Development Of A Thermal Neutron Imaging Facility For Real Time Neutron Radiography And Computed Tomography PDF Download

Neutron imaging is one of the most powerful non-invasive investigation modalities that finds many applications in various fields such as nuclear industry, homeland security, battery research, and archeology. It provides information about the internal structures of an object as neutrons are absorbed and transmitted at different levels in the object in question. It is complementary to X-ray imaging due to the different interaction mechanisms of X-ray and neutrons with materials. Neutron imaging applications can employ neutrons with different energies. Fast (MeV) neutrons have some advantages such as causing less radioactive transmutation in samples and providing deeper penetration which allows for exploring thicker and denser samples. It also provides good contrast images of objects made of a mix of hydrogenous and metallic elements. With the advent of the CCD technology, new opportunities have become available to perform neutron radiography, which provides 2D images of objects, and tomography, which yields information about objects in 3D, more efficiently. Digital neutron radiography allows the collection and storage the information in a digital environment, which enables a better and quicker data analysis. However, performing neutron imaging requires a well-characterized neutron source and a proper neutron imager, especially if fast neutrons are utilized as their interaction cross sections with materials are relatively smaller. This work explores the advancement of the fast neutron radiograph and tomography. The work includes studies comprising characterizing neutron detectors for fast imaging application, characterizing a fast neutron beam facility, performing fast neutron tomography using various imaging phantoms, and investigating spatial resolution in a fast neutron imaging system. In one study, Polyvinyl Toluene (PVT) (C9H10) based plastic scintillators with different dimensions and flours were investigated in terms of their relative light outputs and spatial resolutions. Due to high hydrogen concentration, PVT scintillators would be a suitable candidate since fast neutrons deposit higher energy as they interact mainly via elastic scattering reactions. This study resulted that thicker scintillators yield higher light output due to a higher amount of scintillation materials whereas they performed worse in spatial resolution because of more neutrons and light scattering. Another study was related to characterizing the Ohio State University Research Reactor (OSURR)’ recently built fast neutron beam facility. Models of the beamline and beam stop were created, and Monte-Carlo simulations were performed to determine the neutron energy spectrum, neutron, and gamma-ray flux, and dose rate distributions. Various thermal neutron filters were investigated, and Cadmium ratios that they provide were experimentally determined. Gamma-ray content in the beam was obtained using Optically Stimulated Luminescence (OSL) dosimeters. Another study investigates fast neutron computed tomography (nCT) using custom-made multi-material complex objects. Total nCT data collection time was as low as 2 hours for some objects. Various materials were resolved in 3D reconstructed images of the objects. Low-Z materials comprising the objects were revealed while being shielded behind high-Z materials. The last study in this work is about the investigation of the possible effects on the system's spatial resolution in a fast neutron imaging system. Results showed that the thicker imaging objects deteriorate the spatial resolution due to the neutron scattering. Simulations provided that the neutron interaction kinematics in PVTs would also degrade the spatial resolution.