energy was to start with such a pittance: they haven’t forgiven me yet.’

After the meeting, Szilard – who would shortly estimate that they needed at least $33,000 for the graphite alone – nearly murdered Teller for the modesty of his impromptu request.