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J.R. Oppenheimer and General Groves
Events People Places Processes Science Resources

Time Periods

1890s-1939:
Atomic Discoveries

1939-1942:
Early
Government Support

1942:
Difficult
Choices

1942-1944:
The Uranium
Path to
the Bomb

1942-1944:
The Plutonium
Path to
the Bomb

1942-1945:
Bringing It All Together

1945:
Dawn of the
Atomic Era

1945-present:
Postscript --
The Nuclear Age


Norris Bradbury, Robert Oppenheimer, Richard Feynman, Enrico Fermi, and others, Los Alamos, 1946BASIC RESEARCH AT LOS ALAMOS
(Los Alamos: Laboratory, 1943-1944)
Events > Bringing It All Together, 1942-1945

Enrico FermiThe first few months at Los Alamos were occupied with briefings on nuclear physics for the technical staff and with planning research priorities and organizing the laboratory.  Leslie Groves called once again on Warren Lewis to head a committee, this time to evaluate the Los Alamos program.  The committee's recommendations resulted in the coordinated effort envisioned by those who advocated a unified laboratory for bomb research.  Enrico Fermi (left) took control of critical mass experiments and standardization of measurementHans Bethe techniques.  Plutonium purification work, begun at the Met Lab, became high priority at Los Alamos, and increased attention was paid to metallurgy.  The committee also recommended that an engineering division be organized to collaborate with physicists on bomb design and fabrication.  The laboratory was thus organized into four divisions: theoretical (Hans A. Bethe, right); experimental physics (Robert F. Bacher); chemistry and metallurgy (Joseph W. Kennedy); and ordnance (Navy Captain William S. "Deke" Parsons).  Like other Manhattan Project installations, Los Alamos soon began to expand beyond initial expectations.  

J. Robert OppenheimerAs director, J. Robert Oppenheimer (left) shouldered burdens both large and small, managing the numerous mundane matters such as living quarters, mail censorship, salaries, promotions, and other "quality of life" issues that were inevitable in an intellectual pressure-cooker with few social amenities. Oppenheimer relied on a group of advisers to help him keep the "big picture" in focus, while a committee made up of Los Alamos group leaders provided day-to-day communications between divisions.  

Emilio Segrè, 1954Early experiments on both uranium and plutonium provided welcome results.  Uranium emitted neutrons in less than a billionth of a second -- just enough time, in the world of nuclear physics, for an efficient explosion.  Emilio Segrè (right) later provided an additional cushion with his discovery in December 1943 that, if cosmic rays were eliminated, the subcritical uranium masses would not have to be brought together as quickly as previously thought; nor would the uranium have to be as pure.  Muzzle velocity for the scaled-down artillery piece could be lower, and the gun could be shorter and lighter.  Segrè's tests on the first samples of plutonium demonstrated that plutonium emitted even more neutrons than uranium due to the spontaneous fission of plutonium-240.  Both theory and experimental data now agreed that a bomb using either element would detonate if it could be designed and fabricated into the correct size and shape.  But many details remained to be worked out, including calculations to determine how much uranium-235 or plutonium would be needed for an explosive device.  

An experiment to determine the cross section of uranium-235 for fast neutrons. The target is the small pile of cubes of uranium hydride. The uranium target is surrounded by larger blocks of beryllium tamper.Bacher's experimental physics division patiently generated the essential cross section measurements needed to calculate critical and efficient mass.  The same group utilized particle accelerators to produce the large numbers of neutrons needed for its cross section experiments.  Bacher's group also compiled data that helped identify tamper materials that would most effectively push neutrons back to the core and enhance the efficiency of the explosion.  Despite Los Alamos's postwar reputation as a mysterious retreat where brilliant scientists performed miracles of nuclear physics, much of the work that led to the atomic bombs was extremely tedious.  

Cubes of uranium metal, Los Alamos, 1945The chemists' job was to purify the uranium-235 and plutonium, reduce them to metals, and process the tamper material.  Only highly purified uranium and plutonium would be safe from predetonation.  Fortunately, purification standards for uranium were relatively modest, and the chemical division was able to focus its effort on the lesser known plutonium and make substantial progress on a multi-step precipitation process by summer 1944.  The metallurgy division had to turn the purified uranium-235 and plutonium into metal.  Here, too, significant progress was made by summer as the metallurgists adapted a stationary-bomb technique initially developed at Iowa State College.  

William S. "Deke" ParsonsParsons (right), in charge of ordnance engineering, directed his staff to design two artillery pieces of relatively standard specifications except for their extremely light barrels -- one for a uranium bomb and one for a plutonium bomb.  The guns needed to achieve high velocities, but they would not have to be durable since they would only be fired once.  Here again early efforts centered on the more problematic plutonium weapon, which required a higher velocity due to its higher risk of predetonation.  Two plutonium guns arrived in March 1944 and were field-tested successfully.  In the same month, two uranium guns were ordered.

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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), 39-40.  Click here for more information on the group photograph of scientists at Los Alamos.  The photograph of Enrico Fermi is courtesy the Argonne National Laboratory.  The photograph of Hans Bethe is courtesy the Los Alamos National Laboratory (LANL).  The photograph of Robert Oppenheimer in front of a blackboard is reproduced by permission of the J. Robert Oppenheimer Memorial Committee.  The photograph of Emilio Segrè is courtesy the Lawrence Berkeley National Laboratory.  The photographs of the neutron cross section experiment and of the blocks of uranium are courtesy LANL; they are 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), 99 and 109.  The photograph of Deke Parsons is reproduced from Los Alamos Scientific Laboratory, Los Alamos: Beginning of an Era, 1943-1945 (Los Alamos: Public Relations Office, Los Alamos Scientific Laboratory, ca. 1967-1971), 59.

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