Hitler's Terror Weapons

by Brooks, Geoffrey

  Many millions of words have been written by learned historians in an attempt to make some sense of the German uranium project during the Second World War. Thousands of documents have been inspected, but the story lacks coherency. The muddle over graphite is a good example. Many commentators have attempted to show that the German reactor project failed because of an error in materials measurement prejudicial to the use of graphite as a moderator. If the Germans had had graphite instead of heavy water, so their argument goes, then they would have had a critical reactor in 1941 and plutonium and a bomb, and so on. The evidence does not support this line of reasoning because in 1944 there was enough heavy water available for a working reactor but no attempt was made to reach the critical point. Moreover, to build and run such a reactor would have required a tremendous effort, and it would not have proceeded in Germany as rapidly as it did in the United States, where large manpower and enormous material resources free from constant aerial bombardment had still not produced a plutonium bomb which worked by the end of hostilities in Europe.

  There is in any case rather more to the error in the graphite evaluation than meets the eye. Professor Walther Bothe (1891–1957) was an opponent of the Nazi regime and, after having been roughed up by the Nazis at Heidelberg in 1933, spent a long period in convalescence at a Badenweiler sanatorium from where he did not return to the University until 1937. Bothe had been frequently accused of scientific fraud by the Nazis before the war. There being no smoke without fire, he was probably quite prepared to repay them in his own currency if he got the chance. Professor Peter Jensen, his assistant, was a member of Heisenberg’s intimate circle of three and was aware that Heisenberg wanted heavy water as the official moderator and not graphite.

  In his first pioneering paper in December 1940 Heisenberg had said that graphite was suitable as a moderator and he was visualizing in his mind’s eye a reactor of 25 tonnes of uranium oxide and 30 tonnes of graphite. He even had a Most Favourable Design of Reactor in which there would be layers of uranium oxide, heavy water and graphite. In his supplementary report of 29 February 1940 he had changed his mind and said that “the properties of graphite make it unsuitable as a neutron moderator”. Nobody else thought this at the time and he submitted nothing in writing to substantiate his statement about a matter which was obviously critical to the development of the reactor project.

  How convenient it was then that when Bothe and Jensen reported their results on graphite in a bulletin under the title G-71 The Absorption of Thermal Neutrons in Electro-Graphite49 dated 20 January 1941, their paper should state that experiments on the purest carbon available showed the rate of neutron capture to be so high that it was of no use as a moderator. Actually the measurements were incorrect. Professor Heisenberg explained in an interview postwar:

  “Bothe’s Heidelberg people got about a ton of graphite. An error slipped into his experiment. His values were too high but we assumed they were correct and so we did not think that graphite could be used. He had built a pile of graphite pieces but in between the graphite pieces there was always some air and the nitrogen of the air has high neutron absorption. Somehow he must have forgotten this. I don’t know why but it is understandable.”50

  So Professor Bothe, who had been involved in neutron research since it began in 1931, had forgotten that fresh air absorbs neutrons. Without Heisenberg’s knowledge, Professors Joos and Hanle at the University of Göttingen submitted research papers to the Heereswaffenamt even though they were not affiliated to the official programme. Their treatises G-46/G-85 Concerning the Existence of Boron and Cadmium in Graphite51 dated 18 April 1941 contradicted Bothe’s erroneous opinion. The Heereswaffenamt admitted the contradictory report and accepted that graphite was suitable as a moderator in papers G39(24) and G(40)(a) but decided against it on economic grounds.

  Heavy Water

  Neutrons are absorbed by the hydrogen atoms in ordinary water (H2O). Heavy water (D2O) is water with the hydrogen atoms removed so that the liquid which is left consists only of its deuterium atoms. Neutrons collide with deuterium molecules and lose much of their velocity but are not absorbed, which is why heavy water is an exceptionally efficient moderator when used with natural uranium in reactor processes. The production of D2O is extremely costly: at Vemork near Rjukan in Norway, 200 kilometres west of Oslo, in 1940 the world’s only large production facility, 1000 KwH of electricity was required to turn out a single gramme of heavy water. Heavy water cannot be contaminated in the normal course of events and can be used indefinitely.

  A Hypothetical Meeting

  There is no record of Professor Heisenberg ever having met Hitler. Nevertheless, since the latter preferred to be addressed by people who knew exactly what they were talking about instead of intermediaries, it seems logical that Hitler would have asked him to call at some stage. In broad terms Professor Heisenberg would have explained why he believed a working reactor to be impossible. As the obvious corollary, a plutonium bomb would not be possible. A U235 bomb would be prohibitively expensive. The Führer would then have asked, “Well what is possible?” to which Heisenberg could hardly have replied, “Nothing,” for such an answer would have jeopardized the continued existence of the Uranium Project. We simply have to assume that something was put on the table. Following the logical track proposed in this book, I suggest that Heisenberg would have informed him: “I could produce a very low-yield atom bomb built in the laboratory for a rocket hitting the ground at Mach 3.5.” Whereupon Hitler, rising and offering his hand in parting, would have replied, “Then we’ll have to settle for that, won’t we?” Actually the talk would probably have been far more circuitous than I have expressed it here, but certainly the gist must have been along those lines. Sometime in 1940 a schedule of experiments was drawn up by Heisenberg which were more useful for bomb-making than reactor-building.

  Heisenberg’s Mysterious Leipzig Experiments

  In May 1940, at Leipzig, Professor Heisenberg took delivery of a tonne of uranium oxide sent by the Heereswaffenamt with which he was to start his reactor theory experiments. In G23 Determination of the Diffusion Length of Thermal Neutrons in Heavy Water 52, dated 7 August 1940, Heisenberg described the results of examining how the velocity of neutrons emitted by a 480 mg radium-beryllium source was reduced during their passage through 9 litres of heavy water. In G22 Determination of the Diffusion Length of Thermal Neutrons in Preparation 3853, submitted in December 1940, he reported on work involving neutrons emitted into small samples of uranium oxide in a sphere of 12-cm radius. These experiments represented the preliminary work preparatory to designing a hypothetical uranium oxide reactor moderated by heavy water.

  In March 1941 Heisenberg carried out experiment L-I at Leipzig University.54 He said he wanted to establish constants using uranium oxide with paraffin. The two materials were layered alternately in a cylindrical tank. Paraffin is so rich in hydrogen atoms that it is useless as a moderator. From the experiment there was, of course, no neutron multiplication to report. Paraffin is very useful as a reflector. Used to enclose the reactor core, it prevents neutrons escaping. If the uranium fuel is arranged in alternate layers with paraffin, there could never be a chain reaction, because neutrons would be confined and absorbed within their respective layers. If one had in aggregate a critical mass of uranium, as in a bomb for example, and wanted to keep the material in sub-critical quantities with no passage of neutrons between the layers, paraffin would be a good way to do it.

  There was now a long wait of five months before sufficient heavy water would be available for experiment L-II at Leipzig. During this respite there suddenly burst on the scene a scientific paper which resurrected the spectre of Harteck’s radioactive weapons and worse.

  CHAPTER 5

  The Open Road to the Atom Bomb

  EARLY IN SEPTEMBER 1941 Professor Heisenberg received a copy of a scientific paper of 29 pages entitled On the Question of Initiating Chain Reactions 55 which explained how, from a practical standpoint, a chain reacti
on could be simply and effectively brought about by the use of methane as the moderator in a very low temperature reactor. The author was Professor Fritz Houtermans, a Jewish scientist who was perhaps the most brilliant physicist in the field of chain reaction theory in the Third Reich. He was employed at the Post Office Research Institute under Baron Manfred von Ardenne at the Lichterfelde-Ost laboratory in Berlin. The Post Office research was funded independently. The Postmaster-General, Dr Wilhelm Ohnesorge, was known as a pro-Bomb Cabinet Minister close to Hitler.

  The primary purpose of the report was to show the quickest and most effective means of having a working nuclear pile. It was the obvious first stepping stone into the atomic age. Houtermans also explained how a new explosive U239 (plutonium) could be bred in his reactor. He argued that if heat and energy were not required, then the use of heavy water or graphite as a moderator was not necessary. If the reactor was required only for the production of radioisotopes, then a carbon-based substance in a very low temperature was all that was needed.

  What he had observed in the course of his experiments with moderators was that hydrogen molecules in carbon compounds absorbed far fewer neutrons in extreme sub-zero temperatures. It was almost certainly attributable to the nuclear Doppler Effect: he thought it probable that the Doppler Effect alone would enable a liquid carbon substance to be used as moderator. He considered the most favourable to be liquid methane, CH4, a colourless, odourless, flammable gas which is liquid in the temperature range – 164°C to – 186°C.

  In common with all other Reich physicists, Houtermans was unaware of the stabilizing influence of the delayed neutrons of fission, but he had no concerns regarding the safety and stability of his design. Since the machine only operated at a very low temperature, the chain reaction would automatically collapse once the temperature began to rise substantially towards freezing point. While in theory he planned to use regulating rods to control neutron multiplication, in practice Houtermans would have found his reactor unexpectedly stable due to the unsuspected effect of the delayed neutrons.

  Summarizing the paper in a section headed The Significance of a Chain Reaction in a Low Temperature Environment as a Neutron Source and as an Apparatus for Transforming Isotopes, Houtermans confirmed that because U239 (i.e. Pu239 plutonium) is a different element from uranium and, therefore, chemically distinct, concentrations of Pu239 should be obtainable relatively easily by chemical separation from the spent reactor fuel, and this would be an explosive, since it was also fissionable.

  Houtermans’ paper was the first properly argued scientific thesis to enter circulation in Hitler’s Germany proposing a simple, lowtemperature atomic pile for the production of large quantities of radioisotopes and the bomb material plutonium. It was perfectly clear to Heisenberg that Houtermans’ methane pile would run. Once Ohnesorge, the Postmaster-General, saw the report he would have brought it to the attention of Hitler.

  Within a tamper of U238, plutonium has a critical mass of about 11 kilos, but substantially less is required depending on the force of the implosion designed to detonate the bomb. Where the compression factor is three, for example, then only a few kilos of pure plutonium are needed. Five kilos of plutonium isotopes were produced during the fission cycle of a single reactor for every 20 tonnes of U238.

  In a biography Heisenberg was quoted as saying that a faction comprising von Weizsäcker, Karl Wirtz, Peter Jensen and Houtermans met on several occasions to discuss the implications of the report, for:

  “We were not absolutely sure, but we now saw that it was almost certain. Von Weizsäcker in particular and I were deeply disturbed. It now looked like it was definitely possible to make a reactor. We agreed that if we could make them, then the Americans could too. If they could make reactors, then plutonium was probably possible too, and so on. It was from September 1941 that we saw before us an open road leading to the atom bomb.”56

  In another work, Heisenberg explained how he saw things at the time:

  “The situation for physicists in the United States, especially for emigrés from Germany, was totally different from our own. In America they would be convinced that they are fighting for a just cause and against an evil one. The emigrés, precisely because they had been welcomed so hospitably by the Government of the United States, would have felt obliged to contribute to the best of their ability to the American cause. But is an atom bomb, which can kill 100,000 civilians at a stroke, a weapon like all the others? Can one justify the atom bomb with the dictum ‘A just end, but not an evil end, justifies the means’? Is it ethical therefore to build atom bombs for a just cause but not for an evil one? And if one accepts the principle, and it is a principle which has been imposed repeatedly throughout history, who decides what is a just cause and what an evil one? It is easy enough to establish that Hitler and National Socialism is evil. But is the American cause good? Is the principle not also valid that a society is to be condemned as evil by its choice of means? I replied, ‘Why don’t you speak with Niels Bohr in Copenhagen about this? If he is of the opinion that we are wrong to do this work on uranium and we should abandon it, I would find that a persuasive argument.’”57

  The Meeting in Copenhagen with Bohr

  Niels Bohr (1885–1962) had been appointed Professor of Physics at the University of Copenhagen in 1916 and was awarded the Nobel Prize in Physics in 1922. Heisenberg had been his pupil during the period 1924 to 1926. He was Jewish and emigrated from Denmark in 1943 in advance of the pogrom of Danish Jews. Bohr was understandably suspicious of Heisenberg, but, as the latter was unaware of this, it was arranged for the two of them to meet during a lecture at Bohr’s Copenhagen Institute in 1941. Heisenberg’s ostensible purpose seems to have been to let Bohr know in roundabout terms that senior German atom physicists would prevent an atom bomb being built in the hope that the Americans, with whom Germany was not yet at war, might abandon their own development, if they had one, thus keeping the war non-nuclear. Exactly how all this was to be arranged and agreed between the respective physicists defies the imagination. It seemed more likely to Bohr that this was either a warning to the United States that Germany was close to the atom bomb and the Americans should stay out of the war, or that Heisenberg was spying for Germany. The visit was dangerous and ill-advised58 and Heisenberg came away empty-handed. It might be unwise to read too much into the developments in late 1941, but the feasibility of building an atomic bomb, particularly if it would be used against Germany, must have given Heisenberg food for thought. Whereas previously he might have been against the idea completely, maybe at this juncture he began to see reasons for rectifying his outlook.

  The Leipzig L-II Experiment

  Professor Heisenberg returned to Leipzig University where, on 28 October 1941, he carried out experiment L-II59. An aluminium sphere was filled with concentric alternate layers of uranium oxide and 150 litres of heavy water. In the centre was placed a nickel ball of 1.95-cm diameter containing a radium-beryllium Präparat which sprayed neutrons in all directions. Instruments were positioned around the sphere to measure the neutron multiplication and a neutron increment of 10% was recorded in the interior of the sphere, although this was lost to the aluminium at the periphery.

  This stacked design of pile has been described as a little peculiar. Since the Präparat was a ball at the very centre of the sphere, the neutrons would radiate out through the alternating layers diagonally. This would not be a good way to obtain neutron multiplication, but what a central neutron source does permit, however, is for the pile to be dismantled at the end of the experiment so that isotopic transformations in each of the uranium layers can be measured along their length and with reference to their position in the sphere. Such a proceeding would be more useful for gauging where plutonium formed than for planning a heat reactor. Heisenberg’s explanation was that neutrons radiating diagonally passed through alternate layers in which the heavy water layers were three to four times wider than the uranium layers. This presented a better opportunity for the ne
utrons to be slowed optimally for fission, since losses of medium-fast, or partially slowed, neutrons to the U238 capture bands were high. The measurements would also show where neutron capture was thickest in the uranium, and, if such was the information required, enable the optimum velocity of a neutron for capture by the U235 resonances to be established. That would also be useful information if the real purpose of the experiment was to work out the best arrangement for breeding plutonium in the uranium material.

  Heisenberg admitted that the Leipzig experiments were “unusual” insofar as:

  “we put the neutron source in the middle of a sphere of heavy water, then we measured the capture of the neutrons in the middle of the sphere. It was the diffusion of neutrons from the centre to the outside which we measured.”60

  The purpose of placing the neutron source at the centre of the apparatus, according to a scientific paper he wrote in 1943, was:

  “to measure the volume of neutrons escaping at the surface of the sphere so as to determine whether there was a surplus over the neutrons emitted by the Präparat. If there was a surplus, then therein would lie the proof that a bigger construction would eventually lead to a critical uranium pile.”61

  Further Concerns about the Houtermans Paper

  On 28 November 1941 Heisenberg led a stream of notable visitors to the Lichterfelde-Ost laboratory of Professor von Ardenne. According to the Swiss historian Robert Jungk62 he had a further heart-to-heart talk with Professor Houtermans in company with Professor von Weizsäcker. This was a long, frank discussion about the work Houtermans had been doing for von Ardenne, and in conclusion it seems to have been agreed that their overwhelming priority was to conceal from the Government departments involved “the imminent feasibility of manufacturing atomic weapons” by which, as we have seen, he meant Pu239 in a low temperature reactor for a bomb.