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Wu, Chien-Shiung

nuclear direction beta predicted

(1912–97) Chinese–US physicist: team member of group who confirmed experimentally that parity is not conserved by the weak nuclear force.

Born in Shanghai, Wu moved to the USA in 1936, having completed her degree in China. Under she obtained her doctorate from the University of California at Berkeley in 1940 and took up a post at Princeton. From 1946 she taught at Columbia University, becoming professor of physics in 1957.

Wu had intended to return to China after her postgraduate work in physics, but war and afterwards communism stopped that. Columbia became her home, and she and her husband became US citizens in 1954; but she retained her Chinese dress style, her Chinese name and her preference for Chinese food.

From 1946 she became expert on beta-decay in radioactive atoms, the process whereby an electron and a neutrino are ejected from a neutron in the nucleus, leaving behind a proton.

In 1957 she developed her research on nuclear decay by emission of beta particles by observing that the direction of emission is closely tied to the direction of the spin of the emitting nucleus. The critical experiments used radioactive cobalt-60, (half-life 10.5 min) cooled close to absolute zero. In a difficult experiment, she and four colleagues showed that beta particles are not emitted in equal numbers when the cobalt nuclei are aligned in a magnetic field: more are emitted in the direction opposite the field (and therefore opposite to the direction of nuclear spin). The emission process, therefore, is not identical for a mirror image system; and the physical laws do not remain unchanged under a parity change. This extraordinary result had however been predicted by , who had deduced that the weak nuclear interaction would not be identical under a parity change. The results of this work were far-reaching and many basic assumptions in physics were called into question.

Wu then set out to confirm theory of beta decay (1958), which predicted conservation of a vector current. She confirmed this in 1963. She also observed that electromagnetic radiation that is polarized is released on electron–positron annihilation, as predicted by theory of the electron.

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