Operational optimization of the Necsa SAFARI-1 neutron diffraction strain scanner

Abstract

This thesis reports on research performed by D. Marais during the period February 2012 to November 2016 at the South African Nuclear Energy Corporation (Necsa) SOC Limited. During this period, the Radiation Science division of the Research and Development department embarked on upgrading the residual strain neutron diffraction instrument, MPISI, at the neutron diffraction facility. Performing an experiment on a neutron diffraction instrument is a time-consuming process when taking the limited amount of available beam time into account. Un-optimized sub-systems or miss-aligned components of diffraction instruments exacerbate the problem by, for instance, reducing the maximum achievable beam intensity. With the premise to identify systems and procedures that would have an impact on beam utilization, a literature survey was performed on the components comprising diffraction instruments used in residual stress measurements together with current methods used for instrument and sample alignment. Attention was also given to data acquisition, control, analysis and simulation systems. It was concluded that most strain scanners are unique due to site-specific requirements as well as the complexity and variety of components available on the market. For this reason, the various software systems as well as alignment, calibration and operating procedures are developed in-house for each instrument in order to maximize efficiency and beam utilization. The following research was performed which leads to the increase in beam utilization: * By simulating the neutron optical path of MPISI using Monte Carlo neutron ray-tracing software, a geometrical change was identified which would lead to a flux increase of 3% at the sample position. * A software system named ScanManipulator was created to streamline data correction, reduction and visualisation on MPISI. By coupling it to the data acquisition and control system, real-time data analysis enables experiments to be conducted with respect to set statistical criteria, instead of a fixed measurement time. * By employing best practice used at various international neutron strain scanners, instrument alignment procedures were developed to attain the optimization between maximum neutron intensity and smallest wavelength spread at the sample position. A number of instrument alignment procedures were automated by creating alignment scripts directly in the instrument control system. * An alternative sample positioning method to be used in conjunction with sample positioning and experiment planning software systems deployed on some neutron diffraction strain scanners were developed. By implementing this new methodology on MPISI, samples exhibiting an arbitrary shape can be aligned using less beam time than previously required, thereby improving beam utilization.