Polarized 3He gas target

The report briefly outlines a technique for nuclear polarization of 3He, which is based on optical pumping by laser. Much attention has recently been drawn to this technique. To polarize the nuclear spin of an atomic nucleus, atomic hyperfine interaction is generally used to select a specific nuclear spin state. The present technique is also based on this mechanism. Two different processes are available: direct use of optical pumping between different hyperfine structural levels (direct process) and the use of spin exchange reaction caused by polarizing other atoms by optical pumping (indirect process). There are some factors that can have an adverse effect on optical pumping, affecting the polarity and atomic density. Such factors include the relaxation between two ground states and radiation trapping. In the former, exchange reaction occurs between the two states to decrease the optical pumping efficiency. Large power laser is required to eliminate it. In the latter, photons resulting from natural emission during optical pumping are re-absorbed by the polarized atoms. The mechanism of and some problems with the direct and indirect processes are also discussed briefly. (N.K.)