EMU, the cold-neutron backscattering spectrometer at the BRAGG Institute

The Bragg Institute is presently commissioning a cold-neutron backscattering spectrometer in the ANSTO OPAL research reactor neutron guide hall. First neutron spectra from its commissioning are presented. The 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 is realized through implementation of spherical focusing between the Si (111) crystal monochromator and analyser arrays, so as to achieve a high FWHM energy resolution, in order of 1.2 μeV. EMU also features a unique 7-meter long focusing guide between its two HOPG premonochromators, allowing measurements over the ± 31 μeV energy transfer range enabled by its Si monochromator Doppler drive, all the while maintaining high count rates at its 3He linear-position sensitive detector arrays. EMU is well suited to characterizing diffusion and tunneling processes as occurring in e.g. polymer chains, membranes, proteins, molecular crystals, between interstitial crystal lattice sites, as well as observing hyperfine splitting of nuclear energy levels. Relaxation times from a few 10 ps to over 1 ns are accessible for momentum transfers up to 1.95 Å^-1, and over temperatures ranging from 50 mK up to 800 K. Other sample environments such as pressure, magnetic fields, controlled gas delivery, etc. are also available upon request.