LOW-TEMPERATURE HE-3 NMR-STUDIES IN AGED PALLADIUM TRITIDE

Experimental results from a NMR study of He-3 in nanometer-sized microbubbles in aged palladium tritide over the temperature range 4-300 K are presented. A determination of the rigid-lattice-line-shape second moment at 4 K provides a value for the density of He-3 that is in good agreement with the value inferred for the same sample from an analysis of He-3 spin-lattice relaxation data over the range 50-300 K. This latter analysis is based on the hypothesis of a melting transition for a distribution of densities, and assumes that the bulk helium equation of state applies to helium in the nanometer-sized bubbles. This latter assumption is supported by the agreement between the two independent density determinations. Line-shape analyses reveal narrowing of the He-3 spectrum at about 35 K, which agrees with a prediction of motional narrowing based on an extrapolation of He-3 diffusivity data determined over the range 100-250 K in an earlier study of aged palladium tritide. The observation of coexisting broad and narrow components in the free-induction decay at 40 K provides direct evidence for a distribution of He-3 densities in aged palladium tritide. Because the melting temperature and the rigid-lattice second moment are both strong functions of density, the NMR techniques described here provide a sensitive measure of the density distribution and related properties.