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Preparative Scale Mass Spectrometry: a Brief History of the Calutron

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Preparative Scale Mass Spectrometry: a Brief History of the Calutron ACCOUNT AND PERSPECTIVE Preparative Scale Mass Spectrometry: A Brief History of the Calutron Alfred L. Yergey National Institute of Child Health and Human Development, Bethesda, Maryland, USA A. Karl Yergey Department of History, West Virginia University, Morgantown, West Virginia, USA Calutrons were developed in the laboratory of E. O. Lawrence at the University of California at Berkeley. They were a modification of the cyclotrons he had invented and used in his Noble Prize winning investigations of the atomic nucleus. At the time their construction was undertaken, calutrons represented the only certain means of preparing enriched uranium isotopes for the construction of a fission bomb. The effort was successful enough that every atom of the 42 kg of 235U used in the first uranium bomb had passed through at least one stage of calutron separation. At peak production, the first stage separators, a tanks, yielded an aggregate 258-g/d 235U enriched to about 10 at. % from its natural abundance level of 0.72 at. %. The second stage separators, b tanks, used the 10 at. % material as feedstock and produced a total 204-g/d 235U enriched to at least 80 at. %. The latter, weapons grade, material was used in fission bombs. Under typical operating conditions, each a tank operated at a uranium beam intensity at the collectors of approximately 20 mA and each b tank at a beam intensity of approximately 215 mA at the collectors. Bulk separation of isotopes for bomb production ceased in 1945. Since that time calutrons have been used to separate stable isotopes, but on a more limited scale than wartime weapons production. Stable isotope separations since 1960 have taken place using one modified beta tank. (J Am Soc Mass Spectrom 1997, 8, 943–953) © 1997 American Society for Mass Spectrometry he purpose of this Account and Perspective is to separation in 1941, and produced 200 g of 12 at. % 235U recount the story of the development and use of by February, 1942. While this separation represents Tpreparative scale mass spectrometers, the more than a 15-fold increase in enrichment from the calutrons. The initial development of these electromag- natural abundance level of 0.72 at. %, it was only about netic isotope separators occurred as part of the Manhat- one seventh of the enrichment estimated at that time to tan Project. Since this critical war effort, the Atomic be necessary for an atomic weapon. Remarkably, within Energy Commission and its successor, the Department 18 months, 43 kg of weapons grade, i.e., $85 at. % of Energy, have used calutrons to separate isotopes that enrichment, 235U had been produced. This amount of are used for physical and biomedical research. For fissionable isotope required more than 5000 kg of nat- example, calutrons have been operated, under ISO 9000 ural abundance uranium to have passed through the conditions, to separate an isotope for a cancer treatment calutrons! protocol approved by the Food and Drug Administra- Despite the fact that the events recounted below tion. occurred more than 50 years ago, complete details of Using calutrons for peaceful purposes yields tens to how this astounding feat was accomplished cannot be hundreds of grams per year of highly enriched, physi- cally and biomedically significant isotopes which are obtained by the public. We were initially surprised at often minor components of an element’s natural isoto- our inability to have access to such materials pertinent pic distribution. However, this important and impres- to the work done during World War II. We found that sive contribution to research pales in significance to the while few, if any, of the individual aspects of the magnitude of the accomplishments in the period of calutron development and production work are classi- 1940–1945 [1]. The Y-12 project, the wartime name for fied today, aspects of the wartime work that would calutrons, originated as a concept for large scale isotope provide detailed information about production rates and operational efficiency are not available to the Address reprint requests to Alfred L. Yergey, 10 Center Drive, MSC 1580, public. When one realizes that during the recent Persian Room 6C208, Bethesda, MD 20892. E-mail: [email protected] Gulf War, calutrons were found to be part of the Iraqi © 1997 American Society for Mass Spectrometry. Published by Elsevier Science Inc. Received January 16, 1997 1044-0305/97/$17.00 Revised May 12, 1997 PII S1044-0305(97)00123-2 Accepted May 12, 1997 944 YERGEY AND YERGEY J Am Soc Mass Spectrom 1997, 8, 943–953 government’s atomic weapons development program nuclear fragments and a great deal of energy. Hahn and [2], the basic wisdom for such restrictions is apparent. Strassman’s investigations allowed them to identify This account of the development and use of radioactive barium as a product, measure an energy calutrons is a composite of both the history of a portion release of about 200 MeV and observe the production of of mid-20th century physics and the history of the additional neutrons [9]. Their colleague Lise Meitner Manhattan Project. It differs from accounts of those calculated that these results were entirely consistent areas because it considers events in either of them only with the equivalence of mass and energy [10]. Within a as they bear on calutron development. In this sense, the week of the publication of Hahn and Strassman’s work, following account is unique. For those interested in a there was wide recognition that such results might lead broader picture of early 20th century physics or the to an atomic bomb [11, pp. 252–261; 12, p. 7]. Manhattan Project, an annotated reading list is given as In this same year, Nazi Germany expanded rapidly. an Appendix. Germany annexed Austria in March and the Czechoslo- vak Sudetenland in September. By March, 1939, the Historical Background remainder of Czechoslovakia had been seized. The potential consequences of an atomic weapon in the Between 1927 and 1931 Ernest Lawrence invented the hands of the Third Reich prompted grave concerns cyclotron in order to generate protons with sufficient among a number of scientists who had fled the Nazi energy to disrupt the atomic nucleus and explore inter- regime and settled in the United States. Prominent actions between those protons and the nucleus. Theo- among this group were Leo Szilard, Enrico Fermi, retical calculations at the time estimated that 1-MeV Edward Teller, and Eugene Wigner. While the plans of protons would be required for this task. In 1927 Law- German physicists were not known to this group, it was rence saw reports of European work using a linear certain that the Germans, in principle, were very capa- accelerator to generate high energy particles. Lawrence ble of developing an atomic weapon. The group began realized that to obtain 1-MeV protons from such devices to plan ways to circumvent Germany’s acquisition of would require prohibitively long tubes. In his biogra- such a weapon. phy, Herbert Childs describes Lawrence’s next step in a Planning by the group of concerned scientists re- manner that makes it clear that the step was pure sulted in a letter from Albert Einstein to President inspiration [3, p. 139]. Lawrence realized that using a Franklin D. Roosevelt [11, pp. 305–315]. Dated August magnetic field could confine accelerating particles to 2, 1939, but not delivered until October 11, the letter circular trajectories, thereby giving all particles constant was presented to the President by his long-time infor- angular velocity. The radius of the particle trajectories mal advisor, Alexander Sachs, an economist trained in would, however, depend on their energy. The particles biology and interested in physics. The delay in deliver- underwent successive resonant acceleration through ing the letter was caused by the pressure of world the application of radiofrequency voltage across the events. Between the letter’s writing and delivery, gaps of magnet pole pieces, the “Dees.” In 1929 Law- Adolph Hitler and Josef Stalin signed the German– rence and Niels Edlefsen, his first graduate student, Soviet Non-Aggression Pact, and Germany invaded assembled a 4-in.-diameter version of the device [4]. Poland on September 1. The President’s initial response Initially this device produced 1300-eV protons. Use of a to the letter and to Szilard’s appended comments has stronger magnet led to 80-kV protons. By July, 1931, been described as one of “skeptical interest” [12, p. 14]. Lawrence had designed and constructed a 1-MeV pro- Another report states that Roosevelt immediately called ton instrument. in his aide, General Edward M. “Pa” Watson, and told In 1932 Cockroft and Walton reported that a Li target him that “this requires action” [11, p. 314]. In either bombarded with high energy protons yielded a pair of case, Lyman Briggs, then director of the National Bu- a particles accompanied by the release of 8.62 MeV [5]. reau of Standards, was directed to organize what be- Within a few months, the work had been replicated at came known as the Advisory Committee on Uranium. the University of California at Berkeley [6]. The accel- At the initial meeting of the Advisory Committee held erator results were used by Kenneth Bainbridge in on October 12, 1939, Army and Navy ordinance experts, conjunction with mass spectrometric measurements to Lt. Col. Keith F. Adamson and Cdr. Gilbert C. Hoover, demonstrate the equivalence of mass and energy [7]. were openly skeptical of the possibility of an atomic Bainbridge used his exact mass measurement of 7Li and weapon or of its possibility of being used in the present F. W. Aston’s measurements of the masses of hydrogen war [11, pp. 315–317]. Their cautious attitude out- and helium [8] for the calculation.
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