in the other. We were looking through the periscopes and all that happened was that it blew a lot of dust in our eyes. And then — we hadn't thought about this possibility at all — the whole forest around us caught on fire. These pieces of white-hot metal went flying off into the wild blue yonder setting trees on fire. We were almost surrounded.
Implosion lens development had begun the previous winter, says Bethe, when John von Neumann “very quickly designed an arrangement which was obviously correct from the theoretical point of view — I had tried and failed.” Now in the fall and winter of 1944-45 Kistiakowsky had to make the theoretical arrangement work.
An optical lens takes advantage of the fact that light travels at different velocities in different media. Light traveling through air slows when it encounters glass. If the glass curves convexly, as a magnifying glass is curved, the light that encounters the thicker center must follow a longer path than the light that encounters the thinner edges. The effect of these differing path lengths is to direct the light toward a focal point.
The implosion lens system von Neumann designed was made up of truncated pyramidal blocks about the size of car batteries. The assembled lenses formed a sphere with their smaller ends pointing inward. Each lens consisted of two different explosive materials fitted together — a thick, fast-burning outer layer and a shaped slow-burning solid inclusion that extended to the surface of the face of the block that pointed toward the bomb core:
The fast-burning outer layer functioned for the detonation wave as air around an optical lens functions for light. The slower-burning shaped inclusion functioned as a magnifying glass, directing and reshaping the wave. A detonator would ignite the fast-burning explosive. That material would develop a spherical detonation wave. When the apex of the wave advanced into the apex of the inclusion, however, it would begin burning more slowly. The delay would give the rest of the wave time to catch up. As the detonation wave encountered and burned through the inclusion it thus reshaped itself from convex to concave, from a spherical wave expanding from a point to a spherical wave converging on a point, emerging fitted to the convex curve of the spherical tamper. Before the reshaped wave reached the tamper it passed through a second layer of solid blocks of fast-burning explosive to add to its force. The heavy natural-uranium tamper then served to smooth out any minor irregularities as the spherical shock wave compressed it passing through to the plutonium core.
Kistiakowsky would apologize after the war for a research program “too frequently reduced to guesswork and empirical shortcuts” because the field had been grossly neglected. “Prior to this war the subject of explosives attracted very little scientific interest,” he wrote in an introduction to a technical history of X Division's work, “these materials being looked upon as blind destructive agents rather than precision instruments; the level of fundamental knowledge concerning detonation waves — and strong shock waves induced by them in the adjacent non-explosive media — was distressingly low.” To support its experiments X Division expanded an explosives-casting site a few miles south of Anchor Ranch, constructing roughhewn earth-sheltered timber buildings because hauling in concrete would have delayed the work.
Not until mid-December 1944 did a lens test look promising; the eighteen 5-kilogram bombs Groves told George Marshall he hoped to have on hand by the second half of 1945 he also thought might explode so inefficiently that each would be equivalent to no more than 500 tons of TNT, down from the 1,000 tons Conant had heard estimated in October.
Kistiakowsky had to fight once more with Parsons before he won the field. “So much pessimism was developing about our ability to build satisfactory lenses,” he recalls, “that Captain Parsons began urging (and he was not alone in this) that we give up lenses completely and try somehow to patch up the non-lens type of implosion.” Kistiakowsky thought that alternative hopeless. Early in 1945 Groves came out to monitor the debate. In the end Oppenheimer took Kistiakowsky's side and decided for lenses. Parsons' Ordnance Division then restricted its work to the uranium gun, Little Boy, and to engineering the weapons for the battlefield. X and G Divisions worried about implosion.
Finishing the high-explosive castings by machining them was the most dramatic innovation Kistiakowsky introduced. He wanted to shape the HE components entirely by machining from solid pre-cast blocks but lacked sufficient time to develop and build the elaborate remote-controlled machinery the innovative technology would have required. He settled instead for precision casting with machine finishing and used his limited supply of machinists primarily to turn out the necessary molds. Molds gave him “the greatest agony,” he remembers; the HE components of the bomb totaled “something in the nature of a hundred or so pieces, which had to fit together to within a precision of a few thousandths of an inch on a total size of five feet and make a sphere. So we had to have very precise molds.” Eventually mold procurement paced Fat Man's testing and delivery.
But even with the necessary molds on hand, casting HE was far from simple, another technology that had to be learned by trial and error. In February 1945 Kistiakowsky chose an explosive called Composition B to serve as the fast-burning component of Fat Man's lenses and a mixture he had commissioned from a Navy research laboratory, Baratol, for the slow-burning component. Composition B was poured as a hot slurry of wax, molten TNT and a non-melting crystalline powder, RDX, that was 40 percent more powerful than TNT alone. Baratol slurried barium nitrate and aluminum powder with TNT, stearoxyacetic acid and nitrocellulose:
We learned gradually that these large castings, fifty pounds and more each, had to be cooled in just certain ways, otherwise you get air bubbles in the middle or separations of solids and liquids, all of which screwed up the implosion completely. So it was a slow process. The explosive was poured in and then people sat over that damned thing watching it as if it was an egg being hatched, changing the temperature of the water running through the various cooling tubes built into the mold.
The wilderness reverberated that winter to the sounds of explosions, gradually increasing in intensity as the chemists and physicists applied small lessons at larger scale. “We were consuming daily,” says Kistiakowsky, “something like a ton of high performance explosives, made into dozens of experimental charges.” The total number of castings, counting only those of quality sufficient to use, would come to more than 20,000. X Division managed more than 50,000 major machining operations on those castings in 1944 and 1945 without one explosive accident, vindication of Kistiakowsky's precision approach. A RaLa test on February 7, 1945, showed definite improvement in implosion symmetry. On March 5, after a strained round of conferences, Oppenheimer froze lens design. However scarce plutonium might be, no one doubted that Fat Man would have to be tested at full scale before a military weapon could be trusted to work.
A problem small in scale but difficult of solution was the initiator, the minuscule innermost component of the bombs. The chain reaction required a neutron or two to start it off. No one wanted to trust a billion dollars' worth of uranium or several hundred million dollars' worth of plutonium to spontaneous fission or a passing cosmic ray. Neutron sources had been familiar laboratory devices for more than a decade, ever since James Chadwick bombarded beryllium with alpha particles from polonium and broke the elusive neutral particle free in the first place. In his early lectures at Los Alamos Robert Serber had discussed using a radium-beryllium source in a gun bomb with the radium attached to one piece of core material and the beryllium to the other, arranged to smash together when the gun was fired and the two core components mated to complete a critical assembly. Radium released dangerous quantities of gamma radiation, however, and Edward Condon noted in the
The challenge of initiator development was to design a source of sufficient neutron intensity that released those neutrons only at the precise moment they were needed to initiate the chain reaction. In the case of the uranium gun that requirement would be relatively easy to meet, since the alpha source and the beryllium could be separated with the bullet and the target core. But the implosion bomb offered no such convenient arrangement for separation and for mixing. Polonium and beryllium had to be intimately conjoined in Fat Man at the center of the plutonium core but inert as far as neutrons were concerned until the fraction of a microsecond when the imploding shock wave squeezed the plutonium to maximum density. Then the two materials needed instantaneously to mix.
Polonium, element 84 on the periodic table, was a strange metal. Marie and Pierre Curie had isolated it by hand from pitchblende residues (at backbreaking concentrations of a tenth of a milligram per ton of ore) in 1898 and
