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

Diffusion of hyperpolarised 3He in a silica aerogel sample with 98% porosity was measured by NMR, and systematically studied as a function of pressure P in the range 20 mbar -1 bar. Spin-echo amplitudes, recorded with pulsed 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 show a power-law (1/D proportional to P(0.55)), which could be consistent with a fairly broad mean free path distribution, implying some inhomogeneity in the aerogel structure. We have observed a spatial non-uniformity of D, and some deviation to the quadratic dependence of the decay rate with the gradient intensity, possibly related to sample structure inhomogeneity.