Edward Teller, who was recognized then and later as one of the most imaginative, creative physicists alive. Teller took over direction of a small implosion group in January 1944 and made valuable contributions through the rest of the winter. But as winter turned to spring he began to neglect implosion calculations. He believed he had more important work to do, including early theoretical study of the possibility of using an atomic bomb to ignite a mass of deuterium, a weapon he called the Super that might explode with force equivalent not to thousands of tons of TNT but to millions of tons. “[Bethe] wanted me to work on calculational details at which I am not particularly good,” Teller wrote later, “while I wanted to continue not only on the hydrogen bomb, but on other novel subjects.”
Bethe knew that Rudolf Peierls was in New York working with Kellex. He requested that Peierls transfer to Los Alamos to help out on implosion. Peierls agreed provided that he be allowed to bring along two assistants: a young Englishman named Tony Skyrme and Klaus Fuchs. If the god of war had wanted to provide Igor Kurchatov with a clear channel directly into the heart of the most important and secret work then underway at Los Alamos, he could not have chosen a more providential channel than Klaus Fuchs, Robert Oppenheimer, who had become the wartime director of Los Alamos, said much the same thing later, after Fuchs had been exposed. General Groves had complained that Los Alamos was not compartmentalized adequately for security. “If Fuchs had been infinitely compartmentalized,” Oppenheimer countered, “what was inside his compartment would have done the damage.”
Fuchs arrived at Los Alamos on August 14,1944. “One of the most valuable men in my division,” Hans Bethe would call him, ruefully. Nicholas Metropolis, a mathematician in the Theoretical Division whose office was next to Fuchs's, noticed the German's diligence. “Whenever I walked in — and I would walk in early, like eight o'clock — he was always there. And when I left at night at five o'clock, five thirty, he was still in his office working away. He worked long, long hours.” In October, Oppenheimer led a colloquium that Fuchs attended on a new approach to implosion using three-dimensional “lenses” of high explosives. The radical new concept, proposed the previous summer by British physicist James Tuck, offered a possible way to overcome the interference between detonation waves that made such a mess of steel pipes. A detonator stuck in a piece of explosive started a wave that expanded outward through the HE equally in every direction, convexly, like a swelling dome; but it might be possible to design a complex arrangement of carefully fitted pieces of faster- and slower-burning explosives that would retard or accelerate the passage of the convex detonation wave so as to allow the sides of the dome time to catch up with and pass the peak — like turning a beanie or a yarmulke inside out. With the right combination of shapes and explosives, a detonation wave diverging outward from a point might be converted to a detonation wave converging inward on a point: an explosion might be converted to an implosion, eliminating detonation-wave interference and smoothly squeezing a subcritical ball of plutonium to su-percriticality.
As he had when consulting with Kellex on gaseous diffusion, Fuchs at Los Alamos once again produced a series of significant papers, but these dealt with the crucial question of how to make plutonium efficiently explode. The titles of some of the papers Fuchs wrote in his two years at Los Alamos reveal the extent to which he had tunneled fortuitously to the very center of the plutonium problem:
Formation of Jets in Plane Slabs
Jet Formation in Cylindrical Implosion
Efficiency for Very Slow Assembly
Theory of Implosion, Part I
Theory of Implosion, Part II
Theory of Implosion, Part III
Theory of Implosion, Part IV
Theory of Implosion, Part V
Fuchs also worked on theoretical studies concerning a small but crucial component of an implosion bomb, a device Los Alamos called an “initiator.” In September 1944, the physicist Robert Christy had proposed reducing the jetting problem by using as a bomb core not a shell of plutonium but a nearly solid subcritical ball (in the form of two fitted hemispheres). With a solid instead of a shell, nothing would be collapsing; the imploding detonation wave would simply squeeze the solid mass to criticality. It was a conservative, brute-force solution that would be much less efficient than a shell system and more dangerous as well — in its final incarnation it would be barely subcritical within a heavy natural-uranium tamper and would have to be safed with a removable cadmium wire — but it was a far simpler design.
Unfortunately, a solid core would necessitate adding in another complicated component. Implosion would reduce the core diameter by half, increasing the density of the solid metal by a factor of eight. In the few millionths of a second when the shock wave had squeezed the implosion assembly to maximum density, before the assembly began to rebound and disassemble, it needed a squirt of neutrons to start the chain reaction. The initiator was the first device used in atomic bombs to supply those neutrons, by knocking them out of a shell of beryllium foil with alpha particles from another shell of hot, highly alpha-radioactive polonium. It was a small nugget of exotic metals to be set exactly at the center of the bomb, nested in a cavity within the two hemispheres of plutonium. It was difficult to design because it had to remain inert, releasing no neutrons, until precisely the right moment and then unfailingly do its work. If it produced neutrons prematurely it might cause the bomb to predetonate. If it produced neutrons belatedly they would fly out uselessly through the rebounding wreckage. The initiator was nearly as difficult to design as the larger bomb around it, layers within layers, and its ingenuities were compressed within a gadget no bigger than a walnut. Fuchs would write three papers on initiator theory.
Fuchs attended seminars that winter on various alternatives to implosion. By February 11, 1945, when he left the mesa in northern New Mexico to visit his sister and her family in Massachusetts, he knew as much as anyone at Los Alamos about plutonium bomb design.
Sometime after Fuchs arrived in Cambridge, Kristel told him about Gold's November approach. Her brother “seemed surprised and somewhat annoyed,” she remembered, “… but… he did not comment beyond saying, ‘Oh, it's all right.’” She gave him the envelope Gold had left. He called the contact in Manhattan.[15]
Before seven, one weekday morning, Yatzkov telephoned Gold just as Gold was getting ready to leave for work:
With some difficulty he described to me the fact that he was in a gasoline station, near what I finally determined to be [the] Oxford Circle section of Philadelphia. John wanted to know if I would come down there and meet him. I did so. It was a very snowy morning, I recall it well, and John was wet. We got on the [street] car again and went down to the terminal in Frankford, where John told me that he had just the previous day received notification that Fuchs was now at Cambridge… He then told me that I must, as soon as possible, go to Cambridge. I did so. I believe that I met John on a Tuesday or a Wednesday, and that I arrived in Cambridge on most likely a Friday.
In 1945 there was one Friday between Sunday, February 11, and Thursday, February 22, the day Fuchs left Cambridge to return to New Mexico; he and Gold most likely met on February 16.
“I went directly to the Heineman home,” Gold remembered. “This was in the morning, and when I knocked I was admitted by, I believe, a servant girl. Klaus was there and welcomed me.”
7
‘Mass Production’
If Klaus Fuchs was the most productive spy delivering information on the Anglo-American atomic-bomb program to the Soviet Union from North America, he was by no means the only agent at work. Not many were ever exposed. Only a few of those who became known were brought to trial and convicted. But the collective record, limited and fragmentary though it is, corroborates Igor Gouzenko's characterization of Soviet espionage during and after the Second World War as “mass production,” demonstrates its methodology and reveals patterns and practices that tend to support espionage revelations that many Americans understandably questioned in the
