Angular distribution of $\gamma$ rays from the p-wave resonance of $^{118}$Sn

TL;DR

This study measures the angular distribution of gamma rays emitted from a specific neutron resonance in $^{118}$Sn, revealing an angular-dependent asymmetry that influences the resonance shape.

Contribution

It provides the first detailed measurement of gamma-ray angular distribution and asymmetry in the p-wave resonance of $^{118}$Sn, highlighting angular dependence in resonance asymmetry.

Findings

Angular distribution causes asymmetric resonance shape.

Asymmetry $A_{LH}$ depends on gamma-ray emission angle.

Quantitative parameters A and B describe the angular dependence.

Abstract

The neutron energy-dependent angular distribution of $\gamma$ rays from $^{117}{\rm Sn}(n,\gamma)$ reaction was measured with germanium detectors and a pulsed neutron beam. The angular distribution was clearly observed in $\gamma$-ray emissions with an energy of 9327 keV which corresponds to the transition from a neutron resonance of $^{117}{\rm Sn}+n$ to the ground state of $^{118}{\rm Sn}$. The angular distribution causes an angular-dependent asymmetric resonance shape. An asymmetry $A_{\rm LH}$ was defined as $(N_{\rm L}-N_{\rm H})/(N_{\rm L}+N_{\rm H})$, where $N_{\rm L}$ and $N_{\rm H}$ are integrated values for lower- and higher-energy regions of a neutron resonance, respectively. We found that the $A_{\rm LH}$ has the angular dependence of $(A \cos \theta_\gamma +B)$, where $\theta_\gamma$ is the $\gamma$-ray emission angle with respect to the incident neutron momentum, with $A=0.394 \pm 0.073$ and $B = 0.118 \pm 0.029$ in the 1.33 eV p-wave resonance.