Neutron strain scanning using a radially collimated diffracted beam

Although neutron diffraction provides a unique tool for the sub-surface measurement of elastic strain within engineering components, the technique is characterized by slow rates of data acquisition. The use of a radially collimated diffracted beam for defining the gauge volume enables large, position-sensitive detectors (PSD) to be used, with the acceleration of the data collection process. As a result, focusing collimators are being used or are being considered for use on many new and existing neutron strain scanning instruments. The gauge volume is of key importance when measuring strain or stress by any technique. In this paper we introduce a formalism for quantifying the gauge volume and a phase space representation for its visualization. These are used to describe and analyze the performance of radial (sometimes loosely called `focusing') collimation systems in general, and to compare their performance with traditional and cheaper aperture-based volume definition methods. It is shown that radial collimation of the diffracted beam is an essential companion to a PSD at pulsed sources if a high level of spatial discrimination is to be achieved and can be of value at constant flux sources. Geometrical aberrations, which have previously been well documented for slit gauge definition systems, are a necessary consequence of the gauge definition process and give rise to apparent strains when scanning through a surface. Knowing the radial collimator geometry, the geometrical shifts in the peak positions can be predicted, and the shifts corrected for to provide accurate residual strain measurements, even near surface.