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Optical pumping on the 2 $$^3$$ S–2 $$^3$$ P transition (1083 nm) of metastable $$^3$$ He or $$^4$$ He atoms is used for fundamental science and applications. We report on its combination with an optical probe on the 2 $$^3$$ P–3 $$^3$$ S 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 absorption lines were experimentally obtained in low-pressure gas samples. The line weights and 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 2 $$^3$$ P state was established.

International audience; Tuned pick-up coils with high quality factors $Q$ are used in NMR and MRI for high-sensitivity and low-noise detection. However, large $Q$-factors introduce bandwidth issues at low frequency and the associated enhanced currents may cause significant radiation damping effects, especially with hyperpolarised samples. Signal feedback can be used to actively control these currents and adjust the detection bandwidth without resistive losses. Capacitive and inductive coupling methods are compared using detailed models and the operating conditions for efficient feedback with negligible noise penalty are discussed. Several high-impedance commercial preamplifiers have been found to affect the resonance characteristics of tuned coils in a gain-dependent way, or could not be used in low-frequency NMR because of oscillations at large positive gain. This is attributed to an undocumented internal feedback, and could be neutralised using external feedback. The implementation of an inductive coupling scheme to feed a suitably amplified phase-adjusted signal back into the PU coils of low-field NMR systems is described, and three experimental applications are reported. One system is used for NMR studies of distant dipolar field effects in highly polarized liquid $^3$He without or with radiation damping. The moderate intrinsic $Q$-factor ($\approx$7) could be reduced (down to 1) or increased (up to 100) to control transient maser oscillations. Another system was used for MRI of water samples around 2~mT with $Q\approx$190 Litz-wire detection coils. The detection bandwidth was increased by actively reducing the $Q$-factor to obtain uniform sensitivities in images and avoid artifacts introduced by intensity corrections. Finally, parallel acquisition in MRI was performed using two separately tuned detection coils placed above and below the sample. They were actively decoupled using two feedback systems. For an imaging field of view smaller than the sample, artifact-free unfolded images demonstrate the efficiency of this active coil decoupling scheme.

International audience

International audience; We describe a method, referred to as PAMP (polarization of atoms in a magnetized plasma), that allowshyperpolarization 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 theirgyration radius in the rf gas discharge. Investigations of PAMP are carried out in the 1–15-mbar pressure rangewith rf excitation around 100 MHz. Quantitative NMR measurements at 4.7 T and room temperature showthat, 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 involvingalignment-to-orientation conversion in the excited 23P state is proposed. The PAMP method appears as a veryattractive alternative to established laser polarization techniques (spin exchange or metastability exchange opticalpumping). Application to 3He nuclear magnetometry with a relative precision of 10^(−12) is demonstrated.

International audience; We describe an actively shielded cylindrical RF transmit coil producing a highly uniform internal field (±0.5%) over an extended volume and a strongly suppressed (÷20) external field. Direct field mapping and experimental checks using in-situ NMR and MRI of polarised 3 He at low temperature demonstrate performance consistent with numerical field computations.

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., Accepted for publication in the Journal of Low Temperature Physics

International audience; 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.

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.

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% 3 He in 4 He at 1–2K). We focus on the relative influence of magnetisation density and applied field gradients on FID signals induced by 90° 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.

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.

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.

We report on systematic low-field NMR measurements on room temperature 3 He 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 3 He ranging from 19 to 0.19 μm 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 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 3 He- 3 He and 3 He-silica collisions occur with comparable frequencies at this density. However, a stong 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 μm. In our sample, results are not consistent with a single geometrical mean free path for 3 He-silica collisions.

Optical pumping of helium makes use of the 2 3S–2 3P 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.

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 3 He 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.

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 3 He and bulk samples of dilute 3 He- 4 He mixtures are prepared with polarisations up to 40%. Xenon is liquefied (at 166 K) within a narrow U-shaped tube with polarisations of order 6%. In all these highly magnetised anisotropic samples, dipolar magnetic couplings have a strong influence on spin dynamics. Spectral 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° 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.

As a first step towards thermodynamic measurements in highly polarised dense3He fluids, an accurate determination of the4He chemical potential μ4was performed in unpolarised phase-separated3He-4He liquid mixtures at low pressures (0-1 bar) over a temperature range 0.12 - 0.65 K. A method introduced by H. London and relying on heat flush effects was used. Two volumes containing : a) a cold, phase-separated helium mixture and b) a warmed, pure4He liquid are connected by a narrow tube, and their temperatures are recorded under various conditions. The results agree with existing data obtained by the same technique for T >0.2K, but cannot be analysed with the simple regular solution model fitting vapour pressure data at T >0.6K. The sensitivity of the technique is shown to be sufficient to observe expected effects of nuclear polarisation on μ4.

We report the first in vivo NMR experiments performed with hyperpolarised 3He gas at 0.1 T. Hyperpolarisation is achieved by laser optical pumping of 3He 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.

Chen et al. [Phys. Rev. A 81, 033422 (2010)] claim that adding ${}^{4}\mathrm{He}$ to spin-exchange optical pumping (SEOP) cells confines ${}^{3}\mathrm{He}$ atoms to a diffusion-limited region, which effectively reduces the wall relaxation factor $X$. Here $X$ is a phenomenological parameter used to characterize typical SEOP cells; it quantifies the observation that the measured linear slope of the relaxation rate with alkali-metal-atom density exceeds the spin-exchange rate. In addition they claim to demonstrate that magnetic-field gradients cause a diffusive motion of ${}^{3}\mathrm{He}$. We argue that these claims and other central conclusions drawn by Chen et al. [Phys. Rev. A 81, 033422 (2010)] are inconsistent with the physics of diffusion or should not be made from the data as presented. We also draw attention to several additional issues associated with the results, analysis, and presentation of data in Chen et al. [Phys. Rev. A 81, 033422 (2010)] that confound clear interpretation.

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 a3He gas up toM = 50% over the pressure range 0.4–1.6 torr. Larger nuclear polarizationsM of3He nuclei, of order 80%, can be obtained in3He-4He mixtures when the laser frequency is tuned to a4He line. A standard optical measurement of nuclear polarizationM has been extended to the case of3He-4He mixtures. The effect of various parameters on the steady-state polarizationM and on the pumping timeT p is discussed.

Optical measurements of the nuclear polarizationM of an optically pumped pure3He gas have been performed. They consist in monitoring the circular polarization of various helium lines emitted by the discharge. We have demonstrated the possibility to use the orange line (587.5 nm) to measureM and we have calibrated this technique over the pressure range 0.4–6.5 Torr. The circular polarization of the light has been analyzed using a new device which includes a static polarimeter and photodiodes as light sensors. The intensity of the discharge was modulated and the detection was made at the frequency of modulation.

Laser optical pumping in low magnetic field provides very high nuclear polarizations in gaseous helium mixtures, and is used to prepare polarized liquid. Wall relaxation in glass cells is effiently 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 with3He concentrations of order a few percents or larger. When phase separation occurs, the3He-rich phase retains a high polarization.

International audience; MRI of hyperpolarised gases is usually performed with fast data acquisition to achieve high spatial resolutions despite rapid diffusion-induced signal attenuation. We describe a double-cross k-space sampling scheme suitable for slow low angle shot (SLASH) acquisition and yielding an increased SNR. It consists of a series of anisotropic partial acquisitions with a reduced resolution in the read direction, which alleviates signal attenuation and still provides a high isotropic resolution. The advantages of SLASH imaging over conventional FLASH imaging are evaluated analytically, using numerical lattice calculations, and experimentally in phantom cells filled with hyperpolarised 3He-N2 gas mixtures. Low-field MRI is performed (here 2.7 mT), a necessary condition to obtain long T2* values in lungs for slow acquisition. Two additional benefits of the SLASH scheme over FLASH imaging have been demonstrated: it is less sensitive to the artefacts due to concomitant gradients and it allows measuring apparent diffusion coefficients for an extended range of times.

Typos and contact address corrected.; International audience; 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.

Version 2 (final one) includes a new figure and a new table. Typos of Version 1 have been corrected, in particular in Fig. 3 and in Table 2 (now Table3). The final publication is available at link.springer.com (with open access).; International audience; 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.

NMR free induction decay signals obtained in highly polarized liquid3He-4He mixtures in a U-shaped cell and in a coil-shaped cell significantly differ from the signal, one would obtain from independently precessing spins. We have analyzed the spin dynamics using two different models. Both consider the precession of the local magnetization in the dipolar field produced by the remainder of the sample. The first model explains the observed phenomena qualitatively; the second, solved numerically, reproduces satisfactorily the structure of experimental spectra.

An efficient technique for maintaining large3He nuclear spin polarizations in liquid3He-4He mixtures has been developed and used to polarize samples contained in narrow tubes. Results obtained in a U-shaped cell and a coil-shaped cell are compared. A systematic study of the effects of dipolar fields within the sample on the characteristics of the observed long-lived magnetostatic modes is in progress. NMR signals obtained in the coil-shaped cell reveal phase separation and the existence of a highly polarized3He-rich region in the sample.

Wetting properties of liquid helium on rubidium metal have been investigated in the temperature range 1.0–1.7 K using a heat conduction method. The rubidium surface is found to be wetted under saturated vapor conditions. Prewetting transitions have been observed for rather weak offsets from saturation, indicating that rubidium is close to the nonwetting limit at T = 0 K. Presumably because of substrate inhomogeneities, the prewetting transitions are experimentally found to be continuous and hysteretic. No evidence for the prewetting critical point has been found up to 1.7 K. The prewetting line verifies a simple linear relation between the liquid-vapour surface tension and the two third power of the chemical potential offset, but does not conform quantitatively to the simple model currently used.

This article comprises the first part of a study concerning the effect that a flow of gaseous4He has on the concentration distribution of3He atoms in the presence of a super fluid film. We present a simple model in which hydrodynamic effects in the gas phase (diffusion and viscosity), and local thermodynamic equilibrium with a superfluid film are considered. Results are derived and discussed for the simple case in which a heat flow is sustained along a cylindrical tube lined with a helium film. This heat flux drives a superfluid flow in the film, and a corresponding counterflow of4He in the vapour. The pressure, temperature,3He concentration, and film thickness profiles along the tube are computed. Over a wide range of conditions, a dramatic reduction of the3He concentration, a large temperature increase, and a spectacular film thinning towards hotter regions are predicted to result from a heat flow. The results of a series of experiments supporting this model will be presented in a forthcoming article.

We have piloted mechanical compression of spinpolarized 3 He by a titanium piston compressor. Questions of materials and design are discussed, followed by a thorough investigation of relaxation sources in the course of compression. The latter are traced mainly to regions with large surface to volume ratio, through which fast passage is demanded, therefore. We conclude from this feasibility study that polarized 3 He may be compressed this way up to many bars without serious polarization losses.

7 pages; International audience; In low field nuclear magnetic resonance (NMR) it is desirable to combine proper characteristics of the detection scheme with a good signal to noise ratio. For example, a reduced coupling between the sample and the detection coil is needed for NMR with highly magnetized samples and a large bandwidth is required in magnetic resonance imaging (MRI). We discuss a solution based on a simple active feedback circuit that preserves the signal to noise ratio as opposed to traditional solutions which do not. We give illustrations of its use in experiments on low temperature hyperpolarized liquid 3He-4He mixtures and in hyperpolarized 3He gas MRI.

12 pages; International audience; 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.

21 pages; International audience; 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.

The effect of a refluxing4He gas facing a Rollin film on the concentration distribution of3He atoms is described. In some situations, a large concentration enhancement of the colder part can result. Possible applications are briefly discussed.

We have prepared a large sample of spin polarized liquid 3 He- 4 He at 500 mK. Polarization is obtained by laser optical pumping, and studied by pulsed NMR. We have observed a nuclear magnetization lifetime T 1 longer than 1 hour, and an anomalously long, non linear decay of tranverse magnetization.

Alkali metal surfaces have been suggested not to be wetted by helium at zero temperature. Several experiments have been realized to check if these surfaces, as well as other weak binding substrates, indeed exhibits this particularly interesting property at any finite temperature. Although works are still in progress, essential results are (i) substrates non wetted by helium films do exist (ii) prewetting jumps from low to high coverage have been observed for weakly wetted ones (iii) the consequences of substrate roughness and chemical impurities are suspected to be important and remain to be assessed. These results are reviewed and discussed. Applications of those substrates for both3He and4He isotopes are considered.

3 He-gas is spin polarized by the method of optical pumping of metastables and metastability exchange in a low pressure gas discharge. At a pressure of p ≈ 1.5 Torr a volume of 1 l is polarized within about 30 s to a degree of 50% with 300 mW of incident light from an argon-ion laser pumped LNA laser, tuned to the λ = 1.083 μm resonance line. The polarized gas is compressed by a Toepler pump into a target cell of 120 cm 3 volume. In a first attempt a steady state polarization of 30% has been achieved in the target at a pressure of 685 Torr. The paper analyses the essential parameters governing this technique and pilotes its experimental realization.

International audience; 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.

International audience; Two rather similar historical evolutions are evoked, each one originating in fundamental spin studies by physicists, and ending as magnetic resonance imaging (MRI), a set of invaluable tools for clinical diagnosis in the hands of medical doctors. The first one starts with the early work on nuclear magnetic resonance, the founding stone of the usual proton-based MRI, of which the basic principles are described. The second one starts with the optical pumping developments made to study the effects of spin polarization in various fundamental problems. Its unexpected outcome is a unique imaging modality, also based on MRI, for the study of lung physiology and pathologies.

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., Comment: Accepted for publication in C. R. Chim. (2007), doi:10.1016/j.crci.2007.07.005 v2: typo corrected in Eq. (2)

A dilute gas of3He exhibits an interesting dependence of its transport properties on the nuclear spin polarization, resulting from the indistinguishability of the fermions during binary collisions. This article reports experimental results on the heat conductivity of gaseous3He in the 1.2–4.2 K range, the gas being polarized by laser optical pumping. The heat conductivity is found to change with the nuclear polarization. Compared to preliminary data already reported, the present work has been carried out with more care to control all the heat fluxes reaching the measurement cell, and the temperature range has been extended. The results are compared to recently improved calculations. The temperature dependence of the effect is now found to be in satisfactory agreement with theory.

We have operated an electrooptical modulator to generate sidebands to the laser used for optical pumping of a helium-3 gas. Spreading the laser power over the Doppler absorption profile of the metastable helium atoms can enhance the efficiency of the optical pumping process. This improvement is discussed in terms of velocity changing collisions and compared to the result of a velocity selective optical pumping experiment.

International audience; 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.

The main findings of the european PHIL project (Polarised Helium to Image the Lung) are reported. State of the art optical pumping techniques for polarising ^3He gas are described. MRI methodological improvements allow dynamical ventilation images with a good resolution, ultimately limited by gas diffusion. Diffusion imaging appears as a robust method of lung diagnosis. A discussion of the potential advantage of low field MRI is presented. Selected PHIL results for emphysema are given, with the perspectives that this joint work opens up for the future of respiratory medicine., Comment: To be published in Proc. ICAP 2004 (19th Int. Conf. on Atomic Physics, Rio, July 26-30 2004)

Hyperpolarized 3He is used to non-destructively 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 (2 mT) even with small amounts 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 nonuniform 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.

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., Comment: Submitted July 2003, published online December 2003 (http://www.springerlink.com). V3 symbols corrected in Figs 10-11; minor final changes included

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., Comment: accepted for publication in Europhysics Letters

Optical pumping of 3He 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 3He at very low field (3 mT). Furthermore, measurements of transverse relaxation in zero applied gradient are shown to accurately track pulmonary O2 partial pressure, opening the way for novel imaging sequences.