Fig. 1. Diagram of detection of SNM expected measurement signals based on DDAA technology^[15]

Fig. 2. Schematic diagram of DDAA-based SNM detection system

Fig. 3. Schematic diagram of Geant4 simulation optimization process

Fig. 4. Diagram of thermal neutron induced SNM fission neutron spectrum

Fig. 5. Geometric model (a), relationship between neutron detection efficiency and ³He tube dimensions and pressure (b)

Fig. 6. The relationship between neutron detection efficiency and the density and thickness of moderators

Fig. 7. Neutron energy spectra of moderated polyethylene with different thicknesses (color online)

Fig. 8. Diagram of geometric model and the relationship between the neutron fission probability and the density and thickness of the moderating body(a) Geometric model, (b) Neutron fission probability

Fig. 9. Diagram of optimized detection systems in different geometric configurations Geant4 simulation (color online)

Fig. 10. The neutron count and time curves obtained by different geometric configurations (color online)

Fig. 11. FOM calculation results under different geometric configurations of detection system

Fig. 12. Detection response for SNM samples of different masses (color online)