Methods for analytically estimating the resolution and intensity of neutron time-of-flight spectrometers. The case of the TOFTOF spectrometer

摘要

An analytical method is presented with allows to estimate the energyresolution of time-of-flight neutron spectrometers, as well as its partialcontributions, over a dynamical range that extends from the elastic line to theaccessible inelastic regions. Such a method, already successfully applied inthe past to the TOSCA and HET neutron inelastic scattering spectrometersinstalled at the ISIS neutron spallation source [A M Gaspar, PhD Thesis,Universidade Tecnica de Lisboa, 2004], is here applied to the high resolutiontime-of-flight spectrometer TOFTOF, mainly dedicated to quasi-elastic neutronscattering studies and installed at the new neutron reactor FRM II. To makesuch calculations easily understandable, the principle of work of the TOFTOFinstrument and of each of its components is explained in detail. A simplymethod that can be used to estimate the instrument intensity, i.e. of thenumber of neutrons arriving at the sample position per unit time, is alsobriefly outlined. To the benefit of the TOFTOF users, graphs displaying the dependencies of theinstrument resolution at the elastic line and of the instrument intensity onthe relevant instrument parameters, i.e the wavelength of the incidentneutrons, the choppers speed of rotation and the frame overlap ratio, arepresented, in the form of iso-resolution or iso-intensity lines. The method ofestimation of the frame overlap ratio that is commonly used at time-of-flightinstruments such as TOFTOF is also explained and alternative options concerningthis parameter, depending on the dynamical range of interest, are brieflyaddressed.