me yet.” Szilard, who would write Briggs on October 26 that the graphite alone for a large-scale experiment would cost at least $33,000, must have been appalled.) Adamson had anticipated just such a raid on the public treasury. “At this point,” says Szilard, “the representative of the Army started a rather longish tirade”:
He told us that it was naive to believe that we could make a significant contribution to defense by creating a new weapon. He said that if a new weapon is created, it usually takes two wars before one can know whether the weapon is any good or not. Then he explained rather laboriously that it is in the end not weapons which win the wars, but the morale of the troops. He went on in this vein for a long time until suddenly Wigner, the most polite of us, interrupted him. [Wigner] said in his high-pitched voice that it was very interesting for him to hear this. He always thought that weapons were very important and that this is what costs money, and this is why the Army needs such a large appropriation. But he was very interested to hear that he was wrong: it's not weapons but the morale which wins the wars. And if this is correct, perhaps one should take a second look at the budget of the Army, and maybe the budget could be cut.
“All right, all right,” Adamson snapped, “you'll get your money.”
The Uranium Committee produced a report for the President on November 1. It narrowly emphasized exploring a controlled chain reaction “as a continuous source of power in submarines.” In addition, it noted, “If the reaction turns out to be explosive in character, it would provide a possible source of bombs with a destructiveness vastly greater than anything now known.” The committee recommended “adequate support for a thorough investigation.” Initially the government might undertake to supply four tons of pure graphite (this would allow Fermi and Szilard to measure the capture cross section of carbon) and, if justified later, fifty tons of uranium oxide.
Briggs heard from Pa Watson on November 17. The President had read the report, Watson wrote, and wanted to keep it on file. On file is where it remained, mute and inactive, well into 1940.
Even with Szilard and Fermi stalled, fission studies continued at many other American laboratories. Prodded by a late-October letter from Fermi, for example, Alfred Nier at the University of Minnesota finally began preparing to separate enough U235 from U238, using his mass spectroscope, to determine experimentally which isotope is responsible for slow-neutron fission. But to American physicists and administrators in and out of government a bomb of uranium seemed a remote possibility at best. However intense their sympathies, the war was still a European war.
11
Cross Sections
In the days before the war, Otto Frisch remembers, in Hamburg with Otto Stern, he used to run experiments by day and think intensely about physics well into the night. “I regularly came home,” Frisch told an interviewer once, “had dinner at seven, had a quarter of an hour's nap after dinner, and then I sat down happily with a sheet of paper and a reading lamp and worked until about one o'clock at night — until I began to have hallucinations… I began to see queer animals against the background of my room, and then I thought, ‘Oh, well, better go to bed.’” The young Austrian's hypnagogic visions were “unpleasant feelings” but otherwise “it was an ideal life. I'd never had such a pleasant life, ever — this concentrated five hours work every night.”
Through the spring of 1939, in contrast, after his early experiments with fission, Frisch found himself “in a state of complete doldrums. I had a feeling war was coming. What was the use of doing any research? I simply couldn't brace myself. I was in a pretty bad state, feeling, ‘Nothing I start now is going to be any good.’” As his aunt, Lise Meitner, worried about her isolation in Stockholm, Frisch worried about his vulnerability in Copenhagen; when British colleagues visited he uncharacteristically campaigned among them:
I first spoke to Blackett and then Oliphant when they passed through Copenhagen and said that I had a fear that Denmark would soon be overrun by Hitler, and if so, would there be a chance for me to go to England in time, because I'd rather work for England than do nothing or be compelled in some way or other to work for Hitler or be sent to a concentration camp.
Mark Oliphant directed the physics department at the University of Birmingham. Rather than initiate some complicated sponsorship he simply invited Frisch to visit him that summer to talk over the problem. “So I packed two small suitcases and traveled by ship and train, just like any tourist.” The war overtook him safe in the English Midlands but with nothing more of his possessions on hand than the contents of his two small suitcases. His friends in Copenhagen had to store his belongings and arrange the repossession of the piano he was buying.
Oliphant found him work as an auxiliary lecturer. In that relative security he began to think about physics again. Fission still intrigued him. He lacked the neutron source he would need for direct attack. But he had followed Bohr's theoretical work: the distinction between the fissile characteristics of U235 and U238 in February; the major Bohr-Wheeler paper in September just as the German invasion of Poland brought war, “a great feeling of tense sobriety.” He wondered if Bohr was right that U235 was the isotope responsible for slow-neutron fission. He conceived a way to find out: by preparing “a sample of uranium in which the proportions of the two isotopes were changed.” That meant at least partly separating the isotopes, as Fermi and Dunning had encouraged Nier to do for the same reason. Frisch read up on methods. The simplest, he decided, was gaseous thermal diffusion, a technique developed by the German physical chemist Klaus Clusius. For equipment it required little more than a long tube standing on end with a heated rod inside running down its center. Fill the tube with some gaseous form of the material to be separated, cool the tube wall by flushing it with water, and “material enriched in the lighter isotope would accumulate near the top… while the heavier isotope would tend to go to the bottom.”
Frisch set out to assemble his Clusius tube. Progress was slow. He planned to make the tube of glass, but the laboratory glassblower's first priority was Oliphant's secret war work, work about which Frisch, technically an enemy alien, was not supposed to know. Two physicists on Oliphant's staff, James Randall and H. A. H. Boot, were in fact developing the cavity magnetron, an electron tube capable of generating intense microwave radiation for ground and airborne radar — in C. P. Snow's assessment “the most valuable English scientific innovation in the Hitler war.”
Meanwhile the British Chemical Society asked Frisch to write a review of advances in experimental nuclear physics for its annual report. “I managed to write that article in my bed-sitter where in daytime, with the gas fire going all day, the temperature rose to 42° Fahrenheit… while at night the water froze in the tumbler at my bedside.” He wore his winter coat, set his typewriter on his lap and pulled his chair close to the fire. “The radiation from the gas fire stimulated the blood supply to my brain, and the article was completed on time.”
Frisch's review article mentioned the possibility of a chain reaction only to discount it. He based that conclusion on Bohr's argument that the U238 in natural uranium would scatter fast neutrons, slowing them to capture-resonance energies; the few that escaped capture would not suffice, he thought, to initiate a slow-neutron chain reaction in the scarce U235. Slow neutrons in any case could never produce more than a modest explosion, Frisch pointed out; they took too long slowing down and finding a nucleus. As he explained later:
That process would take times of the order of a sizeable part of a millisecond [i.e., a thousandth of a second], and for the whole chain reaction to develop would take several milliseconds; once the material got hot enough to vaporize, it would begin to expand and the reaction would be stopped before it got much further. So the thing might blow up like a pile of gunpowder, but no worse, and that wasn't worth the trouble.
Not long from Nazi Germany, Frisch found his argument against a violently explosive chain reaction reassuring. It was backed by the work of no less a theoretician than Niels Bohr. With satisfaction he published it.
It had seen the light of day before, most notably in an August 5, 1939, letter from Member of Parliament Winston Churchill to the British Secretary of State for Air. Concerned that Hitler might bluff Neville Chamberlain with threats of a new secret weapon, Churchill had collected a briefing from Frederick Lindemann and written to caution the cabinet not to fear “new explosives of devastating power” for at least “several years.” The best authorities, the
