Continuously operating compact 3He-based neutron spin filter

Polarized 3He can be used as a spin filter to polarize a broad energy spectrum of neutrons. As a prototype for use on the single-crystal diffractometer (SCD) at the spallation neutron source (SNS), we have built a compact system to continuously polarize a 3He spin filter by spin-exchange optical pumping. Polarizing in the neutron beam provides a constant neutron polarization and reduces the sensitivity to relaxation mechanisms. The capability to operate in the presence of non-optimal magnetic field homogeneity allowed us to employ a highly compact solenoid only 9.5 cm in diameter and 20 cm long. Using only 7 W of laser light we maintained 44% 3He polarization in an 11cm3 cell, despite an overall cell relaxation time of ≈10h. Results from a test on the SCD at IPNS are discussed.