Neutron strain scanning using a radially collimated diffracted beam

Although neutron diffraction provides a unique tool for the sub-surface mea surement of elastic strain within engineering components, the technique is characterised by slow rates of data acquisition. The use of a radially coll imated diffracted beam for defining the gauge volume enables large, positio n-sensitive detectors (PSD) to be used, with the acceleration of the data c ollection process. As a result, focusing collimators are being used or are being considered for use on many new and existing neutron strain scanning i nstruments. The gauge volume is of key importance when measuring strain or stress by any technique. In this paper we introduce a formalism for quantif ying the gauge volume and a phase space representation for its visualisatio n. These are used to describe and analyse the performance of radial (someti mes loosely called 'focusing') collimation systems in general, and to compa re their performance with traditional and cheaper aperture-based volume def inition 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 sp atial discrimination is to be achieved and can be of value at constant flux sources. Geometrical aberrations, which have previously been well document ed for slit gauge definition systems, are a necessary consequence of the ga uge definition process and give rise to apparent strains when scanning thro ugh a surface. Knowing the radial collimator geometry, the geometrical shif ts in the peak positions can be predicted, and the shifts corrected for to provide accurate residual strain measurements, even near surface. (C) 2000 Elsevier Science B.V. All rights reserved.