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

The neutron energy-dependent angular distribution of $γ$ rays from $^{117}{\rm Sn}(n,γ)$ reaction was measured with germanium detectors and a pulsed neutron beam. The angular distribution was clearly observed in $γ$-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 $(A \cos θ_γ+B)$, where $θ_γ$ is the $γ$-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.

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