Hitler's Terror Weapons (7 page)

CHAPTER 3

Heisenberg's Pioneering Paper

A
S 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 activities 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 elsewhere
³⁶
.

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 Uranium
³⁸
.

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 Gowing
³⁹
added for good measure that the critical size of the U
²³⁵
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 book
⁴⁰
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 U
²³⁵
using fast neutrons. That's why U
²³⁵
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 U
²³⁸
. This isotope captures free neutrons in the uranium, and this is why natural uranium cannot explode. The ‘fissile' isotope U
²³⁵
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 U
²³⁵
atoms. If the material is sufficiently rich in U
²³⁵
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 U
²³⁵
isotope. If the U
²³⁵
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 U
²³⁵
atoms can only occur in almost pure U
²³⁵
, because the capture bands of the U
²³⁸
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 U
²³⁵
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 U
²³⁵
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 U
²³⁵
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 U
²³⁵
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 U
²³⁵
enrichment question, since once the fuse had been developed it cut the bomb material required to a few kilos of U
²³⁵
. 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:

“Certainly 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.

In the preamble to the report, Heisenberg cited as his principal source of reactor theory an article published in the 9 June 1939 edition of the scientific periodical
Die Naturwissenschaften
⁴²
under the title
Can Technical Use Be Made of the Energy Content of Nuclei?
written by the physicist Dr Siegfried Flügge of the KWI for Physics at Berlin-Dahlem. Under a sub-title
The Control of Chain Reactions
Flügge had stated:

“The decisive question for the technical application of the mechanism is manifestly this: is it possible to slow the chain cascade? Adler and Halban
(Nature,
Vol 143, 1939, p.739) have entered the debate and suggested the addition of cadmium salts to the mixture beforehand. In the absence of cadmium, the reaction would soar straightaway to a stationary temperature of 100,000 degrees C.”

In the mentioned article Adler and Halban had warned:

“The danger that a system containing uranium in high concentration might explode once the chain starts is considerable.”

The idea of the instantaneous explosive chain reaction in a reactor is grounded in an error of theory caused by failing to take into the mathematical reckoning the small fraction of relatively long-lived neutrons which are emitted up to a minute after fission. What should have been done in the mathematical theory was to average the slowing down and diffusion time of the lifetime of the prompt neutrons liberated within a micro-second of fission added to the mean lifetime of the 0.75% of neutrons which emerge up to 80 seconds after fission occurs. That calculation would have shown that while neutron density increases exponentially with time, the stable Period of the Reactor is not “less than 1 second” but is about 54 seconds.

The delayed neutrons play the decisive role in the safe control of modern atomic energy plants and without them nuclear power reactors would not be feasible. Heisenberg may have been genuinely under a misapprehension. On the other hand, he may have realized that this would be a useful error to have in hand if he wanted to obstruct the development of a nuclear reactor.