Hitler's Terror Weapons

by Brooks, Geoffrey

  Since 1934 Otto Hahn, Lise Meitner and Fritz Straßmann had been subjecting uranium to neutron bombardment in experiments at the Kaiser Wilhelm Institute for Chemistry in Berlin-Dahlem. In an article appearing in the scientific journal Die Wissenschaften of 6 January 1939,31 Hahn and Straßmann announced that they had demonstrated nuclear fission in uranium. When a U235 isotope of uranium was struck by a neutron, U235 +1n = U236, a new unstable compound, was formed and split up almost instantaneously. As this occurred, the highly charged fragments repelled each other with a violent kinetic energy which was also very radioactive.

  Of this discovery Heisenberg’s former pupil Carl Friedrich Freiherr von Weizsäcker wrote:

  “I recall that for a week in February 1939 or maybe at the beginning of March I thought through the technical possibilities of atom bombs and atomic engines which all physicists had to get to know about, and discussing theoretically with a close circle of friends the major political consequences of the discovery.”32

  A chain reaction was still not absolutely certain until there appeared in the 22 April 1939 edition of the scientific periodical Nature the findings of three experimental physicists at the College de France, Joliot, von Halban and Kowarski, reporting that at least two neutrons eject during fission, followed in the next few minutes by a small supplementary number from the decaying fragments of the atom. As the collision between one neutron and a U235 nucleus brought about the creation of more than two fresh neutrons, it would probably be possible to arrange for the surplus neutrons to cause a chain reaction.

  Two days after publication of this article the Heereswqffenamt 33 in Berlin received the first letters from scientific institutes and universities pointing out that:

  “the newest developments in nuclear physics which will probably make it possible to produce an explosive many orders of magnitude more powerful than conventional ones: that country which first makes use of [nuclear fission] has an unsurpassable advantage over the others.”

  Professor Heisenberg Explains his Stance

  In the spring of 1939 Heisenberg made a two-month lecture tour in the United States. By now he had decided not to defect on the grounds that he would almost certainly be co-opted to build the atomic bomb which, if ready in time, was likely to be dropped on Germany. Another reason was that he thought it would be difficult to campaign to rebut Aryan Physics as an expatriate. He also felt the need to explain why he wanted to remain in Germany in the coming war, believing that friendships could outlast political differences between nations. His Italian colleague Fermi was at least able to express understanding for his decision while not agreeing with it. When Fermi suggested that Heisenberg should defect he was told:

  “History teached us that sooner or later, every century is shaken by revolutions and wars, and whole populations obviously cannot emigrate every time there is a threat of an upheaval. People must learn to prevent catastrophes, not to run away from them. I have decided to stay in Germany, even if my decision is wrong”34

  and in an interview with Robert Jungk Heisenberg explained:

  “Under a dictatorship, active resistance can only be practised by those who pretend to collaborate with the regime…. I have always been very much ashamed when I think of the people, some of them friends of my own, who sacrificed their lives on 20 July 1944 and thereby put up a really serious resistance to the regime. But even their example shows that effective resistance can only come from those who pretend to collaborate.”35

  Heisenberg was a patriot and a cultural imperialist of the old school who was rooted in Germany and had no desire to be anywhere else. In an interview immediately after the war with Professor Samuel Goudsmit, head of the Alsos US Scientific Mission to Europe, he stated that in his opinion physicists in the Reich had, on the whole, done only the work necessary to preserve their university positions and hold together what remained of the great German tradition in physics. His own small circle had dominated the uranium project and steered the research away from the production of nuclear weapons. To him the war was an interlude. Emphasizing that Germany had not constructed atomic weapons, he made a public ritual of contrasting the moral character of German scientists, who had deliberately obstructed the research, with that of Allied scientists and politicians who had not only built those weapons, but also used them. It is clear that Goudsmit did not accept this explanation and remained convinced that Heisenberg had been in some way involved in a German atom bomb project.

  This book concludes that Professor Goudsmit was right. The fact of the matter seems to be that somebody in Germany designed an ‘atomic-type’ weapon, somebody then built it, a scientist described how he had seen it tested and eighty lead cases containing enough material for two more of these small atom bombs turned up on a Tokyo-bound U-boat when searched after being surrendered to the US Navy at the end of the war. Heisenberg directed the German uranium project, but as regards an atomic-type bomb he was quite certain that he had not designed it and nor could he personally have been involved in the experimental foundation work for it in any way whatsoever because there was no such thing.

  As a means of correcting Press inaccuracies following the announcement of the American atomic attack on the city of Hiroshima, the contingent of German atom scientists confined at Farm Hall, Cambridgeshire, England, issued a memorandum on 7 August 1945 presenting an outline in brief of the official German Uranium Project. The document was drafted by Heisenberg, Dr Karl Wirtz, his experimental assistant, and Professor Walter Gerlach, last Plenipotentiary for Nuclear Science, after consultation with Professors Diebner, Hahn, von Weizsäcker, von Laue, Korsching, Harteck and Bagge. Three scientists present abstained from signing.

  The second paragraph alone alluded to the philosophy of developing nuclear weapons, asserting in a single sentence that “it did not appear feasible at the time to produce a bomb with the technical possibilities available in Germany”. There was an explanatory footnote:

  “As to the question of the atom bomb, the undersigned confirm that they have no knowledge of any other group in Germany which had the production of the bomb as an immediate goal. However, if such an attempt was in fact undertaken, then it was made by dilettantes, and should not be taken seriously.”

  If the interned group of physicists had no knowledge of a bomb project there was no need to add the final sentence. Ultimately all that is denied is the existence of a group working towards the atom bomb as an immediate goal. If we suppose for a moment that there was a group which had been working on an experiment whose methodology and materials could be adapted if so desired to create a small atomic bomb, then that group would fall outside the German physicists’ denial. Heisenberg was very careful with words. When he took up his pen to draft this crafty document all nine of his colleagues knew exactly which group of ‘amateurs’ they had in mind. As Baron Manfred von Ardenne had defected to the Russians and was building their nuclear weapons for them, they all thought it was a very good idea to point the finger in his direction early on just in case any awkward questions were asked about who built a small-scale German atom bomb during the period 1941–1944.

  CHAPTER 3

  Heisenberg’s Pioneering Paper

  AS A CONSCRIPTED reservist, Professor Heisenberg had served . two months in the previous two summers with the Gebirgsjäger. Obeying a mobilization order late on 25 September 1939, he travelled by train from Urfeld to Berlin and reported to Hardenbergstrasse 12 at ten next morning. There he was informed that his call-up telegram had been a deception and that he should attend next door for a conference of nuclear physicists considering the possible applications of nuclear energy.

  The Heereswaffenamt had seized the Kaiser Wilhelm Institute (KWI) for Physics at Berlin-Dahlem as the scientific centre for its uranium research project. Professor Erich Schumann was its coordinating head and he had appointed Dr Kurt Diebner, a second-rank scientist engaged on conventional explosives research, to direct it. The aim was to concentrate and coordinate at Berlin-Dahlem the secret activitie
s of the Uranium Project. In Diebner’s appointment there lay the danger that the project was vulnerable to penetration by political functionaries, as had happened elsewhere. Diebner saw the weakness of his position and agreed that the survival of the Institute depended on it having an authoritative Director. Accordingly he consented to Professor Heisenberg being invited to join the Institute as scientific adviser, travelling from Leipzig University to Berlin once a week. On these visits Heisenberg could intervene to forestall undesirable developments in research in Berlin and elsewhere36.

  Professor Schumann’s address to the group emphasized the defence aspects of the enquiry. The German Reich being at war, it was of the greatest importance that Germany should be forewarned of all possible eventualities; this was the purpose of the technical appraisal they would be undertaking. Even a negative conclusion was valuable, for the military could then be reassured that no unpleasant surprises were in store.

  The experimentalists were commissioned to undertake a variety of materials measurements in specified areas of research, while Heisenberg was given the written task:

  “to consider whether, under the known circumstances of the characteristics of fission processes in uranium, a chain reaction is at all possible, and if so please commit your ideas to paper.”37

  Whereas the majority of physicists were willing to affiliate with the Heereswaffenamt group, a large number was not prepared to relocate under one roof at Berlin-Dahlem, and thus from its inception the programme was structured with a Headquarters and three provincial satellites at Leipzig, Heidelberg and Hamburg. From about 1942 onwards there were other groups in Czechoslovakia, Germany and Austria, mainly SS who kept themselves aloof from the professors, but as early as 1941 the Reich Post Office nuclear project in Berlin had equipment for nuclear research including high voltage installations and a cyclotron.

  Heisenberg completed his assignment within two months, and on 6 December 1939 he submitted his findings in the first of two pioneering papers, G-39 The Possibility of Obtaining Energy from Fission in Uranium38.

  Throughout the Second World War the spectre of a German atom bomb haunted many people, but from the outset Heisenberg made no attempt to disguise the possibility that one could be built.

  “If a chain reaction is possible, then the bomb is possible. Its intensity would depend on the rate of liberation of energy before the chain reaction collapsed,”

  he advised the Heereswaffenamt in his paper. As part of the deliberate process to denigrate the German nuclear project, various historians have stated that the Germans appear not to have considered the question of the fast fission chain reaction. The official British UKAEA historian Margaret Gowing39 added for good measure that the critical size of the U235 bomb appeared not to have been investigated either. Piecemeal transcripts of the German physicists’ secretly tape-recorded private conversations in internment at Farm Hall, England, in 1945 were included by General Leslie Groves, head of the Manhattan Project, in his book40 published in 1962. Following the release of the full documentation by the London Public Record Office in February 1992, it became apparent that Groves had lied when reporting what was supposed to have been said in these conversations. Groves stated that the transcripts proved how Heisenberg

  “had not thought of using the bomb designs we had used: ours took advantage of fast neutrons … the Germans thought they would have to drop a whole reactor.”

  In fact Heisenberg was recorded in conversation as saying in 1945:

  “I knew it could be done with U235 using fast neutrons. That’s why U235 alone can be used as an explosive. They can never make an explosive with slow neutrons, not even with the heavy water reactor.”

  The purpose of Groves’ lie was to pervert history by proving falsely that there never could have been a German atom bomb because the top scientist did not know the principle. This had to be done because a small atom bomb actually had been built and tested by Germany.

  In his pioneering paper of December 1939 Heisenberg spoke of “enrichment”. Natural uranium consists essentially of two isotopes. 99.3% of the material is U238. This isotope captures free neutrons in the uranium, and this is why natural uranium cannot explode. The ‘fissile’ isotope U235 exists in natural uranium in the proportion of 0.7%. If the ratios can be changed, and the major isotope physically reduced in the material, then neutrons will be more free to act on the U235 atoms. If the material is sufficiently rich in U235 atoms, say above 50%, then it can be arranged for an explosive chain reaction to occur, although even 7% will be sufficient for an explosion of some sort.

  In his report Heisenberg explained:

  “An increase in temperature results from enriching the U235 isotope. If the U235 were to be enriched sufficiently to obtain a temperature corresponding to a neutron energy of 3.5 million degrees C, … the mass for the release of all available fissile atoms at once, would be: R=10∏ l cms = 31.41 cms.

  “This explosive transformation of the U235 atoms can only occur in almost pure U235, because the capture bands of the U238 isotopes, even when present in reduced quantities, still absorb the neutrons.”

  This is not a formal expression of critical size, but merely a general statement for discussion based on the idea that a fast fission atom bomb is possible provided one can obtain huge amounts of the U235 isotope. In Heisenberg’s equation, the unknown element is l, the diffusion length or mean free path. This term is the mean distance travelled by a neutron between release from an atom at fission to absorption and fission in another U235 atom. It can be a variable: the American physicists, when first putting their minds to the problem, came up with estimates of critical mass ranging from Feyman’s 50 kilos to Robert Oppenheimer’s 100 kilos, and the Americans had far better tools at their disposal for making the measurements than Heisenberg.

  The Implosion Method

  In the implosion method of detonating an atom bomb, the bomb core is a subcritical mass surrounded by a uniform layer of high explosive. When the HE detonates, a massive uniform pressure of millions of pounds p.s.i. is created which compresses the core material to a supercritical density, thus causing an implosion. Obviously, since the fissile material is compressed into a much smaller volume, the diffusion length is much reduced. If it were to be reduced by a factor of three, i.e. uniformly compressed by the explosion to a third of its original volume, then the critical mass required for implosion is smaller by the power of nine. This might have put U235 within Germany’s capabilities with a stupendous investment and the help of the electrical giants. The diffusion length is a variable depending on the quality of material and other factors. The more efficient the implosive force for the compression, the less U235 bomb material needed.

  A crucial question is whether Germany had an effective implosion fuse in 1939. It is claimed that they were close to it, and by 1941 had made such advances in the technology of implosion fuses that they were already working on an ultra-violet type. Sources allege that an efficient implosion fuse which could set off all detonators around the bomb sphere at the speed of light was invented by Prof. (Ing) Friedrich Lachner in Vienna pre-war and a model exhibited at the Wiener Technische Hochschule (University) to a gathering of home and foreign physicists at a lecture evening. Professor Lachner was a colleague of Professor Adolf Smekal and had obtained his inspiration for the work after hearing one of Smekal’s talks. Later Lachner joined Professor Stetter’s team of SS physicists at Innsbruck. This group was studying the possibilities of the plutonium bomb and allegedly the implosion fuse was perfected in the course of the work.

  Knowledge of the existence of this fuse in 1939, if only at the design stage, would explain Heisenberg’s persisting concern about the U235enrichment question, since once the fuse had been developed it cut the bomb material required to a few kilos of U235. If explosives technologists could improve on a fourfold compression factor the amount of material would be reduced by substantially more.

  Professor Carl-Friedrich von Weizsäcker explained:

  “C
ertainly we made no attempt to build a bomb. This decision was made easy because we recognized the impossibility of manufacturing a bomb in Germany under war conditions. If people now say that we set out to avoid or obstruct the building of a bomb that is a dramatization, since we knew that we were not in a position to do so.”41

  That is what we are supposed to believe. The more important aspect of Heisenberg’s G-39 paper, however, concentrated on what was to occupy him for the rest of the war, the atomic pile that never was and the saga of the heavy water moderator.

  Reactor Theory

  It has to be stressed that uranium work did not lie at the centre of Heisenberg’s interests. When confronted with the assignment, he succeeded in familiarizing himself with the semi-technological field and during the war came to be regarded as the leading expert in the German Uranium Project. If, as he seems to have claimed, he was proposing, initially at least, to be a saboteur of Nazi nuclear science, Heisenberg was in a unique position to direct the uranium work along a false path, since, as the acknowledged senior theoretical physicist, he had been entrusted with the task of formulating the theory from the outset and, having set the guidelines, continued to influence the experimental side of matters until Germany’s final surrender. At the end of 1939, having been cut off from most foreign literature since the outbreak of war, he was entering new territory in attempting to establish a theory for the working uranium pile or Uranbrenner. His summaries in the two papers dated 6 December 1939 and 29 February 1940 respectively are still accepted today as completely correct, or at least in so far as what they actually say. But there is a material omission in these two reports. One cannot at this late stage discount the possibility of an error in his theoretical workings, but all along one has the impression that Heisenberg did not want a working nuclear reactor, and it does not seem unreasonable to suspect that he deliberately drew a false conclusion on which he was to rely later.