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13io,qraphicat Memoirs VOLUME 76

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SAMUEL KING ALLISON November I3, I900-September I5, 1965 BY RO GER H. HILDEBRAND SAMUEL K. ALLISON began his professional life at a time of intense interest in the properties en c! interactions of X rays. His contributions to the field! were immecliately recog- nizec! by the scientific community en c! especially by A. H. Compton, who was responsible for bringing him back to his alma mater, the University of Chicago. It was also near the time when Cockroft-Walton accelerators en c! then Van cle Graaff machines began producing beams of protons en c! cleuterons. His contributions to nuclear en c! atomic phys- ics, using these accelerators, were well recognizec! cluring his lifetime, but they have grown in significance with the emergence of new fielcis, especially nuclear astrophysics. FAMILY AND EARLY YEARS Allison always regarclec! himself as a product of the Uni- versity of Chicago en c! its surrounding community, Hyde Park. He attenclec! the John Fiske Grammar School en c! Hycle Park High School. His father Samuel Buell Allison was the principal of an elementary school in the Chicago Public School System. The family owner! one of the first automobiles in the neighborhood. When school was out they wouic! drive with their friencis to the family summer home near Three Lakes, Wisconsin. There young Sam cle 3

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4 BIOGRAPHICAL MEMOIRS velopec! a love of the North Woocis, which continues! through- out his life en c! lee! in his aclult years to strenuous canoe trips into the Canaclian wilclerness with friends, inclucling his clistinguishec! colleagues William H. Zachariasen en c! John H. Williams. Allison enrollee! in the University of Chicago in 1917. As he later reminiscec! for the benefit of his younger colleagues, it was a time when attendance at chapel was compulsory. He competec! on the varsity swimming en c! water basketball teams while cloing honors work in chemistry en c! mathematics. He was introclucec! to quantum theory by R. A. Millikan, one of the university's first great teachers, and graduated in 1921. Two years later he receiver! his Ph.D. in chemistry uncler W. D. Harkins. His dissertation was on "Atomic Sta- bility III, the Effects of Electrical Discharge and High Tem- peratures. " His performance in Harkin's laboratory earnec! him an appointment as a National Research fellow at Harvarc! (1923- 25~. From there he went to a fellowship at the Carnegie Institution in Washington (1925-26) en c! then to a faculty appointment at the University of California, Berkeley, where he acivancec! from an instructorship to an associate professor- ship (1926-30~. While at Berkeley he married Helen Campbell. Their chilciren Samuel en c! Catherine were born in Chicago after the family mover! permanently to Hycle Park. X RAYS Except for a brief introduction to nuclear physics at the Cavenclish Laboratory (to be cliscussec! later), Allison's prin- cipal research from the time of his graduation until he returnee! to Chicago in 1935 at the invitation of A. H. Compton was in the properties and interactions of X rays by means of precision spectroscopy. It was a time when X rays were the primary means of studying the atom.

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SAMUEL KING ALLISON 5 Allison later sail! that he was hirer! at Chicago because "the university neeclec! a chemist, I was available, en c! the records showed that I usually operated well within my break- age allowance." A review by Robert S. Shanklanc! gives a different perspective of Compton's invitation to Allison: In Professor Wm. Duane's laboratory at Harvard, [Allison] became involved in the famous controversy between Duane and Arthur H. Compton on the validity of the X-ray scattering experiments that were basic for the "Compton effect." Compton's now classic experiments conducted at Washington Uni- versity in St. Louis had been challenged by several X-ray physicists, includ- ing C. G. Barkla and Bergen Davis, but especially by Duane, for they were in conflict with the accepted classical theory of X-ray scattering of Profes- sor Thomson. Duane had interpreted the experiments carried on in col- laboration with students in his laboratory as being adequately explained as "tertiary radiation" produced from carbon and oxygen in the box enclos- ing the X-ray tube by impact of photoelectrons ejected by the primary X rays. Compton, however, had explained his results by the quantum theory- by no means accepted at that time. When Allison joined Duane's group at Harvard, the experiments were repeated with greater care and precision, and the earlier results were shown to be due to secondary X rays produced by scattering of the primary beam by the walls of the box [1925]. When these definitive results were [ob- tained], Professor Duane strongly supported Compton's work at the next meeting of the American Physical Society. The close lifelong association of Allison and Arthur Compton began at this time. The best-known result of the collaboration between Compton en c! Allison was their book X Rays in Theory and E'cteriment (1935), which servec! as an authoritative refer- ence for many years. Much of Allison's major work in X rays was facilitates! by his clesign en c! construction of a high- resolution clouble-crystal spectrometer. He chose John H. Williams, one of his first students at Berkeley, to be his collaborator in that project. Allison applier! the instrument to measurements of unprececlentec! accuracy of the widths en c! intensities of X-ray lines. Among the results was his confirmation of the clynamical theory of X-ray diffraction

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6 BIOGRAPHICAL MEMOIRS by C. G. Darwin en c! P. P. Ewing. He proviclec! the crucial measurements en c! pointer! out funciamental errors in ear- lier theories. He also renclerec! a physical interpretation to relate the rather complex mathematical treatment to the experimental results. WARTIME ACTIVITIES During the war years Allison took on a series of responsi- bilities. He was a consultant to the National Defense Re- search Council (October 1940 to January 1941) en c! then was a member of the Uranium Committee of the Office of Scientific Research en c! Development January 1941 to {anu- ary 1942~. In January 1942 he became director of the Chem- istry Division of the Metallurgical Laboratory, then chair- man of the Project Council, en c! finally director of the laboratory June 1943 to November 1944~. This was the laboratory that first achiever! the controller! release of nuclear energy (December 2, ~942). Alvin Weinberg, once a student in Allison's class in elec- tricity en c! magnetism en c! later director of Oak Ridge Na- tional Laboratory, was among the scientists in the Metallur- gical Laboratory. At a memorial service for Allison in 1965 he clescribec! Allison's work in the laboratory in these worcis: Sam Allison's contribution to the controlled release of nuclear energy went much beyond holding people's hands and submerging his own technical aspirations to the interest of his country and of mankind. He did the earli- est experiments on the multiplication of neutrons in a beryllium-moder- ated chain reactor here at Chicago even before the Metallurgical Labora- tory was begun. [His relatively small exponential pile came closer to the critical value, k = 1, than was achieved by the Fermi group, then at Colum- bia.] This work has remained of fundamental interest, and serves now as the basis for certain major lines of nuclear reactor development both in the United States and abroad. His was the first experimental group at the newly formed Met Lab, and indeed was the nucleus of the wartime lab [around which grew] the final 3,000-man institution.

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SAMUEL KING ALLISON 7 Weinberg clescribec! Allison's administrative burdens in the laboratory as follows: The laboratory had its giants Enrico Fermi and Arthur Compton, and Leo Szilard, and Eugene Wigner; it had its pessimists and bureaucrats; and it had a lot of somewhat bewildered young people undertaking their first scientific jobs. It was Sam Allison who, with his extraordinary patience and insight, kept this disparate crew focused on the main job, which was to achieve success ahead of the Nazi competitors. If the project was faced with a technical crisis, as when the multiplica- tion factor appeared too small to sustain a chain reaction, or when the canning of the uranium slugs seemed to be impossible; or if the project was confronted with a personnel crisis as when the most senior and desper- ately needed physicist handed in his resignation, it was always Sam Allison upon whom much of the burden fell, and it was he, with his gentle and appropriate humor and technical knowledge who saved the day. By the enc! of 1944 the center of activity mover! to Los Alamos en c! Allison was caller! on to go there as chairman of the Technical en c! Schecluling Committee (November 1944-lanuary 1946~. When the first atomic crevice was ex- ploclec! in the desert at Alamogorclo, New Mexico, in July 1945, it was Sam Allison's voice that was heart! counting clown the last seconds before the explosion. That count- clown receiver! a great clear of attention in descriptions of the event, en c! Allison jokes! that he became famous for his ability to count backwards. In a ceremony at the University of Chicago on January 12, 1946, he was awarclec! the Mecial of Merit by Major General Leslie R. Groves. President Harry S. Truman signet! the citation. POSTWAR SCIENTIFIC LEADERSHIP The Mecial of Merit ceremony market! the ens! of his official cluties at the Metallurgical Laboratory en c! the be- ginning of a new phase of public service, administrative accomplishment, en c! scientific success. He was an eloquent en c! effective spokesman in the drive for civilian control of

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8 BIOGRAPHICAL MEMOIRS atomic energy and a staunch defender of individuals under attack during the "Red scare" led by Senator McCarthy. Allison became the first director of the Institute for Nuclear Studies (now the Enrico Fermi Institute), a peacetime suc- cessor to the Metallurgical Laboratory and among the first interdisciplinary institutes. The Institute for Nuclear Stud- ies was formed on the conviction inspired by the wartime example that physicists, chemists, and astrophysicists could benefit by working together. Among the senior members were Enrico Fermi, Willard Libby, Joseph and Maria Mayer, Leo Szilard, Edward Teller, Harold Urey, and later S. Chandesekhar and Gregor Wentzel. The younger faculty included Richard Garwin, Marvin Goldberger, Murray Gell- Mann, Yoichiro Nambu, Eugene Parker, John Simpson, Nathan Sugarman, Anthony Turkevich, and Valentine Telegdi. The students of that era included James Cronin, Jerome Friedman, T. D. Lee, Jack Steinberger, and C.-N. Yang. It was an array of talent seldom, if ever, matched by any labo- ratory in any decade. At a luncheon in the Shoreland Hotel announcing the creation of the institute, Allison fired the opening gun In the struggle against continuation of military censorship, when he said, "We are determined to return to free research as before the war. If secrecy is imposed on scientific research in physics, we will find all first-rate scientists working on subjects as innocuous as the colors of butterfly wings." This speech, delivered at the founding of a prominent institute, caught the attention of a wide audience and was credited with hastening the re-establishment of open scientific in qulry. ~ NUCLEAR AND ATOMIC PHYSICS Allison's contributions to nuclear physics began in the mid-1930s while he was visiting the Cavendish Laboratory

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SAMUEL KING ALLISON 9 as a Guggenheim fellow. In a paper presenting the results of his "Experiments on the Efficiencies of Procluction en c! the Half-Lives of Radio-Carbon and Radio-Nitrogen," he thanked! "Dr. l. D. Cockroft for instruction in the use of the high-voltage apparatus at the Cavendish Laboratory Eand] Lorc! Rutherforc! for permission to work in the laboratory." When he returnee! to Chicago he built his own Cockroft- Walton accelerator in Eckhart Laboratory, home of the Physics Department. He soon hac! some five students measuring the energies of particles produced in lithium targets bom barclec! with protons en c! cleuterons. lust as this work was achieving its initial success it was interruptec! by war. When he was free to return to the fielcI, he reconstructec! the accelerator in the new Research Institutes Builcling, which hac! just been built to house the Institute for Nuclear StucI- ies. He caller! his accelerator the "kevatron" to emphasize its moclest peak energy (400 Key) at a time when his associ- ates were builcling machines in the million- ant! then bil lion-voTt range with names like "cosmotron" en c! "bevatron." The kevatron stool! on the basement floor of the builcling, extenclec! through a very large hole in the first floor, en c! reacher! almost to the level of the seconc! floor. Access to the ion source was by way of a plank thrown across the gaping hole some 10 feet above the basement floor. His students tell of hair-raising adventures in coping with that feature of the laboratory. The high-voltage apparatus was operates! from an adjacent room with a haywire but smoothly efficient rig of mirrors, pulleys, en c! strings culminating in an array of broomsticks you turner! the brooms that pullet! the strings that worker! the levers that macle the beams. The research hac! two objectives: the study of Tow energy nuclear reactions inclucec! by light projectiles (protons, cleu- terons, helium ions, lithium ions) en c! the eluciciation of the phenomena associates! with the interaction of atomic

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0 BIOGRAPHICAL MEMOIRS en c! ionic beams with matter, in particular the energy Toss en c! the capture en c! loss of electrons by the beam particles. A by-product of the research effort was the clevelopment of sophisticated apparatus for the production of monoenergetic beams of particles en c! for the precise measurement of their energy. Allison's postwar studies of Tow-energy nuclear reactions in light nuclei were concernec! at first with the energy re- lease as cleterminec! by measurement of the kinetic energy of the reaction products. These studies incluclec! measure- ments of the energy levels of unstable reaction products, such as 7Be, i3B, i5C, en c! i7N. These light nuclei en c! the reactions leacling to their formation later prover! to be of great cosmological significance because of their role in the procluction of stellar energy en c! in nucleosynthetic pro cesses. In the kevatron, Allison's projectiles were protons or deu- terons, the targets were lithium, beryllium, en c! boron. The reaction products were stucliec! with his electrostatic or mag- netic analyzers. Later, Allison acquirer! a 2-MeV Van cle Graaff accelerator, which he equipped to accelerate lithium ions to energies sufficient to cause nuclear reactions in light nuclei. With his moclest apparatus, first the kevatron en c! then the Van cle Graaff, he was an early pioneer in a field! of research that wouic! later be known as "heavy ion phys- ics." His projectiles were too light to qualify as heavy ions by moclern stanciarcis, but they were heavier than conic! be fount! in other laboratories of that era. Edwin Norbeck, then one of Allison's students, clescribec! the venture into lithium projectiles as follows: By 1953 it was difficult to come up with good nuclear physics experiments that could be done with a low-energy accelerator. I remember a brain- storming session he had arranged to uncover promising projects. The con- clusion of the meeting was that any new experiment would be difficult,

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SAMUEL KING ALLISON 11 either because it required high precision, had a low cross-section, or used exotic beams or targets. After this meeting Prof. Allison and I met in his office to discuss the situation. He recalled seeing an article, published many years earlier in Review of Scientific Instruments, that described a method for making a beam of lithium ions: The authors, l. P. Blewett and E. l. ~ones, had produced lithium ions by heating the lithium aluminum silicates, spodumene and beta-eucryptite, on a filament of platinum gauze. Eucryptite gave twice as much lithium current as spodumene. Allison contacted friends who were geologists and soon we had some spodumene, a semiprecious jewel, and then some alpha- eucryptite. These natural minerals gave good ion currents, but soon we were making our own beta-eucryptite using separated isotopes. We put the source in a Van de Graaff accelerator and brought out a 1.2-MeV 7Li beam. This was more difficult than it sounds, but Allison had a good solution to every problem that arose. When the big day came to bring out the beam, we had a variety of detectors. If there were any nuclear reactions at such a low energy we wanted to be sure that we would not miss them. We had a gamma ray detector and a neutron survey meter. We used a thick target of LiF in a chamber with a thin window on one side. Outside the thin window we had a phototube coated on the end with a ZnS phos- phor and covered with a thin aluminum foil. When the beam hit the target I was pleased to see lots of gamma rays and neutrons, but what caught Prof. Allison's attention were the charged particles. He put a sheet of paper in front of the ZnS and found only a slight reduction in the counting rate. He commented that such a large number of high-energy protons could only come from the reaction 7Li(7Li,p) i3B. He then noted that the only trouble with that explanation was that the nucleus i3B [was not supposed] to exist. The discovery of this nucleus was only the beginning. It was soon follower! by further studies of lithium-inclucec! nuclear reactions. The stucly of reactions with lithium beams was a new branch of nuclear physics. Even with a maximum beam energy of only 2 MeV, the Van cle Graaff accelerator conic! be user! to stucly reactions of 6Li en c! 7Li with all of the stable isotopes of Li, Be, B. C, N. en c! 0. The lithium ions proclucec! nuclei far from stability, of which i3B was the first example. Reactions observer! at energies near or below

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12 BIOGRAPHICAL MEMOIRS the Coulomb barrier incluclec! "fusion-like" processes such as 7Li(7Li,p)~3B en c! 9Be(7Li,p)~5C en c! "stripping or trans- fer" processes such as 9Be(7Li,~Li)~Be. Measurements of the products of various reactions macle it possible to determine the masses of the grounc! en c! low-lying exciter! states of i2B, i3B, i5C, en c! i7N. The last of his nuclear studies in- volvec! eTuciciation of the mechanisms of complex reactions such as 6Li + 6Li yielcling three alpha particles, en c! investi- gation of the role of intermediate nuclei (e.g., ~Be) in these reactions. Using ciata on 9Be(7Li,~Li)~Be from an experiment by Norbeck et al. at the University of Minnesota, Allison calculates! the neutron density out to 40 Em. The words "halo nuclei," now in common use, clic! not appear until much later. Allison introclucec! the precision techniques he hac! cle- velopec! for nuclear reaction spectroscopy to stucly the in- teraction of particles with matter. He commentec! that everyone wan tee! quantitative information about the pas- sage of beams through matter, but no one wantec! to make the measurements. Using the apparatus clevelopec! for pre- cise determination of the energies en c! products of nuclear reactions he en c! his associates were able to measure the changes in energy, the "stopping power," and the charge- changing cross-sections as a function of energy, ionic spe- cies, en c! stopping material. The early work on the energy loss of slow protons, cleuterons, alpha particles, en c! Li6 nuclei passing through thin aluminum and gold films was pioneering en c! establishec! Allison en c! his collaborators as the leaclers in this fielcI. The work was extenclec! to gaseous targets. The results of the measurements of cross-sections for electron capture and Toss in hydrogen and air were out- stancling. This work was follower! by extensive studies of helium ions in gasses where neutral atoms and both the

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SAMUEL KING ALLISON singly en c! cloubly charger! ions coexist. The work was then extenclec! to 2-MeV lithium. In this atomic beam work Allison was without peer. The review article "Passage of Heavy Particles Through Matter" by Allison en c! Warshaw (1956) was the clefinitive work on stopping powers for at least a clecacle. The measurements of atomic capture cross-sections became important in appli- cations, such as neutral injection into plasma machines en c! procluction of H- ions in tandem Van cle Graaff machines. In the experiments on light nuclei it was often necessary to subtract a background! clue to a contamination of the targets by clecomposec! pump oil. Allison iclentifiec! the un- welcome scattering nuclei by measuring the difference in energy between the incident en c! recoiling projectiles. That experience lee! him to suggest to his colleague Anthony Turkevich that this technique couic! be user! to analyze sur face materials where conventional chemical analysis was not feasible. Turkevich en c! his colleague Anthony Tuzzolino built an instrument on this principle using the recently clevelopec! silicon detectors. Their scattering analysis instrument was carrier! to the moon on the last three Surveyor missions en c! macle the first chemical analyses of the lunar surface. More recently, a successor to that instrument built by Tom Economu has analyze c! the surface of Mars. STUDENTS Among Allison's major interests was the training of Ph.D. cancliciates in the techniques of research. Tociay many of his students pursue clistinguishec! careers, in some cases working in fielcis far remover! from their thesis problems. They recall his gift for making h are! things clear en c! his emphasis on putting effort where it counts, a point he drove home with a turn of phrase: "If it's not worth cloing, it's not

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4 BIOGRAPHICAL MEMOIRS worth cloing well." His numerous overseas contacts resultec! in a flow of foreign students en c! postclocs. George Morrison, a postcloc who playact a leacling role in the work with lithium at the Van cle Graaff, relates, "In Looking back, I have to say that my period at Chicago was the most rewarding and enjoyable research time of my life. . . Lithium beams, even at 2 MeV were opening up new physics en c! there was Sam himself encouraging, ebullient, luminous, en c! larger than life. " James Cronin began working in Allison's laboratory when he was still uncertain about what sort of physics to clo, en c! Sam Allison's personality played a dominant role in his de- cision to clo a thesis on nuclear physics. He says, "Sam was easy to work with, but Ehe] hac! his subtle ways of pushing his students. One Christmas, while I was away visiting my family, Sam built a proportional counter detector for my thesis experiment. It was clone complete with a flowing gas system en c! a preamplifier. This shower! his impatience with my slowness (anc! even reticence) to built! this particular piece of equipment." On Memorial Day weekends Allison brought his students en c! staff to his cabin in the North Woocis. Everyone was expecter! to help clear brush en c! wincifall accumulates! over the winter, en c! Leo Herzenberg was among those who learner! on those occasions to pacicIle a canoe, catch a fish, en c! wield an ax. Recalling an incident that was typical of Sam Allison's style, Herzenberg recounts, "One of the graduate students was attempting to cut clown a small tree. He kept swinging the ax with much energy but harcIly scratching the bark with each stroke. After a while he just stool! there, covered with sweat, with a Took of extreme frustration. Allison came over, took the ax, en c! with a single seemingly effort- less swing cut right through the tree. The student stood

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SAMUEL KING ALLISON 15 there, mouth wicle open, en c! askocI, "How clic! you clo that?" Allison, repliecI, "Fifty-seven years of experience!" LAST YEARS Allison went to CuTham, EnglancI, near Oxford, in 1965 as the U.S. clelegate to the Plasma Physics en c! Controller! Nuclear Fusion Research Conference sponsorec! by the In- ternational Atomic Energy Agency. He cliec! there of com- plications following an aortic aneurism on September 15, ~ 965. In a memorial service at Chicago, William H. Zachariasen commentec! on Allison's last years en c! on the character of his life in words that provide a fitting conclu- sion to this memoir. Despite heavy demands on his time by other duties in postwar years, Sam continued as an active scientist and teacher. But the combination of ad- ministrative duties and personal research taxed his strength in increasing measure as he grew older. When he resigned as director of the Fermi Institute in 1957, he felt relieved and looked forward with anticipation to many years of fruitful scientific inquiry under less stressful conditions. However, two years [before his death] his colleagues in the Fermi Institute appealed so strongly to Sam's sense of duty that he reluctantly agreed to serve yet another term. Surely . . . a younger man should have been found to do the job so that Sam, who had already given so much unselfish service, could have been spared this burden. Sam had a good life. He was at peace with himself and with the world, and he had much happiness at home and in his work. He had a simple approach to his research. The only motivation was the job and excitement of satisfying intellectual curiosity. He had no thought of other rewards. However, . . . Sam was pleased and somewhat surprised that fellow scientists had such high opinions of his work. While he tended to belittle his own accomplishments, he was most liberal in praising those of other workers in the same field . . . [He was] a great and noble man. AM GRATEFUL TO many of Allison's friends, family members, stu- dents, and colleagues who have contributed material to and com- mented on drafts of this memoir. Among these are James Cronin,

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6 BIOGRAPHICAL MEMOIRS Carol Herzenberg (Caroline Littlejohn)), Leo Herzenberg, Tanera Marshall, George Morrison, Paul Murphy, Edwin Norbeck, Gilbert Perlow, Tohn Schiffer, Tohn Simpson, and Anthony Turkevich. I have used copies of the tributes by H. L. Anderson, R. S. Shankland, A. Weinberg, T. H. Williams, and W. H. Zachariasen, and excerpts from anonymous notes, possibly by N. Sugarman, found in the files of the Enrico Fermi Institute. I have also used material from a booklet "Samuel K. Allison: The Frank P. Hixon Distinguished Ser- vice Professorship," edited by C. Daly (University of Chicago Devel- opment Office). I have given all of the documents used in prepar- ing this memoir to the Special Collections Department of the University of Chicago's Toseph Regenstein Library, which was an additional source.

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SAMUEL KING ALLISON SELECTED BIBLIOGRAPHY 1925 17 With W. Duane. On scattered radiation due to X rays from molyb- denum and tungsten targets. Proc. Natl. Acad. Sci. U. S. A. 11:25-27. 1927 The reflection of X rays by crystals as a problem in the reflection of radiation by parallel planes. Phys. Rev. 29:375-79. 1929 With J. H. Williams. Design of a double X-ray spectrometer. 7. Opt. Soc. Am. 18:473-78. 1935 With A. H. Compton. X Rays in Theory and Experiment. New York: D. Van Nostrand Company. Experiments on the efficiencies of production and the half-lives of radio-carbon and radio-nitrogen. Camb. Phil. Soc. Proc. 32:179-82. 1939 The masses of Li6, Li7, Be8, Be9, Bin, and Bit. Phys. Rev. 55:624-27. 1956 With S. D. Warshaw. Passage of heavy particles through matter. Rev. Mod. Phys. 25:779-817. With P. G. Murphy and E. Norbeck, Jr. Mass of Bi3 from the nuclear reaction Li7 (Li7,p) Bi3. Phys. Rev. 102:1182-83. With C. S. Littlejohn. Stopping power of various gasses for lithium ions of 100-450 KeV kinetic energy. Phys. Rev. 104:959-61. 1958 Experimental results on charge-changing collisions of hydrogen and helium ions at kinetic energies above 0.2 Key. Rev. Mod. Phys. 30:1137-68. 1960 Classical analysis of the reaction Be9(Li7,Li8)Be8. Phys. Rev. 119:1975-81. With J. Cuevas and M. Garcia-Munoz. Experiments on charge-changing collisions of lithium ionic and atomic beams. Phys. Rev. 120:1266-78.