WU, CHIEN-SHIUNG

Chien-Shiung Wu ranks as one of the foremost physicists of the twentieth century. Her pioneering experiments in beta decay and weak interactions were the preeminent tests of new paradigms and models of subatomic physics.

Chien-Shiung Wu was born to Wu Zhongyi and Fan Funhua on May 31, 1912, in Liu He, a small town near Shanghai. Her father, an engineer, was the headmaster of the School for Girls, one of the first schools to admit girls in China. Wu graduated from this school in 1922 and continued her studies at the Soochow School for Girls in Nanjing. As was generally expected for women at the time, she enrolled in

the Normal School program, which led to a teaching career. Pursuing her interest in physics and mathematics, she enrolled in 1930 at the National Central University in Nanjing, from which she graduated in 1934 at the head of her class. She taught for a year at the National Chekiang University and started research in X-ray crystallography at the National Academy of Sciences (Academia Sinica) in Shanghai in 1935 and 1936. Wu immigrated to the United States in 1936 to pursue graduate studies at the University of Michigan. However, she first visited the University of California at Berkeley where she met another Chinese physics student, Luke Chia Yuan, who introduced her to Professor Ernest Lawrence. Lawrence immediately recognized Wu's intelligence and potential and convinced her to stay at Berkeley. This encounter with Luke Yuan was the beginning of a long and warm relationship, which was further strengthened by marriage in 1942.

Wu worked under the direct supervision of Professor Emilio Segrè. Her success hinged on her appreciation of the importance of careful and accurate measurements. Wu's Ph.D. thesis (1940) involved studies of fission products of uranium, a topic of major interest at the time. In particular, the identification of two Xe isotopes earned her wide recognition a few years later, during World War II, when the development of nuclear piles depended critically on the knowledge and avoidance of materials that would poison and shut down the reactors. Her very careful work enabled her to identify ¹³⁵Xe as the culprit in reactor malfunction and made it possible to devise techniques to control the operation of reactors.

After graduation, Wu taught at Smith College and Princeton University. By then the nation was at war, and she was invited to join the Manhattan District Project at Columbia University. She first worked on gaseous diffusion of uranium and later on measurements by time-of-flight of the energy dependence of neutron reaction cross sections.

The end of the war in 1945 allowed Wu to finally take control of her career and focus on a problem that was to make significant advances in the under-standing of nature. Enrico Fermi had developed a mathematical theory to explain the radioactive process of beta decay. This approach involved a new force called the weak interaction. In this process a neutral particle, named neutrino by Fermi, and postulated simultaneously by Enrico Fermi and Wolfgang Pauli, was emitted. This particle was assumed to have remarkable properties, namely, it was massless; had spin just like protons, neutrons, and electrons; and barely interacted with matter. Understanding the weak interaction and the nature of the neutrino became Wu's life-long commitment. Existing experiments disagreed with Fermi's theory, but Wu quickly under-stood the experimental problems and assembled the most suitable apparatus to measure, with exquisite precision, the shapes of the electron spectra resulting from these decays. She investigated different types of beta decay, the so-called "allowed" and "forbidden" transitions, and showed unambiguously that the Fermi theory of beta decay was correct in all its details.

These experiments brought Wu worldwide recognition. She was now poised to handle the next challenge. It came in the form of a puzzle in particle physics. Two of the newly discovered particles, the τ and θ, had the same mass, spin, and lifetime, and yet one decayed into two pions while the other decayed into three pions. These two decay modes were of opposite parity, meaning that reflection symmetry was not obeyed. This symmetry, a property of physical systems obeyed in all observed interactions studied up to that time, requires that a mirror image of a process obtained by reversing all directions and velocities be identical to the original process. The τ - θ puzzle led Professors Tsung Dao Lee and Chen-Ning Yang to question the accumulated evidence for conservation of parity in various decay processes. They realized, after extensive discussions with Wu, the undisputed experimentalist in beta decay and weak interaction physics, that there was no evidence for parity conservation or nonconservation in weak interactions. Wu designed the experiment that would test directly this symmetry principle. She formed collaboration with the experts in low-temperature spin polarization at the National Bureau of Standards in Washington, D. C., to measure the forward-backward asymmetry of electron emission of spin-polarized ⁶⁰Co nuclei. Again, in this instance, as in many cases in her work, she contributed a crucial element to the experiment, namely, the large crystals of paramagnetic salts necessary for the polarization of the ⁶⁰Co nuclei. The historic paper describing this work, "Experimental Test of Parity Conservation in Beta Decay," has become a classic.

Wu's beautiful and definitive work on beta decay established the Fermi theory of weak interactions. The elegant and momentous experiment, which established the nonconservation of parity and the violation of particle-antiparticle charge conjugation symmetry in physics, altered forever the view of the universe. As T. D. Lee described her and her work, "C. S. Wu was one of the giants of physics. In the field of beta-decay, she has no equal" (p. 7).

Wu continued research on fundamental problems. In 1963 she observed the phenomenon of weak magnetism, which confirmed the symmetry between the weak and the electromagnetic currents and set the cornerstone for the unification of these two basic forces into the electroweak force. Wu's interest in the nature of the neutrino led her to studies of double beta decay in which either two neutrinos or none are emitted, depending on their characteristics. She examined the radiations of "exotic" atoms with muons or pions replacing electrons in order to determine nuclear charge radii with higher accuracy than previously measured. She used new techniques such as the Mössbauer effect to study another fundamental property, time-reversal invariance. Wu also conducted research in condensed matter physics through the examination of magnetic transitions and relaxation effects in materials, and she explored a current problem in biology, the structure of sickle cell hemoglobin. In later years she devoted much effort to educational programs in both the Republic of China and Taiwan, as well as to the development of new facilities such as synchrotron radiation light sources.

Wu was frequently honored. She was promoted to a full professorship at Columbia in 1958, the first woman to hold a tenured faculty position in the physics department. She was appointed the first Michael I. Pupin Professor of Physics in 1973 and retired in 1981. She was elected to the National Academy of Sciences (1958) and received the National Medal of Science (1958), the Research Corporation Award (1958), and the John Price Wetherill Medal of the Franklin Institute (1962). Many distinguished awards followed, most notably the Cyrus B. Comstock Award (1964), the Tom Bonner Prize of the American Physical Society (1975), and the Wolf Prize from the State of Israel (1978). She was inducted in 1998 into the American National Women's Hall of Fame. She was the first woman to receive an honorary doctorate from Princeton University, and the first woman to serve as President of the American Physical Society(1975).

Beauty and aesthetics defined her work, her demeanor, and her relationships with family and friends. She was proud of the intellectual development of her son, Vincent Yuan, who as a physicist also worked in parity nonconservation in compound nuclei, and of the academic achievements of her grand daughter. She nurtured about thirty-three graduate students and many visiting scientists and postdoctoral fellows.

Wu died in New York, following a stroke, on February 16, 1997. Her remarkable life can be portrayed by an ancient Chinese poem by Qu Yuan (∼340B.C.E.): "Although the road is long and arduous, I am determined to explore its entire length."

See also:Parity, Nonconservation of; Pauli, Wolfgang

Bibliography

Lee, T. D. "Chien-Shiung Wu, 84, Dies; Top Experimental Physicist." The New York Times (February 18, 1997) p. 7.

McGrayne, S. B. Nobel Prize Women in Science (Carol Publishing Group, Secaucus, NJ, 1992).

Wu, C. "Recent Investigations of the Shapes of Beta-Ray Spectra." Reviews of Modern Physics22 , 386–398 (1950).

Wu, C. Ambler, E.; Hayward, R. W.; Hoppes, D. D.; and Hudson, R. P. "Experimental Test of Parity Conservation in Beta Decay." Physical Review105 , 1413–1415 (1957).

Wu, C. and Moszkowski, S A. Beta Decay (Wiley, New York, 1966).

Noemie Benczer Koller