NMR diffusion of hyperpolarised He-3 in aerogel: a systematic pressure study

Diffusion of hyperpolarised He-3 in a silica aerogel sample with 98% porosi ty was measured by NMR, and systematically studied as a function of pressur e P in the range 20 mbar - I bar. Spin-echo amplitudes, recorded with pulse d gradients in the usual CMPG sequence, follow a monoexponential decay, and the decay rate varies quadratically with the gradient intensity: thus the apparent diffusion coefficient of the gas D can be evaluated. Our results s how a power-law (1/D proportional to P-0.55), which could be consistent wit h a fairly broad mean free path distribution, implying some inhomogeneity i n the aerogel structure. We have observed a spatial non-uniformity of D, an d some deviation to the quadratic dependence of the decay rate with the gra dient intensity, possibly related to sample structure inhomogeneity. (C) 20 01 Elsevier Science Inc. All rights reserved.