“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 suff
icient 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.1397 Working 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 po
sitive 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
12
A Communication from Britain
James Bryant Conant traveled to London in the winter of 1941 to open a liaison office between the British government and the National Defense Research Council.1403 Conant was the first American scientist of administrative rank to visit the beleaguered nation following the ad hoc exchanges of the Tizard Mission and he came to count the trip “the most extraordinary experience of my life.” “I was hailed as a messenger of hope,” he writes in his autobiography. “I saw a stouthearted population under bombardment. I saw an unflinching government with its back against the wall. Almost every hour I saw or heard something that made me proud to be a member of the human race.”1404
Making of the Atomic Bomb Page 49