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Wide-Angle Neutron Polarization Analysis

Summary:

The capabilities of many neutron scattering instruments would be greatly enhanced by neutron polarization analyzers that can cover a wide angular range. 

Description:

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 has been demonstrated on MACS and the first physics experiment was performed in Nov. 2010 and the results published in Physical Review Letters.

MACS Instrument (top view)

Figure: Apparatus for wide-angle polarization analysis.

The 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.

Analyzer Cells

Photograph: Six Analyzer cells constructed from GE180 glass
(Photograph by: Neutron Physics Group)

The development of the analyzer cells revealed a new phenomenon in the relaxation of 3He. Whereas our 3He cells are normally constructed from a boron-free aluminosilicate glass (GE180), 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 an unexpected temperature dependence for the 3He relaxation. For the Nov. 2010 experiment we employed aluminosilicate cells with reduced angular range (about 30 deg). During the recent long shutdown of the NIST reactor, the NIST glass/optical shop developed a method for fabricating wide-angle cells from GE180 glass (see Fig. 9). We have successfully obtained relaxation times of 100 h to 400 h in these cells and 3He polarization values of 75 %. In addition we have constructed a new neutron-compatible solenoid with improved magnetic field homogeneity and decreased losses for the NMR-based spin-flip. New software has been developed to facilitate operation on the beam line. The entire improved apparatus will be employed on MACS in 2013. Successful demonstration of wide-angle polarization analysis is the first step towards implementation for neutron scattering. This capability will greatly enhance fundamental and applied studies of magnetic materials.

Lead Organizational Unit:

pml

Staff:

Neutron Physics Group

Thomas R. Gentile