named it in honor of Marie Curie's native Poland. Physically and chemically it resembled bismuth, the next element down the periodic table, except that it was a softer metal and emitted five thousand times as much alpha radiation as an equivalent mass of radium, which caused the ionized, excited air around a pure sample to glow with an unearthly blue light.
Po210, the isotope of polonium that interested Los Alamos, decayed to lead 206 with the emission of an alpha particle and a half-life of 138.4 days. The range of Po210's alphas was some 38 millimeters in air but only a few hundredths of a millimeter in solid metals; the alphas gave up their energies ionizing atoms along the way and finally came to a stop. That meant the polonium for an initiator could be safely confined within a sandwich of metal foils. Sandwiching the foils in turn might be concentric shells of light, silvery beryllium. The entire unit need be no larger than a hazelnut.
“I think I probably had the first idea [for an initiator design],” Bethe remembers, “and Fermi had a different idea, and I thought mine was better for once, and then I was the chairman of a committee of three to watch the development of the initiator.” Segregating the Po210 from the beryllium was straightforward. Making sure the two elements mixed thoroughly at the right instant was not, and the primary difference between initiator designs — many were invented and tested during the winter of 1944-45 — was their differing mixing mechanisms. A quantity of Po210 equivalent in alpha activity to 32 grams of radium, thoroughly mixed with beryllium, would produce some 95 million neutrons per second, but that would be no more than nine or ten neutrons in the brief ten-millionth of a second when they would be useful in an imploding Fat Man to start the chain reaction; therefore the mixing had to be certain and thorough. Initiator design has never been declassified, but irregularities machined into the beryllium outer surface that induced turbulence in the imploding shock wave probably did the job: the Fat Man initiator may have been dimpled like a golf ball.
To supply ten neutrons to initiate a chain reaction men labored for years. Bertrand Goldschmidt, a French chemist who had once been Marie Curie's personal assistant and who came to the United States after the invasion of France to work with Glenn Seaborg at the Met Lab, extracted the first half-curie of initiator polonium from old radon capsules at a New York cancer hospital (polonium is a daughter product of radium decay). Quantity production required using scarce neutrons from the Oak Ridge air-cooled pile to transmute bismuth one step up the periodic table to Po. Charles A. Thomas, research director for the Monsanto Chemical Company, a consultant on chemistry and metallurgy, took responsibility for purifying the Po, for which purpose he borrowed the indoor tennis court on his mother-in-law's large and securely isolated estate in Dayton, Ohio, and converted it to a laboratory.
Thomas shipped the Po on platinum foil in sealed containers, but another nasty characteristic of polonium caused shipping troubles: for reasons never satisfactorily explained by experiment, the metal migrates from place to place and can quickly contaminate large areas. “This isotope has been observed to migrate upstream against a current of air,” notes a postwar British report on polonium, “and to translocate under conditions where it would appear to be doing so of its own accord.” Chemists at Los Alamos learned to look for it embedded in the walls of shipping containers when Thomas' foils came up short.
Initiator studies proceeded in G Division at a test site established in Sandia Canyon, one mesa south of the Hill. The Initiator group drilled blind holes in large turbine ball bearings — screwballs, the experimenters called them — inserted test initiators and plugged the holes with bolts. After imploding the screwballs they recovered the remains and examined them to see how well the Po and Be had mixed. Mixing, unfortunately, could not be a conclusive measure of effectiveness. Bethe's committee selected the most promising design on May 1, 1945, but only a full-scale test culminating in a chain reaction could prove definitively that the design worked.
Progress toward a Japanese atomic bomb, never rapid, slowed to frustration and futility across the middle years of the Pacific war. After the Imperial Navy had bowed out of atomic energy research Yoshio Nishina had continued patriotically to pursue it even though he privately believed that Japan in challenging the United States had invited certain disaster. On July 2, 1943, Nishina had met with his Army liaison, a Major General Nobuuji, to report that he had “great expectations” for success. He noted that the Air Force had asked him to study uranium as a possible aircraft fuel, as an explosive and as a source of power, and he had recently received a request for assistance from another Army laboratory, which had contributed 2,000 yen to his expenses. Nobuuji promptly discouraged such consultations. “The main point,” Nishina agreed, “is to complete the project as rapidly as possible.” His calculations, he told Nobuuji, indicated that 10 kilograms of U235 of at least 50 percent purity should make a bomb, although cyclotron experiments would be necessary to determine “whether 10 kg. will be sufficient, or whether it will require 20 kg. or even 50 kg.” He wanted help finishing his 60-inch cyclotron:
The 250-ton, 1.5 meter accelerator is ready for operation except for certain components which are unavailable as they are being used in the construction of munitions. If this accelerator is completed we believe we can accomplish a great deal. At this moment the U.S. plans to construct an accelerator ten times as great but we are unsure as to whether they can accomplish this.
The previous March Nishina had discarded as impractical under wartime conditions in Japan all methods of isotope separation except gaseous thermal diffusion. Otto Frisch had tried gaseous thermal diffusion (differing from Philip Abelson's
Nishina did not meet again with Nobuuji until seven months later, in February 1944, when he reported difficulty producing uranium hexa-fluoride. His team had managed to develop a method for generating elemental fluorine but had not yet been able to combine the gas with uranium using an old and inefficient process that Abelson in the United States had discarded before he began his thermal-diffusion studies. Nishima also had a problem with his diffusion column that Abelson would have appreciated: it leaked. “To achieve an airtight system,” Nishina told Nobuuji, “we used [sealing] wax and finally achieved our goal. Solder could not be used because of the corrosive properties of the fluorine.” He was “in the middle of developing this [hexafluoride-generating] process but can see the end in sight.” His 1.5-meter cyclotron was now in operation but only at low energy; his explanation for that compromise comments pointedly on the condition of the Japanese industrial economy by 1944:
We have been unable to obtain any superior, high-frequency-generating vacuum tubes… for the cyclotron… As a result of this constraint, the low operating voltages limit the population of neutrons we can produce… In order to liberate many high-energy neutrons, a high-voltage vacuum tube is required. But, unfortunately, they are difficult to acquire.
By summer Nishina's group had manufactured some 170 grams of uranium hexafluoride — in the United States hex was now being produced by the ton — and in July attempted a first thermal separation. Gauges at the top and bottom of the column, intended to measure a difference in pressure — showing that separation was taking place — indicated no difference at all. “Well, don't worry,” Nishina told his team. “Just keep on with it, just keep giving it more gas.”
He reconvened with Nobuuji on November 17, 1944, to report that “since February of this year there has not been a great deal of progress.” He was losing as much as half his hexafluoride to corrosion effects:
We thought the materials we had used to make this apparatus for working with the [hexafluoride] were made of impure metals. Therefore we next used the most highly-refined metals available for the system. However, they were still eaten away. It was therefore necessary to reduce the pressure of the system… to compensate for this erosion.
The cyclotron was operating at higher but not yet full power; Nishina was using it, he told Nobuuji, “to assay the concentrated, separated material.” Significantly missing from the November 17 conference report is any mention of measurable separation of U235 from U238. Nishina's staff had understood for more than a year that he did not believe his country could build an atomic bomb in time to affect the outcome of the war. Whether he continued research out of loyalty, or because he thought such knowledge would be valuable after the war, or to win support
