Angular Distribution of ${\hat{I}}^3$-rays from Neutron-Induced Compound States of ${}^{140}$La

Abstract

Angular distribution of individual ${\hat{I}}^3$-rays, emitted from a neutron-induced compound nuclear state via radiative capture reaction of ${}^{139}$La(n,${\hat{I}}^3$) has been studied as a function of incident neutron energy in the epithermal region by using germanium detectors. An asymmetry $A_{\mathrm{LH}}$ was defined as $(N_{\mathrm{L}}-N_{\mathrm{H}})/(N_{\mathrm{L}}+N_{\mathrm{H}})$, where $N_{\mathrm{L}}$ and $N_{\mathrm{H}}$ are integrals of low and high energy region of a neutron resonance respectively, and we found that $A_{\mathrm{LH}}$ has the angular dependence of $(A\cos \hat{I}{¸}_{\hat{I}}^3+B)$, where $\hat{I}{¸}_{\hat{I}}^3$ is emitted angle of ${\hat{I}}^3$-rays, with $A=-0.3881\pm 0.0236$ and $B=-0.0747\pm 0.0105$ in 0.74 eV p-wave resonance. This angular distribution was analyzed within the framework of interference between s- and p-wave amplitudes in the entrance channel to the compound nuclear state, and it is interpreted as the value of the partial p-wave neutron width corresponding to the total angular momentum of the incident neutron combined with the weak matrix element, in the context of the mechanism of enhanced parity-violating effects. Additionally we used the result to quantify the possible enhancement of the breaking of the time-reversal invariance in the vicinity of the p-wave resonance.

14pages, 25 figures