Above is the "Gadget" Just before the Trinity test July 16, 1945.
During the first week of April 1943, J. Robert Oppenheimer and the first staff members to arrive at Los Alamos set up experimental equipment, organized their work areas and moved into the newly completed and, in many cases, uncompleted facilities in the technical area. In the midst of these arrangements, Oppenheimer's assistant, Robert Serber, delivered a series of lectures summarizing what was then known about the design of nuclear weapons. This information included not only the early work that had been done at the University of California by theorists assigned to the electromagnetic separation project led by Nobel Laureate Ernest O. Lawrence in the Radiation Laboratory, but also the results of the work of a June 1942 conference held in Berkeley. At this conference, Oppenheimer, Serber, Hans Bethe from Cornell University's physics department, John Van Vleck from the University of Wisconsin's physics department, Edward Teller who was on leave from Washington University to the University of Chicago's metallurgical laboratory, Felix Bloch from Stanford University's physics department, Richard Tolman, the California Institute of Technology's dean of physical sciences, and Emil Konopinski from the University of Chicago had discussed the work of British and American theorists and the possibility of a "super" bomb conceived by Teller and Enrico Fermi.
During the nine months that elapsed between the summer conference at Berkeley and the opening of the laboratory, both theoretical and experimental work had gone forward at a variety of academic and non-profit laboratories, and it was to summarize the results of this work that Serber conducted his lectures. Los Alamos scientists tore themselves away from setting up their laboratories to attend, but mindful of the work that had to be done, Serber made his summaries as terse as possible. At the end of each day, he met with Edward Condon, whom Oppenheimer had brought from the Westinghouse Research Laboratories to serve as associate director of the Laboratory, to write up the lectures and supplement them. The ultimate result was LA-1, "The Los Alamos Primer.
" It was not easy to lecture about the fundamentals of nuclear weapons design in a laboratory still under construction, with carpenters and plumbers in the immediate vicinity of the reading room of the Administration Building where the lectures were given.
In his first lecture, Serber began, "The object of the project is to produce a practical military weapon in the form of a bomb in which the energy is released by a fast-neutron chain reaction in one or more of the materials known to show nuclear fission." Oppenheimer sent John Manley, the experimental physicist from the University of Illinois who had helped him organize Los Alamos, up to Serber with a note that he should use the word "gadget" instead of "bomb" because the workmen might overhear the lectures. The name stuck. Throughout the project, the device was known as a "gadget."
In his lectures, Serber set forth the energy to be expected from fission processes, the nature of the fast-neutron chain reaction, the "target size" or cross section for a neutron causing fission in uranium and plutonium, he energies and numbers of neutrons to be expected from the fissioning atoms and the nature of the newly discovered element, plutonium, which with uranium 235 was expected to be one of the fuels for the bombs they would design.
Although the cross sections for fission for uranium 235 were low and those for plutonium were high compared to modern values, and the estimates of neutrons to be expected in the fission processes were low, these errors canceled each other out so that the chain reaction was accurately predicted. Based on these numbers, Serber also estimated the minimum size of the bomb and the effect of various tamper materials that would surround the fissionable material in reflecting neutrons to enhance the efficiency of the chain reaction. The efficiency would be limited by the explosion of the active material, which would terminate the chain reaction and would be very low, since very few of the atoms would fission before the fissionable material was blown apart.
In addition, Serber estimated the damage to be expected from the neutrons, residual radiation and blast of the bomb. However, he overlooked the damage from gamma rays and the fireball that would be produced. Gamma rays from the Hiroshima bomb would ultimately produce casualties 4,000 feet from ground zero and the Nagasaki bomb from 5,000 feet from ground zero. Approximately 5 to 15 percent of the casualties were due to gamma rays. The fireball, which was three-and-a-half times as bright as the sun and half again as hot as its surface, produced skin burns approximately three times farther from ground zero and accounted for 20 to 30 percent of the fatalities at Hiroshima and Nagasaki.
Very little damage would be produced if the weapon were detonated prematurely by a stray neutron causing an early chain reaction before the material was properly assembled.
This made it necessary to assemble the material very rapidly by firing it together with a gun that produced muzzle velocities exceeding 3,000 feet a second, and to provide a source of neutrons to initiate the chain reaction at the precise moment when the material was assembled. Other means of assembling the critical material had also been considered but were, for the time being, subordinated to gun design.
Serber's lectures made clear the challenges that faced the new laboratory. He concluded, "the immediate experimental program is largely concerned with measuring the neutron properties of various materials and with the ordnance problem. It is also necessary to start new studies on techniques for direct experimental determination of critical size and time scale, working with large but subcritical amounts of active material." This would require the use of particle accelerations that could produce fast neutrons like the Harvard cyclotron, Wisconsin Van de Graff and Illinois Cockcroft-Walton machines that Manley and University of California professor Edwin McMillan had acquired for the Laboratory, but which were, as yet, in pieces waiting reassembly. The ordnance problem would require experts in military ordnance like Capt. W.S. "Deke" Parsons, who visited the small ordnance group in May 1943 and came to head the ordnance engineering division a month later.
The provision of large but subcritical amounts of active material awaited the completion of the uranium isotope separation plants at Oak Ridge, Tenn., and the production reactors at Hanford, Wash. To prepare this material for the experiments that would determine the critical sizes of a chain-reacting assembly and the times required for chain reaction, the chemistry and metallurgy staff of the Laboratory would also have to be augmented. The small group of theoretical and experimental physicists Oppenheimer and Manley had thought might suffice to design nuclear weapons would give way to a large, multidisciplinary organization.
- Robert Seidel
Next:. The University of California contract to operate Los Alamos.
