Differential die-away analysis (DDAA) is a technique utilized for the active detection and analysis of Special Nuclear Materials (SNM). This method can identify the presence of nuclear materials irrespective of shielding by inducing fission through external pulsed neutrons.

This study aims to design and optimize a detection system based on DDAA technology by Monte Carlo simulation to achieve the highest sensitivity in detecting SNM.

The Geant4 Monte Carlo simulation software was employed to design the SNM detection system with optimization of its key parameters. The size, pressure, thickness, and density of the ³He detector, as well as the thickness and density of the moderator were calculated for obtaining the geometric parameters required to achieve optimal neutron detection efficiency. Then, different geometric arrangements of the system were tested to identify the configuration that delivers optimal sensitivity for SNM detection. Finally, based on the DDAA principal and simulation optimization results, the detection system was evaluated using a pulsed neutron with an energy of 14 MeV and a pulse width of 100 μs.

The simulation results indicate that the detection system achieves high sensitivity for detecting 1 g of ²³⁵U nuclear material when configured with the following geometric parameters: a pressure of 0.405 3 MPa, a 5.08 cm diameter ³He tube, an outer wrapping density of 1.10 g·cm^-3 for the ³He tube, a 4 cm thick moderator, and a 7 cm thick moderator with a density of 0.85 g·cm^-3 surrounding the testing sample.

The optimized geometric configuration significantly enhances the sensitivity of SNM detection systems based on DDAA technology. These findings provide crucial insights for the parameter design of effective SNM detection systems, thereby contributing to improved nuclear material security and non-proliferation efforts.