Manhattan Project - Prologue - UCSB Department of History

Manhattan Project - Prologue
• The race for the Bomb…all the elements of
high drama: earth-shaking discoveries,
matters of life and death, decisions that
would affect the lives of millions and the
future of humanity all played out on a
global stage.
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• Remember the bigger picture: Virtually all
ideas associated with nuclear weapons that
shaped the Cold War derive from the
attitudes and assumptions formulated 1941-
45:
• Nuclear bargaining chips
• Bomb as a diplomatic tool
• Need for secrecy and speed
• Science-state-public-industry relations
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Manhattan Project
• The scale of Manhattan Project
– $2 billion cost ($225 billion today)
– Over 120,000 people employed to build the
atomic bombs
– Work done at over three dozen sites in 19 states
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Manhattan Project Scientists:
Soldiers out of Uniform
• Their participation in Manhattan Project –
The single most profound experience in the
history of the American scientific
community.
– Intellectual, financial, political, professional,
and moral implications
– Linked basic research with military applications
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Scientific Understanding Continues to Grow
• Bohr’s sudden realization about U-235
(Feb. 1939)
– Publishes paper in Sept. 1939 with Princeton’s
John Wheeler that shows only U-235 will
fission easily. One of last openly published
papers on fission research
• Scientists “looking for neutrons” in Paris
and New York (winter 1939)
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• French research; publication in April 1939
gives proof that enough neutrons are
produced to sustain a chain reaction.
• British and German governments begin to
take increasing interest (unlike in the U.S.)
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• Szilard wires Lewis Strauss: “ESTIMATE
CHANCES FOR REACTION NOW
ABOVE 50”
• Szilard: “That night, there was very little
doubt in my mind that the world was
headed for grief.”
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Uranium Overview
• There are two main forms of uranium
–U-238
• Much more common; can both capture neutrons and
be fissioned by them
–U-235
• Represents only 0.7% of uranium
• Weighs slightly less than U-238
• Fissions much more easily than U-238; doesn’t
capture neutrons.
• Much more unstable.
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• To make a nuclear reactor, U-238 will work
fine.
• But, to make a bomb, you need pure U-235.
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• The problem is how to separate the U-235
from the U-238.
– This process is called “enrichment”
• Scientists still skeptical about how to get
enough U-235 (several hundred pounds are
thought necessary as of 1939):
– As Niels Bohr said: “It can never be done
unless you turn the United States into one huge
factory.”
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Einstein’s Letter to FDR
• Leo Szilard is worried; meets summer of 1939
with:
• Edward Teller (Hungarian, refugee, and later “father” of
the U.S. hydrogen bomb)
• Eugene Wigner (Hungarian, 1902-1995; Nobel 1963)
• The three scientists visit Einstein on Long Island
and draft letter to Franklin D. Roosevelt
– Einstein signs it.
• Irony:
• Einstein himself played no role in the Manhattan Project
and was denied security clearance as a German supporter
of left-wing causes.
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Reconstruction of the letter
signing
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Analyzing The Letter
• Historical significance
• Contains key themes that would define the
Manhattan Project (and the Cold War arms
race).
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“Einstein’s” Letter
• What does it say?
• “it may be possible to set up a nuclear chain
reaction in a large mass of uranium, by
which vast amounts of power and large
quantities of new radium-like elements
would be generated.”
• “…you may think it desirable to have some
permanent contact maintained between the
Administration and the group of physicists
working on chain reactions in America.”
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• “a) to approach Government Departments, keep
them informed of the further development, and put
forward recommendations for Government action,
giving particular attention to the problem of
securing a supply of uranium ore for the United
States. b) to speed up the experimental work, which
is at present being carried on within the limits of the
budgets of University laboratories, by providing
funds…”
• “I understand that Germany has actually stopped the
sale of uranium from the Czechoslovakian mines
which she has taken over.”
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Initial U.S. Interest in Atomic Power
• Enrico Fermi goes to Washington, DC
(March 1939)
• Military’s interest is not a bomb but nuclear
power…little support given.
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• Later in 1939 – after Einstein’s letter – FDR
creates the Uranium Advisory Committee
(off-shoot of the National Bureau of
Standards)
• October 1939: Wigner, Szilard, Teller meet
with the military.
– Result? Finally, money continue fission
research and commit the U.S. government to it.
How much?
• $6,000 !!?
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Meanwhile, in England…
• Otto Frisch and Rudolf Peierls are studying
the idea of critical mass
• The minimum mass of a fissionable material that
will just maintain a fission chain reaction under
precisely specified conditions.
• Instead of several tons, they calculate a few pounds
of U235 is needed.
• Spring of 1940: Frisch and Peierls write a
memo to English government…
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Scientific Understanding Continues to Grow, II
• Later (summer 1940) scientists
at UC-Berkeley discover that
uranium can be bombarded
with neutrons to make
plutonium (a new element).
• Glenn Seaborg (American
chemist, 1912-1999; shared
Nobel in chemistry in 1951).
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The Possibility of Plutonium
• What happens to U-238 when it captures a
neutron?
– U-238 + neutron -> U-239 -> Np-239 -> Pu-239
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• Scientists predict that Pu-239 has the same
fission characteristics as U-235 and can be
used in a bomb.
• Also, scientists expect Pu-239 can be
produced in a uranium-fueled nuclear
reactor.
– The problem is how to separate the plutonium
from the uranium. This can be done chemically
– easier yet expensive and messy.
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British Investigations
• After Frisch/Peierls memo, British form a
small group of scientists in 1940– MAUD
committee (Thomson, Chadwick,
Cockcroft…)
• “We entered the project with more
skepticism than belief…” they said later.
• After further investigation, the skepticism
vanished.
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• Committee devotes attention to how to
separate U-235
– One possibility: gaseous diffusion
separation…what’s this?
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• By spring of 1941, Chadwick recalled:
– “I realized that a nuclear bomb was not only
possible – it was inevitable…And I had then to
start taking sleeping pills.”
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The MAUD Report - July 1941
• What does it say?
– Bomb is possible
– Pursue w/o delay
– Cost to make U-235 to be £5,000,000
– Possible to have ready by 1943
– Germans are working on this too.
• Report outlines the basic idea of how a
bomb would work with about 22 pounds of
U-235
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• Conclusions/Recommendations:
– Bomb possible
– Work needed on increased scale at highest
priority
– Cooperate with the US.
• **MAUD report and Frisch/Peierls memo is
what spurs U.S. to action.
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Meanwhile in Nazi Germany…
• German bomb research proceeding
– Unaware of potential possibilities of plutonium
– Using heavy water (instead of graphite as in US) as a
moderator
• Moderator – slows down neutrons so fission can take place.
Graphite better and cheaper than heavy water.
• September 1941 – Werner Heisenberg’s infamous
meeting in Copenhagen with Niels Bohr.
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Back in the U.S…
• Vannevar Bush and the National Defense Research
Council (NRDC) take over US uranium work. James
Conant (chemist and pres. of Harvard put in charge)
Arthur Compton
Bush
Karl Compton
Lawrence
Conant
A. Loomis
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• Money begins to flow
– July 1940 - $40,000 more put into work
• Efforts directed not for bomb but for Enrico
Fermi’s studies of uranium reactor.
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• By fall of 1941, Compton, Conant,
Lawrence, Bush – encouraged by MAUD
report – urge FDR to undertake large bomb
project.
• FDR separates scientists from policy
decisions.
• FDR (informally) authorizes $1.2 million
and urges crash program.
• December 6, 1941: FDR authorizes the
Manhattan Engineering District.
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And then…
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