|
50th Anniversary Article:
New Weapons Laboratory Gives
Birth to the "Gadget"
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.
Above is the "Gadget" just before the Trinity test July 16, 1945.
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.
|
Related Reading
Interchange of Information Between Chicago and Los Alamos June 17, 1943 (PDF 329 KB)
John von Neumann discusses calculating machines used to design the bombs August 1, 1944 (PDF 440 KB)
Implosion of the 4th of July (PDF 4.02 MB)
A German dirty bomb?: Radioactive Poison in Rocket Propelled, Unmanned Aircraft (PDF 127 KB)
Robert Bacher and Robert Wilson's Recommendations for Nuclear Physics Research (PDF 640 KB)
50th Anniversary Articles
The Berkeley Summer Study
Experimenting with Tanks and Barns
Evolving from Calculators to Computers
Plutonium Complicates Early Gun Work
Emilio Segre' Leads the Research on Spontaneous Fission
Implosion Becomes the Key to the 'Gadget'
New Weapons Laboratory Gives
Birth to the "Gadget"
|