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EARLY BOMB DESIGN (Los Alamos: Laboratory, 1943-1944)
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Bringing it All Together, 1942-1945
Early work on the
design of the atomic bomb began even as
scientists continued to arrive at
Los Alamos
throughout 1943. The
properties of uranium were reasonably
well understood, those of plutonium less
so, and knowledge of fission
explosions entirely theoretical.
That 2.2 secondary neutrons were produced when uranium-235
fissioned was accepted, but while
Glenn Seaborg's team had proven in March
1941 that plutonium underwent
neutron-induced fission, it was not known
yet if plutonium released secondary neutrons during
bombardment. Further, the exact sizes of the
"cross sections" of various fissionable
substances had yet to be determined in experiments using
the various particle accelerators then
being shipped to Los Alamos. The theoretical
consensus was that fission
chain reactions
(left) did take place with sufficient speed to produce
powerful releases of energy (and not simply result in the
explosion of the critical mass itself),
but only experiments could test this theory. The
optimum size of the critical mass remained to be
established, as did the optimum shape. When enough
data were gathered to establish optimum critical mass,
optimum effective mass still had to be determined.
That is, it was not enough simply to start a chain
reaction in a critical mass; it was necessary to start one
in a mass that would release the greatest possible amount
of energy before it was destroyed in the
explosion.
In addition to calculations on uranium and plutonium
fission, chain reactions, and critical and effective
masses, work needed to be done on the ordnance aspects of
the bomb, or the "Gadget" as it came to be known.
Two subcritical masses of fissionable material would have
to come together to form a supercritical mass for an
explosion to occur. Furthermore, they had to come
together in a precise manner and at high speed.
Measures also had to be taken to ensure that the highly
unstable subcritical masses did not predetonate because of
spontaneously emitted neutrons or neutrons produced by
alpha particles reacting with lightweight
impurities. The chances of predetonation could be
reduced by purification of the fissionable material and by
using a high-speed firing system capable of achieving
velocities of 3,000 feet per second. A conventional
artillery method of firing one subcritical mass into the
other (above) was under consideration for uranium-235, but
this method would work for plutonium only if absolute
purification of plutonium could be achieved.
A
"gun-type" design
of this sort was thus designed for uranium. Unable
to solve the purification problem, however, bomb designers
feared that they would have to turn instead to the
relatively unknown
implosion method (right) for
plutonium. With implosion, symmetrical shockwaves
directed inward would compress a subcritical mass of
plutonium into a smaller, now-critical sphere. This
sphere would be surrounded by a heavy
"tamper" that would reflect neutrons back
into the active volume and restrain the explosion for a
few crucial moments, thereby increasing the
efficiency of the blast. An
initiator placed at the center of the sphere would ensure
that the chain reaction began at precisely the right
moment (rather than relying on "background neutrons"
caused by spontaneous fission or background
radiation).
Always in the background loomed the
hydrogen bomb, a thermonuclear device
considerably more powerful than either a uranium or
plutonium device. Any hydrogen bomb would likely
require an atomic fission bomb as a detonator,
however. For this reason research on the hydrogen
bomb, or "Superbomb," was always a distant second in
priority at Los Alamos, but
Robert Oppenheimer concluded that it was
too important to ignore. After considerable thought,
he gave Edward Teller permission to
devote himself to the nuclear weapon that ultimately would
dominate the
Cold War.
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Sources and notes for this page.
The text for this page was adapted from, and portions
were taken directly from the
Office of History and Heritage Resources
publication:
F. G. Gosling,
The Manhattan Project: Making the Atomic Bomb
(DOE/MA-0001; Washington: History Division, Department
of Energy, January 1999), 38-39. The photograph of "Little Boy" is
courtesy the U.S. Army Corps of Engineers (via the
National Archives). The fission chain graphic is
adapted from graphics originally produced by the
Washington State Department of Health; the modifications are original to the Department of
Energy's Office of History and Heritage Resources.
The sketches of the gun-type and
implosion approaches to bomb design are
reproduced from Robert Serber's
April 1943 "Los Alamos Primer,"
21-22. The photograph of the "Ivy Mike"
hydrogen bomb test is courtesy the
Department of Energy's
Nevada Site Office.
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