Probing silica aerogel structure with spin-polarised helium-3 gas

We report on systematic low-field NMR measurements on room temperature He-3 gas imbibed into a 98% porous silica aerogel. Laser polarised helium is us ed to obtain large NMR signals at 0.1 T even for small amounts of gas. WE u se a multi-echo technique in a controlled magnetic field gradient to charac terize spin diffusion. Measurements are carried out for pressures P ranging from 10 mbar to 1 bar (mean free path of free He-3 ranging from 19 to 0.19 mum respectively). In a free gas, the damping rate 1/T-2 of the amplitude of the echoes is checked to be proportional to the known diffusion coeffici ent (T-2 proportional to P). Damping rates 2.5 times smaller than those in a free gas are measured within the aerogel. This reduction is quite signifi cant at 1 bar, suggesting that He-3-He-3 and He-3-silica collisions occur w ith comparable frequencies at this density. However, a strong dependence of T-2 on P is still observed at low pressure, revealing a significant reduct ion of diffusion due to binary atomic collisions even for the lowest gas de nsity. Measurements over such a large range of gas densities indirectely pr obe collisional effects over a wide range of sizes up to tens of mum. In ou r sample, results are not consistent with a single geometrical mean free pa th for He-3-silican collisions.