The Discovery of Spontaneous Fission in Plutonium

It was the spring of 1944. In a secluded canyon in New Mexico, 14 miles from the bustling technical area of the wartime Los Alamos Laboratory, three physics graduate students were working inside a Forest Service log cabin filled with electronics. For the past eight months, they had been driving there each day by jeep to search for evidence of “spontaneous fission,” a naturally occurring process in which certain heavy atomic nuclei split of their own accord, emitting neutrons. Anxiously, they puzzled over a startling oscilloscope trace produced by a sample of plutonium. Why were these students studying the phenomenon of spontaneous fission in this canyon? What caused their concern?

The professor in charge of the work, nuclear physicist Emilio Segrè, had fled Italy in 1938 and joined Ernest Lawrence's nuclear physics laboratory in Berkeley, California. In 1943, at the request of theoretical physicist J. Robert Oppenheimer, Segré had moved several of his Berkeley experiments to Los Alamos to be part of Project Y – the secret project to build the first atomic bombs. Jointly directed by Oppenheimer and military engineer Gen. Leslie R. Groves, Project Y was a part of the Manhattan Project (the Manhattan Engineer District). Before World War II, Los Alamos, a small New Mexico town on a high mesa, had been the site of a ranch school for boys.

By late summer 1944, the implosion program was among the laboratory's highest priorities. It had started out as a small, informally run, back burner effort of a handful of researchers surrounding the reserved Seth Neddermeyer (Chapter 4). Between the fall of 1943 and the summer of 1944, it was transformed into a well-coordinated, multidisciplinary research effort of more than fourteen groups operating within T-Division and the newly created Gadget (G) and Explosives (X) Divisions.

The shift began with a visit in late September 1943 by the great mathematician and physicist John von Neumann. On learning about Neddermeyer's test implosions of small cylindrical metal shells, von Neumann pointed out that their efficiency could be increased using a substantially higher ratio of explosive to metal mass, which would promote more rapid assembly. The suggestion excited leading Los Alamos theorists, including Bethe, Oppenheimer, and Teller, who could now envision an atomic weapon requiring active material having less mass and a lower level of purity than was needed in the gun device – advantages of particular interest to General Groves.

Theorists, particularly Bethe and Teller, spent more and more time on implosion questions, while von Neumann continued to work on theoretical aspects of the implosion in Washington, D.C. The new implosion theory group was set up in March 1944 under Teller to develop the mathematical description of implosion. Additional experimentalists joined the program. Neddermeyer's E-Division group expanded from five to roughly fifty.

The United States would not have been able to complete the atomic bomb project without its vigorous economy and substantial industrial facilities. However, the scientific resources of the nation were just as important, given the existing gaps in scientific knowledge at the time Los Alamos opened its doors. President Roosevelt's decision to support atomic bomb research preceded the first demonstration of a divergent chain reaction, the development of an industrial-scale method for separating 235U, and determination of plutonium's chemical and physical properties. In organizing the American atomic bomb project, Vannevar Bush drew on a sizable community of well-trained scientists having a wide repertoire of techniques and approaches. In bringing these tools to bear on the wartime problem of building the atomic bomb, the Los Alamos scientists developed a new approach to research.

What were the elements of this approach? First, the research was bound even more tightly than was conventional science to the behavior of artifacts and apparatus. The bombs had to explode, the detonators to fire, and the shape of the gadgets was constrained by that of the B-29 bomb bays. The technology had, in principle, to be totally reliable. Malfunctioning meant failure – it could no longer be construed as but another step in the process of understanding the physical world. In a context in which the lack of funding was not a constraint on research, one result was that solutions were often approached in several ways at once.

The Japanese began surrender negotiations one day after the Nagasaki bombing. Communities everywhere experienced the war's end with heartfelt relief. Los Alamos scientists were particularly proud of the unique role they had played in bringing the war to a close. The relief – and pride – were short-lived, for most of those who had worked on the bomb suffered loss of focus, while confronting an array of difficult choices, for example, whether to feel guilty for adding atomic bombs to the world's arsenal, and whether to continue working at Los Alamos. For a short time, the technical work of the laboratory slowed down, almost to a halt.

Responses to the war's end at Los Alamos varied a great deal. Laura Fermi recalls children parading through the streets, banging on pots and pans and joyfully making mini-explosions, while their parents grappled with the sobering implications of their achievement. Depression typically followed a short period of relief. Richard Feynman recalls that while he sat on a jeep and pounded on drums during one of the many end-of-the-war parties held at Los Alamos, he noticed that Robert Wilson was not jubilant. Feynman also became depressed soon afterward. Only a few of the scientists saw hope in the fact that the bomb was so destructive – believing that nuclear weapons might actually end all wars because the second use of so terrible a weapon was unlikely.

The story of the production of fissionable materials at Los Alamos is about the challenges of working with little-known, scarce substances under difficult experimental conditions, as well as the excitement of discoveries and unexpected turns in the course of all-out efforts to achieve practical results quickly. The interplay between the plutonium and uranium efforts within CM-Division reflects the wartime strategy of pairing complicated and straightforward tasks. In this way, personnel, equipment, and time could be focused on the most demanding problems. Thus, the relatively simple effort to produce uranium gun parts at Los Alamos complemented the more difficult effort to produce plutonium spheres, just as the relatively simpler gun program as a whole later complemented the more complex implosion program. Those implementing the less intricate effort were under pressure to proceed rapidly and produce absolutely reliable results meeting all contingencies so that more of the group's resources could be diverted to the thornier problem. Consequently, the uranium program was remarkably fast-paced and rigorous. The need to make the most of resources and save time weighed especially on Joseph Kennedy, Arthur Wahl, and Cyril Stanley Smith in CM-Division, because they had to adjust to the changing requirements of the other divisions, for whom they provided support services, while at the same time working to achieve their own goals.