In our experiments, the nuclear polarization is produced by optical pumping of the helium gas at 300K. Solid state LNA lasers, used and developed in our laboratory, deliver several watts at 1083 nm. Polarization M as high as 85% have been obtained. Commercially available laser diodes are also used, but their performance is limited by their low output power (50mW) [1]. It has been improved via combination with a doped silica fiber amplifier [2], and fiber lasers for 1083 nm are now available. Our efforts concentrate on studies of helium optical pumping in non-standard conditions of pressure and magnetic field. Production of highly polarized gas at 300K and pressure above 1 bar [3] permits use of helium as a probe in biomedical magnetic resonance imaging investigations [4]. In-vivo experiments in humans in low field (0.1T) are performed in collaboration with NMR experts at Univ. Paris-Sud [5], and  lung images have been obtained. Clinical evaluation of this new diagnosis tool will soon be conducted in a standard 1.5T tomograph in the Kremlin-Bicêtre Hospital, in collaboration with medical doctors from the Trousseau Hospital.

    Simultaneously, our fundamental research program on liquid phases of polarized helium-3 is actively pursued. Experiments are done using double cells. Polarization is produced in a room temperature volume. It is connected by a tube to a low temperature part where the helium sample is liquefied and studied. Spin relaxation on the wall at low temperature is reduced by using cesium coatings. The magnetization lifetime is determined by dipole-dipole relaxation in bulk liquid. It is 5 min. at 0.5K in pure liquid helium-3, rapidly increasing as the atomic density decreases through dilution in liquid helium-4 (up to a few hours) [6].
    Using sensitive pressure gauges and a temperature stabilized helium-3 cryostat specially developed in our laboratory, we aim at to measure the chemical potential of polarized liquid, through the determination of its saturating vapor pressure. From the variation of the binding energy of liquid helium-3 or its entropy as a function of M, one hopes to extract interesting information about the microscopic structure of this liquid, for which diverging hypotheses have been proposed.

    To study spin polarized solutions at temperatures where the system is significantly Fermi degenerate, we use a home-made dilution refrigerator as well. Unique results have been obtained by NMR on the dynamics of the macroscopic magnetization of the system [7,8]. Study of the phase separation of the solutions, and systematic measurement of their chemical potential, should reveal the role of spin correlations, just as the vapor pressure would in the case of pure liquid helium-3.

 

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(updated on May 9th, 2001 by P.J. Nacher )

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References

[1] E.  Stoltz, M. Meyeroff, N.Bigelow, M. Leduc, P.-J. Nacher, G. Tastevin, Appl. Phys. B 63 (1996) 629-633.

[2] S. Chernikov, J. Taylor, N. Platonov, V. Gaponstev, P.-J. Nacher, G. Tastevin, M. Leduc, M. Barlow, Electr.Lett. 33 (1997) 787-788.

[3] Brevets CNRS :  96/01973, 97/11553 et 98/16366.

[4] A. Constantinesco, P. Choquet, M. Wioland, M. Leduc, P.-J. Nacher, G. Tastevin, Méd. Nucl :Imagerie Fonctionnelle et Métabolique21 (1997) 285-292.

[5] L. Darrasse, G. Guillot, P.-J. Nacher, G. Tastevin, C. R. Acad. Sci. Paris324 (1997) 691-700.

[6] G. Tastevin, B. Villard, P.-J. Nacher, Czesch. J. Phys. 46-S1 (1996) 127-238 (Proc. of LT21 conf.).

[7] P.-J. Nacher, E. Stoltz, G. Tastevin, Czesch. J. Phys. 46-S6 (1996) 3025-3032 (Proc. of LT21 conf.).

[8] E. Stoltz, Thèse de Doctorat de l'Univ. Paris VI, Déc.1996.

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