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

TL;DR

This study investigates the angular distribution of gamma rays emitted from neutron-induced compound states of $^{140}$La, revealing interference effects and potential parity violation near a p-wave resonance.

Contribution

It provides the first detailed analysis of gamma-ray angular distributions in $^{140}$La neutron capture, linking observed asymmetries to interference between s- and p-wave amplitudes.

Findings

Angular distribution follows a cosθ pattern with specific coefficients.

Interference analysis yields p-wave neutron width related to total angular momentum.

Results suggest possible enhancement of time-reversal symmetry breaking.

Abstract

Angular distribution of individual $\gamma$-rays, emitted from a neutron-induced compound nuclear state via radiative capture reaction of ${}^{139}$La(n,$\gamma$) 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 $(A\cos\theta_\gamma+B)$, where $\theta_\gamma$ is emitted angle of $\gamma$-rays, with $A= -0.3881\pm0.0236$ and $B=-0.0747\pm0.0105$ 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.