Military history

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 purely 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.

The USSR opportunistically invaded Finland at the end of November. In the rest of Europe the strange standoff prevailed that isolationist Idaho senator William Borah would label the “phony war.” The Peierlses moved to a larger house; early in the new year they generously invited Frisch to live with them. Genia Peierls, who was Russian, took the bachelor Austrian in hand. She “ran her house,” writes Frisch, “with cheerful intelligence, a ringing Manchester voice and a Russian’s sovereign disregard of the definite article. She taught me to shave every day and to dry dishes as fast as she washed them, a skill that has come in useful many times since.”1261 Life at the Peierlses was entertaining, but Frisch walked home through ominous blackouts so dark that he sometimes stumbled over roadside benches and could distinguish fellow pedestrians only by the glow of the luminous cards they had taken to wearing in their hatbands. Thus reminded of the continuing threat of German bombing, he found himself questioning his confident Chemical Society review: “Is that really true what I have written?”1262

Sometime in February 1940 he looked again. There had always been four possible mechanisms for an explosive chain reaction in uranium:

(1) slow-neutron fission of U238;

(2) fast-neutron fission of U238;

(3) slow-neutron fission of U235; and

(4) fast-neutron fission of U235.

Bohr’s logical distinction between U238 and thorium on the one hand and U235 on the other ruled out (1): U238 was not fissioned by slow neutrons. (2) was inefficient because of scattering and the parasitic effects of the capture resonance of U238. (3) was possibly applicable to power production but too slow for a practical weapon. But what about (4)? Apparently no one in Britain, France or the United States had asked the question quite that way before.

If Frisch now glimpsed an opening into those depths he did so because he had looked carefully at isotope separation and had decided it could be accomplished even with so fugitive an isotope as U235. He was therefore prepared to consider the behavior of the pure substance unalloyed with U238, as Bohr, Fermi and even Szilard had not yet been. “I wondered—assuming that my Clusius separation tube worked well—if one could use a number of such tubes to produce enough uranium-235 to make a truly explosive chain reaction possible, not dependent on slow neutrons. How much of the isotope would be needed?”1263

He shared the problem with Peierls. Peierls had his critical-mass formula. In this case it required the cross section for fast-neutron fission of U235, a number no one knew because no one had yet separated a sufficient amount of the rare isotope to determine its cross section by experiment, the only way the number could be reliably known. Nevertheless, says Peierls, “we had read the paper of Bohr and Wheeler and had understood it, and it seemed to convince us that in those circumstances for neutrons in U235 the cross-section would be dominated by fission.” Peierls could state simply what followed: “If a neutron hit the [U235] nucleus something was bound to happen.”1264

What followed thus made the cross section intuitively obvious: it would be more or less the same as the familiar cross section that expressed the odds of hitting the uranium nucleus with a neutron at all—the geometric cross section, 10-23 square centimeters, an entire order of magnitude larger than the fission cross sections previously estimated for natural uranium that were small multiples of 10-24.

“Just sort of playfully,” Frisch writes, he plugged 10-23 cm21265 into Peierls’ formula.1266 “To my amazement” the answer “was very much smaller than I had expected; it was not a matter of tons, but something like a pound or two.”1267 A volume less than a golf ball for a substance so heavy as uranium.

But would that pound or two explode or fizzle? Peierls easily produced an estimate. The chain reaction would have to proceed faster than the vaporizing and swelling of the heating metal ball. Peierls calculated the time between neutron generations, between 1×2×4×8×16×32×64 . . . , to be about four millionths of a second, much faster than the several thousandths of a second Frisch had estimated for slow-neutron fission.1268

Then how destructive was the consequent explosion? Some eighty generations of neutrons—as many as could be expected to multiply before the swelling explosion separated the atoms of U235 enough to stop the chain reaction—still millionths of a second in total, gave temperatures as hot as the interior of the sun, pressures greater than the center of the earth where iron flows as a liquid. “I worked out the results of what such a nuclear explosion would be,” says Peierls. “Both Frisch and I were staggered by them.”1269

And finally, practically: could even a few pounds of U235 be separated from U238? Frisch writes:

I had worked out the possible efficiency of my separation system with the help of Clusius’s formula, and we came to the conclusion that with something like a hundred thousand similar separation tubes one might produce a pound of reasonably pure uranium-235 in a modest time, measured in weeks. At that point we stared at each other and realized that an atomic bomb might after all be possible.1270

“The cost of such a plant,” Frisch adds for perspective, “would be insignificant compared with the cost of the war.”1271

“Look, shouldn’t somebody know about that?” Frisch then asked Peierls.1272 They hastened their calculations to Mark Oliphant. “They convinced me,” Oliphant testifies.1273 He told them to write it all down.

They did, succinctly, in two parts, one part three typewritten pages, the other even briefer. Talking about it made them nervous, Peierls recalls (by then it was March and the exceptional cold had given way to warmer weather):

I remember we were writing our memorandum . . . together in my room in the Physics Lab on the ground floor; it was a fine day and the window was open . . . and while we were discussing the wording a face suddenly appeared in the open window. And we were a little worried! It turned out that just underneath the window (which was facing south) people were growing some tomato plants, and somebody had been there bending down inspecting what these plants were doing.1274

The first of the two parts they titled “On the construction of a ‘superbomb’; based on a nuclear chain reaction in uranium.”1275 It was intended, they wrote, “to point out and discuss a possibility which seems to have been overlooked in . . . earlier discussions.”1276They proceeded to cover the same ground they had previously covered together in private, noting that “the energy liberated by a 5 kg bomb would be equivalent to that of several thousand tons of dynamite.” They described a simple mechanism for arming the weapon: making the uranium sphere in two parts “which are brought together first when the explosion is wanted. Once assembled, the bomb would explode within a second or less.”1277 Springs, they thought, might pull the two small hemispheres together. Assembly would have to be rapid or the chain reaction would begin prematurely, destroying the bomb but not much else. A byproduct of the explosion—about 20 percent of its energy, they thought—would be radiation, the equivalent of “a hundred tons of radium” that would be “fatal to living beings even a long time after the explosion.” Effective protection from the weapon would be “hardly possible.”

The second report, “Memorandum on the properties of a radioactive ‘super-bomb,’ ” a less technical document, was apparently intended as an alternative presentation for nonscientists.1278 This study explored beyond the technical questions of design and production to the strategic issues of possession and use; it managed at the same time both seemly innocence and extraordinary prescience:

1. As a weapon, the super-bomb would be practically irresistible. There is no material or structure that could be expected to resist the force of the explosion. . . .

2. Owing to the spreading of radioactive substances with the wind, the bomb could probably not be used without killing large numbers of civilians, and this may make it unsuitable as a weapon for use by this country. . . .

3. . . . It is quite conceivable that Germany is, in fact, developing this weapon. . . .

4. If one works on the assumption that Germany is, or will be, in the possession of this weapon, it must be realised that no shelters are available that would be effective and could be used on a large scale. The most effective reply would be a counter-threat with a similar weapon.

Thus in the first months of 1940 it was already clear to two intelligent observers that nuclear weapons would be weapons of mass destruction against which the only apparent defense would be the deterrent effect of mutual possession.

Frisch and Peierls finished their two reports and took them to Oliphant. He quizzed the men thoroughly, added a cover letter to their memoranda (“I have considered these suggestions in some detail and have had considerable discussion with the authors, with the result that I am convinced that the whole thing must be taken rather seriously, if only to make sure that the other side are not occupied in the production of such a bomb at the present time”) and sent letter and documents off to Henry Thomas Tizard, an Oxford man, a chemist by training, the driving force behind British radar development, the civilian chairman of the Committee on the Scientific Survey of Air Defense—better known as the Tizard Committee—which was the most important British committee at the time concerned with the application of science to war.1279

“I have often been asked,” Otto Frisch wrote many years afterward of the moment when he understood that a bomb might be possible after all, before he and Peierls carried the news to Mark Oliphant, “why I didn’t abandon the project there and then, saying nothing to anybody. Why start on a project which, if it was successful, would end with the production of a weapon of unparalleled violence, a weapon of mass destruction such as the world had never seen? The answer was very simple. We were at war, and the idea was reasonably obvious; very probably some German scientists had had the same idea and were working on it.”1280

Whatever scientists of one warring nation could conceive, the scientists of another warring nation might also conceive—and keep secret. That early in 1939 and early 1940, the nuclear arms race began. Responsible men who properly and understandably feared a dangerous enemy saw their own ideas reflected back to them malevolently distorted. Ideas that appeared defensive in friendly hands seen the other way around appeared aggressive. But they were the same ideas.

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Werner Heisenberg sent his considered conclusions to the German War Office on December 6, 1939, while Fermi and Szilard waited for the $6,000 the Briggs Uranium Committee had allocated to them for graphite studies and Frisch prepared his pessimistic Chemical Society review. Heisenberg thought fission could lead to energy production even with ordinary uranium if a suitable moderator could be found. Water would not do, but “heavy water [or] very pure graphite would, on the other hand, suffice on present evidence.” The surest method for building a reactor, Heisenberg wrote, “will be to enrich the uranium-235 isotope. The greater the degree of enrichment, the smaller the reactor can be made.” Enrichment—increasing the proportion of U235 to U238—was also “the only method of producing explosives several orders of magnitude more powerful than the strongest explosives yet known.”1281 (The phrase indicates Heisenberg understood the possibility of fast-neutron fission even before Frisch and Peierls did.)

During the same period Paul Harteck in Hamburg was building a Clusius separation tube; in December he tested it by successfully separating isotopes of the heavy gas xenon. He traveled to Munich at Christmastime to discuss design improvements with Clusius, who was professor of physical chemistry at the university there. Auer, the thorium specialists, purveyors of gas mantles and radioactive toothpaste, delivered the first ton of pure uranium oxide processed from Joachimsthal ores to the War Office in January 1940. German uranium research was thriving.

Acquiring a suitable moderator looked more difficult. The German scientists favored heavy water, but Germany had no extraction plant of its own. Harteck calculated at the beginning of the year that a coal-fired installation would require 100,000 tons of coal for each ton of heavy water produced, an impossibility in wartime. The only source of heavy water in quantity in the world was an electrochemical plant built into a sheer 1,500-foot granite bluff beside a powerful waterfall at Vemork, near Rjukan, ninety miles west of Oslo in southern Norway. Norsk Hydro-Elektrisk K vaelstofaktieselskab produced the rare liquid as a byproduct of hydrogen electrolysis for synthetic ammonia production.1282

I.G. Farben, the German chemical cartel assembled by Bayer’s Carl Duisberg in the 1920s, owned stock in Norsk Hydro; learning of the War Office’s need it approached the Norwegians with an offer to buy all the heavy water on hand, about fifty gallons worth some $120,000, and to order more at the rate of at least thirty gallons a month. Norsk Hydro was then producing less than three gallons a month, enough in the prewar years to glut the small physics-laboratory market. It wanted to know why Germany needed so vast a quantity. I.G. Farben chose not to say. In February the Norwegian firm refused either to sell its existing stock or to increase production.

Heavy water also impressed the French team, a fact Joliot pased on to the French Minister of Armament, Raoul Dautry. When Dautry heard about the German bid for Norsk Hydro’s supply he decided to win the water for France. A French bank, the Banque de Paris et des Pays Bas, controlled a majority interest in the Norwegian company and a former bank officer, Jacques Allier, was now a lieutenant in Dautry’s ministry.1283 Dautry briefed the balding, bespectacled Allier with Joliot on hand on February 20: the minister wanted the lieutenant to lead a team of French secret-service agents to Norway to acquire the heavy water.

Allier slipped into Oslo under an assumed name and met with the general manager of Norsk Hydro at the beginning of March. The French officer was prepared to pay up to 1.5 million kroner for the water and even to leave half for the Germans, but once the Norwegian heard what military purpose the substance might serve he volunteered his entire stock and refused payment. The water, divided among twenty-six cans, left Vemork by car soon afterward on a dark midnight. From Oslo Allier’s team flew it to Edinburgh in two loads—German fighters forced down for inspection a decoy plane Allier had pretended to board at the time of the first loading—and then transported it by rail and Channel ferry to Paris, where Joliot prepared through the winter and spring of the phony war to use it in both homogeneous and heterogeneous uranium-oxide experiments.

Nuclear research in the Soviet Union during this period was limited to skillful laboratory work. Two associates of Soviet physicist Igor Kurchatov reported to the Physical Review in June 1940 that they had observed rare spontaneous fissioning in uranium. “The complete lack of any American response to the publication of the discovery,” writes the American physicist Herbert F. York, “was one of the factors which convinced the Russians that there must be a big secret project under way in the United States.”1284 It was not yet big, but by then it had begun to be secret.

Japanese studies toward an atomic bomb began first within the military.1285 The director of the Aviation Technology Research Institute of the Imperial Japanese Army, Takeo Yasuda, a lieutenant general and an alert electrical engineer, conscientiously followed the international scientific literature that related to his field; in the course of his reading in 1938 and 1939 he noticed and tracked the discovery of nuclear fission.1286 In April 1940, foreseeing fission’s possible consequences, he ordered an aide who was scientifically trained, Lieutenant Colonel Tatsusaburo Suzuki, to prepare a full report. Suzuki went to work with a will.

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Niels Bohr had returned from Princeton to Copenhagen at the beginning of May 1939, preoccupied with the gathering European apocalypse. His friends had urged him to send for his family and remain in the United States. He had not been tempted. Refugees still escaping from Germany and now fleeing Central Europe as well needed him; his institute needed him; Denmark needed him. Hitler proposed on May 31 to compromise the neutrality of the Scandinavian countries with nonagression pacts. The pragmatic Danes alone accepted, fully aware the pact was worthless and even demeaning but unwilling to invite invasion for a paper victory. By autumn, when the John Wheelers offered to shelter one of Bohr’s sons in Princeton for the duration of the conflict, Bohr reserved the offer against future need. “We are aware that a catastrophe might come any day,” he wrote in the midst of Poland’s agony.1287

Catastrophe for Denmark waited until April 1940 and came then with brutal efficiency. Bohr was lecturing in Norway. The British had announced their intention to mine Norwegian coastal waters against shipment of Norwegian iron ore to Nazi Germany. On the final evening of his lecture tour, April 8, Bohr dined with the King of Norway, Haakon VII, and found King and government lost in gloom at the prospect of a German attack. After dinner he boarded the night train for Copenhagen. A train ferry carried the cars across the Óresund at night to Helsingør while the passengers slept. Danish police pounding on compartment doors woke them to the news: the Germans had invaded not only Norway but Denmark as well. Two thousand German troops hidden in coal freighters moored near Langelinie, the Copenhagen pier of Hans Christian Andersen’s Little Mermaid, had stormed ashore in the early morning, so unexpected a sight that night-shift workers bicycling home thought a motion picture was being filmed. A major German force had marched north through Schleswig-Holstein onto the Danish peninsula as well, crossing the border before dawn. German aircraft marked with black crosses dominated the air. German warships commanded the Kattegat and Skagerrak passages that open Denmark and southern Norway to the North Sea.

The Norwegians fought back, determined that their King, court and parliament must escape to exile. The Danes, in their flat country where Panzers might roll, did not. Rifle fire crackled in the streets of Copenhagen in the early morning, but King Christian X ordered an immediate ceasefire, which took effect at 6:25 A.M. By the time Bohr’s train arrived in the capital city what Churchill would call “this ruthless coup” was complete, the streets littered with green surrender leaflets, the King preparing to receive the German chief of staff.1288 Danish resistance would be dedicated and effective, but it would take less suicidal forms than open battle with the Wehrmacht.

The American Embassy quickly passed word that it could guarantee the Bohrs safe passage to the United States. Bohr again chose duty. His immediate concern was to burn the files of the refugee committee that had helped hundreds of emigres to escape to exile. “It was characteristic of Niels Bohr,” his collaborator, Stefan Rozental, writes, “that one of the first things he did was to contact the Chancellor of the University and other Danish authorities in order to protect those of the staff at the Institute whom the Germans might be expected to persecute.”1289 Those were Poles first of all, but Bohr also sought out government leaders to argue for concerted Danish resistance to any German attempt to install anti-Semitic laws in Denmark.

He even found time on the day of the occupation to worry about the large gold Nobel Prize medals that Max von Laue and James Franck had given him for safekeeping.1290 Exporting gold from Germany was a serious criminal offense and their names were engraved on the medals.1291, 1292 George de Hevesy devised an effective solution—literally: he dissolved the medals separately in acid. As solutions of black liquid in unmarked jars they sat out the war innocently on a laboratory shelf. Afterward the Nobel Foundation recast them and returned them to their owners.

Norsk Hydro was a prime German objective and there was heavy fighting around Rjukan, which held out until May 3, the last town in southern Norway to surrender. Then a management under duress reported to Paul Harteck that its heavy-water facility, the Vemork High Concentration Plant, could be expanded to increase production of the ideal neutron moderator to as much as 1.5 tons per year.

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“What I should like,” Henry Tizard wrote Mark Oliphant after he had studied the Frisch-Peierls memoranda, “would be to have quite a small committee to sit soon to advise what ought to be done, who should do it, and where it should be done, and I suggest that you, Thomson, and say Blackett, would form a sufficient nucleus for such a committee.” Thomson was G. P. Thomson, J.J.’s son, the Imperial College physicist who had ordered up a ton of uranium oxide the previous year to study and felt ashamed at the absurdity. He had concluded after neutron-bombardment experiments that a chain reaction in natural uranium was unlikely and a war project therefore impractical. Tizard, who had been skeptical to begin with and had taken Thomson’s conclusions as support for his skepticism, appointed Thomson chairman of the small committee; James Chadwick, now at Liverpool, his assistant P. B. Moon and Rutherford protege John Douglas Cockcroft were added to the list. Blackett was busy with other war work, although he would join the committee later. The group met informally for the first time on April 10 in the Royal Society’s quarters at Burlington House.

It probably met as much to hear a visitor, the ubiquitous Jacques Allier of the Banque de Paris and the French Ministry of Armament, as to discuss the Frisch-Peierls work. Allier warned the British physicists about the German interest in heavy-water production and bid for collaboration on nuclear research between Britain and France. Only then, Thomson notes in the minutes he kept, did they consider “the possibility of separating isotopes . . . and it was agreed that the prospects were sufficiently good to justify small-scale experiments on uranium hexafluoride [a gaseous uranium compound].” They proposed rather ungenerously to remind Frisch to avoid “any possible leakage of news in view of the interest shown by the Germans.”1293 They were willing to inform him that his memorandum was being considered but not to supply details. (Peierls’ name seems not yet to have made an impression on Thomson, and Tizard apparently retained the second Frisch-Peierls memorandum in his files.) “We entered the project with more scepticism than belief,” the committee would report later, “though we felt it was a matter which had to be investigated.”1294 Thomson’s minutes make that skepticism evident. Tizard for his part wrote Lindemann’s brother Charles, a science adviser to the British Embassy in Paris, that he considered the French “unnecessarily excited” about the perils of German nuclear research.1295 “I still . . . think that [the] probability of anything of real military significance is very low,” he estimated in a note written the same week to the British War Cabinet staff.1296

It might have been as unpromising a start as the first meeting of the Briggs Uranium Committee had been, but the men on the Thomson committee were active, competent physicists, not military ordnance specialists, and whatever their initial skepticism they understood where the numbers Frisch and Peierls had used came from and what they might mean. At a second meeting on April 24 Thomson recorded laconically that “Dr. Frisch produced some notes to show that the uranium bomb was feasible.”1297 Many years later Oliphant recalled a more expansive response: “The Committee generally was electrified by the possibility.” Chadwick’s good opinion helped. He had just begun exploring fast-neutron fission himself with his new Liverpool cyclotron, the first in England, when he saw the Frisch-Peierls memorandum. At the April 24 meeting he awarded the emigres’ work chagrined confirmation: he “was embarrassed,” says Oliphant, “confessing that he had reached similar conclusions, but did not feel justified in reporting them until more was known about the neutron cross sections from experiments. Peierls and Frisch had used calculated values. However, this confirmatory evidence led the Committee to pay great attention to the development of techniques for . . . separation.”1298

Chadwick agreed to undertake the necessary studies. For several more weeks, until their protests through Oliphant registered with Thomson, Frisch and Peierls would be walled off from their own secrets. But work toward a bomb of chain-reacting uranium was now fairly begun, and this time it had found the right—fast—track.

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Szilard chafed. The months after the first Uranium Committee meeting became “the most curious period of my life.” No one called. “We heard nothing from Washington at all. . . . I had assumed that once we had demonstrated that in the fission of uranium neutrons are emitted, there would be no difficulty in getting people interested; but I was wrong.”1299 The Uranium Committee’s November 1 report had in fact been languishing in Roosevelt’s files; Watson finally decided on his own in early February 1940 to bring it up again.1300 He asked Lyman Briggs if he had anything to add. Briggs reported the transfer, finally, of the $6,000 for Fermi’s work on neutron absorption in graphite. That was “a crucial undertaking,” Briggs said; he imagined it would determine “whether or not the undertaking has a practical application.”1301 He proposed to wait for results.

Something other than Briggs’ penurious methodology triggered a new burst of activity from Szilard. He had spent the winter preparing a thorough theoretical study, “Divergent chain reactions in systems composed of uranium and carbon”—divergent in this case meaning chain reactions that continue to multiply once begun (the document’s first footnote, numbered zero, cited “H.1302 G. Wells, The World Set Free [1913]”). Early in the new year Joliot’s group reported a uranium-water experiment that “seemed to come so close to being chain-reacting,” says Szilard, “that if we improved the system somewhat by replacing water with graphite, in my opinion we should have gotten over the hump.” He arranged lunch with Fermi to discuss the French paper. “I asked him, ‘Did you read Joliot’s paper?’ He said he did. I asked him, ‘What did you think of it?’ and Fermi said, ‘Not much.’ ” Szilard was furious. “At which point I saw no reason to continue the conversation and went home.”1303

He traveled again to Princeton to see Einstein. They worked up another letter and sent it under Einstein’s signature to Sachs. It emphasized the secret German uranium research at the Kaiser Wilhelm Institutes, about which they had learned from the physical chemist Peter Debye, the 1936 Nobel laureate in chemistry and director of the physics institute at Dahlem, who had been expelled recently to the United States, ostensibly on leave of absence, when he refused to give up Dutch citizenship and join the Nazi Reich. Sachs sent the Einstein letter on to Pa Watson for FDR. But Watson thought it sensible to check first with the Uranium Committee. Adamson responded, echoing Briggs: everything depended on the graphite measurements at Columbia. Watson proposed to wait for the official report. Sachs may have rebutted; Roosevelt wrote the gadfly economist on April 5 emphasizing that the Briggs committee was “the most practical method of continuing this research” but also calling for another committee meeting that Sachs might attend.1304 Briggs dutifully scheduled it for Saturday afternoon, April 27.

In the meantime another development intervened. Alfred Nier at the University of Minnesota had gone to work, after Fermi wrote urging him again to do so, to prepare to separate measurable samples of U235 and U238. John Dunning sent him uranium hexafluoride, a highly corrosive compound that is a white solid at room temperature but volatilizes to a gas when heated to 140°F. “I worked with this for a couple of months in late 1939,” Nier remembers.1305 Unfortunately the gas was too volatile; it dispersed through Nier’s three-foot glass spectrometer tube despite the best efforts of his vacuum pump to clear it and contaminated the collector plates:

Finally I said, “This won’t do.” A new instrument was built in about 10 days in February, 1940. Our glass blower bent the horseshoe-shaped mass spectrometer tube for me; I made the metal parts myself. As a source of uranium, I used the less volatile uranium tetrachloride and tetrabromide left over from [his earlier] Harvard experiments. The first separation of U-235 and U-238 was actually accomplished on February 28 and 29, 1940. It was a leap year, and on Friday afternoon, February 29,1 pasted the little samples [collected on nickel foil] on the margin of a handwritten letter and delivered them to the Minneapolis Post Office at about six o’clock. The letter was sent by airmail special delivery and arrived at Columbia University on Saturday. I was aroused early Sunday morning by a long-distance telephone call from John Dunning [who had worked through the night bombarding the samples with neutrons from the Columbia cyclotron]. The Columbia test of the samples clearly showed that U-235 was responsible for the slow neutron fission of uranium.

The demonstration vindicated Bohr’s hypothesis, but it also led Briggs to even greater suspicion of the value of natural uranium; it was “very doubtful,” he reported to Watson on April 9 “whether a chain reaction can be established without separating 235 from the rest of the uranium.”1306 Nier, Dunning and their collaborators Eugene T. Booth and Aristide von Grosse had written much the same thing in the Physical Review on March 15: “These experiments emphasize the importance of uranium isotope separation on a larger scale for the investigation of chain reaction possibilities in uranium.”1307 But isotope separation was Dunning’s approach to the problem in the first place and his enthusiasm as well; the slow-neutron finding hardly ruled out the Fermi-Szilard system. More misleading may have been the measurements Nier and the Columbia team published on April 15 using larger (but still microscopic) samples: “Furthermore, the number of fissions/microgram of U238 observed under these neutron intensity conditions, is sufficient to account for practically all the fast neutron fission observed in unseparated U.”1308 The statement was correct within the limits of measurement for such small samples, but its wording seems to deprecate U235 fast-neutron fission. In fact, Nier had not collected enough U235 to allow Columbia to measure that possibility. All anyone knew by then was that the U235 cross section for fast-neutron fission was less than the isotope’s cross section for slow-neutron fission. But that cross section, as the first Nier/Columbia paper reported, was a whopping 400 to 500 × 10−24cm2.1309

Predictably, then, when the Uranium Committee met on April 27, with Sachs, Pegram, Fermi, Szilard and Wigner in attendance, it listened to the renewed debate, squared its shoulders at Sachs’ exhortation to plunge ahead—and never wavered in its adamant conviction that a large-scale uranium-graphite experiment should await the outcome of Fermi’s graphite measurements.

*   *   *

Now that the $6,000 had been paid, Columbia was able to buy the graphite Szilard had tracked down for Fermi’s use. “Cartons of carefully-wrapped graphite bricks began to arrive at the Pupin Laboratory,” Herbert Anderson remembers, four tons in all. “Fermi returned to the chain reaction problem with enthusiasm. This was the kind of physics he liked best. Together we stacked the graphite bricks in a neat pile. We cut narrow slots in some of the bricks for the rhodium foil detectors we wanted to insert, and soon we were ready to make measurements.”1310

“So the physicists on the seventh floor of Pupin Laboratories started looking like coal miners,” adds Fermi, “and the wives to whom these physicists came back tired at night were wondering what was happening.”1311

The arrangement was designed to determine how far neutrons from a radon-beryllium source set in paraffin on the floor under the graphite column would diffuse up the column through the graphite after first slowing down in scattering collisions: the farther the neutrons traveled, the smaller was carbon’s absorption cross section and therefore the better moderator it would be. The Pupin seventh floor became a racetrack like the second floor of the institute in Rome. Anderson describes the scene:

A precise schedule was followed for each measurement. With the rhodium in place in the graphite, the source was inserted in its position inside the pile and removed after a one-minute exposure. To get the rhodium foil under the Geiger counter in the allotted 20 seconds [because its induced half-life is only 44 seconds] took coordination and some fast legwork. The division of labor was typical. I removed the source on signal; Fermi, stopwatch in hand, grabbed the rhodium and raced down the hall at top speed. He had just enough time to place the foil carefully into position, close the lead shield and, at the prescribed moment, start the count. Then with obvious satisfaction at seeing everything go right, he would watch the flashing lights on the scaler, tapping his fingers on the bench in time with the clicking of the register. Such a display of the phenomenon of radioactivity never failed to delight him.1312

The absorption cross section, as Fermi and Anderson subsequently calculated it, proved usefully small: 3 × 10−27cm2.1313 And could be made smaller still, they thought, with purer graphite. The measurement strongly supported Fermi’s and Szilard’s plan to attempt to induce a slow-neutron chain reaction in natural uranium.

But while such a plan might demonstrate a potential future source of power, the American scientists and administrators who were advising Briggs could not yet identify any military use. In April the British Thomson committee asked A. V. Hill, a scientific adviser to the British Embassy in Washington, to find out what the Americans were doing about fission. According to the official history of the British atomic energy program, Hill talked to unidentified “scientists of the Carnegie Institution,” whose opinions he reported pungently:1314

It is not inconceivable that practical engineering applications and war use may emerge in the end. But I am assured by American colleagues that there is no sign of them at present and that it would be a sheer waste of time for people busy with urgent matters in England to turn to uranium as a war investigation. If anything likely to be of war value emerges they will certainly give us a hint of it in good time. A large number of American physicists are working on or interested in the subject; they have excellent facilities and equipment: they are extremely well disposed towards us: and they feel that it is much better that they should be pressing on with this than that our people should be wasting their time on what is scientifically very interesting, but for present practical needs probably a wild goose chase.1315

The opinion from the Carnegie may have been hardheaded, but it was based on more than prejudice. Roberts, Hafstad and fellow DTM physicist Norman P. Heydenburg had improved their measurements of cross sections for fast-neutron fission, scattering and capture in natural uranium. Using their numbers, Edward Teller in one of the many calculations he made during this period arrived at a critical mass in excess of thirty tons, the same order of magnitude as Perrin and Peierls had calculated before him.1316 With only slightly more pessimistic assumptions Roberts concluded that “the cross-section for capture [in natural uranium] is sufficiently large that it now seems impossible for a fast-neutron chain reaction to occur, even in an infinitely large block of pure uranium.”1317 By the spring of 1940 experiments at Columbia and the DTM had thus ruled out both slow- and significant fast-neutron fission in U238 and ruled in slow-neutron fission in U235. The asymmetry might have been a clue. No one picked it up.

*   *   *

Since at least the time of Einstein’s first letter to FDR, Edward Teller had debated within himself the morality of weapons work. His life had twice been cruelly uprooted by totalitarianism. He understood Germany’s frightening technological advantages at the outset of the war. “I came to the United States in 1935,” he notes. “ . . . The handwriting was on the wall. At that time, I believed that Hitler would conquer the world unless a miracle happened.”1318 But pure science still pacified him. “To deflect my attention from physics, my full-time job which I liked, to work on weapons, was not an easy matter. And for quite a time I did not make up my mind.”1319

The accidental juxtaposition of two events led him to decision. “In the spring of 1940 it was announced that President Roosevelt would speak to a Pan American Scientific Congress in Washington, and as one of the professors of George Washington University I was invited. I did not intend to go.”1320 The other event of that crucial day, May 10, 1940, reversed his intention: the phony war abruptly ended. With seventy-seven divisions and 3,500 aircraft Germany without declaration or warning invaded Belgium, the Netherlands and Luxembourg to make way for the invasion of France. Teller thought Roosevelt might speak to that outrage. In his voluntary prewar isolation he had never bothered, Teller says, to visit the Capitol or listen to one of FDR’s radio talks or otherwise involve himself in the political life of his adopted country, but he wanted now to see the President of the United States in person.1321

Alone among the scientists at the congress Teller knew about the Einstein letter. It was a direct link, he was an emotional man and the encounter with Roosevelt was eerily personal: “We had never met, but I had an irrational feeling he was talking to me.”1322The President mentioned the German invasion, its challenge to “the continuance of the type of civilization” the people of the Americas valued, the distances of the modern world shortened by modern technology to timetables that removed the “mystic immunity” Americans once felt from European war.1323 “Then he started to talk about the role of the scientist,” Teller recalls, “who has been accused of inventing deadly weapons.1324 He concluded: ‘If the scientists in the free countries will not make weapons to defend the freedom of their countries, then freedom will be lost.’ ” Teller believed Roosevelt was not proposing what scientists may do “but something that was our duty and that we must do—to work out the military problems, because without the work of the scientists the war and the world would be lost.”1325

Teller’s memory of Roosevelt’s speech differs from its text. The President said that most people abhor “conquest and war and bloodshed.”1326 He said that the search for truth was a great adventure but that “in other parts of the world, teachers and scholars are not permitted” that search—an observation of which Teller had personal knowledge. And then, cannily, Roosevelt offered absolution in advance for war work:

You who are scientists may have been told that you are in part responsible for the debacle of today . . . but I assure you that it is not the scientists of the world who are responsible. . . . What has come about has been caused solely by those who would use, and are using, the progress that you have made along lines of peace in an entirely different cause.

“My mind was made up,” Teller reports, “and it has not changed since.”1327

*   *   *

Vannevar Bush made a similar choice that spring. The sharp-eyed Yankee engineer, who looked like a beardless Uncle Sam, had left his MIT vice presidency for the Carnegie Institution in the first place to position himself closer to the sources of government authority as war approached. Karl Compton had offered to move up to chairman of the MIT corporation and give him the presidency to keep him, but Bush had larger plans.

As a young man, with a doctorate in engineering behind him jointly from MIT and Harvard earned in one intense year, Bush in 1917 had gone patriotically to work for a research corporation developing a magnetic submarine detector. The device was effective, and one hundred sets got built; but because of bureaucratic confusion they were never put to use against German submarines. “That experience,” Bush writes in a memoir, “forced into my mind pretty solidly the complete lack of proper liaison between the military and the civilian in the development of weapons in time of war, and what that lack meant.”1328

In Washington after the invasion of Poland the Carnegie president gathered with a group of fellow science administrators—Frank Jewett, president of Bell Telephone Laboratories and the National Academy of Sciences; James Bryant Conant, the young president of Harvard, a distinguished chemist; Richard Tolman of Caltech, the theoretician who had wooed Einstein; Karl Compton—to worry about the approaching conflict:

It was during the period of the “phony” war. We were agreed that the war was bound to break out into an intense struggle, that America was sure to get into it in one way or another sooner or later, that it would be a highly technical struggle, that we were by no means prepared in this regard, and finally and most importantly, that the military system as it existed . . . would never fully produce the new instrumentalities which we would certainly need.1329

They devised a national organization to do the job. Bush had learned his way around Washington and took the lead. The organization Bush wanted needed independent authority. He thought it should report directly to the President rather than through military channels and should have its own source of funds. He drafted a proposal. Then he arranged an introduction to Harry Hopkins.

A small-town Iowa boy, idealistic and energetic, Harry Lloyd Hopkins had fallen into New York social work after four years at Grinnell and won appointment at the beginning of the Depression administering emergency state relief. When the governor of New York was elected President, Hopkins moved with Roosevelt to Washington to help out with the New Deal. He ran the vast Works Progress Administration, then took over as Secretary of Commerce. His performance moved him closer and closer to the President, who picked up talent wherever he could find it; as war approached, Roosevelt invited Hopkins to dinner at the White House one evening and moved the man in for the duration as his closest adviser and aide. Hopkins was tall, a chain smoker and emaciated to the point of cachexia, his ghastly health the result of cancer surgery that took most of his stomach and left him unable to absorb much protein and therefore slowly starving to death. He kept an office in the White House basement but usually worked out of a cluttered bedroom suite—the Lincoln Bedroom—down the hall from FDR’s.

When Bush met Hopkins, though the presidential aide was a liberal Democrat and the Carnegie president an admirer of Herbert Hoover and a self-styled Tory, “something meshed,” writes Bush, “and we found we spoke the same language.”1330 Hopkins had a scheme for an Inventors Council. Bush countered with his more comprehensive National Defense Research Council. “Each of us was trying to sell something to the other.”1331 Bush won. Hopkins liked his plan.

In early June Bush made the rounds of Washington touching bases: the Army, the Navy, Congress, the National Academy of Sciences. On June 12 “Harry and I then went in to see the President. It was the first time I had met Franklin D. Roosevelt. . . . I had the plan for N.D.R.C. in four short paragraphs in the middle of a sheet of paper. The whole audience lasted less than ten minutes (Harry had no doubt been there before me). I came out with my ‘OK-FDR’ and all the wheels began to turn.”

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.1332, 1333 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.1334

Not only native-born Englishmen felt that surge. So did the emigré 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.”1335, 1336 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.1337 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 100,000 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.’ ”1338

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.”1339 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.”1340

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.1341 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.”1342 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.”1343 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 sodawater”—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.1344 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.”1345 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.1346 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.1347 Negotiations with the Dutch advanced, the air raid was canceled, but the abort message arrived too late to stop half the hundred Heinkel lll’s ordered into action from dropping 94 tons of bombs. The bombs started massive fires in stores of fats and margarine. The first official Dutch statement, issued from the embassy in Washington, placed casualties in the devastated city at 30,000, and the Western democracies responded with outrage. Actual deaths totaled about 1,000; some 78,000 people went homeless.

The British retaliated on May 15 by dispatching ninety-nine bombers to attack railway centers and supply depots in the Ruhr. Busy with the Battle of France, Hitler did not immediately strike back, but he issued a directive that prepared the way. He ordered the Luftwaffe “to undertake a full-scale offensive against the British homeland as soon as sufficient forces are available.”1348

The initial German air attack, the Battle of Britain, began in mid-August: a month of ferocious daylight contests between the Luftwaffe and British Fighter Command for air supremacy in advance of Operation Sea Lion, Germany’s planned cross-Channel invasion. It was not yet an attack on cities. British airfields and aircraft factories were primary targets. Hitler had reserved for himself the decision to bomb London, just as the Kaiser had done before him.1349 Cities would soon go on the targeting list, however; the Luftwaffe was scheduled to raid Liverpool at night on August 28. Accident again intervened: German bombers aiming for oil storage tanks along the Thames overflew their targets on August 24 and bombed central London instead.

Churchill immediately retaliated, hurling four bombing raids in one week at Berlin. They accomplished little physical damage but incited Hitler to hysterical revenge:

And if the British air force drops two or three or four thousand kilograms of bombs, then we will drop in a single night 150,000, 180,000, 230,000, 300,000, 400,000, a million kilograms. If they announce that they will attack our cities on a large scale, then we shall wipe their cities out!1350

The Luftwaffe was losing the Battle of Britain in any case, taking unacceptable losses—some 1,700 German aircraft compared to about 900 British. Night bombing would alleviate the losses, curtaining the bombers in dark asylum. But night bombing was notably less accurate than daylight bombing in those days before effective radar and required correspondingly larger targets. Cities and their civilian populations thus fell victim partly by default, because the technology necessary for more accurate targeting was not yet at hand. In any case terror was a weapon that Hitler especially prized, the destruction of what he called the enemy’s “will-to-resist,” and early in September he told his Sea Lion planners that “a systematic and long-drawn-out bombardment of London might produce an attitude in the enemy which will make Sea Lion unnecessary.”1351, 1352 He ordered the bombardment. Since it rained from the skies for months, it was hardly Blitzkrieg, lightning war, but the citizens exposed beneath it were not in the mood for fine distinctions, and they soon named it the Blitz.

Gresham’s Law operated with air raid shelters as it operates with good and bad money: the basements of better department stores like Dickens and Jones, where clerks carried around refreshments—chocolates and ice cream—filled up first. Because the bombing followed regularly, night after night, Londoners had time to get used to it, but adjustment could go either way, the confident beginner slowly unraveling, the frightened beginner moving beyond fear.

More Londoners by far lived out the dangerous raids in their homes than in shelters: 27 percent fled to corrugated-iron Anderson shelters in back gardens, 9 percent to street shelters, only 4 percent into the Tube. By mid-November 13,700 tons of high explosives had fallen and 12,600 tons of incendiary canisters, an average of 201 tons per night; for the entire Blitz, September to May, the total tonnage reached 18,800—18.8 kilotons by modern measure, spread across nine months.1353 London civilian deaths in 1940 and 1941 totaled 20,083, civilian deaths elsewhere in Britain 23,602, for a total death by Blitz in the second and third year of the war (about which the United States was still officially neutral) of 43,685.1354 After that the bombing went the other way. Only twenty-seven Londoners lost their lives to bombs in 1942.

At Oxford in December 1940, Franz Simon, now officially working for the MAUD Committee, produced a report nearly as crucial to the future of uranium-bomb development as the original Frisch-Peierls memoranda had been.1355 It was titled “Estimate of the size of an actual separation plant.” Its aim, Simon wrote, was “to provide data for the size and costs of a plant which separates 1 kg per day of 235U from the natural product.”1356 He estimated such a plant would cost about £5,000,000 and outlined its necessities in careful detail.

Simon had never trusted the mails. He trusted them even less at the height of the Blitz. He duplicated some forty copies of his report, accumulated enough rationed gasoline for a round trip and shortly before Christmas drove from Oxford into bomb-threatened London to deliver the fruit of half a year’s hard work, his whole force in the struggle for his country, to G. P. Thomson.

*   *   *

The Germans may have been collecting radium, as Cockcroft thought MAUD RAY KENT signaled. They were certainly laying in industrial stocks of uranium. In June 1940, about the time Simon was hammering out his kitchen strainer, Auer ordered sixty tons of refined uranium oxide from the Union Miniére in occupied Belgium.1357 Paul Harteck in Hamburg tried that month to measure neutron multiplication in an ingenious arrangement of uranium oxide and dry ice—frozen carbon dioxide, a source of carbon free from any impurity other than oxygen—but was unable to convince Heisenberg to lend him enough uranium to guarantee unambiguous results. Heisenberg had larger plans. He had allied himself with von Weizsácker at the KWI. In July they began designing a wooden laboratory building to be constructed on the grounds of the Kaiser Wilhelm Institute for Biology and Virus Research, next to the physics institute. To discourage the curious they named the building the Virus House. They intended to build a subcritical uranium burner there.

Germany had access to the world’s only heavy-water factory and to thousands of tons of uranium ore in Belgium and the Belgian Congo. It had chemical plants second to none and competent physicists, chemists and engineers . It lacked only a cyclotron for measuring nuclear constants. The Fall of France—Paris was occupied June 14, an armistice signed June 22—filled that need. Kurt Diebner, the War Office’s resident nuclear physics expert, rushed to Paris. Perrin, von Halban and Kowarski, he found, had escaped to England and taken Allier’s twenty-six cans of heavy water with them, but Joliot had chosen to remain in France.1358 (The French laureate would become president of the Directing Committee of the National Front, the largest Resistance organization of the war.)

German officers interrogated Joliot at length when he returned to his laboratory after the occupation began. Their interpreter, sent along from Heidelberg, turned out to be Wolfgang Gentner, the former Radium Institute student who had confirmed that Joliot’s Geiger counter was working properly when Joliot discovered artificial radioactivity in 1933. Gentner arranged a secret meeting one evening at a student café and warned Joliot that the cyclotron he was building might be seized and shipped to Germany. Rather than allow that outrage Joliot negotiated a compromise: the cyclotron would stay but German physicists could use it for purely scientific experiments; Joliot would be allowed in turn to continue as laboratory director.

The Virus House was finished in October. Besides a laboratory the structure contained a special brick-lined pit, six feet deep, a variant of Fermi’s water tank for neutron-multiplication studies. By December Heisenberg and von Weizsäcker had prepared the first of several such experiments. With water in the pit to serve as both reflector and radiation shield they lowered down a large aluminum canister packed with alternating layers of uranium oxide and paraffin. A radium-beryllium source in the center of the canister supplied neutrons, but the German physicists were able to measure no neutron multiplication at all. The experiment confirmed what Fermi and Szilard had already demonstrated: that ordinary hydrogen, whether in the form of water or paraffin, would not work with natural uranium to sustain a chain reaction.

That understanding left the German project with two possible moderator materials: graphite and heavy water.1359 In January a misleading measurement reduced that number to one. At Heidelberg Walther Bothe, an exceptional experimentalist who would eventually share a Nobel Prize with Max Born, measured the absorption cross section of carbon using a 3.6-foot sphere of high-quality graphite submerged in a tank of water. He found a cross section of 6.4 × 10−27 cm2, more than twice Fermi’s value, and concluded that graphite, like ordinary water, would absorb too many neutrons to sustain a chain reaction in natural uranium. Von Halban and Kowarski, now at Cambridge and in contact with the MAUD Committee, similarly overestimated the carbon cross section—the graphite in both experiments was probably contaminated with neutron-absorbing impurities such as boron—but their work was eventually checked against Fermi’s. Bothe could make no such check. The previous fall Szilard had assaulted Fermi with another secrecy appeal:

When [Fermi] finished his [carbon absorption] measurement the question of secrecy again came up. I went to his office and said that now that we had this value perhaps the value ought not to be made public. And this time Fermi really lost his temper; he really thought this was absurd. There was nothing much more I could say, but next time when I dropped in his office he told me that Pegram had come to see him, and Pegram thought that this value should not be published. From that point the secrecy was on.1360

It was on just in time to prevent German researchers from pursuing a cheap, effective moderator. Bothe’s measurement ended German experiments on graphite. Nothing in the record indicates the overestimate was deliberate, but it is worth noting that Walther Bothe, a protege of Max Planck, had been hounded from the directorship of the physics institute of the University of Heidelberg in 1933 because he was anti-Nazi. “These galling fights so affected my health,” he wrote later in a brief unpublished memoir, “that I had to spend a long period in a Badenweiler sanitorium.” When Bothe was well again Planck appointed him to the Kaiser Wilhelm Society’s Heidelberg physics institute, but “the Nazis continued to harass me, even to the accusation of scientific fraud.”1361

At nearly the same time—early 1941—Harteck learned at Hamburg what Otto Frisch had recently learned at Liverpool. Frisch had moved to the industrial port city in the northwest of England to work with Chadwick and Chadwick’s cyclotron. He built a Clusius tube there with a student assistant Chadwick assigned him—they moved in such energetic coordination through the laboratory that they won the nickname “Frisch and Chips”—and discovered, says Frisch, that “uranium hexafluoride is one of the gases for which the Clusius method does not work.”1362 The discovery set the British program back not at all, since Simon was already hard at work on gaseous barrier diffusion. But the German researchers had placed such faith in thermal diffusion that they had not bothered to develop alternatives. They quickly began doing so and identified several promising methods; oddly enough, barrier diffusion was not among them. Restudying the separation problem made it even clearer that U235 and U238 could only be separated by brute-force methods and at great expense.

When Harteck reported to the War Office in March 1941, following a conference with his colleagues, he stressed their consensus that isotope separation would be feasible “only for special applications in which cheapness is but a secondary consideration.”1363Only for a bomb, he meant—so he told the historian David Irving after the war. The German physicists gave “special applications” second place on their list; they recommended urgent work first of all on the production of heavy water. Like Fermi and Szilard, they opted initially for a slow-neutron chain reaction in natural uranium. Make that reaction work and “special applications” might follow. Knowing no more than they knew, they hardly had a choice.

*   *   *

Lieutenant Colonel Suzuki reported back to Lieutenant General Yasuda in October 1940.1364 He confined his report to a basic issue: the availability to Japan of uranium deposits. He looked beyond Japan to Korea and Burma and concluded that his country had access to sufficient uranium. A bomb was therefore possible.

Yasuda turned then to the director of Japan’s Physical and Chemical Research Institute, who passed the problem on to his country’s leading physicist, Yoshio Nishina. Nishina, born late in the Meiji era and fifty years old in 1940, known for theoretical work on the Compton Effect, had studied with Niels Bohr in Copenhagen, where he was remembered as a cosmopolitan and exceptional man. He had built a small cyclotron at his Tokyo laboratory, the Riken, and with help from an assistant who had trained at Berkeley was building in 1940 a 60-inch successor with a 250-ton magnet, the plans for which had been donated by Ernest Lawrence. More than one hundred young Japanese scientists, the cream of the crop, worked under Nishina at the Riken; to them he was Oyabun,“the old man,” and he ran his laboratory Western-style with warmth and informality.

The Riken began measuring cross sections in December. In April 1941 the official order came through: the Imperial Army Air Force authorized research toward the development of an atomic bomb.

*   *   *

Leo Szilard was known by now throughout the American physics community as the leading apostle of secrecy in fission matters. To his mailbox, late in May 1940, came a puzzled note from a Princeton physicist, Louis A. Turner. Turner had written a Letter to the Editor of the Physical Review, a copy of which he enclosed.1365 It was entitled “Atomic energy from U238” and he wondered if it should be withheld from publication. “It seems as if it was wild enough speculation so that it could do no possible harm,” Turner told Szilard, “but that is for someone else to say.”1366

Turner had published a masterly twenty-nine-page review article on nuclear fission in the January Reviews of Modern Physics citing nearly one hundred papers that had appeared since Hahn and Strassmann reported their discovery twelve months earlier; the number of papers indicates the impact of the discovery on physics and the rush of physicists to explore it.1367 Turner had also noted the recent Nier/Columbia report confirming the attribution of slow-neutron fission to U235. (He could hardly have missed it; theNew York Times and other newspapers publicized the story widely. He wrote Szilard irritably or ingenuously that he found it “a little difficult to figure out the guiding principle [of keeping fission research secret] in view of the recent ample publicity given to the separation of isotopes.”1368) His reading for the review article and the new Columbia measurements had stimulated him to further thought; the result was his Physical Review letter.

Since U235 is responsible for slow-neutron fission, the letter pointed out, and ordinary uranium contains only one part in 140 of that isotope, “it is natural to conclude that only 1/140 of any quantity of U can be considered as a possible source of atomic energy if slow neutrons are to be used.”1369 But the truth may be otherwise, Turner went on. The fission energy of most of the U238, if it could not be used directly, might yet find indirect release.

Turner was referring to the possibility that bombarding uranium with neutrons converted some of the uranium to transuranic elements, the transuranics that Bohr had hoped might have been banished by the discovery of fission. When an atom of U238 captured a neutron it became the isotope U239. That substance itself might fission, Turner suggested. But whether or not U239 did so, it was energetically unstable and would probably decay by beta emission to new elements heavier than uranium. And one or more of those new elements might be fissionable by slow neutrons—which would thereby indirectly put U238 to work.

The next element up the periodic table from uranium would be element 93. Turner selected as the likeliest candidate for fission not 93 X 239, however, but the element next along, the element that 93 would probably decay to, 94 X 239, which he called “eka-osmium.”1 And 94 EkaOs 239, Turner proposed, changing from an odd to an even number of neutrons when it absorbed a neutron preparatory to fissioning (239 nucleons—94 protons = 145 neutrons + 1 = 146) just as U235 changed to U236, ought to be even more fissionable than the lighter uranium isotope: “In 94EkaOs240 . . . the excess energy would be even larger than in 92 U 236 and a large cross section for fission would be expected.”1370, 1371

While Turner was thinking these theories through, two Berkeley men, Edwin M. McMillan and Philip M. Abelson, were moving independently toward demonstrating them. McMillan, a slim, freckled, California-born experimentalist, had been one of the men most responsible in the 1930s for improving Ernest Lawrence’s cyclotrons to the point where they worked steadily and produced reliable results. Soon after the news of the discovery of fission reached Berkeley in late January 1939 he had devised an elegantly simple experiment to explore the phenomenon. “When a nucleus of uranium absorbs a neutron and fission takes place,” McMillan told an audience later, “the two resulting fragments fly apart with great violence, sufficient to propel them through the air, or other matter, for some distance. This distance, called the ‘range,’ is a quantity of some interest, and I undertook to measure it.” He did so first with thin sheets of aluminum foil “like the pages of a book” stacked on a layer of uranium oxide backed with filter paper.1372He bombarded the uranium with slow neutrons. Some of the fission fragments recoiled up into the stack of foils; each fragment embedded itself in a single sheet of foil at the end of its range, which depended on its mass; McMillan could then simply check successive sheets of foil in an ionization chamber, look for the characteristic half-lives of various fission products and read out the range (the uranium nucleus splits in many different ways, producing many different lighter-element nuclei).

But aluminum itself is activated by neutron bombardment, which made half-life measurements difficult. So McMillan replaced the foils with a stack of cigarette papers previously treated with acid to remove any trace of minerals that might develop radioactivity under bombardment. “Nothing very interesting about the fission fragments came out of this,” he comments. The uranium coating on the filter paper under the stack of cigarette papers, on the other hand, “showed something very interesting.”1373 It showed two half-life activities different from those of the fission products that had recoiled away. And since whatever had remained in the uranium layer had not recoiled, the two different activities were probably not fission products. They were probably radioactivities induced in the uranium by captured neutrons. McMillan suspected that one of the two activities, the one with a half-life of 23 minutes, was one that Hahn, Meitner and Strassmann had identified in the 1930s as U239, “a uranium isotope produced by resonance neutron capture.”1374 The other activity left behind in the uranium layer had a longer half-life, about 2 days. In his report on his foil and cigarette-paper experiments McMillan chose not to speculate on what that second activity might be, but privately, he remembers, he thought “the two-day period could . . . be the product of the beta-decay of U-239, and therefore an isotope of [transuranic] element 93; in fact, this was the most reasonable explanation.”1375

To check that explanation McMillan needed some hint of the substance’s chemical identity. He expected that element 93 would behave chemically like the metal rhenium, element 75, next to osmium on the periodic table—would be “eka-rhenium” in the old terminology. He bombarded a larger uranium sample and enlisted the aid of Emilio Segré, who was now working as a research associate at Berkeley. “Segré was very familiar with the chemistry of [rhenium], since he and his co-workers [studying rhenium] had discovered [a similar element], now called technetium, in 1937.” Segrè began a chemical analysis of the irradiated uranium; in the meantime McMillan sharpened his half-life measurement to 2.3 days. Segrè, says McMillan, “showed that the 2.3-day material had none of the properties of rhenium, and indeed acted like a rare earth instead.” The rare earths, elements 57 (lanthanum) to 71 (lutetium), form a chemically closely related and odd series between barium and hafnium. Because of their middle-table atomic weights near barium, they often turn up as fission products. When Segrè found the 2.3-day activity acting not like rhenium, as expected, but like a rare earth, McMillan assumed that was what it was: “Since rare earths are prominent among the fission products, this discovery seemed at the time to end the story.” Segrè even published a paper on his work titled “An unsuccessful search for transuranic elements.”1376

McMillan might have left it there, but the fact that the 2.3-day substance did not recoil away from the uranium layer nagged at him. “As time went on and the fission process became better understood, I found it increasingly difficult to believe that one fission product should behave in a way so different from the rest, and early in 1940 I returned to the problem.”1377 The 60-inch cyclotron, with a massive rectangular-framed magnet spacious enough to shelter Lawrence’s entire crew between its poles for a photograph—twenty-seven men, two rows seated on the lower jaw of the beast, Lawrence prominent at center, and a third row standing inside its maw—was up and running by then; McMillan used it to study the 2.3-day activity in more detail. He studied the activity chemically as well and managed the significant observation that it did not always fractionally crystallize out of solution as a rare earth would.

“By now it was the spring of 1940,” McMillan continues, “and Dr. Philip Abelson came to Berkeley for a short vacation.” Abelson was the young experimentalist for whose benefit Luis Alvarez had vacated his Berkeley barber chair half-shorn to pass along the news of the discovery of fission. He had finished his Berkeley Ph.D. and signed on with Merle Tuve at the DTM. Like McMillan, he had become suspicious of the conclusion that the 2.3-day activity was merely another rare-earth fission product. He found time in April 1940 to begin sorting out its chemistry—although he was a physicist by graduate training, he had earned his B.S. in chemistry at Washington State. But he needed a bigger sample of bombarded uranium than he could produce with DTM equipment. “When he arrived for his vacation,” says McMillan, “and our mutual interest became known to one another, we decided to work together.”1378 McMillan made up a new batch of irradiated uranium. Abelson pursued its chemistry.

“Within a day,” Abelson recalls, “I established that the 2.3-day activity had chemical properties different from those of any known element. . . . [It] behaved much like uranium.”1379 Apparently the transuranics were not metals like rhenium and osmium but were part of a new series of rareearth-like elements similar to uranium. For a rigorous proof that they had found a transuranic the two men isolated a pure uranium sample with strong 23-minute U239 activity and demonstrated with half-life measurements that the 2.3-day activity increased in intensity as the 23-minute activity declined. If the 2.3-day activity was different chemically from any other element and was created in the decay of U239, then it must be element 93. McMillan and Abelson wrote up their results. McMillan had already thought of a name for the new element—neptunium, for the next planet out beyond Uranus—but they chose not to offer the name in their report. They mailed the report, “Radioactive element 93,” to the Physical Review on May 27, 1940, the same day Louis Turner sent Szilard his transuranic theories: anticipation and discovery can cut that close in science.1380

Presumably Szilard did not yet know of the Berkeley work (published June 15) when he answered Turner’s letter on May 30, since he makes no mention of it, but he recognized the logic of Turner’s argument, told him “it might eventually turn out to be a very important contribution”—and proposed he keep it secret.1381 Szilard saw beyond what Turner had seen. He saw that a fissile element bred in uranium could be chemically separated away: that the relatively easy and relatively inexpensive process of chemical separation could replace the horrendously difficult and expensive process of physical separation of isotopes as a way to a bomb. But unstable element 93, neptunium, was not yet that fissile element and Szilard did not yet realize how small a quantity of pure fissile material was needed to make a critical mass. (Turner was first with his observation, but he was not alone. The idea occurred independently to von Weizsácker one day in July, before the June Physical Review reached him in Germany with the McMillan-Abelson news, while he was riding the Berlin subway, though he assumed element 93 would do the job; he offered the idea to the War Office in a five-page report.1382 A British team at the Cavendish worked it out and presented it to the MAUD Committee early in 1941. But the Germans thought only heavy water could make a uranium burner go in which the new elements might breed, and the British had become optimistic about isotope separation. Neither group therefore pursued the Turner approach.)

After Abelson returned to Washington, McMillan pressed on. Unstable neptunium decayed by beta emission with a 2.3-day half-life; he suspected it decayed to element 94. By analogy with uranium, which emits alpha particles naturally, element 94 should also be a natural alpha emitter. McMillan therefore looked for alphas with ranges different from the uranium alphas coming off his mixed uranium-neptunium samples. By autumn he had identified them. He tried some chemical separations, “finding that the alpha-activity did not belong to an isotope of protactinium, uranium or neptunium.”1383 He was that close.

But American science, spurred on by British appeals, was finally gearing up for war. Churchill had sent over Henry Tizard in the late summer of 1940 with a delegation of experts and a black-enameled metal steamer trunk, the original black box, full of military secrets. The prize specimen among them was the cavity magnetron developed in Mark Oliphant’s laboratory at Birmingham. John Cockcroft, a future Nobel laureate with a vital mission, traveled along to explain the high-powered microwave generator. The Americans had never seen anything like it before. Cockcroft got together one weekend in October with Ernest Lawrence and multimillionaire physicist-financier Alfred Loomis, the last of the gentlemen scientists, at Loomis’ private laboratory in the elegant suburban New York colony of Tuxedo Park. That meeting laid the groundwork for a major new NDRC laboratory at MIT. To keep its work secret it was named the Radiation Laboratory, as if serious scientists might actually be pursuing applications so dubious as those bruited by visionaries from nuclear physics. Loomis wanted Lawrence to direct the new laboratory. Lawrence preferred to stay at Berkeley laying plans and raising funds for a new 184-inch cyclotron but was willing to encourage his best people to move to Cambridge. He convinced McMillan: “I left Berkeley in November 1940 to take part in the development of radar for national defense.”1384 Lawrence’s and McMillan’s priorities are a measure of the priorities of American science in late 1940. Peacetime cyclotrons and radar for air defense came first before superbombs. With a different perspective on the matter, James Chadwick at Liverpool was so uncharacteristically incensed by the publication of the McMillan-Abelson paper reporting element 93 that he asked for, and got, an official protest through the British Embassy. An attache was duly dispatched to Berkeley to scold Ernest Lawrence, the 1939 Nobel laureate in physics, for giving away secrets to the Germans in perilous times.

Laura and Enrico Fermi and their two children had moved from a Manhattan apartment in the summer of 1939 across the George Washington Bridge and beyond the Palisades to the pleasant suburb of Leonia, New Jersey. Harold Urey, a short, intense, enthusiastic man, was a resident along with other Columbia families and had convinced the Fermis to buy a house there, praising Leonia’s “excellent public schools,” Laura writes, and extolling “the advantages of living in a middle-class town where one’s children may have all that other children have.”1385 Among much good advice Urey cautioned the Italian couple to wage eternal war on crabgrass. Fermi was a product of Roman apartments; he quickly identified Digitaria sanguinalis neutrally as “an unlicensed annual” and chose to ignore it.1386 Laura prepared to do battle but was unable to distinguish crabgrass from sod. Urey dropped by one day to give her counsel and identified the problem. “D’you know what’s wrong with your lawn, Laura?” the chemistry laureate asked her compassionately.1387 “It’s all crab grass.” Life was pleasant in Leonia; Fermi practiced fitting in. Segré remembers that his friend “purposely studied contemporary Americana and read the comic strips. . . .1388 Among adult immigrants, I have never seen a comparably earnest effort toward Americanization.”

Segrè traveled to Indiana toward the end of 1940 to interview at Purdue, perfunctory interviewing because he meant to stay at Berkeley—“the machine was so good, I could do these things that nowhere else could I do.”1389, 1390 He continued eastward to visit the Fermis in Leonia. Independently of Turner, Segré recalls, both he and Fermi had been thinking about element 94. On December 15, he writes, “we had a long walk along the Hudson, in freezing weather, during which we spoke of the possibility that the isotope of mass 239 of element 94 . . . might be a slow neutron fissioner. If this proved to be true, [it] could substitute for 235U as a nuclear explosive. Furthermore, a nuclear reactor fueled with ordinary uranium would produce [the new element]. This gave an entirely new perspective on the making of nuclear explosives, eliminating the need to separate uranium isotopes, at that time a truly scary problem.”1391

Lawrence happened to be visiting New York. “Fermi, Lawrence, Pegram and I met in Dean Pegram’s office at Columbia University and developed plans for a cyclotron irradiation that could produce a sufficient amount of [element 94].”1392 After Christmas Segré returned to Berkeley.

A young chemist there, Glenn T. Seaborg, had already begun working toward identifying and isolating element 94. Born in Michigan of Swedish-American parents, Seaborg had grown up in Los Angeles and taken his Ph.D. at Berkeley in chemistry in 1937, when he was twenty-five. He was exceptionally tall, thin, guarded in the Swedish way but gifted and comfortable at work. The published record of Otto Hahn’s 1933 Cornell lectures, Applied Radiochemistry, had been his guidebook in graduate school: radiochemistry was his passion. He had been practicing it at Berkeley in January 1939 when the news of fission arrived; like Philip Abelson, he was excited by the discovery and chagrined to have missed it and had walked the streets for hours the night he heard.

As early as the end of August he had bombarded a sample of uranium to produce neptunium and had assigned one of his second-year graduate students, Arthur C. Wahl, to study its chemistry. His other collaborator in the search for 94 was Joseph W. Kennedy, like Seaborg a Berkeley chemistry instructor. By late November the group had progressed through four more bombardments, unraveling enough of neptunium’s chemistry to devise techniques for isolating highly purified samples. Seaborg then wrote McMillan at MIT, a letter he summarizes in a careful history he wrote later that he cast as a contemporary diary: “I suggested that since he has now left Berkeley . . . and is therefore not in a position to continue this work [of studying neptunium and looking for element 94], that we would be very glad to carry on in his absence as his collaborators.”1393 McMillan acceded in mid-December; by the time Segré returned to Berkeley Seaborg had separated out significant fractions of material from his bombarded samples, including uranium, fission products, purified neptunium and a rare-earth fraction that might contain 94.

Two searches were thus to proceed simultaneously.1394 Seaborg’s team would follow one especially intense alpha emitter it had identified in the hope of demonstrating that it was an isotope of 94, chemically different from all other known elements. At the same time, Segré and Seaborg would produce neptunium 239 in quantity, look for its decay product (which ought to be 94239) and attempt to measure that substance’s fissibility.

Segrè and Seaborg bombarded ten grams of a solid uranium compound, uranyl nitrate hexahydrate (UNH), for six hours in the 60-inch cyclotron on January 9. They bombarded five more grams for an hour the next morning. By afternoon they knew from ionization-chamber measurements that they could make 94 by cyclotron bombardment; one kilogram of UNH, they calculated, suitably irradiated, should produce about 0.6 microgram (one millionth of a gram) from neptunium after allowing time for beta decay.1395

Seaborg’s team identified an alpha-emitting daughter of Np238 on January 20. Definitive proof that it was 94 required chemical separation, and that delicate, tedious work proceeded during February. The crucial breakthrough came at the beginning of a week when everyone routinely labored past midnight to pursue the difficult fractionations to their end. On Sunday afternoon, February 23, Wahl discovered he could precipitate the alpha emitter from acid solution using thorium as a carrier. But he was not then able to separate the alpha emitter from the thorium. He talked to a Berkeley chemistry professor who suggested using a more powerful oxidizing agent.

That evening Seaborg and Segré began bombarding 1.2 kilograms of UNH in the 60-inch cyclotron to transmute some of its uranium into neptunium. They packed the UNH into glass tubes, set the tubes in holes drilled into a 10-inch block of paraffin and set the paraffin in a wooden box. Then they arranged the wooden box behind the beryllium target of the big cyclotron, which battered copious quantities of neutrons from the beryllium with powerful 16 MeV deuterons—favorite cyclotron projectiles, deuterium nuclei from heavy water. With the UNH in place in the cyclotron Seaborg climbed the stairs to the third floor of Gilman Hall where Wahl brewed fractionations under the roof in a cramped room relieved by a small balcony. Wahl tried the new oxidation chemistry that evening with Seaborg at his side. It worked; the thorium precipitated from solution and the alpha emitter stayed behind, enough of it to read out about 300 kicks per minute on the linear amplifier. That, writes Seaborg, was the “key step in its discovery,” but they still needed a precipitate of the alpha emitter and they pushed on through the night.1396 Seaborg remembers noticing the new day—lightning over San Francisco to the west across the Bay—when he stepped out onto the balcony to clear his lungs of fumes.1397Working again past midnight on Tuesday, Wahl filtered out a precipitate cleared of thorium. “With this final separation from Th,” Seaborg records with emphasis, “it has been demonstrated that our alpha activity can be separated from all known elements and thus it is now clear that our alpha activity is due to the new element with the atomic number 94”1398

The bombardment of Segrè’s and Seaborg’s kilogram sample, interrupted from time to time by other experiments that commanded the cyclotron, continued for a week. The UNH was rendered more intensely radioactive; the radioactivity would increase dangerously as they concentrated the Np239 they had made. They began working with goggles and lead shielding, dissolving the uranium first in two liters of ether and then proceeding through a series of laborious precipitations.

Their fifth and sixth reprecipitations they finished on Thursday, March 6. From 1.2 kilograms of UNH they had now separated less than a millionth of a gram of pure Np239 mixed with sufficient carrier to stain a miniature platinum dish that measured two-thirds of an inch across and half an inch deep. When they had dried this speck of matter God had not welcomed at the Creation they simply snipped off the sides of the platinum dish, covered the sample with a protective layer of Duco Cement, glued the dish to a piece of cardboard labeled Sample A and set it aside until it decayed completely to 94239.

On Friday, March 28 (of the week when Field Marshal Erwin Rommel, commander of the Afrika Korps, opened a major offensive in North Africa; when the British meat ration was reduced to six ounces per person per week; when British torpedo bombers successfully attacked the Italian fleet as it returned from the Aegean, a performance that greatly interested the Japanese), Seaborg recorded:

This morning Kennedy, Segrè and I made our first test for the fissionability of 94239 using Sample A. . . .1399

Kennedy has constructed during the past few weeks a portable ionization chamber and linear amplifier suitable for detecting fission pulses. . . . Sample A (estimated to contain 0.25 micrograms of 94239) was placed near the screened window of the ionization chamber embedded in paraffin near the beryllium target of the 37-inch cyclotron. The neutrons produced by the irradiation of the beryllium target with 8 MeV deuterons give a fission rate of 1 count per minute per microampere. When the ionization chamber is surrounded by a cadmium shield, the fission rate drops to essentially zero. . . .

This gives strong indications that 94239 undergoes fission with slow neutrons.

Not until 1942 would they officially propose a name for the new element that fissioned like U235 but could be chemically separated from uranium. But Seaborg already knew what he would call it. Consistent with Martin Klaproth’s inspiration in 1789 to link his discovery of a new element with the recent discovery of the planet Uranus and with McMillan’s suggestion to extend the scheme to Neptune, Seaborg would name element 94 for Pluto, the ninth planet outward from the sun, discovered in 1930 and named for the Greek god of the underworld, a god of the earth’s fertility but also the god of the dead: plutonium.

*   *   *

Frisch and Peierls had calculated a small U235 critical mass on the basis of sensible theory.1400 Through the winter Merle Tuve’s group at the DTM had continued to refine its cross-section measurements; in March Tuve was able to send to England a measured U235 fast-fission cross section that the British used to confirm a critical mass somewhat larger than the Frisch-Peierls estimate: about eighteen pounds untamped, nine or ten pounds surrounded by a suitably massive and reflective tamper. “This first test of theory,” Peierls wrote triumphantly that month, “has given a completely positive answer and there is no doubt that the whole scheme is feasible (provided the technical problems of isotope separation are satisfactorily solved) and that the critical size for a U sphere is manageable.”1401

Chadwick had also made further cross-section measurements. He was already a sober man; when he saw the new numbers a more intense sobriety seized him. He described the change in 1969 in an interview:

I remember the spring of 1941 to this day. I realized then that a nuclear bomb was not only possible—it was inevitable. Sooner or later these ideas could not be peculiar to us. Everybody would think about them before long, and some country would put them into action. And I had nobody to talk to. You see, the chief people in the laboratory were Frisch and [Polish experimental physicist Joseph] Rotblat. However high my opinion of them was, they were not citizens of this country, and the others were quite young boys. And there was nobody to talk to about it. I had many sleepless nights. But I did realize how very very serious it could be. And I had then to start taking sleeping pills. It was the only remedy. I’ve never stopped since then. It’s 28 years, and I don’t think I’ve missed a single night in all those 28 years.1402

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