Angular distribution of γ rays from the p -wave resonance of Sn118

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