Isotope-Sensitive Imaging of Special Nuclear Materials Using Computer Tomography Based on Scattering Nuclear Resonance Fluorescence

Abstract

The characterization and radiography of special nuclear materials (SNMs) such as plutonium and uranium is essential in various scenarios in the context of nonproliferation and national security. However, existing techniques are not sufficient in and of themselves for rapid isotope-sensitive interrogation of SNMs. In the present study, we propose an interrogation method, called scattering-nuclear-resonance-fluorescence-based computer tomography (SNRF CT), to reveal concealed SNMs with simultaneous measurements of the isotopic composition and tomographic distribution. Monte Carlo simulations show that three NRF signatures at 1802, 1846, and 1862 keV from a uranium target composed of ${ }^{235}\mathrm{U}$ and ${ }^{238}\mathrm{U}$, irradiated by a quasimonochromatic $\ensuremath{\gamma}$-ray beam of high intensity, can be identified. For different target configurations, sinograms of the SNRF signatures are obtained, and the tomographic distributions are reconstructed with appreciable image contrast and spatial precision. It is found that the tomographic patterns of ${ }^{235}\mathrm{U}$ and ${ }^{238}\mathrm{U}$ hidden in a 30-mm-diameter iron rod can be clearly visualized. Compared with transmission NRF CT (TNRF CT), SNRF CT has the advantage that it does not need a witness target or calibration of the notch-refill effect. Furthermore, the imaging time for SNRF CT is 20 times shorter than that for TNRF CT, assuming a low missed-detection rate of ${10}^{\ensuremath{-}4}$. We conclude that, using an intense $\ensuremath{\gamma}$-ray beam, the proposed SNRF CT technique is capable of simultaneous identification and tomographic imaging of the isotopic content of SNMs in a realistic measurement time. This imaging method could be extended to other applications in spent-fuel management and screening of cargo for explosives, drugs, and toxic chemicals.