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Contributions à l'étude expérimentale de l'hélium-3 liquide polarisé

Abstract : Spin-polarized liquid helium-3 is prepared by laser optical pumping in low magnetic field and at room temperature, prior to fast liquefaction of the polarized sample. The use of a new helium-3 cryostat enabled us to obtain liquid helium-3 with polarization rates up to 25 % at well-stabilized temperatures (around 0.5 K). We could thereby study the effect of nuclear polarization on liquid-vapour equilibrium, and particularly on the saturated vapour pressure. Very sensitive capacitive gauges were developed. We estimated (to first order in M^2) the expected effects when the polarization M is suddenly destroyed. These effects were experimentally observed in helium-3/helium-4 mixtures ; in pure helium-3, only a transient increase in pressure has been recorded. We then describe in a third part a preliminary experiment which aimed at determining the longitudinal relaxation time T1 in mixtures. Relaxation on the walls is efficiently reduced by a cesium coating and T1s of order 20 minutes were observed. A careful determination of the helium-3 concentration in the liquid phase was made. Finally we studied the effects of dipolar field on transverse polarisation decay in our strongly polarized samples. We observed the free precession of polarization after a NMR pulse, and analysed in detail its decay time constant as a fonction of different parameters. This time constant drastically varied with the tipping angle, an effect which could be linked to NMR dynamical instabilities.
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  • HAL Id : tel-00011770, version 1

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Bérengère Villard. Contributions à l'étude expérimentale de l'hélium-3 liquide polarisé. Matière Condensée [cond-mat]. Université Pierre et Marie Curie - Paris VI, 1999. Français. ⟨tel-00011770⟩

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