Organic scintillators for active detection using an intense pulsed bremsstrahlung source

Various active interrogation techniques are under investigation with the goal of increasing sensitivity to the presence of special nuclear material (SNM) in an unknown target. The Intense Pulsed Active Detection approach uses a single very short but intense pulse of high-energy photons to interrogate a target. Interrogation with short pulses opens the possibility of accessing the strong prompt neutron signal from induced fission in the target. In the current experimental campaign, a bremsstrahlung spectrum of hard x-rays - endpoint energies between 8 MeV and 16 MeV - is provided by the HERMES-III pulsed power facility at Sandia National Laboratories. We fielded a combination of neutron-sensitive organic scintillator detectors at the HERMES campaign, including liquid, plastic, and crystalline scintillators at ~45 m from the target. We have investigated the sensitivity of these organic scintillators to the neutron and gamma signatures at three timescales of interest in the active interrogation context: the prompt signal from photofission at O(μs), the “delayed prompt” signal from thermalized neutrons inducing fission in fissile material at O(ms), and the delayed gammas and beta-delayed neutrons from fission daughters at O(s).