17O NMR spin-lattice relaxation in NpO2: Field-dependent cross-relaxation process driven by 237Np spins

We have performed 17O NMR spin-lattice relaxation measurements with conventional NMR techniques on 17O in NpO2. The results have been found to exhibit a field-dependent cross-relaxation effect from 237Np in addition to the field-independent exchange-fluctuation background rate which dominates at high fields. Analysis of the cross-relaxation process reveals a temperature-independent relaxation rate as well as a greatly enhanced unlike spin-spin coupling between the 237Np spins and their 17O nearest neighbors. We discuss the possibility of observing the cross-relaxation effects in other actinide dioxides.