Focusing conditions for a triple-axis neutron spectrometer

Abstract

Two methods are described for determining the magnitude of the focusing effects that are observed in phonon measurements using the triple-axis crystal spectrometer technique. Both methods, one purely graphical and the other numerical, can be used to predict the spectrometer conditions that will give optimum focusing for a given phonon. They are illustrated by their application to measurements of a phonon in magnesium oxide, and their relative merits are discussed. These measurements show a variation by a factor of 12.5 in the observed peak intensities due solely to focusing effects (numerically calculated value 13.0), thus emphasizing the practical importance of being able to predict the focusing conditions.