Polarized He spin filters for neutron science

The large spin dependence of the absorption cross section for neutrons by He gas provides a method to polarize neutron beams. For certain applications, such polarized He-based neutron ”spin filters” have advantages over conventional neutron optical polarizing methods. Spin filters operate at all neutron wavelengths, can cover a large angular range and/or a large energy range, and decouple neutron polarization from energy selection. Both spin-exchange optical pumping (SEOP) and metastability-exchange optical pumping (MEOP) are currently being employed to polarize He spin filters at various neutron facilities worldwide. I will focus on the development and application of SEOP-based neutron spin filters at the National Institute of Standards and Technology, Center for Neutron Research (NCNR) [1]. The combination of long relaxation time spin filter cells, high power spectrally narrowed diode lasers, and the use of Rb/K mixtures have allowed us to reach He polarizations up to 85 % in spin filter cells ≈1 liter in volume. Studies have revealed limits to the achievable polarization from temperature-dependent relaxation [2] and unexplained magnetic field dependence for relaxation in SEOP cells [3]. Applications include neutron scattering methods such as triple-axis spectrometry and small angle neutron scattering, and fundamental neutron physics. In most neutron scattering applications, cells are transported to the beam line and stored in a magnetically shielded solenoid or box. A recent focus has been apparatus for wide-angle neutron polarization analysis. A measurement of the spin-dependence of the neutron-He scattering length was performed with a small, polarized He cell in a neutron interferometer and a He spin filter for accurate neutron polarimetry [4]. Use of spin filters in high flux neutron beams have revealed beam-induced alkali-metal relaxation and long term effects on SEOP spin filter cells [5].