A precision measurement of the longitudinal polarization of betas following P 32 decay

Abstract

The method of Mott scattering has been used to determine the longitudinal polarization of 616 keV betas from the allowed Gamow-Teller decay of P 32. After rotation of the spin from longitudinal to transverse by a spherical electrostatic analyser, the up-down asymmetry was measured over a wide range of gold target thickness. Source depolarization, plate scattering depolarization, multiple and plural scattering in gold and aluminium targets and instrumental asymmetry effects were carefully investigated experimentally. Corrections were made for various backgrounds, wall scattering, finite geometry and fringing fields. The final value of P = (−0.990±0.009)υ/c for υ/ c = 0.891 depends upon a calculated value of the Most-scattering function S( θ), which has not been corrected for screening, inelastic scattering, Schwinger radiative loss, or finite nuclear size effects. These are estimated to be the order of 2 % or less. Neglecting these effects and small theoretical corrections to the polarization which result from such effects as Δl-forbiddenness and finite nuclear size, this experimental result can be interpreted in terms of the beta-decay coupling constants. If time reversal invariance and the two-component neutrino theory are assumed, the ratio C T C A = 0.011±0.010 is computed from the measured polarization. On the other hand, assuming time reversal invariance and C T = C′ T = O, then the ratio C′ A C A is limited to the range +0.82 to +1.22 by the result reported here.