NMR of hyperpolarised He-3 gas in aerogel

We report preliminary low-field (2 mT and 0.1 T) NMR measurements on room t emperature laser polarised He-3 gas imbibed into a 98% porous silica aeroge l. The hyperpolarised helium is introduced into the cell in which the aerog el 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 v ery weak specific wall relaxation in the cell. NMR of hyperpolarised He-3 g as is thus a potential tool to measure physical parameters inside the aerog els which are used to study quantum fluids in confined geometries. This has been demonstrated using a multi-echo technique to characterize spin diffus ion in an applied magnetic field gradient. Spin diffusion was found to be i sotropic and its pressure dependence was studied. An estimate of the mean f ree path for gas atoms in the aerogel is deduced from this behaviour. (C) 2 000 Elsevier Science B.V. All rights reserved.