Low-Flux Neutron Imaging for ²⁵²Cf-Based Thermal Neutron Radiography

Abstract

A transportable exposure device capable of using 1 mg of ²⁵²Cf was designed and constructed to evaluate the practical effectiveness of the ²⁵²Cf-based thermal neutron radiography with a small source. More refined neutron beams are extracted through three diverging collimators with large outlet aperture of 200 mm in diameter. A thermal neutron flux of 3.0×10³cm^-2s^-1 was obtained at a collimation ratio of 25. The neutron/gamma ratio was 2.9×l0⁴cm^-2mR^-1. A high sensitivity scintillation converter, ZnS-⁶LiF was selected because of the limited thermal neutron flux from the exposure device. Various combinations of the converter and X-ray films were evaluated in order to provide resonable results. Radiographic images were analyzed by Klasens’ method with the aid of a computerized microdensitometer. It was shown that a combination of the converter and Kodak-SR film is appropriate choice for the high quality radiographs. The combination was effectively applied to the measurement of uranium enrichment in uranium dioxide pellets. On the other hand, a simple imaging device which consists of an image intensifier and Polaroid camera was developed to decrease exposure times in such low-flux neutron imaging.