Abstract
For pt.XI see J. Less-Common Met., vol.129, p.335 (1987). The theory of nuclear spin relaxation due to dipolar coupling between diffusing spins in crystals requires the calculation of spectral density functions for particular jump models of the diffusion. The results of such calculations have always been presented in a tabular form that is inconvenient to use compared with the simpler, but much less accurate, Bloembergen-Purcell-Pound theory. An approximate analytic function is proposed which, when fitted to the results of detailed calculations, has comparable accuracy to these calculations but has the simplicity of use of the BPP theory.