Hitler's Terror Weapons, Brooks, Geoffrey [cat reading book .txt] 📗

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 reaction could be simply and effectively brought about by the use of methane as the moderator in a very low temperature