Isotopic Composition Estimation of Special Nuclear Materials Based on Photofission Reaction and Its Delayed Neutrons Using a Bremsstrahlung Photon Source

Nondestructive assay (NDA) techniques for the detection and quantification of special nuclear materials (SNMs) are a key issue for the worldwide expansion of nuclear energy utilization. For this background, the photofission reaction rate ratio (PFRR) method was proposed. Although the PFRR method has been validated through numerical simulations, its experimental verification has not yet been successful. In the PFRR method, signals from photofission reactions must be measured to detect and quantify SNMs; however, the detection of fission neutrons is challenging due to intense gamma flashes and background neutrons originating from other photonuclear reactions, such as (γ, n) and (γ, 2n). To overcome these difficulties, the authors proposed a novel method for detecting and quantifying SNMs using delayed neutrons. In the previous paper, this method was successfully validated for the estimation of uranium enrichment using monochromatic photon energy. However, the actual system is expected to use bremsstrahlung X-rays generated by a compact electron accelerator. Therefore, the present paper examines the applicability of the PFRR method using bremsstrahlung X-rays. As a result, the present method estimated uranium enrichment within a 2.5% difference from actual enrichment for a thin target. As the next step, detectability of delayed neutrons in the actual system was evaluated. Quantitatively, 60 μA of the electron current leads to 105 delayed neutrons with 1 min irradiation and 1 min neutron counting. This level of neutron production is sufficient to detect and quantify SNMs. An experimental verification will be conducted in future work.