From Physics Today Online - July 2000 Volume 53, Number 7
The German Uranium Project
The Farm Hall tapes show that Werner Heisenberg did not know how to calculate the critical mass in 1945, indicating that he did not work on atomic bombs during the war.
Hans A. Bethe
During World War II, the German atomic scientists were trying to produce electric power by creating a chain reaction in uranium. Werner Heisenberg hoped that this achievement would impress the allies if the war was lost.
To get a chain reaction in natural uranium you need a moderator to slow down the neutrons that are generated from fission. This was widely known. Two moderators were believed to be possible: carbon or heavy water.
Walter Bothe, the leading experimental nuclear physicist in Germany, did the crucial experiment and concluded that carbon in the form of graphite would not work. In America, Enrico Fermi did a similar experiment and concluded that graphite was marginal. He suspected that an impurity in the graphite was responsible for the problem. Leo Szilard, who was working alongside Fermi, had studied chemical engineering before going into physics. He remembered that electrodes of boron carbide were commonly used in the manufacture of graphite. It was known that one atom of boron absorbs about as many slow neutrons as 100 000 atoms of carbon. Very small boron impurities would "poison" the graphite for use as a nuclear reaction moderator. Szilard therefore went around to the American graphite manufacturers and convinced one of them to make boron-free graphite. Using this pure graphite as the moderator, the American group achieved a chain reaction on 2 December 1942.
The German team, however, needed to use heavy water, D2O. Ordinary water contains heavy water at a rate of about 1 part in 10 000. The two can be separated by repeated electrolysis, which requires large amounts of electric power in close proximity to a water source. The Germans had this at a hydroelectric plant in occupied Norway, and they set up a separation facility there. The British alerted the Norwegian underground that heavy water was useful for the war, without telling them why. Courageous Norwegians sabotaged production as best they could. As a result the Germans had only about half the heavy water they needed by the end of the war.
Despite the shortage of heavy water, Heisenberg continued to work toward a chain reaction to the very end of the war. What else could he have done? Graphite was not an alternative; he had no reason to doubt Bothe's measurement. Bothe was the recognized authority in the field and Germans believed strongly in authority. Even if another German had repeated the experiment, the result would have been unchanged. No German physicist would have consulted a chemical engineer: The barrier between the two disciplines was too great. It would have been equally impossible to accelerate heavy water production. To do so would have required additional electric power sources in an already power-constrained economy.
The best evidence we have suggests that Heisenberg had no interest in building an atomic bomb. In mid-1942, Albert Speer, the weapons minister, asked Heisenberg whether he could produce a weapon in nine months. With a clear conscience he could answer "No." And he did not know how much fissionable material he would need. When, on occasion, friends asked him, his replies were varied and vague, ranging from 10 kilograms to a few tons.
Why didn't he know? Why hadn't pure intellectual curiosity led him to investigate the properties of uranium-235 with fast neutrons? He could have made a small amount using the cyclotrons available in both Paris and Copenhagen. But he never asked that these properties be measured. The best proof of his lack of interest came at the end of the war. Heisenberg and about ten other German nuclear scientists were interned at Farm Hall, a country estate in England. All of their conversations were secretly taped. When the news of the Hiroshima atomic bomb was broadcast, these scientists could not believe it. When they recognized that it was real, they asked Heisenberg how it could have been done. His first attempt at explanation was totally wrong! He hypothesized something like a nuclear reactor, with the neutrons slowed by many collisions with a moderator.
That he was capable of doing the work was shown about a week later when, in another lecture, he corrected himself and presented a theory similar to that worked out by Rudolf Peierls and Otto Frisch in 1940. He estimated the necessary amount of uranium-235 at about 20 kg, which is nearly correct. These two lectures prove to me that Heisenberg, the scientific leader of the German effort, did not work on a bomb. They show that he did not know critical information, and that he could have derived the information if he had tried.
The British government kept the Farm Hall transcripts secret for nearly half a century. Why? Allegedly the German government had requested the concealment, but it should have been eager to have them published. Unless you assume that they were afraid of what the transcripts would show, the secrecy was foolish. Farm Hall is the real proof that the Heisenberg group had not worked on an atomic bomb. Around 1990 several fellows of the Royal Society urged publication. They convinced the Lord Chancellor, and so the transcripts are now in the public domain. (See the article, "Bomb Apologetics: Farm Hall, August 1945," by Jeremy Bernstein and David Cassidy in Physics Today, August 1995, page 32.)
If he wasn't trying to build a bomb, then why did Heisenberg work on the uranium project? He told me, in 1948, that he wanted to save the lives of a few young German physicists for the post-war period. I believe that this was indeed an important argument for him. And in judging his behavior during the war, we must remember that he was under enormous pressure, pressure virtually unimaginable for Americans or Englishmen.
So Heisenberg didn't try to build a bomb, and apparently had no intention of doing so. Why then the visit to Niels Bohr in 1941? What was he trying to communicate? What did he hope to accomplish?
When Bohr came to Los Alamos at the end of 1943, he told J. Robert Oppenheimer that Heisenberg had talked to him about an atomic bomb. Bohr reproduced frommemory a rough drawing that Heisenberg had shown him. The drawing was given to Edward Teller and me, and we immediately recognized it as a nuclear reactor with many control rods. What did Heisenberg intend to say with this drawing? Perhaps, "Look, this is what we are trying to build and you will recognize that this is a reactor, not a bomb." If so, he overestimated Bohr's knowledge of atomic power. Perhaps he was trying to get Bohr to be a messenger of conscience, and wanted Bohr to persuade the allied scientists also to refrain from working on a bomb. According to journalist Thomas Powers, this message was repeated by Wolfgang Gentner, another non-Nazi German physicist, some time later.1 But Bohr didn't understand--and neither he nor the US would have trusted the message in any case.
I recently became aware of a statement made by Heisenberg in 1965, in which he said approximately: "It was from September 1941 that we saw an open road ahead of us, leading to the atomic bomb." How is this to be reconciled with my assessment and with Heisenberg's statement at Farm Hall that he did not work on the bomb, but only on the "engine"--that is, a reactor?
The explanation is that the Germans rejected the separation of uranium isotopes as too difficult. They saw the fissionability of plutonium as the key to the entire project. Once you had a chain reaction you could make plutonium, and once you had plutonium, you could make a bomb. However, if they had achieved the reactor, they would have found that the road from there to a bomb was still full of obstacles.