Studies on optical pumping cells (OPC) to polarize 3He

The technique applied at HMI to obtain nuclear-spin-polarized 3He, used in neutron spin filters (NSFs), is metastability-exchange optical pumping. To prepare efficient NSF, one must highly polarize 3He nuclei in the optical pumping volume (OPV) and reduce the polarization losses during the compression phase. Great progress has been achieved in reducing of depolarization due to the recent development of both, large polarization preserving piston compressors and long relaxation time filter cells. It is even more important to significantly enhance the 3He polarization rate during optical pumping in order to increase NSF efficiency. Different cells materials were tested, such as Duran and quartz glass. In order to use the laser light more efficiently and to decrease the risk of 3He depolarization due to unfavorable reflections, antireflection (AR) coatings were used on cell windows made of quartz glass. They were compared with the ones without coating, made of quartz, Duran and BK7 glass. The comparison of various techniques to mount the windows such as blowing, gluing or molecular diffusion was also conducted. It indicated that the molecular diffusion is the most suitable technique because of a better purity of the gas in the cell and the preservation of the optical flatness of the windows. Cells, for practical reasons each entirely made from the same material (Duran, Quartz glass) with windows mounted using this method, showed the best polarization performance