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The alignment and calibration of neutron strain instruments directly affect the reliability and accuracy of neutron stress experiments. Procedures for the purpose of instrument optimization and characterization were established at Necsa’s angular dispersive neutron strain scanner but may also be applicable to other diffraction instruments. Attention is given to the following aspects: sample table center of rotation, horizontally focusable multi-wafer silicon single crystal monochromator, neutron area detector offset and the instrument gauge volume. Introduction The Materials Probe for Internal Strain Investigations (MPISI) neutron diffraction strain scanner [1] is situated at the SAFARI-1 research reactor of the South African Nuclear Energy Corporation (Necsa) SOC Limited and is depicted in Fig. 1. A commissioning phase followed after a recent major instrument upgrade to improve measurement accuracy and precision. In order to maximize the available neutron flux at the sample position and reduce the error in the measured diffraction angle (and consequently in the dhkl and calculated stress value), all components contributing to the neutron optical path of the instrument must be aligned to as high an accuracy as practically achievable. Complete instrument alignment is generally an iterative process as the system comprises of a number of interdependent components. Center of rotation The instrument reference point is measurable and is defined as the intersection of the sample table’s center of rotation (CoR) axis with the diffraction plane. In order to determine the CoR, the linear x and y-axes (motors sx and sy) of the sample table should be adjusted such that an accurately machined calibration pin is not be displaced by more than 10 % of the applicable gauge volume dimension when Fig. 1: Diagram of MPISI indicating selected motor names and components. Alignment sample Center of rotation (CoR) Incident neutron counter Monochromator chamber shielding Monochromator collimator Reactor beam Sample rotation (som) Monochromator crystal χ Monochromator tilt (mchi) Sample (Detector) 2θ rotation (stth) Sample y axis (sy) Sample x axis (sx) Sample z axis (sz) Primary slit horisonal offset x (psho) Primary slit position y (psp) Monochromator horizontal focus (mf1) Monochromator x axis (mx) In-pile collimator Optical alignment camera Sample goniometer Primary slit Position sensitive neutron detector (PSD) Reactor wall Monochromator fixed vertical focus Secondary slit position (ssp)

Alignment and Calibration Procedures of the Necsa Neutron Strain Scanner