The Making of the Atomic Bomb

time when I dropped in his office he told me that Pegram had come to see him, and Pegram thought that this value should not be published.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.

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 Walfher Bothe, a prot6ge 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 “These galling fights so affected my health,” he wrote later in a brief unpublished memoir, “that I had to spend a long period in a Badenweiler sanitorium.”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.”

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.” 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.” Only 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. 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. It was entitled “Atomic energy from Tj²³⁸” 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.”

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. Turner had also noted the recent Nier/Columbia report confirming the attribution of slow-neutron fission to U235. (He could hardly have missed it; the New 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.”) 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.” 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 ₉₃X²³⁹, however, but the element next along, the element that 93 would probably decay to, ₉₄X²³⁹, which he called “eka-osmium.” And ₉₄EkaOs²³⁹, 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 ₉₂U²³⁶ and a large cross section for fission would be expected.”

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. He 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.” 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.” The other activity left behind in the uranium layer had a longer half-life,