The OSIRIS diffractometer and polarisation-analysis backscattering spectrometer

The OSIRIS instrument at ISIS combines a longwavelength powder diffractometer with a polarisation-analysis backscattering spectrometer. The diffractometer can access wavelengths up to 70 A with a resolution of better than 1% Deltad/d. The very high counting rate at shorter wavelengths is ideal for in situ, real-time and parametric experiments. The spectroscopy section incorporates an array of graphite crystals arranged in near-backscattering to give a high counting rate with 25-mueV energy resolution. The incident beam is polarised using a supermirror bender and the scattered beam will be polarisation-analysed by a He-3 spin filter that is in the process of being constructed. The spin-filter system consists of a fibre laser, a peristaltic or rotary-diaphragm pump and a wide-angle banana-shaped quartz cell in a continuous-flow setup. The scattered beam passes twice through the spin-filter cell, thus doubling the optical path length in the cell. The aim is to achieve 70% nuclear polarisation with no variation in time.