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Search Results: 1 - 10 of 189873 matches for " G. Tastevin "
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Magnetic Field Effects on the 1083 nm Atomic Line of Helium. Optical Pumping of Helium and Optical Polarisation Measurement in High Magnetic Field
E. Courtade,F. Marion,P. -J. Nacher,G. Tastevin,K. Kiersnowski,T. Dohnalik
Physics , 2002, DOI: 10.1140/epjd/e2002-00176-1
Abstract: The structure of the excited $2^{3}$S and $2^{3}$P triplet states of $^{3}$He and $^{4}$He in an applied magnetic field B is studied using different approximations of the atomic Hamiltonian. All optical transitions (line positions and intensities) of the 1083 nm $2^{3}$S-$2^{3}$P transition are computed as a function of B. The effect of metastability exchange collisions between atoms in the ground state and in the $2^{3}$S metastable state is studied, and rate equations are derived, for the populations these states in the general case of an isotopic mixture in an arbitrary field B. It is shown that the usual spin-temperature description remains valid. A simple optical pumping model based on these rate equations is used to study the B-dependence of the population couplings which result from the exchange collisions. Simple spectroscopy measurements are performed using a single-frequency laser diode on the 1083 nm transition. The accuracy of frequency scans and of measurements of transition intensities is studied. Systematic experimental verifications are made for B=0 to 1.5 T. Optical pumping effects resulting from hyperfine decoupling in high field are observed to be in good agreement with the predictions of the simple model. Based on adequately chosen absorption measurements at 1083 nm, a general optical method to measure the nuclear polarisation of the atoms in the ground state in an arbitrary field is described. It is demonstrated at $B\sim$0.1 T, a field for which the usual optical methods could not operate.
MRI of the lung using hyperpolarized He-3 at very low magnetic field (3 mT)
C. P. Bidinosti,J. Choukeife,G. Tastevin,A. Vignaud,P. -J. Nacher
Physics , 2004,
Abstract: Optical pumping of He-3 produces large (hyper) nuclear-spin polarizations independent of the magnetic resonance imaging (MRI) field strength. This allows lung MRI to be performed at reduced fields with many associated benefits, such as lower tissue susceptibility gradients and decreased power absorption rates. Here we present results of 2D imaging as well as accurate 1D gas diffusion mapping of the human lung using He-3 at very low field (3 mT). Furthermore, measurements of transverse relaxation in zero applied gradient are shown to accurately track pulmonary oxygen partial pressure, opening the way for novel imaging sequences.
NMR measurements of hyperpolarized 3He gas diffusion in high porosity silica aerogels
Geneviève Tastevin,Pierre-Jean Nacher
Physics , 2005, DOI: 10.1063/1.1997130
Abstract: Hyperpolarized 3He is used to nondestructively probe by NMR the structure of custom-made and commercial silica aerogels (97% and 98.5% porous). Large spin-echo signals are obtained at room temperature and very low magnetic field (2mT) even with small mounts of gas. Attenuation induced by applied field gradients results from the combined effects of gas diffusion and confinement by the porous medium on atomic motion. Nitrogen is used as a buffer gas to reach equivalent 3He pressures ranging from 5 mbars to 3.5 bars. The observed pressure dependence suggests a non-uniform structure of the aerogels on length scales up to tens of micrometers. A description by broad phenomenological distributions of mean free paths is proposed, and quantitatively discussed by comparison to numerical calculations. The investigated aerogel samples exhibit different effective diffusion characteristics despite comparable nominal porosities.
NMR Time Reversal as a Probe of Incipient Turbulent Spin Dynamics
Michael E. Hayden,Emmanuel Baudin,Geneviève Tastevin,Pierre-Jean Nacher
Physics , 2007, DOI: 10.1103/PhysRevLett.99.137602
Abstract: We demonstrate time reversal of nuclear spin dynamics in highly magnetized dilute liquid 3He-4He mixtures through effective inversion of long-range dipolar interactions. These experiments, which involve using magic sandwich NMR pulse sequences to generate spin echoes, probe the spatiotemporal development of turbulent spin dynamics and promise to serve as a versatile tool for the study and control of dynamic magnetization instabilities. We also show that a repeated magic sandwich pulse sequence can be used to dynamically stabilize modes of nuclear precession that are otherwise intrinsically unstable. To date, we have extended the effective precession lifetimes of our magnetized samples by more than three orders of magnitude.
Nonlinear NMR dynamics in hyperpolarized liquid 3He
Emmanuel Baudin,Michael E. Hayden,Geneviève Tastevin,Pierre-Jean Nacher
Physics , 2007,
Abstract: In a highly polarized liquid (laser-polarized 3He-4He mixtures in our experiment), dipolar magnetic interactions within the liquid introduce a significant nonlinear and nonlocal contribution to the Bloch equation that leads to instabilities during NMR evolution. We have launched a study of these instabilities using spin echo techniques. At high magnetizations, a simple 180 degree rf pulse fails to refocus magnetization, so we use a standard solid-state NMR pulse sequence: the magic sandwich. We report an experimental and numerical investigation of the effect of this sequence on unstable NMR evolution. Using a series of repeated magic sandwich sequences, the transverse magnetization lifetime can be increased by up to three orders of magnitude.
NMR Time Reversal Experiments in Highly Polarised Liquid 3He-4He Mixtures
Emmanuel Baudin,Michael E. Hayden,Geneviève Tastevin,Pierre-Jean Nacher
Physics , 2007, DOI: 10.1007/s10909-007-9532-5
Abstract: Long-range magnetic interactions in highly magnetised liquids (laser-polarised 3He-4He dilute mixtures at 1 K in our experiment) introduce a significant non-linear and non-local contribution to the evolution of nuclear magnetisation that leads to instabilities during free precession. We recently demonstrated that a multi-echo NMR sequence, based on the magic sandwich pulse scheme developed for solid-state NMR, can be used to stabilise the magnetisation against the effect of distant dipolar fields. Here, we report investigations of echo attenuation in an applied field gradient that show the potential of this NMR sequence for spin diffusion measurements at high magnetisation densities.
In-Vivo NMR of Hyperpolarized Helium-3 in the Human Lung at Very Low Magnetic Fields
Christopher P. Bidinosti,Jamal Choukeife,Pierre-Jean Nacher,Geneviève Tastevin
Physics , 2002, DOI: 10.1016/S1090-7807(02)00198-2
Abstract: We present NMR measurements of the diffusion of hyperpolarized helium-3 in the human lung performed at fields much lower than those of conventional MRI scanners. The measurements were made on standing subjects using homebuilt apparatus operating at 3 mT. Oxygen-limited transverse relaxation (T2 up to 15-35 s) could be measured in-vivo. Accurate global diffusion measurements have been performed in-vivo and in-vitro. 1D ADC mapping with high SNR demonstrates the real possibility of performing quality lung imaging at extremely low fields.
A broadband Ytterbium-doped tunable fiber laser for 3He optical pumping at 1083 nm
Genevieve Tastevin,Sebastien Grot,Emmanuel Courtade,Sylvain Bordais,Pierre-Jean Nacher
Physics , 2003, DOI: 10.1007/s00340-003-1325-1
Abstract: Large amounts of hyperpolarized 3He gas with high nuclear polarization rates are required for use in neutron spin filters or nuclear magnetic resonance imaging of human lung. Very high efficiency can be obtained by metastability exchange optical pumping using multimode lasers to excite the 23S-23P transition at 1083 nm. Broadband Ytterbium-doped tunable fiber lasers have been designed for that particular application. Different options for the architecture of the fiber oscillator are presented and compared. Emphasis is given to a linear cavity configuration that includes a high reflectivity fiber mirror and a low reflectivity tunable fiber Bragg grating. Optical measurements are performed to finely characterize the spectral behavior of the lasers. Atomic response is also quantitatively probed to assess the optimal design of the oscillator for optical pumping. Multimode operation matching the 2 GHz Doppler-broadened helium resonance line and tunability over more than 200 GHz are demonstrated. Boosting the output of this fiber laser with a Yb-doped fiber power amplifier, all-fiber devices are built to provide robust, high power turn-key sources at 1083 nm for improved production of laser polarized 3He.
High nuclear polarization of helium-3 at low and high pressure by metastability exchange optical pumping at 1.5 Tesla
Marie Abboud,Alice Sinatra,Xavier Maitre,Genevieve Tastevin,Pierre-Jean Nacher
Physics , 2003, DOI: 10.1209/epl/i2004-10237-y
Abstract: We perform metastability exchange optical pumping of helium-3 in a strong magnetic field of 1.5 T. The achieved nuclear polarization, from 80% at 1.33 mbar to 25% at 67 mbar, shows a substantial improvement at high pressures with respect to standard low-field optical pumping. The specific mechanisms of metastability exchange optical pumping at high field are investigated, advantages and intrinsic limitations are discussed. From a practical point of view, our results open the way to alternative technological solutions for polarized helium-3 applications and in particular for magnetic resonance imaging of human lungs.
Metastability Exchange Optical Pumping of Helium-3 at High Pressures and 1.5 T: Comparison of two Optical Pumping Transitions
Marie Abboud,Alice Sinatra,Geneviève Tastevin,Pierre-Jean Nacher,Xavier Ma?tre
Physics , 2005,
Abstract: At low magnetic field, metastability exchange optical pumping of helium-3 is known to provide high nuclear polarizations for pressures around 1 mbar. In a recent paper, we demonstrated that operating at 1.5 T can significantly improve the results of metastability exchange optical pumping at high pressures. Here, we compare the performances of two different optical pumping lines at 1.5 T, and show that either the achieved nuclear polarization or the production rate can be optimized.
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