The First War of Physics by Jim Baggott Page A

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 hadbeen 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