High Resolution Residual Strain/Stress Measurements on Three Axis Neutron Diffractometer

Focusing 3-axis diffractometer set-up equipped with bent perfect crystal (BPC) monochromator and analyzer offers the sensitivity in determination of strains Dd/d < 10^-4 in polycrystalline materials which is about one order of magnitude higher with respect to that of conventional 2-axis neutron scanners. It also offers possibility of line profile analysis for reasonable sample volumes and counting times. In this paper, the feasibility of using the 3-axis set-up even for measurements of rather large bulk polycrystalline samples with an acceptable resolution is presented. As the 3-axis set-up exploits focusing in real and momentum space, by a proper adjustment of the curvature of the analyzer, a high-resolution determination of the lattice changes can also be achieved even on large irradiated gauge volumes, though with a slightly relaxed resolution. It can be successfully exploited namely, in the strain/stress measurements on samples exposed to an external load, e.g. in tension/compression rig, in aging machine etc. In addition to the original performance where the analysis is carried out by rocking the BPC analyzer and the neutron signal registered by a point detector, a new alternative is offered which uses a fixed rocking angle position of the analyzer and the detector signal is registered by a one-dimensional position sensitive detector (PSD). This trick makes possible in some cases the elastic strain/stress measurements considerably faster and thus reduces the drawback of the time consuming step-by-step analysis.