Editing Calutron (section)

== Origins ==
News of the [[discovery of nuclear fission]] by German chemists [[Otto Hahn]] and [[Fritz Strassmann]] in 1938, and its theoretical explanation by [[Lise Meitner]] and [[Otto Frisch]], was brought to the United States by [[Niels Bohr]].{{sfn|Hewlett|Anderson|1962|pp=10–12}} Based on his [[liquid drop model]] of the nucleus, he theorized that it was the [[uranium-235]] isotope and not the more abundant [[uranium-238]] that was primarily responsible for fission with [[thermal neutron]]s.{{sfn|Stuewer|1985|pp=211–214}} To verify this [[Alfred O. C. Nier]] at the [[University of Minnesota]] used a [[mass spectrometer]] to create a microscopic amount of [[Enriched uranium|enriched]] uranium-235 in April 1940. [[John R. Dunning]], [[Aristid von Grosse]] and [[Eugene T. Booth]] were then able to confirm that Bohr was correct.{{sfn|Smyth|1945|p=172}}<ref>{{cite journal  |title=Nuclear Fission of Separated Uranium Isotopes  |last1=Nier |first1=Alfred O. |author-link=Alfred O. C. Nier  |last2=Booth |first2=E. T. |author-link2=Eugene T. Booth  |last3=Dunning |first3=J. R. |author-link3=John R. Dunning   |last4=von Grosse |first4=A. |author-link4=Aristid von Grosse  |journal=[[Physical Review]]  |volume=57  |issue=6   |page=546 |date=March 1940    |doi=10.1103/PhysRev.57.546 |bibcode = 1940PhRv...57..546N |s2cid=4106096 }}</ref> [[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]] could be initiated, and therefore that the development of an [[atomic bomb]] was a theoretical possibility.{{sfn|Hewlett|Anderson|1962|pp=10–14}} There were fears that a [[German atomic bomb project]] would develop one first, especially among scientists who were refugees from [[Nazi Germany]] and other [[fascist]] countries.{{sfn|Jones|1985|p=12}}

[[File:Diagram of uranium isotope separation in the calutron.png|thumb|left|upright=1.5|alt=Diagram showing the source, the particle stream being deflected 180°, and it being caught in the collector|Diagram of uranium isotope separation in the calutron]]
At the [[University of Birmingham]] in Britain, the Australian physicist [[Mark Oliphant]] assigned two refugee physicists—Otto Frisch and [[Rudolf Peierls]]—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]].{{sfn|Rhodes|1986|pp=322–325}} Their March 1940 [[Frisch–Peierls memorandum]] indicated that the [[critical mass]] of uranium-235 was within an [[order of magnitude]] of 10&nbsp;kg, which was small enough to be carried by a [[bomber]] of the day.{{sfn|Hewlett|Anderson|1962|p=42}} The British [[Maud Committee]] then unanimously recommended pursuing the development of an atomic bomb.{{sfn|Hewlett|Anderson|1962|pp=39–40}} Britain had offered to give the United States access to its scientific research,{{sfn|Phelps|2010|pp=126–128}} so the [[Tizard Mission]]'s [[John Cockcroft]] briefed American scientists on British developments. He discovered that the American project was smaller than the British, and not as far advanced.{{sfn|Phelps|2010|pp=281–283}}

A disappointed Oliphant flew to the United States to speak to the American scientists. These included [[Ernest Lawrence]] at the [[University of California, Berkeley|University of California]]'s [[Lawrence Berkeley National Laboratory|Radiation Laboratory]] in [[Berkeley, California|Berkeley]].{{sfn|Hewlett|Anderson|1962|pp=43–44}} The two men had met before the war, and were friends.{{sfn|Cockburn|Ellyard|1981|pp=74–78}} Lawrence was sufficiently impressed to commence his own research into uranium.{{sfn|Hewlett|Anderson|1962|pp=43–44}} Uranium-235 makes up only about 0.72% of natural uranium,<ref>{{cite journal|title = Atomic weights of the elements. Review 2000 (IUPAC Technical Report)|journal = Pure and Applied Chemistry|date = 1 January 2003|volume = 75|issue = 6|pages = 683–800|doi = 10.1351/pac200375060683|first1 = John R.|last1 = de Laeter|first2 = John Karl|last2 = Böhlke|first3 = P. De|last3 = Bièvre|first4 = H.|last4 = Hidaka|first5 = H. S.|last5 = Peiser|first6 = K. J. R.|last6 = Rosman|first7 = P. D. P.|last7 = Taylor|s2cid = 96800435|doi-access = free}}</ref> so the separation factor of any uranium enrichment process needs to be higher than 125 to produce 90% uranium-235 from natural uranium.{{sfn|Smyth|1945|pp=156–157}} The Maud Committee had recommended that this be done by a process of [[gaseous diffusion]],{{sfn|Hewlett|Anderson|1962|p=42}} but Oliphant had pioneered another technique in 1934: electromagnetic separation.<ref>{{cite journal |first1=M. L. E. |last1=Oliphant |first2=E. S. |last2=Shire |first3=B. M. |last3=Crowther  |journal=Proceedings of the Royal Society A |title=Separation of the Isotopes of Lithium and Some Nuclear Transformations Observed with them |date=15 October 1934 |volume=146 |issue=859 |pages=922–929 |doi=10.1098/rspa.1934.0197 |bibcode=1934RSPSA.146..922O |doi-access=free }}</ref> This was the process that Nier had used.{{sfn|Hewlett|Anderson|1962|pp=43–44}}

The principle of electromagnetic separation is that charged [[ion]]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]] (OSRD), had passed over the electromagnetic method was that while the mass spectrometer was capable of separating isotopes, it produced very low yields.{{sfn|Smyth|1945|pp=164–165}} The reason for this was the so-called [[space charge]] 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 beam]]s from his work with his invention, the [[cyclotron]], 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 {{convert|37|inch|cm|adj=on}} cyclotron into a giant mass spectrometer for [[isotope separation]].{{sfn|Hewlett|Anderson|1962|pp=43–44}}

[[File:Calutron emitter.jpg|thumb|alt=Four men in suits bend over a piece of machinery.|[[Frank Oppenheimer]] (center right) and [[Robert Lyste Thornton|Robert Thornton]] (right) examine the 4-source emitter for the improved Alpha calutron.]]
The 37-inch cyclotron at Berkeley was dismantled on 24 November 1941, and its magnet used to create the first calutron.{{sfn|Smyth|1945|pp=188–189}} Its name came from '''Cal'''ifornia '''U'''niversity and cyclo'''tron'''.{{sfn|Jones|1985|p=119}} 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.{{sfn|Hiltzik|2015|p=238}} The calutron consisted of an ion source, in the form of a box with a slit in it and hot [[heating element|filaments]] inside. [[Uranium tetrachloride]] was ionized by the filament, and then passed through a {{convert|0.04|by|2|in|adj=on}} 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.{{sfn|Albright|Hibbs|1991|p=18}}{{sfn|Hewlett|Anderson|1962|pp=56–58}}

When the calutron was first operated on 2 December 1941, just days before the [[Japanese attack on Pearl Harbor]] brought the United States into [[World War II]], a uranium beam intensity of 5 [[microampere]]s (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.{{sfn|Hewlett|Anderson|1962|pp=56–58}}

Other researchers also investigated electromagnetic isotope separation. At [[Princeton University]], a group led by [[Henry D. Smyth]] and [[Robert R. Wilson]] developed a device known as an isotron. Using a [[klystron]], they were able to separate isotopes using high-voltage electricity rather than magnetism.{{sfn|Hewlett|Anderson|1962|p=59}} 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.{{sfn|Smyth|1945|pp=188–189}} At [[Cornell University]] 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.{{sfn|Parkins|2005|pp=45–46}}<ref>{{cite journal  |title=On the Separation of Isotopes in Quantity by Electromagnetic Means  |last1=Smith |first1=Lloyd P.  |last2=Parkins |first2=W. E.  |last3=Forrester  |first3= A. T.  |journal=[[Physical Review]]  |volume=72  |issue=11  |pages=989–1002 |date=December 1947  |doi=10.1103/PhysRev.72.989|bibcode = 1947PhRv...72..989S }}</ref>