Characterization of a reactor-based fast neutron beam facility for fast neutron imaging

The Ohio State University Research Reactor's (OSURR) fast neutron beamline is aimed to meet the growing demand for high flux and well-collimated neutron sources for fast neutron radiography and tomography applications. The beam facility consists of two collimators, separated by a neutron-gamma shutter, and a movable beam stop, sitting on a rail system for back/forth and up/down motion to provide an adjustable working space. The beam facility provides a beam diameter of 3.2-cm and has a calculated geometric L/D ratio of ~62. The collimator closer to reactor core includes a 10.16-cm thick polycrystalline Bismuth for filtering gamma-rays, which provides ~2 orders of magnitude reduction in gamma flux at 2-MeV, and a 15.24-cm thick graphite with a 3.2-cm diameter aperture. Various Monte Carlo N-Particle (MCNP) simulations were performed to obtain neutron energy spectrum, neutron and gamma flux distributions, and dose rate values. Simulations showed a fast neutron (@1.6 MeV) flux ~5.4 × 10⁷ n·cm^-2·s^-1 at the collimator exit. While the simulations of neutron and gamma flux distributions have verified that the beam shutter and beam stop provide a decent neutron and gamma shielding, a neutron radiograph of the beam was experimentally obtained using a Polyvinyl Toluene (PVT) based plastic scintillator and a lens-based imaging setup which has further validated the simulated radiographs of the beam. Simulations also provided neutron dose rates around the beam stop with a close agreement with experimental values. However, disagreements were found between experimental and simulated gamma flux dose rates, which needs further validation.