The First War of Physics (10 page)

Chapter 3

CRITICAL MASS

September 1939–November 1940

I
n January 1939 Otto Frisch at last received some good news. He learned that his father, though still imprisoned in Dachau concentration camp, had been granted a Swedish visa. Shortly afterwards he was released to rejoin Frisch’s mother in Vienna. Both then made their way to Stockholm and safety.

However, even this good news could not relieve the sense of dark foreboding that had begun to overwhelm him. He became increasingly depressed by the prospect of impending war, and he saw no value in continuing his research in Copenhagen. His sense of vulnerability grew. When British physicist Patrick Blackett and Australian Mark Oliphant came to visit Bohr’s laboratory, Frisch asked them for help.

Oliphant was a native of Adelaide whose initial leanings towards medicine and dentistry had been diverted towards physics while at university. A speech by New Zealander Ernest Rutherford had further directed the impressionable student towards nuclear physics. He joined Rutherford’s research group at the Cavendish Laboratory in Cambridge in 1927, where he witnessed at first hand many of the remarkable discoveries in nuclear physics of the early 1930s. In 1934 he had published a paper with
Rutherford on nuclear fusion reactions involving deuterium, or heavy hydrogen.
¹
German chemist Paul Harteck was a co-author.

In 1937 Oliphant had been appointed to a professorship at the University of Birmingham in England and was now head of the physics department. He was very sympathetic to Frisch’s appeals and subsequently wrote to suggest that Frisch visit Birmingham in the summer of 1939 to see what could be done. Oliphant’s confidence and calmness had impressed the now desperate Frisch, and he did not need a second invitation. He packed two small suitcases and made his way to England, ‘just like any tourist’.

Oliphant found Frisch a job as an assistant lecturer. It was a very informal position. Oliphant would deliver a lecture to a group of students and at the end would hand over to Frisch those who had struggled to comprehend the subject matter. Frisch would sit with a few dozen students who would fire questions at him. The discussions were very lively and much to Frisch’s taste.

At Birmingham Frisch joined fellow émigré Rudolf Peierls.
²
Peierls had been born in Berlin, of assimilated Jewish parents, and had studied physics in Berlin, Munich and Leipzig, where he gained his doctorate under Heisenberg in 1928. He moved to Zurich in Switzerland before taking a Rockefeller Scholarship in 1932 to study first with Fermi in Rome, then in Cambridge, England, to work with the theoretician Ralph Fowler. He was in Cambridge when Hitler came to power in 1933, and it became obvious shortly thereafter that he would not be able to return to Germany. At the end of his scholarship he moved to Manchester to work with Lawrence Bragg, then back to Cambridge for a couple of years before successfully applying for a professorship in mathematics at Birmingham University in 1937.

As war commenced in September 1939 the laboratory facilities at Birmingham were largely given over to essential – and secret – war-related research. Much of this activity centred on the development of the cavity magnetron, used to generate intense microwave radiation for ground and airborne radar systems in what C.P. Snow later called ‘the most valuable English scientific innovation in the Hitler war’.

As enemy aliens, Frisch and Peierls were not meant to know anything about this work, but the secrecy surrounding it was all a bit of a charade. Occasionally, Oliphant would ask Peierls hypothetical questions beginning, ‘If you were faced with the problem …’ Years later Frisch wrote: ‘Oliphant knew that Peierls knew, and I think that Peierls knew that Oliphant knew that he knew. But neither of them let on.’

Frisch’s teaching commitments were relatively light and, with time on his hands, he turned his attention back to problems related to nuclear fission. He used what spare laboratory space he could find to carry out some small-scale experiments. Bohr and Wheeler had argued that the fission discovered in uranium was primarily due to the less stable U-235 isotope. Frisch decided to try to gain some experimental evidence for this, by making measurements on samples that were slightly enriched in the minor isotope. He set up an apparatus to separate a small quantity of U-235 based on the thermal diffusion technique developed by Clusius and Dickel. Progress was slow.

In the meantime he had received a request from the British Chemical Society for a review on recent progress in nuclear science that would be of interest to chemists. He wrote the article sitting in his bedsit, his typewriter on his knees, wearing a greatcoat, huddled over a gas fire in an attempt to stay warm as the winter temperature dipped, sometimes to minus 18 Celsius. On cold nights the tumbler of water at his bedside would freeze solid.

In the section on fission he repeated the consensus view that while it might one day be possible to create a self-sustaining chain reaction, its dependence on slow neutrons meant that it would build too slowly to make an effective bomb. ‘The result would be no worse than setting fire to a similar quantity of old-fashioned gunpowder’, he concluded. Frisch did not believe an atomic bomb was possible.

But the task of writing the review article had set him thinking. The problem that had been identified by Bohr and Wheeler related to slow neutrons. Because of the tendency for U-238 to capture fast neutrons
with certain characteristic ‘resonant’ energies, or speeds, slow neutrons are necessary to achieve a chain reaction in naturally-occurring uranium. Slow neutrons mean a slow build-up of energy. The energy released in a slow-neutron reaction would heat up the uranium, possibly melting or even evaporating it long before it could explode. As the uranium heated up, more and more neutrons would escape the surface and eventually the chain reaction would grind to a halt.

The Uranverein physicists had come to precisely the same conclusion. But, Frisch now wondered, what would happen if
fast
neutrons were used? U-235 was expected to be fissioned by fast as well as slow neutrons. Fast secondary neutrons generated by fission of U-235 would be of no use in mixtures containing large amounts of U-238, as a potentially high proportion would be removed through resonance capture by U-238. However, there would be no such constraint if pure or nearly pure U-235 were used. Frisch had set up his small Clusius-Dickel apparatus to separate U-235 without much difficulty. This was clearly not a technique that could be expected to produce quantities of pure U-235 measured in tons, but might it be possible that a much smaller amount of U-235 would be required to support a fast-neutron chain reaction?

A fast-neutron chain reaction in pure U-235. Insofar as the atomic bomb ever had a ‘secret’, Frisch had just found it.

Frisch shared his thoughts with Peierls, who had in early June 1939 refined a mathematical formula for calculating the critical mass of material required to support a nuclear chain reaction, a formula that had originally been developed by French theoretician Francis Perrin. For mixtures of isotopes with a high proportion of U-238, Peierls had used his revised formula to calculate a critical mass of the order of tons, totally unsuitable for a weapon.

Now Frisch was demanding a rather different calculation, based on fast rather than slow neutrons in pure U-235. The problem was that nobody knew the rate at which U-235 would be fissioned by fast neutrons as nobody had ever separated enough of the isotope to measure it.

They had no choice but to speculate. It was clear from the work of Bohr and Wheeler that U-235 nuclei could be fissioned rather easily by slow
neutrons, so it made sense to assume that fast neutrons would be just as effective, perhaps even to the point that fission would occur every time a U-235 nucleus was hit by a fast neutron. As Peierls later put it: ‘The work of Bohr and Wheeler seemed to suggest that every neutron that hits a [U-]235 nucleus should produce fission.’ This assumption greatly simplified the calculation. The rate they needed to estimate was just the rate at which fast neutrons would hit the U-235 nuclei.

They plugged the numbers into Peierls’ formula, and were profoundly shocked by the result. This was no longer a matter of tons. They had estimated a critical mass of only a
few pounds.
For a substance as dense as uranium this was a critical mass about the size of a golf ball.
³
Frisch estimated that this much U-235 could be separated in a matter of weeks using about 100,000 Clusius-Dickel tubes like the one he had assembled in the laboratory in Birmingham.

‘At that point we stared at each other and realised that an atomic bomb might after all be possible.’

Fast-neutron fission

In Liverpool, Polish-born physicist Joseph Rotblat had reached much the same conclusion. He had read about the discovery of nuclear fission and had conducted his own experiments at the University of Warsaw to verify the production of secondary neutrons. He had quickly realised the potential for a bomb, and grew greatly concerned about what the Nazis would do with such a weapon: ‘I had no doubt that the Nazis would not hesitate to use any device, however inhumane, if it gave their doctrine world domination.’

He had very little modern equipment with which to perform nuclear experiments at the University of Warsaw. He was aware that James Chadwick, winner of the 1935 Nobel prize in physics for his discovery of the neutron and Britain’s leading experimental nuclear physicist, was building Britain’s first cyclotron – based on Lawrence’s design – in the basement of his laboratory at the University of Liverpool. Rotblat dreamed of one day building a cyclotron in Warsaw. He had approached Chadwick in the spring of 1939 with a request to join his group for a short period to observe the latter stages of construction of the cyclotron. Chadwick had agreed and, with a small stipend from Warsaw, Rotblat had leapt at the opportunity to make what was to be his first trip outside his native Poland. He left his new wife, Tola, behind. Just for now, or so he thought.

Although he battled with the English language, the drab slums of Liverpool and the rather primitive conditions of the laboratory, he settled in and was quick to impress Chadwick with his skills as an experimentalist. So much so that in August 1939 Chadwick offered him an Oliver Lodge Fellowship, the department’s most prestigious award. This was the first time the Fellowship had been awarded to a foreign scientist. The stipend associated with it was enough for Rotblat to bring his wife to England.

Rotblat returned to Poland towards the end of August, but Tola was recovering from appendicitis and could not travel. The news blackout in Poland meant that neither of them realised how dangerous the situation had become. Rotblat left for England only a few days before the German invasion began, on what was one of the last trains to leave Poland. His young wife was now trapped. Despite repeated attempts to get her out, he never saw her again. She died in Nazi-occupied Poland. Rotblat never remarried.

The brutal invasion of his native country led him in late November to suggest to Chadwick that they work together on developing an atomic bomb. He feared that those physicists who had stayed on in Nazi Germany might already be working on such a bomb, which Hitler could then use to conquer the world. ‘It was a terrible time for me, perhaps the worst dilemma a scientist could experience’, he said later. ‘Working on a weapon of mass destruction was against all my ideas – all my ideas of what science should do – but those ideas were in danger of being eradicated if Hitler acquired the bomb.’

Chadwick himself had been caught on the hop by the outbreak of war.
⁴
He had been on a trout-fishing holiday in a remote part of northern Sweden with his wife and daughters. When news of the German invasion of Poland reached them, they had immediately headed for Stockholm only to discover that all flights to London had been cancelled. They flew to Holland instead, encountering the author H.G. Wells in their Amsterdam hotel before finally making their way across the North Sea on a tramp steamer.

Rotblat had independently come to the conclusion that slow neutron fission of U-235 would not be sufficient to support an explosive release of nuclear energy, but that fast-neutron fission might. Late one evening in November 1939 he approached Chadwick, who just grunted. But Chadwick’s own early scepticism about the prospects for a bomb had given way to interest, and Rotblat’s logic might have simply served to confirm Chadwick’s own thoughts on the subject. The Liverpool cyclotron had come on stream a few months before. A few days later, Chadwick sat down with Rotblat and together they agreed what experiments needed to be done.

Frisch-Peierls memorandum

Frisch and Peierls discussed their results with Oliphant, who was immediately convinced by their arguments. He recommended they write it all up in a short memorandum. They produced two short typewritten notes, both dated March 1940. The first was largely concerned with the physical principles and practical feasibility of a super-bomb based on U-235. The second, titled
Memorandum on the Properties of a Radioactive ‘Super-bomb’
, was to prove remarkably prescient. It argued that development of an atomic weapon would be ‘practically irresistible’; that it could not be used without ‘killing large numbers of civilians’; that it was ‘quite conceivable that Germany is, in fact, developing this weapon’, though Frisch and Peierls went on to admit that it was possible that ‘nobody in Germany has yet realised that separation of uranium isotopes would make the construction of a super-bomb possible’. The memorandum hints at the threat to come: