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harge 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 of the ions can be calculated according to the strength of the field and the charge of the ions during world war ii 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 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 edit 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 1 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 neutrons 2 to verify this alfred o c nier at the university of minnesota used a mass spectrometer to create a microscopic amount of 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 3 4 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 5 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 6 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 7 their march 1940 frisch peierls memorandum indicated that the critical mass of uranium 235 was within an order of magnitude of 10 kg which was small enough to be carried by a bomber of the day 8 the british maud committee then unanimously recommended pursuing the development of an atomic bomb 9 britain had offered to give the united states access to its scientific research 10 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 11 a disappointed oliphant flew to the united states to speak to the american scientists these included ernest lawrence at the university of california s radiation laboratory in berkeley 12 the two men had met before the war and were friends 13 lawrence was sufficiently impressed to commence his own research into uranium 12 uranium 235 makes up only about 0 72 of natural uranium 14 so the separation factor of any uranium enrichment process needs to be higher than 125 to produce 90 uranium 235 from natural uranium 15 the maud committee had recommended that this be done by a process of gaseous diffusion 8 but oliphant had pioneered another technique in 1934 electromagnetic separation 16 this was the process that nier had used 12 the principle of electromagnetic separation is that charged ions 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 17 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 beams 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 37 inch 94 cm cyclotron into a giant mass spectrometer for isotope separation 12 frank oppenheimer center right and 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 18 its name came from cal ifornia u niversity and cyclo tron 19 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 20 the calutron consisted of an ion source in the form of a box with a slit in it and hot filaments inside uranium tetrachloride was ionized by the filament and then passed through a 0 04 by 2 inch 1 0 by 50 8 mm 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 21 22 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 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 22 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 23 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 18 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 24 25 research edit 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 26 edward condon donald cooksey 27 a theodore forrester 28 irving langmuir kenneth ross mackenzie frank oppenheimer j robert oppenheimer william e parkins bernard peters and joseph slepian 27 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 29 30 the xax development unit at oak ridge was used for research development and training lawrence had a large cyclotron under construction at berkeley one with a 184 inch 470 cm magnet 31 this was converted into a calutron that was switched on for the first time on 26 may 1942 32 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 33 34 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 35 these were sheets of iron about 3 feet 1 m 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 33 34 the main calutron patents were methods of and apparatus for separating materials lawrence 36 magnetic shims oppenheimer and frankel 35 and calutron system lawrence 37 burhop and bohm later studied the characteristics of electric discharges in magnetic fields today known as bohm diffusion their papers on the properties of plasmas under magnetic containment would find usage in the post war world in research into controlled nuclear fusion 38 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 39 the chemists had to find a way of producing quantities of uranium tetrachloride ucl 4 from uranium oxide 40 nier had used uranium bromide 41 initially they produced it by using hydrogen to reduce uranium trioxide uo 3 to uranium dioxide uo 2 which was then reacted with carbon tetrachloride ccl 4 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 ucl 5 and phosgene cocl 2 while nowhere near as volatile as the uranium hexafluoride used by the gaseous diffusion process uranium tetrachloride is hygroscopic so work with it had to be undertaken in gloveboxes that were kept dry with phosphorus pentoxide p 4 o 10 the presence of phosgene a lethal gas responsible for 85 000 deaths as a chemical weapon during world war i required that the chemists wear gas masks when handling it 40 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 johns hopkins university and purdue university and by the tennessee eastman corporation 42 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 by the middle of 1944 there were nearly 1 200 people working at the radiation laboratory 43 design edit 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 the director of the osrd which was overseeing the project visited berkeley in february 1942 and found the atmosphere there stimulating and refreshing 44 on 9 march 1942 he reported to the president franklin d roosevelt 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 45 46 control panels and operators for calutrons at the oak ridge y 12 plant the operators mostly women worked in shifts covering 24 hours a day 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 47 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 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 48 the shasta dam area in california remained under consideration for the electromagnetic plant until september 1942 by which time lawrence had dropped his objection 49 the 25 june meeting also designated stone webster as the primary contractor for the design and engineering 50 the army assumed responsibility for the manhattan project on 17 september 1942 with brigadier general leslie r groves jr as director 51 although the army did not formally take over the contracts with the university of california from the osrd until 1 may 1943 52 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 53 54 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 51 between october 1942 and november 1943 groves paid monthly visits to the radiation laboratory in berkeley 46 reports indicated that compared to the alternatives of a gaseous diffusion plant or a plutonium producing nuclear reactor 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 55 groves cancelled the pilot plant on 14 november in favor of proceeding immediately with the production plant 56 the radiation laboratory forwarded the preliminary designs for a produ...
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