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DYNAMIQUE DE L'AIMANTATION DANS LES SOLUTIONS HELIUM3-HELIUM4 HYPERPOLARISEES

Abstract : We present a detailed study of the evolution of the nuclear magnetisation in 3He-4He polarised solutions at saturated vapour pressure at temperatures of order 1 K. Precise NMR measurements are made at low magnetic field (~mT), on 3He-4He hyperpolarised solutions which were produced by liquefaction of optically pump polarised gas. We have measured two intrinsic quantities: the longitudinal relaxation time and the coefficient of diffusion, which provide informations about effective atomic interactions in non-degenerate solutions. Precautions were taken (in particular, the use of cesium coating) to limit the effect of wall relaxation thereby allowing us to extract the relaxation due to dipolar interaction between the nucleus of 3He atoms. Relaxation times up to 8 hours have been measured in the most dilute solutions. At fixed temperature, dipolar relaxation time was found to be inversely proportionnal to the density of 3He, in agreement with the theory of dilute quantum mixtures. However, the temperature dependence does not agree with existing theoretical predictions, which should be revised. We have measured values of the coefficient of diffusion with great precision and determined the experimental conditions where the evolution of transverse magnetisation is not affected by the dipolar field. We have systematically studied the evolution of this magnetisation following a single 90° pulse as a function of magnetisation density of the sample and the applied gradient. We have observed the development of instabilities in the evolution of magnetisation due to the dipolar field. It was also found that the refocalisation of transverse magnetisation following a second 180° pulse is strongly affected by the dipolar field.
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Contributor : Nathalie Piegay <>
Submitted on : Friday, July 4, 2003 - 10:42:47 AM
Last modification on : Monday, December 14, 2020 - 9:49:31 AM
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Nathalie Piegay. DYNAMIQUE DE L'AIMANTATION DANS LES SOLUTIONS HELIUM3-HELIUM4 HYPERPOLARISEES. Matière Condensée [cond-mat]. Université Pierre et Marie Curie - Paris VI, 2002. Français. ⟨tel-00003086⟩

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