NMR of hyperpolarised 3He gas in aerogel

We report preliminary low-field (2 mT and 0.1 T) NMR measurements on room temperature laser polarised He-3 gas imbibed into a 98% porous silica aerogel. The hyperpolarised helium is introduced into the cell in which the aerogel had been grown, and large NMR signals can be recorded. Long polarisation lifetimes (T-1 similar or equal to 50 s) have been observed, revealing a very weak specific wall relaxation in the cell. NMR of hyperpolarised He-3 gas is thus a potential tool to measure physical parameters inside the aerogels which are used to study quantum fluids in confined geometries. This has been demonstrated using a multi-echo technique to characterize spin diffusion in an applied magnetic field gradient. Spin diffusion was found to be isotropic and its pressure dependence was studied. An estimate of the mean free path for gas atoms in the aerogel is deduced from this behaviour. (C) 2000 Elsevier Science B.V. All rights reserved.