Backscattering spectroscopy at the NIST Center for Neutron Research

We discuss the design of a new backscattering spectrometer that is soon to become operational at the NIST Center for Neutron Research. Both monochromator and analyzer are composed of spherically bent Si (1 1 1) crystals that focus the incident and scattered neutron beams. The bending increases the intrinsic lattice gradient of Si beyond its Darwin limit, resulting in an energy resolution of about 0.75 mu eV FWHM. The Doppler-driven monochromator should provide a wide dynamic range of +/- 50 mu eV. The elastic Q-range will cover 0.15-1.8 Angstrom(-1). The most novel component of this spectrometer is the phase-space-transform chopper. This device rotates at 4730 rpm while neutrons are Bragg-diffracted from sets of HOPG crystals affixed to its periphery. The process enhances the neutron flux at the backscattered energy of 2.08 meV, but at the expense of a larger horizontal divergence. This effectively reduces the divergence mismatch between primary and secondary spectrometers, a long standing problem with backscattering instruments. Simulations suggest a resultant flux increase of order 3 should be obtained. (C) 1998 Elsevier Science B.V. All rights reserved.