Speaker
Description
Ensuring the security of nuclear reactor facilities is one of the most pressing challenges in the nuclear field. Theft or illegal transport of nuclear materials, and sabotage of nuclear facilities, are serious threats to safety and stability. Addressing these risks requires technologies that can detect and identify nuclear materials without damaging them. However, existing approaches have been constrained by the absence of practical photon sources that are simultaneously compact, affordable, and produce minimal background radiation.
To overcome this limitation, the present study makes use of high-energy gamma rays produced through the 7Li(p, $\gamma$)8Be reaction as a novel photon source [1]. When these photons strike nuclear materials, they can induce photonuclear fission, generating fast neutrons in the process. By measuring the emitted neutrons, the presence and quantity of nuclear material can be inferred [2]. Based on this principle, this research proposes a new detection concept that utilizes photonuclear reactions for non-destructive nuclear material identification.
In the present experiment, 14.6 MeV and 17.6 MeV high-energy gamma rays were generated via the 7Li(p, $\gamma$)8Be reaction and irradiated onto a gold target to induce the 197Au($\gamma$, n)196Au reaction. The emitted photonuclear neutrons were successfully observed. We found that a major issue encountered during the gamma-ray irradiation was the strong background radiation produced by the 0.478 MeV gamma rays from the (p, p’) reaction in the Li target. To reduce this inelastic scattering background, an experiment was performed at a proton energy of 0.5 MeV, which is lower than the reaction threshold energy of the inelastic reaction, i.e. 0.546 MeV. Furthermore, the feasibility of the proposed non-destructive detection method based on the 7Li(p, $\gamma$)8Be reaction was evaluated using neutron transport simulations with the MCNP code. Because we are not authorized to handle nuclear fuel materials in our accelerator facility, we plan to use a ²³⁷Np sample as a substitute material to test the present method to detect photofission neutrons. The experimental feasibility was evaluated in calculations with the MCNP code.
References:
[1] T. Saito et al., “Measurement of thick-target gamma-ray production yields of the 7Li (p, p’) 7Li and 7Li(p, $\gamma$)8Be reactions in the near-threshold energy region for the 7Li (p, n)7Be reaction”, J. Nucl. Sci. and Tech., 54, 253–259, (2016)
[2] R. Kimura et al., “Principle validation of nuclear fuel material isotopic composition measurement method based on photofission reactions”, J. Nucl. Sci.and Tech., 53, 1978-1987, (2016).
