Time-focused crystal analyzer spectrometer

John M. Carpenter, G�za Zsigmond
2005 Physical Chemistry, Chemical Physics - PCCP  
We outline a linearized analysis of pulsed-source time-of-flight crystal analyzer spectrometers (CAS) based on scattered neutron analysis by crystal monochromators and intended for use in high-resolution quasielastic scattering and Brillouin scattering measurements. Spectrometers of this kind, often called "backscattering spectrometers," provide for high resolution measurements of excitations with typical energies in the range 1-100 meV in both steady source and pulsed source applications. The
more » ... applications. The analysis is fully general, treating threedimensionally the geometric conditions for time focusing of the orientations of the source (moderator), sample, monochromator crystal and detector, considered as thin elements having large areas, and allows for the use of mosaic crystal monochromators. We report the results of VITESS Monte Carlo simulations, which verify the analytical focusing conditions for dispersionless excitations, reveal higher-order (than linear) contributions to the resolution widths, and provide absolute intensity estimates. Motivation We deal with pulsed-source crystal analyzer spectrometers, for example, IRIS, TOSCA, QENS, and the LAM instruments at ISIS, IPNS, and KENS, starting from a generalized concept of this class of machines. The aim of the present analysis is to provide a basis for design of spectrometers of the highest possible resolution with the highest possible counting rates, meanwhile to reveal new flexibilities for the design. The principle is to increase the areas of components and the range of wavelengths accepted, maintaining precise time resolution by "time-focusing." Variations of neutron flight path lengths and angles, correlated through the analyzer Bragg condition, compensate variations in the selected wavelengths. The theory of time focusing of crystal analyzer spectrometers is the subject of a pair of recent papers. 1,2
doi:10.1039/b415165h pmid:19791341 fatcat:wxcqzn3binbellhkvzswzzdtpu