A background suppression chopper for the High-Flux Backscattering Spectrometer at NIST

Abstract

A characteristic of backscattering spectroscopy is the limited flux in achieving high resolution. This means that one requires a very high signal-to-noise ratio by keeping the background to a minimum. The high-flux backscattering spectrometer at the NIST Center for Neutron Research uses a phase-space transformation (PST) chopper to enhance the flux by a factor of 4.2. The chopper has six equal size sectors, alternating between open and pyrolytic graphite. The background in this spectrometer arises from neutrons from the main guide. Measurements show that neutrons which pass through an open sector of the PST chopper at the same time that the monochromatized beam passes through on its way to the sample, contribute to 60% of the background. In order to reduce this background while retaining 80% of the present neutron flux, we propose a second chopper placed between the local shutter and the entrance to the converging guide, synchronized with the PST chopper.