Distillation of Nuclear Polarization of 3He-4He Mixtures in Very High Magnetic Fields

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We have used the method of fractional distillation to produce an enhanced nuclear polarization of a saturated 3He-4He mixture. For the experimental study reported in this paper, we have developed an all plastic distillation setup, which allows high 3He circulation rates at low temperatures (18 μmol/s at 150 mK) up to the highest magnetic fields (22.5 T). The magnetization has been measured with a torque magnetometer. The nuclear magnetic relaxation time T1 of the 3He-4He mixture was found to increase with the square of the magnetic field, indicating that T1 is dominated by surface relaxation. As a consequence, the enhancement of the magnetization due to the distillation process, which is amongst others proportional to T1, increased also with the square of the magnetic field. At the highest field at 140 mK, a polarization of 8% of the 3He-4He mixture has been obtained, 1.6 times the equilibrium value. Various ways to improve the set-up are discussed.

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