the Manhattan Project. Based solely on Gold's partial recollection of the contact's name, a candidate for this contact might be Judith Coplon, a 1943 Barnard graduate whom Robert Lamphere later established to be involved in Soviet espionage. Coplon was living in New York at this time, working in the Justice Department Economic Warfare Section. She has not previously been identified in this context.)
Gold remembered carrying John's message to Cambridge to leave with Kristel Heineman in early November 1944. Fuchs's sister remembered Gold visiting her for the first time in late January or early February 1945- Neither Kristel, Fuchs nor Gold ever quite straightened out when their various Cambridge meetings occurred, but other records make it possible to establish some of them at least approximately.
Whenever it was that Gold visited her that winter, Kristel remembered looking out the window of her house and noticing a man whom she did not know walking down the street. It was just before noon. The man came to her door and rang the bell. She answered the door. The man asked her if she was Mrs. Heineman, the sister of Klaus Fuchs. She said she was and the man gave his name. She was never able to remember his name, but six years later, when she was shown a photograph of Harry Gold, she immediately and positively identified him as the man who rang her bell that day and returned twice more to her house in Cambridge.
Harry told Fuchs's sister that he was a chemist who had worked at one time with her brother. He was anxious to see Klaus, he said. The Heineman children came home for lunch then and Kristel invited Gold to join them. He mentioned that he was tired from a long train ride.
Kristel Heineman remembered telling Gold during lunch, in FBI paraphrase, “the approximate dates between which Klaus Fuchs would visit the Heineman home” — dates presumably in February 1945. Gold, to the contrary, remembers her mentioning Christmas:
Mrs. Heineman told me that Klaus had been transferred somewhere in the Southwest United States, but that she expected him here about Christmastime. I believe that she indicated that she had received several letters from him. She said that she thought that he would certainly be home about Christmas, as he usually made a great event of bringing presents for the children.
If Kristel did not yet know that Fuchs would not visit Cambridge for Christmas, then she had not yet received a letter Fuchs wrote her from Post Office Box 1663, Santa Fe, New Mexico, on December 15:
Dear Kristel,
Many thanks for your letter. I am afraid I have been very busy during the last few weeks and I expect that will go on for a little time longer. But I do hope that I shall be able to take a holiday some time at the end of January. I have not even been able to do any Christmas shopping… I expect Marcia and Steve will be cross if my Christmas parcel does not arrive on time. But I trust you will be able to pacify them.
We have lots of snow around here and I am itching to get on skis. But before I do so I shall have to pacify my conscience as an uncle and get the parcel for your kids off.
With best wishes
Klaus
Placing Gold's visit in November or early December would also explain Yatzkov's urgency in dispatching him later, when word from Fuchs finally came, a month and a half after Christmas. But whenever Gold visited Cambridge, he accomplished his mission — he left the sealed envelope and went on his way.
Fuchs was indeed “very busy.” The previous summer, on July 14, 1944, the German emigre physicist had met in Washington with James Chadwick, the Nobel laureate discoverer of the neutron and the head of the British Mission in the United States. Chadwick had informed Fuchs that his services had been requested at Los Alamos, the secret laboratory in northern New Mexico where the first atomic bombs were being designed, “provisionally until the end of December.” Los Alamos was in turmoil and needed help.
The laboratory had been planning to build weapons that assembled critical masses of U235 or plutonium239 using a gun configuration: firing one subcritical piece of nuclear material up the barrel of a cannon to join it with a subcritical ring fitted to the muzzle. The worry with such an assembly mechanism was predetonation. Both uranium and plutonium fissioned spontaneously, as Georgi Flerov and K. A Petrzhak had first demonstrated in the case of uranium. Secondary neutrons released by such random spontaneous fission might start a chain reaction prematurely within the barrel of the cannon, as the “bullet” approached the target ring, before the two pieces had time fully to assemble. If the mass of nuclear material thus predetonated, it would still explode, but it might do so inefficiently. Instead of exploding with a force equivalent to ten thousand tons or more of TNT, it might fizzle at the equivalent to no more than a few hundred pounds of TNT — no better than a conventional high-explosive bomb could do. The United States was spending some $2 billion to make three atomic bombs; a fizzle would be an unconscionable waste of money.
Pu239 was known to fission spontaneously at more than double the rate of U235- Another isotope of plutonium, Pu240, which turned up as a contaminant in Pu239, was even more unstable. Assembling a critical mass of Pu239 within the barrel of a cannon had appeared from the beginning to be problematic. The plutonium bullet would have to travel up the barrel several thousand feet per second faster than would the bullet in the uranium gun. Until April 1944, a plutonium gun assembly had looked barely attainable. But the experiments so far conducted at Los Alamos had used microgram quantities of plutonium transmuted laboriously in a cyclotron, which produced primarily Pu239- The first gram quantities of reactor-produced plutonium arrived at Los Alamos early in the spring of 1944 from Oak Ridge. A nuclear reactor generates far more neutrons than a cyclotron. That higher neutron flux had transmuted more of the uranium in the reactor to Pu240. The spontaneous fission rate of reactor-produced Pu239, with its greater admixture of Pu240, turned out to be five times greater than that of cyclotron-produced plutonium, unacceptably high for gun assembly. Even at the highest attainable muzzle velocities, a plutonium bullet would melt before it had time to mate with a target assembly.
By July 1944, when Fuchs talked to Chadwick, Los Alamos had decided that the plutonium gun would have to be scrapped. The uranium gun, Little Boy, a conservative and reliable but inefficient design, would require as much of the rare uranium isotope as could be separated through 1945. Unless Los Alamos worked out a way to assemble a critical mass of plutonium without predetonation, the Manhattan Project, which by then was approaching the US automobile industry in number of employees and capital investment, would be able to deliver only one atomic bomb.
An alternative to the gun system had been proposed soon after the lab had opened its doors in April 1943, though many had doubted that it could be made to work. It was called implosion. In its first incarnation it depended on the fact that whether or not a mass of fissionable material is critical is determined not only by its volume but also by its geometry. Six kilograms of plutonium cast as two solid hemispheres would begin chain-reacting as soon as they were brought into contact; but the same six kilograms of plutonium configured as a hollow shell, from which secondary neutrons would more easily escape, would be essentially inert. Pack high explosives (HE) around such a shell, figure out a way to detonate the HE from a number of different points simultaneously, thus collapse the shell inward into a solid ball, and critical assembly might be achieved so rapidly that spontaneous fission would not have time to spoil the chain reaction. Slammed with high explosives, the walls of the shell would have to move only a short distance inward, and the HE would accelerate them together far faster than a cannon could do.
No one had ever used explosives to assemble something before; their normal use was blowing things apart. The first experiments conducted at Los Alamos using two-dimensional arrangements — pinching steel pipes with collar rings of HE — had been disastrous. Navy Captain William “Deke” Parsons, who was in charge of explosives research, scoffed that implosion was like trying to “blow in a beer can without splattering the beer.” From each point of detonation a convex detonation wave moved through the explosive; when the various waves spread into contact they interfered with each other in complex patterns like the interference waves that passing boats produce when their wakes collide. Instead of uniformly closing the steel pipes down to a solid pinch, the colliding shock waves liquified jets of hot metal and blew the pipes cockeyed.
Implosion phenomena were too complex for cut-and-try; the experimenters needed theory to guide them. Someone needed to go to work calculating the hydrodynamics — the complex, dynamic fluid motions — of implosion. Someone needed to work out the number and best placement of detonators around the outside of the HE sphere. Someone needed to calculate the ideal geometry of the plutonium shell, whether larger or smaller, whether thicker-walled or thin. The head of the Theoretical Division at Los Alamos, emigre physicist Hans Bethe, turned to
