The Making of the Atomic Bomb

The National Defense Research Council immediately absorbed the Uranium Committee. That had been part of its purpose. Briggs was a cautious and frugal man, but his committee had also lacked the authority of a source of funds independent of the military. The white-haired director of the National Bureau of Standards would continue to be responsible for fission work. He would report now to James Bryant Conant, Harvard's wiry president, boyish in appearance but in practice cool and reserved, whom Bush had enlisted as soon as FDR authorized the new council.

The NDRC gave research in nuclear fission an articulate lobby within the executive branch. But though Bush and Conant felt challenged by German science — “the threat of a possible atomic bomb,” writes Bush, “was in all our minds” — both men, concerned about scarce scientific resources, were initially more interested in proving the impossibility of such a weapon than in rushing to build one: the Germans could not do what could not be done. When Briggs wrapped up his pre-NDRC committee work in a report to Bush on July 1 he asked for $140,000, $40,000 of it for research on cross sections and other fundamental physical constants, $100,000 for the Fermi- Szilard large-scale uranium-graphite experiment (the military had decided to grant $100,000 on its own through the Naval Research Laboratory to isotope-separation studies). Bush allotted Briggs only the $40,000. Once again Fermi and Szilard were left to bide their time.

Winston Churchill had accepted George VI's invitation to form a government upon Neville Chamberlain's resignation the day Germany invaded the Lowlands; he shouldered the prime ministership calmly but felt the somber weight of office. C. P. Snow recalls a more paradoxical mood:

I remember — I shall not forget it while I live — the beautiful, cloudless, desperate summer of 1940… Oddly enough, most of us were very happy in those days. There was a kind of collective euphoria over the whole country. I don't know what we were thinking about. We were very busy. We had a purpose. We were living in constant excitement, usually, if we examined the true position, of an unpromising kind. In one's realistic moments, it was difficult to see what chance we had. But I doubt if most of us had many realistic moments, or thought much at all. We were all working like mad. We were sustained by a surge of national emotion, of which Churchill was both symbol and essence, evocator and voice.

Not only native-born Englishmen felt that surge. So did the emigre scientists whom Britain had sheltered. Franz Simon, an outstanding chemist whom Frederick Lindemann had extracted from Germany in 1933 for the Clarendon, wrote his old friend Max Born on the eve of the Battle of France that he longed to “use my whole force in the struggle for this country.” Though he may not yet have realized it, Simon's opportunity had already arrived. Early in the year, when Frisch and Peierls were first beginning to discuss the ideas that would lead to their important memoranda, Peierls had consulted Simon about methods of isotope separation. Frisch had chosen to work with gaseous thermal diffusion — his Clusius tube — because it seemed to him the simplest method, but Simon had begun then to think about other systems. Half a dozen approaches had been tried in the past. You couldn't spit on the floor without separating isotopes, Simon joked; the problem was to collect them. He wanted to find a method adaptable to mass production, because with a 1:139 isotope ratio, uranium separation would have to proceed on a vast scale, as Frisch's calculation of 100,00 °Clusius tubes demonstrated. Frisch dramatized the difficulty with a simile: “It was like getting a doctor who had after great labour made a minute quantity of a new drug and then saying to him: ‘Now we want enough to pave the streets.’”

The surge of national emotion sustained Mark Oliphant as well, and in that mood he found even less patience than usual for obstructive rules. When P. B. Moon questioned the assumption that gaseous thermal diffusion was the method of choice for isotope separation, he won no encouragement from the Thomson committee, but back in Birmingham Oliphant simply told him to go ahead and talk it over with Peierls. “Within a week or two,” writes Moon, “Peierls identified ordinary diffusion as a logically superior process and wrote directly to Thomson on the matter.” Peierls proposed that the Thomson committee consult with Simon, the best man around. The committee hesitated, even though Simon was a naturalized citizen. Oliphant then authorized Peierls out of hand to visit Simon at Oxford.

Simon in the meantime had been working to convert a skeptical Lindemann. At Simon's suggestion Peierls had written to Lindemann on June 2. Together at Oxford later in June they approached Lindemann in person. “I do not know him sufficiently well to translate his grunts correctly,” Peierls reported of the meeting. But he felt sure he had “convinced him that the whole thing ought to be taken seriously.”

Like Peierls, Simon had settled on “ordinary” gaseous diffusion (as opposed to gaseous thermal diffusion) as the best method of isotope separation after winnowing through the alternatives. Gases diffuse through porous materials at rates that are determined by their molecular weight, lighter gases diffusing faster than heavier gases. Francis Aston had applied this principle in 1913 when he separated two isotopes of neon by diffusing a mixed sample several thousand times over and over through pipe clay — that is, unglazed bisque of the sort used to make clay pipes. Thick materials like pipe clay worked too slowly to be effective at factory scale; Simon sought a more efficient mechanism and concluded that a metal foil punctured with millions of microscopic holes would work faster. Divide a cylinder down its length with such a foil barrier, pump a gas of mixed isotopes into one side of the divided cylinder, and gas would diffuse through the barrier as it flowed from one end of the cylinder to the other. Compared to the gas left behind, the gas that diffused through the barrier would be selectively enriched in lighter isotopes. In the case of uranium hexafluoride the enrichment factor would be slight, 1.0043 under ideal conditions. But with enough repetitions of the process any degree of enrichment was possible, up to nearly 100 percent.

The immediate problem, Simon saw, was barrier material. The smaller the holes, the higher the pressures a separation system could sustain, and the higher the pressure, the smaller the equipment could be. Whatever the material, it would have to resist corrosion by uranium hexafluoride — “hex,” they were beginning to call it, not necessarily in tribute to its evil contrarities — or the gas would clog its microscopic pores.

One morning that June, inspired, Simon took a hammer to a wire strainer he found in his kitchen. He carried the results to the Clarendon and called together two of his assistants — a Hungarian, Nicholas Kurti, and a big Rhodes scholar from Idaho, H. S. Arms. “Arms, Kurti,” Simon announced, holding up the strainer, “I think we can now separate the isotopes.” He had hammered the wires flat in demonstration, reducing the spaces between to pinholes.

“The first thing we used,” Kurti recalls, “was ‘Dutch cloth,’ as I think it is called — a very fine copper gauze which has many hundreds of holes to the inch.” The assistants hammered the holes even finer by hand. They tested the copper barrier not with hex but with a mixture of water vapor and carbon dioxide, “in other words something much like ordinary soda-water” — the first in an urgent series of experiments carried out through the summer and fall to study materials, pore size, pressures and other basic parameters preliminary to any equipment design.

In late June G. P. Thomson gave his committee a new name to disguise its activities: MAUD. The initials appear to form an acronym but do not. They arrived as a mysterious word in a cable from Lise Meitner to an English friend: met niels and margrethe recently both well but unhappy ABOUT EVENTS PLEASE INFORM COCKCROFT AND MAUD RAY KENT. Meitner's friend passed the message to Cockcroft, who decided, he wrote Chadwick, that maud ray kent was “an anagram for ‘radium taken.’ This agrees with other information that the Germans are getting hold of all the radium they can.” Thomson borrowed the first word of Cockcroft's mysterious anagram for a suitably misleading name. The committee members did not learn until 1943 that Maud Ray was the governess who had taught Bohr's sons English; she lived in Kent.

The war crossed the Channel first in the air. As a result of the German bombing of Warsaw in the autumn of 1939, an act Germany represented as tactical because the Polish city was heavily fortified, the British Air Ministry had repudiated its pledge to refrain from strategic bombing. But neither belligerent was eager to exchange bombing raids, and although nightly blackouts added inconvenience and apprehension to the wartime burden of the people of both nations, the implicit truce held until mid-May 1940. Then within a week two events triggered British action. German raiders targeted for French airfields at Dijon lost their way and bombed the southern German city of Freiburg instead, killing fifty-seven people; the German Ministry of Propaganda brazenly denounced the bombing as British or French and threatened fivefold retaliation. Blacker and more violent non sequitur destroyed the city center of Rotterdam. Dutch forces were holding out stubbornly as late as May 14 in the northern section of that old Netherlands port. The German commanding general ordered a “short but devastating air raid” that he hoped might decide the battle. Negotiations with the Dutch advanced, the air raid was canceled, but the abort message arrived too late to stop half the hundred Heinkel Ill's ordered into action from dropping 94 tons of bombs. The bombs