Quantum Tunneling of $$^3$$ 3 He in Solid $$^4$$ 4 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 $$^3$$ He as isotopic impurities in solid $$^4$$ 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.