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

Abstract : 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 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 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 coefficient (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 significant at 1 bar, suggesting that He-3-He-3 and He-3-silica collisions occur with comparable frequencies at this density. However, a strong 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 mum. In our sample, results are not consistent with a single geometrical mean free path for He-3-silican collisions.
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https://hal.archives-ouvertes.fr/hal-02018524
Contributor : Geneviève Tastevin <>
Submitted on : Wednesday, February 13, 2019 - 9:26:53 PM
Last modification on : Saturday, March 23, 2019 - 1:38:30 AM

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  • HAL Id : hal-02018524, version 1

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G. Tastevin, P. J. Nacher, G. Guillot. Probing silica aerogel structure with spin-polarised helium-3 gas. Journal of Low Temperature Physics, 2000, 121 (5-6), pp.773-778. ⟨hal-02018524⟩

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