EMU, the cold-neutron backscattering spectrometer at the Bragg Institute, ANSTO

Abstract

The Bragg Institute is currently in the final installation stage of a cold-neutron backscattering spectrometer in the ANSTO OPAL research reactor neutron guide hall. This spectrometer, called EMU, is based on Si (111) crystal backscattering and extracts neutrons from a cold neutron guide via a double HOPG (002) crystal premonochromator setup. Backscattering occurs through implementation of spherical focusing between the Si (111) crystal monochromator and analyser arrays, aiming to deliver a spectrometer FWHM energy resolution in the order of 1.2 μeV. EMU also features a 7-metre long focusing guide located between the two premonochromators, a so-called graphite chopper alternating beam delivery to the backscattering crystal monochromator and then into the secondary spectrometer, and a linear Doppler drive modulating incident neutron energies over ± 31 μeV. Scattered, analysed neutrons are counted in 3He LPSD arrays. EMU is provisioned for future extensions of its dynamic range via higher-resolution, undeformed Si (111) crystal analyser arrays, and variable HOPG (002) crystal premonochromator reflection angles. Access to the EMU spectrometer will be via beam-time requests to the OPAL neutron-beam user facility. EMU is ideally suited for measuring relaxation times from a few 10 ps to over 1 ns, for momentum transfers up to 2 Å-1, and readily from cryogenic temperatures up to 700 K.