High-energy-neutron spectroscopy of crystal-field excitations inNpO2

Neutron spectroscopy has been applied to study the crystal-field potential in ${\mathrm{NpO}}_2$. The inelastic-scattering cross section shows a broad magnetic peak that is split into two components and is centered at about 55 meV. A quasielastic magnetic signal extending up to about 30 meV is also observed at temperatures below the phase transition at 25 K. The origin of the inelastic peak is attributed to excitations between the ${\mathrm{\Gamma }}_8^{\mathrm{}\left(2\right)\mathrm{}}$ and ${\mathrm{\Gamma }}_8^{\mathrm{}\left(1\right)\mathrm{}}$ crystal-field quartets. A mechanism that could be responsible for the observed splitting is proposed.