Neutron Spin Filters

³He Neutron Spin Filters

Nuclear spin polarized ³He gas, produced by optical pumping, can be used to polarize or analyze neutron beams because of the strong spin dependence of the neutron absorption cross section for ³He. Compared to commonly used neutron polarizers such as supermirrors and Heusler crystals, neutron spin filters (NSF) have these advantages: 1) they are broadband and can polarize cold, thermal or hot neutrons effectively; 2) they can polarize large area and large divergence neutron beams without adding beam divergence; 3) they are in-line add-on devices, and 4) they can efficiently flip the neutron polarization by reversing the ³He nuclear polarization. This last capability can be performed using the adiabatic fast passage nuclear magnetic resonance technique, thus integrating the polarizer and the flipper into a single neutron polarizing device.

Polarized ³He is used to provide polarized beam capability for various instruments at the NIST Center for Neutron Research. The process involves the polarization of ³He gas contained within a glass cell which is placed on the instrument. By this method, polarized ³He can be used to polarize the neutron beam (polarizer) and/or select out the chosen spin state (analyzer). The high density of polarized ³He gas (10¹⁹ to 10²⁰ cm⁻³) required for an effective neutron polarizer can be produced using two optical pumping methods: spin-exchange (SEOP) {Walker97 , BCV60}, which is performed directly at high pressure (1-10 bar), and metastability-exchange (MEOP){CSW63}, in which the gas is polarized at low pressure (1 mbar) and then compressed. More detail is provided through the reference links below about neutron spin filters.

The NIST Center for Neutron Research (NCNR) initiated a polarized ³He neutron spin filter program in 2006. The goal is to polarize and/or analyze neutron beams for neutron scattering instruments where ³He NSFs are advantageous. At the NCNR, polarized ³He NSF devices have been applied to diffuse reflectometry, small-angle neutron scattering (SANS), and thermal neutron triple-axis spectrometry (TAS). Applications to more instruments (Multi Axis Crystal Spectrometer, Disk Chopper time-of-flight spectrometer, BT-1 powder diffractometer, Materials Diffractometer, MAGIK reflectometer) are under development. For more information about the capabilities specific to the NCNR neutron spin filter program, please select the provided links.