Photoneutron Detection in a Pulsed High Photon Field using Helium-4 Scintillators

Abstract

Photon active interrogation techniques offer improved detection of shielded special nuclear material (SNM). Shielded SNM detection is challenging due to limited passive emissions, especially by highly-enriched uranium. Photon active interrogation can improve detection capabilities because a high-energy photon beam can penetrate shielding materials and induce photonuclear reactions in SNM, greatly increasing radiation emissions. The University of Michigan is developing economical active interrogation techniques for homeland security applications using an off-the-shelf medical linear accelerator (linac) and economical detector technology. The linac produces a bremsstrahlung spectrum with a 9-MeV endpoint, which is sufficient to induce photonuclear reactions in actinides. To detect the neutrons produced in the photonuclear reactions with high confidence, detectors with robust gamma-rejection abilities must be employed. For this work, we use a commercial off-the-shelf (COTS) 4He scintillation detector developed by Arktis to detect photoneutrons during photon active interrogation of depleted uranium. 4He gas is inert, abundant, and has desirable gamma rejection capabilities. We first show measurements of isotopic photon sources, then several isotopic neutron sources in addition to two neutron generator experiments. Lastly, we show results from the photon active interrogation experiments.