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Longitudinal nuclear spin relaxation in ³HE gas at low temperatures Chapman, Ross

Abstract

The first measurements of the temperature dependence of the intrinsic dipolar relaxation time T[sub 1B] due to binary collisions in dilute ³HE gas are reported. Sufficiently pure experimental conditions to observe T[sub 1B] were prepared by coating bare pyrex sample cells with clean neon gas. The experiments are performed at low temperatures (1.7 K to 19.0 K) where the colliding atoms have very low energy, so the effect of the long-range attractive forces is strongly felt and the measurements are sensitive to the depth and nature of the helium attractive well. Calculations of T[sub 1B] using the best available helium potentials have been fitted to the experimental results to test the parameters which describe the potential. The data favours a potential of the Bruch-McGee form, but having a slightly deeper attractive well of 11.5 K. The experiment also probes the adsorbed phase of ³HE via wall relaxation. Both wall relaxation and bulk gas relaxation are present in all measurements, but they can be identified by their density dependence. Measurements of wall relaxation time T[sub 1W] have been made on strongly relaxing and weakly relaxing surfaces, and the field dependence has been studied between 0.5 kG and 9.3 kG at temperatures of 2.6 K, 4.2 K and 8.0 K. The results of the field dependence are interpreted in terms of a model which considers relaxation of ³HE atoms diffusing on a plane and interacting via the dipolar coupling.

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