Position-sensitive analysis in curved-crystal three-axis neutron spectrometry (quasielastic scattering case)

The use of bent perfect crystals in focusing three-axis neutron spectrometry with position-sensitive detection (PSD) is analyzed on the basis of a phase-space theory. With PSD, the usual sequential scans are replaced by simultaneous scans. The case of high resolution in energy transfer is considered in detail. With commercial thin silicon wafers, the achievable resolutions are in the 10-150 µeV range, depending on neutron energy. Resolutions around 10 µeV are obtained on the peak of cold-source spectra. To take advantage of the possibilities offered by silicon wafers, PSD with spatial resolution well below 1 mm will be needed. With PSD analysis, a new kind of focusing exists that allows the thickness of bent perfect analyzer crystals to be increased, providing an intensity gain at no resolution loss. With multilamella assemblies a gain in count rate by the number of lamellae in the packet is achievable. Results of a demonstration experiment are presented, confirming that under the right conditions, a multilamella analyzer may resemble a single bent wafer, the individual curves of many wafers having been combined to provide the intensity gain.