Numerical analysis of instrumental resolution effects on strain measurements by diffraction near surfaces and interfaces

The inherent instrumental optics related problems in using neutron diffraction to characterise stress gradients near surfaces or interfaces, are treated by a numerical approach. By ray-tracing relevant ranges of wavelengths and divergence angles through a diffraction set-up, it is demonstrated that the artificial peak shifts observed near surfaces and interfaces are mainly attributed to the shift of the gage volume centre of gravity away from the centre of the sample table, and to the inhomogeneity of the wavelength distribution across the irradiated gage volume.

It has been demonstrated that these two effects are either competing or reinforcing effects depending on the experimental configuration. This insight can be used to reduce the errors but can not remove them completely. Some recommendations are given for optimum reduction of errors and a few ideas are provided for a complete elimination of the errors encountered.