Instrument description

SHARPER is a time focussing time-of-flight spectrometer designed for quasielastic and inelastic scattering for incident wavelengths in the range of 4 to 6 Å.

An intense beam is extracted from the H15 guide by a vertically focussing monochromator array. It consists of three composite pyrolytic graphite monochromators using the full height (20 cm) of the guide and focussing the beam at the sample position. In order to minimise the interference with the subsequent instruments, the monochromator can deliver only four wavelengths: 4.1; 4.6; 5.1; and 5.9 Å. The second order reflection from the graphite monochromator is removed by a beryllium-filter cooled at liquid nitrogen temperature.

To achieve the time-focussing condition, the beam is pulsed by a Fermi chopper. It has a small slot length to ensure a good transmission. The normal distance between the Fermi chopper and the sample is 38 cm. To prevent frame-overlap when the chopper is rotating faster than 7500 rpm, a suppressor chopper is placed before the Fermi chopper and rotates in phase with the latter at a lower speed. 

The secondary spectrometer consists first of an evacuated sample area. This allows us if necessary to remove the external wall of a cryostat or an oven. The vertical positioning of the sample box is achieved by a motor. A helium filled box between the sample and the detectors minimises the background. The sample box is equipped with an oscillating collimator which prevents parasitic reflections (from the cryostat walls for example) reaching the detectors. The detector bank is entirely covered with detector boxes, thus avoiding the inconvenience of moving the counters.