  Hyperpolarized 3He is used to nondestructively probe by NMR the structure of
custom-made and commercial silica aerogels (97% and 98.5% porous). Large
spin-echo signals are obtained at room temperature and very low magnetic field
(2mT) even with small mounts of gas. Attenuation induced by applied field
gradients results from the combined effects of gas diffusion and confinement by
the porous medium on atomic motion. Nitrogen is used as a buffer gas to reach
equivalent 3He pressures ranging from 5 mbars to 3.5 bars. The observed
pressure dependence suggests a non-uniform structure of the aerogels on length
scales up to tens of micrometers. A description by broad phenomenological
distributions of mean free paths is proposed, and quantitatively discussed by
comparison to numerical calculations. The investigated aerogel samples exhibit
different effective diffusion characteristics despite comparable nominal
porosities.
