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The Los Alamos Laboratory, also known as Project Y, was a secret laboratory established by the
Manhattan Project The Manhattan Project was a research and development Research is " creative and systematic work undertaken to increase the stock of knowledge". It involves the collection, organization, and analysis of information to increase understa ...
and operated by the
University of California The University of California (UC) is a public university, public Land-grant university, land-grant research university, research university system in the U.S. state of California. The system is composed of the campuses at University of Californ ...
during
World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a global war A world war is "a war War is an intense armed conflict between states State may refer to: Arts, entertainment, and media Literatur ...
. Its mission was to design and build the first atomic bombs.
Robert Oppenheimer J. Robert Oppenheimer (; April 22, 1904 – February 18, 1967) was an American theoretical physicist and professor of physics at the University of California, Berkeley. Oppenheimer was the wartime head of the Los Alamos Laboratory and is among ...
was its first director, serving from 1943 to December 1945, when he was succeeded by
Norris Bradbury Norris Edwin Bradbury (30 May 1909 – 20 August 1997), was an American physicist A physicist is a scientist A scientist is a person who conducts Scientific method, scientific research to advance knowledge in an Branches of science, ar ...
. In order to enable scientists to freely discuss their work while preserving security, the laboratory was located in a remote part of
New Mexico ) , population_demonym = New Mexican ( es, Neomexicano, Neomejicano, Nuevo Mexicano) , seat = Santa Fe , LargestCity = Albuquerque , LargestMetro = Greater Albuquerque , OfficialLang = None , Languages = English English usually refer ...

New Mexico
. The wartime laboratory occupied buildings that had once been part of the
Los Alamos Ranch School Los Alamos Ranch School was a private ranch schoolA ranch school is a type of school A school is an educational institution designed to provide learning spaces and learning environments for the teaching of students (or "pupils") under the d ...
. The development effort initially concentrated on a
gun-type fission weapon Gun-type fission weapons are fission-based nuclear weapons A nuclear weapon (also called an atom bomb, nuke, atomic bomb, nuclear warhead, A-bomb, or nuclear bomb) is an explosive device that derives its destructive force from nuclear reactio ...

gun-type fission weapon
using
plutonium Plutonium is a radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material co ...

plutonium
called Thin Man. In April 1944, the Los Alamos Laboratory determined that the rate of
spontaneous fission Spontaneous fission (SF) is a form of radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy b ...
in plutonium bred in a nuclear reactor was too great due to the presence of
plutonium-240 Plutonium-240 (, Pu-240) is an isotope Isotopes are variants of a particular chemical element which differ in neutron number, and consequently in nucleon number. All isotopes of a given element have the same number of protons but different numb ...

plutonium-240
and would cause a predetonation, a
nuclear chain reaction In nuclear physics Nuclear physics is the field of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the ord ...
before the
core Core or cores may refer to: Science and technology * Core (anatomy) In common parlance, the core of the body is broadly considered to be the torso. Functional movements are highly dependent on this part of the body, and lack of core muscular dev ...
was fully assembled. Oppenheimer then reorganized the laboratory and orchestrated an all-out and ultimately successful effort on an alternative design proposed by
John von Neumann John von Neumann (; hu, Neumann János Lajos, ; December 28, 1903 – February 8, 1957) was a Hungarian-American Hungarian Americans (Hungarian language, Hungarian: ''amerikai magyarok'') are United States, Americans of Hungarian p ...

John von Neumann
, an
implosion-type nuclear weapon Nuclear weapon designs are physical, chemical, and engineering arrangements that cause the physics package of a nuclear weapon to detonate. There are three existing basic design types: * pure fission weapons, the simplest and least technically ...
, which was called
Fat Man "Fat Man" (also known as Mark III) is the codename for the type of nuclear bomb A nuclear weapon (also called an atom bomb, nuke, atomic bomb, nuclear warhead, A-bomb, or nuclear bomb) is an explosive device that derives its destructive f ...

Fat Man
. A variant of the gun-type design known as
Little Boy "Little Boy" was the codename for the type of atomic bomb A nuclear weapon (also known as an atom bomb, atomic bomb, nuclear bomb or nuclear warhead, and colloquially as an A-bomb or nuke) is an explosive device that derives its destructi ...

Little Boy
was developed using
uranium-235 Uranium-235 (235U) is an Isotopes of uranium, isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a nuclear chain reaction. It is the only fissile isotope t ...

uranium-235
. Chemists at the Los Alamos Laboratory developed methods of purifying uranium and plutonium, the latter a metal that only existed in microscopic quantities when Project Y began. Its metallurgists found that plutonium had unexpected properties, but were nonetheless able to cast it into metal spheres. The laboratory built the Water Boiler, an
aqueous homogeneous reactor Aqueous homogeneous reactors (AHR) are a type of nuclear reactor in which soluble nuclear fuel, nuclear salts (usually uranium sulfate or uranium nitrate) are dissolution (chemistry), dissolved in water. The fuel is mixed with the coolant and the ne ...
that was the third reactor in the world to become operational. It also researched the Super, a
hydrogen bomb lenses2) Uranium-238 ("tamper") lined with beryllium reflector3) Vacuum ("levitated core")4) Tritium "boost" gas (blue) within plutonium or uranium hollow core 5) Radiation channel filled with polystyrene foam6) Uranium ("pusher/tamper")7) Lithium ...

hydrogen bomb
that would use a fission bomb to ignite a
nuclear fusion Nuclear fusion is a nuclear reaction, reaction in which two or more atomic nuclei are combined to form one or more different atomic nuclei and subatomic particles (neutrons or protons). The difference in mass between the reactants and products ...

nuclear fusion
reaction in
deuterium Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two stable isotopes The term stable isotope has a meaning similar to stable nuclide, but is preferably used when speaking of nuclides of a specific elemen ...

deuterium
and
tritium Tritium ( or , ) or hydrogen-3 (symbol T or H) is a rare and radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration or nuclear disintegration) is the process by which an unstable atomic nucl ...

tritium
. The Fat Man design was tested in the
Trinity nuclear test The Christian doctrine of the Trinity (, from "threefold") holds that God is one God, but three coeternal and consubstantial persons A person (plural people or persons) is a being that has certain capacities or attributes such as rea ...
in July 1945. Project Y personnel formed pit crews and assembly teams for the
atomic bombings of Hiroshima and Nagasaki The United States detonated two nuclear weapons A nuclear weapon (also known as an atom bomb, atomic bomb, nuclear bomb or nuclear warhead, and colloquially as an A-bomb or nuke) is an explosive device that derives its destructive fo ...
and participated in the bombing as weaponeers and observers. After the war ended, the laboratory supported the
Operation Crossroads Operation Crossroads was a pair of nuclear weapon A nuclear weapon (also known as an atom bomb, atomic bomb, nuclear bomb or nuclear warhead, and colloquially as an A-bomb or nuke) is an explosive device that derives its destructive f ...
nuclear tests at
Bikini Atoll
Bikini Atoll
. A new Z Division was created to control testing, stockpiling and bomb assembly activities, which were concentrated at
Sandia Base Sandia Base was the principal nuclear weapons A nuclear weapon (also called an atom bomb, nuke, atomic bomb, nuclear warhead, A-bomb, or nuclear bomb) is an explosive device that derives its destructive force from nuclear reactions, either nucl ...
. The Los Alamos Laboratory became
Los Alamos Scientific Laboratory Los Alamos National Laboratory (Los Alamos or LANL for short) is a United States Department of Energy National Labs, United States Department of Energy national laboratory initially organized during World War II for the design of nuclear weapons ...
in 1947.


Origins


Nuclear fission and atomic bombs

The discovery of the
neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behav ...

neutron
by
James Chadwick Sir James Chadwick, (20 October 1891 – 24 July 1974) was a British physicist A physicist is a scientist A scientist is a person who conducts scientific research The scientific method is an Empirical evidence, empirical m ...

James Chadwick
in 1932, followed by the
discovery of nuclear fission Nuclear fission In nuclear physics Nuclear physics is the field of physics that studies atomic nuclei and their constituents and interactions. Other forms of nuclear matter are also studied. Nuclear physics should not be confused with atom ...
by chemists
Otto Hahn Otto Hahn (; 8 March 1879 – 28 July 1968) was a German chemist A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval Latin Medieval Latin was the form of Latin Latin (, or , ) is a classical language ...

Otto Hahn
and
Fritz Strassmann Friedrich Wilhelm "Fritz" Strassmann (''german: Fritz Straßmann'', ; 22 February 1902 – 22 April 1980) was a German chemist who, with Otto Hahn in early 1939, identified the element barium Barium is a chemical element with the Symbol (chemis ...
in 1938, and its explanation (and naming) by physicists
Lise Meitner Elise Meitner ( , ; 7 November 1878 – 27 October 1968) was a leading Austrian-Swedish physicist A physicist is a scientist A scientist is a person who conducts scientific research The scientific method is an Empirical evidence, ...

Lise Meitner
and
Otto Frisch Otto Robert Frisch FRS FRS may also refer to: Government and politics * Facility Registry System, a centrally managed Environmental Protection Agency database that identifies places of environmental interest in the United States * Family Resou ...
soon after, opened up the possibility of a controlled
nuclear chain reaction In nuclear physics Nuclear physics is the field of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the ord ...
using
uranium Uranium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

uranium
. At the time, few scientists in the United States thought that an
atomic bomb A nuclear weapon (also known as an atom bomb, atomic bomb, nuclear bomb or nuclear warhead, and colloquially as an A-bomb or nuke) is an explosive device that derives its destructive force from nuclear reactions, either nuclear fission, fiss ...

atomic bomb
was practical, but the possibility that a German atomic bomb project would develop atomic weapons concerned refugee scientists from
Nazi Germany Nazi Germany, (lit. "National Socialist State"), ' (lit. "Nazi State") for short; also ' (lit. "National Socialist Germany") officially known as the German Reich from 1933 until 1943, and the Greater German Reich from 1943 to 1945, was ...

Nazi Germany
and other fascist countries, leading to the drafting of the Einstein–Szilard letter to warn President
Franklin D. Roosevelt Franklin Delano Roosevelt (, ; January 30, 1882April 12, 1945), often referred to by his initials FDR, was an American politician who served as the 32nd president of the United States from 1933 until his death in 1945. A member of the De ...

Franklin D. Roosevelt
. This prompted preliminary research in the United States, beginning in late 1939. Progress was slow in the United States, but in Britain, Otto Frisch and
Rudolf Peierls Sir Rudolf Ernst Peierls, (; ; 5 June 1907 – 19 September 1995) was a German-born British physicist who played a major role in both Tube Alloys, Britain's nuclear weapon programme, as well as the subsequent Manhattan Project The Manh ...
, two refugee physicists from Germany at the
University of Birmingham The University of Birmingham (informally Birmingham University) is a public university, public research university located in Edgbaston, Birmingham, United Kingdom. It received its royal charter in 1900 as a successor to Queen's College, Birmingh ...

University of Birmingham
, examined the theoretical issues involved in developing, producing and using atomic bombs. They considered what would happen to a sphere of pure uranium-235, and found that not only could a
chain reaction A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. In a chain reaction, positive feedback Positive feedback (exacerbating feedback, self-reinforcing feedback) is a pro ...
occur, but it might require as little as of
uranium-235 Uranium-235 (235U) is an Isotopes of uranium, isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a nuclear chain reaction. It is the only fissile isotope t ...

uranium-235
to unleash the energy of hundreds of tons of
TNT Trinitrotoluene (; TNT), or more specifically 2,4,6-trinitrotoluene, is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more ...

TNT
. Their superior,
Mark Oliphant Sir Marcus Laurence Elwin "Mark" Oliphant, (8 October 1901 – 14 July 2000) was an Australian physicist and Humanitarianism, humanitarian who played an important role in the first experimental demonstration of nuclear fusion and in the History ...
, took the Frisch–Peierls memorandum to Sir
Henry Tizard Sir Henry Thomas Tizard (23 August 1885 – 9 October 1959) was an English chemist A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval Latin ''alchemist'') is a scientist A scientist is a person who conducts S ...
, the chairman of the Committee for the Scientific Survey of Air Warfare (CSSAW), who in turn passed it on to
George Paget Thomson Sir George Paget Thomson, FRS (; 3 May 189210 September 1975) was a British physicist and Nobel laureate in physics recognized for his discovery of the wave properties of the electron by electron diffraction. Education and early life Thomson ...

George Paget Thomson
, to whom the CSSAW had delegated responsibility for uranium research. CSSAW created the
MAUD Committee The MAUD Committee was a British scientific working group formed during the Second World War World War II or the Second World War, often abbreviated as WWII or WW2, was a global war that lasted from 1939 to 1945. It involved the vast ...
to investigate. In its final report in July 1941, the MAUD Committee concluded that an atomic bomb was not only feasible, but might be produced as early as 1943. In response, the British government created a nuclear weapons project known as
Tube Alloys Tube Alloys was the research and development Research and development (R&D, R+D), known in Europe Europe is a continent A continent is any of several large landmasses. Generally identified by convention (norm), convention ra ...
. There was still little urgency in the United States, which unlike Britain was not yet engaged in
World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a global war A world war is "a war War is an intense armed conflict between states State may refer to: Arts, entertainment, and media Literatur ...
, so Oliphant flew there in late August 1941, and spoke to American scientists including his friend
Ernest Lawrence Ernest Orlando Lawrence (August 8, 1901 – August 27, 1958) was a pioneering American nuclear scientist and winner of the Nobel Prize in Physics in 1939 for his invention of the cyclotron. He is known for his work on uranium-isotope sepa ...

Ernest Lawrence
at the
University of California The University of California (UC) is a public university, public Land-grant university, land-grant research university, research university system in the U.S. state of California. The system is composed of the campuses at University of Califor ...

University of California
. He not only managed to convince them that an atomic bomb was feasible, but inspired Lawrence to convert his cyclotron into a giant
mass spectrometer Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is used i ...
for
isotope separation Isotope separation is the process of concentrating specific isotope Isotopes are two or more types of atoms that have the same atomic number 300px, The Rutherford–Bohr model of the hydrogen atom () or a hydrogen-like ion (). In this mod ...
, a technique Oliphant had pioneered in 1934. In turn, Lawrence brought in his friend and colleague
Robert Oppenheimer J. Robert Oppenheimer (; April 22, 1904 – February 18, 1967) was an American theoretical physicist and professor of physics at the University of California, Berkeley. Oppenheimer was the wartime head of the Los Alamos Laboratory and is among ...
to double-check the physics of the MAUD Committee report, which was discussed at a meeting at the
General Electric Research Laboratory General Electric Research Laboratory was the first industrial research facility in the United States. Established in 1900, the lab was home to the early technological breakthroughs of General Electric General Electric Company (GE) is an Americ ...
in
Schenectady, New York Schenectady () is a city A city is a large .Goodall, B. (1987) ''The Penguin Dictionary of Human Geography''. London: Penguin.Kuper, A. and Kuper, J., eds (1996) ''The Social Science Encyclopedia''. 2nd edition. London: Routledge. It can be de ...
, on 21 October 1941. In December 1941, the
S-1 Section The Uranium Committee was a committee of the National Defense Research Committee The National Defense Research Committee (NDRC) was an organization created "to coordinate, supervise, and conduct scientific research on the problems underlying the ...
of the
Office of Scientific Research and Development The Office of Scientific Research and Development (OSRD) was an agency of the United States The United States of America (USA), commonly known as the United States (U.S. or US), or America, is a country Contiguous United States, primarily l ...
(OSRD) placed Arthur H. Compton in charge of the design of the bomb. He delegated the task of bomb design and research into
fast neutron calculations The neutron detection temperature, also called the neutron energy, indicates a free neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a ...
—the key to calculations of critical mass and weapon detonation—to
Gregory Breit Gregory Breit (russian: Григорий Альфредович Брейт-Шнайдер, ''Grigory Alfredovich Breit-Shneider''; July 14, 1899, Mykolaiv, Kherson Governorate The Kherson Governorate (1802–1922) (russian: Херсонская ...

Gregory Breit
, who was given the title of "Co-ordinator of Rapid Rupture", and Oppenheimer as an assistant. But Breit disagreed with other scientists working at the
Metallurgical Laboratory The Metallurgical Laboratory (or Met Lab) was a scientific laboratory at the University of Chicago The University of Chicago (UChicago, U of C, or Chicago) is a private research university in Chicago (''City in a Garden''); I Will ...
, particularly
Enrico Fermi Enrico Fermi (; 29 September 1901 - 28 November 1954) was an Italian (later naturalized American) physicist and the creator of the world's first nuclear reactor, the Chicago Pile-1. He has been called the "architect of the nuclear age" and ...

Enrico Fermi
, over the security arrangements, and resigned on 18 May 1942. Compton then appointed Oppenheimer to replace him. John H. Manley, a physicist at the Metallurgical Laboratory, was assigned to assist Oppenheimer by contacting and coordinating experimental physics groups scattered across the country. Oppenheimer and
Robert Serber Robert Serber (March 14, 1909 – June 1, 1997) was an American physicist who participated in the Manhattan Project. Serber's lectures explaining the basic principles and goals of the project were printed and supplied to all incoming scientific sta ...
of the
University of Illinois The University of Illinois Urbana-Champaign (U of I, Illinois, or colloquially the University of Illinois or UIUC) is a public university, public land-grant university, land-grant research university in Illinois in the twin cities of Champaign ...

University of Illinois
examined the problems of neutron diffusion—how neutrons moved in a nuclear chain reaction—and
hydrodynamics In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. ...
—how the explosion produced by a chain reaction might behave.


Bomb design concepts

To review this work and the general theory of fission reactions, Oppenheimer and Fermi convened meetings at the
University of Chicago The University of Chicago (UChicago) is a private Private or privates may refer to: Music * "In Private "In Private" was the third single in a row to be a charting success for United Kingdom, British singer Dusty Springfield, after an abse ...
in June and at the University of California in Berkeley, in July with theoretical physicists
Hans Bethe Hans Albrecht Bethe (; July 2, 1906 – March 6, 2005) was a German-American German Americans (german: Deutschamerikaner, ) are Americans who have full or partial Germans, German ancestry. With an estimated size of approximately 43 million ...

Hans Bethe
, John Van Vleck,
Edward Teller Edward Teller ( hu, Teller Ede; January 15, 1908 – September 9, 2003) was a Hungarian-American Hungarian Americans (Hungarian language, Hungarian: ''amerikai magyarok'') are United States, Americans of Hungarian people, Hungarian ...
,
Emil Konopinski Emil John (Jan) Konopinski (December 25, 1911 in Michigan City, Indiana – May 26, 1990 in Bloomington, Indiana Indiana () is a U.S. state in the Midwestern United States The United States of America (USA), commonly known as the ...
, Robert Serber,
Stan Frankel Stanley Phillips Frankel (1919 – May, 1978) was an American computer scientist. He worked in the Manhattan Project The Manhattan Project was a research and development undertaking during World War II World War II or the Second ...
, and Eldred C. Nelson, the latter three former students of Oppenheimer, and
experimental physicistExperimental physics is the category of disciplines and sub-disciplines in the field of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natura ...
s
Emilio Segrè Emilio Gino Segrè (1 February 1905 – 22 April 1989) was an Italian-American physicist A physicist is a scientist A scientist is a person who conducts scientific research The scientific method is an Empirical evidence, empirica ...
,
Felix Bloch Felix Bloch (23 October 1905 – 10 September 1983) was a Swiss Swiss may refer to: * the adjectival form of Switzerland *Swiss people Places *Swiss, Missouri *Swiss, North Carolina *Swiss, West Virginia *Swiss, Wisconsin Other uses *Swiss-sy ...
,
Franco Rasetti Franco Dino Rasetti (August 10, 1901 – December 5, 2001) was an Italian-born American physicist, paleontologist and botanist. Together with Enrico Fermi Enrico Fermi (; 29 September 1901 - 28 November 1954) was an Italian (later natura ...
, John Manley, and
Edwin McMillan Edwin Mattison McMillan (September 18, 1907 – September 7, 1991) was an American physicist and Nobel laureate credited with being the first-ever to produce a transuranium element, neptunium. For this, he shared the Nobel Prize in Chemistry with ...
. They tentatively confirmed that a fission bomb was theoretically possible. There were still many unknown factors. The properties of pure uranium-235 were relatively unknown; even more so those of
plutonium Plutonium is a radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material co ...

plutonium
, a
chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo du ...
that had only recently been discovered by
Glenn Seaborg Glenn Theodore Seaborg (; April 19, 1912February 25, 1999) was an American chemist whose involvement in the synthesis, discovery and investigation of ten transuranium elements earned him a share of the 1951 Nobel Prize in Chemistry. His work i ...
and his team in February 1941, but which was theoretically
fissile In nuclear engineering Nuclear engineering is the branch of engineering Engineering is the use of scientific method, scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, veh ...
. The scientists at the Berkeley conference envisioned breeding plutonium in
nuclear reactor A nuclear reactor, formerly known as an atomic pile, is a device used to initiate and control a fission nuclear chain reaction 300px, A possible nuclear fission chain reaction: 1) A uranium-235 atom absorbs a neutron">uranium-235.html" ;"ti ...

nuclear reactor
s from
uranium-238 Uranium-238 (238U or U-238) is the most common isotope Isotopes are two or more types of atoms that have the same atomic number 300px, The Rutherford–Bohr model of the hydrogen atom () or a hydrogen-like ion (). In this model it is an ...
atoms that absorbed neutrons from fissioning uranium-235 atoms. At this point no reactor had been built, and only microscopic quantities of plutonium were available that had been produced by
cyclotron A cyclotron is a type of particle accelerator , a synchrotron collider type particle accelerator at Fermi National Accelerator Laboratory (Fermilab), Batavia, Illinois, USA. Shut down in 2011, until 2007 it was the most powerful particle ac ...

cyclotron
s. There were many ways of arranging the fissile material into a critical mass. The simplest was shooting a "cylindrical plug" into a sphere of "active material" with a "
tamper Tamper may refer to: *Tamper, to use a tamp A tamp is a device used to compact or flatten an Aggregate (composite), aggregate or another powdered or granular material, typically to make it resistant to further compression or simply to increase it ...
"—dense material that would focus neutrons inward and keep the reacting mass together to increase its efficiency. They also explored designs involving
spheroid A spheroid, also known as an ellipsoid of revolution or rotational ellipsoid, is a quadric In mathematics, a quadric or quadric surface (quadric hypersurface in higher dimensions), is a generalization of conic sections (ellipse In math ...

spheroid
s, a primitive form of "implosion" suggested by
Richard C. Tolman
Richard C. Tolman
, and the possibility of autocatalytic methods, which would increase the efficiency of the bomb as it exploded. Considering the idea of the fission bomb theoretically settled—at least until more experimental data was available—the Berkeley conference then turned in a different direction. Edward Teller pushed for discussion of a more powerful bomb: the "Super", usually referred to today as a "
hydrogen bomb lenses2) Uranium-238 ("tamper") lined with beryllium reflector3) Vacuum ("levitated core")4) Tritium "boost" gas (blue) within plutonium or uranium hollow core 5) Radiation channel filled with polystyrene foam6) Uranium ("pusher/tamper")7) Lithium ...

hydrogen bomb
", which would use the explosive force of a detonating fission bomb to ignite a
nuclear fusion Nuclear fusion is a nuclear reaction, reaction in which two or more atomic nuclei are combined to form one or more different atomic nuclei and subatomic particles (neutrons or protons). The difference in mass between the reactants and products ...

nuclear fusion
reaction between
deuterium Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two stable isotopes The term stable isotope has a meaning similar to stable nuclide, but is preferably used when speaking of nuclides of a specific elemen ...

deuterium
and
tritium Tritium ( or , ) or hydrogen-3 (symbol T or H) is a rare and radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration or nuclear disintegration) is the process by which an unstable atomic nucl ...

tritium
. Teller proposed scheme after scheme, but Bethe rejected each one. The fusion idea was set aside to concentrate on producing fission bombs. Teller also raised the speculative possibility that an atomic bomb might "ignite" the atmosphere because of a hypothetical fusion reaction of nitrogen nuclei, but Bethe calculated that this could not happen, and a report co-authored with Teller showed that "no self-propagating chain of nuclear reactions is likely to be started".


Bomb laboratory concept

Oppenheimer's deft handling of the July conference impressed his colleagues; his insight and ability to handle even the most difficult people came as a surprise even to those who knew him well. In the wake of the conference, Oppenheimer saw that while they had come to grips with the physics, considerable work was still required on the engineering, chemistry, metallurgy and ordnance aspects of building an atomic bomb. He became convinced that bomb design would require an environment where people could freely discuss problems and thereby reduce wasteful duplication of effort. He reasoned that this could best be reconciled with security by creating a central laboratory in an isolated location.
Brigadier General #REDIRECT Brigadier general #REDIRECT Brigadier general Brigadier general (Brig. Gen.) or brigade general is a military rank used in many countries. It is the lowest ranking general officer in some countries, usually sitting between the ranks of co ...
Leslie R. Groves Jr. became director of the
Manhattan Project The Manhattan Project was a research and development Research is " creative and systematic work undertaken to increase the stock of knowledge". It involves the collection, organization, and analysis of information to increase understa ...
on 23 September 1942. He visited Berkeley to look at Lawrence's
calutron A calutron is a mass spectrometer Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-ch ...
s, and met with Oppenheimer, who gave him a report on bomb design on 8 October. Groves was interested in Oppenheimer's proposal to establish a separate bomb design laboratory. When they met again in Chicago a week later, he invited Oppenheimer to discuss the issue. Groves had to catch a train to New York, so he asked Oppenheimer to accompany him so that they could continue the discussion. Groves, Oppenheimer, and
Colonel Colonel (; abbreviated as Col., Col or COL) is a senior military officer An officer is a person who has a position of authority In the fields of sociology Sociology is the study of society, human social behaviour, patterns of social rel ...
James C. Marshall
James C. Marshall
and
Lieutenant Colonel Lieutenant colonel ( or ) is a rank of commissioned officer An officer is a person who holds a position of authority as a member of an armed force A military, also known collectively as armed forces, is a heavily armed, highly orga ...
Kenneth Nichols Major General Major general (abbreviated MG, maj. gen. and similar) is a military rank Military ranks are a system of hierarchical relationships in armed forces A military, also known collectively as armed forces, is a heavily ar ...

Kenneth Nichols
all squeezed into a single compartment where they talked about how a bomb laboratory could be created, and how it would function. Groves subsequently had Oppenheimer come to
Washington, D.C. ) , image_skyline = , image_caption = Clockwise from top left: the Washington Monument The Washington Monument is an obelisk within the National Mall The National Mall is a Landscape architecture, landscaped ...
, where the matter was discussed with
Vannevar Bush Vannevar Bush ( ; March 11, 1890 – June 28, 1974) was an American engineer, inventor and science administrator, who during World War II, World War II headed the U.S. Office of Scientific Research and Development (OSRD), through which almo ...

Vannevar Bush
, the director of the OSRD, and
James B. Conant James Bryant Conant (March 26, 1893 – February 11, 1978) was an American chemist A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval Latin ''alchemist'') is a scientist A scientist is a person who conduc ...
, the chairman of the
National Defense Research Committee The National Defense Research Committee (NDRC) was an organization created "to coordinate, supervise, and conduct scientific research on the problems underlying the development, production, and use of mechanisms and devices of warfare" in the Uni ...
(NDRC). On 19 October, Groves approved the establishment of a bomb laboratory. While Oppenheimer seemed the logical person to direct the new laboratory, which became known as Project Y, he had little administrative experience; Bush, Conant, Lawrence and
Harold Urey Harold Clayton Urey ( ; April 29, 1893 – January 5, 1981) was an American physical chemist Physical chemistry is the study of macroscopic The macroscopic scale is the length scale on which objects or phenomena are large enough to be visi ...

Harold Urey
all expressed reservations about this. Moreover, unlike his other project leaders—Lawrence at the
Berkeley Radiation Laboratory Lawrence Berkeley National Laboratory (LBNL), commonly referred to as Berkeley Lab, is a United States Department of Energy National Labs, United States national laboratory that conducts scientific research on behalf of the United States Depart ...
, Compton at the Metallurgical Project in Chicago, and Urey at the
SAM Laboratories K-25 was the codename given by the Manhattan Project The Manhattan Project was a research and development undertaking during World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a World w ...
in New York—Oppenheimer did not have a
Nobel Prize The Nobel Prizes ( ; sv, Nobelpriset ; no, Nobelprisen ) are five separate prizes that, according to Alfred Nobel Alfred Bernhard Nobel ( , ; 21 October 1833 – 10 December 1896) was a Swedish chemist, engineer, inventor, busines ...
, raising concerns that he might not have the prestige to deal with distinguished scientists. There were also security concerns; many of Oppenheimer's closest associates were active members of the
Communist Party A communist party is a political party A political party is an organization that coordinates candidates to compete in a particular country's elections. It is common for the members of a party to hold similar ideas about politics, and part ...
, including his wife
Kitty Kitty or Kittie may refer to: Animals * Cat, a small, domesticated carnivorous mammal ** Kitten, a young cat Film * Kitty Films, an anime production company in Japan * Kitty (1929 film), ''Kitty'' (1929 film), based on the Deeping novel; the fir ...
, girlfriend Jean Tatlock, brother
Frank Frank may refer to: People As a name * Frank (given name) * Frank (surname) Groups of people * A member of the medieval Germanic people, the Franks * Crusaders in medieval Middle Eastern history * Levantines (Latin Christians) known as Franco ...

Frank
, and Frank's wife Jackie. In the end, Groves personally issued instructions to clear Oppenheimer on 20 July 1943.


Site selection

The idea of locating Project Y at the Metallurgical Laboratory in Chicago, or the
Clinton Engineer Works
Clinton Engineer Works
in
Oak Ridge, Tennessee Oak Ridge is a city in Anderson County, Tennessee, Anderson and Roane County, Tennessee, Roane counties in the East Tennessee, eastern part of the U.S. state of Tennessee, about west of downtown Knoxville, Tennessee, Knoxville. Oak Ridge's popula ...
, was considered, but in the end it was decided that a remote location would be best. A site in the vicinity of
Los Angeles Los Angeles ( ; xgf, Tovaangar; es, Los Ángeles, , ), commonly referred to by the initialism An acronym is a word In linguistics, a word of a spoken language can be defined as the smallest sequence of phonemes that can be u ...

Los Angeles
was rejected on security grounds, and one near
Reno, Nevada Reno ( ) is a city in the northwest section of the U.S. state of Nevada, along the Nevada-California border, about from Lake Tahoe, known as "The Biggest Little City in the World". Known for its casino and tourism industry, Reno is the county ...
as being too inaccessible. On Oppenheimer's recommendation, the search was narrowed to the vicinity of
Albuquerque, New Mexico Albuquerque ( , ), ; kee, Arawageeki; tow, Vakêêke; zun, Alo:ke:k'ya; apj, Gołgéeki'yé. abbreviated as ABQ, is the most populous city in the U.S. state of New Mexico ) , population_demonym = New Mexican ( es, Neomexicano, Neomejica ...
, where Oppenheimer owned a ranch in the
Sangre de Cristo Range , country= United States The United States of America (USA), commonly known as the United States (U.S. or US), or America, is a country Contiguous United States, primarily located in North America. It consists of 50 U.S. state, states, a ...
. The climate was mild, there were air and rail connections to Albuquerque, it was sufficiently distant from the
West Coast of the United States#REDIRECT West Coast of the United States The West Coast of the United States, also known as the Pacific Coast, Pacific states, and the western seaboard, is the coastline along which the Western United States The Western United States (also calle ...
for a Japanese attack not to be an issue, and the population density was low. In October 1942, Major
John H. Dudley
John H. Dudley
of the Manhattan District (the military component of the Manhattan Project) surveyed sites around Gallup, New Mexico, Gallup, Las Vegas, New Mexico, Las Vegas, La Ventana, Jemez Springs, New Mexico, Jemez Springs, and Otowi Historic District, Otowi, and recommended the one near Jemez Springs. On 16 November, Oppenheimer, Groves, Dudley and others toured the site. Oppenheimer feared that the high cliffs surrounding the site would make people feel claustrophobic, while the engineers were concerned with the possibility of flooding. The party then moved on to the Otowi site, the vicinity of the
Los Alamos Ranch School Los Alamos Ranch School was a private ranch schoolA ranch school is a type of school A school is an educational institution designed to provide learning spaces and learning environments for the teaching of students (or "pupils") under the d ...
. Oppenheimer was impressed by and expressed a strong preference for the site, citing its natural beauty and views of the Sangre de Cristo Mountains, which, he hoped, would inspire those who would work on the project. The engineers were concerned about the poor access road, and whether the water supply would be adequate, but otherwise felt that it was ideal. The United States Under Secretary of War, Robert P. Patterson, approved the acquisition of the site on 25 November 1942, authorizing $440,000 for the purchase of the site of , all but of which were already owned by the Federal Government. Secretary of Agriculture Claude R. Wickard granted use of some of United States Forest Service land to the United States Department of War, War Department "for so long as the military necessity continues". The need for land for a new road, and later for a right of way for a power line, eventually brought wartime land purchases to , but only $414,971 was ultimately spent. The big ticket items were the school, which cost $350,000, and the Anchor Ranch, which cost $25,000. Both hired lawyers to negotiate deals with the government, but Hispanic homesteaders were paid as little as $7 an acre (). Grazing permits were withdrawn, and private land was purchased or condemned under eminent domain using the authority of the War Powers Act of 1941, Second War Powers Act. Petitions of condemnation were worded to cover all mineral, water, timber and other rights, so private individuals would have no reason whatsoever to enter the area. The site acquired an irregular shape due to abutting the Bandelier National Monument and a Native American sacred burial ground.


Construction

An important consideration in the acquisition of the site was the existence of the Los Alamos Ranch School. This consisted of 54 buildings, of which 27 were houses, dormitories or other quarters providing of accommodation. The remaining buildings included a sawmill, ice house (building), ice house, barns, carpentry shop, stables and garage (residential), garages, all totalling . At the nearby Anchor Ranch there were four houses and a barn. Construction work was supervised by the United States Army Corps of Engineers#Divisions and districts, Albuquerque Engineer District until 15 March 1944, when the Manhattan Engineer District assumed responsibility. Willard C. Kruger and Associates of Santa Fe, New Mexico, was engaged as architect and engineer. Black & Veatch was brought in for the design of utilities in December 1945. The former was paid $743,706.68 and the latter $164,116 by the time the Manhattan Project ended at the end of 1946. The Albuquerque District supervised $9.3 million of construction at Los Alamos, and the Manhattan District, another $30.4 million. The initial work was contracted to the M. M. Sundt Company of Tucson, Arizona, with work commenced in December 1942. Groves initially allocated $300,000 for construction, three times Oppenheimer's estimate, with a planned completion date of 15 March 1943. It soon became clear that the scope of Project Y was far greater than expected, and by the time Sundt finished on 30 November 1943, over $7 million had been spent. The Zia Company took over responsibility for maintenance in April 1946. Oppenheimer initially estimated that the work could be performed by 50 scientists and 50 technicians. Groves tripled this number to 300. The actual population, including family members, was about 3,500 by the end of 1943, 5,700 by the end of 1944, 8,200 by the end of 1945, and 10,000 by the close of 1946. The most desirable accommodation were the six existing log and stone cottages that had once housed the headmaster and the Los Alamos Ranch School faculty. They were the only dwellings at Los Alamos that had bathtubs, and became known as "Bathtub Row". Oppenheimer lived on Bathtub Row; his next-door neighbor was Captain (United States O-6), Captain William Sterling Parsons, W. S. "Deak" Parsons, the head of the Ordnance and Engineering Division. Parsons' house was slightly larger, because Parsons had two children and Oppenheimer, at that point, had only one. After Bathtub Row, the next most desirable accommodation was the apartments built by Sundt. A typical two-storey building held four families. Each Sundt apartment had two or three bedrooms, a kitchen with a cranky black coal stove, and a small bathroom. J. E. Morgan and Sons supplied 56 prefabricated dwellings that became known as "Morganville". The Robert E. McKee Company built a part of the town known as "McKeeville". In June through October 1943, and again in June and July 1944, numbers outstripped the available accommodation and personnel were temporarily lodged in Frijoles Canyon. The houses at CEW and HEW were basic but of a higher standard (as specified by Kenneth Nichols, Nichols) than the houses at Los Alamos (as specified by Leslie Groves, Groves), but Nichols said to Los Alamos scientists that housing there was Groves' problem not his. Rents were set based on the income of the occupant. Transient visitors to Los Alamos were accommodated in the Fuller Lodge, the Guest Cottage or the Big House, which had once been part of the Los Alamos Ranch School. A school was established in 1943, catering for both grade school and high school, and 140 children were enrolled; 350 by 1946. Education was free, as was a nursery school for working mothers. With 18 grade-school teachers, 13 high-school teachers, and a superintendent, it enjoyed an excellent teacher:pupil ratio. Numerous technical buildings were constructed. Most were of a semi-permanent type, using gypsum board. They were heated from a central heating plant. Initially this was Boiler House No. 1, which had two coal-fired Boiler (power generation), boilers. This was replaced by Boiler House No. 2, which had six oil-fired boilers. In addition to the main site at Los Alamos, some 25 outlying sites were developed for experimental work. The growth of the town outpaced the sewage system, and by late 1945 there were electrical outages. Lights had to be shut off during the day, and between 7 and 10 pm. Water also ran short. During the autumn of 1945, consumption was per day, but the water supply could furnish only . On 19 December, pipes that had been laid above ground to save time in 1943 froze, cutting off the supply completely. Residents had to draw water from 15 tanker trucks that carried per day. Because its name was secret, Los Alamos was referred to as "Site Y"; to residents it was known as "The Hill". Because they lived on Federal land, the state of New Mexico did not allow residents of Los Alamos to vote in elections, although it did require them to pay state income taxes. A drawn-out series of legal and legislative battles lay ahead before the residents of Los Alamos became fully-fledged citizens of New Mexico on 10 June 1949. Birth certificates of babies born in Los Alamos during the war listed their place of birth as PO Box 1663 in Santa Fe. All letters and packages came through that address. Initially Los Alamos was to have been a military laboratory with Oppenheimer and other researchers commissioned into the Army. Oppenheimer went so far as to order himself a lieutenant colonel's uniform, but two key physicists, Robert Bacher and Isidor Rabi, balked at the idea. Conant, Groves and Oppenheimer then devised a compromise whereby the laboratory was operated by the University of California. Financial and procurement activities were the responsibility of the University of California under a 1 January 1943 letter of intent from the OSRD. This was superseded by a formal contract with the Manhattan District on 20 April 1943, which was backdated to 1 January. Financial operations were directed by the resident business officer, J. A. D. Muncy. The intent was that it would be militarized when the time came to finally assemble the bomb, but by this time the Los Alamos Laboratory had grown so large that this was considered both impractical and unnecessary, as the anticipated difficulties regarding civilians working on dangerous tasks had not occurred.


Organization


Military

Colonel John M. Harman was the first post commander at Los Alamos. He joined the Santa Fe office as a lieutenant colonel on 19 January 1943, and was promoted to colonel on 15 February. Los Alamos officially became a military establishment on 1 April 1943, and he moved to Los Alamos on 19 April. He was succeeded by Lieutenant Colonel C. Whitney Ashbridge, a graduate of the Los Alamos Ranch School, in May 1943. In turn, Ashbridge was succeeded by Lieutenant Colonel Gerald R. Tyler in October 1944, Colonel Lyle E. Seaman in November 1945, and Colonel Herb C. Gee in September 1946. The post commander was answerable directly to Groves, and was responsible for the township, government property and the military personnel. Four military units were assigned to the post. The MP Detachment, 4817th Service Command Unit, arrived from Fort Riley, Kansas, in April 1943. Its initial strength was 7 officers and 196 enlisted men; by December 1946 it had 9 officers and 486 men, and was manning 44 guard posts 24 hours a day. The Provisional Engineer Detachment (PED), 4817th Service Command Unit, was activated at Camp Claiborne, Louisiana, on 10 April 1943. These men performed jobs around the post such as working in the boiler plant, the motor pool and the mess halls. They also maintained the buildings and roads. It reached a peak strength of 465 men, and was disbanded on 1 July 1946. The 1st Provisional Women's Army Corps, Women's Army Auxiliary Corps (WAAC) Detachment was activated at Fort Sill, Oklahoma, on 17 April 1943. Its initial strength was just one officer and seven auxiliaries. The WAAC became the Women's Army Corps (WAC) on 24 August 1943, and the detachment became part of the 4817th Service Command Unit, with a strength of two officers and 43 enlisted women. They were sworn into the United States Army by Ashbridge. It reached a peak strength of about 260 women in August 1945. The WACs did a wider variety of jobs than the PED; some were cooks, drivers and telephone operators, while others served as librarians, clerks and hospital technicians. Some performed highly specialized scientific research inside the Technical Area. The Special Engineer Detachment (SED) was activated in October 1943 as part of the 9812th Technical Service Unit. It was made up of men with technical skills or advanced education, and was mostly drawn from the defunct Army Specialized Training Program. War Department policy forbade giving deferments from the draft to men under 22, so they were assigned to the SED. It reached a peak strength of 1,823 men in August 1945. SED personnel worked in all areas of the Los Alamos Laboratory.


Civilian

As director of the Los Alamos Laboratory, Oppenheimer was no longer answerable to Compton, but reported directly to Groves. He was responsible for the technical and scientific aspects of Project Y. He assembled the nucleus of his staff from the groups that had been working for him on neutron calculations. These included his secretary, Priscilla Greene, Serber and McMillan from his own group, and Emilio Segrè and Joseph W. Kennedy's groups from the University of California, John Harry Williams, J. H. Williams' group from the University of Minnesota, Joe McKibben's group from the University of Wisconsin, Felix Bloch's group from Stanford University and Marshall Holloway's from Purdue University. He also secured the services of Hans Bethe and Robert Bacher from the Radiation Laboratory at MIT, Edward Teller, Robert F. Christy, Darol K. Froman, Alvin C. Graves and John H. Manley and his group from the Manhattan Project's Metallurgical Laboratory, and Robert R. Wilson and his group, which included Richard Feynman, that had been performing Manhattan Project research at Princeton University. They brought with them a great deal of valuable scientific equipment. Wilson's group dismantled the cyclotron at Harvard University and had it shipped to Los Alamos; McKibben's brought two Van de Graaff generators from Wisconsin; and Manley's brought the Cockcroft–Walton accelerator from the University of Illinois. Communications with the outside world went through a single Forest Service line until April 1943, when it was replaced by five Army telephone lines. This was increased to eight in March 1945. There were also three teletypewriters with encoding machines. The first was installed in March 1943, and two more were added in May 1943. One was removed in November 1945. There were telephones in the offices, but none in private residences, as the Army regarded this as a security hazard. There were some public phones in the township for emergencies. Since there was no way to prevent the lines being tapped, classified information could not be discussed over the phone lines. Initially the phone lines were operable only during business hours until enough WACs arrived to man the switchboard around the clock. Women at Los Alamos were encouraged to work, due to the shortage of labor and security concerns over bringing in local workers. About 60 wives of scientists were at work in Technical Area by September 1943. About 200 of the 670 workers in the laboratory, hospital and school were women in October 1944. Most worked in administration, but many women such as Lilli Hornig, Jane Hamilton Hall, and Peggy Titterton worked as scientists and technicians. Charlotte Serber headed the A-5 (Library) Group. A large group of women worked on numerical calculations in the T-5 (Computations) Group. Dorothy McKibbin ran the Santa Fe office, which opened at 109 East Palace Avenue on 27 March 1943. The Los Alamos Laboratory had a governing board, the members of which were Oppenheimer, Bacher, Bethe, Kennedy, D. L. Hughes (Personnel Director), D. P. Mitchell (Procurement Director) and Deak Parsons. McMillan, George Kistiakowsky and Kenneth Bainbridge were later added. The laboratory was organized into five divisions: Administration (A), Theoretical (T) under Bethe, Experimental Physics (P) under Bacher, Chemistry and Metallurgy (CM) under Kennedy, and Ordnance and Engineering (E) under Parsons. All the divisions expanded during 1943 and 1944, but T Division, despite trebling in size, remained the smallest, while E Division grew to be the largest. Security clearance was a problem. Scientists (including, at first, Oppenheimer) had to be given access to the Technical Area without proper clearance. In the interest of efficiency, Groves approved an abbreviated process by which Oppenheimer vouched for senior scientists, and three other employees were sufficient to vouch for a junior scientist or technician. The Los Alamos Laboratory was reinforced by a British contribution to the Manhattan Project#Los Alamos Laboratory, British Mission under James Chadwick. The first to arrive were Otto Frisch and Ernest Titterton; later arrivals included Niels Bohr and his son Aage Bohr, and Sir G. I. Taylor, Geoffrey Taylor, an expert on hydrodynamics who made a major contribution to the understanding of the Rayleigh–Taylor instability. This instability at the Interface (chemistry), interface between two fluids of different density, densities occurs when the lighter fluid is pushing the heavier, and was vital to the interpretation of experiments with explosives, predicting the effects of an explosion, the design of the neutron initiators, and the design of the atomic bomb itself. Chadwick remained only for a few months; he was succeeded as head of the British Mission by Rudolf Peierls. The original idea, favored by Groves, was that the British scientists would work as a group under Chadwick, who would farm out work to them. This was soon discarded in favor of having the British Mission fully integrated into the laboratory. They worked in most of its divisions, only being excluded from plutonium chemistry and metallurgy. With the passage of the Atomic Energy Act of 1946, known as the McMahon Act, all British government employees had to leave. All had left by the end of 1946, except for Titterton, who was granted a special dispensation, and remained until 12 April 1947. The British Mission ended when he departed.


Gun-type weapon design


Research

In 1943, development efforts were directed to a
gun-type fission weapon Gun-type fission weapons are fission-based nuclear weapons A nuclear weapon (also called an atom bomb, nuke, atomic bomb, nuclear warhead, A-bomb, or nuclear bomb) is an explosive device that derives its destructive force from nuclear reactio ...

gun-type fission weapon
using plutonium called Thin Man. The names for all three atomic bomb designs—
Fat Man "Fat Man" (also known as Mark III) is the codename for the type of nuclear bomb A nuclear weapon (also called an atom bomb, nuke, atomic bomb, nuclear warhead, A-bomb, or nuclear bomb) is an explosive device that derives its destructive f ...

Fat Man
, Thin Man, and
Little Boy "Little Boy" was the codename for the type of atomic bomb A nuclear weapon (also known as an atom bomb, atomic bomb, nuclear bomb or nuclear warhead, and colloquially as an A-bomb or nuke) is an explosive device that derives its destructi ...

Little Boy
—were chosen by Serber based on their shapes. Thin Man was a long device, and its name came from the Dashiell Hammett The Thin Man, detective novel and The Thin Man (film), series of movies of the same name. The Fat Man was round and fat, and was named after Sydney Greenstreet's "Kasper Gutman" character in ''The Maltese Falcon (1941 film), The Maltese Falcon''. Little Boy came last, and was named after Elisha Cook, Jr.'s character in the same film, as referred to by Humphrey Bogart. A series of conferences in April and May 1943 laid out the laboratory's plan for the rest of the year. Oppenheimer estimated the critical mass of a uranium-235 gadget with a formula based on diffusion theory derived at Berkeley by Stan Frankel and E. C. Nelson. This gave a value for a uranium-235 gadget with a perfect tamper of 25 kg; but this was only an approximation. It was based on simplifying assumptions, notably that all neutrons had the same speed, that all collisions were elastic collision, elastic, that they were scattered isotropy, isotropically, and that the mean free path of neutrons in the core and tamper were the same. Bethe's T Division, particularly Serber's T-2 (Diffusion Theory) Group and Feynman's T-4 (Diffusion Problems) Groups, would spend the next few months working on improved models. Bethe and Feynman also developed a formula for the efficiency of the reaction. No formula could be more accurate than the values put into it; the values for the cross sections were dubious, and had not yet been determined for plutonium. Measurement of these values would be a priority, but the laboratory possessed just 1 gram of uranium-235, and only a few micrograms of plutonium. This task fell to Bacher's P Division. Williams P-2 (Electrostatic Generator) Group carried out the first experiment in July 1943, when it used the larger of the two Van de Graaff generators to measure the ratio of the neutron per fission in plutonium against that of uranium-235. This involved some negotiation with the Metallurgical Laboratory to obtain 165 μg of plutonium, which was received at Los Alamos on 10 July 1943. Bacher was able to report that the number of neutrons per fission of plutonium-239 was 2.64 ± 0.2, about 1.2 times as much as uranium-235. Titterton and Boyce McDaniel of Wilson's P-1 (Cyclotron) Group attempted to measure the time it took for prompt neutrons to be emitted from a uranium-235 nucleus when it fissions. They calculated that most were emitted in less than 1 nanosecond. Subsequent experiments demonstrated that fission took less than a nanosecond too. Confirmation of the theorists' contention that the number of neutrons emitted per fission was the same for both neutron temperature, fast and slow neutrons took longer, and was not completed until the autumn of 1944.
John von Neumann John von Neumann (; hu, Neumann János Lajos, ; December 28, 1903 – February 8, 1957) was a Hungarian-American Hungarian Americans (Hungarian language, Hungarian: ''amerikai magyarok'') are United States, Americans of Hungarian p ...

John von Neumann
visited the Los Alamos Laboratory in September 1943 and participated in discussions of the damage that an atomic bomb would do. He explained that while the damage done by a small explosion was proportional to the Impulse (physics), impulse (the average pressure of the explosion times its duration), the damage from large explosions such as an atomic bomb would be determined by the peak pressure, which depends on the cube root of its energy. Bethe then calculated that a explosion would result in an overpressure of at , and therefore result in severe damage within that radius. Von Neumann also suggested that, because pressure increases when shock waves bounce off solid objects, the damage could be increased if the bomb was detonated at an altitude comparable to the damage radius, approximately .


Development

Parsons was appointed the head of Ordnance and Engineering Division in June 1943 on the recommendation of Bush and Conant. To staff the division, Tolman, who acted as a coordinator of the gun development effort, brought in John Streib, Charles Critchfield and Seth Neddermeyer from the National Bureau of Standards. The division was initially organized into five groups, with original group leaders being McMillan of the E-1 (Proving Ground) Group, Kenneth Bainbridge of the E-2 (Instrumentation) Group, Robert Brode of the E-3 (Fuse Development) Group, Critchfield of the E-4 (Projectile, Target, and Source) Group and Neddermeyer of the E-5 (Implosion) Group. Two more groups were added in the autumn of 1943, the E-7 (Delivery) Group under Norman Ramsey and the E-8 (Interior Ballistics) Group under Joseph O. Hirschfelder. A proving ground was established at the Anchor Ranch. The gun would be an unusual one, and it had to be designed in the absence of crucial data about the critical mass. The design criteria were that the gun would have a muzzle velocity of ; that the tube would weigh only instead of the conventional for a tube with that energy; that, as a consequence it would be made of alloyed steel; that it should have a maximum breech pressure of ; and that it should have three independent Primer (firearm), primers. Because it would need to be fired only once, the barrel could be made lighter than the conventional gun. Nor did it require rifling or recoil mechanisms. Pressure curves were computed under Hirschfelder's supervision at the Geophysical Laboratory prior to his joining the Los Alamos Laboratory. While they waited for the guns to be fabricated by the Naval Gun Factory, various propellants were tested. Hirschfelder sent John L. Magee to the United States Bureau of Mines, Bureau of Mines' Experimental Mine, U.S. Bureau of Mines, Experimental Mine at Bruceton, Pennsylvania to test the propellant and ignition system. Test firing was conducted at the Anchor Ranch with a 3"/50 caliber gun, /50 caliber gun. This allowed the fine-tuning of the testing instrumentation. The first two tubes arrived at Los Alamos on 10 March 1944, and test firing began at the Anchor Ranch under the direction of Thomas H. Olmstead, who had experience in such work at the Naval Surface Warfare Center Dahlgren Division, Naval Proving Ground in Dahlgren, Virginia. The primers were tested and found to work at pressures up to . Brode's group investigated the fusing systems, testing radar altimeters, proximity fuses and barometric altimeter fuses. Tests were conducted with a frequency-modulated continuous-wave radar, frequency modulated type radar altimeter known as AYD and a pulse type known as 718. The AYD modifications were made by the Norden Laboratories Corporation under an OSRD contract. When the manufacturer of 718, RCA, was contacted, it was learned that a new tail warning radar, AN/APS-13, later nicknamed ''Archie'', was just entering production, which could be adapted for use as a radar altimeter. The third unit to be made was delivered to Los Alamos in April 1944. In May it was tested by diving an Beechcraft Model 18, AT-11. This was followed by full-scale drop testing in June and July. These were very successful, whereas the AYD continued to suffer from problems. Archie was therefore adopted, although the scarcity of units in August 1944 precluded wholescale destructive testing. Testing of Silverplate Boeing B-29 Superfortress aircraft with Thin Man bomb shapes was carried out at Edwards Air Force Base, Muroc Army Air Field in March and June 1944.


Plutonium

At a meeting of the S-1 Executive Committee on 14 November 1942, Chadwick had expressed a fear that the alpha particles emitted by plutonium could produce neutrons in light elements present as impurities, which in turn would produce fission in the plutonium and cause a Fizzle (nuclear test), predetonation, a chain reaction before the core was fully assembled. This had been considered by Oppenheimer and Seaborg the month before, and the latter had calculated that neutron emitters like boron had to be restricted to one part in a hundred billion. There was some doubt about whether a chemical process could be developed that could ensure this level of purity, and Chadwick brought the matter to the S-1 Executive Committee's attention for it to be considered further. Four days later, though, Lawrence, Oppenheimer, Compton and McMillan reported to Conant that they had confidence that the exacting purity requirement could be met. Only microscopic quantities of plutonium were available until the X-10 Graphite Reactor at the Clinton Engineer Works came online on 4 November 1943, but there were already some worrying signs. When plutonium fluoride was produced at the Metallurgical Laboratory, it was sometimes light colored, and sometimes dark, although the chemical process was the same. When they managed to reduce it to plutonium metal in November 1943, the density was measured at 15 g/cm3, and a measurement using X-ray scattering techniques pointed to a density of 13 g/cm3. This was bad; it had been assumed that its density was the same as uranium, about 19 g/cm3. If these figures were correct, far more plutonium would be needed for a bomb. Kennedy disliked Seaborg's ambitious and attention-seeking manner, and with Arthur Wahl had devised a procedure for plutonium purification independent of Seaborg's group. When they got hold of a sample in February, this procedure was tested. That month the Metallurgical Laboratory announced that it had determined that there were two different fluorides: the light colored plutonium tetrafluoride (PuF4) and the dark plutonium trifluoride (PuF3). The chemists soon discovered how to make them selectively, and the former turned out to be easier to reduce to metal. Measurements in March 1944 indicated a density of between 19 and 20 g/cm3. Eric Jette's CM-8 (Plutonium Metallurgy) Group began experimenting with plutonium metal after gram quantities were received at the Los Alamos Laboratory in March 1944. By heating it, the metallurgists discovered five temperatures between at which it suddenly started absorbing heat without increasing in temperature. This was a strong indication of multiple allotropes of plutonium; but was initially considered too bizarre to be true. Further testing confirmed a state change around ; it entered the δ phase, with a density of 16 g/cm3. Seaborg had claimed that plutonium had a melting point of around , about that of uranium, but the metallurgists at the Los Alamos Laboratory soon discovered that it melted at around . The chemists then turned to techniques for removing light element impurities from the plutonium; but on 14 July 1944, Oppenheimer informed Kennedy that this would no longer be required. The notion of
spontaneous fission Spontaneous fission (SF) is a form of radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy b ...
had been raised by Niels Bohr and John Archibald Wheeler in their 1939 treatment of the mechanism of nuclear fission. The first attempt to discover spontaneous fission in uranium was made by Willard Libby, but he failed to detect it. It had been observed in Britain by Frisch and Titterton, and independently in the Soviet Union by Georgy Flyorov and Konstantin Petrzhak in 1940; the latter are generally credited with the discovery. Compton had also heard from the French physicist Pierre Victor Auger, Pierre Auger that Frédéric Joliot-Curie had detected what might have been spontaneous fission in polonium. If true, it might preclude the use of polonium in the neutron initiators; if true for plutonium, it might mean that the gun-type design would not work. The consensus at the Los Alamos Laboratory was that it was not true, and that Joliot-Curie's results had been distorted by impurities. At the Los Alamos Laboratory, Emilio Segrè's P-5 (Radioactivity) Group set out to measure it in uranium-234, −235 and −238, plutonium, polonium, protactinium and thorium. They were not too worried about the plutonium itself; their main concern was the issue Chadwick had raised about interaction with light element impurities. Segrè and his group of young physicists set up their experiment in an old Forest Service log cabin in Pajarito Canyon, about from the Technical Area, in order to minimize background radiation emanating for other research at the Los Alamos Laboratory. By August 1943, they had good values for all the elements tested except for plutonium, which they were unable to measure accurately enough because the only sample they had was five 20 μg samples created by the 60-inch cyclotron at Berkeley. They did observe that measurements taken at Los Alamos were greater than those made at Berkeley, which they attributed to cosmic rays, which are more numerous at Los Alamos, which is above sea level. While their measurements indicated a spontaneous fission rate of 40 fissions per gram per hour, which was high but acceptable, the error margin was unacceptably large. In April 1944 they received a sample from the X-10 Graphite Reactor. Tests soon indicated 180 fissions per gram per hour, which was unacceptably high. It fell to Bacher to inform Compton, who was visibly shaken. Suspicion fell on
plutonium-240 Plutonium-240 (, Pu-240) is an isotope Isotopes are variants of a particular chemical element which differ in neutron number, and consequently in nucleon number. All isotopes of a given element have the same number of protons but different numb ...

plutonium-240
, an isotope that had not yet been discovered, but whose existence had been suspected, it being simply created by a plutonium-239 nucleus absorbing a neutron. What had not been suspected was its high spontaneous fission rate. Segrè's group measured it at 1.6 million fissions per gram per hour, compared with just 40 per gram per hour for plutonium-239. This meant that reactor-bred plutonium was unsuitable for use in a gun-type weapon. The plutonium-240 would start the chain reaction too quickly, causing a predetonation that would release enough energy to disperse the critical mass before enough plutonium reacted. A faster gun was suggested but found to be impractical. So too was the possibility of separating the isotopes, as plutonium-240 is even harder to separate from plutonium-239 than uranium-235 from uranium-238.


Implosion-type weapon design

Work on an alternative method of bomb design, known as implosion, had begun by Neddermeyer's E-5 (Implosion) group. Serber and Tolman had conceived implosion during the April 1943 conferences as a means of assembling pieces of fissionable material together to form a critical mass. Neddermeyer took a different tack, attempting to crush a hollow cylinder into a solid bar. The idea was to use explosives to crush a subcritical amount of fissile material into a smaller and denser form. When the fissile atoms are packed closer together, the rate of neutron capture increases, and they form a critical mass. The metal needs to travel only a very short distance, so the critical mass is assembled in much less time than it would take with the gun method. At the time, the idea of using explosives in this manner was quite novel. To facilitate the work, a small plant was established at the Anchor Ranch for casting explosive shapes. Throughout 1943, implosion was considered a backup project in case the gun-type proved impractical for some reason. Theoretical physicists like Bethe, Oppenheimer and Teller were intrigued by the idea of a design of an atomic bomb that made more efficient use of fissile material, and permitted the use of material of lower purity. These were advantages of particular attraction to Groves. But while Neddermeyer's 1943 and early 1944 investigations into implosion showed promise, it was clear that the problem would be much more difficult from a theoretical and engineering perspective than the gun design. In July 1943, Oppenheimer wrote to John von Neumann, asking for his help, and suggesting that he visit Los Alamos where he could get "a better idea of this somewhat Buck Rogers project". At the time, von Neumann was working for the Navy Bureau of Ordnance, Princeton University, the Army's Aberdeen Proving Ground and the NDRC. Oppenheimer, Groves and Parsons appealed to Tolman and Rear Admiral (United States), Rear Admiral William R. Purnell to release von Neumann. He visited Los Alamos from 20 September to 4 October 1943. Drawing on his recent work with blast waves and shaped charges used in armor-piercing shells, he suggested using a high-explosive shaped charge to implode a spherical core. A meeting of the Governing Board on 23 September resolved to approach George Kistiakowsky, a renowned expert on explosives then working for OSRD, to join the Los Alamos Laboratory. Although reluctant, he did so in November. He became a full-time staff member on 16 February 1944, becoming Parsons' deputy for implosion; McMillan became his deputy for the gun-type. The maximum size of the bomb was determined at this time from the size of the bomb bay of the B-29. By July 1944, Oppenheimer had concluded that plutonium could not be used in a gun design, and opted for implosion. The accelerated effort on an implosion design, codenamed
Fat Man "Fat Man" (also known as Mark III) is the codename for the type of nuclear bomb A nuclear weapon (also called an atom bomb, nuke, atomic bomb, nuclear warhead, A-bomb, or nuclear bomb) is an explosive device that derives its destructive f ...

Fat Man
, began in August 1944 when Oppenheimer implemented a sweeping reorganization of the Los Alamos laboratory to focus on implosion. Two new groups were created at Los Alamos to develop the implosion weapon, X (for explosives) Division headed by Kistiakowsky and G (for gadget) Division under Robert Bacher. Although Teller was head of the T-1 (Implosion and Super) Group, Bethe considered that Teller was spending too much time on the Super, which had been given a low priority by Bethe and Oppenheimer. In June 1944, Oppenheimer created a dedicated Super Group under Teller, who was made directly responsible to Oppenheimer, and Peierls became head of the T-1 (Implosion) Group. In September, Teller's group became the F-1 (Super and General Theory) Group, part of the Enrico Fermi's new F (Fermi) Division. The new design that von Neumann and T Division, most notably Rudolf Peierls, devised used explosive lenses to focus the explosion onto a spherical shape using a combination of both slow and fast high explosives. A visit by Sir Geoffrey Taylor in May 1944 raised questions about the stability of the interface between the core and the depleted uranium tamper. As a result, the design was made more conservative. The ultimate expression of this was the adoption of Christy's proposal that the core be solid instead of hollow. The design of lenses that detonated with the proper shape and velocity turned out to be slow, difficult and frustrating. Various explosives were tested before settling on composition B as the fast explosive and baratol as the slow explosive. The final design resembled a soccer ball, with 20 hexagonal and 12 pentagonal lenses, each weighing about . Getting the detonation just right required fast, reliable and safe electrical detonators, of which there were two for each lens for reliability. It was therefore decided to use exploding-bridgewire detonators, a new invention developed at Los Alamos by a group led by Luis Walter Alvarez, Luis Alvarez. A contract for their manufacture was given to Raytheon. To study the behavior of converging shock waves, Robert Serber devised the RaLa Experiment, which used the short-lived radioisotope lanthanum-140, a potent source of gamma radiation. The gamma ray source was placed in the center of a metal sphere surrounded by the explosive lenses, which in turn were inside in an ionization chamber. This allowed the taking of an X-ray movie of the implosion. The lenses were designed primarily using this series of tests. In his history of the Los Alamos project, David Hawkins (philosopher), David Hawkins wrote: "RaLa became the most important single experiment affecting the final bomb design". Within the explosives was the thick aluminum pusher, which provided a smooth transition from the relatively low density explosive to the next layer, the thick tamper of natural uranium. Its main job was to hold the critical mass together as long as possible, but it would also reflect neutrons back into the core. Some part of it might fission as well. To prevent predetonation by an external neutron, the tamper was coated in a thin layer of boron. A polonium-beryllium modulated neutron initiator, known as an "urchin" because its shape resembled a sea urchin, was developed to start the chain reaction at precisely the right moment. This work with the chemistry and metallurgy of radioactive polonium was directed by Charles Allen Thomas of the Monsanto Company and became known as the Dayton Project. Testing required up to 500 Curie (unit), curies per month of polonium, which Monsanto was able to deliver. The whole assembly was encased in a duralumin bomb casing to protect it from bullets and flak. The ultimate task of the metallurgists was to determine how to cast plutonium into a sphere. The brittle α phase that exists at room temperature changes to the plastic β phase at higher temperatures. Attention then shifted to the even more malleable δ phase that normally exists in the range. It was found that this was stable at room temperature when alloyed with aluminum, but aluminum emits neutrons when bombarded with alpha particles, which would exacerbate the pre-ignition problem. The metallurgists then hit upon a plutonium–gallium alloy, which stabilized the δ phase and could be Hot pressing, hot pressed into the desired spherical shape. As plutonium was found to corrode readily, the sphere was coated with nickel. The work proved dangerous. By the end of the war, half the experienced chemists and metallurgists had to be removed from work with plutonium when unacceptably high levels of the element appeared in their urine. A minor fire at Los Alamos in January 1945 led to a fear that a fire in the plutonium laboratory might contaminate the whole town, and Groves authorized the construction of a new facility for plutonium chemistry and metallurgy, which became known as the DP-site. The hemispheres for the first plutonium Pit (nuclear weapon), pit (or core) were produced and delivered on 2 July 1945. Three more hemispheres followed on 23 July and were delivered three days later.


Little Boy

Following Oppenheimer's reorganization of the Los Alamos Laboratory in July 1944, the work on the uranium gun-type weapon was concentrated in Francis Birch (geophysicist), Francis Birch's O-1 (Gun) Group. The concept was pursued so that in case of a failure to develop an implosion bomb, at least the enriched uranium could be used. Henceforth the gun-type had to work with enriched uranium only, and this allowed the Thin Man design to be greatly simplified. A high-velocity gun was no longer required, and a simpler weapon could be substituted, one short enough to fit into a B-29 bomb bay. The new design was called
Little Boy "Little Boy" was the codename for the type of atomic bomb A nuclear weapon (also known as an atom bomb, atomic bomb, nuclear bomb or nuclear warhead, and colloquially as an A-bomb or nuke) is an explosive device that derives its destructi ...

Little Boy
. After repeated slippages, the first shipment of slightly enriched uranium (13 to 15 percent uranium-235) arrived from Oak Ridge in March 1944. Shipments of highly enriched uranium commenced in June 1944. Criticality experiments and the Water Boiler had priority, so the metallurgists did not receive any until August 1944. In the meantime, the CM Division experimented with uranium hydride. This was considered by T Division as a prospective active material. The idea was that the hydrogen's ability as a neutron moderator would compensate for the loss of efficiency, but, as Bethe later recalled, its efficiency was "negligible or less, as Feynman would say", and the idea was dropped by August 1944. Frank Spedding's Ames Project had developed the Ames process, a method of producing uranium metal on an industrial scale, but Cyril Stanley Smith, the CM Division's associate leader in charge of metallurgy, was concerned about using it with highly enriched uranium due to the danger of forming a critical mass. Highly enriched uranium was also far more valuable than natural uranium, and he wanted to avoid the loss of even a milligram. He recruited Richard D. Baker, a chemist who had worked with Spedding, and together they adapted the Ames Process for use at the Los Alamos laboratory. In February Baker and his group made twenty 360 gram reductions and twenty-seven 500 gram reductions with highly enriched uranium tetrafluoride. Two types of gun design were produced: Type A was of high alloy steel, and Type B of more ordinary steel. Type B was chosen for production because it was lighter. The primers and propellant were the same as those previously chosen for Thin Man. Scale test firing of the hollow projectile and target insert was conducted with the 3-inch/50 caliber gun and a 20 mm Hispano cannon, Hispano cannon. Starting in December, test firing was done full-scale. Amazingly, the first test case produced turned out to be the best ever made. It was used in four test firings at the Anchor Ranch, and ultimately in the Little Boy used in the bombing of Hiroshima. The design specifications were completed in February 1945, and contracts were let to build the components. Three different plants were used so that no one would have a copy of the complete design. The gun and breech were made by the Naval Gun Factory in Washington, D.C.; the target, case and some other components were by the Naval Ordnance Plant in Center Line, Michigan; and the tail fairing and mounting brackets by the Expert Tool and Die Company in Detroit, Michigan. Birch's tidy schedule was disrupted in December by Groves, who ordered Oppenheimer to give priority to the gun-type over implosion, so that the weapon would be ready by 1 July 1945. The bomb, except for the uranium payload, was ready at the beginning of May 1945. The uranium-235 projectile was completed on 15 June, and the target on 24 July. The target and bomb pre-assemblies (partly assembled bombs without the fissile components) left Hunters Point Naval Shipyard, California, on 16 July aboard the cruiser , arriving 26 July. The target inserts followed by air on 30 July. Although all of its components had been tested in target and drop tests, no full test of a gun-type nuclear weapon occurred before Hiroshima. There were several reasons for not testing a Little Boy type of device. Primarily, there was insufficient uranium-235. Additionally, the weapon design was simple enough that it was only deemed necessary to do laboratory tests with the gun-type assembly. Unlike the implosion design, which required sophisticated coordination of shaped explosive charges, the gun-type design was considered almost certain to work. Thirty-two drop tests were conducted at Wendover Air Force Base, Wendover, and only once did the bomb fail to fire. One last-minute modification was made, to allow the powder bags of propellant that fired the gun to be loaded in the bomb bay. The danger of accidental detonation made safety a concern. Little Boy incorporated basic safety mechanisms, but an accidental detonation could still occur. Tests were conducted to see whether a crash could drive the hollow "bullet" onto the "target" cylinder resulting in a massive release of radiation, or possibly nuclear detonation. These showed that this required an impact of 500 times that of gravity, which made it highly unlikely. There was still concern that a crash and a fire could trigger the explosives. If immersed in water, the uranium halves were subject to a neutron moderator effect. While this would not have caused an explosion, it could have created widespread radioactive contamination. For this reason, pilots were advised to crash on land rather than at sea.


Water boiler

The Water Boiler was an
aqueous homogeneous reactor Aqueous homogeneous reactors (AHR) are a type of nuclear reactor in which soluble nuclear fuel, nuclear salts (usually uranium sulfate or uranium nitrate) are dissolution (chemistry), dissolved in water. The fuel is mixed with the coolant and the ne ...
, a type of nuclear reactor in which the nuclear fuel in the form of soluble uranium sulfate is dissolution (chemistry), dissolved in water. Uranium sulfate was chosen instead of uranium nitrate because sulfur's neutron capture cross section is less than that of nitrogen. The project was proposed by Bacher in April 1943 as part of an ongoing program of measuring critical masses in chain-reacting systems. He saw it also as a means of testing various materials in critical mass systems. T Division were opposed to the project, which was seen as a distraction from studies related to the form of chain reactions found in an atomic bomb, but Bacher prevailed on this point. Calculations related to the Water Boiler did take up an inordinate amount of T Division's time in 1943. The reactor theory developed by Fermi did not apply to the Water Boiler. Little was known about building reactors in 1943. A group was created in Bacher's P Division, the P-7 (Water Boiler) Group, under the leadership of Donald Kerst, that included Charles P. Baker, Gerhart Friedlander, Lindsay Helmholz, Marshall Holloway and Raemer Schreiber. Robert F. Christy from the T-1 Group provided support with the theoretical calculations, in particular, a calculation of the critical mass. He calculated that 600 grams of uranium-235 would form a critical mass in a tamper of infinite size. Initially it was planned to operate the Water Boiler at 10 kW, but Fermi and Samuel K. Allison visited in September 1943, and went over the proposed design. They pointed out the danger of decomposition of the uranium salt, and recommended heavier shielding. It was also noted that radioactive fission products would be created that would have to be chemically removed. As a consequence, it was decided that the Water Boiler would only run at 1 kW until more operating experience had been accumulated, and features needed for high power operation were shelved for the time being. Christy also calculated the area that would become contaminated if an accidental explosion occurred. A site in Los Alamos Canyon was selected that was a safe distance from the township and downstream from the water supply. Known as Omega, it was approved by the Governing Board on 19 August 1943. The Water Boiler was not simple to construct. The two halves of the stainless steel sphere that was the boiler had to be arc welding, arc welded because solder would be corroded by the uranium salt. The CM-7 (Miscellaneous Metallurgy) Group produced beryllia bricks for the Water Boiler's tamper in December 1943 and January 1944. They were hot pressed in graphite at at for 5 to 20 minutes. Some 53 bricks were made, shaped to fit around the boiler. The building at Omega Site was ready, if incomplete, by 1 February 1944, and the Water Boiler was fully assembled by 1 April. Sufficient enriched uranium had arrived by May to start it up, and it went critical on 9 May 1944. It was only the third reactor in the world to do so, the first two being the Chicago Pile-1 reactor at the Metallurgical Laboratory and the X-10 Graphite Reactor at the Clinton Engineer Works. Improved cross-section measurements allowed Christy to refine his criticality estimate to 575 grams. In fact, only 565 grams were required. The accuracy of his prediction surprised Christy more than anyone. In September 1944, the P-7 (Water Boiler) Group became the F-2 (Water Boiler) Group, part of Fermi's F Division. On completion of the planned series of experiments in June 1944, it was decided to rebuild it as a more powerful reactor. The original goal of 10 kW power was discarded in favor of 5 kW, which would keep the cooling requirements simple. It was estimated that it would have a neutron flux of 5 x 1010 neutrons per square centimeter per second. Water cooling was installed, along with additional control rods. This time uranium nitrate was used instead of uranium sulfate because the former could more easily be decontaminated. The tamper of beryllia bricks was surrounded with graphite blocks, as beryllia was hard to procure, and to avoid the (γ, n) reaction in the beryllium, in which gamma rays produced by the reactor-generated neutrons: : + → + - 1.66 MeV The reactor commenced operation in December 1944.


Super

From the first, research into the Super was directed by Teller, who was its most enthusiastic proponent. Although this work was always considered secondary to the objective of developing a fission bomb, the prospect of creating more powerful bombs was sufficient to keep it going. The Berkeley summer conference had convinced Teller that the Super was technologically feasible. An important contribution was made by
Emil Konopinski Emil John (Jan) Konopinski (December 25, 1911 in Michigan City, Indiana – May 26, 1990 in Bloomington, Indiana Indiana () is a U.S. state in the Midwestern United States The United States of America (USA), commonly known as the ...
, who suggested that deuterium could more easily be ignited if it was mixed with tritium. Bethe noted that a tritium-deuterium (T-D) reaction releases five times as much energy as a deuterium-deuterium (D-D) reaction. This was not immediately followed up, because tritium was hard to obtain, and there were hopes that deuterium could be easily ignited by a fission bomb, but the cross sections of T-D and D-D were measured by Manley's group in Chicago and Holloway's at Purdue. By September 1943, the values of the D-D and T-D had been revised upwards, raising hopes that a fusion reaction could be started at lower temperatures. Teller was sufficiently optimistic about the Super, and sufficiently concerned about reports that the Germans were interested in deuterium, to ask the Governing Board to raise its priority. The board agreed to some extent, but ruled that only one person could be spared to work on it full-time. Oppenheimer designated Konopinski, who would spend the rest of the war working on it. Nonetheless, in February 1944, Teller added Stanislaw Ulam, Jane Roberg, Geoffrey Chew, and Harold and Mary Argo to his T-1 Group. Ulam calculated the Compton scattering, inverse Compton cooling, while Roberg worked out the ignition temperature of T-D mixtures. Maria Goeppert joined the group in February 1945. Teller argued for an increase in resources for Super research on the basis that it appeared to be far more difficult than anticipated. The board declined to do so, on the grounds that it was unlikely to bear fruit before the war ended, but did not cut it entirely. Indeed, Oppenheimer asked Groves to breed some tritium from deuterium in the X-10 Graphite Reactor. For some months Teller and Bethe argued about the priority of the Super research. In June 1944, Oppenheimer removed Teller and his Super Group from Bethe's T Division and placed it directly under himself. In September, it became the F-1 (Super) Group in Fermi' s F Division. Over the following months, Super research continued unabated. It was calculated that burning of liquid deuterium would release the energy of , enough to devastate . The Super Group was transferred back to T Division on 14 November 1945. A colloquium on the Super was held at the Los Alamos Laboratory in April 1946 to review the work done during the war. Teller gave an outline of his "Classic Super" concept, and Nicholas Metropolis and Anthony L. Turkevich presented the results of calculations that had been made concerning thermonuclear reactions. The final report on the Super, issued in June and prepared by Teller and his group, remained upbeat about the prospect of the Super being successfully developed, although that impression was not universal among those present at the colloquium. Work had to be curtailed in June 1946 due to the loss of staff. By 1950, calculations would show that the Classic Super would not work; that it would not only be unable to sustain thermonuclear burning in the deuterium fuel, but would be unable to ignite it in the first place.


Trinity

Because of the complexity of an implosion-style weapon, it was decided that, despite the waste of fissile material, an initial test would be required. Groves approved the test, subject to the active material being recovered. Consideration was therefore given to a controlled fizzle, but Oppenheimer opted instead for a full-scale nuclear test, codenamed "Trinity". In March 1944, responsibility for planning the test was assigned to Kenneth Bainbridge, a professor of physics at Harvard, working under Kistiakowsky. Bainbridge selected the Alamogordo Bombing Range, bombing range near Alamogordo Army Airfield as the site for the test. Bainbridge worked with Captain Samuel P. Davalos on the construction of the Trinity Base Camp and its facilities, which included barracks, warehouses, workshops, an explosive Magazine (artillery), magazine and a commissary (store), commissary. Groves did not relish the prospect of explaining the loss of a billion dollars worth of plutonium to a Senate committee, so a cylindrical containment vessel codenamed "Jumbo" was constructed to recover the active material in the event of a failure. Measuring long and wide, it was fabricated at great expense from of iron and steel by Babcock & Wilcox in Barberton, Ohio. Brought in a special railroad car to a siding in Pope, New Mexico, it was transported the last to the test site on a trailer pulled by two tractors. By the time it arrived, confidence in the implosion method was high enough, and the availability of plutonium was sufficient, that Oppenheimer decided not to use it. Instead, it was placed atop a steel tower from the weapon as a rough measure of how powerful the explosion would be. In the end, Jumbo survived, although its tower did not, adding credence to the belief that Jumbo would have successfully contained a fizzled explosion. A pre-test explosion was conducted on 7 May 1945 to calibrate the instruments. A wooden test platform was erected from Ground Zero and piled with of TNT spiked with nuclear fission products in the form of an irradiated uranium slug from the Hanford Site, which was dissolved and poured into tubing inside the explosive. This explosion was observed by Oppenheimer and Groves's new deputy commander, Brigadier General Thomas Farrell (general), Thomas Farrell. The pre-test produced data that proved vital for the Trinity test. For the actual test, the device, nicknamed "the gadget", was hoisted to the top of a steel tower, as detonation at that height would give a better indication of how the weapon would behave when dropped from a bomber. Detonation in the air maximized the energy applied directly to the target, and generated less nuclear fallout. The gadget was assembled under the supervision of
Norris Bradbury Norris Edwin Bradbury (30 May 1909 – 20 August 1997), was an American physicist A physicist is a scientist A scientist is a person who conducts Scientific method, scientific research to advance knowledge in an Branches of science, ar ...
at the nearby McDonald Ranch House on 13 July, and precariously winched up the tower the following day. Observers included Bush, Chadwick, Conant, Farrell, Fermi, Groves, Lawrence, Oppenheimer and Tolman. At 05:30 on 16 July 1945 the gadget exploded with an TNT equivalent, energy equivalent of around 20 kilotons of TNT, leaving a crater of Trinitite (radioactive glass) in the desert wide. The shock wave was felt over away, and the mushroom cloud reached in height. It was heard as far away as El Paso, Texas, so Groves issued a cover story about an ammunition magazine explosion at Alamogordo Field.


Project Alberta

Project Alberta, also known as Project A, was formed in March 1945, absorbing existing groups of Parsons's O Division that were working on bomb preparation and delivery. These included Ramsey's O-2 (Delivery) Group, Birch's O-1 (Gun) Group, Bainbridge's X-2 (Development, Engineering, and Tests) Group, Brode's O-3 (Fuse Development) Group and George Galloway's O-4 (Engineering) Group. Its role was to support the bomb delivery effort. Parsons became the head of Project Alberta, with Ramsey as his scientific and technical deputy, and Ashworth as his operations officer and military alternate. In all, Project Alberta consisted of 51 Army, Navy and civilian personnel. The 1st Technical Service Detachment, to which the personnel of Project Alberta were administratively assigned, was commanded by
Lieutenant Colonel Lieutenant colonel ( or ) is a rank of commissioned officer An officer is a person who holds a position of authority as a member of an armed force A military, also known collectively as armed forces, is a heavily armed, highly orga ...
Peer de Silva, and provided security and housing services on Tinian. There were two bomb assembly teams, a Fat Man Assembly Team under Commander Norris Bradbury and Roger Warner, and a Little Boy Assembly Team under Birch. Philip Morrison was the head of the Pit Crew, Bernard Waldman and Luis W. Alvarez, Luis Alvarez led the Aerial Observation Team, and Sheldon Dike was in charge of the Aircraft Ordnance Team. Physicists Robert Serber and William Penney, and US Army Captain (United States O-3), Captain James F. Nolan, a medical expert, were special consultants. All members of Project Alberta had volunteered for the mission. Project Alberta proceeded with the plan to have the Little Boy ready by 1 August, and the first Fat Man ready for use as soon as possible after that. In the meantime, a series of twelve combat missions were flown between 20 and 29 July against targets in Japan using high-explosive pumpkin bombs, versions of the Fat Man with the explosives, but not the fissile core. Project Alberta's Sheldon Dike and Milo Bolstead flew on some of these missions, as did the British observer Group Captain Leonard Cheshire. Four Little Boy pre-assemblies, L-1, L-2, L-5 and L-6 were expended in test drops. The Little Boy team had the live bomb completely assembled and ready for use on 31 July. The final item of preparation for the operation came on 29 July 1945. Orders for the attack were issued to General (United States), General Carl Spaatz on 25 July under the signature of General Thomas T. Handy, the acting Chief of Staff of the United States Army, since General of the Army George C. Marshall was at the Potsdam Conference with President of the United States, President Harry S. Truman. The order designated four targets: Hiroshima, Kokura, Niigata, Niigata, Niigata, and Nagasaki, and ordered the attack to be made "as soon as weather will permit after about 3 August". Assembly of a Fat Man unit was a complex operation involving personnel from the High Explosive, Pit, Fusing and Firing teams. To prevent the assembly building from becoming overcrowded and thereby causing an accident, Parsons limited the numbers allowed inside at any time. Personnel waiting to perform a specific task had to wait their turn outside the building. The first Fat Man pre-assembly, known as F13, was assembled by 31 July, and expended in a drop test the next day. This was followed by F18 on 4 August, which was dropped the next day. Three sets of Fat Man pre-assemblies, designated F31, F32, and F33, arrived on B-29s of the 509th Composite Group and 216th Army Air Forces Base Unit on 2 August. On inspection, the high explosive blocks of F32 were found to be badly cracked and unserviceable. The other two were assembled, with F33 earmarked for a rehearsal and F31 for operational use. Parsons, as the weaponeer, was in command of the Hiroshima mission. With Second Lieutenant Morris R. Jeppson of the 1st Ordnance Squadron, he inserted the Little Boy's powder bags in the ''Enola Gay''s bomb bay in flight. Before climbing to altitude on approach to the target, Jeppson switched the three safety plugs between the electrical connectors of the internal battery and the firing mechanism from green to red. The bomb was then fully armed. Jeppson monitored its circuits. Four other members of Project Alberta flew on the Hiroshima mission. Luis Walter Alvarez, Luis Alvarez, Harold Agnew and Lawrence H. Johnston were on the instrument plane ''The Great Artiste (B-29), The Great Artiste''. They dropped "Bangometer" canisters to measure the force of the blast, but this was not used to calculate the yield at the time. Bernard Waldman was the camera operator on the Necessary Evil (aircraft), observation aircraft. He was equipped with a special high-speed Fastax movie camera with six seconds of film in order to record the blast. Unfortunately, Waldman forgot to open the camera shutter, and no film was exposed. Other members of the team flew to Iwo Jima in case ''Enola Gay'' was forced to land there, but this was not required. Purnell, Parsons, Paul Tibbets, Spaatz and Curtis LeMay met on Guam on 7 August, the day after the Hiroshima attack, to discuss what should be done next. Parsons said that Project Alberta would have a Fat Man bomb ready by 11 August, as originally planned, but Tibbets pointed to weather reports indicating poor flying conditions on that day due to a storm, and asked if it could be readied by 9 August. Parsons agreed to do so. For this mission, Ashworth was the weaponeer, with Lieutenant (navy), Lieutenant Philip M. Barnes, of the 1st Ordnance Squadron as assistant weaponeer on the B-29 ''Bockscar''. Walter Goodman and Lawrence H. Johnston were on board the instrumentation aircraft, ''The Great Artiste''. Leonard Cheshire and William Penney were on the observation plane ''Big Stink (B-29), Big Stink''. Robert Serber was supposed to be on board but was left behind by the aircraft commander because he had forgotten his parachute.


Health and safety

A medical program was established at Los Alamos under Captain James F. Nolan of the United States Army Medical Corps. Initially, a small five-bed infirmary was established for civilians, and a three-bed infirmary for military personnel. More serious cases were handled by the Army's Bruns General Hospital in Santa Fe, but this was soon regarded as unsatisfactory due to the loss of time due to the long trip, and security risks. Nolan recommended that the infirmaries be consolidated and expanded into a 60-bed hospital. A 54-bed hospital was opened in 1944, staffed by Army personnel. A dentist arrived in March 1944. A Veterinary Corps (United States Army), Veterinary Corps officer, Captain J. Stevenson, had already been assigned to look after the guard dogs. Laboratory facilities for medical research were limited, but some research was conducted into the effects of radiation, and the absorption and toxic effects of metals, particularly plutonium and beryllium, mainly as a result of accidents. The Health Group began conducting urine tests of laboratory workers in early 1945, and many of these revealed dangerous levels of plutonium. Work on the Water Boiler also occasionally exposed workers to dangerous fission products. There were 24 fatal accidents at Los Alamos between its opening in 1943 and September 1946. Most involved construction workers. Four scientists died, including Harry Daghlian and Louis Slotin in criticality accidents involving the demon core.


Security

On 10 March 1945, a Japanese fire balloon struck a power line, and the resulting power surge caused the Manhattan Project's reactors at the Hanford site to be temporarily shut down. This generated great concern at Los Alamos that the site might come under attack. One night found everyone staring at a strange light in the sky. Oppenheimer later recalled this demonstrated that "even a group of scientists is not proof against the errors of suggestion and hysteria". With so many people involved, security was a difficult task. A special Counter Intelligence Corps detachment was formed to handle the Manhattan Project's security issues. By 1943, it was clear that the Soviet Union was attempting to penetrate the project. The most successful Soviet spy was Klaus Fuchs of the British Mission. The 1950 revelation of his espionage activities damaged the United States' nuclear cooperation with Britain and Canada. Subsequently, other instances of espionage were uncovered, leading to the arrest of Harry Gold, David Greenglass and Ethel and Julius Rosenberg. Other spies like Theodore Hall remained unknown for decades.


Post-war

After the war ended on 14 August 1945, Oppenheimer informed Groves of his intention to resign as director of the Los Alamos Laboratory, but agreed to remain until a suitable replacement could be found. Groves wanted someone with both a solid academic background and a high standing within the project. Oppenheimer recommended Norris Bradbury. This was agreeable to Groves, who liked the fact that as a naval officer Bradbury was both a military man and a scientist. Bradbury accepted the offer on a six-month trial basis. Groves announced this at a meeting of division leaders on 18 September. Parsons arranged for Bradbury to be quickly discharged from the Navy, which awarded him the Legion of Merit for his wartime services. He remained in the Naval Reserve, though, ultimately retiring in 1961 with the rank of captain. On 16 October 1945, there was a ceremony at Los Alamos at which Groves presented the laboratory with the Army-Navy "E" Award, and presented Oppenheimer with a certificate of appreciation. Bradbury became the laboratory's second director the following day. The first months of Bradbury's directorship were particularly trying. He had hoped that Atomic Energy Act of 1946 would be quickly passed by Congress and the wartime Manhattan Project would be superseded by a new, permanent organization. It soon became clear that this would take more than six months. President Harry S. Truman did not sign the act creating the United States Atomic Energy Commission, Atomic Energy Commission into law until 1 August 1946, and it did not become active until 1 January 1947. In the meantime, Groves' legal authority to act was limited. Most of the scientists at Los Alamos were eager to return to their laboratories and universities, and by February 1946 all of the wartime division heads had left, but a talented core remained. Darol Froman became head of Robert Bacher's G division, now renamed M Division. Eric Jette became responsible for Chemistry and Metallurgy, John H. Manley for Physics, George Placzek for Theory, Max Roy for Explosives, and Roger Wagner for Ordnance. Z Division was created in July 1945 to control testing, stock piling, and bomb assembly activities. It was named after Jerrold R. Zacharias, its leader until 17 October 1945, when he returned to MIT, and was succeeded by Roger S. Warner. It moved to
Sandia Base Sandia Base was the principal nuclear weapons A nuclear weapon (also called an atom bomb, nuke, atomic bomb, nuclear warhead, A-bomb, or nuclear bomb) is an explosive device that derives its destructive force from nuclear reactions, either nucl ...
between March and July 1946, except for its Z-4 (Mechanical Engineering) Group, which followed in February 1947. The number of personnel at the Los Alamos Laboratory plummeted from its wartime peak of over 3,000 to around 1,000, but many were still living in substandard temporary wartime accommodation. Despite the reduced staff, Bradbury still had to provide support for
Operation Crossroads Operation Crossroads was a pair of nuclear weapon A nuclear weapon (also known as an atom bomb, atomic bomb, nuclear bomb or nuclear warhead, and colloquially as an A-bomb or nuke) is an explosive device that derives its destructive f ...
, the nuclear tests in the Pacific. Ralph A. Sawyer was appointed the Technical Director with Marshall Holloway from B Division and Roger Warner from Z Division as associate directors. Two ships were assigned for Los Alamos Laboratory personnel, the and . Operation Crossroads cost the Los Alamos Laboratory over one million dollars, and the services of 150 personnel (about one-eighth of its staff) for nine months. As the United States had only about ten atomic bombs in mid-1946 about one fifth of the stockpile was expended. The Los Alamos Laboratory became the
Los Alamos Scientific Laboratory Los Alamos National Laboratory (Los Alamos or LANL for short) is a United States Department of Energy National Labs, United States Department of Energy national laboratory initially organized during World War II for the design of nuclear weapons ...
in January 1947. The contract with the University of California that had been negotiated in 1943 allowed the University to terminate it three months after the end of hostilities, and it served notice. There were concerns about the university operating a laboratory outside the state of California. The university was persuaded to rescind its notice, and the operating contract was extended until July 1948. Bradbury would remain director until 1970. The total cost of Project Y up to the end of 1946 was $57.88 million (equivalent to $ million in ).


Notes


References

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * {{authority control Code names Former nuclear research institutes History of the Manhattan Project, Y Los Alamos, New Mexico Military projects of the United States Military history of New Mexico Military in New Mexico