Design and Performance of a Thermal Neutron Imaging Facility at the North Carolina State University PULSTAR Reactor

A thermal neutron imaging facility has been set up at the North Carolina State University PULSTAR reactor. The PULSTAR is an open pool light water moderated 1 MWth research reactor with six beam tubes. The present facility is set up on beam tube # 5 of the reactor. The facility is intended to have radiographic and tomographic capabilities. The design of the neutron collimator was performed using MCNP5. The collimator includes a 4-in bismuth filter followed by a 6-in single-crystal sapphire filter. Thermal neutron scattering cross-section libraries for sapphire and bismuth were generated and used in the MCNP simulation of the system. Based on the current design, the L/D of the facility ranges between 100 and 150. The neutron flux at the image plane can be varied from 1.8times10⁶ to 7times10⁶ n/cm²middots with a Cd-ratio of ~450. The resolution of the system for different imaging media was also estimated and found to be ~33 mum for conventional radiography film and ~110 mum for digital image plates. Initial measurements, using ASTM standards, show that the imaging facility achieves a beam quality classification of I^A