Polarized $^{3}$He Gas for Neutron Scattering

Nuclear spin polarized $^{3}$He gas, produced by either metastability-exchange (MEOP) or spin-exchange optical pumping (SEOP), can be used to polarize or analyze neutron beams because of the strong spin dependence of the absorption cross section for neutrons by $^{3}$He. Such neutron spin filters (NSFs) have applications in both neutron scattering and fundamental neutron physics. The advent of practical NSFs with $^{3}$He polarization values exceeding 70{\%} has led to substantial increase in these devices in the neutron scattering community worldwide. $^{3}$He NSFs are advantageous over conventionally used neutron polarizers in that they can polarize a broad wavelength band of neutrons and polarize large area and widely divergent neutron beams. $^{3}$He cells are often polarized off-line and transported to neutron instruments, hence long lifetime cells are critical. In addition NSFs are required to be compact and relatively insensitive to $^{3}$He relaxation induced by external magnetic fields. In the talk I will present the current status of the polarized $^{3}$He user program for neutron instruments at the NIST Center for Neutron Research. Topics include the production rate of highly polarized $^{3}$He by SEOP, cell development, magnetostatic cavity development, and interfaces between $^{3}$He spin filter devices and a variety of instruments.