2022
Journal articles
- titre
- Generating accurate tip angles for NMR outside the rotating-wave approximation
- auteur
- Christopher P Bidinosti, Geneviève Tastevin, Pierre-Jean Nacher
- article
- Journal of Magnetic Resonance, 2022, 345, pp.107306. ⟨10.1016/j.jmr.2022.107306⟩
- resume
- The generation of accurate tip angles is critical for many applications of nuclear magnetic resonance. In low static field, with a linear rather than circular polarized rf field, the rotating-wave approximation may no longer hold and significant deviations from expected trajectories on the Bloch sphere can occur. For rectangular rf pulses, the effects depend strongly on the phase of the rf field and can be further compounded by transients at the start and end of the pulse. The desired terminus can be still be achieved, however, through the application of a phase-dependent Bloch-Siegert shift and appropriate consideration of pulse timings. For suitably shaped rf pulses, the Bloch-Siegert shift is largely phase independent, but its magnitude can vary significantly depending on details of the pulse shape as well as the characteristics of the rf coil circuit. We present numerical simulations and low-field NMR experiments with 1 H and 3 He that demonstrate several main consequences and accompanying strategies that one should consider when wanting to generate accurate tip angles outside the validity of the rotating-wave approximation and in low static field.
- DOI
- DOI : 10.1016/j.jmr.2022.107306
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2021
Journal articles
- titre
- Doppler-free spectroscopy of the lowest triplet states of helium using double optical resonance
- auteur
- Ali Dia, Marie Abboud, Pierre-Jean Nacher, Geneviève Tastevin
- article
- The European Physical Journal D : Atomic, molecular, optical and plasma physics, 2021, 75, pp.223. ⟨10.1140/epjd/s10053-021-00233-1⟩
- resume
- Optical pumping on the 23S-23P transition (1083 nm) of metastable 3He or 4He atoms is used for science and applications. We report on its combination with an optical probe on the 23P{33S transition (706.5 nm) in a ladder-type double optical resonance scheme, with cw single-frequency pump and probe diode lasers. Theoretical expectations for level structure and absorption spectra were computed for both isotopes. Narrow Doppler-free spectral lines were experimentally obtained in low-pressure gas samples. The line weights and their Zeeman structure in weak magnetic field agreed with expectations. The precision of the line positions and line splittings was limited by generic errors of our commercial Fizeau wavemeter, which were independently characterised. The potential of this double resonance scheme for spectroscopic measurements on the three lowest triplet states of He was evaluated. The relevance of velocity- and sublevel-selective pumping combined with polarisation spectroscopy of probe absorption for a study of pressure-dependent population transfers in the 23P state was established.
- DOI
- DOI : 10.1140/epjd/s10053-021-00233-1
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- titre
- The outcomes of measurements in the de Broglie–Bohm theory
- auteur
- Geneviève Tastevin, Franck Laloë
- article
- Comptes Rendus. Physique, 2021, 22 (1), pp.99-116. ⟨10.5802/crphys.81⟩
- resume
- Within the de Broglie-Bohm (dBB) theory, the measurement process and the determination of its outcome are usually discussed in terms of the effect of the Bohmian positions of the measured system S. %} This article shows that the Bohmian positions associated with the measurement apparatus M can actually play a crucial role in the determination of the result of measurement. Indeed, in many cases, the result is practically independent of the initial value of a Bohmian position associated with S, and determined only by those of M. The measurement then does not reveal the value of any pre-existing variable attached to S, but just the initial state of the measurement apparatus. Quantum contextuality then appears with particular clarity as a consequence of the dBB dynamics for entangled systems.
- DOI
- DOI : 10.5802/crphys.81
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2019
Journal articles
- titre
- A fast MOSFET rf switch for low-field NMR and MRI
- auteur
- Pierre-Jean Nacher, Sashika Kumaragamage, Geneviève Tastevin, Christopher P. Bidinosti
- article
- Journal of Magnetic Resonance, 2019, 310, pp.106638. ⟨10.1016/j.jmr.2019.106638⟩
- resume
- TRansmit Array Spatial Encoding (TRASE) MRI uses trains of rf pulses alternately produced by distinct transmit coils. Commonly used coil switching involving PIN diodes is too slow for low- and ultra-lowfield MRI and would introduce wait times between pulses typically as long as each individual pulse in a few mT. A MOSFET-based rf switch is described and characterised. Up to hundreds of kHz, it allows for sub-ls switching of rf currents from a single amplifier to several coils with sufficient isolation ratio and negligible delay between pulses. Additionally, current switching at null current and maximum voltage can be used to abruptly stop or start pulses in series-tuned rf coils, therefore avoiding the rise and fall times associated with the Q-factors. RF energy can be efficiently stored in tuning capacitors for times as long as several seconds. Besides TRASE MRI, this energy storage approach may find applications in fast repeated spin-echo experiments. Here, a threefold acceleration of TRASE phase-encoding is demonstrated when MOSFET switches are used instead of fast reed relays.
- DOI
- DOI : 10.1016/j.jmr.2019.106638
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2018
Journal articles
- titre
- Nuclear hyperpolarization of He3 by magnetized plasmas
- auteur
- A. Maul, P. Blümler, P.J. Nacher, E. Otten, G. Tastevin, W. Heil
- article
- Physical Review A : Atomic, molecular, and optical physics, 2018, 98 (6), ⟨10.1103/PhysRevA.98.063405⟩
- resume
- We describe a method, referred to as PAMP (polarization of atoms in a magnetized plasma), that allows hyperpolarization of 3He nuclear spins at high magnetic field solely by excitation of a rf gas discharge. A magnetized plasma is obtained when the mean free path of the free electrons is much larger than their gyration radius in the rf gas discharge. Investigations of PAMP are carried out in the 1–15-mbar pressure range with rf excitation around 100 MHz. Quantitative NMR measurements at 4.7 T and room temperature show that, for different cell sizes and gas densities, 3He nuclear polarizations in the 1 to 9% range are achieved (i.e., larger than the Boltzmann equilibrium spin polarization of the free electrons). A description involving alignment-to-orientation conversion in the excited 23P state is proposed. The PAMP method appears as a very attractive alternative to established laser polarization techniques (spin exchange or metastability exchange optical pumping). Application to 3He nuclear magnetometry with a relative precision of 10^(−12) is demonstrated.
- DOI
- DOI : 10.1103/PhysRevA.98.063405
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- titre
- Surrealistic Bohmian trajectories do not occur with macroscopic pointers
- auteur
- G. Tastevin, F. Laloë
- article
- The European Physical Journal D : Atomic, molecular, optical and plasma physics, 2018, 72, pp.183. ⟨10.1140/epjd/e2018-90129-4⟩
- resume
- We discuss whether position measurements in quantum mechanics can be contradictory with Bohmian trajectories, leading to what has been called \textquotedblleft surrealistic trajectories\textquotedblright\ in the literature. Previous work has considered that a single Bohmian position can be ascribed to the pointer. Nevertheless, a correct treatment of a macroscopic pointer requires that many particle positions should be included in the dynamics of the system, and that statistical averages should be made over their random initial values. Using numerical as well as analytical calculations, we show that these surrealistic trajectories exist only if the pointer contains a small number of particles; they completely disappear with macroscopic pointers. With microscopic pointers, non-local effects of quantum entanglement can indeed take place and introduce unexpected trajectories, as in Bell experiments; moreover, the initial values of the Bohmian positions associated with the measurement apparatus may influence the trajectory of the test particle, and determine the result of measurement. Nevertheless, a detailed observation of the trajectories of the particles of the pointer can still reveal the nature of the trajectory of the test particle; nothing looks surrealistic if all trajectories are properly interpreted.
- DOI
- DOI : 10.1140/epjd/e2018-90129-4
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-
Conference papers
- titre
- A Fast MOSFET Switch for RF Pulse Management and Shaping in Low-Field NMR and MRI
- auteur
- Pierre-Jean Nacher, Sashika Kumaragamage, Geneviève Tastevin, Christopher Bidinosti
- article
- European Magnetic Resonance Meeting EUROMAR 2018, Jul 2018, Nantes, France
- resume
- TRansmit Array Spatial Encoding (TRASE) is a MRI method that achieves spatial encoding through repeated pulses of radiofrequency (RF) B1 fields from different phase-gradient transmit coils. It does not require the application of B0 gradients and is a distinct alternative to the standard MRI approach.1 An evaluation of the benefits and limitations of TRASE MRI at low field (a few mT) is under way using hyperpolarised gas samples.2,3 Fast switching between two or more transmit coils with negligible wait time between pulses is required to reduce the total RF encoding time. PIN diodes are often used for coil switching in MRI but bias control with RF isolation involves response times (several RF periods) which are too slow for the targeted low-frequency application. We have designed bidirectional MOSFET switches with floating and optically isolated gate control.4 A similar device was recently reported to be suitable as Q-switch in low-field NMR.5 Our tests have been performed with different RF power amplifiers and different loads at various frequencies (in the DC – 200 kHz range). An Apollo Tecmag console was used to manage RF pulses and TTL signals controlling the switches. RF currents were recorded using a digital oscilloscope by monitoring the voltage drop across a 1-Ω resistor inserted on the ground line at the output of the RF amplifier. Our MOSFET switch (with components rated for 500V and 32 A or for 1.7kV and 9A) has switching times below 1 μs. It has a low ON impedance (R~1 Ω) and a frequency-dependent OFF impedance (~500 pF capacitance), yielding efficient RF current switching in resistive loads and in transmit coils (various situations will be reported). Additionally, current switching at null current and maximum voltage can be used to abruptly stop or start pulses in series-tuned RF coils, therefore avoiding the rise and fall times associated with the Q-factors (Fig. 1). RF energy can be efficiently stored in tuning capacitors for times as long as hundreds of milliseconds. Besides TRASE MRI, this energy storage approach may find applications in fast repeated spin-echo experiments.
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- titre
- Exploration of TRASE MRI at Low Magnetic Field: Potential Performance and Limitations
- auteur
- Pierre-Jean Nacher, Geneviève Tastevin, Christopher P. Bidinosti
- article
- European Magnetic Resonance Meeting EUROMAR 2018, Jul 2018, Nantes, France. pp.574
- Accès au bibtex
-
- titre
- A Fast MOSFET RF Switch for TRASE MRI at Low Magnetic Field
- auteur
- P.-J Nacher, S Kumaragamage, G. Tastevin, C P Bidinosti
- article
- Joint annual meeting ISMRM-ESMRMB 2018, Proc. of the Intl. Soc. Mag. Reson. Med., Jun 2018, Paris, France. pp.1699
- resume
- TRansmit Array Spatial Encoding (TRASE) MRI uses trains of B1 pulses alternatively produced by distinct transmit coils. Commonly used coil switching involving PIN diodes is too slow for low-field MRI and would introduce wait times between pulses typically as long as each individual pulse (hence, significant diffusion-induced resolution loss in TRASE MRI of gas samples). A MOSFET-based RF switch is described and characterised. Up to 200 kHz, it allows for sub-µs switching of RF currents from a single amplifier to several coils with sufficient isolation ratio and no delay between pulses.
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- titre
- Unconventional trajectories on the Bloch Sphere: A closer look at the effects and consequences of the breakdown of the rotating wave approximation
- auteur
- Christopher B. Bidinosti, Pierre-Jean Nacher, Geneviève Tastevin
- article
- Joint annual meeting ISMRM-ESMRMB 2018, Proc. of the Intl. Soc. Mag. Reson. Med., Jun 2018, Paris, France. pp.2701
- resume
- TRASE MRI uses rapid π-pulses of phase gradient fields, and in general requires as many as two distinct phase-gradient coils per encoding direction. This tends to restrict one to large amplitude, linear B1 fields, which in low B0 field leads to a breakdown of the rotating wave approximation. We have studied this regime both numerically and experimentally. Our results show a rich behavior involving a complex interplay of the Bloch-Siegert shift, the B1 start and stop phase, and B1 amplitude transients.
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- titre
- Concomitant B1 Field in Low-Field MRI: Potential Contributions to TRASE Image Artefacts
- auteur
- Christopher B. Bidinosti, Pierre-Jean Nacher, Geneviève Tastevin
- article
- Joint annual meeting ISMRM-ESMRMB 2018, Proc. of the Intl. Soc. Mag. Reson. Med., Jun 2018, Paris, France. pp.2670
- resume
- TRansmit Array Spatial Encoding (TRASE) MRI uses trains of rf pulses produced by transmit coils which generate transverse fields of uniform magnitude and spatially varying directions. These coils also unavoidably generate concomitant rf fields, which in turn affect magnetisation dynamics during rf flips in low-field NMR. Bloch's equation are numerically solved to show that π-pulses imperfectly reverse transverse magnetisation and that the resulting error in azimuthal angle linearly increases with B1/B0, with the number of pulses in the TRASE pulse train, and with distance from the coil axis in the sample. This may induce significant image distortions or artefacts. Supporting experiments performed at 2 mT will be reported.
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2013
Journal articles
- titre
- Metastability exchange optical pumping of 3He gas up to hundreds of millibars at 4.7 Tesla
- auteur
- A. Nikiel-Osuchowska, Guilhem Collier, Bartosz Glowacz, Tadeusz Palasz, Z. Olejniczak, W. Weglarz, Geneviève Tastevin, Pierre-Jean Nacher, Tomasz Dohnalik
- article
- The European Physical Journal D : Atomic, molecular, optical and plasma physics, 2013, 67 (9), pp.200. ⟨10.1140/epjd/e2013-40153-y⟩
- resume
- Metastability exchange optical pumping (MEOP) is experimentally investigated in 3He at 4.7 T, at room temperature and for gas pressures ranging from 1 to 267 mbar. The 23S - 23P transition at 1083 nm is used for optical pumping and for detection of the laser-induced orientation of 3He atoms in the rf discharge plasmas. The collisional broadening rate is measured (12.0 ± 0.4 MHz.mbar-1 FHWM) and taken into account for accurate absorption-based measurements of both nuclear polarization in the ground state and atom number density in the metastable state. The results lay the ground for a comprehensive assessment of the efficiency of this method, by comparison with achievements at lower magnetic field strengths (1 mT - 2 T) over an extended range of operating conditions. Stronger hyperfine decoupling in the optically pumped metastable state is observed to systematically lead to slower build-up of nuclear orientation in the ground state, as expected. The nuclear polarizations obtained at 4.7 T still decrease at high gas pressures but in a less dramatic way than observed at 2 T in the same sealed glass cells. To date, thanks to the linear increase in gas densities, they correspond to the highest nuclear magnetizations achieved by MEOP in pure 3He gas. The improved efficiency puts less demanding requirements for compression stages in polarized gas production systems and makes high-field operation particularly attractive for magnetic resonance imaging of the lungs, for instance.
- DOI
- DOI : 10.1140/epjd/e2013-40153-y
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- titre
- Multiple echoes due to distant dipolar fields in NMR of hyperpolarized noble gas solutions
- auteur
- Steven W. Morgan, Emmanuel Baudin, Gaspard Huber, Patrick Berthault, Geneviève Tastevin, Maurice Goldman, Pierre-Jean Nacher, Hervé Desvaux
- article
- The European Physical Journal D : Atomic, molecular, optical and plasma physics, 2013, pp.67:29. ⟨10.1140/epjd/e2012-30470-0⟩
- resume
- We report on multiple echo measurements in hyperpolarized liquids using optically pumped spin-1/2 noble gas atoms: either129Xe dissolved in cyclohexane or 3He dissolved in superfluid 4He. An NMR pulse sequence 90° -tau -90° (with slice-selective flipping pulses for 129Xe experiments) was used and long echo trains have been observed in the presence of applied gradients due to average dipolar fields typically one order of magnitude larger than those of bulk water in high magnetic fields. We show that a mean field description is valid for explaining the multiple echoes observed in these liquids, even for spin temperatures as low as 10mK for 129Xe or 10µK for 3He, and the echoes originate from the distant dipolar fields within the samples. Numerical lattice simulations have been used to assess the effects of slice selection and of finite sample size in addition to those of atomic diffusion. They account for the observed echo widths and amplitudes much better than previously published models which disregard finite size effects that appear to be of key importance. This opens the way to using multiple echoes resulting from distant dipolar fields for the determination of the absolute magnetization in hyperpolarized liquids without signal calibration.
- DOI
- DOI : 10.1140/epjd/e2012-30470-0
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2010
Conference papers
- titre
- Fundamentals of metastability exchange optical pumping in helium
- auteur
- Marion Batz, Pierre-Jean Nacher, Geneviève Tastevin
- article
- JCNS Workshop on Modern Trends in Production and Applications of Polarized 3He, Jul 2010, Ismaning, Germany. pp.012002, ⟨10.1088/1742-6596/294/1/012002⟩
- resume
- Advances in metastability exchange optical pumping (MEOP) at high laser powers, but also at high gas pressures and high magnetic field strengths, has provided strong motivation for revisiting the understanding of the limitations of this powerful technique. A comprehensive model has been developped for improved description of the combined effects of OP, ME, and relaxation, and of detailed MEOP features observed over the broad range of operating conditions. A brief description is provided, with illustrative comparisons of computed and experimental results. This improved tool is used to explain the excellent photon efficiency of OP obtained at all field strengths. It is combined with an angular momentum budget approach to quantitatively investigate the newly discovered strong OP-enhanced polarisation losses that currently limits MEOP performance.
- DOI
- DOI : 10.1088/1742-6596/294/1/012002
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- titre
- An accurate optical technique for measuring the nuclear polarisation of 3He gas
- auteur
- Cavin Talbot, Marion Batz, Pierre-Jean Nacher, Geneviève Tastevin
- article
- JCNS Workshop on Modern Trends in Production and Applications of Polarized 3He, Jul 2010, Ismaning, Germany. pp.012008, ⟨10.1088/1742-6596/294/1/012008⟩
- resume
- In the metastability exchange optical pumping cells of our on-site production unit and of our other experimental set-ups, we use a light absorption technique to measure the 3He nuclear polarisation. It involves weak probe beams at 1083 nm, that are either perpendicular or parallel to the magnetic field and cell axis, with suitable light polarisations. When metastability exchange collisions control the populations of the sublevels in the 23S state, absolute values of the 3He ground state nuclear polarisation are directly inferred from the ratio of the absorption rates measured for these probe beams. Our report focuses on the transverse detection scheme for which this ratio, measured at low magnetic field for sigma and pi light polarisations, hardly depends on gas pressure or the presence of an intense pump beam. This technique has been systematically tested both in pure 3He and isotopic mixtures and it is routinely used for accurate control of the optical pumping efficiency as well as for calibration of the NMR system.
- DOI
- DOI : 10.1088/1742-6596/294/1/012008
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2009
Journal articles
- titre
- Using a cw 465nm laser to probe metastable He2 molecules
- auteur
- Geneviève Tastevin, Bartosz Glowacz, Pierre-Jean Nacher
- article
- Journal of Low Temperature Physics, 2009, pp.Online first. ⟨10.1007/s10909-009-0050-5⟩
- resume
- A cw 465~nm laser has been built for sensitive detection of helium metastable molecules by resonant absorption on the a3Σu+(v=0)-e3Πg(v=0) transition band. The frequency-mixing radiation is obtained from commercial laser diodes in a periodically-poled KTP non-linear crystal. The 1.3-nm tuning range includes the main rotational lines of 3He2, 4He2, and 3He-4He dimers. Measurements of absolute molecular densities down to a few 109 cm-3 are reported in low pressure (1-400 mbar) room temperature He gas excited by a weak rf discharge. Unsophisticated detection techniques provide signals with good signal-to-noise ratios thanks to the narrow absorption linewidths (a few GHz, due to Doppler and moderate collisional broadenings) in the fully resolved spectrum. Prospects for use or upgrade of this blue laser to probe the broadened and shifted molecular lines in condensed He are discussed.
- DOI
- DOI : 10.1007/s10909-009-0050-5
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2008
Journal articles
- titre
- Nonlinear NMR dynamics in hyperpolarized liquid 3He
- auteur
- Emmanuel Baudin, Michael E. Hayden, Geneviève Tastevin, Pierre-Jean Nacher
- article
- Comptes Rendus. Chimie, 2008, 11 (4-5), pp.560 - 567. ⟨10.1016/j.crci.2007.07.005⟩
- resume
- 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° 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.
- DOI
- DOI : 10.1016/j.crci.2007.07.005
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- titre
- NMR Time Reversal Experiments in Highly Polarised Liquid 3He-4He Mixtures
- auteur
- Emmanuel Baudin, Michael E. Hayden, Geneviève Tastevin, Pierre-Jean Nacher
- article
- Journal of Low Temperature Physics, 2008, 150 (3-4), pp.168 - 173. ⟨10.1007/s10909-007-9532-5⟩
- resume
- 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.
- DOI
- DOI : 10.1007/s10909-007-9532-5
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2007
Journal articles
- titre
- NMR Time Reversal as a Probe of Incipient Turbulent Spin Dynamics
- auteur
- Michael E. Hayden, Emmanuel Baudin, Geneviève Tastevin, Pierre-Jean Nacher
- article
- Physical Review Letters, 2007, 99, pp.137602. ⟨10.1103/PhysRevLett.99.137602⟩
- resume
- 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.
- DOI
- DOI : 10.1103/PhysRevLett.99.137602
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- titre
- Optical measurement of He-3 nuclear polarization for metastable exchange optical pumping studies at high magnetic field
- auteur
- K. Suchanek, M. Suchanek, A. Nikiel, T. Palasz, Marie Abboud, A. Sinatra, Pierre-Jean Nacher, G. Tastevin, Z. Olejniczak, T. Dohnalik
- article
- The European Physical Journal. Special Topics, 2007, 144, pp.67-74. ⟨10.1140/epjst/e2007-00109-8⟩
- resume
- An accurate optical method to measure the nuclear polarization of He-3 atoms in the 1(1)S ground state is described. The absorption of a weak, probe laser beam is used to measure the relative populations of two hyper. ne sublevels of the 2(3)S metastable state that are not addressed by the pumping laser beam. Since a common spin temperature between the ground and metastable states is established by metastable exchange collisions, the nuclear polarization can be derived from these absorption measurements. The method is highly sensitive, robust, and can be used to monitor the dynamics of optical pumping and relaxation processes without interfering with them. It was successfully implemented and tested in the 0.45-2.0 T magnetic field range at the He-3 gas pressure up to 67 mbar.
- DOI
- DOI : 10.1140/epjst/e2007-00109-8
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- titre
- Metastability exchange optical pumping of He-3 at high pressure and high magnetic field for medical applications
- auteur
- A. Nikiel, T. Palasz, M. Suchanek, Marie Abboud, A. Sinatra, Z. Olejniczak, T. Dohnalik, G. Tastevin, Pierre-Jean Nacher
- article
- The European Physical Journal. Special Topics, 2007, 144, pp.255-263. ⟨10.1140/epjst/e2007-00138-3⟩
- resume
- Systematic studies of the metastability exchange optical pumping (MEOP) process in He-3 gas at high magnetic fields (0.45, 0.9, 1.5, and 2 T) and pressures ( 32 and 64 mbar) are performed. The impact of experimental parameters such as laser power, beam pro. le, and shape of the pumping cell is evaluated. By varying the discharge intensity in the cell, the density of metastable state atoms and the plasma-induced nuclear relaxation rate are also controlled, and their effect on the MEOP efficiency can be investigated. Very accurate experimental results are obtained, opening the way to quantitative tests of a recently proposed model of the MEOP process at high magnetic field. We report selected MEOP results with nuclear polarizations exceeding 50 % at 64 mbar and 2 T, which represents a dramatic improvement in performances over MEOP at low magnetic field. The present findings suggest that still higher polarizations can be achieved in higher magnetic fields, and motivate investigations at higher gas pressures. New ways of producing hyperpolarized 3He for magnetic resonance imaging and medical applications can be envisaged, as most clinical whole-body scanners operate at 1.5 T.
- DOI
- DOI : 10.1140/epjst/e2007-00138-3
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2005
Journal articles
- titre
- Metastability Exchange Optical Pumping of Helium-3 at High Pressures and 1.5 T: Comparison of two Optical Pumping Transitions
- auteur
- Marie Abboud, Alice Sinatra, Geneviève Tastevin, Pierre-Jean Nacher, Xavier Maître
- article
- Laser Physics, 2005, 15, 4, pp.475-479
- resume
- 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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- titre
- NMR measurements of hyperpolarized 3He gas diffusion in high porosity silica aerogels
- auteur
- Geneviève Tastevin, Pierre-Jean Nacher
- article
- Journal of Chemical Physics, 2005, 123, pp.064506. ⟨10.1063/1.1997130⟩
- resume
- 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.
- DOI
- DOI : 10.1063/1.1997130
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2004
Journal articles
- titre
- High nuclear polarization of helium-3 at low and high pressure by metastability exchange optical pumping at 1.5 Tesla
- auteur
- Marie Abboud, Alice Sinatra, Xavier Maître, Geneviève Tastevin, Pierre-Jean Nacher
- article
- EPL - Europhysics Letters, 2004, 68, pp.480. ⟨10.1209/epl/i2004-10237-y⟩
- resume
- 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.
- DOI
- DOI : 10.1209/epl/i2004-10237-y
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-
- titre
- A broadband Ytterbium-doped tunable fiber laser for 3He optical pumping at 1083 nm
- auteur
- Geneviève Tastevin, Sébastien Grot, Emmanuel Courtade, Sylvain Bordais, Pierre-Jean Nacher
- article
- Applied Physics B - Laser and Optics, 2004, 78 (2), pp.145-156
- resume
- 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.
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-
- titre
- MRI of the lung using hyperpolarized He-3 at very low magnetic field (3 mT)
- auteur
- Christopher P. Bidinosti, Jamal Choukeife, Geneviève Tastevin, Alexandre Vignaud, Pierre-Jean Nacher
- article
- Magnetic Resonance Materials in Physics, Biology and Medicine, 2004, 16, pp.255-258
- resume
- 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.
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2003
Journal articles
- titre
- Restricted diffusion of polarised He-3 gas in aerogels
- auteur
- G. Tastevin, Pierre-Jean Nacher, J. Choukeife
- article
- Physica B: Condensed Matter, 2003, 329-333, pp.303-304
- resume
- Polarised He-3 is used to non-destructively probe by NMR the structure of silica aerogels. Using laser optical pumping large spin-echo signals are obtained with small amounts of gas, even at low magnetic field. Attenuation induced by applied field gradients is measured for equivalent He-3 pressures ranging from 5 mbar to 3 bar. Systematic diffusion studies are performed by NMR on custom-made (98% porous) and commercial (97% porous) aerogels used by research groups to study quantum fluids in confined geometries. The observed pressure dependence suggests a non-uniform structure of both aerogel samples on length scales up to tens of microns. (C) 2003 Elsevier Science B.V. All rights reserved.
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- titre
- Anomalous spin echoes in highly polarised liquid helium mixtures
- auteur
- G. Tastevin, N. Piegay, F. Marion, Pierre-Jean Nacher
- article
- Physica B: Condensed Matter, 2003, 329-333, pp.187-188
- resume
- NMR experiments are performed on laser polarised liquid helium mixtures in low field (2 mT) above 1 K. Concentrations range from 0.5% to 10% and nuclear polarisations up to 0.1. For large magnetisations, following a 90degrees-tau-180degrees RF pulse pair the spin echo signal detected at time 2tau is significantly smaller than expected from attenuation due to diffusion in the applied static field gradient. Unexpected echoes are also observed at times 3tau, 4tau, 5tau, etc. The first feature may be related to other effects of large dipolar fields in bulk samples (dynamic instabilities, altered transverse decay). Numerical simulations on lattices have been performed to quantitatively interpret all spin echo results. (C) 2003 Elsevier Science B.V. All rights reserved.
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- titre
- In-vivo NMR of hyperpolarized 3He in the human lung at very low magnetic fields
- auteur
- Christopher B. Bidinosti, J. Choukeife, Pierre-Jean Nacher, Geneviève Tastevin
- article
- Journal of Magnetic Resonance, 2003, 162, p.122-132
- resume
- We present NMR measurements of the diffusion of hyperpolarized 3He 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 3mT. O2-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 a plastic bag; the average apparent diffusion coefficient (ADC) in-vivo was 14.2±0.6 mm2/s, whereas the diffusion coefficient in the bag (3He diluted in N2) was 79.5±1 mm2/s. 1D ADC mapping with high SNR (~200- 300) demonstrates the real possibility of performing quality lung imaging at extremely low fields.
- Accès au bibtex
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2002
Journal articles
- titre
- NMR instabilities in highly magnetised liquid helium solutions
- auteur
- P. J. Nacher, N. Piegay, F. Marion, G. Tastevin
- article
- Journal of Low Temperature Physics, 2002, 126 (1-2), pp.145-150
- resume
- Spin dynamics have been shown to be dramatically altered in anisotropic samples when the internal dipolar field dominates the external static field gradients. In recent NMR experiments on laser polarised liquids (He or Xe), spectral clustering and precession instabilities have been observed. Early analyses have suggested the generic nature of these effects, and tentatively addressed their existence in bulk systems. We report here on. NMR. experiments conducted in spherical samples of laser-polarised liquid helium mixtures (0.5-10% He-3 in He-4 at 1-2K). We focus on the relative influence of magnetisation density and applied field gradients on FID signals induced by 90degrees tipping pulses. Numerical simulations are used to study the effects of distant dipolar couplings on local and global magnetisation. A discussion of the features of FID and spin echo signals is presented.
- Accès au bibtex
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- titre
- Magnetic field effects on the 1083 nm atomic line of helium
- auteur
- Emmanuel Courtade, François Marion, Pierre-Jean Nacher, Geneviève Tastevin, K. Kiersnowski, Tomasz Dohnalik
- article
- The European Physical Journal D : Atomic, molecular, optical and plasma physics, 2002, 21, pp.25-55. ⟨10.1140/epjd/e2002-00176-1⟩
- resume
- 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.
- DOI
- DOI : 10.1140/epjd/e2002-00176-1
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-
- titre
- Study of bulk longitudinal spin relaxation in He-3-He-4 liquid mixtures
- auteur
- N. Piegay, G. Tastevin
- article
- Journal of Low Temperature Physics, 2002, 126 (1-2), pp.157-162
- resume
- Longitudinal spin relaxation has been investigated in spin polarised liquid He-3-He-4 mixtures in cesium-coated glass cells. The contribution of wall relaxation is found to be either negligible or easy to separate, and bulk relaxation times can be derived front systematic studies. We report on relaxation measurements at saturated vapour pressure, for temperatures ranging from 1.1 to 3K. He-3 densities vary from 0.07 to 11 mmol.cm(-3) (He-3 concentrations between x(3) similar or equal to 0.3 and 34%), so that both normal and superfluid He-3-He-4 solutions are studied. At fixed temperature, the longitudinal relaxation time is inversely proportional to the He-3 number density over the whole range of investigated concentrations. In contrast, it is almost independent of temperature (changes do not exceed 10%). Comparison with published data and expected behaviours is discussed.
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- titre
- CPMG measurements and ultrafast imaging in human lungs with hyperpolarized helium-3 at low field (0.1 T).
- auteur
- E. Durand, G. Guillot, L. Darrasse, Geneviève Tastevin, A. Vignaud, D. Vattolo, J. Bittoun, Pierre-Jean Nacher
- article
- Magnetic Resonance in Medicine, 2002, 47 (1), pp.75-81
- resume
- This work reports the use of single-shot spin echo sequences to achieve in vivo diffusion gas measurements and ultrafast imaging of human lungs, in vivo, with hyperpolarized He-3 at 0.1 T. The observed transverse relaxation time of He-3 lasted up to 10 s, which made it possible to use long Carr-Purcelf-Meiboom-Gill echo trains. Preliminary NMR studies showed that the resolution of lung images acquired with hyperpolarized He-3 and single-shot sequences is limited to about 6 mm because of the diffusion of the gas in applied field gradients. Ultrafast images of human lungs in normal subjects, achieved in less than 0.4 s with the equivalent of only 130 mu mol of fully polarized He-3, are presented. Comparison with other studies shows that there is no SNR penalty by using low fields in the hyperpolarized case. Advantage was taken of the self diffusion-weighting of the rapid acquisition with relaxation enhancement (RARE) sequence to acquire apparent diffusion coefficient (ADC) images of the lungs. Time scales of seconds could be explored for the first time because there is no hindrance from T-2(star) as with the usual approaches. At 0.1 T, 180 degrees RF pulses can be repeated every 10 ms without exceeding specific absorption rate limits, which would not be the case for higher fields. Moreover, at low field, susceptibility-induced phenomena are expected to be milder. This supports the idea that low-field imagers can be used for hyperpolarized noble gas MRI of lungs and may be preferred for ADC measurements. Magn Reson Med 47:75-81, 2002. (C) 2002 Wiley-Liss, Inc.
- Accès au bibtex
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- titre
- NMR dynamics with large dipolar interactions in liquid helium-3 films
- auteur
- P. J. Nacher, F. Marion, B. Villard, G. Tastevin
- article
- Journal of Low Temperature Physics, 2002, 126 (1-2), pp.145-150
- resume
- In a growing number of experimental situations NMR dynamics is found to be deeply affected by strong dipolar couplings. We have developed models based on approximations of dipolar interactions and applied them to a thin film situation. Weakly damped, robust eigenmodes are found as solutions of the simplified Bloch equations in 2D systems. 3D numerical simulations also reveal the existence of spectral clustering (collapse of NMR spectra into sets of narrow lines at low tip angles. Systematic pulsed NMR experiments previously performed in thin films of laser-polarised helium-3 have been re-analysed. The resulting spectra consist of several well-resolved lines with the expected frequencies. The effect of applied field gradients on the characteristics of the modes has been studied. The results demonstrate the robustness of spectral clustering in highly polarised thin films.
- Accès au bibtex
-
Conference papers
- titre
- Optical pumping of helium-3 at high pressure and magnetic field
- auteur
- Pierre-Jean Nacher, Emmanuel Courtade, Marie Abboud, Alice Sinatra, Geneviève Tastevin, Tomasz Dohnalik
- article
- 2002, pp.2225-2236
- resume
- At low magnetic field, the efficiency of metastability-exchange optical pumping of helium-3 is known to be optimal for pressures around 1 mbar. We demonstrate on several examples (up to 32 mbar) that operating in a higher magnetic field (up to 0.12 T) can significantly increase the nuclear polarisations achieved at higher pressures. Since polarisation measurements cannot be made with the standard technique, we use a general optical method based on absorption measurements at 1083 nm to measure the polarisation of the atoms in the ground state.
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-
2001
Journal articles
- titre
- NMR instabilities and spectral clustering in laser-polarized liquid xenon
- auteur
- Karen L. Sauer, François Marion, Pierre-Jean Nacher, Geneviève Tastevin
- article
- Physical Review B: Condensed Matter and Materials Physics (1998-2015), 2001, 63, pp.184427
- resume
- To study how highly magnetized liquids behave in NMR experiments, we have performed low field NMR on laser-polarized liquid 129Xe, with nuclear polarization of up to 6%. Contrary to conventional NMR results, we find that instabilities develop after a large pulse, leading to an unexpectedly abrupt decay of the signal. In contrast, the NMR spectrum after a small pulse collapses to a series of unusually sharp lines (spectral clustering), whose widths correspond to precession lifetimes longer than a half minute. We discuss the key role of long-range dipolar coupling in such spin dynamics.
- Accès au texte intégral et bibtex
-
- titre
- NMR diffusion of hyperpolarised He-3 in aerogel: a systematic pressure study
- auteur
- G. Guillot, P. J. Nacher, G. Tastevin
- article
- Magnetic Resonance Imaging, 2001, 19 (3-4), pp.391--394
- resume
- Diffusion of hyperpolarised He-3 in a silica aerogel sample with 98% porosity was measured by NMR, and systematically studied as a function of pressure P in the range 20 mbar - I bar. Spin-echo amplitudes, recorded with pulsed gradients in the usual CMPG sequence, follow a monoexponential decay, and the decay rate varies quadratically with the gradient intensity: thus the apparent diffusion coefficient of the gas D can be evaluated. Our results show a power-law (1/D proportional to P-0.55), which could be consistent with a fairly broad mean free path distribution, implying some inhomogeneity in the aerogel structure. We have observed a spatial non-uniformity of D, and some deviation to the quadratic dependence of the decay rate with the gradient intensity, possibly related to sample structure inhomogeneity. (C) 2001 Elsevier Science Inc. All rights reserved.
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2000
Journal articles
- titre
- Kinetics of helium-3 laser optical pumping
- auteur
- M. Leduc, Pierre-Jean Nacher, Geneviève Tastevin, Emmanuel Courtade
- article
- Hyperfine Interactions, 2000, 127 (1-4), pp.443--449
- resume
- We report on optical pumping experiments in helium-3 using a new broad-band, all-fiber laser at 1083 nm. The kinetics of polarization build-up are compared to the numerical results of an appropriate model. Effects of laser power and bandwidth are discussed.
- Accès au bibtex
-
- titre
- Measurement of intrinsic longitudinal spin relaxation in He-3-He-4 solutions
- auteur
- N. Piegay, Geneviève Tastevin
- article
- Physica B: Condensed Matter, 2000, 284-288, pp.315-316
- resume
- Bulk longitudinal spin relaxation time T-1 has been studied in hyperpolarised liquid He-3-He-4 mixtures at saturated vapour pressure and at low magnetic field (2 mT). Wall relaxation is efficiently prevented by using a cesium-coated glass cell, and T(1)s as long as 10h have been observed. T-1 is found to vary with He-3 density n(3) as n(3)(alpha) with alpha = -0.90 +/- 0.02 at T = 1.1 K. This was obtained for several densities it, ranging from 0.07 to 11 mol/l, which correspond to He-3 concentrations alpha(3) ranging from 0.20 to 34%. An increase of T-1 with increasing temperature has also been observed at x(3) = 8% from 2 to 3.7 K, as expected in the non-degenerate regime. (C) 2000 Elsevier Science B.V. All rights reserved.
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-
- titre
- Spectroscopy of the helium 2 3S - 2 3P transition above 0.01 Tesla - Application to optical pumping studies
- auteur
- K. Kiersnowski, Emmanuel Courtade, F. Marion, Pierre-Jean Nacher, Geneviève Tastevin
- article
- Hyperfine Interactions, 2000, 127 (1-4), pp.451--454
- resume
- Optical pumping of helium makes use of the 2 S-3-2 P-3 transition at 1083 nm. We report on a study of this transition in magnetic fields up to 1.5 T. Based on these results, an optical method to measure nuclear polarisation in arbitrary field has been developed. Preliminary results on optical pumping at 0.1 T are presented.
- Accès au bibtex
-
- titre
- Spin polarised He-3 gas for NMR imaging
- auteur
- Pierre-Jean Nacher, Geneviève Tastevin
- article
- PHYSICA B, 2000, 284-288, pp.2053-2054
- resume
- Polarised He-3 gas has been used since 1996 to make in vivo lung NMR images of increasing quality. For the methodological studies performed by various groups so far, hyperpolarised (HP) gas is obtained by laser optical pumping. We present examples of HP gas MRI investigations with potential diagnostic applications. We report on our MRI work at low field (0.1 T), which could result in reduced requirements in equipment and He-3 gas. The growing interest in He-3 gas MRI should lead to the development of cost-efficient schemes to produce large amounts of HP gas and to efficiently recycle He-3. Cryogenic techniques have been proposed to polarise or compress 3He gas. Their potential interest is discussed considering recent technical developments. (C) 2000 Elsevier Science B.V. All rights reserved.
- Accès au bibtex
-
- titre
- Vapour pressure changes in spin-polarised liquid He-3 and He-3-He-4 mixtures
- auteur
- B. Villard, P. J. Nacher, G. Tastevin
- article
- Physica B: Condensed Matter, 2000, 284, pp.178-179
- resume
- We prepare spin-polarised samples of liquid He-3 and He-3-He-4 mixtures by fast liquefaction of a vapour highly polarised by laser optical pumping. Nuclear polarisations of order 25% are routinely obtained in the liquid. We perform temperature and pressure measurements and induce sudden polarisation changes by pulsed NMR. Heat is expected to be released when the polarisation is reduced, and the saturated vapour pressure is expected to be lower for an unpolarised liquid at fixed temperature. In this experiment, the liquid is weakly coupled to a 0.5 K heat sink in order to successively observe both effects of polarisation changes. We report the first observation of these effects when the polarisation is suddenly destroyed in a liquid mixture (x=10%). In pure He-3, the observed saturated vapour pressure changes systematically differ from those expected. (C) 2000 Elsevier Science B.V. All rights reserved.
- Accès au bibtex
-
- titre
- Spin diffusion measurements in spin-polarised liquid He-3-He-4 mixtures
- auteur
- N. Piegay, Geneviève Tastevin, Pierre-Jean Nacher
- article
- Journal of Low Temperature Physics, 2000, 121 (5-6), pp.785-790
- resume
- We report on investigations of spin diffusion in isotopic helium liquid mixtures using a multi-echo technique in applied magnetic field gradients. Samples (up to 1 cm(3) in volume) are obtained by liquefaction of optically polarised gas. NMR measurements are performed at saturated vapour pressure in a low magnetic field (2.2 mT). Signal-to-noise ratios stay high down to He-3 concentrations of order a few parts per thousand at 1K for instance. We demonstrate that the experimental accuracy of spin diffusion measurements is substantially improved using optically polarised mixtures. We have observed that the dynamics of the precession of the transverse magnetisation can be altered due to large dipolar fields at high magnetisation densities. Effects on spin-echo signals and implications for the extension of spin diffusion measurements with our technique are discussed.
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-
- titre
- Optically polarized helium-3 for NMR imaging in medicine
- auteur
- Geneviève Tastevin
- article
- Physica Scripta, 2000, T86, pp.46-50
- resume
- Recent investigations in magnetic resonance imaging in animals and human subjects have demonstrated that laser polarized helium-3 can be used to visualize airways and air spaces. This report presents the current performances and possible improvements of optical pumping techniques for the production of polarized gas. The specific properties and dynamics of the polarization inside the lungs am also discussed. They impact both the development of optimal imaging methods and the analysis of the data to extract physiological information. Applications to medical assessment of lung ventilation and function are considered.
- Accès au bibtex
-
- titre
- NMR instabilities in spin-polarised liquids: He-3, He-3-He-4 mixtures and Xe-129
- auteur
- Pierre-Jean Nacher, Geneviève Tastevin, B. Villard, N. Piegay, K. Sauer
- article
- Journal of Low Temperature Physics, 2000, 121 (5-6), pp.743-748
- resume
- We report on pulsed NMR experiments performed in low magnetic field (1-2 mT) on various spin-polarised liquid samples obtained from optically pumped gas. Thin films of pure He-3 and bulk samples of dilute He-3-He-4 mixtures are prepared with polarisations up to 40%. Xenon is liquefied (at 166 K) within a narrow U-shaped tube with polarisations of older 6%. In all these highly magnetised anisotropic samples, dipolar magnetic couplings have a strong influence on spin dynamics. Spect? al clustering is obtained for small tipping angles and usually results in precession lifetimes of several seconds. Much shorter precession lifetimes (down to 10 ms) are obtained for larger tipping angles in highly polarised samples. In all systems, 90 degrees tipping pulses are followed by precession signals which abruptly decay after a variable delay: as long as 1 s for moderate polarisations in xenon. Use of different liquids permits us to vary the effects of dipolar couplings, quantum exchange and spin diffusion, and to probe their influence on the observed phenomena.
- Accès au bibtex
-
- titre
- NMR of hyperpolarised He-3 gas in aerogel
- auteur
- G. Tastevin, P. J. Nacher, G. Guillot
- article
- Physica B: Condensed Matter, 2000, 284-288, pp.291-292
- resume
- We report preliminary low-field (2 mT and 0.1 T) NMR measurements on room temperature laser polarised He-3 gas imbibed into a 98% porous silica aerogel. The hyperpolarised helium is introduced into the cell in which the aerogel had been grown, and large NMR signals can be recorded. Long polarisation lifetimes (T-1 similar or equal to 50 s) have been observed, revealing a very weak specific wall relaxation in the cell. NMR of hyperpolarised He-3 gas is thus a potential tool to measure physical parameters inside the aerogels which are used to study quantum fluids in confined geometries. This has been demonstrated using a multi-echo technique to characterize spin diffusion in an applied magnetic field gradient. Spin diffusion was found to be isotropic and its pressure dependence was studied. An estimate of the mean free path for gas atoms in the aerogel is deduced from this behaviour. (C) 2000 Elsevier Science B.V. All rights reserved.
- Accès au bibtex
-
- titre
- Probing silica aerogel structure with spin-polarised helium-3 gas
- auteur
- G. Tastevin, P. J. Nacher, G. Guillot
- article
- Journal of Low Temperature Physics, 2000, 121 (5-6), pp.773-778
- resume
- We report on systematic low-field NMR measurements on room temperature He-3 gas imbibed into a 98% porous silica aerogel. Laser polarised helium is used to obtain large NMR signals at 0.1 T even for small amounts of gas. WE use a multi-echo technique in a controlled magnetic field gradient to characterize spin diffusion. Measurements are carried out for pressures P ranging from 10 mbar to 1 bar (mean free path of free He-3 ranging from 19 to 0.19 mum respectively). In a free gas, the damping rate 1/T-2 of the amplitude of the echoes is checked to be proportional to the known diffusion coefficient (T-2 proportional to P). Damping rates 2.5 times smaller than those in a free gas are measured within the aerogel. This reduction is quite significant at 1 bar, suggesting that He-3-He-3 and He-3-silica collisions occur with comparable frequencies at this density. However, a strong dependence of T-2 on P is still observed at low pressure, revealing a significant reduction of diffusion due to binary atomic collisions even for the lowest gas density. Measurements over such a large range of gas densities indirectely probe collisional effects over a wide range of sizes up to tens of mum. In our sample, results are not consistent with a single geometrical mean free path for He-3-silican collisions.
- Accès au bibtex
-
1998
Journal articles
- titre
- Magnetic resonance imaging using polarized noble gases
- auteur
- M. Leduc, Pierre-Jean Nacher, G. Tastevin
- article
- Laser Physics, 1998, 8 (3), pp.799--802
- resume
- A new technique of medical imaging based on NMR has been developed, making use of spin polarized gases, He-3 or Xe-129, introduced inside the human body, where the polarization lives long enough to allow images to be taken. 3D images have been recorded for the lungs of living animals and humans with a resolution of a few mm(3). The polarization methods are based on optical pumping, for which we discuss the possible improvements in the technology of the required cw infrared lasers. We also display the advantages opened up by MRT in low magnetic field. Finally we comment on the perspectives of this MRI method in the field of medicine and biophysics, especially in functional ventilation and perfusion imaging.
- Accès au bibtex
-
1997
Journal articles
- titre
- 1083 nm ytterbium doped fibre amplifier for optical pumping of helium
- auteur
- S. V. Chernikov, J. R. Taylor, N. S. Platonov, V. P. Gapontsev, Pierre-Jean Nacher, G. Tastevin, M. Leduc, M. J. Barlow
- article
- Electronics Letters, 1997, 33 (9), pp.787--789
- resume
- A combination of a narrow linewidth, tunable DBR diode laser and high power ytterbium fibre amplifier in a master oscillator power fibre amplifier (MOPFA) configuration is demonstrated for efficient optical pumping of helium. A compact and efficient diode pumped prototype system is shown to compare favourably with alternative laser sources for production of hyperpolarised He-3 using direct pumping for medical application in MRI.
- Accès au bibtex
-
- titre
- MRI of hyperpolarized gases in competition with nuclear medicine?
- auteur
- Pierre-Jean Nacher, Geneviève Tastevin, A. Constantinesco, P. Choquet, M. Wioland, M. Leduc
- article
- Medecine Nucleaire: Imagerie Fonctionnelle Et Metabolique, 1997, 21 (5), pp.285--292
- resume
- Optical pumping techniques with laserlight were recently used to polarize the nuclear spins of gaseous xenon and helium in order to achieve magnetic resonance imaging of ventilatory lung spaces. Due to their properties, hyperpolarized noble gases open new windows in functional ventilation and perfusion imaging. This article presents the state of the art of these techniques and discusses their perspectives with respect to standard nuclear medicine methods.
- Accès au bibtex
-
- titre
- Low-field He-3 nuclear magnetic resonance in human lungs
- auteur
- L. Darrasse, G. Guillot, Pierre-Jean Nacher, G. Tastevin
- article
- Comptes rendus de l’Académie des sciences. Série 2, 1997, 324 (11), pp.691--700
- resume
- We report the first in vivo NMR experiments performed with hyperpolarised He-3 gas at 0.1 T. Hyperpolarisation is achieved by laser optical pumping of He-3 atoms in the metastable triplet state. Longitudinal relaxation times of nuclear magnetisation of the order of 35 s are measured in human lungs. Very long transverse relaxation times of a few seconds are also observed. This work establishes the possibility of magnetic resonance imaging of human lung airways at low field.
- Accès au bibtex
-
1996
Journal articles
- titre
- Study of moderately concentrated spin-polarized He-3-He-4 mixtures above TF
- auteur
- G. Tastevin, B. Villard, Pierre-Jean Nacher
- article
- Czechoslovak Journal of Physics, 1996, 46, pp.237-238
- resume
- Laser optical pumping in low magnetic field provides high nuclear polarizations in gaseous helium mixtures, which are liquefied without substantial loss of spin orientation. Polarized liquid mixtures with He-3 concentrations from 0.05 to 0.30 have been studied between 0.5 and 2.4 K. Coating the inner surface of the glass sample container with cesium strongly reduces wall relaxation, so that bulk longitudinal relaxation times could be measured. Evaporation of polarized liquid samples allowed to retrieve large amounts of high pressure spin polarized room temperature gas into a detachable storage cell.
- Accès au bibtex
-
- titre
- High nuclear polarization in He-3 and He-3-He-4 gas mixtures by optical pumping with a laser diode
- auteur
- E. Stolz, M. Meyerhoff, N. Bigelow, M. Leduc, Pierre-Jean Nacher, G. Tastevin
- article
- Applied Physics B - Laser and Optics, 1996, 63 (6), pp.629--633
- resume
- New diode lasers delivering 50 mW output power at 1083 nm are shown to be efficient sources for optical pumping of helium. They can polarize nuclei in a He-3 gas up to M = 50% over the pressure range 0.4-1.6 torr. Larger nuclear polarizations M of He-3 nuclei, of order 80%, can be obtained in He-3-He-4 mixtures when the laser frequency is tuned to a He-4 line. A standard optical measurement of nuclear polarization M has been extended to the case of He-3-He-4 mixtures. The effect of various parameters on the steady-state polarization M and on the pumping time T-p is discussed.
- Accès au bibtex
-
- titre
- Recent results on hyperpolarized He-3-He-4 liquid mixtures
- auteur
- Pierre-Jean Nacher, E. Stoltz, G. Tastevin
- article
- Czechoslovak Journal of Physics, 1996, 46 (Supplement 6), pp.3025-3032
- resume
- Laser optical pumping in law magnetic field provides very high nuclear polarizations in gaseous helium mixtures, and is used to prepare polarized liquid. Wall relaxation in glass cells is efficiently reduced using cesium coatings, and bulk longitudinal relaxation times are measured. In highly magnetized samples, dipolar fields control the spin dynamics in anisotropic volumes and weak external magnetic field inhomogeneities. Long lived magnetostatic modes are observed by pulsed NMR. Detailed analysis of their frequency and damping gives information on magnetization density and spin diffusion coefficient in polarized mixtures. Experiments are performed above 0.2 K on mixtures with He-3 concentrations of order a few percents or larger. When phase separation occurs, the He-3-rich phase retains a high polarization.
- Accès au bibtex
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1994
Journal articles
- titre
- Vapour Pressure Measurements on Spin-Polarized Liquid He-3
- auteur
- G. Tastevin
- article
- Physica B: Condensed Matter, 1994, 194-196, pp.911-912
- resume
- Experimental investigation of liquid/vapour equilibrium of spin-polarized He-3 is under progress, using laser optical pumping techniques to achieve high nuclear polarization rates. So far, the vapour pressure has been observed to increase when the polarization is destroyed. Preliminary measurements on the temperature and polarization dependance of this increase are reported. They can be analyzed assuming that the rapid change in the polarization of the system is adiabatic.
- Accès au bibtex
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1992
Journal articles
- titre
- Investigation of Adsorption and Wetting of He-3 on Cesium and Cesiated Glass
- auteur
- G. Tastevin
- article
- Journal of Low Temperature Physics, 1992, 89 (3-4), pp.669--672
- resume
- Experiments have been carried out to investigate the binding of He-3 on cesium substrates, using optical pumping to spin-polarize the atoms. The behaviour of He-3 on the walls at low temperature can be analyzed through the evolution of the nuclear magnetization of the sample. Preliminary results are presented, including: (i) adsorption studies of gaseous He-3 on cesiated glass; (ii) magnetic relaxation time of polarized liquid He-3 on cesium and cesiated glass; (iii) evidence for wetting of liquid He-3 on cesium.
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- titre
- NMR Characterization of Phase Separation in Highly Polarized He-3 - He-4 Solutions above 450 mK
- auteur
- G. Tastevin
- article
- Journal of Low Temperature Physics, 1992, 89 (1-2), pp.317--324
- resume
- Laser optical pumping techniques have been successfully applied to concentrated He-3-He-4 gaseous mixtures. Using a He-3 cryostat, these mixtures were subsequently liquefied and studied down to 450 mK, using pulsed NMR techniques. Observation of highly nuclear polarized solutions and of phase separation (demixion) in the mixtures is reported. Measurements of magnetic relaxation times and the dynamics of coexisting phases in this system are discussed.
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1991
Journal articles
- titre
- Kinetic theory for quantum gases
- auteur
- Geneviève Tastevin, Pierre-Jean Nacher, Franck Laloë
- article
- Annalen der Physik, 1991, 48, pp.149
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- titre
- A kinetic equation for quantum gases (spin and statistics)
- auteur
- P. Nacher, G. Tastevin, F. Laloë
- article
- Journal de Physique I, 1991, 1 (2), pp.181-209. ⟨10.1051/jp1:1991124⟩
- resume
- We generalize our previous work on the compatibilty of kinetic equations with second virial corrections to the inclusion of spin and particle indistinguishability; the system is supposed to be sufficiently dilute for higher order virial density corrections (interactions and statistics) to be negligible. We show that the general idea of the “free Winger transform” can be extended to this situation; the function which appears in the kinetic equation becomes here a matrix which acts in the space of spin states of the particles. Assuming that the collisions are described by a hamiltonian which does not act on the spins (a very good approximation for nuclear spins), we write explicitly a kinetic equation which is valid for this case. The right hand side of the equation is an 18 dimension integral, as for spinless distinguishable particles, but here it contains an additional term due to statistics, which introduces commutators and anticommutators. We discuss the local conservation laws in this formalism and find, as expected, a total number of 8 conserved quantities for spin 1/2 particles (including three components of the magnetization). When the gas is at equilibrium, we obtain a pressure dependence which is in agreement with known calculations on spin polarized gases. We finally study the gradient expansion of the collision integral, and show that the zero-order (local) part is identical with the 4 terms (including identical spin rotation terms) obtained previously by Lhuillier et al. The first order (non-local) part contains many terms, wich we compare with those obtained by Silin in a context more closely related to the Landau theory.
- DOI
- DOI : 10.1051/jp1:1991124
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- titre
- A Kinetic Equation for Quantum Gases
- auteur
- Pierre-Jean Nacher, G. Tastevin, F. Laloë
- article
- Annalen der Physik, 1991, 48 (1-3), pp.149--168
- resume
- We apply the formalism of the free Wigner transform to the derivation of a kinetic equation valid for dilute quantum gases; this formalism is compatible with a precise treatment of two-body correlations and allows the inclusion of second virial corrections. We generalize the results obtained in previous articles where the calculation of the collision integral was limited to zero ("local") and first order gradient terms; here we use a more general ("non-local") form for the interaction term which is an integral of higher dimension (18). The conservation laws are studied in this formalism, which implies non-linear expressions for the local hydrodynamic quantities in terms of the function appearing in the kinetic equation. The pressure at thermal equilibrium is then evaluated and found to be equal to the value predicted by the Beth-Uhlenbeck formula, without adjusting any free parameter of the theory.
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1990
Journal articles
- titre
- Heat Conduction in Spin-Polarized Gaseous He-3 at Low Temperature
- auteur
- C. Larat, M. Leduc, Pierre-Jean Nacher, G. Tastevin, G. Vermeulen
- article
- Journal of Low Temperature Physics, 1990, 80 (5-6), pp.299--314
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1989
Journal articles
- titre
- Une équation cinétique pour les gaz quantiques
- auteur
- Geneviève Tastevin, Pierre-Jean Nacher, Franck Laloë
- article
- Journal de Physique, 1989, 50, pp.1879
- resume
- This article is the second of a series where we propose a theory of the dynamics of quantum gases with a precise treatment of the short range two body correlations. For this purpose, we use a variant of the Wigner transform, the "free transform" , which has a useful property in this context: if, before collision, two particles are uncorrelated, the incoming part of the free Wigner transform remains always exactly factorized. Therefore, instead of asssuming a factorization of the two particle distribution, it is less restrictive to make a similar assumption on the free transform. Time symmetry is broken when the correlations of the outgoing part of the free transform are ignored; this approximation, which is well in the spirit of Boltzmann, amounts to considering successive collisions as independent processes (the correlations present in the outgoing part could play a role if the same particles rapidly collided again, an unlikely event in dilute gases). Therefore, the succession of collisions is not treated exactly, but each collision is, even during interaction. One then obtains a closed set of equations which leads directly to a kinetic equation for the free distribution $ f $. The equation includes retardation and quantum refraction effects, and provides a generalization of the Boltzmann equation. The reconstruction of the two particle distribution function $ f_{II} $ from $ f $ reintroduces the effects of short range correlations and, more generally, gives the values of all one particle and two particle physical observables. The price to pay for not using the usual one particle distribution is that the relation between the local physical quantities (density of particles, energy density, etc.) becomes more complicated than in the usual theory, and includes terms which are non linear in $ f $. We nevertheless show how this formalism automatically satisfies the local conservation of all hydrodynamic quantities: particle, momentum and energy densities. The pressure tensor includes quadratic terms which correspond to second virial corrections. The temperature of the gas is defined from the kinetic energy of the particles when they are far apart from each other. We finally briefly discuss the connections of this formalism with the classical Enskog theory, or mean field type theories such as the Landau theory (in particular, we note the presence of recoil effects in the "molecular field")
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- titre
- Dynamique des gaz quantiques (particules discernables sans spin)
- auteur
- Pierre-Jean Nacher, Geneviève Tastevin, Franck Laloë
- article
- Journal de Physique, 1989, 50, pp.1907
- resume
- Nous appliquons les résultats de deux articles précédents, en particulier uneéquation cinétique généralisant l'équation de Boltzmann, à l'étude des gaz dilués quantiques hors d'équilibre. Par une simple extension de la méthode de Chapman Enskog appliquée à cette équation cinétique, nous calculons les propriétés hydrodynamiques du gaz. A l'ordre le plus bas (hydrodynamique d'Euler), nous obtenons des résultats pour la pression, la vitesse du son et la capacité calorifique qui, contrairement à ceux de la théorie de Boltzmann, contiennent les secondes corrections du viriel. A l'ordre suivant (hydrodynamique visqueuse, ou de Navier Stokes), nous obtenons les corrections du premier ordre en densité aux coefficients de transport, et montrons qu'il peut exister un terme de seconde viscosité, quadratique en densité. Ce formalisme donne accès aux parties des corrections du viriel des coefficients de transport qui dépendent des collisions à deux corps, mais pas à celles qui sont dues aux collisions ternaires. Il est quantique et permet de réobtenir l'expression de Beth et Uhlenbeck de la seconde correction du viriel en fonction des déphasages.
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-
- titre
- Dynamique des gaz quantiques (particules discernables sans spin)
- auteur
- P.J. Nacher, G. Tastevin, F. Laloë
- article
- Journal de Physique, 1989, 50 (14), pp.1907-1941. ⟨10.1051/jphys:0198900500140190700⟩
- resume
- Nous appliquons les résultats de deux articles précédents, en particulier une équation cinétique généralisant l'équation de Boltzmann, à l'étude des gaz dilués quantiques hors d'équilibre. Par une simple extension de la méthode de Chapman Enskog appliquée à cette équation cinétique, nous calculons les propriétés hydrodynamiques du gaz. A l'ordre le plus bas (hydrodynamique d'Euler), nous obtenons des résultats pour la pression, la vitesse du son et la capacité calorifique qui, contrairement à ceux de la théorie de Boltzmann, contiennent les secondes corrections du viriel. A l'ordre suivant (hydrodynamique visqueuse, ou de Navier Stokes), nous obtenons les corrections du premier ordre en densité aux coefficients de transport, et montrons qu'il peut exister un terme de seconde viscosité, quadratique en densité. Ce formalisme donne accès aux parties des corrections du viriel des coefficients de transport qui dépendent des collisions à deux corps, mais pas à celles qui sont dues aux collisions ternaires. Il est quantique et permet de réobtenir l'expression de Beth et Uhlenbeck de la seconde correction du viriel en fonction des déphasages.
- DOI
- DOI : 10.1051/jphys:0198900500140190700
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-
- titre
- Dynamics of Quantum Gases (Distinguishable Particles without Spin)
- auteur
- Pierre-Jean Nacher, G. Tastevin, F. Laloe
- article
- Journal de Physique, 1989, 50 (14), pp.1907--1941
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-
Conference papers
- titre
- Kinetic theory of quantum gases: beyond the Boltzmann equation
- auteur
- Geneviève Tastevin, Pierre-Jean Nacher, Franck Laloë
- article
- SPOQS III, 1989, Torino, Italy. pp.310
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1988
Journal articles
- titre
- Obtaining polarized liquid helium 3 from optically oriented gas
- auteur
- G. Tastevin, P.J. Nacher, L. Wiesenfeld, M. Leduc, F. Laloë
- article
- Journal de Physique, 1988, 49 (1), pp.1-6. ⟨10.1051/jphys:019880049010100⟩
- resume
- Spin polarized liquid helium three can be obtained by liquefaction of a gas polarized by laser optical pumping. We show that this method can lead to nuclear polarizations exceeding 40 % in a liquid at about 400 mK and almost zero pressure. The relaxation time of the orientation is a few minutes, comparable to the relaxation time of weakly polarized bulk liquid. The NMR signals given by the liquid are strongly shifted in frequency ; this provides a precise and convenient way to measure the polarization and to follow the motion of the liquid inside the experimental cell.
- DOI
- DOI : 10.1051/jphys:019880049010100
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- titre
- Non-Magnetic Metal and Glass He-3 Cryostat Immersed in a 4.2 K Bath
- auteur
- Pierre-Jean Nacher, G. Tastevin, L. Wiesenfeld
- article
- Review of Scientific Instruments, 1988, 59 (2), pp.304--306
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- titre
- Obtaining polarized liquid 3He from optically oriented gas
- auteur
- Geneviève Tastevin, Pierre-Jean Nacher, Laurent Wiesenfeld, Michèle Leduc, Franck Laloë
- article
- Journal de Physique, 1988, 49, pp.1
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1987
Journal articles
- titre
- Liquefaction of Highly Polarized He-3
- auteur
- Pierre-Jean Nacher, M. Leduc, G. Tastevin, L. Wiesenfeld
- article
- Japanese Journal of Applied Physics Part 1: Regular Papers Short Notes & Review Papers, 1987, 26, pp.205--206
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- titre
- Nuclear polarization and heat conduction changes in gaseous 3He
- auteur
- Michèle Leduc, Pierre-Jean Nacher, David Betts, James M. Daniels, Geneviève Tastevin, Franck Laloë
- article
- EPL - Europhysics Letters, 1987, 4, pp.59
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-
Theses
- titre
- Hélium trois polarisé: ondes de spin et liquéfaction du gaz
- auteur
- Geneviève Tastevin
- article
- Physique Atomique [physics.atom-ph]. Université Pierre et Marie Curie - Paris VI, 1987. Français. ⟨NNT : ⟩
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1986
Journal articles
- titre
- Quantum Properties of Spin-Polarized He-3 Optically Oriented by a LNA Laser
- auteur
- M. Leduc, F. Laloe, Pierre-Jean Nacher, G. Tastevin, J. M. Daniels, D. Betts
- article
- Journal of the Optical Society of America B, 1986, 3 (8), pp.P138--P140
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1985
Journal articles
- titre
- Direct detection of spin waves in gaseous 3He↑
- auteur
- G. Tastevin, P.J. Nacher, M. Leduc, F. Laloë
- article
- Journal de Physique Lettres, 1985, 46 (6), pp.249-254. ⟨10.1051/jphyslet:01985004606024900⟩
- resume
- In gaseous spin-polarized 3He at low temperature, spin-diffusion becomes oscillatory and gives rise to spin waves; these waves have a quality factor μM, where M is the (relative) nuclear magnetization and μ a dimensionless coefficient characteristic of the importance of exchange effects in binary collisions. We describe here an NMR technique where these oscillatory modes are directly excited and detected, with the use of two sets of radiofrequency coils (for induction and detection), each connected in opposition in order to create or detect pure magnetization gradients over the sample. The measurements of μ are in good agreement with theory as well as with previous experiments, but only at temperatures above 2 K; below, the measurements are affected by processes which are not fully understood, probably related to the formation of a 3He monolayer on the surface of the coating of solid hydrogen which covers the inner surface of the cell.
- DOI
- DOI : 10.1051/jphyslet:01985004606024900
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1984
Journal articles
- titre
- Spin rotation effects and spin waves in gaseous 3He↑
- auteur
- P.J. Nacher, G. Tastevin, M. Leduc, S.B. Crampton, F. Laloë
- article
- Journal de Physique Lettres, 1984, 45 (9), pp.441-448. ⟨10.1051/jphyslet:01984004509044100⟩
- resume
- Spin polarized gases such as H↑ or 3He↑ can propagate damped spin waves at low temperatures; their origin is the so-called « identical spin rotation effect » in collisions between identical particles. We report here the observation of such effects in 3He↑ in the temperature range 2 < T < 6 K, and preliminary measurements of the coefficient μ which characterizes the quality factor of the spin oscillations. The measurements are in agreement with the numerical calculations of C. Lhuillier.
- DOI
- DOI : 10.1051/jphyslet:01984004509044100
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Conference papers
- titre
- Observation of spin waves in gaseous 3He
- auteur
- Pierre-Jean Nacher, Geneviève Tastevin, Michèle Leduc, Stuart B. Crampton, Franck Laloë
- article
- LT 17, 1984, Karlsruhe, Germany
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