Pure nuclear reflections in case of nuclear level crossing

Measurements of the time-differential decay of the collective nuclear excitation in yttrium iron garnet have been performed to observe the (002) pure nuclear reflection versus the splitting within two d sites. Even when the energy levels of the two sites are identical, the reflectivity amplitude does not vanish. This is a new realization of a pure nuclear reflection. It arises in case of nuclear level crossing of two d sites because their eigenvectors are not identical.