Hitler's Terror Weapons (10 page)

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 U
²³⁸
, 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 U
²³⁸
.

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-advised
⁵⁸
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-II
⁵⁹
. 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 neutrons to be slowed optimally for fission, since losses of medium-fast, or partially slowed, neutrons to the U
²³⁸
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 U
²³⁵
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 Jungk
⁶²
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 Pu
²³⁹
in a low temperature reactor for a bomb.

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 Hahn
⁶³
, 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 U
²³⁵
using slow neutrons, he had the idea of seeing what the effect would be of using unslowed neutrons on the U
²³⁸
capture bands. He explained:

“The object of the experiment was to determine the neutron multiplication brought about solely by fissioning U
²³⁵
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 U
²³⁸
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 U
²³⁸
resonances for plutonium breeding purposes.

On 6 January 1942 at Berlin-Dahlem, with four helpers, he embarked on experiment B-III
⁶⁵
. 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 U
²³⁵
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 moderator
⁶⁶
, 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: