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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-charge ratio. Mass spectrometry is us ...
originally designed and used for separating the isotopes of
uranium Uranium is a chemical element; it has chemical symbol, symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Ura ...
. It was developed by
Ernest Lawrence Ernest Orlando Lawrence (August 8, 1901 – August 27, 1958) was an American accelerator physicist who received the Nobel Prize in Physics in 1939 for his invention of the cyclotron. He is known for his work on uranium-isotope separation for ...
during the
Manhattan Project The Manhattan Project was a research and development program undertaken during World War II to produce the first nuclear weapons. It was led by the United States in collaboration with the United Kingdom and Canada. From 1942 to 1946, the ...
and was based on his earlier invention, the
cyclotron A cyclotron is a type of particle accelerator invented by Ernest Lawrence in 1929–1930 at the University of California, Berkeley, and patented in 1932. Lawrence, Ernest O. ''Method and apparatus for the acceleration of ions'', filed: Januar ...
. Its name was derived from California University Cyclotron, in tribute to Lawrence's institution, 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. Headquartered in Oakland, California, Oakland, the system is co ...
, where it was invented. Calutrons were used in the industrial-scale Y-12
uranium enrichment Enriched uranium is a type of uranium in which the percent composition of uranium-235 (written 235U) has been increased through the process of isotope separation. Naturally occurring uranium is composed of three major isotopes: uranium-238 (23 ...
plant at the 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 po ...
. The
enriched uranium Enriched uranium is a type of uranium in which the percent composition of uranium-235 (written 235U) has been increased through the process of isotope separation. Naturally occurring uranium is composed of three major isotopes: uranium-238 (23 ...
produced was used in the
Little Boy Little Boy was a type of atomic bomb created by the Manhattan Project during World War II. The name is also often used to describe the specific bomb (L-11) used in the bombing of the Japanese city of Hiroshima by the Boeing B-29 Superfortress ...
atomic bomb A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission or atomic bomb) or a combination of fission and fusion reactions (thermonuclear weapon), producing a nuclear expl ...
that was detonated over Hiroshima on 6 August 1945. The calutron is a type of sector mass spectrometer, an instrument in which a sample is
ionized Ionization or ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule i ...
and then accelerated by
electric field An electric field (sometimes called E-field) is a field (physics), physical field that surrounds electrically charged particles such as electrons. In classical electromagnetism, the electric field of a single charge (or group of charges) descri ...
s and deflected by
magnetic field A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...
s. The ions ultimately collide with a plate and produce a measurable
electric current An electric current is a flow of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is defined as the net rate of flow of electric charge through a surface. The moving particles are called charge c ...
. Since the ions of the different isotopes have the same electric charge but different masses, the heavier isotopes are deflected less by the magnetic field, causing the beam of particles to separate into several beams by mass, striking the plate at different locations. The
mass Mass is an Intrinsic and extrinsic properties, intrinsic property of a physical body, body. It was traditionally believed to be related to the physical quantity, quantity of matter in a body, until the discovery of the atom and particle physi ...
of the ions can be calculated according to the strength of the field and the charge of the ions. During
World War II World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...
, calutrons were developed to use this principle to obtain substantial quantities of high-purity uranium-235, by taking advantage of the small mass difference between uranium isotopes. Electromagnetic separation for uranium enrichment was abandoned in the post-war period in favor of the more complicated, but more efficient,
gaseous diffusion Gaseous diffusion is a technology that was used to produce enriched uranium by forcing gaseous uranium hexafluoride (UF6) through microporous membranes. This produces a slight separation (enrichment factor 1.0043) between the molecules containi ...
method. Although most of the calutrons of the Manhattan Project were dismantled at the end of the war, some remained in use to produce isotopically enriched samples of naturally occurring elements for military, scientific and medical purposes.


Origins

News of the
discovery of nuclear fission Nuclear fission was discovered in December 1938 by chemists Otto Hahn and Fritz Strassmann and physicists Lise Meitner and Otto Robert Frisch. Fission is a nuclear reaction or radioactive decay process in which the atomic nucleus, nucleus of a ...
by German chemists
Otto Hahn Otto Hahn (; 8 March 1879 – 28 July 1968) was a German chemist who was a pioneer in the field of radiochemistry. He is referred to as the father of nuclear chemistry and discoverer of nuclear fission, the science behind nuclear reactors and ...
and Fritz Strassmann in 1938, and its theoretical explanation by
Lise Meitner Elise Lise Meitner ( ; ; 7 November 1878 – 27 October 1968) was an Austrian-Swedish nuclear physicist who was instrumental in the discovery of nuclear fission. After completing her doctoral research in 1906, Meitner became the second woman ...
and
Otto Frisch Otto Robert Frisch (1 October 1904 – 22 September 1979) was an Austrian-born British physicist who worked on nuclear physics. With Otto Stern and Immanuel Estermann, he first measured the magnetic moment of the proton. With his aunt, Lise M ...
, was brought to the United States by
Niels Bohr Niels Henrik David Bohr (, ; ; 7 October 1885 – 18 November 1962) was a Danish theoretical physicist who made foundational contributions to understanding atomic structure and old quantum theory, quantum theory, for which he received the No ...
. Based on his liquid drop model of the nucleus, he theorized that it was the
uranium-235 Uranium-235 ( or U-235) is an 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 that exists in nat ...
isotope and not the more abundant
uranium-238 Uranium-238 ( or U-238) is the most common isotope of uranium found in nature, with a relative abundance of 99%. Unlike uranium-235, it is non-fissile, which means it cannot sustain a chain reaction in a thermal-neutron reactor. However, it i ...
that was primarily responsible for fission with
thermal neutron The neutron detection temperature, also called the neutron energy, indicates a free neutron's kinetic energy, usually given in electron volts. The term ''temperature'' is used, since hot, thermal and cold neutrons are moderated in a medium wit ...
s. To verify this Alfred O. C. Nier at the
University of Minnesota The University of Minnesota Twin Cities (historically known as University of Minnesota) is a public university, public Land-grant university, land-grant research university in the Minneapolis–Saint Paul, Twin Cities of Minneapolis and Saint ...
used 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-charge ratio. Mass spectrometry is us ...
to create a microscopic amount of enriched uranium-235 in April 1940. John R. Dunning,
Aristid von Grosse Aristid von Grosse (January 1905 – July 21, 1985) was a Germans, German nuclear chemist. During his work with Otto Hahn, he got access to waste material from radium production, and with this starting material he was able in 1927 to isolate ...
and Eugene T. Booth were then able to confirm that Bohr was correct. Leo Szilard and Walter Zinn soon confirmed that more than one neutron was released per fission, which made it almost certain that a
nuclear chain reaction In nuclear physics, a nuclear chain reaction occurs when one single nuclear reaction causes an average of one or more subsequent nuclear reactions, thus leading to the possibility of a self-propagating series or "positive feedback loop" of thes ...
could be initiated, and therefore that the development of an
atomic bomb A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission or atomic bomb) or a combination of fission and fusion reactions (thermonuclear weapon), producing a nuclear expl ...
was a theoretical possibility. There were fears that a German atomic bomb project would develop one first, especially among scientists who were refugees from
Nazi Germany Nazi Germany, officially known as the German Reich and later the Greater German Reich, was the German Reich, German state between 1933 and 1945, when Adolf Hitler and the Nazi Party controlled the country, transforming it into a Totalit ...
and other
fascist Fascism ( ) is a far-right, authoritarian, and ultranationalist political ideology and movement. It is characterized by a dictatorial leader, centralized autocracy, militarism, forcible suppression of opposition, belief in a natural soci ...
countries. At the
University of Birmingham The University of Birmingham (informally Birmingham University) is a Public university, public research university in Birmingham, England. It received its royal charter in 1900 as a successor to Queen's College, Birmingham (founded in 1825 as ...
in Britain, the Australian physicist
Mark Oliphant Sir Marcus Laurence Elwin Oliphant, (8 October 1901 – 14 July 2000) was an Australian physicist and humanitarian who played an important role in the first experimental demonstration of nuclear fusion and in the development of nuclear weapon ...
assigned two refugee physicists—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 Tube Alloys, Britain's nuclear weapon programme, as well as the subsequent Manhattan Project, the combined Allied ...
—the task of investigating the feasibility of an atomic bomb, ironically because their status as enemy aliens precluded their working on secret projects like
radar Radar is a system that uses radio waves to determine the distance ('' ranging''), direction ( azimuth and elevation angles), and radial velocity of objects relative to the site. It is a radiodetermination method used to detect and track ...
. Their March 1940 Frisch–Peierls memorandum indicated that the
critical mass In nuclear engineering, critical mass is the minimum mass of the fissile material needed for a sustained nuclear chain reaction in a particular setup. The critical mass of a fissionable material depends upon its nuclear properties (specific ...
of uranium-235 was within an
order of magnitude In a ratio scale based on powers of ten, the order of magnitude is a measure of the nearness of two figures. Two numbers are "within an order of magnitude" of each other if their ratio is between 1/10 and 10. In other words, the two numbers are ...
of 10 kg, which was small enough to be carried by a
bomber A bomber is a military combat aircraft that utilizes air-to-ground weaponry to drop bombs, launch aerial torpedo, torpedoes, or deploy air-launched cruise missiles. There are two major classifications of bomber: strategic and tactical. Strateg ...
of the day. The British Maud Committee then unanimously recommended pursuing the development of an atomic bomb. Britain had offered to give the United States access to its scientific research, so the
Tizard Mission The Tizard Mission, officially the British Technical and Scientific Mission, was a delegation from the United Kingdom that visited the United States during World War II to share secret research and development (R&D) work that had military applicat ...
's
John Cockcroft Sir John Douglas Cockcroft (27 May 1897 – 18 September 1967) was an English nuclear physicist who shared the 1951 Nobel Prize in Physics with Ernest Walton for their splitting of the atomic nucleus, which was instrumental in the developmen ...
briefed American scientists on British developments. He discovered that the American project was smaller than the British, and not as far advanced. A disappointed Oliphant flew to the United States to speak to the American scientists. These included
Ernest Lawrence Ernest Orlando Lawrence (August 8, 1901 – August 27, 1958) was an American accelerator physicist who received the Nobel Prize in Physics in 1939 for his invention of the cyclotron. He is known for his work on uranium-isotope separation for ...
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. Headquartered in Oakland, California, Oakland, the system is co ...
's Radiation Laboratory in Berkeley. The two men had met before the war, and were friends. Lawrence was sufficiently impressed to commence his own research into uranium. Uranium-235 makes up only about 0.72% of natural uranium, so the separation factor of any uranium enrichment process needs to be higher than 125 to produce 90% uranium-235 from natural uranium. The Maud Committee had recommended that this be done by a process of
gaseous diffusion Gaseous diffusion is a technology that was used to produce enriched uranium by forcing gaseous uranium hexafluoride (UF6) through microporous membranes. This produces a slight separation (enrichment factor 1.0043) between the molecules containi ...
, but Oliphant had pioneered another technique in 1934: electromagnetic separation. This was the process that Nier had used. The principle of electromagnetic separation is that charged
ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
s are deflected by a magnetic field, and lighter ones are deflected more than heavy ones. The reason the Maud Committee, and later its American counterpart, the S-1 Section of the
Office of Scientific Research and Development The Office of Scientific Research and Development (OSRD) was an agency of the United States federal government created to coordinate scientific research for military purposes during World War II. Arrangements were made for its creation during May ...
(OSRD), had passed over the electromagnetic method was that while the mass spectrometer was capable of separating isotopes, it produced very low yields. The reason for this was the so-called
space charge Space charge is an interpretation of a collection of electric charges in which excess electric charge is treated as a continuum of charge distributed over a region of space (either a volume or an area) rather than distinct point-like charges. Thi ...
limitation. Positive ions have positive charge, so they tend to repel each other, which causes the beam to scatter. Drawing on his experience with the precise control of charged-particle beams from his work with his invention, the
cyclotron A cyclotron is a type of particle accelerator invented by Ernest Lawrence in 1929–1930 at the University of California, Berkeley, and patented in 1932. Lawrence, Ernest O. ''Method and apparatus for the acceleration of ions'', filed: Januar ...
, Lawrence suspected that the air molecules in the vacuum chamber would neutralize the ions, and create a focused beam. Oliphant inspired Lawrence to convert his old cyclotron into a giant mass spectrometer for
isotope separation Isotope separation is the process of concentrating specific isotopes of a chemical element by removing other isotopes. The use of the nuclides produced is varied. The largest variety is used in research (e.g. in chemistry where atoms of "marker" n ...
. The 37-inch cyclotron at Berkeley was dismantled on 24 November 1941, and its magnet used to create the first calutron. Its name came from California University and cyclotron. The work was initially funded by the Radiation Laboratory from its own resources, with a $5,000 grant from the Research Corporation. In December Lawrence received a $400,000 grant from the S-1 Uranium Committee. The calutron consisted of an ion source, in the form of a box with a slit in it and hot filaments inside.
Uranium tetrachloride Uranium tetrachloride is an inorganic compound, a salt of uranium and chlorine, with the formula UCl4. It is a hygroscopic olive-green solid. It was used in the electromagnetic isotope separation (EMIS) process of uranium enrichment. It is one ...
was ionized by the filament, and then passed through a slot into a vacuum chamber. The magnet was then used to deflect the ion beam by 180°. The enriched and depleted beams went into collectors. When the calutron was first operated on 2 December 1941, just days before the
Japanese attack on Pearl Harbor The attack on Pearl HarborAlso known as the Battle of Pearl Harbor was a surprise military strike by the Empire of Japan on the United States Pacific Fleet at its naval base at Pearl Harbor on Oahu, Hawaii, on December 7, 1941. At the tim ...
brought the United States into
World War II World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...
, a uranium beam intensity of 5 microamperes (μA) was received by the collector. Lawrence's hunch about the effect of the air molecules in the vacuum chamber was confirmed. A nine-hour run on 14 January 1942 with a 50 μA beam produced 18 micrograms (μg) of uranium enriched to 25% uranium-235, about ten times as much as Nier had produced. By February, improvements in the technique allowed it to generate a 1,400 μA beam. That month, 75 μg samples enriched to 30% were shipped to the British and the Metallurgical Laboratory in Chicago. Other researchers also investigated electromagnetic isotope separation. At
Princeton University Princeton University is a private university, private Ivy League research university in Princeton, New Jersey, United States. Founded in 1746 in Elizabeth, New Jersey, Elizabeth as the College of New Jersey, Princeton is the List of Colonial ...
, a group led by Henry D. Smyth and Robert R. Wilson developed a device known as an isotron. Using a
klystron A klystron is a specialized linear-beam vacuum tube, invented in 1937 by American electrical engineers Russell and Sigurd Varian,Pond, Norman H. "The Tube Guys". Russ Cochran, 2008 p.31-40 which is used as an amplifier for high radio frequenci ...
, they were able to separate isotopes using high-voltage electricity rather than magnetism. Work continued until February 1943, when, in view of the greater success of the calutron, work was discontinued and the team was transferred to other duties. At
Cornell University Cornell University is a Private university, private Ivy League research university based in Ithaca, New York, United States. The university was co-founded by American philanthropist Ezra Cornell and historian and educator Andrew Dickson W ...
a group under Lloyd P. Smith that included William E. Parkins, and A. Theodore Forrester devised a radial magnetic separator. They were surprised that their beams were more precise than expected, and, like Lawrence, deduced that it was a result of stabilization of the beam by air in the vacuum chamber. In February 1942, their team was consolidated with Lawrence's in Berkeley.


Research

While the process had been demonstrated to work, considerable effort was still required before a prototype could be tested in the field. Lawrence assembled a team of physicists to tackle the problems, including
David Bohm David Joseph Bohm (; 20 December 1917 – 27 October 1992) was an American scientist who has been described as one of the most significant Theoretical physics, theoretical physicists of the 20th centuryDavid Peat Who's Afraid of Schrödinger' ...
,
Edward Condon Edward Uhler Condon (March 2, 1902 – March 26, 1974) was an American nuclear physicist, a pioneer in quantum mechanics, and a participant during World War II in the development of radar and, very briefly, of nuclear weapons as part of the Ma ...
, Donald Cooksey, A. Theodore Forrester,
Irving Langmuir Irving Langmuir (; January 31, 1881 – August 16, 1957) was an American chemist, physicist, and metallurgical engineer. He was awarded the Nobel Prize in Chemistry in 1932 for his work in surface chemistry. Langmuir's most famous publicatio ...
,
Kenneth Ross MacKenzie Kenneth Ross MacKenzie (June 15, 1912 – July 3, 2002) was an American nuclear physicist. Together with Dale R. Corson and Emilio Segrè, he synthesized the element astatine, in 1940. MacKenzie received his PhD under Ernest Lawrence at L ...
, Frank Oppenheimer,
J. Robert Oppenheimer J. Robert Oppenheimer (born Julius Robert Oppenheimer ; April 22, 1904 – February 18, 1967) was an American theoretical physics, theoretical physicist who served as the director of the Manhattan Project's Los Alamos Laboratory during World ...
, William E. Parkins, Bernard Peters and Joseph Slepian. In November 1943 they were joined by a British Mission headed by Oliphant that included fellow Australian physicists Harrie Massey and Eric Burhop, and British physicists such as Joan Curran and Thomas Allibone. Lawrence had a large cyclotron under construction at Berkeley, one with a magnet. This was converted into a calutron that was switched on for the first time on 26 May 1942. Like the 37-inch version, it looked like a giant C when viewed from above. The operator sat in the open end, whence the temperature could be regulated, the position of the electrodes adjusted, and even components replaced through an airlock while it was running. The new, more powerful calutron was not used to produce enriched uranium, but for experiments with multiple ion sources. This meant having more collectors, but it multiplied the throughput. The problem was that the beams interfered with each other, producing a series of oscillations called hash. An arrangement was devised that minimized the interference, resulting in reasonably good beams being produced, in September 1942. Robert Oppenheimer and Stan Frankel invented the magnetic shim, a device used to adjust the homogeneity of a magnetic field. These were sheets of iron about in width that were bolted to the top and bottom of the vacuum tank. The effect of the shims was to slightly increase the magnetic field in such a way as to help focus the ion beam. Work would continue on the shims through 1943. The main calutron patents were ''Methods of and apparatus for separating materials'' (Lawrence), ''Magnetic shims'' (Oppenheimer and Frankel), and ''Calutron system'' (Lawrence). Burhop and Bohm later studied the characteristics of electric discharges in magnetic fields, today known as
Bohm diffusion The diffusion of plasma across a magnetic field was conjectured to follow the Bohm diffusion scaling as indicated from the early plasma experiments of very lossy machines. This predicted that the rate of diffusion was linear with temperature and in ...
. Their papers on the properties of plasmas under magnetic containment would find usage in the post-war world in research into controlled
nuclear fusion Nuclear fusion is a nuclear reaction, reaction in which two or more atomic nuclei combine to form a larger nuclei, nuclei/neutrons, neutron by-products. The difference in mass between the reactants and products is manifested as either the rele ...
. Other technical problems were more mundane but no less important. Although the beams had low intensity, they could, over many hours of operation, still melt the collectors. A water cooling system was therefore added to the collectors and the tank liner. Procedures were developed for cleaning the "gunk" that condensed inside the vacuum tank. A particular problem was blockage of the slits by "crud", which caused the ion beams to lose focus, or stop entirely. The chemists had to find a way of producing quantities of uranium tetrachloride () from
uranium oxide Uranium oxide is an oxide of the element uranium. The metal uranium forms several oxides: * Uranium dioxide or uranium(IV) oxide (UO2, the mineral uraninite or pitchblende) * Diuranium pentoxide or uranium(V) oxide (U2O5) * Uranium trioxide or ...
. (Nier had used uranium bromide.) Initially, they produced it by using hydrogen to reduce
uranium trioxide Uranium trioxide (UO3), also called uranyl oxide, uranium(VI) oxide, and uranic oxide, is the hexavalent oxide of uranium. The solid may be obtained by heating uranyl nitrate to 400 °C. Its most commonly encountered polymorph is amorphous ...
() to
uranium dioxide Uranium dioxide or uranium(IV) oxide (), also known as urania or uranous oxide, is an oxide of uranium, and is a black, radioactive, crystalline powder that naturally occurs in the mineral uraninite. It is used in nuclear fuel rods in nuclear reac ...
(), which was then reacted with
carbon tetrachloride Carbon tetrachloride, also known by many other names (such as carbon tet for short and tetrachloromethane, also IUPAC nomenclature of inorganic chemistry, recognised by the IUPAC), is a chemical compound with the chemical formula CCl4. It is a n ...
() to produce uranium tetrachloride. Charles A. Kraus proposed a better method for large-scale production that involved reacting the uranium oxide with carbon tetrachloride at high temperature and pressure. This produced uranium pentachloride () and
phosgene Phosgene is an organic chemical compound with the formula . It is a toxic, colorless gas; in low concentrations, its musty odor resembles that of freshly cut hay or grass. It can be thought of chemically as the double acyl chloride analog of ...
(). While nowhere near as nasty as the uranium hexafluoride used by the gaseous diffusion process, uranium tetrachloride is
hygroscopic Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption (chemistry), absorption or adsorption from the surrounding Natural environment, environment, which is usually at normal or room temperature. If water mol ...
, so work with it had to be undertaken in
glovebox A glovebox (or glove box) is a sealed container that is designed to allow one to manipulate objects where a separate atmosphere is desired. Built into the sides of the glovebox are gloves arranged in such a way that the user can place their hand ...
es that were kept dry with
phosphorus pentoxide Phosphorus pentoxide is a chemical compound with molecular formula Phosphorus, P4Oxygen, O10 (with its common name derived from its empirical formula, P2O5). This white crystalline solid is the anhydride of phosphoric acid. It is a powerful desic ...
(). The presence of phosgene, a lethal gas responsible for 85,000 deaths as a
chemical weapon A chemical weapon (CW) is a specialized munition that uses chemicals formulated to inflict death or harm on humans. According to the Organisation for the Prohibition of Chemical Weapons (OPCW), this can be any chemical compound intended as ...
during
World War I World War I or the First World War (28 July 1914 – 11 November 1918), also known as the Great War, was a World war, global conflict between two coalitions: the Allies of World War I, Allies (or Entente) and the Central Powers. Fighting to ...
, required that the chemists wear gas masks when handling it. Of the $19.6 million spent on research and development of the electromagnetic process, $18 million (92 percent) was spent at the Radiation Laboratory in Berkeley, and further work conducted at
Brown University Brown University is a Private university, private Ivy League research university in Providence, Rhode Island, United States. It is the List of colonial colleges, seventh-oldest institution of higher education in the US, founded in 1764 as the ' ...
,
Johns Hopkins University The Johns Hopkins University (often abbreviated as Johns Hopkins, Hopkins, or JHU) is a private university, private research university in Baltimore, Maryland, United States. Founded in 1876 based on the European research institution model, J ...
and
Purdue University Purdue University is a Public university#United States, public Land-grant university, land-grant research university in West Lafayette, Indiana, United States, and the flagship campus of the Purdue University system. The university was founded ...
, and by the Tennessee Eastman corporation. During 1943, the emphasis shifted from research to development, engineering, and the training of workers to operate the production facilities at the 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 po ...
. By the middle of 1944, there were nearly 1,200 people working at the Radiation Laboratory.


Design

Much of the great progress on the electromagnetic process can be attributed to Lawrence's leadership style. His audacity, optimism and enthusiasm were contagious. His staff put in long hours, and University of California administrators sliced through red tape despite not knowing what the project was about. Government officials began to view the development of atomic bombs in time to affect the outcome of the war as a genuine possibility.
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 ...
, the director of the OSRD, which was overseeing the project, visited Berkeley in February 1942, and found the atmosphere there "stimulating" and "refreshing". On 9 March 1942, he reported to the president,
Franklin D. Roosevelt Franklin Delano Roosevelt (January 30, 1882April 12, 1945), also known as FDR, was the 32nd president of the United States, serving from 1933 until his death in 1945. He is the longest-serving U.S. president, and the only one to have served ...
, that it might be possible to produce enough material for a bomb by mid-1943, based on new estimates from Robert Oppenheimer that the critical mass of a sphere of pure uranium-235 was between 2.0 and 2.5 kilograms. The experiments with the 184-inch magnet led to the construction of a prototype calutron called the XA. It contained a rectangular, three-coil magnet with a horizontal field in which the calutron tanks could stand side by side, with four vacuum tanks, each with a double source. At the 25 June 1942 meeting of the S-1 Executive Committee, which had superseded the S-1 Uranium Committee on 19 June, there was a proposal to build the electromagnetic plant at Oak Ridge, where the other
Manhattan Project The Manhattan Project was a research and development program undertaken during World War II to produce the first nuclear weapons. It was led by the United States in collaboration with the United Kingdom and Canada. From 1942 to 1946, the ...
uranium separation facilities would be located, for reasons of economy and security. Lawrence lodged an objection due to his desire to have the electromagnetic separation plant located much nearer to Berkeley. The Shasta Dam area in California remained under consideration for the electromagnetic plant until September 1942, by which time Lawrence had dropped his objection. The 25 June meeting also designated Stone & Webster as the primary contractor for the design and engineering. The Army assumed responsibility for the Manhattan Project on 17 September 1942, with Brigadier General Leslie R. Groves, Jr., as director, although the Army did not formally take over the contracts with the University of California from the OSRD until 1 May 1943. Major Thomas T. Crenshaw, Jr., became California Area Engineer in August 1942, with Captain Harold A. Fidler, who soon replaced him, as his assistant. Crenshaw established his office in the Donner Laboratory at the University of California. In September 1942, the S-1 Executive Committee recommended that a five-tank pilot plant be built along with a 200-tank section of a production plant. Between October 1942 and November 1943, Groves paid monthly visits to the Radiation Laboratory in Berkeley. Reports indicated that compared to the alternatives of a gaseous diffusion plant or a
plutonium Plutonium is a chemical element; it has symbol Pu and atomic number 94. It is a silvery-gray actinide metal that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exhibits six allotropes and four ...
-producing
nuclear reactor A nuclear reactor is a device used to initiate and control a Nuclear fission, fission nuclear chain reaction. They are used for Nuclear power, commercial electricity, nuclear marine propulsion, marine propulsion, Weapons-grade plutonium, weapons ...
, an electromagnetic plant would take longer and require more scarce materials to build, and need more manpower and more electricity to operate. The cost of a kilogram of fissile material would therefore be much greater. On the other hand, while the alternative processes still faced considerable technical obstacles, the electromagnetic process was proven to work, and could be built in stages that would immediately start producing fissile material. Groves cancelled the pilot plant on 14 November, in favor of proceeding immediately with the production plant. The Radiation Laboratory forwarded the preliminary designs for a production plant to Stone & Webster before the end of the year, but one important issue remained unsettled. Oppenheimer contended that
weapons-grade Weapons-grade nuclear material is any fissionable nuclear material that is pure enough to make a nuclear weapon and has properties that make it particularly suitable for nuclear weapons use. Plutonium and uranium in grades normally used in nuc ...
uranium would have to be 90% pure uranium-235. Edward Lofgren and
Martin Kamen Martin David Kamen (August 27, 1913, Toronto – August 31, 2002, Montecito, California) was an American chemist who, together with Sam Ruben, co-discovered the synthesis of the isotope carbon-14 on February 27, 1940, at the University of ...
thought that this could not be achieved without a second stage of enrichment. The two stages became known as Alpha and Beta. In March 1943, Groves approved the construction of five Alpha and two Beta racetracks. In September, he authorized four more Alpha racetracks, which became known as Alpha II, along with two more Beta racetracks to process their product.


Construction

Construction of the electromagnetic plant at Oak Ridge, codenamed Y-12, commenced 18 February 1943. The facility would eventually comprise nine major process buildings and 200 other structures covering almost of floor space. The site in Bear Creek Valley southwest of the Oak Ridge township was selected in the hope that the surrounding ridge lines might contain a major explosion or nuclear accident. Problems with the substratum required the excavation crews to perform more blasting and excavation to provide adequate foundations for the heavy machinery in the facilities. Supplies and materials of all kinds poured in: 2,157 carloads of electrical equipment, 1,219 of heavy equipment, 5,389 of lumber, 1,407 of pipe and fittings, 1,188 of steel, 257 of valves, and 11 of welding electrodes. The racetracks required 85,000
vacuum tube A vacuum tube, electron tube, thermionic valve (British usage), or tube (North America) is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference has been applied. It ...
s. Where possible, off-the-shelf components were used, but all too many components of the calutrons were unique. Two purchasing departments were established, one in Boston near Stone & Webster for facility equipment, and the other at Oak Ridge for construction supplies. The Chief Engineer of the Manhattan District,
Colonel Colonel ( ; abbreviated as Col., Col, or COL) is a senior military Officer (armed forces), officer rank used in many countries. It is also used in some police forces and paramilitary organizations. In the 17th, 18th, and 19th centuries, a colon ...
James C. Marshall, and his deputy, Lieutenant Colonel Kenneth D. Nichols, discovered that the electromagnetic isotope separation process would require of
copper Copper is a chemical element; it has symbol Cu (from Latin ) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orang ...
, which was in desperately short supply. However, they realized that
silver Silver is a chemical element; it has Symbol (chemistry), symbol Ag () and atomic number 47. A soft, whitish-gray, lustrous transition metal, it exhibits the highest electrical conductivity, thermal conductivity, and reflectivity of any metal. ...
could be substituted, in an 11:10 ratio of copper:silver. On 3 August 1942, Nichols met with the Under Secretary of the Treasury, Daniel W. Bell, and asked for the transfer of silver bullion from the West Point Bullion Depository. Nichols later recalled the conversation: Eventually, of silver were used, then worth over $600 million. Nichols had to provide a monthly accounting to the Treasury. The silver bars were taken under guard to the Defense Plant Corporation in Carteret, New Jersey, where they were cast into cylindrical billets, and then to Phelps Dodge in Bayway, New Jersey, where they were extruded into strips thick, wide and long. Some 258 carloads were shipped under guard by rail to
Allis-Chalmers Allis-Chalmers was a United States, U.S. manufacturer of machinery for various Industry (economics), industries. Its business lines included list of agricultural machinery, agricultural equipment, heavy equipment, construction equipment, electric ...
in
Milwaukee, Wisconsin Milwaukee is the List of cities in Wisconsin, most populous city in the U.S. state of Wisconsin. Located on the western shore of Lake Michigan, it is the List of United States cities by population, 31st-most populous city in the United States ...
, where they were wound onto magnetic coils and sealed into welded casings. Finally, they moved by unguarded flatcars to the Clinton Engineer Works. There, special procedures were instituted for handling the silver. When they had to drill holes in it, they did so over paper so that the filings could be collected. After the war, all the machinery was dismantled and cleaned and the floorboards beneath the machinery were ripped up and burned to recover minute amounts of silver. In the end, or less than 0.036% was lost. In May 1970, the last of silver was replaced with copper and returned to the Treasury. The XAX racetrack with two tanks and three coils was ready to train workers in August 1943. Bugs were discovered, but were not aggressively followed up. The first Alpha process building, 9201-1, was completed on 1 November 1943. When the first racetrack was started up for testing on schedule in November, the 14-ton vacuum tanks crept out of alignment by as much as because of the power of the magnets and had to be fastened more securely. A more serious problem arose when the magnetic coils started shorting out. In December Groves ordered a magnet broken open, and handfuls of rust were found inside. Moisture was also a problem in its own right, as was the winding of the wire, which was too tight. Groves ordered the racetracks to be torn down and the magnets sent back to the factory to be cleaned and rewound. Rigid standards for preparation and cleanliness were instituted to prevent a recurrence of these problems. Training for the Beta tracks shifted from the XAX to the XBX training and development racetrack in November 1943. A second Alpha I racetrack became operational in January 1944. The first Beta racetrack and the third and first Alpha racetracks, now repaired, became operational in March 1944, and the fourth Alpha racetrack in April 1944. A third building, 9201-3, contained a fifth racetrack that incorporated some modifications, and was known as Alpha I. This became operational on 3 June 1944. Work on the Alpha and Beta chemistry buildings, 9202 and 9203, commenced in February 1943, and was completed in September. Work on the Beta process building, 9204-1, began in May 1943, and was ready for operation on 13 March 1944, but was not completed until September 1944. Groves authorized Alpha II in September 1943. This consisted of two new Alpha process buildings, 9201-4 and 9201-5, another Beta, 9204-2, an extension to the Alpha chemistry building, and a new Beta chemistry building, 9206. When 9206 opened, the old Beta chemistry building, 9203, was converted into a laboratory. Work started on the new Alpha II process buildings on 2 November 1943; the first racetrack was completed in July 1944, and all four were operational by 1 October 1944. The Alpha II racetracks were configured in a linear layout rather than an oval, although they were still called racetracks. In all, there were 864 Alpha calutrons, arranged in nine racetracks of 96. There were only 36 calutrons in each Beta racetrack, for a total of 288 calutrons, although only 216 of them were ever operated. Work on the new Beta process building commenced on 20 October 1943. Equipment installation began on 1 April 1944, and it was ready for use on 10 September 1944. A third Beta process building, 9204-3, was authorized in May 1944 to process the output of the K-25 gaseous diffusion plant. It was completed on 15 May 1945. A fourth Beta process building, 9204-4, was authorized on 2 April 1945 and was completed by 1 December 1945. A new group of Alpha chemistry buildings known as the 9207 group was commenced in June 1944, but work was halted in June 1945 before they were completed. Along with these main buildings, there were offices, workshops, warehouses and other structures. There were two steam plants for heating, and a power plant for electricity.


Operations

The Alpha racetracks were a 24-fold magnification of the XA calutron that could hold 96 calutron Alpha tanks. The calutrons were upright and arrayed facing each other in pairs of inner and outer machines. To minimize magnetic losses, and to economize on steel consumption, the assembly was curved into an oval shape that formed a closed magnetic loop long, wide and high, in the shape of a racetrack; hence the name. The two Alpha I buildings, 9201-1 and 9201-2, each contained two racetracks, with only one in the Alpha I, 9201-3. The Beta racetracks were smaller, linear in shape, and optimized for recovery rather than production, with only 36 instead of 96 process bins. The four Alpha II racetracks were also linear in configuration. They incorporated many improvements, the most important being that they had four sources instead of just two. They also had improved magnets and vacuum systems. Tennessee Eastman was hired to manage Y-12 on the usual cost plus fixed fee basis, with a fee of $22,500 per month plus $7,500 per racetrack for the first seven racetracks and $4,000 per additional racetrack. Workers were recruited in the Knoxville area. The typical recruit was a young woman, a recent graduate of a local high school. Training was initially conducted at the
University of Tennessee The University of Tennessee, Knoxville (or The University of Tennessee; UT; UT Knoxville; or colloquially UTK or Tennessee) is a Public university, public Land-grant university, land-grant research university in Knoxville, Tennessee, United St ...
. Training switched to Berkeley from April to September 1943, where it was conducted on the XA calutron and a 1:16 scale model of the Alpha racetrack, and then to Oak Ridge when the XAX calutron became available. Some 2,500 operators would be required once all the Alpha II calutrons were available. The Tennessee Eastman payroll at Y-12 ballooned from 10,000 in mid-1944 to 22,482 in August 1945. For security reasons, the trainees were not informed of the purpose of the equipment they were taught to operate. The calutrons were initially operated by scientists from Berkeley to remove bugs and achieve a reasonable operating rate. Then the Tennessee Eastman operators took over. Nichols compared unit production data, and pointed out to Lawrence that the young "hillbilly" girl operators were outproducing his Ph.Ds. They agreed to a production race and Lawrence lost, a morale boost for the " Calutron Girls" (called Cubicle Operators at the time) and their supervisors. The women were trained like soldiers not to reason why, while "the scientists could not refrain from time-consuming investigation of the cause of even minor fluctuations of the dials". For a while, the calutrons suffered from a series of debilitating breakdowns and equipment failures, exacerbated by a shortage of spare parts. Hopes that the Alpha II racetracks would be more reliable soon faded, as they were plagued by insulator failures. These problems were gradually overcome. The first shipments of enriched uranium to the Manhattan Project's
Los Alamos Laboratory The Los Alamos Laboratory, also known as Project Y, was a secret scientific laboratory established by the Manhattan Project and overseen by the University of California during World War II. It was operated in partnership with the United State ...
were made in March 1944, consisting of Alpha product enriched to 13 to 15 percent uranium-235. While of no use in a bomb, it was urgently required for experiments with enriched uranium. The last shipment of Alpha product was made on 11 May 1944. On 7 June 1944, Y-12 made its first delivery of
weapons-grade Weapons-grade nuclear material is any fissionable nuclear material that is pure enough to make a nuclear weapon and has properties that make it particularly suitable for nuclear weapons use. Plutonium and uranium in grades normally used in nuc ...
Beta product, enriched to as high as 89% uranium-235. A major problem was that of loss of feed material and product. Only 1 part in 5,825 of the feed material became finished product. About 90 percent was splattered over the feed bottles or vacuum tanks. The problem was particularly acute with the enriched feed of the Beta calutrons. Extraordinary efforts were made to recover product, including burning the carbon receiver liners to recover the uranium in them. Despite everything, some 17.4 percent of Alpha product and 5.4 percent of Beta product was lost. Frank Spedding from the Manhattan Project's Ames Laboratory and Philip Baxter from the British Mission were sent to advise on improvements to recovery methods. The death of a worker from exposure to phosgene also prompted a search for a safer production process. In February 1945, slightly enriched 1.4 percent uranium-235 feed material began arriving from the S-50 liquid thermal diffusion plant. Shipments of product from S-50 were discontinued in April. S-50 product was fed into K-25 instead. In March 1945, Y-12 began receiving feed enriched to 5 percent from K-25. The output of these plants was in the form of uranium hexafluoride (). It was converted to uranium trioxide, which then went into the usual process for conversion to uranium tetrachloride. On 5 August 1945, K-25 started producing feed enriched to 23 percent, enough to be fed straight into the Beta racetracks. The remaining Alpha product was then fed into K-25. By September 1945, the calutrons had produced 88 kilograms of product with an average enrichment of 84.5 percent, and the Beta racetracks turned out another 953 kilograms enriched to 95 percent by the end of the year. Enriched uranium from the calutrons provided the fissile component of the
Little Boy Little Boy was a type of atomic bomb created by the Manhattan Project during World War II. The name is also often used to describe the specific bomb (L-11) used in the bombing of the Japanese city of Hiroshima by the Boeing B-29 Superfortress ...
atomic bomb used in the atomic bombing of Hiroshima in August 1945. With the war over, the Alpha tracks began to suspend operations on 4 September 1945, and ceased operation completely on 22 September. The last two Beta tracks went into full operation in November and December 1945, processing feed from K-25 and the new K-27 gaseous diffusion plant. By May 1946, studies suggested that the gaseous diffusion plants could fully enrich the uranium by themselves without accidentally creating a critical mass. After a trial demonstrated that this was the case, Groves ordered all but one Beta track shut down in December 1946. The total cost of the electromagnetic project up to the end of the Manhattan Project on 31 December 1946 was $ (equivalent to $ in ).


Postwar years

The workforce at Y-12 dropped from a wartime peak of 22,482 on 21 August 1945 to less than 1,700 in 1949. All the calutrons were removed and dismantled, except for the XAX and XBX training tracks in Building 9731, and the Beta 3 racetracks in Building 9204–3. In 1947,
Eugene Wigner Eugene Paul Wigner (, ; November 17, 1902 – January 1, 1995) was a Hungarian-American theoretical physicist who also contributed to mathematical physics. He received the Nobel Prize in Physics in 1963 "for his contributions to the theory of th ...
, the director of the
Oak Ridge National Laboratory Oak Ridge National Laboratory (ORNL) is a federally funded research and development centers, federally funded research and development center in Oak Ridge, Tennessee, United States. Founded in 1943, the laboratory is sponsored by the United Sta ...
(ORNL), asked the United States Atomic Energy Commission, Atomic Energy Commission for permission to use the Beta calutrons to produce isotopes for physics experiments. Permission was granted, and a wide range of isotopes was produced. Lithium-6 from the Beta calutrons was used for research into thermonuclear weapons. Many other isotopes were used for peaceful scientific and medical purposes. The Beta 3 racetracks were transferred to the ORNL in March 1950. By the mid-1950s, the Beta calutrons had produced quantities of all the naturally occurring stable isotopes except those of osmium, which had to wait until April 1960. The calutrons continued to produce isotopes until 1998. , they are still on standby.


Other countries


Soviet Union and China

Like the United States, the Soviet Union (USSR) carried out research on multiple enrichment technologies for the Soviet atomic bomb project. A trial electromagnetic process was carried out in 1946 with a calutron using a magnet taken from Germany. A site was chosen for an electromagnetic plant at Sverdlovsk-45 in 1946. The pilot plant, known as Plant 418, was completed in 1948. A more efficient design was developed in which the particle beams were bent by 225° instead of 180° as in the American calutron. It was used to complete the uranium enrichment process after technical difficulties were encountered with the gaseous diffusion process. Uranium enriched to about 40 percent uranium-235 was brought to Sverdlovsk-45 for final enrichment to between 92 and 98 percent. After the problems with the gaseous diffusion process were resolved in 1950, it was decided not to proceed with a full-scale electromagnetic plant. , it remains operational. In 1969, a research calutron known as S-2 was built at All-Russian Scientific Research Institute of Experimental Physics, Arzamas-16 for high-efficiency separation of isotopes of heavy elements like plutonium. Four research and production calutrons were built at the China Institute of Atomic Energy in Beijing of identical design to those of the USSR in the early 1960s.


United Kingdom

In 1945, the Nuclear weapons and the United Kingdom, British atomic bomb project built a 180° calutron, similar in design to an American Beta calutron, at the Atomic Energy Research Establishment at Harwell, Oxfordshire. Owing to the success of the gaseous diffusion plant at Capenhurst, electromagnetic separation was not pursued by the United Kingdom, and the calutron was used to separate isotopes for research. The 180° design was not ideal for this purpose, so Harwell built a 90° calutron, HERMES, the "Heavy Elements and Radioactive Material Electromagnetic Separator". It was inspired by France's SIDONIE and PARIS separators at the Laboratoire René Bernas of the University of Paris, University of Paris IX in Orsay, and PARSIFAL at the military research laboratory of the Commissariat à l'énergie atomique et aux énergies alternatives in Bruyères-le-Châtel.


Israel, Japan, and France

Israel, Japan and France also built some research calutrons, including the SOLIS and MEIRA separators at the Soreq Nuclear Research Center. There is also CERN's On-Line Isotope Mass Separator, Isotope Separator On-Line Detector (ISOLDE), which was built in 1967.


India

A calutron at the Saha Institute of Nuclear Physics at Salt Lake City (Bidhan Nagar), Kolkata, Bidhan Nagar in India was used to produce plutonium for Smiling Buddha, India's first nuclear test on 18 May 1974.


Iraq

After the 1990–91 Gulf War, UNSCOM determined that Iraq had been pursuing a calutron program to enrich uranium. Iraq chose to develop the electromagnetic process over more modern, economic, and efficient methods of enrichment because calutrons were easier to build, with fewer technical challenges, and the components required to build them were not subject to export controls. At the time the program was discovered, Iraq was estimated to be two or three years away from producing enough material for nuclear weapons. The program was destroyed in the Gulf War. Consequently, the Nuclear Suppliers Group added the electromagnetic separation equipment to its guidelines for transfers of nuclear-related dual-use equipment, material and technology.


Notes


References

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Further reading

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External links

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Link to a site about a Calutron operator (Gladys Owens), and a famous picture of her at work.
{{Portal bar, History of Science, Nuclear technology Isotope separation History of the Manhattan Project Mass spectrometry Particle physics facilities University of California, Berkeley Uranium