Prompt neutrons from photofission and its use in homeland security applications

Photofission is the process in which a nucleus disintegrates into two daughter products after absorbing a photon. Photofission near threshold in actinides is very similar to spontaneous fission in terms of the number of emitted decay neutrons and their energy distribution. Most of the neutrons are in the ~2 MeV energy range, and can be efficiently detected with liquid scintillator detectors. Thus, Prompt Neutrons from Photofission (PNPF) near threshold can be used as an excellent tool for the detection of actinides. Since the photofission cross section for most fissionable materials drops to near zero for incident photon energies of less than 6 MeV, a source of photons with a higher energy is needed, for example 9 MeV. At this energy interference from (γ, n) processes is minimal. Photon sources in this energy range are well suited for other non-intrusive inspection applications as well as searching for fissionable materials. Passport Systems, Inc. is currently operating a continuous wave (CW) 9 MeV electron accelerator and an array of liquid scintillator detectors to achieve this goal. Pulse shape discrimination (PSD) techniques are used determine the particle type. The remaining neutrons are also filtered through an in-house developed pileup rejection algorithm. The resulting neutron count is compared with the known background to determine the confidence level for possible shielded Special Nuclear Material identification. Initial testing of this system has been performed and the results will be presented. The results show the utility of a CW photon source as well as the ability to fuse the PNPF data with other data to reduce the dose to cargo, or scan times.