Longitudinal asymmetry and γ-ray angular distribution in neutron-radiative-capture reactions

The parity-nonconserving effect was investigated in low-energy (n, γ) reactions for several nuclei with mass numbers of ∝100. The longitudinal asymmetry (AL) was measured in p-wave resonances for 139La and 81Br taking into account the possible angular dependence within the framework of the sp mixing model. AL for 139La was found to be independent of the γ-ray energy threshold within the experimental errors. From these results the relation between a γ-ray detection experiment and a neutron-transmission experiment is discussed. AL was measured in the p-wave resonances for 81Br, 93Nb, 108Pd, 111Cd, 124Sn and 139La. The obtained nonzero values of AL are AL=(9.8±0.3)%, (2.1±0.1)% and -(1.3(+0.7-0.4))% for 139La(Ep=0.734 eV), 81Br(Ep=0.88 eV) and 111Cd(Ep=4.53 eV), respectively. A theoretical interpretation is discussed in terms of the interference between two amplitudes of the s- and p-wave incident neutrons. (orig.)