Phonon measurements on a filter-detector spectrometer and its resolution function

Abstract

The use of a filter-detector spectrometer for accurate measurements of phonon dispersion curves is demonstrated. Two possible approaches - the "average energy" and the "cut-off energy" - have been examined experimentally as well as theoretically and it is shown that the former approach is consistent and reliable. Expressions for the resolution function and phonon shapes have been derived. The calculations reproduce the observed neutron groups in most cases. Focussing of the neutron groups, both in constant-Q and constant-E modes of operation has been demonstrated by a number of examples and supported by theoretical calculations.