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Dynamique RMN non linéaire et renversement temporel dans les mélanges d'3He-4He hyperpolarisés à basse température

Abstract : This thesis deals with the control of the effects on the dynamics of the NMR precession of the non-linear dipolar couplings between the sample and the detection coil (radiation damping), and of the distant dipolar interaction in a hyperpolarized liquid sample. Conventional treatments of nuclear magnetic resonance (NMR) ignore collective effects for spin dynamics and the evolution of local magnetization is described by a simple linear Bloch equation. This approach, usually justified in weakly polarised systems, turns out to be inappropriate in a highly polarised liquid. In this case, dipolar magnetic interactions introduce a significant non-linear and non-local contribution to the Bloch equation that leads to precession instabilities NMR experiments at low field (2 mT) are performed on spheroidal samples (0.5 cm³) filled with liquid helium mixtures at 1-2K. These mixtures consist in hyperpolarised ³He atoms (nuclearly oriented at room temperature bymetastability optical pumping in the gas phase) condensed in superfluid ⁴He. In these mixtures, intrinsic longitudinal relaxation times are many hours long at low ³He concentrations [4]. Dipolar fields reach a few µT (to be compared to the few nT obtained for proton at equilibrium in a standard 12T spectrometer), and bring a dominant contribution to the evolution of magnetisation. This model system allows a great freedom for the control of key experimental parameters (diffusion coefficient, dipolar field strength), though the choice of operating conditions (temperature, concentration, and nuclear polarisation). A simulation program of the non linear dynamic has also been used to brighten our understanding on the involved phenomena observed experimentally. The most important points presented in this work are : * The study and control of the coupling between the sample and the detection coil (radiation damping, cavity pulling) using a feedback electronic device. * The study of the dipolar echo produced by the NMR pulse sequence called magic sandwich inspired from solid state NMR and used here on a liquid sample for the first time. This pulse sequence reverse the effective dipolar interactions leading to a time-reversal evolution of the NMR precession. * The control and suppression of the dipolar instability using repeated magic sandwiches; the free induction decay lifetime has been increased by 3 orders of magnitude using this technique. * The demonstration of new composite pulse sequences allowing to out-perform the original magic sandwich pulse sequence. The tools developed during this work should allow a better control of the NMR dynamics in liquid systems where dipolar effects are important.
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https://theses.hal.science/tel-00623612
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Submitted on : Tuesday, February 7, 2012 - 5:43:14 PM
Last modification on : Thursday, March 17, 2022 - 10:08:09 AM
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  • HAL Id : tel-00623612, version 2

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Emmanuel Baudin. Dynamique RMN non linéaire et renversement temporel dans les mélanges d'3He-4He hyperpolarisés à basse température. Physique Atomique [physics.atom-ph]. Université Pierre et Marie Curie - Paris VI, 2010. Français. ⟨NNT : ⟩. ⟨tel-00623612v2⟩

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