“Alex,” said Roosevelt, quickly understanding, “what you are after is to see that the Nazis don’t blow us up.”1221
“Precisely,” Sachs said.
Roosevelt called in Watson. “This requires action,” he told his aide.
Meeting afterward with Sachs, Watson went by the book. He proposed a committee consisting initially of the director of the Bureau of Standards, an Army representative and a Navy representative. The Bureau of Standards, established by Act of Congress in 1901, is the nation’s physics laboratory, charged with applying science and technology in the national interest and for public benefit. Its director in 1939 was Dr. Lyman J. Briggs, a Johns Hopkins Ph.D. and a government scientist for forty-three years who had been nominated by Herbert Hoover and appointed by FDR. The military representatives were Lieutenant Colonel Keith F. Adamson and Commander Gilbert C. Hoover, both ordnance experts.
“Don’t let Alex go without seeing me again,” Roosevelt had directed Watson.1222 Sachs met the same evening with Briggs, briefed him and proposed he and his committee of two get together with the physicists working on fission. Briggs agreed. Sachs saw the President again and declared himself satisfied. That was good enough for Roosevelt.
Briggs set a first meeting of the Advisory Committee on Uranium for October 21 in Washington, a Saturday. Sachs proposed to invite the emigrés; to counterbalance them Briggs invited Tuve, who found a schedule conflict and deputized Richard Roberts as his stand-in.1223 Fermi, still nursing his Navy grievance, refused to attend but was willing to allow Teller to speak in his behalf. On the appointed day the Hungarian conspiracy breakfasted with Sachs at the Carleton Hotel, the out-of-towners having arrived the night before.1224 From the hotel they proceeded to the Department of Commerce. The meeting then counted nine participants: Briggs, a Briggs assistant, Sachs, Szilard, Wigner, Teller, Roberts, Adamson for the Army and Hoover for the Navy.
Szilard began by emphasizing the possibility of a chain reaction in a uranium-graphite system.1225 Whether such a system would work, he said, depended on the capture cross section of carbon and that was not yet sufficiently known. If the value was large, they would know that a large-scale experiment would fail. If the value was extremely small, a large-scale experiment would look highly promising. An intermediate value would necessitate a large-scale experiment to decide. He estimated the destructive potential of a uranium bomb to be as much as twenty thousand tons of high-explosive equivalent. Such a bomb, he had written in the memorandum Sachs carried to Roosevelt, would depend on fast neutrons and might be “too heavy to be transported by airplane,” which meant he was still thinking of exploding natural uranium, not of separating U235.1226
Adamson, openly contemptuous, butted in. “In Aberdeen,” Teller remembers him sneering, “we have a goat tethered to a stick with a ten-foot rope, and we have promised a big prize to anyone who can kill the goat with a death ray. Nobody has claimed the prize yet.”1227 As for twenty thousand tons of high explosive, the Army officer said, he’d been standing outside an ordance depot once when it blew up and it hadn’t even knocked him down.1228
Restraining himself, Wigner spoke after Szilard, supporting his compatriot’s argument.
Roberts raised serious objection.1229 He was convinced that Szilard’s optimism for a chain reaction was premature and his notion of a fast-neutron weapon made of natural uranium misguided. Roberts had co-authored a review of the subject just one month before. It agreed with Szilard that “there are not yet sufficient data to say definitely whether or not a uranium powerhouse is a possibility.”1230 But it also assessed—because the DTM had begun assessing—the question of the fast-neutron fission of natural uranium and found, because of resonance capture and extensive scattering of fast neutrons, that it was “very unlikely that the fast neutrons can produce a sufficient number of fissions to maintain a [chain] reaction.”1231, 1232
The DTM physicist also pointed out that other lines of research might be more promising than a slow-neutron chain reaction in natural uranium. He meant isotope separation. At the University of Virginia Jesse Beams, formerly Ernest Lawrence’s colleague at Yale, was applying to the task the high-speed centrifuges he was developing there. Roberts thought answers to these questions might require several years of work and that research should be left in the meantime to the universities.
Briggs spoke up to defend his committee.1233 He argued vigorously that any assessment of the possibilities of fission at a time when Europe was at war had to include more than physics; it had to include the potential impact of the development on national defense.
Szilard was “astonished,” as he told Pegram the next day, at Sachs’ “active and enthusiastic” participation in the meeting.1234 Sachs seconded Briggs and the Hungarians. “The issue was too important to wait,” he recalled his argument, “and the important thing was to be helpful because if there was something to it there was danger of our being blown up. We had to take time by the forelock, and we had to be ahead.”1235
Then it was Teller’s turn. For himself, he announced in his deep, heavily accented voice, he strongly supported Szilard. But he had also been given the task of serving as messenger for Fermi and Tuve, who had discussed these issues in New York and had come to some agreement about them. “I said that this needed a little support. In particular we needed to acquire a good substance to slow down the neutrons, therefore we needed pure graphite, and this is expensive.”1236 Jesse Beams’ centrifuge work also required support, Teller added.
“How much money do you need?” Commander Hoover wanted to know.1237
Szilard had not planned to ask for money. “The diversion of Government funds for such purposes as ours appears to be hardly possible,” he explained to Pegram the next day, “and I have therefore myself avoided to make any such recommendation.”1238 But Teller answered Hoover promptly, probably speaking for Fermi: “For the first year of this research we need six thousand dollars, mostly in order to buy the graphite.” (“My friends blamed me because the great enterprise of nuclear energy was to start with such a pittance,” Teller reminisces; “they haven’t forgiven me yet.”1239 Szilard, who would write Briggs on October 26 that the graphite alone for a largescale experiment would cost at least $33,000, must have been appalled.1240)
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.1241
“All right, all right,” Adamson snapped, “you’ll get your money.”1242
The Uranium Committee produced a report for the President on November 1.1243 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.1244, 1245 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.”1246
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.’ ”1247 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.1248
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.”1249 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.”1250 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.”1251 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.”1252
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.”1253
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.”1254
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.1255
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 distinguished M.P. emphasized with a nod to Niels Bohr, held that “only a minor constituent of uranium is effective in these processes.” That constituent would need to be laboriously extracted for any large-scale effects. “The chain process can take place only if the uranium is concentrated in a large mass,” Churchill continued, slightly muddling the point. “As soon as the energy develops, it will explode with a mild detonation before any really violent effects can be produced. It might be as good as our present-day explosives, but it is unlikely to produce anything very much more dangerous.” He concluded optimistically: “Dark hints will be dropped and terrifying whispers will be assiduously circulated, but it is to be hoped that nobody will be taken in by them.”1256
Frisch found a friend that year in a fellow emigré at Birmingham, the theoretician Rudolf Peierls. A well-off Berliner, a slender man with a boyish face, a notable overbite and a mind of mathematical austerity, Peierls was born in 1907 and had arrived in England in 1933 on a Rockefeller Fellowship to Cambridge. With the Nazi purge of the German universities he chose to remain in England. He would be naturalized as a British citizen in February 1940, but until then he was technically an enemy alien. When Oliphant consulted with him from time to time on the mathematics of resonant cavities—important for microwave radar—both men were careful to pretend that the question was pure
ly academic.1257
Peierls had already contributed significantly to the debate on the possible explosive effects of fission. The previous May one of Frederic Joliot’s associates in Paris, Francis Perrin, had published a first approximate formula for calculating the critical mass of uranium—the amount of uranium necessary to sustain a chain reaction. A lump smaller than a critical mass would be inert; a lump of critical size would explode spontaneously upon assembly.
The possibility of a critical mass is anchored in the fact that the surface area of a sphere increases more slowly with increasing radius than does the volume (as nearly r2 to r3). At some particular volume, depending on the density of the material and on its cross sections for scattering, capture and fission, more neutrons should find nuclei to fission than find surface to escape from; that volume is then the critical mass. Estimating the several cross sections of natural uranium, Francis Perrin put its critical mass at forty-four tons. A tamper around the uranium of iron or lead to bounce back neutrons might reduce the requirement, Perrin calculated, to only thirteen tons.
Peierls saw immediately that he could sharpen Perrin’s formula.1258, 1259 He did so in a theoretical paper he worked out in May and early June 1939 that the Cambridge Philosophical Society published in its Proceedings in October. Because a critical-mass formula based on slow-neutron fission would be mathematically complicated, requiring that the characteristics of the moderator be taken into account, Peierls proposed to consider “a simplified case”: fission by unmoderated fast neutrons. Plugging in the fission cross section of natural uranium, which was essentially the fission cross section of U238, gave a critical mass, notes Peierls, “of the order of tons.” As a weapon, an object of that size was too unwieldy to take seriously. “There was of course no chance of getting such a thing into any aeroplane, and the paper appeared to have no practical significance.”1260 Peierls was aware of the British and American concern for secrecy, but in this case he saw no reason not to publish.
Making of the Atomic Bomb Page 44