Dark Sun: The Making Of The Hydrogen Bomb

[espionage] material that this method makes it possible to increase the relative velocities of particles up to 10,000 meters per second, providing that symmetry is achieved, and hence, this method is preferable to the gun method.

Now, it is difficult to assess whether this conclusion is correct or not, but without doubt “implosion” is of great interest, is correct in principle and should be subjected to serious theoretical and experimental study.

If the information that plutonium bred in a natural-uranium reactor could be a shortcut to the bomb was the first Anglo-American breakthrough that the Soviet espionage network delivered to Soviet scientists, the information that implosion was superior to gun assembly was the second. But whether this information came from Alan Nunn May or from some other source, as yet unknown, the declassified Soviet record does not reveal. It almost certainly did not come from Klaus Fuchs, who arrived at Los Alamos after the hydride gun was abandoned, and who knew, by the time he visited his sister in Cambridge in February 1945, what the documents Kurchatov reviewed on March 16 apparently failed to report: that implosion was not only desirable for plutonium assembly but also necessary, because all Pu239 bred in a reactor, whether American or Soviet, would be contaminated with Pu240, and a gun bomb loaded with such material would detonate prematurely.

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“I went up to Cambridge and saw Klaus there,” Harry Gold remembered of his February 16, 1945, meeting with Klaus Fuchs. It was winter in Massachusetts and there was heavy snow on the ground. Gold stopped along the way to buy a book for Kristel Heineman — a piece of froth titled Mrs. Palmer's Honey — and candy for the Heineman children. Gold had bragged of his own children on one of his earlier visits, imaginary children made up as a cover but elaborated into a fantasy of the family life that the lonely bachelor chemist never knew. Essie and David, Gold would confide to Abe Broth-man's secretary, those were the children's names — twins, a boy and a girl, and his wife was a former Gimbel's model. Long afterward, defenders of the Rosenbergs would cite Gold's family fantasies as evidence that he had concocted his tales of espionage, but not even Harry Gold could have invented Harry Gold.

“Mrs. Heineman stated that she brought the chemist into her living room,” the FBI paraphrases, “where Fuchs was then sitting.” Kristel excused herself, Gold testified, “saying ‘I have to pick up the children from the school.’ Klaus asked me to go upstairs with him to his room, which as I recall was the front one looking out on the street, and we sat there for possibly fifteen or twenty minutes.”[18] Fuchs briefed Gold on his move to Los Alamos and described the place. They had made tremendous progress, Fuchs said. Gold remembered that “he… made mention of a lens, which was being worked on as a part of the atom bomb.” Fuchs told Harry “that he was getting along very well [at Los Alamos], but that he was strictly limited in regard to being able to leave… He said that it had only been with the greatest difficulty and due to the fact that he had gotten a bit ahead of schedule on his work, as regards the rest of the group, that he had been able to wangle time off to come to Cambridge.” Gold proposed meeting again in Boston along the Charles River, a prearrangement with Fuchs that Yatzkov had mentioned to Gold when they had met a few days previously in Philadelphia. “[Klaus] told me that such would be impossible; that he was certain that it would be a very long time, possibly even a year, before he could again leave Los Alamos, and that the next meeting would have to take place in Santa Fe.” Fuchs mentioned April. Gold told him “that I could not possibly get to Santa Fe in April.” They settled on early June.

To identify a location for the June meeting, Fuchs gave Gold a map: “a yellow folder,” the FBI describes it. “Outside of this folded circular are printed the words ‘Santa Fe The Capital City Different in the Land of Enchantment.’ Both sides of this circular contain maps. One side contains a Chamber of Commerce map of the City of Santa Fe, New Mexico, which was compiled April 1940. This side of the folder shows a complete layout of the Santa Fe streets, public buildings, churches, hotels, restaurants, and auto courts. On the reverse side of this pamphlet is a map of the area surrounding Santa Fe.” Fuchs pointed out the Castillo Street Bridge (over the Santa Fe River) and proposed to meet there at four in the afternoon on the first Saturday in June.

At that point, Fuchs passed Gold, in Gold's words, “a quite considerable packet of information.” The contents of the packet, Fuchs would confess, covered everything he knew up to that time about bomb design:

Fuchs wrote a report… summarizing the whole problem of making an atomic bomb as he then saw it. This report included a statement on the special difficulties that would have to be overcome in making a plutonium bomb. He reported the high spontaneous fission rate of plutonium[240] and the deduction that a plutonium bomb would have to be detonated by using the implosion method rather than the relatively simple gun method… He also reported that the critical mass for plutonium was less than that for U-235 and that about five to fifteen kilograms would be necessary for a bomb. At this time the issue was not clear as to whether uniform compression of the core could be better obtained with a high-explosive lens system, or with multipoint detonation over the surface of a uniform sphere of high explosives. He reported the current ideas as to the need for an initiator, though these, at the time, were very vague, and it was thought that a constant neutron source might be sufficient. Finally… he referred only to the hollow plutonium core for the atomic bomb as he did not then know anything about the possibility of a solid core.

Fuchs also reported the outer dimensions of the high-explosive lens system (which were effectively the outer dimensions of the bomb), the timing sequence for implosion and the plans for building and producing bombs at Los Alamos to the extent he knew them. From memory, he incorporated into his report portions of his two most recent Los Alamos technical studies: Jet formation in cylindrical implosion with 16 detonation points and Formation of jets in plane slabs. These studies were based on Walter Koski's work, in which David Greenglass was participating; the drawing Greenglass gave Julius Rosenberg in January depicted a mold for a cylindrical implosion lens with four-point detonation. Fuchs's and Greenglass's common references would have served Yatzkov as independent confirmation of the authenticity of the information his Los Alamos spies were passing.

“Mrs. Heineman had returned” by then, Gold says, “and one of the children peered curiously into the room. Mrs. Heineman called the child back… ” With the information in hand that he had come for, Gold was ready to leave, but he had one more duty to perform.

It was standard NKVD practice to try to buy even the organization's most high-minded spies. Yatzkov, probably nervous about the long hiatus between contacts with Fuchs, had given Gold the munificent sum of 1,500 1945 dollars — about $30,000 in 1995, more than poor Harry ever got at one time — to pass to Fuchs, with the caution “that I must proceed very delicately… so as not to offend him and that under no circumstance must I insist upon or make an issue of this matter.” Harry also had an NKVD “Christmas present” for Fuchs, in the tradition that bemused Elizabeth Bentley, “a wallet of the very thin dress or opera type.” Fuchs accepted the wallet, “but looked somewhat bewildered, and when I made some very tentative inquiries concerning whether he needed any money either for himself or possibly for his sister, the reply was so cold and final that I went no further with the matter. It was quite obvious that by even mentioning this, I had offended the man.” “Fuchs held the envelope containing the 1,500 dollars as if it were an unclean thing,” Gold remembered at another time, “and flatly refused to accept it.” Five years later, Fuchs was still insulted. “He turned down this offer,” he told the FBI, “and stated he would not do such a thing.” Gold backed off: “I left shortly thereafter and returned to New York.” Gold passed Fuchs's report to Yatzkov/Yakovlev and told him about the high-explosive lens that Fuchs had mentioned. At their next regular meeting in March, the Soviet rezident was hungry for more. “[He] told me to try to remember anything else that Fuchs had mentioned during our Cambridge meeting about the lens. Yakovlev was very agitated and asked me to scour my memory clean so as to elicit any possible scrap of information about this lens.”

Yatzkov/Yakovlev was following the right trail. The day when Fuchs had returned to work at Los Alamos, February 28, 1945, the leaders of the Manhattan Project — including Groves, Office of Scientific Research and Development section head and Harvard president James Bryant Conant, Hans Bethe, George Kistiakowsky and Richard Tolman — had met in Robert Oppenhei-mer's office and decided tentatively to develop the lensed, solid- core Christy implosion design as a combat weapon. Exploding-wire electric detonators — physicist Luis Alvarez's new invention, far more reliable than lead azide or fulminate of mercury — would fire the complex arrangement of HE lenses. The “Christy gadget” would need a modulated initiator, a device still being engendered that drew on Walter Koski's studies of jet formation (as interpreted by Klaus Fuchs) for its design; the group agreed to review its decision May 1, by which time it hoped a reliable initiator would be in hand. “Now we have our bomb,” Oppenheimer