Investigation of parity mixing in 203Tl by measurement of the forward-backward asymmetry of γ-rays after β-decay

A multiple detector arrangement of fixed geometry and high inherent accuracy has been used to measure the forward backward asymmetry coefficients A1 in the β-γ transition of 203Hg → 203Tl. Moreover, a control value has been determined which should exhibit no symmetry. A non-zero coefficient A1 is indicative of small admixture of opposite parity in nuclear states. The properties of a multiple detector arrangement and systematic asymmetries are discussed in detail. Sources of 203Hg of high quality have been prepared with a mass separator. To avoid a systematic bias due to non-negligible quadratic terms in the statistical errors a new technique of data analysis has been applied. The measurements of several sources yielded for the asymmetry coefficient A1 = -(2.6+- 0.7) x 10-4 and for the control value (1.3 +- 0.7) x 10-4, where the quoted errors represent one standard deviation. (Auth.)