On 10 December Otto Hahn and three colleagues from the KWI for Chemistry also called in, but what was discussed was not revealed in an official biography of Hahn63, although it appears likely that Houtermans had already been told by Ohnesorge by that date what the Führer required from him. Early in December 1941 Houtermans had a soul-searching conversation with his protector Professor Max von Laue on the subject of a secret assignation he had been given and in connection with this task he is supposed to have sent a cryptic telegram (from a wartime Germany controlled by the Gestapo) to his former colleague Eugene P. Wigner in the United States. The message read, “Hurry! We’re nearly there!” Obviously Houtermans believed in living dangerously.
Despite all the long faces, the Jewish Houtermans must have known that, at least as far as he was concerned, there was a certain inevitability about it all and so he knuckled down to the task in hand.
The Nuclear Project is Wound Down
On 3 December 1941, the Minister for Armaments, Dr Fritz Todt, notified Hitler of the faltering state of the military economy and advised him bluntly that any unplanned future expansion would have to be financed from the budgets of other departments. Consequently Professor Erich Schumann, the Director of Military Research at the Heereswaffenamt, ordered a reappraisal of the uranium project and warned leading scientists that the continued financial support of the War Ministry for the nuclear project was dependent on the promise of a definite military application in the short term.
On 16 December 1941, following a conference of Directors of the various Physics Institutes at the Heereswaffenamt HQ, an enquiry into the progress of the uranium research was started for the information of the Head of Army Ordnance, General Leeb. Whereas the report was positive in recommending that the industrial exploitation of nuclear power would benefit both the general economy and the Wehrmacht, Professor Schumann was not convinced, and all the signs were clearly visible that the military would relinquish the project even before the review had been completed.
In January 1942 it was agreed between Schumann, Leeb and Dr Vögler, President of the KW Foundation, that the latter organization would take over the research in harness with the Reich Research Council, an agency of the Ministry of Education and Science. Professor Esau, an anti-Bomb Nazi, was appointed its scientific head while the Education Minister, Bernhard Rust, would be its President.
The Army research team led by Dr Kurt Diebner was to retain a measure of independence under the restructuring and would continue to be subject to War Ministry control. As if to underline the insignificance which the Heereswaffenamt now attached to the uranium project, Professors von Weizsäcker and Harteck were served conscription papers that month for military service on the Russian Front and Heisenberg was obliged to exert all his influence to persuade Professor Schumann to rescind the orders and restore the two physicists to their reserved occupations.
More Odd Experiments by Heisenberg
Heisenberg himself was to continue with his intriguing experiments begun at Leipzig. At the end of 1940 the Heereswaffenamt had decided that uranium experiments were to use cast metal instead of uranium oxide powder. The Auer Company of Berlin had the contract for refining the uranium oxide confiscated at Oolen in Belgium, and a small plant had been set up at Oranienburg which turned out about a tonne of refined uranium oxide per month. The product was contaminated with boron, a neutron absorber, and therefore unsatisfactory. A quantity of uranium metal powder of good quality had been placed at Heisenberg’s disposal and he set to work on two small experiments: L-III, to investigate the scale of neutron losses in the outer shell and separating panels of the aluminium container, and a second to see the effect of fast neutrons sprayed directly into uranium metal powder. By coincidence, Professor Stetter, an SS-physicist with a special interest in plutonium as an explosive, submitted a paper announcing his own preparations for a similar experiment, but Heisenberg decided to forge ahead because the sphere he was using was larger.
Experiment L-II had shown a slight interim neutron multiplication prior to losses in the aluminium vessel. As all previous investigations had concentrated on fissioning U235 using slow neutrons, he had the idea of seeing what the effect would be of using unslowed neutrons on the U238 capture bands. He explained:
“The object of the experiment was to determine the neutron multiplication brought about solely by fissioning U235 with fast neutrons.”64
Within a square tin box a simple 15-cm diameter sphere was fitted which contained the uranium powder and 500 mgs of the radium-beryllium element at its centre. The outcome showed that U238 could not be used as of itself to produce energy. And if he wanted to know the fact, the experiment also demonstrated that fast neutrons had to be decelerated by a moderator such as heavy water before appreciable amounts would be captured by the U238 resonances for plutonium breeding purposes.
On 6 January 1942 at Berlin-Dahlem, with four helpers, he embarked on experiment B-III65. This was a repetition of the L-I uranium oxide/paraffin test examining the advantages of having uranium metal powder instead of the oxide. It had several additional goals: to test the effectiveness of the layer thicknesses as proposed by theory, and to establish to what degree the rare U235 in the material should be enriched to enable a working atomic pile to be built using ordinary water or paraffin instead of the heavy water moderator66, this being, as he knew, a very optimistic proposition.
The configuration was an aluminium sphere of 28.5-cm radius containing 551 kilos of uranium metal powder in nineteen concentric layers alternate with eighteen layers of paraffin stacked horizontally, the Präparat ball being placed at the centre. As with L-I there was no neutron multiplication and nor was any expected, for:
“The experimental assemblies containing paraffin as a moderator were not suitable for neutron multiplication. They were to measure important constants for later designs.”67
Measurement was made of the neutron distribution inside the sphere and in the ordinary water surrounding it. It was concluded that uranium metal powder was better than uranium oxide and
“confirmed other experiments in that when evaluating the neutron figure, account should be taken of neutron capture by U238 isotopes”.
In other words, experiments L-I and B-III with layered uranium and paraffin were concerned primarily with measuring the generation of neutrons and their capture by U238 isotopes, for on concluding an experiment the amount of U239 (which eventually decays into plutonium) forming in the uranium material was to be measured.
Speer Attempts to Resurrect the Official Project
On 7 February 1942 Dr Todt was killed when his personal Heinkel bomber crashed taking off in poor visibility at Rastenburg. Hitler decided to appoint Speer at once as successor to Todt. As has been mentioned, Speer was an enthusiast for the atom bomb, but soon found that the Völkischer Beobachter began to stir against him whenever he approached Hitler on the subject of atomic physics. The first convention of the Reich Research Council was held at its Berlin Steglitz HQ on 26 February 1942 under the chairmanship of Reich Minister Rust. Substantial funds became available to the project for the first time once Rust had become convinced that the heat reactor could definitely be built. In what must have been a speech glowing with optimism Heisenberg assured him not only that atomic reactors for energy production were “undoubtedly possible” but that an enormously powerful explosive could be bred in them.
In early May 1942 Speer arranged for Goering to be appointed head of a newly independent Reich Research Council. The restructuring was intended to emphasize the importance of the new committee which Speer was planning should oversee a progressive research programme for military purposes. Goering’s presidential council consisted of 21 members who were either Ministers, Chiefs of Staff or high Party officials and thus the uranium project was taken over completely by the political side after all.
The Last Two Leipzig Experiments
Heisenberg’s paper G-136: The Proof by Experiment of Effective Neutron Multiplication in a Laye
red arrangement of Heavy Water and Uranium Metal in a Sphere68 was undated but delivered with a covering letter dated July 1942, the month following an accident which destroyed Heisenberg’s Leipzig apparatus. The report was stamped Geheime Kommandosache (i.e. very secret), an unusual classification which may possibly be explained by the conclusion in the summary that:
“A spherical arrangement of 17-cm wide layers of heavy water and 4-cm wide layers of uranium metal density 10 separated by 2 to 5mm thick aluminium support material has a negative coefficient for neutron absorption. The mere enlargement of the layered arrangement described herein will lead to a uranium pile for the production of nuclear energy.”
In L-II the previous year, using uranium oxide, the experiment had given a positive neutron coefficient which was very marginal: there had been a neutron multiplication but this had been lost to the aluminium material of the sphere.
L-IV consisted of two aluminium hemispheres bolted together at the equator. A chimney was fitted through which the Präparat would be dropped into the centre to initiate the reaction. The apparatus was bedded on a foundation of waterproofed wooden beams in a zinc tub filled with ordinary water. The internal arrangement was a 17-cm radius aluminium sphere with walls 1.2 mm thick. This contained 140 kilos of heavy water and the Präparat. The inner sphere rested on a lower hemispherical shell of 5-mm aluminium plate containing 90 kilos of uranium powder of density 10.8 and this in turn rested on a larger hemisphere filled with 660 kilos of uranium powder of density 9.34.
The measurements showed a neutron multiplication of 13%, and with this apparatus Heisenberg had succeeded in generating more neutrons than provided by the Präparat source. Simply by increasing the size of the sub-reactor one would eventually have a working uranium pile for the production of nuclear energy.
For the reactor builder the next step was an experiment in which the materials were increased and on 31 July 1942 in G-161 Observations on the Planned Intermediate Experiment with 1.5 tonnes Heavy Water and 3 tonnes Uranium Metal Heisenberg did precisely that. It was logical. But what was not logical was that meanwhile he had set up a duplicate of L-IV. He knew what would happen. The uranium fuel would gradually become more radioactive with the products of fission and plutonium being formed in the U238 resonances, but what would that have to do with reactor technology?
On 2 June 1942 the experiment began. 750 kilos of uranium metal powder in the outer shere surrounded a central sphere of heavy water. As before, the apparatus was immersed in a vat of ordinary water, the Präparat was dropped into the centre via the chimney, and Heisenberg, Mr and Mrs Döpel, master mechanic W. Paschen and technician G. Kunze, who monitored the gamma-radiation instrumentation, all sat back for a few months to see what would happen next.
In the U-metal/paraffin experiment B-III, measurements were made of neutron capture by the resonances along the diagonals outward from the central Präparat. From this information it was a straightforward matter to plot where the diminishing neutron velocities coincided with the greatest incidences of neutron capture, and this was in fact known as Factor e/w where w = probability of resonance capture.
The concentric inner sphere of Heisenberg’s L-V experiment at Leipzig contained 220 litres of heavy water, the small nickel ball with the radiumberyllium neutron source being at its centre. This Präparat played a twofold role. It emitted neutrons into the heavy water where they lost a degree of momentum before proceeding into the surrounding uranium metal powder. Here they fissioned U235 atoms to release more neutrons into the reaction or were captured by U238 isotopes to decay into plutonium. Additionally, gamma radiation from the neutron source generated photoneutrons in the heavy water and these also entered the uranium. Over a period of many months this ‘experiment’ would be a subtle means of uranium enrichment, since a measureable proportion of the U238 capture band converted into the fissile isotopes of plutonium. After his return to Leipzig University on 23 June 1942, it was noticed that the sphere, which had been quietly fissioning for three weeks, was leaking bubbles. The equipment was raised from the water and an access hatch opened to inspect the interior. A hissing sound was followed by a jet of flame. The sphere was hosed down with water until the fire appeared to be extinguished. The heavy water was then drained from the inner sphere to prevent its accidental contamination, after which the main sphere was re-sealed and lowered back into the water tank for safety. A few hours later the apparatus began to give off bubbles once more and the water in the containing tank began to boil. On closer inspection it was seen that the sphere was vibrating and beginning to swell in size. The laboratory was evacuated and shortly after there was an explosion involving a hailstorm of burning uranium powder. The fire brigade succeeded in dowsing all fires except that in the sphere which was allowed to burn out over the next few days.
How did this fire start? Possibly fissioned material in the powdered uranium began to warm up. As it expanded, the pressure cracked the seal holding the two aluminium hemispheres together, allowing water to enter from the shielding tank. This would have oxidized the uranium, generating more heat. But the ignition source is a mystery. The material burnt out and the attempt to do whatever it was that was being attempted failed. So terminated this series of experiments, the purpose of the last of which seems not to have been questioned too closely previously.
Professor Goudsmit Not Deceived
The only military scientist on record as believing that Heisenberg had been involved in an attempt to build a German atom bomb is Professor Samuel Goudsmit. In his book Alsos- The Failure in German Science (Sigma, London, 1947) on page 183 Professor Goudsmit reproduced two sketches contained in Heisenberg’s official German wartime report respecting the B-III uranium/paraffin experiment. Under a drawing of the chimneyed sphere appeared the caption “Germany’s Atom Bomb” and the words “Germany’s experimental uranium pile which they believed would make a bomb”. Beside a slightly adulterated version of the cut-through diagram he wrote, “Diagram for the experimental ‘bomb’ which consisted of layers of uranium and paraffin”. Professor Goudsmit was Jewish and had lost both parents at Auschwitz. His book was a non-scientific publication very popular at the time and it is possible that he was merely attempting to ridicule Heisenberg’s scientific circle. But one can interpret it in another sense. Perhaps this was as far as Goudsmit, who was restrained by the various US secrecy laws, was permitted to go in print with his allegation. Joke or not, this device would have worked as the warhead of a V-2 rocket.
After five or six months in the sub-reactor sphere, the uranium powder was enriched with plutonium. In the bomb casing the material would be stacked in alternate layers with paraffin. The paraffin in the top hemisphere layers prevent premature fission by the highly radioactive Pu240 plutonium isotopes which would otherwise spoil the reaction when the bomb is detonated. The bottom hemisphere would be filled with iron ballast. The weight of bomb core, casing and ballast material acting as an anvil would have been limited to one tonne, the payload of a V-2, the speed of the rocket at impact rendering superfluous the four or so tonnes of HE necessary to set off the device in normal circumstances. Impact was at Mach 3.5. Because detonation would not be uniform around the bomb sphere, this method would have resulted in a ‘fizzle’ equivalent to several dozen tonnes of TNT, an earthquake effect and meltdown with radioactive fallout.
It was a brilliant concept, cheap to manufacture anywhere and not difficult to produce in numbers. The failure of the Leipzig experiment after only three weeks signalled the end of Professor Heisenberg’s participation in the project and in midsummer 1942 the project was transferred elsewhere.
CHAPTER 6
The German Post
Office Takes Over
“I, of all people, did in fact lead the way for the great advance in atomic development in the German Reich.”
Wilhelm Ohnesorge, Postmaster General, 1937-1945 Quoted from his obituary, Soldatenzeitung, East Berlin, 10 March 1962.
A LBERT SPEER69 recalled that, altho
ugh Hitler did speak to him occasionally of the possibilities of the atom bomb, the strategic benefits of having it eluded the Führer. The subject was a source of irritation to Speer, for he knew that there was in existence some sort of secret arrangement involving the Post Office about which he was being left completely in the dark, and this appeared to be a matter on which, as Armaments Minister, he really ought to have been consulted.
Explaining that there were 2200 recorded points of reference in his conferences with Hitler, and that there was only a single occasion when the subject of nuclear research appeared on the agenda, being passed over “with laconic brevity”, Speer noted Hitler’s strengthening resolve not to pursue the matter.
Hitler’s objection to the atom bomb was fundamental. He had read somewhere (almost certainly the article by Professor Jean Thibaud of the Sorbonne published on 12 March 1941) that a nuclear explosion might proceed to ignite all the hydrogen atoms in the atmosphere, transforming the world into a glowing star. German physicists could not guarantee that the theory was definitely wrong: even at Los Alamos in July 1945 the Italian-American physicist Fermi wondered aloud whether the test bomb he was about to ignite might trigger the heavens, destroying every living thing on earth. Speer thus concluded:
“Even if Hitler had not been against nuclear research on doctrinal grounds [i.e. Aryan Physics]: even if the stage we had reached in investigating the principles in June 1942 had provided the atomic physicists with an objective for the investment of thousands of millions of marks towards producing the atom bomb, it would have been impossible for our strained war economy to have brought together the technicians, materials and priorities for the project.” 70
It is an odd thing that Hitler should have appeared to shun atomic physics when speaking to Speer whilst openly affirming his enthusiasm for its future prospects to his closer companions at table. Martin Bormann’s stenographer Henry Picker recorded71 that Hitler considered the splitting of the atom to be the most important of all scientific achievements for Germany’s future to the extent that it was the Führer himself who was inspirational in having the short documentary film Gold starring Hans Albers exhibited repeatedly in cinemas in order to popularize the subject of nuclear science. Anything short of a chain reaction was Aryan Physics.
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