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

Abstract

We report on systematic low-field NMR measurements on room temperature 3 He gas imbibed into a 98% porous silica aerogel. Laser polarised helium is used to obtain large NMR signals at 0.1 T even for small amounts of gas. We use a multi-echo technique in a controlled magnetic field gradient to characterize spin diffusion. Measurements are carried out for pressures P ranging from 10 mbar to 1 bar (mean free path of free 3 He ranging from 19 to 0.19 μm 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 coefficient (T 2 P). Damping rates 2-5 times smaller than those in a free gas are measured within the aerogel. This reduction is quite significant at 1 bar, suggesting that 3 He- 3 He and 3 He-silica collisions occur with comparable frequencies at this density. However, a stong dependence of T 2 on P is still observed at low pressure, revealing a significant reduction of diffusion due to binary atomic collisions even for the lowest gas density. Measurements over such a large range of gas densities indirectely probe collisional effects over a wide range of sizes up to tens of μm. In our sample, results are not consistent with a single geometrical mean free path for 3 He-silica collisions.