Making of the Atomic Bomb

by Richard Rhodes

  “That I was named to head the [Theoretical] division,” Bethe comments, “was a severe blow to Teller, who had worked on the bomb project almost from the day of its inception and considered himself, quite rightly, as having seniority over everyone then at Los Alamos, including Oppenheimer.” Bethe believed he was chosen because his “more plodding but steadier approach to life and science would serve the project better at that stage of its development, where decisions had to be adhered to and detailed calculations had to be carried through, and where, therefore, a good deal of administrative work was inevitable.”2048 Teller saw his old friend’s steadier approach differently: “Bethe was given the job to organize the effort and, in my opinion, in which I may well have been wrong, he overorganized it. It was much too much of a military organization, a line organization.”2049 On the other hand, Teller has repeatedly praised Oppenheimer’s direction of Los Alamos, direction which included Bethe’s appointment and ratified Bethe’s decisions:

  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.2050

  “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. . . .2051 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.”2052, 2053 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 Segrè 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 Segrè 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. Segrè 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.2054

  In December at this Pajarito Canyon field station Segrè 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. Segrè 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.”2055 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.2056

  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.2057 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.2058 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.2059 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.”2060 He had alre
ady won von Neumann to his view, which had prepared the Hungarian mathematician in turn to endorse the precision instrument of implosion.2061 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.2062

  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.2063 The two never agreed about anything and they certainly didn’t want me interfering.

  While Kistiakowsky struggled with that dilemma the theoreticians began to glimpse how a successful implosion mechanism might be designed.

  The previous spring the Polish mathematician Stanislaw Ulam, then thirty-four years old and a member of the faculty at the University of Wisconsin, had found himself unhappy merely teaching in the midst of war: “It seemed a waste of my time; I felt I could do more for the war effort.”2064 He had noticed that letters from his old friend John von Neumann often bore Washington rather than Princeton postmarks and deduced that von Neumann was involved in war work; now he wrote asking for advice. Von Neumann proposed they meet between trains in Chicago to talk and turned up impressively chaperoned by two bodyguards. Eventually Hans Bethe sent along an official invitation. In the winter of 1943 Ulam and his wife Françoise, who was then two months pregnant, rode the Sante Fe Chief to New Mexico as so many others had done before them. “The sun shone brilliantly, the air was crisp and heady, and it was warm even though there was a lot of snow on the ground—a lovely contrast to the rigors of winter in Madison.”2065

  The day of his arrival Ulam met Edward Teller for the first time—he was assigned to Teller’s group—who “talked to me on that first day about a problem in mathematical physics that was part of the necessary theoretical work in preparation for developing the idea of a ‘super’ bomb.”2066 Teller’s preemption of Ulam’s first days at Los Alamos for Super calculations was symptomatic of the discord that had been widening between him and Hans Bethe, who needed every available theoretical physicist and mathematician to concentrate on the difficult problem of implosion. Teller had contributed enthusiastically and crucially to the most interesting part of the work. “However,” Bethe complains, “he declined to take charge of the group which would perform the detailed calculations on the implosion. Since the theoretical division was very shorthanded, it was necessary to bring in new scientists to do the work that Teller declined to do.”2067 That was one reason the British team had been invited to Los Alamos.

  Teller recalls no specific refusal. “[Bethe] wanted me to work on calculational details at which I am not particularly good,” he counters, “while I wanted to continue not only on the hydrogen bomb, but on other novel subjects.”2068

  The Los Alamos Governing Board reevaluated the Super once again in February 1944, learning that despite deuterium’s more favorable cross section it would still be difficult to ignite. A Super would almost certainly require tritium. The small tritium samples studied so far had been transmuted in a cyclotron by bombarding lithium with neutrons. Large-scale tritium production, like large-scale plutonium production, would require production reactors, but the piles at Hanford were unfinished and previously committed. “Both because of the theoretical problems still to be solved and because of the possibility that the Super would have to be made with tritium,” reports the Los Alamos technical history, “it appeared that the development would require much longer than originally anticipated.” Work could continue—the Super was too portentous a weapon to ignore—but only to the extent that it “did not interfere with the main program.”2069

  Von Neumann soon drafted Ulam to help work out the hydrodynamics of implosion. The problem was to calculate the interactions of the several shock waves as they evolved through time, which meant trying to reduce the continuous motion of a number of moving, interacting surfaces to some workable mathematical model. “The hydrodynamical problem was simply stated,” Ulam comments, “but very difficult to calculate—not only in detail, but even in order of magnitude.”2070

  He remembers in particular a long discussion early in 1944 when he questioned “all the ingenious shortcuts and theoretical simplifications which von Neumann and other . . . physicists suggested.” He had argued instead for “simpleminded brute force—that is, more realistic, massive numerical work.”2071 Such work could not be done reliably by hand with desktop calculating machines. Fortunately the laboratory had already ordered IBM punchcard sorters to facilitate calculating the critical mass of odd-shaped bomb cores. The IBM equipment arrived early in April 1944 and the Theoretical Division immediately put it to good use running brute-force implosion numbers. Hydrodynamic problems, detailed and repetitious, were particularly adaptable to machine computation; the challenge apparently set von Neumann thinking about how such machines might be improved.

  Then a member of the newly arrived British mission made a proposal that paid his mission’s way. James L. Tuck was a tall, rumpled Cherwell protégé from Oxford who had worked in England developing shaped charges for armor-piercing shells. A shaped charge is a charge of high explosive arranged in such a way—usually hollowed out like an empty ice cream cone with the open end pointed forward—that its normally divergent, bubble-shaped shock wave converges into a high-speed jet. Such a ferocious jet can punch its way through the thick armor of a tank to spray death inside.

  It had just become clear from theoretical work that the several diverging shock waves produced by multiple detonators in Neddermeyer’s experiments reinforced each other where they collided and produced points of high pressure; such pressure nodes in turn caused the jets and irregularities that spoiled the implosion. Rather than continue trying to smooth out a colliding collection of divergent shock waves, Tuck sensibly proposed that the laboratory consider designing an arrangement of explosives that would produce a converging wave to begin with, fitting the shock wave to the shape it needed to squeeze. Such explosive arrangements were called lenses by analogy with optical lenses that similarly focus light.

  No one wanted to tackle anything so complex so late in the war. Geoffrey Taylor, the British hydrodynamicist, arrived in May to offer further insight into the problem. He had developed an understanding of what came to be called Raleigh-Taylor instabilities, instabilities formed at the boundaries between materials. Accelerate heavy material against light material, he demonstrated mathematically, and the boundary between the two will be stable. But accelerate light material against heavy material and the boundary between the two will be unstable and turbulent, causing the two materials to mix in ways extremely difficult to predict. High explosive was light compared to tamper. All of the tamper materials under consideration except uranium were significantly lighter than plutonium. Raleigh-Taylor instabilities would constrain subs
equent design. They would also make it difficult to predict bomb yield.

  As the IBM results clarified shock-wave behavior the physicists began seriously to doubt if a uniform wrap of HE could ever be made to produce a symmetrical explosion. Complex though explosive lenses might be, they were apparently the only way to make implosion work. Von Neumann turned to their formulation. “You have to assume that you can control the velocity of the detonation wave in a chemical explosive very accurately,” Kistiakowsky explains, “so if you start the wave at certain points by means of detonators you can predict exactly where it will be at a given time. Then you can design the charge.”2072 It was soon clear that the velocity of the converging shock waves from the several explosive lenses that would surround the bomb core could vary by no more than 5 percent. That was the demanding limit within which von Neumann designed and Kistiakowsky, Neddermeyer and their staffs began to work.2073

  In the spring of 1944 the two difficult personal conflicts—between Teller and Bethe and between Kistiakowsky and Neddermeyer—forced Oppenheimer to intervene. First, Bethe writes, Teller withdrew from fission development:

  With the pressure of work and lack of staff, the Theoretical Division could ill afford to dispense with the services of any of its members, let alone one of such brilliance and high standing as Teller.2074 Only after two failures to accomplish the expected and necessary work, and only on Teller’s own request, was he, together with his group, relieved of further responsibility for work on the wartime development of the atomic bomb.

  A letter from Oppenheimer to Groves on May 1, 1944, seeking to replace Teller with Rudolf Peierls, corroborates Bethe’s account: “These calculations,” it says in part, “were originally under the supervision of Teller who is, in my opinion and Bethe’s, quite unsuited for this responsibility. Bethe feels that he needs a man under him to handle the implosion program.” It was, Oppenheimer notes, a question of the “greatest urgency.”2075