Nuclear polarizationn of 3He gas at low temperatures by optical pumping

Abstract

A colour centre laser system has been developed that can produce over 60% nuclear polarization of 3He atoms in the gas phase at room temperature. We discuss in this particle the efficiency of the optical pumping technique in 3He as a function of the pumping intensity and other important parameters. At low temperatures, the low rate of the metastability exchange collisions limits the maximum nuclear polarization to relativity small values. To obtain higher nuclear polarization at low temperatues, we have developed a polarization transfer technique in which the optical pumping is done in 3He gas at room temperature and the polarization is transported by spin diffusion to a second region of gas at liquid helium temperatures (1.7≤T≤4.2 K). Loss of polarization at low temperautres is strongly reduced by molecular hydrogen which freezes on the cold surfaces. The efficiency of the polarization transfer was measured and found nearly complete under good conditions. Nuclear polarizations up to 50% were obtained in ...