Quantum Tunneling of He in Solid He: A New Analysis

We discuss the analysis of the experimental values of the nuclear spin-lattice and spin-spin relaxation times for the tunneling of He as isotopic impurities in solid He. These two relaxation times cannot be described quantitatively using a unique correlation time although it is often presented as such in the literature. In this paper, we discuss how to distinguish the high-frequency portion of the spectral densities that determine the spin-lattice relaxation rates from the low-frequency components which determine the spin-spin relaxation rates.