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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.

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.

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.

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.

Polarised 3 He 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 3He 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.

We report results of pulsed NMR experiments performed in low magnetic field (1.3 mT) on thin films of liquid 3 He. A high nuclear polarisation is obtained by laser optical pumping in the vapour prior to fast liquifaction. In these highly magnetised anisotropic samples, classical dipolar couplings have a strong influence on spin dynamics. Very narrow NMR spectra, resulting from precession lifetimes of several seconds, can be obtained in situations where inhomogeneous broadening would be expected for independently precessing spins (when a strong gradient is applied). Conversely, precession lifetimes as short as 10 ms are found to result from large tipping angles in highly polarised samples (even in a very uniform field). These results are compared to previous findings in liquid helium mixtures, and to recent predictions on spectral clustering and instabilities induced by dipolar field inhomogeneities.

Polarised 3He 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 3He gas. The growing interest in 3He gas MRI should lead to the development of cost-efficient schemes to produce large amounts of HP gas and to efficiently recycle 3He. Cryogenic techniques have been proposed to polarise or compress 3He gas. Their potential interest is discussed considering recent technical developments.

We report preliminary low-field (2 mT and 0.1 T) NMR measurements on room temperature laser polarised 3He 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 (T1≃50 s) have been observed, revealing a very weak specific wall relaxation in the cell. NMR of hyperpolarised 3He 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.

A new technique for maintaining large 3 He nuclear spin polarizations ( M ≈50%) in liquid 3 He- 4 He mixtures (concentration x ≈5%) is described. 3 He atoms are continuously circulated between the liquid mixture and a room temperature volume in which they are polarized by optical pumping. The experiment takes place in a low, homogeneous magnetic field ( B 0 ⋍3G), so that the magnetization of the sample plays a key role in NMR experiments. When the nuclear spins are polarized in the upper energy Zeeman state, a precessing transverse magnetization can spontaneouly appear in the sample.

Heavy alkali metals are weak binding substrates for liquid helium films. A wetting transition exists on cesium surfaces, while rubidium surfaces are found to be wetted by liquid helium, even at low temperatures. A prewetting transition is predicted to exist on rubidium. An experimental investigation of these phenomena is being pursued using the heat conduction technique to monitor the presence of helium films. We report recent results which provide quantitative data on the wetting properties of helium on these substrates.

We report the results of pressure, temperature, and isotopic concentration measurements in tubes lined with a thermally driven, flowing superfluid film. For pure 4 He films, vapour refluxing generates gradients in both pressure and temperature. With the addition of a few percent of 3 He, pressure gradients are dramatically reduced and the refluxing of 4 He atoms in the vapour couples gradients in temperature and isotopic concentration. We have observed that the 3 He concentration at the hot end of a narrow tube can be decreased by a factor greater than 10 4 . These results can be interpreted in terms of a recently developed model which includes a description of the hydrodynamics of the refluxing vapour.

The transport properties of dilute spin polarized gases exhibit interesting dependences on the polarization M at low temperature, due to quantum indistinguishability between colliding particles. For the heat conduction κ of polarized gaseous helium 3, recent calculations have checked the convergence of the moment expansion of the out of equilibrium density matrix. Four to five moments are necessary to achieve a 1% accuracy. At the same time the changes of κ with M have been reliably measured in the l.3–4.2 K range, with great care to control all the heat fluxes reaching the cell. The temperature dependence of the changes Of κ with M is now found in satisfactory agreement with theory.