As soon as the Los Alamos Laboratory opened its doors, committees were formed to plan the research program and cope with practicalities. Robert Serber offered an indoctrination course early in April 1943 to acquaint scientists with the current state of research on the atomic bomb. Conferences that month laid out specific research objectives. Even though many fission constants were poorly determined and the accuracy of approximations was generally low, Los Alamos physicists were confident that a reasonably efficient gun bomb could be built. Acceptance of the gun as a workable assembly lent optimism to the entire project. As a fallback, Oppenheimer established a small research effort under Seth Neddermeyer to explore implosion assembly.

The Planning Board

Committees helped Oppenheimer make major decisions, with Groves's approval. The first informal committee – Robert Wilson, Edwin McMillan, Oppenheimer, Edward Condon (the associate director), John Manley, and Serber – met on 6 March 1943 and considered practicalities, such as when people and equipment would arrive and who would handle services rendered by the machine and electronics shops.

This initial planning group gave way several weeks later to a larger committee called the Planning Board, which coordinated the technical program over the next month. Oppenheimer, Condon, Dana Mitchell, and Julian Mack provided administrative guidance, while Wilson, Serber, John Williams, McMillan, and Donald Mastick planned the scientific program.

The story of the Los Alamos project to build the first atomic bombs has been told often. Why then another history of Project Y, as it was known during World War II? Three features distinguish this account: it is a history of the technical developments; it is based on the full complement of documents, both classified and unclassified, of wartime Los Alamos; and it explores for the first time the methodology by which researchers at Los Alamos succeeded in their wartime mission.

Unlike earlier histories of Los Alamos, this book treats in detail the research and development that led to the implosion and gun weapons; the research in nuclear physics, chemistry, and metallurgy that enabled scientists to design these weapons; and the conception of the thermonuclear bomb, the “Super.” Although fascinating in its own right, this story has particular interest because of its impact on subsequent developments. Although many books examine the implications of Los Alamos for the development of a nuclear weapons culture, this is the first to study its role in the rise of the methodology of “big science” as carried out in large national laboratories.

Our primary aim is to recount this technical history, but we have not ignored the social context entirely. Although we largely leave for other historians the problem of analyzing the social community at Los Alamos in wartime – for example, the role of women, of foreign scientists, and of military personnel – we do provide an abbreviated account of the establishment and early years of the unique community that grew around the Los Alamos Laboratory.

Between the summers of 1943 and 1944, the Theoretical Division, under Hans Bethe, and the Physics Division, under Robert Bacher, collaborated in studying the nuclear physics of the atomic bomb. T-Division's responsibilities included calculating critical mass and efficiency. The lack of hard nuclear-constant data was particularly troubling. While P-Division worked to improve the experimental data using available detectors, accelerators, and other devices, T-Division developed flexible models based on the changing set of available data. To cross-check their results, researchers often used different methods to solve the same problem. For example, the Water Boiler, a nuclear pile using enriched uranium in a water solution, provided a means of checking critical mass calculations. As a backup, Richard Feynman made calculations on uranium hydride, then being considered as a potential active material. Teller's investigation of the hydrogen bomb (the Super) was an alternative approach to a nuclear weapon. The opportunity to conduct physics research on a larger scale than had ever before been attempted gave the Los Alamos physicists the experience of working in well-funded multi-disciplinary groups, which included both experimentalists and theorists, as well as electronics experts, chemists, and metallurgists.

Nuclear Theory: Critical Mass and Efficiency

In September 1943, T-Division was refining its critical mass and efficiency predictions and calculating the damage the bomb could cause. Up to this point, the division had remained somewhat informal in its organization to accommodate changing priorities, but by October it had begun to subdivide into groups: Bethe took on the problem of implosion, Victor Weisskopf led the calculations of efficiency, Robert Serber spearheaded diffusion theory, Edward Teller assumed responsibility for both the Super and implosion, Feynman led the uranium hydride calculations, and Donald Flanders headed the computational effort.

Just before dawn on 16 July 1945, the area selected for the Trinity test – the desolate Jornada del Muerto region of New Mexico – no longer swarmed with activity, as it had in the past several weeks. The thunder-storms that had worried Groves and Oppenheimer through the night had stopped. The scientists, who had worked almost nonstop in preparing for the first atomic bomb test, waited tensely for the test to begin.

Arranging their apparatuses around the gadget – ionization chambers, seismographs, motion picture cameras, and other devices – they prepared to record physical aspects of the explosion: light, heat, neutrons, gamma rays, and other features. The data would indicate what to expect of combat atomic bombs and how to achieve the most destruction. But even the most careful preparations could not guarantee a successful test, because the weather had to be just right to prevent heavy fallout from reaching populated areas. Completing the test on schedule became of paramount importance when President Harry S. Truman announced that he would meet with Churchill and Stalin at Potsdam on 16 July 1945.

The Experimental Program

Because only a limited number of measurements could be taken at Trinity, the ones to be selected became a critical topic of discussion. A panel consisting of Fussell, Moon, Bernard Waldman, and Victor Weisskopf was assembled to evaluate proposals. Data were needed on both the performance and the effects of the weapon.

During the spring of 1944, Emilio Segrè's group in P-Division made the startling observation that the first samples of pile-produced 239Pu had an unusually high spontaneous fission rate, with a neutron emission approximately five times that of cyclotron-produced 239Pu. This finding confirmed the gnawing suspicions of Fermi, Segrè, Seaborg, and others that the neutron bath in the production piles at Clinton and Hanford might cause the formation of a significant quantity of 240Pu, an as-yet-unobserved spontaneously fissioning isotope of plutonium. However, the alarmingly high rate of the spontaneous fission was unexpected. This rate increased the neutron background enough to make it highly probable for a gun-assembled gadget to predetonate and thus undermined the plutonium gun program.

Determined not to lose the heavy investment made in plutonium production, Groves forced the laboratory to change course. The primary technical objective shifted from developing a gun weapon to developing a plutonium implosion assembly. Within days after Oppenheimer officially announced the spontaneous fission discovery, the laboratory reorganized its work force to focus on implosion. Two new divisions were established – X (Explosives) under Kistiakowsky, and G (Gadget) under Bacher. Most of the groups in these new divisions were moved out of the earlier Research and Ordnance Engineering divisions. Unfortunately, at this point experiments in the implosion diagnostic program were indicating that an implosion weapon would be extremely difficult, if not impossible, to achieve.

By the time a fast-neutron fission laboratory was conceived early in 1942, theorists and experimentalists had made initial calculations for the bomb, including new estimates of critical mass and efficiency. Some progress had been made on designing methods for assembling the weapon and on a program for measuring nuclear constants central to bomb calculations. On the whole, however, research languished because of poor information exchange among the various groups involved, which were scattered throughout the United States. Oppenheimer, who replaced Gregory Breit as coordinator of the fast-fission project, recommended to Groves that the effort be centralized. Groves, who recognized the security benefits of centralization, readily complied, thereby setting in motion plans for establishing the Los Alamos Laboratory.

Groves and Oppenheimer took the first step toward creating the laboratory in early 1943 by recruiting many of the world's best scientists. The temporary nature of the project and the urgency of its mission aided the recruitment effort, but the task was complicated by the delicate issue of whether Los Alamos would be a military or a civilian establishment – an issue never formally resolved. The standard caricature of Oppenheimer as an other-worldly intellectual and Groves as a burly martinet highlights the misalignment between the military and scientific communities that joined in Project Y. Surprisingly, Oppenheimer and Groves developed a collaboration that was both congenial and fruitful.

The accelerated implosion effort, which began in August 1944, made rapid progress. By October 1944, James Conant was giving a lensed implosion device a 50–50 chance of working on schedule, if all went smoothly, for a test at Trinity on 1 May 1945 and a “3:1” chance for a test on 1 July. But he added, “In my opinion, the probabilities of success by the gun method (Mark 1) within the next year are very much greater than by the implosion method. Indeed the gun method seems as nearly certain as any untried new procedure can be.”

Overcoming asymmetries remained the outstanding technical problem of the implosion program. By mid-fall 1944, two experimental strands of the implosion program were converging on this problem: research on the explosive lens and on the electric detonator. In addition, in T-Division, Robert Christy put forth a conservative proposal for overcoming the asymmetry: try to implode a solid sphere rather than a spherical shell of active material. However, calculations indicated that the “Christy gadget” was intrinsically far less efficient than the hollow weapon, and that such a device would require a modulated initiator to activate the explosion at the most favorable moment. The call for the development of the implosion initiator added another thorny problem to the program.

As the time approached when sizable quantities of plutonium would become available, gross design features had to be frozen in order to begin final bomb production.

After the implosion design was frozen at the end of February 1945, the program shifted its emphasis from research to constructing actual bomb components, including explosive lenses, detonators, initiators, and the plutonium hemispheres. Kistiakowsky reported on X-Division's work in April: “One can now state with a reasonable degree of assurance that all major research and design gambles involved in the freeze of the program of the X-Division have been won. Progress is more and more determined by the rate of supply of manufactured items.” By May, he concluded, “The activities in X-Division have lost all semblance to research and have become so largely production and inspection and testing that their brief summary here seems impractical” (at which point his division progress reports stopped abruptly). However, most of the crucial components of the implosion gadget remained problematic, almost to the time of the Trinity test, and most underwent last-minute change.

The Cowpuncher Committee

Oppenheimer launched the final phase of Project Y's implosion effort with his appointment on 1 March 1945 of the powerful Cowpuncher Committee to “ride herd” on the program. Besides Oppenheimer, the members included Bethe, Kistiakowsky, Parsons, Bacher, Samuel Allison, Cyril Smith, and Kenneth Bainbridge. The committee oversaw eight major programs: (1) fabrication and inspection of explosive lenses; (2) design and construction of electric detonators and detonator circuits; (3) diagnostic tests to determine timing, compression, and symmetry; (4) research in chemistry and metallurgy; (5) study of the critical mass and time constant of the plutonium nuclear explosion; (6) design of the inner metal parts of the implosion assembly; (7) coordination of the Trinity program; and (8) assignment of shop priorities.

Following the discovery of nuclear fission in 1938, scientists in Germany, France, Great Britain, the Soviet Union, Japan, and the United States began to investigate the possibility of exploiting this energy source for military purposes. The United States alone was able to draw its governmental, industrial, and scientific capabilities into an efficient bombbuilding collaboration. It had not only the manpower, materials, and industrial support needed for the expensive project – eventually to cost $2.2 billion – but also a sizable and competent physics community well versed in technology, strengthened by talented emigrés, with ties to government and industry, and close international contacts. Some American scientists, like Ernest Lawrence, were experienced in managing large research efforts. This community also included older scientific statesmen, like Vannevar Bush, with political experience and proven abilities in coordinating government-sponsored applied research projects. On 9 October 1941 Bush persuaded President Franklin Roosevelt to authorize American research on the feasibility of a fission bomb.

The Discovery of Nuclear Fission

The events leading to Los Alamos began in 1933, when Frédéric Joliot and Irène Curie produced artificial radioactivity by bombarding aluminum with alpha particles. The next year Enrico Fermi and his co-workers in Rome bombarded a variety of elements with neutrons, the neutral fundamental particles that James Chadwick had discovered in 1932. Upon bombarding uranium, Fermi's group found an unexplained radioactive substance and speculated that they had created a new transuranic element.

Two developments in the spring of 1944 offered hope for overcoming the asymmetry problems of the implosion weapon. The asymmetry arose in part because the detonation waves diverging from the various initiation points met and interacted to produce small regions of markedly increased pressure. Furthermore, the multiple detonations of the surrounding explosive were not adequately simultaneous. The first problem would be dealt with by the three-dimensional explosive lens, suggested by James Tuck in May and given its basic design by John von Neumann. The second problem could be solved, as Luis Alvarez had suggested in May, by replacing the original inherently variable Primacord detonation distribution systems with electric detonators of highly superior reproducibility, thus providing a means of achieving adequately simultaneous detonation at several points. Developing and producing practical explosive lens and electric detonator systems would require a concerted research and development effort right up to the Trinity test in July 1945.

In view of these difficulties, it seemed wise to try testing the device. The decision to do so was made early in 1944. By the fall of that year, the site selection committee had fixed on the Jornada del Muerto region of south central New Mexico.

Explosives

The research and development of high explosives – materials that detonate at supersonic speeds by a process involving chemical reaction and a shock wave – was (arguably) the most pivotal and problematic component of the implosion program.

A comprehensive American program of research on plutonium and 235U isotope separation evolved between mid-1941 and mid-1942. Plans were formulated for the construction of plutonium production reactors, uranium separation plants, and centralized bomb research facilities. The program for producing 235U by gaseous diffusion was slowed both by the technical problem of finding a suitable isotope separation filter, or “barrier,” and by the difficulty of coordinating Kellogg Company employees and Columbia University researchers. Despite such obstacles, plans for providing fissionable materials were well on the way to being implemented by 1943.

Expansion of the American Atomic Bomb Program

In June 1941, Vannevar Bush persuaded President Roosevelt to form the Office of Scientific Research and Development (OSRD), under the aegis of the Office of Emergency Management. With Bush as director, the OSRD assumed responsibility for mobilizing scientific resources and applying research to national defense. James Conant replaced Bush as chairman of the National Defense Research Committee, which now operated as a unit of the OSRD. Recommendations for research contracts were channeled through Conant and placed by Bush. The Advisory Committee on Uranium, essentially a research organization, became the S-1 Section of OSRD and remained in place throughout the war. Bush and Conant established three subsections of S-1: one on theoretical research under Fermi; one on power production under George Pegram, physicist and dean of the graduate faculties at Columbia University; and a third on heavy water and isotope separation under Columbia chemist Harold Urey.

The first months on the mesa required drastic adjustments – to Oppenheimer's style of scientific leadership, to Groves's close administration of the town, and to the unusual partnership between scientists and the military. The laboratory grew more rapidly than anticipated because in the early months of the project Oppenheimer and Warren K. Lewis's advisory committee recognized that a scientific community of some 100 scientists was too small to cope with the complexities of producing an atomic bomb. Broadening the laboratory mission, as the Lewis Committee recommended, implied the absorption of new sub communities, including the Army Special Engineer Detachment (SED) and the British Mission.

Life in wartime Los Alamos was abnormal in almost every respect, but the townspeople strived toward normalcy in their everyday lives, meeting their practical concerns about food, shelter, amusement, and schools with a spirit of adventure. The spartan simplicity and transience of housing and the lack of many community services often turned daily life into a struggle. But as Kathleen Mark reflected, “When one considers that we lived … closely packed together – aware of every detail of our neighbor's lives – even to what they were having for dinner every night – one can't help but marvel that we enjoyed each other so much.” The residents worked and also played hard in the isolated military community to which they were restricted.

Through mid-1945 the Los Alamos Laboratory continued to pursue a strategy of overlapping approaches in its programs to test and refine critical mass and perform other calculations affecting bomb design and deployment. These activities included work in R-, G-, and F-Divisions with critical and subcritical assemblies, nuclear constant and other measurements by R-Division, and theoretical fine-tuning by T-Division, along with a backburner effort on the Super in F-Division. In addition, new projects were begun after mid-1944, among them a series of tests on critical assembly by G-Division, work on a high-power Water Boiler (nicknamed Hypo) to be used by F-Division for critical mass measurements, and the development of a spectacular assembly suggested by Frisch, which, unlike other experimental assemblies, went critical with prompt neutrons alone.

Besides diverting resources from implosion development and preparations for the Trinity test, these projects were sometimes quite dangerous and often risked the loss of precious fissionable materials. Nonetheless, they were mounted with considerable enthusiasm, and characteristically subjected to empirical testing. As had been the case in early nuclear constant measurements by P-Division and in the metallurgy program, researchers attempted to compensate for the lack of 239Pu by working with 239U, in hopes that the results could be extrapolated to reveal the properties of the heavier element.

Continuation of the Critical Mass Studies

Short of the real explosion, there was no way to determine precisely the extent of supercriticality achieved in the gun- or implosion-assembled bomb, or to measure other chain reaction properties, such as the neutron population growth rate.

After abandoning the plutonium gun, Oppenheimer streamlined the uranium gun program. The remaining work, all experimental and straight-forward, was consolidated in one ordnance group under Lt. Comdr. A. Francis Birch. Birch's group completed and tested the uranium gadget design by February 1945. Meanwhile, metallurgists decided that 235U was strong enough to be used in its natural state, so that alloy research could be discontinued. They developed suitable large-scale metal reduction and fabrication techniques. Polonium production for the initiators was on schedule.

As 235U production slowly increased and the laboratory made arrangements for fabricating gun parts, a quiet but dramatic change was occurring in the gun program. Before March 1945, this program had centered on perfecting a reliable method of assembling 235U. Now Birch had to turn the gadget into a bomb that could be delivered by an airplane. The bomb had to contain the gun gadget, offer protection from antiaircraft fire, house all the components (including antenna, fuzing, and circuitry), and provide a reliable, stable flight. The design of the gadget could still be changed slightly, after being frozen in February. But once the drop tests began, extensive changes could not be made without jeopardizing Groves's July deadline.

Conversion to the Uranium Gun

With the cancellation of the plutonium gun in July 1944, the need to develop a multifaceted high-velocity gun suddenly vanished. Fifteen months of intensive effort had gone into the physical design of the plutonium gun's components, into interior ballistics, and the mechanical properties of plutonium.

Although the problem of producing a gun weapon seemed relatively simple as the laboratory entered the summer of 1943, William Parsons and Charles Critchfield soon recognized that the guns needed to assemble uranium and plutonium would be very different from the ordinary ones that shot high-explosive projectiles at targets. The Ordnance and Engineering Division (E) did not know enough about the critical masses of plutonium and uranium, their metallurgical properties, or the speed required by nuclear constraints to assemble an effective plutonium weapon. Three points were clear: the guns would have to be designed from scratch – standard ordnance cannon could not be adapted to this use; extensive testing of both the plutonium and uranium guns and their mockup target and projectile components would be required; and targets and projectiles of active material would have to be designed. No one had yet tried to create an explosion using two or more pieces of fissionable material. Work on the gun gadget proceeded along three paths – interior ballistics research, experimental testing of designs, and target-projectile-initiator development. Each proceeded more or less independently until the reorganization of the laboratory in August 1944 consolidated all gun work.

Between April 1943 and August 1944, Parsons, Critchfield, Edwin McMillan, and Joseph Hirschfelder designed, created, and tested all the principal components of the gun gadget, including the gun mechanism, target and projectile geometries, and initiators.