The capabilities of many neutron scattering instruments would be greatly enhanced by neutron polarization analyzers that can cover a wide angular range.
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.
(Top) Apparatus for wide-angle polarization analysis. (Bottom) Photograpah of a analyzer cell construced from three sections of resized fused quartz tubing. (Photograph by: NI&D group)
Lead Organizational Unit:pml
Neutron Physics Group