Wide-Angle Neutron Polarization Analysis

We have tested a polarizer-analyzer-spin flipper system based solely on ³He spin filters on the Multi-Axis Crystal Spectrometer (MACS) at the NIST Center for Neutron Research. The compact system is housed by a 36 cm diameter, vertical solenoid. Nuclear magnetic resonance (NMR) is used to flip the ³He polarization in the ³He polarizer, thereby inverting the polarization of the incoming neutron beam. The polarization of neutrons scattered into a wide angular range are analyzed by a second ³He spin filter cell. The system as been demonstrated on MACS and the first physics experiment is scheduled for Nov. 2010.

The top figure shows a top view of the MACS instrument, with an enlarged view of the spin filter system:  the neutron beam (red) passes through the neutron-compatible solenoid and is polarized by a ³He spin filter. After being scattered from a sample (located in a cryostat that is lowered into the solenoid), one or two curved ³He cells analyze the polarization of the widely scattered neutrons.

The development of the analyzer cells has revealed a new phenomenon in the relaxation of ³He.  Whereas our ³He cells are normally constructed from a boron-free aluminosilicate glass, the curved wide-angle (120 deg) cells were fabricated from fused quartz. Despite the long relaxation times we achieved, the polarization was lower than expected due to temperature dependence for the ³He relaxation.  For upcoming experiments we will employ aluminosilicate cells with reduced angular range (about 30 deg), which will meet the experimental needs.  We will develop a solution for full angular coverage.

Successful demonstration of wide-angle polarization analysis is the first step towards implementation for neutron scattering.  This capability will greatly enhance the range and depth of both fundamental and applied studies of magnetic materials.