The ABC of powder diffractometer detector coverage

Abstract

This article addresses the question of the most effective detector configuration for neutron spectrometers using a discussion of a particular case - constant-wavelength powder diffractometers at continuous neutron sources. A first variation uses an essentially one-dimensional `banana' detector coupled with out-of-scattering-plane beam divergence before and after the sample. A second variation uses an incident beam tightly defined both in- and out-of-plane coupled to a `4 pi' detector covering all possible scattering angles after the sample. It is widely believed that the 4 pi arrangement is superior for most varieties of neutron spectrometer but is more difficult and more expensive to implement. Starting from a commonly used overall instrument figure-of-merit, this article presents simple arguments leading to the surprising conclusion that this is untrue for these constant-wavelength powder diffractometers, provided only that the cylindrical samples used with the banana detector have a height greater than 2.4 times their diameter. © 2011, Wiley-Blackwell.