The effect of gravity on the resolution of small-angle neutron scattering

The distance through which neutrons fall under gravity in a small-angle diffractometer is not always negligible. If the data are analysed with respect to the machine axis rather than with respect to the real centre of the scattering pattern, as might be the case on white-beam pulsed-source instruments, then the measured spectrum will be distorted compared with the zero-gravity spectrum. It is shown that this gravity effect may be treated as a contribution to the resolution of the instrument, and an analytic form for the gravity resolution function is calculated. The mean and variance of the scattering vector are given, and numerical estimates are made of the severity of the problem under typical experimental conditions. For most instruments in current use the gravity contribution to the Q resolution is not expected to exceed 5%.