Effects of high-flux neutron beams on ³He cells polarized in situ with spin-exchange optical pumping

Polarized ³He produced by spin-exchange optical pumping (SEOP) has potential as a neutron spin filter for polarization and polarization analysis in many neutron-scattering and neutron particle physics applications. The advantage of the SEOP method is its suitability for providing continuous stable polarization over the course of long experiments. However, we have discovered that exposure to high neutron flux leads to additional strong relaxation mechanisms in the optically polarized alkali-metal vapor used to polarize the ³He. At a neutron flux density of 4.7×10⁹ cm⁻² s⁻¹, the alkali-metal relaxation rate increased from 100 to 1000 s⁻¹ leading to reduced alkali-metal polarization. Other effects such as time dependence and gas composition dependence were explored to help understand the processes. In this paper we discuss our observations and present possible solutions for practical use of SEOP as a neutron spin filter for high-flux density applications.