Calibration and optimization of Bragg edge analysis in energy-resolved neutron imaging experiments

The investigation of microstructure of crystalline materials is one of the possible and frequently used applications of energy-resolved neutron imaging. The position of Bragg edges is defined by sharp changes in neutron transmission and can thus be determined by the measurement of the transmission spectra as a function of neutron wavelength. The accuracy of this measurement depends on both the data analysis technique and the quality of the measured spectra. While the optimization of reconstruction methods was addressed in several previous studies, here we introduce an important prerequisite when aiming for high resolution Bragg edge strain imaging - a well calibrated flight path across the entire field of view (FOV). Compared to e.g. powder diffraction, imaging often uses slightly different geometries and hence requires a calibration for each particular setup. We herein show the importance of this calibration across the entire FOV in order to determine the instrumental error correction for pulsed neutron beamlines. In addition, we also consider the precision of Bragg edge reconstruction as a function of integration time and the minimal sample area. We demonstrate that, with a proper calibration procedure, the Bragg edge wavelength distribution across the entire sample can be reconstructed with an accuracy of..../.. = +/- similar to 0.01%. Our experiments indicate that the strain maps of Inconel 625 samples printed by a direct metal laser melting additive manufacturing technique can be reconstructed with the precision of +/- similar to 100..... The full FOV calibration technique becomes even more important with the development of advanced neutron energy-resolved imaging beamlines and detectors with large FOVs.