Positive muons in single crystal sodium fluoride: A simple spin system.

We report on the relaxation of positive muons (mu(+)) stopped in a single crystal of sodium fluoride at 15 +-0.2K. At this temperature the mu(+) is believed to be static, and the observed time dependence of the mu(+) spin polarization is seen to deviate from the familiar Kubo-Toyabe form at late times. Specifically these data exhibit oscillations in the long time tail, which are attributed to hyperfine transitions between the mu(+) and neighboring spins. Quantitative comparison on these data to exact quantum mechanical calculations indicates that most of the time dependence can be explained by considering only interactions with the first shell (i.e., two fluorine and two sodium nuclei), and suggests strongly that the muon occupies a site along the 110> axis, midway between two fluorine nuclei.