Resonance Neutron Radiography Using a Pulsed Neutron Source and a 2-Dimensional Scintillation Detector

Abstract

Resonance neutron radiography is an imaging technique whereby a single exposure to neutrons produces several radiographs each of which shows a distribution of a different isotope in the object. The technique has been shown before to be feasible but it has not been practicable due to the lack of an intense pulsed epithermal neutron source and a suitable active imaging detector. An accelerator-based pulsed spallation source has recently been completed at Argonne National Laboratory (ANL). For energies higher than 1 eV, its peak flux is several orders of magnitude more intense than that of a chopped beam from a high-flux reactor. A 2-dimensional neutron-position scintillation detector was also recently developed here. It consists of a thin (1-2 mm) Li glass scintillator coupled to an array of photomultipliers. The prototype detector has a FWHM resolution of 3 mm and a sensitive area of 30{\texttimes}30 cm2. The neutron source and detector were used in a resonance radiography experiment in which a foil sample consisting of indium and gold were radiographed. Two images were obtained; one due to resonance absorption of 1.46 eV neutrons in 115In showing the extent of indium in the sample, and the other due to absorption of 4.91 eV neutrons in 197Au showing the extent of gold. A radiography experiment on nuclear fuel pellets showed resonances of the uranium and plutonium isotopes in the range of 0.3--10 eV. The area of potential applications of resonance radiography below 20 eV includes isotopes (Z>{\textasciitilde}40) of fission products, rare earths, heavy metals, and actinides. Resonance cross sections are generally 10--1000 times larger than thermal cross sections thereby providing higher isotopic detection sensitivity and better image contrast than in thermal radiography.