Upgrade of the OSIRIS primary spectrometer

Abstract

A proposed upgrade is presented for the primary part of the indirect time-of-flight near-backscattering spectrometer OSIRIS at the ISIS Facility. This upgrade comprises a new supermirror guide with non-linear defocusing and focusing sections, a new wavelength bandwidth chopper system and a flexible slit system to adjust the divergence. The new neutron guide has an elliptical defocusing section, a curved section and an elliptical focusing section. The estimated gain factor in intensity is wavelength dependent between 4 and 7 at the sample position with a homogeneous divergence distribution on a smaller beam spot. The divergence will increase by up to a factor 3 compared to the present guide, but can be reduced by up to a factor 5 through the slit system. The increased guide cross section requires a system of two counter-rotating disc choppers with larger disc diameters than the present ones for the selection of the wavelength bandwidth. Estimates for the performance indicate that the proposed upgrade will keep the OSIRIS spectrometer competitive in the years to come.