The Making of the Atomic Bomb

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.”

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 Segre, who was now working as a research associate at Berkeley. “Segre 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.” Segre began a chemical analysis of the irradiated uranium; in the meantime McMillan sharpened his half-life measurement to 2.3 days. Segre, 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 Segre 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.” Segre even published a paper on his work titled “An unsuccessful search for transuranic elements.”

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.” 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.” 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.” Apparently the transuranics were not metals like rhenium and osmium but were part of a new series of rare-earth-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.

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. 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 Weizsacker one day in July, before the June Physical Review reached him in Germany with the McMil-lan-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. 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.” 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.” 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 attach^ 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.” 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 san-guinalis neutrally as “an unlicensed annual” and chose to ignore it. 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. “It's all crab grass.” Life was pleasant in Leonia; Fermi practiced fitting in. Segre remembers that his friend “purposely studied contemporary Americana and read the comic strips… Among adult immigrants, I have never seen a comparably earnest effort toward Americanization.”

Segre 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.”