The First War of Physics (13 page)

In the meantime, opinion had swung firmly in the direction of heavy water as the moderator of choice. The cost of producing graphite of sufficient purity was deemed prohibitive, compared to the cost of an assumed abundant supply of heavy water from the captured Vemork plant.

Yet by the end of 1940 the German physicists had obtained only eight litres from Vemork. Construction of a heavy water plant in Germany was again debated and deemed uneconomic. Wirtz was despatched to inspect the Vemork plant and discuss ways in which production could be increased.

Critical to future progress

There was more bad news to follow. Harteck and fellow Uranverein scientist Hans Jensen in Hamburg had finally come to accept that the Clusius-Dickel thermal diffusion technique did not work for uranium hexafluoride. They had constructed separation tubes on a larger scale than Frisch in Liverpool, including an apparatus eighteen feet tall installed at I.G. Farben’s Leverkusen works, but the results were identical. It had taken seventeen days to produce a single gram of uranium hexafluoride which contained twice the normal amount of U-235, a separation of a miserly 1 per cent. At temperatures for which uranium hexafluoride remained stable, the separation factor was found to be essentially zero. Separation might have been improved by operating at higher temperatures, but the uranium hexafluoride tended to decompose at higher temperatures. This was clearly not a technique that could be used to separate U-235 or enrich uranium on the scale they were going to need for a reactor, or a bomb.

The meeting of the Uranverein held in March 1941 was a gloomy affair. Harteck subsequently reported to the German War Office that the scientists faced two urgent problems. They needed to produce heavy water for use as a moderator and they needed to come up with an alternative way of separating U-235. Of these two problems, the first was deemed to be the more tractable – a sufficient quantity of heavy water would allow a reactor to be constructed from natural uranium. If sufficient quantities of heavy water were not available, then uranium enrichment would be required to
allow a reactor to be constructed using ordinary water as a moderator. Harteck judged enrichment to be relevant ‘only for special applications in which cheapness is but a secondary consideration’.

In other words, separation of U-235 was viable only if a bomb was to be built. The problem of isotope separation was not abandoned, and a couple of radical alternatives were discussed. Bagge raised the possibility of an electromagnetic separation method, exploiting the subtle differences in the flight paths of the different uranium isotopes as they are propelled in a confined ‘atomic beam’ through an electromagnetic field. Directing the beam through two shutters rotating at different speeds would allow that part of the beam rich in U-235 to pass; that part rich in U-238 would be blocked.

A second suggestion was made by Harteck’s Hamburg colleague Wilhelm Groth. This was based on the use of an ultracentrifuge. Wirtz and Uranverein physicist Horst Korsching had made some promising advances with the application of thermal diffusion methods to liquids. Nobody in the Uranverein thought to suggest gaseous diffusion through a porous membrane, as Simon and Peierls had done in Britain.

The Uranverein physicists had put all their faith in the Clusius–Dickel method, so had no choice but to start working from scratch on alternatives. Access to heavy water was now absolutely critical to any future progress.

They should accelerate the thing

Even in these extraordinary times, Fritz Houtermans was rightly regarded as an extraordinary character. Born in Danzig, he grew up with his part-Jewish mother in Vienna. He rejected his privileged background (his father was a wealthy Dutch banker) and became a political radical. His programme of psychoanalysis with Sigmund Freud was terminated when he admitted that he had been inventing his dreams. He was expelled from school for publicly reciting
The Communist Manifesto
to fellow pupils on May Day.

Drawn to physics, he studied in Göttingen in Germany with James Franck and met a succession of notables passing through the German
university town during this period, including Heisenberg, Fermi and Oppenheimer. In the 1920s and early 1930s, Houtermans established his scientific reputation primarily through his work on the physics of energy production in stars. While at Göttingen he met and courted German physicist Charlotte Riefenstahl
²
(she was also courted by Oppenheimer). He married Charlotte while attending a physics conference in Odessa in August 1931. Rudolf Peierls was a witness.

When Hitler came to power Houtermans had already developed a passionate contempt for Nazism. A brush with the Gestapo and Charlotte’s insistence prompted their departure for Britain, but Houtermans was restless. He busied himself helping Szilard find places for the stream of displaced physicists emerging from Germany. He also worked out how to reproduce pages of
The Times
small enough to be hidden beneath postage stamps and sent these to friends in Germany in an attempt to combat the disinformation which formed the staple diet of the German media.

His Communist sympathies led him to be persuaded to take a job at the Ukrainian Physics Institute in Kharkov, just as the Great Purge was about to begin. Within a few years he was living the Stalinist nightmare. The Kharkov Institute was accused of harbouring German spies and Houtermans was arrested on 1 December 1937. With the help of Soviet physicist Peter Kapitza, Charlotte escaped with their two children first to Copenhagen, then to America.

Houtermans was in prison for two and a half years. Sent first to the notorious Lubyanka prison at NKVD
³
headquarters in Moscow, he was transferred to Butyrka, then Kholodnaya Gora in Kharkov, then the central NKVD prison in Kharkov, where he was tortured. He later vividly described the various methods that had been used by his NKVD interrogators. One form of torture had required him to place both feet on the floor of his cell and lean forward against a wall with his full weight pressing on his fingertips. The pain in his fingertips quickly grew intolerable. And yet, he refused to break.

But when his interrogators threatened to arrest his wife and children (Houtermans did not know that they were by this time safe in America) he agreed to sign a confession, implicating colleagues who he thought were already out of the country and safely out of reach. He was released into the hands of the Gestapo in April 1940, a beneficiary of the Nazi–Soviet pact. He was promptly rearrested as a suspected Soviet spy and imprisoned in Berlin.

His colleague and close friend Max von Laue helped free him in July. He learned about the Uranverein, and was shocked to discover the roles that Heisenberg and Weizsäcker were now playing in the German nuclear project. But Houtermans himself was about to be drawn into work on nuclear fission.

Despite his release from prison he was still under suspicion and under Gestapo surveillance. He was banned from taking an academic position or working on government research projects. Laue found him a position in the research team of Manfred von Ardenne, an independent scientist and entrepreneur who had used an inheritance to found his own laboratory in Lichterfeld, a suburb of Berlin. Ardenne had managed to secure funds for independent research on nuclear fission in uranium from the German Post Office. Both Ardenne and Wilhelm Ohnesorge, head of the Reich Postal Ministry who agreed the funding, were aware of the possibility of atomic bombs based on uranium fission. Ohnesorge had even indirectly advised Hitler.

Houtermans was assigned the task of working out the theory of nuclear chain reactions. By the end of 1940 he had independently come to the conclusion that Weizsäcker, McMillan and Turner had each arrived at over a year before. Resonance capture of a neutron by U-238 would eventually lead to the production of a new fissionable element with 94 protons. If a nuclear reactor could be constructed, it could be used to produce element
94 which could be separated relatively easily from the spent reactor materials and used to make a bomb. Houtermans was horrified.

Ardenne was not an academic scientist. He had studied physics, chemistry and mathematics for only four semesters before leaving university to educate himself. He had set up his laboratory to carry out research on radio and television technology and electron microscopy, but he operated largely outside academic circles. The Uranverein physicists had no choice but to tolerate his activities but tended to keep their distance. Houtermans was an altogether different prospect, however. Unlike Ardenne he understood the physics and its implications. He expressed his concerns about the possibility of building an atom bomb based on element 94 to both Heisenberg and Weizsäcker in early 1941.

The precise details of the exchange between these three physicists were never made clear. Houtermans was not working on the ‘official’ uranium project and the Uranverein physicists would have been well aware of the interest that he still attracted from the Gestapo. It seems that Houtermans came to understand that Heisenberg and Weizsäcker were seeking to ‘make use of warfare for physics’, but, of all those involved in nuclear research, Houtermans also understood only too well how quickly moral resolve could collapse under brutal tyranny.

However, Houtermans also picked up signals that suggested Heisenberg and Weizsäcker were actively trying to play down the significance of element 94. This was a dangerous impression to leave with someone under the Gestapo’s watchful eye. It also contradicted the impression left by Weizsäcker’s own eagerness to communicate the possibility of building a bomb using element 93, which he had reported to the Army Weapons Research Bureau in July 1940. Furthermore, if Weizsäcker was actively trying to downplay the significance of element 94, then a patent application which he drafted sometime during 1941 which spells out how element 94 can be produced in a reactor, separated and used to make a bomb ‘about ten million times’ more powerful than any existing explosive material is, perhaps, difficult to understand.

In any event, Houtermans’ concern spilled over into action. He was alerted by Laue to an opportunity to send a message to America via Fritz
Reiche, a Jewish physicist who had managed to secure the necessary travel permits and visas and in mid-March 1941 was about to depart for New York.
⁴
Houtermans asked Reiche to commit a message to memory. As Reiche later recalled, Houtermans had asked that he:

Please say all this: that Heisenberg will not be able to withstand any longer the pressure from the government to go very earnestly and seriously into the making of the bomb. And say to them, say they should accelerate, if they have already begun this thing … they should accelerate the thing.

Irrespective of Heisenberg’s true motivations, his very involvement in the Uranverein sent all kinds of signals to physicists working in Britain and America, particularly those who were also refugees from Nazi Germany. Now Houtermans was signalling that Nazi desire for a super-weapon could soon overwhelm any internal resistance – real or not – from the German physicists themselves.

But no amount of element 94 could be produced without first building a working nuclear reactor. And no reactor could be built in Germany without first obtaining a reliable supply of heavy water. There would be no progress of any kind until this problem was solved.

Blood is thicker than heavy water

Jomar Brun was head of hydrogen research at Norsk Hydro when in 1933 he realised the potential for large-scale production of heavy water at the Vemork plant, whose primary function was to produce ammonia for use in nitrogen fertiliser. He worked with inorganic chemist Leif Tronstad from the Norwegian Institute of Technology in Trondheim to draw up plans for a heavy water production facility involving hundreds of electrolysis, combustion and condensation cells. It was an incredibly speculative proposal,
but Norsk Hydro had given the go-ahead and the facility met its first order for heavy water – from Birkbeck College in London – in August 1934. Tronstad and Brun published important results on the physical properties of heavy water in the British journal
Nature
in 1935.

Uranverein physicist Karl Wirtz had corresponded with Brun before the war and, as a result of Wirtz’s visits to the plant, they had become friends. Together with Harteck, Wirtz made a further visit to the Vemork plant in May 1941. The German scientists discussed their requirements and proposals to expand production capacity using a new catalytic process devised by Harteck and his colleagues in Hamburg, but they were evasive about what they wanted the heavy water for.

Brun and Tronstad’s suspicions do not seem to have been overly raised by these discussions. However, when Tronstad joined Operation Skylark, based in Trondheim, his suspicions were raised considerably. Skylark was part of a wider network of Norwegian resistance groups that had been established by the British Secret Intelligence Service (SIS)
⁵
to gather intelligence on the movements of German battleships along the Norwegian coastline. Skylark had been in radio contact with the SIS since February 1941. In April, the Norwegians had received the message:

FOR HEAVENS SAKE KEEP THIS UNDER YOUR HAT STOP TRY TO FIND OUT WHAT THE GERMANS ARE DOING WITH THE HEAVY WATER THEY PRODUCE AT RJUKAN STOP PARTICULARLY FIND OUT WHAT ADDRESS THEY SENT IT TO IN GERMANY STOP

The message likely originated with Lieutenant-Commander Eric Welsh, a veteran SIS operative and accomplished dissembler. Welsh spoke fluent Norwegian and had worked for many years in Bergen, where his expertise in industrial paints had been put to good use in the design of special corrosion-resistant floor tiles used at the Vemork heavy water plant. Welsh knew Brun, and was broadly familiar with the layout of the plant.

The reason for Welsh’s enquiry is somewhat less clear. The MAUD Committee physicists were obviously well aware of the German interest in heavy water as a result of the briefing that Thomson, Oliphant and Cockcroft had received from Allier in April 1940, and it is perhaps possible that a request for more intelligence had been passed through to the SIS. It is also quite possible that Welsh had been tipped off by Paul Rosbaud who, after ensuring that his Jewish wife and daughter were safe in Britain in 1938 had returned to Berlin to continue to meet his editorial responsibilities for
Die Naturwissenschaften
and to spy for Britain. Rosbaud had helped Use Meitner to escape and in January 1939 had rushed into print Hahn and Strassman’s paper describing their results on uranium. His continued friendly relations with the German physicists involved in the Uranverein meant that he was aware of the significance of heavy water. Welsh was Rosbaud’s spymaster.