Development of a Thermal Neutron Imaging Facility at the N.C. State University PULSTAR reactor

Abstract

A Thermal Neutron Imaging facility is being set up at the PULSTAR reactor at North Carolina State University. The PULSTAR is an open pool type light water moderated research reactor with a full power of 1-MWth and fuel that is enriched to 4% in U-235. It is equipped with 6 Beam Tubes (BT) to extract the radiation out of the reactor core. BT #5 is being used for the neutron imaging facility.

Neutron imaging has expanded rapidly as a means of Non-Destructive Testing of materials. It offers some very explicit advantages over the usual γ-ray (or x-ray) imaging. Neutron cross-sections, being almost independent of the atomic number (Z) of the material, result in neutron imaging being capable of discerning materials of similar Z and/or low Z materials even when they are present inside high Z surroundings. Also, hydrogen, which is a very important element in determining the properties of materials, can be imaged even if present in minute quantities due to its significant neutron scattering and absorption cross-sections. Neutrons also offer the advantage of being capable to differentiate between isotopes of an element. Furthermore, radioactive materials which cannot be imaged using photons due to fogging of the detector can be imaged with neutrons using the transfer technique.

The facility at the PULSTAR is intended to have both radiographic and tomographic capabilities. The radiography capabilities include using conventional film, digital image plate systems, as well as a real-time radiography system. In the present work the design of the facility is being presented. The collimator constitutes the major part of the imaging facility. The collimator design and its performance were simulated using MCNP. The designed collimator has a poly-crystal bismuth filter that is 4-inches in length, and a single crystal sapphire filter that is 6-inches in length. To aid in the design process, the bismuth and sapphire thermal neutron scattering cross-sections were calculated and implemented as libraries that can be used in MCNP calculations. The L/D of the system ranges from 100 to 150. The filter length can be changed to vary the estimated neutron flux from 1.8x10⁶ to 7x10⁶ n/cm².sec at full power with a sub-cadmium neutron content >98% as estimated by the MCNP simulations. Using the designed collimator, the beam divergence angle is 2° which translates to a beam size of 35-cm at 6-m from the aperture.

Radiography and tomography simulations were also performed using MCNP and the effect of scattering was observed in the image. In addition, the Point Spread Function (PSF) for different detection systems was simulated and the corresponding resolution defined by the FWHM for film, image plate and real time detection systems was obtained and found to be between 33 to 50-μm, 106 to 118-μm and 113 to 118-μm respectively. The results obtained were in good agreement with the measurement performed using a 25-μm thick gadolinium foil. The designed beam was evaluated using the standards of the American Society of Testing and Materials (ASTM) and it was found that the designed beam achieves quality I[superscript A] ranking. Initial radiographs using the facility have been taken and are presented. The real-time radiography and tomography system will be setup in the near future.