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Home , Isotope separation, Kenneth Bainbridge

Reflections on at Its Half-Century

Some Reminiscences of and the

Alfred O. Nier School of and Astronomy, University of Minnesota, Minneapolis, MN 55455

Prior to World War II there were only a few mass spectro- mg two tons and a 5 kW generator with a stabilized output meters in the entire world, and these were built by scientists voltage to power it. Whereas, in my previous apparatus (1) who used them as tools in their own research, studying the the mass spectrometer “tube” was mounted inside of a sole- dissociation and ionization of molecules by impact noid, limiting both the and radius of curva- or determining the relative abundances of . The ture of the ions, and hence limiting the obtainable mass need of the petroleum industry to find better means for resolution, in the new instrument the tube was mounted analyzing complex hydrocarbon mixtures and the between the poles of the magnet. The combination of the separation and atomic bomb production program larger possible radius of curvature and stronger magnetic (known as the Manhattan Project) stimulated the design field gave appreciably higher resolution as well as general and construction of new improved mass spectrometers. Fol- performance. Figure 1 is a schematic drawing of the mass lowing the war such instrumentation became widely avail- spectrometer tube employed (2). able commercially and applicable to a wide range of prob- Cambridge, Massachusetts, was an exciting place to be in lems such as gas analyses, use of isotopic tracers in chemical, the 1930’s for one interested in geological age determina- biological or medical science, and determination of isotopic tions. Alfred Lane, a retired professor of geology from Tufts abundance in samples of geological or cosmological interest. College, was chairman of the National Research Council’s This is an account of some of the author’s experiences during Committee on the Measurement of Geological Time, and the transformation of this rather rare form of instrumenta- very active in promoting the work of the committee. At MIT tion from an exotic tool of specialists to a device having there was Robley Evans and his group, working on the radio- universal application. activity of minerals, and at Harvard, in , there was When I completed my graduate studies in physics at Min- Gregory Baxter, the atomic weight authority and successor nesota in 1936,1 had the good fortune to receive a National to T. W. Richards. Baxter and his students had made atomic Research Council Fellowship, and elected to spend my two weight measurements on numerous lead samples extracted years at Harvard working with Kenneth Bainbridge. For my from uranium and thorium minerals and had a marvelous thesis I had measured the relative abundances of the iso- collection of these samples and ones of common lead as well topes of a few elements—argon, potassium, , rubidium, as material studied earlier by Richards. and cadmium. Bainbridge, who was an authority on the The of Uranium precise measurement of isotopic masses, suggested I might Isotopic Composition wish to extend my abundance measurements to the heavy At the time, the isotopic abundance ratio, 238U/235U, was elements, particularly lead and uranium, of interest in the not known even within a factor of two. 235U had only been determination of the ages of minerals by radioactive decay identified mass spectroscopically by Dempster (3) a few methods. He helped me design a mass spectrometer that years before, and its abundance roughly estimated from the could readily measure isotopic abundance ratios throughout blackening of a line on a photographic plate. An accurate the entire atomic table. It required an electromagnet weigh- knowledge of the isotopic abundance ratio was of great im- portance for radioactive dating. I felt that with a sample of a volatile uranium compound such as UFg a successful deter- Downloaded via CALIFORNIA INST OF TECHNOLOGY on January 18, 2019 at 18:13:40 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. mination might be made, and Lane, armed with a $500 grant he had obtained on my behalf from the Geological Society of America, looked for a prospective provider. He was unsuc- cessful and the money reverted. Fortunately, Baxter came to the rescue, providing samples of UCI4 and UBr4, which are volatile at elevated temperatures. The well-known ratio, 238U/a35U = 139 (now given more accurately as 137.8), result- ed from the measurements. Figure 2 shows a mass spectrum obtained in this early work (4). The measurements were sufficiently good to show even the rare isotope, 234U, in radioactive equilibrium with 23aU, and having an abundance of only 1 part in 17,000 in uranium. The Sector Magnet Mass Spectrometer In the fall of 1938, when my NRC fellowship expired, I was offered and accepted a faculty position back at Minnesota. Figure 1. Schematic views of 180° magnetic deflection mass spectrometer John who had been me started tube, tons are produced by emitted from a heated filament F and Tate, my adviser, helped get in research me with funds so I could passing between plates D and B before being collected on plate E (edge view my again by providing of lube). Mass spectra are obtained by sweeping the ion accelerating potential have a 2-ton magnet similar to the one 1 had at Harvard, and impressed between plates B and C. The analyzed ion currents are measured- Bainbridge allowed me to keep the mass spectrometer tubes with an electrometer tube amplifier connected to collector plate P. I had built at Harvard. As a result, I was back in operation in

Volume 66 Number 5 May 1989 385 . about 6 months and continued some of the lead-age work I after the discovery of nuclear fission, which was a lively topic had started at Harvard. of discussion at the meeting. I already knew John Dunning Now, however, there were other interests as well. Clusius at Columbia University, and he introduced me to Fermi. At and Dickel in Germany (5) had demonstrated that with a the time it had not been experimentally demonstrated that thermal diffusion column one could separate isotopes. This 235U was responsible for the slow neutron fission of uranium, seemed an interesting field to enter, especially since we had a as had been predicted by Bohr and Wheeler (8), so a direct mass spectrometer that could monitor the performance of measurement was of some interest. Because of the high cross isotope separation methods. We built a column that ulti- section for the slow neutron fission it appeared that by mately produced methane enriched by a factor of over 10 in running my 180° mass spectrometer with more intense ion 13C (6). The material was of great interest to biologists, who beams it should be possible to isolate enough 235U to make could use it for tracer studies. As a result I gained many new possible a test. friends, and my students and I, in addition to performing our Following the meeting I returned to Minneapolis, but be- own research, found ourselves producing enriched 13C and tween lecturing 8 hours per week, perfecting the sector mag- running analyses for our colleagues in physiology and bot- net mass spectrometer, trying to separate 13C by thermal any. diffusion, and continuing our study of uranium lead sam- At the time, we had one of the few mass spectrometers in ples, I was not looking for things to do, so the separation of existence capable of making precise isotope analyses. It was 235U was not high on my priority list. Dunning kept after me, clear we needed more instruments—preferably ones which as did Fermi in a letter that is one of my prized possessions, a did not use 2-ton magnets requiring 5 kW voltage-stabilized copy of which is shown as Figure 4. generators for providing the magnet current. This led to the To accomplish the separation I proposed using UF0, a development of the sector magnet mass spectrometer (7), in relatively volatile compound and now available, as a source which a 60° sector magnet took the place of the much larger of uranium that could be ionized. The Columbia group pro- one needed to give a 180° deflection. The upshot was that a vided me with some, which I tried over the Christmas holi- magnet weighing a few hundred pounds, and powered by days and into early February 1940. Unfortunately, UF0, be- several automobile storage batteries, took the place of the 2- ing a highly volatile compound, coated most of the surfaces ton magnet with its 5-kW generator. Figure 3 is a photo- of the mass spectrometer tube, including the ion collector graph of the original instrument, completed in 1939, which targets, so the tests were unsuccessful. Finally, in February I was destined to be the prototype for all subsequent magnetic built an entirely new mass spectrometer tube and went back deflection instruments, including the hundreds used on the to using UBr4, which was volatile only when heated. Fortu- Manhattan Project for a variety of purposes. nately, I had some left over from my Harvard days. An oven was built into the ion source, as had been done in a previous of 235U Mass Separation by Spectrometry isotopic study of low volatility elements (9), and the vapor I attended the American Physical Society meeting in emerging was ionized as it passed through the electron beam Washington, DC, in April 1939. This was only a few months

240 238 236 234 232 ATOMIC MASS UNITS

Figure 3. Photograph of sector magnet mass spectrometer. Shown are the magnet with the 60° sector magnet pole faces and the mass spectrometer Figure 2. Mass spectrum showing the three isotopes of . tube mounted between the poles of the magnet.

386 Journal of Chemical Education of the instrument. None of the vapor reached the collector time we had the only mass spectrometer in the entire world targets, so there was no background contamination. that could measure uranium isotope abundance ratios. In the initial runs, on February 28 and 29, 1940, two By the end of 1941 matters were more formalized, the samples of separated 235U, each of about 1.5 ng, along with Office of Scientific Research and Development (OSRD) had accompanying samples of separated 238U, were collected. come into existence, and I reported to of Co- This was enough so that when my Columbia University lumbia University, who headed the uranium program there colleagues Booth, Dunning, and Grosse bombarded the tar- and who was deeply concerned that better analytical facili- gets with slow neutrons, it was unambiguously clear that it ties were needed to monitor the various separation methods was the 235U that gave the fission fragments {10). being studied. We received funds from the OSRD to con- Subsequently, we collected larger samples, but unfortu- struct several sector instruments for performing uranium nately not large enough to permit a meaningful study of the analyses. In January 1942 Mark Inghram, a graduate stu- fast neutron fission of 235U. It would have been easy to collect dent of A. J. Dempster’s at the University of Chicago, joined much larger samples had we had a few hundred dollars to our group at Minnesota, and he and Edward Ney, one of our buy some improved diffusion pumps, but money was not undergraduates, performed most of the uranium analyses, available, so we attempted no more separations. Pressure including the critical ones that established that the gaseous buildup prevented us from increasing the ion currents to the diffusion method pursued by the Columbia University group level of which the apparatus was capable. The work was was working as predicted. The verification paved the way to picked up in November 1941 by E. O. Lawrence and his the adoption of the method and the eventual construction of colleagues in Berkeley, who converted the 37-in. to the K-25 separation plant in Oak Ridge, Tennessee. a 180° mass spectrometer with an ion source similar to that In June 1942 we sent Mark Inghram and two uranium we had employed. analysis instruments to Columbia University to take over the analyses there and Ed Ney and two instruments to Vir- Mass Spectrometers for the Manhattan Project ginia to help with the centrifuging work. During the year we Meanwhile there was a growing interest in the separation built a total of seven complete instruments for uranium of 235U by other means—the Columbia group experimenting isotope analyses, one of which was sent to the General Elec- with through porous membranes, Jesse tric Company and served as a prototype for the several dozen Beams and his colleagues at the University of Virginia with instruments they built for monitoring the performance of , and Phil Abelson and colleagues at the Naval the Oak Ridge electromagnetic and gaseous diffusion plants, Research Lab with liquid thermal diffusion. Since none of as well as other parts of the Manhattan Project, as the entire these groups had mass spectrometer facilities for checking program was now called. the performance of their enrichment systems, all of their While we were relieved in Minnesota of the uranium anal- samples were sent to us for measurement. Indeed, at the yses, other needs arose as the Manhattan Project developed. Only two companies, Consolidated Electrodynamics in Pas- adena and Westinghouse, manufactured any kind of mass spectrometer. They had gotten into the building of instru- ments for the oil industry and could not respond in a timely Columbia Cliubtrsitp way to the diverse and unusual needs of the Manhattan in thrCiinofllcm Perk Project. By the summer of 1943 we constructed about 12

D'PARTMLNT OF PrtYS’CS complete instruments for analyses in the several water . 5 shows the October 2B, 1939 heavy plants producing Figure glass spectrometer tube from one of these instruments. They, too, employed a 60° sector magnet, in this case some little permanent horseshoe magnets, purchased from Cen- tral Scientific. They were sold for demonstration purposes, Dr. Alfred 0. Nler before we use in our vacuum we had Department of Physics and could them systems, University of Minnesota to remove the red paint that was generally used on demon- Minneapolis, Minnesota stration magnets. Dear Nier: When the decision was made to build the gaseous diffu- sion plant, it was apparent that there were many difficulties Since our discussion last spring In Washington on the possibilities of using a mass spectograph separa- tion of the uranium Isotopes for deciding whether the slow neutron fission is or is not due to E35 isotope, I have convinced myself that this Is actually the best way to decide the question, which Is of a considerable theoretical and possibly practical Interest. I understand that you have lately undertaken such a separation, and I should very much like to know whether and how this work Is progressing. Please give my best regards to Professor Tate. Yours sincerely,

EF:L

Figure 4. October 28, 1939, letter from encouraging the separa- Figure 5. Photograph of glass mass spectrometer tube used for determining tion of 23SU. deuterium concentration in separation plants.

Volume 66 Numbers May 1989 387 the impurities in the process stream in over 50 locations. Initially, we tried to avoid using mass spectrometers since it was still a time when operating a mass spectrometer was more of an art than a science, so it seemed too complicated a method. However, when we were told it had not been decid- ed what refrigerants were going to be used in the cooling systems, and hence what impurities might appear, it seemed safest to employ mass spectrometers, since they had greater versatility than any other type of instrument. We therefore mass sam- Figure 6. Strip chart record obtained from mass spectrometer monitoring developed still another style of spectrometer and impurities in process gas stream in Oak Ridge, Tennessee, gaseous diffusion pling system (12). Because of the criticality of the monitor- plant used for separating 235U. The components monitored included 02, N?, HF, ing, spare instruments were installed in each position, so COj, fluorocarbon fragment CF3, and UFs. The N2 (trace G) was about 6% and over 100 instruments were constructed under contract by increased momentarily to about 8%, as shown, due to a release of nitrogen . Each instrument had its own strip chart during a plant operation procedure. recorder, with a slave recorder mounted in a central control room, and the entire huge plant could be monitored by a to be overcome if the plant was to be successful. Not the least single individual. Figure 6 shows a section of strip chart of these was that it had to be vacuum tight, since it operated record from one of the instruments. N2,02, HF, CO2, CF3 (as below atmospheric pressure and employed UF6 as a process a measure of fluorocarbons), and UF6 were followed. At the gas. UF6 reacts with water, and the nonvolatile compound time it was the largest single installation of mass spectro- formed would plug the diffuser membranes, making the meters ever attempted, and I suspect it has not been plant inoperative. The leaking in of air, always carrying a matched since. certain amount of water was to be minimized at all vapor, Conclusion costs. Conventional leak testing methods were not sensitive enough to meet the stringent requirements we faced. In retrospect, the five years I devoted to the program were On one of my trips to Columbia University in 1942 the interesting and exciting. Our small development group con- topic came up, and John Dunning, Eugene Booth, and I sisted almost entirely of individuals under 25 years of age. (I discussed the possibility of using a portable mass spectrome- was the oldest, 34, when the war ended.) Our background ter as a leak detector, with helium as a probe gas, for check- was the study of the bombardment of and molecules ing components and the assembled plant as it was being put by electrons and the determination of the relative abun- together. The contract for building the big diffusion plant dances of isotopes—hardly subjects considered to be of had been given to the M. W. Kellogg Co., in nearby Jersey much practical importance in the “real” world. We filled a City and, by coincidence, the chief designer of the process void that had to be filled, and could only be filled by individ- system was Manson Benedict, an old friend of mine from uals who had the basic knowledge necessary to master prob- Harvard days. Robert B. Jacobs, the man employed by Kel- lems that were new and beyond normal experience. logg to develop leak testing procedures for the plant, was This was but one of many examples of the importance of also a close friend from Harvard days, so communications basic science to national defense and welfare. I remember in between Minnesota, Columbia, and Kellogg were very late 1945 talking with Captain Conrad, who headed the Na- smooth. vy’s Office of Research and Invention, as it was called. He In any event, I went home and promised to look into the pointed out the realization by the military of the role civilian development of a portable mass spectrometer. Within a few scientists had played in the war effort when called upon to months we had constructed a prototype. The original instru- apply their broad basic knowledge to problems of specific ment employed a glass mass spectrometer tube and glass military importance. It was this realization that led to the diffusion pump very similar to those used for the hydrogen creation of the Office of Naval Research, which began the analyses. Four of the instruments were built in a few months, large-scale support of basic science in the universities and and three were put in the hands of groups in the diffusion other appropriate institutions and served as a model for program concerned with vacuum leak problems. The fourth subsequent government support programs such as those of was sent to General Electric to serve as a prototype for the the National Science Foundation. hundreds were to build as the many they project developed. Literature Cited Before the actual production was begun, our group had re- tube and 1. Nier, A. O. Phys Reu. 1936,50,1041. placed the glass spectrometer diffusion pump by 2. Nier, A. O. Phys Reu. 1937,52,933. all-metal versions (11). 3. Dempster, A. J. Phys. Reu. 1938,53,64. 4. A. 1943 it was clear that there were other Nier, O. Phys. Reu. 1939,55,150. By early problems 5. Clusius, K.; Dickel, G. Naturwiss. 1938,26, 546. in the diffusion plant where mass spectrometry could play a 6. Nier, A. O.; Bardeen, J. J. Chem. Phys. 1941,9, 690. and I agreed to move to New York to head an instru- 7. Nier, A. O. Reu. Sci. Instr. 1940,11, 212. part, 8. Bohr, N.; Wheeler, J. A. Phys. Rev. 1939,56,426 and 1065. ment development laboratory for the Kellex Corporation, as 9. Nier, A. O. Phys. Reu. 1938,53,282. the subsidiary of the Kellogg company concerned with the 10. Nier, A. O.; Booth, E. T.; Dunning, J. R.; Grosse, A. V. Phys. Rev. 1940,57, 546. 11. Nier, A. O.; Stevens, C. M.; Hustrutid, A.; Abbott, T, A. J. Appl. Phys. 1947,18,30. was called. Our diffusion plant construction biggest single 12. Nier, A. O.; Abbott, T. A.; Pickard, J. K.; Leland, W. T.; Taylor, T. I.; Stevens, C. M.; assignment was to develop an on-line system for monitoring Drukey, D. L.; Goertzel, G. Anal. Chem. 1948,20, 188.

388 Journal of Chemical Education