Throughout the war years, Oppie knew in detail what was going on in every part of the Laboratory. He was incredibly quick and perceptive in analyzing human as well as technical problems. Of the more than ten thousand people who eventually came to work at Los Alamos, Oppie knew several hundred intimately, by which I mean that he knew what their relationships with one another were and what made them tick. He knew how to organize, cajole, humor, soothe feelings — how to lead powerfully without seeming to do so. He was an exemplar of dedication, a hero who never lost his humanness. Disappointing him somehow carried with it a sense of wrongdoing. Los Alamos' amazing success grew out of the brilliance, enthusiasm and charisma with which Oppenheimer led it.
“I believe maybe [Teller] resented my being placed on top of him,” Bethe concludes. “He resented even more that there would be an end to free and general discussion… He resented even more that he was removed [by lack of administrative contact] from Oppenheimer.”
The theoretical complexity of the Super challenged Teller as the fission bomb had not; it also offered a line of work along which he might lead. “When Los Alamos was established in the spring of 1943,” he writes and the technical history of the laboratory confirms, “the exploration of the Super was among its objectives.” He accepted the postponement of that exploration through the summer of 1943, helping Bethe with the more immediate problem of developing means to calculate the critical mass and nuclear efficiency of various bomb designs. During the summer, experimental studies at Purdue found that the fusion reaction cross section for deuterium was much larger than expected; Teller cited that result to the Purdue Los Alamos Governing Board in September to propose renewing the Super investigation. Then John von Neumann arrived on the Hill to endorse and extend Seth Neddermeyer's implosion work and for a few months Teller was caught up in reconnoitering that new territory.
Emilio Segre won a new workshop that 1943 autumn. At Berkeley he had measured the rate of spontaneous fission — naturally occurring fission without neutron bombardment — in uranium and plutonium. The measurements were difficult because the rates were low for the small samples Segre had to use, but they were crucial. They determined how cleansed of light-element impurities the bomb cores would have to be — there was no point in purifying past the spontaneous background — and they determined how fast the gun assemblies would have to fire to avoid predetonation. Segre moved off the Los Alamos mesa to protect his new and more capacious measuring instruments from the radiation other experiments generated there:
At this time I acquired a special small laboratory for measuring spontaneous fission, the like of which I have never seen before or since. It was a log cabin that had been occupied by a ranger and it was located in a secluded valley a few miles from Los Alamos. It could be reached only by a jeep trail that passed through fields of purple and yellow asters and a canyon whose walls were marked with Indian carvings. On this trail we once found a large rattlesnake. The cabin-laboratory, in a grove shaded by huge broadleaf trees, occupied one of the most picturesque settings one could dream of.
In December at this Pajarito Canyon field station Segre made a significant discovery. The spontaneous fission rate for natural uranium was much the same at the field station as at Berkeley, but at the field station the rate was seemingly higher for U235. Segre deduced that cosmic-ray neutrons, which were usually too slow to fission U238 but effective to fission U235, caused the difference. Cosmic rays batter neutrons from the upper reaches of the atmosphere and the field station was 7,300 feet nearer that region than was sea-level Berkeley. Shield out such stray neutrons and the U235 bomb core could be purified less rigorously than they had assumed. Predetonation would be less likely: the gun that assembled the U235 to critical mass would need less muzzle velocity and could be significantly shorter and lighter. Thus was Little Boy engendered, Thin Man's modest brother, a gun assembly six feet long instead of seventeen that would weigh less than 10,000 pounds, an easy load for a B- 29: in a log cabin in a grove beyond fields of bright asters, up a trail visited by rattlesnakes.
Gun research was already advanced. “The first task of the gun group,” Edwin McMillan remembers, “was to set up a test stand where experiments could be done. You have to have a gun emplacement, and a gun, and a sand butt, which is nothing but a huge box full of sand that you fire projectiles into so that you can find the pieces afterwards, and because there might be somebody else out there.” The site they chose was Anchor Ranch, a former working ranch three miles southwest of the mesa that the Army had bought as part of the reservation; they fired the first shot on September 17, 1943.
Until the following March the group used a three-inch Navy anti-aircraft gun fitted with unrifled barrels. With it they tested propellants — eventually choosing cordite — and studied scale-model projectiles and targets. Knowing that the uranium bullet would complete a critical assembly they decided that it should not impact upon the target core but pass freely through; within microseconds of its arrival at spherical configuration it would in any case have vaporized.
From the beginning the plutonium gun with its nearly unattainable muzzle velocity of 3,000 feet per second had been a gamble. When von Neumann that autumn celebrated the advantages of implosion the Governing Board gave the novel approach its strong endorsement. Through the fall and early winter of 1943 Neddermeyer's experiments made only slow progress, however. He added few men to his group. He continued to work methodically with metal cylinders wrapped with solid slabs of high explosive. By spacing several detonators symmetrically around the wrap he could start implosion simultaneously at different points on the HE surface. From each point of detonation a detonation wave shaped like an expanding bubble would travel inward toward the metal cylinder; by varying the spacing of the detonators and the thickness of the HE Neddermeyer hoped to find a configuration that smoothed the convex, multiple shock waves to one uniform cylindrical squeeze. He was working to the same end with small metal balls, scale models of an eventual bomb core. But “the first successful HE flash photographs of imploding cylinders,” notes the Los Alamos technical history, “showed that there were… very serious asymmetries in the form of jets which traveled ahead of the main mass. A number of interpretations of these jets were proposed, including the possibility that they were optical illusions.” They were all too real. “Absolutely awful results,” says Bethe. Oppenheimer decided Neddermeyer needed help. Groves agreed. Conant knew just the man,
“Everything in books [about the Manhattan Project] looks so simple, so easy, and everybody was friends with everybody,” George Kistiakowsky told an audience wryly long after the war. He remembered a different Los Alamos. The tall, outspoken Ukrainian-born Harvard chemist had begun studying explosives for the National Defense Research Committee in 1940; “by 1943 I thought I knew something about them.” What he knew about them was original and unorthodox: “that they could be made into precision instruments, a view which was very different from that of military ordnance.” He had already won von Neumann to his view, which had prepared the Hungarian mathematician in turn to endorse the precision instrument of implosion. Conant similarly trusted Kistiakowsky's judgment. In 1941 Conant had abandoned his skepticism toward the atomic bomb because of Kistiakowsky; now the explosives expert found the Harvard president seeking his help to advance Neddermeyer's work:
I began going to Los Alamos as a consultant in the Fall of 1943, and then pressure was put on me by Oppenheimer and General Groves and particularly Conant, which really mattered, to go there on full time. I didn't want to, partly because I didn't think the bomb would be ready in time and I was interested in helping win the war. I also had what looked like an awfully interesting overseas assignment all fixed up for myself. Well, instead, unwillingly, I went to Los Alamos. That gave me a wonderful opportunity to act as a reluctant bride throughout the life of the project, which helped at times.
Kistiakowsky arrived in late January 1944 and took up residence in a small stone cabin that had been the Ranch School's pump house, an accommodation he negotiated in preference to the men's dormitory — he was forty-four years old and divorced. He quickly discovered, as he suspected, that everything was not easy and everybody was not friends:
After a few weeks… I found that my position was untenable because I was essentially in the middle trying to make sense of the efforts of two men who were at each other's throats. One was Captain [Deke] Parsons who tried to run his division the way it is done in military establishments — very conservative. The other was, of course, Seth Neddermeyer, who was the exact opposite of Parsons, working away in a little corner. The two never agreed about anything and they certainly didn't want me interfering.
