ROBERT GREEN SACHS

May 4, 1916–April 14, 1999

BY KAMESHWAR C. WALI

ON SACHS’S EIGHTIETH BIRTHDAY on May 4, 1996, Carolyn Sachs said, “Bob has three loves in his life: family, sailing, and physics.” She went on to say, “However much I wanted, I never had him declare his order of preference, because I was afraid of the outcome.” Indeed for the world of physics at large, although Sachs was known as an avid sailor and a loving patriarch of a large family, the highlight of his life was his loyalty and commitment to physics. Over the course of his scientific career, which spanned six decades, he made fundamental contributions to a wide- ranging area of basic research, including atomic, nuclear, and particle physics. A conscientious and meticulous teacher, Sachs guided the research of several graduates from the University of Wisconsin and the University of Chicago who have made their own mark in research.

To speak in broad terms Sachs believed strongly that “it is experiment what defines physics,” not pure thought or pure mathematics however elegant and beautiful these may be. Secondly, the underpinning of a physical model or a physical theory should be the most general, well-established principles to derive constraints and construct models to test against experiments and provide new tests for further



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ROBERT GREEN SACHS May 4, 1916–April 14, 1999 BY KAMESHWAR C. WALI O N SACHS’ S EIGHTIETH BIRTHDAY on May 4, 1996, Carolyn Sachs said, “Bob has three loves in his life: family, sailing, and physics.” She went on to say, “However much I wanted, I never had him declare his order of preference, because I was afraid of the outcome.” Indeed for the world of physics at large, although Sachs was known as an avid sailor and a loving patriarch of a large family, the highlight of his life was his loyalty and commitment to physics. Over the course of his scientific career, which spanned six de- cades, he made fundamental contributions to a wide- rang- ing area of basic research, including atomic, nuclear, and particle physics. A conscientious and meticulous teacher, Sachs guided the research of several graduates from the University of Wisconsin and the University of Chicago who have made their own mark in research. To speak in broad terms Sachs believed strongly that “it is experiment what defines physics,” not pure thought or pure mathematics however elegant and beautiful these may be. Secondly, the underpinning of a physical model or a physical theory should be the most general, well-established principles to derive constraints and construct models to test against experiments and provide new tests for further 321

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322 BIOGRAPHICAL MEMOIRS measurements. He (his students as well) avoided models based on extensive numerology and numerical analysis, which ultimately did no more than testing the general principles. Sachs characterized his approach as “phenomenological theory,” and over the years with this methodology he and his students worked on groups of problems, each group spanning a period of years but overlapping to some extent either in time or content or both. There have been three such major groups of research activity: (1) electromagnetic interactions of nuclei and their constituents, the nucleons (neutrons and protons); (2) resonances and unstable par- ticles; and (3) time reversal and CP violation. Although Sachs’s primary interest was teaching and re- search, when needed and called upon, he offered himself in service to the physics community. He was recognized for his contributions to questions relating to national and in- ternational energy policies, for his services to high-energy physics panels, and for his successful efforts in creating the Division of Particles and Fields. He served as associate labo- ratory director at Argonne National Laboratory (1964-68) and was in charge of its high-energy experimental research program at the new and then the most powerful 12 GeV accelerator, the Zero Gradient Synchrotron (ZGS). He later served as the director of the laboratory (1973-79). He also put in two terms as the director of the Enrico Fermi Insti- tute (1968-73 and 1983-86). Bob Sachs was born on May 4, 1916, in Hagerstown, Maryland, but his family moved to Baltimore, Maryland, in 1921, when Bob was five years old. Bob’s grandfather came originally from Russia. His family name was Schabershovsky. When he married one of the seven daughters (and no sons) of a man with the family name of Sachs, he was adopted by his wife’s family and acquired the name Sachs. Bob’s fa- ther, Harry Maurice Sachs, was the oldest in a family of

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323 ROBERT GREEN SACHS seven siblings from his father’s second marriage after the death of his first wife. Bob’s mother, Anna Green, was the daughter of an upper-middle-class Jewish family in Rich- mond, Virginia. Anna’s father was an Austrian by birth and her mother was a Lithuanian. Anna married Harry Sachs when she was only 16. Her father was strongly opposed to the marriage because she was too young to marry and, secondly, Harry, who came from a small town, did not measure up to his expectations for a son-in-law. Anna married without his knowledge, but with complicity of her mother, which led to her parents’ divorce. She remained estranged from her father for the rest of her life, but remained close to her mother. In Balti- more Harry Sachs worked in the city printer’s office during the day and took night classes and eventually got a law degree from the University of Maryland. Anna was an ac- complished singer, who sang professionally and during ser- vices at the temple Oheb Shalom to supplement the family income. These were difficult days for the Sachs family as Bob was growing up. Bob Sachs had his elementary education at John Eager Howard School number 61. After completing his high school education at the City College in Baltimore he entered Johns Hopkins University in 1933. By then he had realized that physics was his true calling and opted for a straight six-year Ph.D. degree course, bypassing the usual bachelor’s and master’s degrees. He began his graduate research as a stu- dent of James Franck and Maria Göpert-Mayer. Officially his thesis supervisor was Göpert-Mayer, but Edward Teller at the nearby George Washington University was his true mentor. At Teller’s suggestion Sachs worked on the neu- tron-proton interaction potential in deuteron, which led to his first publication (1938). The values of the parameters of the Yukawa potential that he determined by solving the

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324 BIOGRAPHICAL MEMOIRS deuteron problem numerically and fitting them to the data proved to be extremely useful in nuclear physics of the time. They were used extensively until the advances in me- son theory and the modification of the potential in light of new experiments. After his graduation, during the years 1939-41, Sachs continued his association with Edward Teller as a research fellow at George Washington University. He worked closely with him in research on a great variety of physics, including atomic, molecular, and solid-state physics, as well as nuclear physics. Among several papers during that period there were two that had great impact. One of them established a very general result for the ratio of the frequencies of the trans- verse and longitudinal polar waves in polar crystals in the long wavelength limit (1940). It is known as the Lyddane- Sachs-Teller theorem in solid-state physics literature. The other had to do with the general problem of scattering of slow neutrons by molecular gases (1941). At the time, the results were of interest mainly in determining neutron cross- sections from measurements on molecular gases, but later they played a vital role in the study and analysis of neutron dynamics in gas-cooled reactors that became a part of the Atomic Energy Commission program. In 1941 Sachs had spent just four months (March to June) as an instructor at Purdue University, when he re- ceived a call from Robert Oppenheimer to be “ his ” postdoctoral fellow at the University of California, Berke- ley. He left Purdue, but just as he started research on a problem in meson theory his pure research career came to an abrupt end; following the Pearl Harbor tragedy and America’s entry into World War II he became engaged mostly in classified applied research, the results of which could not be published in regular journals. First, he was called back to Purdue to serve as a house theorist to work on a

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325 ROBERT GREEN SACHS project on crystal rectifiers. In 1943 he moved to Aberdeen Proving Ground in Maryland to become the chief of the air blast section of the terminal ballistics branch of the Ballis- tic Research Laboratory (BRL). The project at Purdue concerned the development of improved detectors for microwave radar. The prevailing art of the manufacture of such detectors was very labor inten- sive and entailed high cost. The needed contact between the point of a metal wire and a silicon crystal in the detec- tor had to be made manually by tickling the crystal with the wire. Sachs’s assignment, under the direction of Karl Lark- Horovitz, was to make detectors that circumvented this prob- lem and performed the way the known simple theory pre- dicted. Although the effort did not lead to the solution of the practical problem in designing better detectors, it did lead to an understanding of why the ideal theory does not work and became an important contribution to the practi- cal use of semiconductors. There were two types of Si and Ge semiconductors known as N type and P type. The theory of impurities explained why some of these semiconductors are of a specific type, but Sachs and Lark-Horovitz realized how specific impurities could be added to highly purified Si and Ge to make on demand either an N- or P-type semi- conductor. Subsequently this discovery became extremely important in the production of transistors. At BRL Sachs led a group that analyzed the tests of bombs and explosives. Aside from managing the group Sachs provided the interpretation of the results in terms of effec- tiveness in actual warfare, proposed new tests, did opera- tions analysis on bombing, and carried out research on shock waves and blasts. During the course of this work Sachs and his group needed to know the effect of ambient pressure and temperature in order to analyze the results of their tests and for the evaluation of blast effects at high altitudes.

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326 BIOGRAPHICAL MEMOIRS Sachs solved this problem in a very simple way by using scaling methods (1944).1 The BRL was a laboratory of the Ordinance Department, which had no access to the work of the Manhattan Project at Los Alamos. The contrary was not true. It turned out that Sachs’s solution was of great impor- tance to the Manhattan Project. It had defied John Kirkwood, a distinguished theoretical chemist working at Los Alamos, who was trying to find the solution by detailed dynamical methods. After the explosion of atomic bombs over Hiroshima and Nagasaki, the Ordinance Department requested Sachs to undertake a study of the comparative effectiveness in terms of both cost and strategies. This resulted in an un- published report titled “Atomic Explosives for Defensive and Offensive Purposes” (1945). It was a far-reaching analy- sis that remained classified until 1964. An extract from this report appeared in the Bulletin of Atomic Scientists.2 At the beginning of 1946 Sachs joined the University of Chicago’s Metallurgical Laboratory to work on the physics of a gas-cooled BeO, ceramic-moderated reactor for elec- tric power generation, proposed by Farrington Daniels and hence known as Daniels Pile. When the laboratory became Argonne National Laboratory on July 1, 1946, and was trans- ferred to the Atomic Energy Commission, Sachs was ap- pointed director of the Theoretical Physics Division. He had multiple responsibilities, including recruitment of theo- retical physicists, research on various designs of power reac- tors, and basic research in nuclear physics. He also com- muted between Chicago and Oak Ridge, Tennessee, to give introductory lectures in the first nuclear engineering pro- gram set up in Oak Ridge. The program included a group of young engineers selected by interested industrial compa- nies from among their employees as promising candidates to become the first industrial nuclear power engineers. In-

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327 ROBERT GREEN SACHS deed many among this group became the principal leaders of the then developing industrial programs in nuclear engi- neering.3 He was also successful in bringing some promi- nent theorists such as his former thesis advisor, Maria Göpert- Mayer, David Inglis, and Morton Hammermesh to Argonne. Sachs’s contributions to applied physics during this pe- riod included calculations of the critical sizes and tempera- ture effects for reactor cores as well as investigations of various control methods. Most of this work and the work during the war years were presented in reports that were classified for a long time. Only a part of that work appeared subsequently in regular publications and found applications in basic research. For example, Sachs’s work on relativistic shock waves during the war was subsequently cleared for publication (1946) and had significant impact on theoreti- cal work in astrophysics.4 Likewise, the paper with Fermi and Sturm (1947) on thermal neutron scattering, which arose from applied work on reactors, illustrated for the first time the power neutron diffraction for studying the proper- ties of condensed matter. In nuclear physics it became in- strumental in determining the nature of effective poten- tials in neutron scattering. In 1947 Sachs joined the faculty of the physics depart- ment of the University of Wisconsin at Madison, marking the true beginning of his academic career in teaching and research. Wisconsin, which was a major center of experi- mental nuclear physics at the time, welcomed Sachs, who began to teach basic theory courses, including a course in nuclear theory to both experimental and theory students. Soon students graduating from Wisconsin found themselves to be leaders in Atomic Energy Commission projects and in nuclear engineering programs at Argonne National Labo- ratory as well as in academic positions throughout the country. The course on nuclear theory led to his writing a textbook

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328 BIOGRAPHICAL MEMOIRS on the subject, Nuclear Theory (Addison-Wesley, Cambridge, Mass., 1953). At the time Sachs entered the field of nuclear physics, nuclear theory was concerned with the nature and origin of strong interactions of nuclear forces. The interpretation of the underlying nuclear structure, however, based on ex- periments that involved only strong interactions, had be- come a difficult task. Sachs’s proposal was to use electro- magnetic probes since the strength of the electromagnetic interaction measured by the fine structure constant was ap- proximately 0.01 of that of the strong interactions. Conse- quently at relatively low energies they were expected to cause little disturbance to the underlying nuclear system. More- over, their dynamical effects could be isolated and calcu- lated using perturbation techniques. Sachs’s first seminal paper in nuclear physics was pub- lished before he joined the faculty at the University of Wis- consin. The paper, “Magnetic Moments of Light Nuclei” (1946), established what became know as the mirror theo- rem for the magnetic moments of mirror nuclei (pairs of nuclei such that neutrons and protons in one are replaced by protons and neutrons in the other). It was based on the assumption that magnetic moments of nucleons in a nucleus are additive and it led to a correlation between the ob- served magnetic moments and the internal angular momen- tum structure of light nuclei. The mirror theorem, with suitable modifications to include relativistic corrections and its application to several experimental situations, led to sev- eral important papers during 1946-48. It soon became evi- dent, however, that the mirror theorem, combined with the assumption that the nucleon moments are additive, failed to give reasonable agreement with experiments. There was mounting evidence that there was a contribution to the total magnetic moment in addition to the sum of the spin

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329 ROBERT GREEN SACHS and orbital moments of individual nucleons inside the nucleus. Sachs recognized that the additional contribution could come from exchange currents due to the exchange of charged pi mesons between the nucleons. This was long before pi mesons were established experimentally. Persuaded by Fermi not to concentrate on a specific model, he developed a general phenomenological theory of exchange currents in nuclei (1948). This was based on a gauge invariant form of the Majorana exchange potential between nucleons. It led in subsequent years to a study of general consequences of gauge invariance. For instance, in the paper on the radia- tive transitions of a nonrelativistic system of particles (1951), Sachs (with Norman Austern) was able to give a general proof of the Siegert theorem5 for electric multipoles of all orders. Among other important results Austern and Sachs presented the formal construction of magnetic multipole moments, including the exchange current effects from any gauge invariant Hamiltonian. They demonstrated that the Rayleigh cross-section for the scattering of long wavelength radiation from a neutral system of charged particles was a direct consequence of gauge invariance.6 Further study of this formulation led to general expressions for nuclear tran- sition amplitudes for various phenomenological forms of exchange and velocity-dependent interactions (1951, 1952) and an understanding of various electromagnetic proper- ties of nuclear systems in light of then available data. While primarily a nuclear theorist, Sachs foresaw high- energy physics as the new frontier field, the physics of the future in the early 1950s. He persuaded the department and the administration to hire faculty in this relatively new area and succeeded by the middle 1950s in making Wiscon- sin a center for high-energy experimental and theoretical research.7 In his own research Sachs turned his attention

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330 BIOGRAPHICAL MEMOIRS from nuclear structure to the structure of nucleons, the presumed elementary constituents of all nuclei. He visual- ized the nucleon as consisting of a bare nucleon surrounded by a cloud of pi mesons. Because the strong interaction between the pi-meson cloud and the bare nucleon prevented the application of techniques based on perturbation theory, a new approach was necessary. Sachs introduced the gen- eral methods of his phenomenological approach to investi- gate the electromagnetic properties of the physical nucleon (1952), in particular the magnetic moments of the neutron and proton and the (static) neutron-electron interaction. His investigations suggested that the pion cloud consisted of highly correlated pairs and led to a model that could, in terms of the electromagnetic energy, account for the neu- tron-proton mass difference (1954). The later discovery of the rho meson in nucleon-nucleon collisions confirmed this feature of the nucleon structure. This was the time in the 1950s when the pioneering experiments of Robert Hofstadter on electron-neutron scat- tering raised fundamental questions regarding the physical interpretation of the electromagnetic form factors and their physical interpretation in terms of charge and magnetic moment distribution inside the nucleon. The conventional Dirac and Pauli form factors had led to a paradoxical result in the interpretation of the measured electron-neutron in- teraction.8 In a paper that had strong impact, Sachs, along with F. J. Ernst and K. C. Wali, derived certain combina- tions of the Dirac and Pauli form factors as the truly physi- cally meaningful expressions, in the sense that they were the Fourier transforms of the spatial charge and magnetiza- tion distributions inside the nucleon (1960). This not only removed the paradox but also led subsequently to new in- sights as regards the high-momentum transfer behavior of the form factors (1962).

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331 ROBERT GREEN SACHS In the late 1940s and early 1950s, when the world of elementary particles was ushered into a new era with the discovery of a host of new elementary particles, Sachs, like many other theorists, turned his attention to the study of their strange properties. He proposed a classification scheme for elementary particles based on a new additive quantum number called “attribute” (1955). This paralleled the Gell- Mann-Nishijima classification scheme for hadrons based on “strangeness,” but went beyond it since it included leptons as well.9 With Treiman he wrote a classic paper on K - K interference (1956) that was to become a forerunner for a number of theoretical and experimental discoveries in K- meson physics. He developed a phenomenological theory based on S-matrix approach to describe their anomalous decay properties (1961), which incidentally proved to be a powerful tool to describe unstable particles in relativistic field theories. The discrete symmetries—the charge conjugation (C), space inversion or parity (P), and time inversion (T)—have presented a great challenge to particle theory. The invari- ance of interactions under their combined operation, known as the CPT theorem, is regarded as one of the sacred prin- ciples of theoretical physics, since it is based on some very general principles, such as locality, Lorentz invariance, and causality. The discovery of parity violation in weak interac- tions in 1957 naturally raised the question regarding the invariance under the other two discrete operations. Sachs became interested in the subject when some preliminary experiments indicated a violation of the ∆S = ∆Q rule in the semi-leptonic decays of neutral K-mesons. Along with Treiman, Sachs analyzed the data, compared it with theory, and showed that the results implied the violation of CP invariance (1962). They suggested several experiments to test the conservation of CP in neutral K decays.10 Then

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338 BIOGRAPHICAL MEMOIRS there were many individual members who, aware of the situation, had been looking into the laboratory’s broader capabilities in energy research. The laboratory had set up panels and task forces to look at energy options other than the nuclear option. One such panel was the solar energy panel that was struggling with the problem of tracking the Sun, using focusing devices to obtain useful solar energy. From his days as associate laboratory director for high-en- ergy physics Sachs recalled the work of Roland Winston, a ZGS user from the University of Chicago. Winston had in- vented a unique nonimaging (nonfocusing) light collector for use in a Cerenkov counter in high-energy physics re- search. Sachs encouraged him to look into the possibility of using it to collect sunlight for solar energy systems. Win- ston and his collaborators soon established that nonimaging concentrators can be designed to focus sunlight efficiently throughout most of the day without ever moving. No track- ing was necessary. It opened up a whole new possibility of large-scale deployment of nontracking solar concentrators. Within a few years the laboratory recovered from its major troubles. The AEC became the Energy Research and Development Administration, followed by the establishment of the Department of Energy. Sachs felt he had accom- plished his task and that it was time to return to the univer- sity to pursue his main interest, namely, research and teach- ing. He did so in 1979, but had to serve another term (1983-86) as the director of the Enrico Fermi Institute be- fore he could devote himself full-time to this pursuit. The fundamental questions concerning the discrete sym- metries C, P, and T were to remain the main focus of his research during the last two decades of his life. In spite of the great success of the Standard Model of strong and electroweak interactions, it is recognized that it is not a fundamental theory. Of its many shortcomings the lack of

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339 ROBERT GREEN SACHS understanding of the origin and the strength of CP viola- tion in weak interactions stands out as one of the great mysteries of theoretical physics. With the advent of quan- tum chromodynamics (QCD) a new problem arose concern- ing what is called “strong CP violation.” Sachs devoted him- self almost exclusively to an understanding of some fundamental questions concerning this problem, which at the root depends on the definition of QCD vacuum (1994). His last two published papers (1994, 1997) delve deeply into the problem. His subsequent detailed exploration re- lating the origin of strong CP violation to the early Uni- verse has remained unpublished. Bob Sachs was devoted to his family just as much as to his academic pursuits and responsibilities. He is survived by his wife, Carolyn L. Sachs; five children (Judith Crow, Portola Valley, California; Joel Sachs, Arlington, Massachusetts; Rebecca Norris, Maynard, Massachusetts; Jennifer Sachs, New York City; and Jeff Sachs, Basking Ridge, New Jersey); three stepchildren (Jacqueline Wolf, West Newton, Massachusetts; Kate Wolf, Lincoln, Massachusetts; and Thomas Wolf, Brookline, Massachusetts); and fourteen grandchildren. Judith and Joel were his adopted children from his mar- riage to Jean Shudofsky (nee Jean Woolf) on December 17, 1950. Jean Sachs died on January 27, 1968, after a pro- longed battle with cancer beginning around 1960. Affec- tionate and charming, Jean made the Sachs home a warm and friendly place for all Bob’s students and associates.17 After her death Franklin Levin, a former student, intro- duced Sachs to Levin’s “beautiful and very intelligent” sis- ter Carolyn Wolf, whose husband had died in a tragic plane crash a year before. They married on August 21, 1968. The extended family came together often—for birth- days, holidays, and sometimes to sail together. “As the pa- triarch of a large brood,” Rebecca Norris says, her father

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340 BIOGRAPHICAL MEMOIRS “spoke willingly and enthusiastically at family occasions such as weddings, anniversaries, and bar and bat mitzvahs. He looked the role—tall and slim with his bushy eyebrows and hair a little wild—and his speeches always had the right touch. They were interesting, warm, funny, impeccably de- livered, and perhaps most important of all, brief. He was the quintessential grandfather.” To his colleagues, students, and associates Sachs was caring, loyal, and responsible. He was known to hold strong opinions, but was always forthright and a man of unques- tionable integrity. He was tireless in working for institu- tional developments. At both Wisconsin and Chicago he enjoyed great respect from deans, presidents, and provosts. He made sure that the teaching faculty was the backbone of a university. Sachs stood for fairness and internationalism in physics. Throughout his scientific career, along with his own re- search efforts, Sachs brought forth opportunities for oth- ers, particularly for the young. He worked closely with his students and his devotion to them was legendary. “I first met Bob Sachs in 1959 at CERN sitting at a lakeshore café,” says R. F. Sawyer. “ He offered me a post-doc job and said it is pretty much like this in Madison. It is a good place to do physics. His enthusiasm and energy had made the depart- ment in Madison a fine place to do particle physics.” Sachs was responsible for Roland Winston’s outstanding career by directing him from high-energy physics to solar energy re- search. The summer institutes Sachs initiated at the University of Wisconsin during the years 1960-64 was a prime example of his concern for the physics community at large. It was a time when even the best of the younger physicists in U.S. universities had to interrupt their research during the sum- mer and seek employment to supplement their income. That

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341 ROBERT GREEN SACHS was an era when few university faculty members had sum- mer support covering their salaries. Sachs’s proposal to miti- gate the situation by forming a summer institute for well- qualified theorists was welcomed by the National Science Foundation. With the necessary support for travel and sala- ries for the participants the summer institutes became unique in fostering free exchange of ideas among a rare combina- tion of senior and distinguished and young, upcoming re- searchers not only from United States but also from all over the world.18 Sachs was a Guggenheim fellow (1959-60); received hon- orary doctor of science degrees from Purdue University (1967), University of Illinois, Circle Campus (1977), and Elmhurst College (1987); was elected to membership in the National Academy of Sciences (1971); and served as the chairman of the Academy’s Class I (Physical and Mathematical Sciences, 1980-83) and chairman of its Physics Section (1977- 80). I WOULD LIKE TO THANK Carolyn Sachs and the Sachs family for all their help in completing this project. I am particularly indebted to Rebecca Norris; Judith Crow; and Jeff, Jennifer, and Frances Sachs for providing me with Bob Sachs’s family history. I have also had extensive help from many of Sachs’s colleagues and associates, most particularly (in alphabetical order) Malcom Derric, Thomas Fields, William F. Fry, Roger Hilderbrand, Wendell G. Holladay, J. M. Nevitt, Gale Pewitt, Jonathan L. Rosner, Raymond F. Sawyer, Roland Win- ston, and Lincoln Wolfenstein. NOTES 1. This paper is still the primary source on the subject for people working on weapons program at Livermore. Although it was avail- able to the public, Sachs was unsuccessful in getting it released for publication in a scientific journal. 2. Power of prediction—an example. Bull. At. Sci. XX(1964):20- 21.

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342 BIOGRAPHICAL MEMOIRS 3. A partial list includes the names of John Simpson, Harry Stevens, Robert Dietrich, and Captain Hyman Rickover, who later became Admiral Rickover. Sachs shared lodgings with Rickover in Oak Ridge and became his private tutor on the subject of nuclear energy. 4. The subject of this paper became quite important in astro- physics after the war. In a footnote of this paper Sachs calls atten- tion to the possibility of thermonuclear detonations. 5. In electromagnetic radiative transitions in nuclear systems, theoretical justification for replacing the current density operator by charge density operator is referred to as Siegert’s theorem (A. J. F. Siegert. Phys. Rev. 52[1927]:787). 6. These results found an important extension to semi-relativis- tic systems, such as pion-nucleon systems with finite source interac- tions by Richard Capps, a student of Sachs at the time (Phys. Rev. 99[1955]:926). 7. The appointment of William F. Fry in 1951 was the beginning of high-energy experimental physics program. It was followed by the appointments of William. D Walker in 1954 and Myron L. Good in 1960. By the middle 1950s the program blossomed into an active center of fundamental discoveries concerning the behavior of the new particles. Theoretical high-energy physics also acquired a boost with K. M. Watson, K. Simon, and later H. W. Lewis and R. F. Sawyer on the faculty. 8. The static electron-neutron interaction could be understood qualitatively in terms of the charge distribution in the pion cloud surrounding the neutron. However, using meson theory, L. Foldy had shown that the observed interaction could be accounted for by the Pauli form factor or the (anomalous) magnetic moment of the neutron instead of the expected Dirac form factor (Phys. Rev. 83[1951]:688). 9. However, the predictions based on this classification were found to conflict with experiments and consequently the classification scheme had to be abandoned. 10. It turned out that the preliminary results indicating the viola- tion of ∆S = ∆Q rule were not sustained with further improved statistics in measurements. Hence the theory of CP violation that was proposed in this paper turned out to be wrong. True CP viola- tion in K-mesons was discovered in 1964. 11. The panel headed by Norman Ramsey, who was appointed by

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343 ROBERT GREEN SACHS the President’s Science Advisory Committee to study and report the U.S. priorities in high-energy physics research. The other members of the panel were E. L. (Ned) Goldwasser, J. H. Williams, F. Seitz, P. H. Abelson, O. Chamberlain, M. Gell-Mann, T. D. Lee, W. Panofsky, and E. M. Purcell (J. M. Holl. Argonne National Laboratory, 1946-96, p. 216. Urbana: University of Illinois Press, 1997). 12. L. Greenbaum. A Special Interest, p. 160. Ann Arbor: University of Michigan Press, 1971. 13. Concluding remarks by R. G. Sachs in the proceedings of a one-day symposium on the 30th anniversary of the ZGS startup, ed. M. Derric, unpublished. 14. R. G. Sachs. P. 38 in History of the ZGS (Argonne 1979), ed. J. S. Day, A. D. Krisch, L. G. Ratner: New York: American Institute of Physics, 1980. 15. For instance, A. Yokosawa’s successful design of a polarized target intended to study the spin dependence of the scattering of protons from polarized nuclei. 16. National Academy of Sciences. Report of the Elementary Par- ticle Physics Panel to the Physics Survey Committee with J. D. Bjorken, J. W. Cronin, L. Hand, D. H. Miller and W. J. Willis, Physics Per- spective II, A, 1-159. Washington, D.C.: National Academy of Sci- ences, 1972. 17. I came to Madison in 1955 to do my Ph.D., leaving behind my family, my pregnant wife, and two little girls. Jean Sachs’s warmth and affection meant a great deal to me. Jennifer Sachs and Rebecca Norris (then Jenny and Rebbi) made up for the girls I had left behind. 18. The long list included, among others, Julian Schwinger, Abdus Salam, John Ward, L. Michel, C. N. Yang, and T.D. Lee. Jeffrey Goldstone did his fundamental work on spontaneous symmetry breaking during the first summer institute. Benjamin Lee and Jonathan Rosner were still students when they got an opportunity to participate in these institutes, which undoubtedly had great influence on their future careers.

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344 BIOGRAPHICAL MEMOIRS SELECTED BIBLIOGRAPHY 1938 With M. Göpert-Mayer. Calculation of a new neutron-proton inter- action potential. Phys. Rev. 53:991-93. 1940 With R. H. Lyddane and E. Teller. Polar vibrations of alkali halides. Phys. Rev. 59:673-76. 1941 With E. Teller. The scattering of slow neutrons by molecular gases. Phys. Rev. 60:18-27. 1944 The Dependence of Blast on Ambient Pressure and Temperature, Ballistic Research Laboratories Report No. 466, May 1944 (Avail- able at Defense Documentation Center, Alexandra, Va., under order No. ATI 39393). 1945 Atomic Explosives for Defensive and Offensive purposes, BRL 590, November 1945. 1946 Some properties of very intense shock waves. Phys. Rev. 69:514-22. Magnetic moments of light nuclei. Phys. Rev. 69:611-15. 1947 With E. Fermi and W. J. Sturm. The transmission of slow neutrons through microcrystalline materials. Phys. Rev. 71:589-94. 1948 Phenomenological theory of exchange currents in nuclei. Phys. Rev. 74:433. Erratum. Phys. Rev. 75(1949):1605.

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345 ROBERT GREEN SACHS 1951 With N. Austern. Consequences of gauge invariance for radiative transitions. Phys. Rev. 81:705-709. Interaction effects on radiative transitions in nuclei. Phys. Rev. 81:710- 16. With M. Ross. Evidence for non-additivity of nucleon moments. Phys. Rev. 84:379-80. 1952 With J. G. Brennan. Nuclear photo processes at high energies. Phys. Rev. 88:824-27. Structure of the nucleon. Phys. Rev. 87:1100-1110. 1954 Structure of the nucleon. II. Pion-nucleon scattering. Phys. Rev. 95:1065- 78. With W. G. Holladay. Neutron-proton mass difference. Phys. Rev. 96:810-11. 1955 Classification of the fundamental particles. Phys. Rev. 99:1573-80. 1956 With S. B Treiman. Alternate modes of decay of neutral K-mesons. Phys. Rev. 103:1545-49. 1960 With F. J. Ernst and K. C. Wali. Electromagnetic form factors of the nucleon. Phys. Rev. 119:1105-14. 1961 With R. Jacob. Mass and lifetime of unstable particles. Phys. Rev. 121:350-56. 1962 With S. B. Treiman. Test of CP conservation in neutral K-meson decay. Phys. Rev. Lett. 8:137-40. High energy behavior of nucleon electromagnetic form factors. Phys. Rev. 126:2256-60.

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346 BIOGRAPHICAL MEMOIRS 1963 Interference phenomena of neutral K-mesons. Ann. Phys. 22:239-62. Methods for testing the CPT theorem. Phys. Rev. 129:2280-85. 1973 With B. G. Kenny. Non-hermitian interactions and the evidence for T-violation. Phys. Rev. D8:1605-1607. 1986 Supplementary evidence for T-violation. Phys. Rev. D1:3283. 1990 CP or T-violation? In CP Violation in Particle Physics and Astrophysics, ed. J. Tran Than Van. Gif-sur-Yvette Cedex, France: Editions Frontieres. With M. J. Booth and R. Briere. Interpretation of the neutron elec- tric dipole moment: Possible relationship to |e¢|. Phys. Rev. D141:177. 1994 Is QCD consistent with quantum field theory? Phys. Rev. Lett. 73:377- 80. 1997 QCD vacuum in the early Universe. Phys. Rev. Lett. 78:420-23.

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