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IMPLOSION BECOMES A NECESSITY (Los Alamos: Laboratory, 1944)
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Bringing It All Together, 1942-1945
Because the gun-type bomb design seemed
so simple and practical, Deke Parsons had assigned
implosion studies a low priority and
placed the emphasis on the more familiar artillery
method. Consequently, Seth H. Neddermeyer performed
his early implosion tests in relative obscurity.
Neddermeyer found it difficult to achieve symmetrical
implosions at the low velocities he had achieved.
When the Princeton mathematician John von Neumann, a
Hungarian refugee, visited
Los Alamos late in 1943, he suggested
that high-speed assembly and high velocities would prevent
predetonation and achieve more symmetrical
explosions. A relatively small, subcritical mass
could be placed under so much pressure by a symmetrical
implosion that an efficient detonation would occur.
Less fissionable material would be required, bombs could
be ready earlier, and extreme purification of
plutonium would be unnecessary. Von
Neumann's theories excited
Robert Oppenheimer, who assigned
Parsons's deputy, George B. Kistiakowsky, the task of
perfecting implosion techniques. (Kistiakowsky would
later become President Dwight D. Eisenhower's science
adviser.) Because Parsons and Neddermeyer did not
get along, it was Kistiakowsky who worked with the
scientists on the implosion project.
While experiments on the gun and implosion methods
continued, Parsons directed much of his effort toward
developing bomb hardware, including arming and wiring
mechanisms and fusing devices. Working with the Army
Air Force, Parsons's group developed two bomb models by
March 1944 and began testing them with B-29s.
According to Robert Serber, in his memoir titled Peace & War: Reminiscences of a Life on the
Frontiers of Science, the names were inspired by characters from Dashiell Hammett novels. The
names "Thin Man," and "Fat Man" referenced the different size and shape of
the bombs, but the names were also used because they worked as potential decoys, referring to
President Roosevelt (Thin Man) and Winston Churchill (Fat Man). Thin Man utilized the plutonium
gun design, while Fat Man (right) was an implosion prototype. Emilio Segrè's lighter,
smaller uranium design became "Little Boy," Thin Man's brother.
In the summer of 1944, however, it became clear that,
because of the plutonium-240 problem, a gun-type design
would not work for the plutonium bomb. The implosion
method was now transformed from an intriguing possibility
into a difficult necessity.
Glenn Seaborg had warned that when
plutonium-239 was irradiated for a length of time it was
likely to pick up an additional neutron,
transforming it into plutonium-240 and increasing the
danger of predetonation, i.e., the bullet and target in
the plutonium weapon would melt before coming
together. Measurements taken at
Oak Ridge confirmed the presence of
plutonium-240 in the plutonium produced in their
experimental pile (X-10). On July
17, the difficult decision was made to cease work on the
plutonium gun method -- there would be no "Thin
Man." Plutonium could be used only in an
implosion device, but in the summer of 1944 an implosion
weapon looked like a long shot.
Abandonment of the plutonium gun project eliminated a
shortcut to the bomb. This necessitated revision of
the estimates of weapon delivery
Vannevar Bush had given the President in
1943. The new timetable, presented to General George
Marshall by Leslie Groves on August 7,
1944 -- two months after "D-Day," the Allied
invasion of France -- promised small implosion weapons of
uranium or plutonium in the second quarter of 1945 if
experiments proved satisfactory. More certain was
the delivery of a uranium gun-type bomb by August 1, 1945,
and the delivery of one or two more by the end of that
year. Marshall and Groves agreed that Germany might
well surrender by the summer of 1945, thus making it
probable that Japan would be the target of any atomic
bombs ready by that time.
Oppenheimer acted quickly to maximize the laboratory's
efforts to master implosion. Only if the implosion
method could be perfected would the plutonium produced at
Hanford come into play. Without
either a plutonium gun bomb or implosion weapon, the
burden would fall entirely on uranium and the less
efficient gun method. Oppenheimer directed a major
reorganization of Los Alamos in July 1944 that prepared
the way for the final development of an implosion
bomb. Robert Bacher took over G Division (for
"Gadget") to experiment with implosion and
design a bomb; Kistiakowsky led X Division (for
"explosives") in work on the explosive
components; Hans Bethe continued to head up theoretical
studies; and Parsons now focused on overall bomb
construction and delivery.
Field tests performed with uranium-235 prototypes in late
1944 eased doubts about the gun-type method to be employed
in the uranium bomb. It was clear that the uranium-235
from Oak Ridge could be used in a gun-type nuclear device
to meet the August 1 deadline Groves had given General
Marshall and the Joint Chiefs of Staff. The
plutonium produced at such expense and effort at Hanford
(right), however, would not fit into wartime planning
unless a breakthrough in implosion technology could be
found.
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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), 40, 42. The diagram illustrating
implosion is reproduced from the
Department of Energy
report
Linking Legacies: Connecting the Cold War Nuclear
Weapons Production Processes to their Environmental
Consequences
(Washington: Center for Environmental Management
Information, Department of Energy, January 1997), 13. The photograph of the implosion experiment
is courtesy the
Los Alamos National Laboratory; it is reprinted in Rachel Fermi and Esther Samra,
Picturing the Bomb: Photographs from the Secret World
of the Manhattan Project (New York: Harry N. Abrams, Inc., Publishers, 1995),
111, 116. The photograph of Fat Man is courtesy the
U.S. Army Corps of Engineers (via the
National Archives). The photograph of
Leslie Groves with
Robert Oppenheimer is courtesy the
Department of Energy. Click
here for more information on the Hanford B Reactor
photograph.
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