Simulation design of the moderator and collimator of a thermal neutron radiography system based on reactors

Neutron imaging technology is a crucial nondestructive testing technique widely used in nuclear, military, medical, and other fields. However, the development time of neutron imaging technology is relatively short, warranting further investigations in many aspects. In neutron imaging devices, the quality of the neutron-slowing collimator strongly affects both the imaging resolution and the required exposure time. Hence, developing and investigating neutron collimation systems are essential for the development of neutron imaging technology. Based on the shortcomings of the current common collimator structure, we propose a new collimator structure consisting of a circular tube-type collimator and an divergent collimator. Taking the reactor as the neutron source, a new neutron collimator system is studied and designed using the Geant4 program, and neutron slowing, collimation, and neutron gamma ratio improvement are studied and designed under this structure. The optimal selection and design of materials and structural dimensions of each part are completed. The designed device is compared with a conventional device, and simple thermal neutron radiography (TNR) is conducted. The simulation results show that the device has a collimation ratio of 62, a normalized thermal neutron flux of 2.07×10-6cm-2·s-1 at the exit, an n/γ ratio of 4.01×1011cm-2·Sv-1, and the inhomogeneity of the radial distribution of neutrons is 7.5%. The neutron beams passing through the new collimator have higher average thermal neutron fluxes and thermal neutron ratios than those passing through conventional cylindrical and divergent collimators. The thermal neutron imaging simulation results of the Image Quality Indicators validate the significance of this study by providing a reference for the design optimization of TNR devices and theoretical support for subsequent experiments.