The First War of Physics by Jim Baggott

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 decaywithin 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.