An Analysis of Intense Pulsed Active Detection (IPAD) for the Detection of Special Nuclear Materials

We propose and analyze computationally the Intense Pulsed Active Detection (IPAD) of fissile materials. This approach employs bremsstrahlung photons from a single intense 100-ns interrogating pulse produced by a pulsed-power generator. The IPAD system requires only 3 s to detect delayed neutrons. In contrast, a multiple-pulse-train LINAC system requires several minutes. Because of the much smaller interrogation time, the IPAD system offers a much greater contrast between the induced signal and the cosmic-ray neutron background. For a similar signal-to-noise ratio as the LINAC system the IPAD offers a greatly reduced tissue dose. For a similar dose the IPAD offers a greater signal-to-noise with a much smaller false alarm rate. Our detailed numerical analysis employs the ITS and MCNPX codes, from which we have calculated critical quantities such as the number of delayed neutrons per unit dose and the number of prompt fission neutrons that can be distinguished from photoneutrons created in benign materials such as would arise from
²⁰⁷
Pb. Some fundamental scaling relations have also been developed from our computations. For bremsstrahlung endpoint energies (E
_end
) well below the giant dipole resonance, the neutrons per dose increase as the 9th power of the E
_end
which transitions to the 2nd power as E
_end
increases beyond the giant dipole resonance. The tissue dose scales as the 2nd power of E
_end
throughout the range. Therefore, for any applications where the total dose is limited, no additional photo-fission neutrons are produced at high bremsstrahlung endpoints.