Energy-dependent angular distribution of individual γ rays in the La 139 ( n , γ ) La 140 * reaction

Neutron energy-dependent angular distributions were observed for individual $\ensuremath{\gamma}$ rays from the 0.74 eV $p$-wave resonance of $^{139}\mathrm{La}+n$ to several lower excited states of $^{140}\mathrm{La}$. The $\ensuremath{\gamma}$-ray signals were analyzed in a two-dimensional histogram of the $\ensuremath{\gamma}$-ray energy, measured with distributed germanium detectors, and neutron energy, determined with the time-of-flight of pulsed neutrons, to identify the neutron energy dependence of the angular distribution for each individual $\ensuremath{\gamma}$ rays. The angular distribution was also found for a photopeak accompanied with a faint $p$-wave resonance component. Our results can be interpreted as interference between $s$- and $p$-wave amplitudes, which may be used to study discrete symmetries of fundamental interactions.