A Parity Violation Experiment for Undergraduate Laboratories

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Ganger, Michael
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Houghton College
The discovery of parity violation in weak interactions was a foundational discovery of the 20th century, first proposed by Lee and Yang in 1956 and experimentally verified by Wu in 1957. Lee and Yang also proposed a simpler experiment which does not require that the 60Co source be polarized. Randomly oriented 60Co beta decays to an excited state of 60Ni, which then deexcites by emitting two gamma rays. Conservation of angular momentum ensures that the spins of all emitted particles are aligned. Therefore, when a gamma ray and a beta particle have antiparallel momenta they necessarily have opposite helicities. In the proposed experiment, these circularly polarized gamma rays are transmitted through a steel rod magnetized along the axis between two collinear detectors, a germanium detector for the gamma rays and a silicon detector for the beta particles. Due to the slight dependence of the Compton scattering cross-section on the relative orientations of the gamma rays and the electron spins in the magnet, a parity violating asymmetry may be observed by comparing beta particle and transmitted gamma ray coincidence count rates for opposite directions of magnetization. An experiment to observe effect this is currently being prepared at Houghton College using modern techniques suitable for an undergraduate laboratory.
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