Characterization of a Stilbene Detector Array for Safeguards.

Stilbene-based multiplicity counting is a promising approach for use in nuclear material assay applications because of its potential advantages compared to 3He and liquid scintillator detectors. Stilbene provides an increased light output, resulting in a higher figure-of-merit for pulse shape discrimination of neutrons and gamma-rays than Xylene- and mineral oil-based systems.Lawrence Livermore National Laboratory (LLNL) developed and evaluated a stilbene-based scintillation detector array for neutron multiplicity counting applications. A detector array with 10 high-efficiency 4” (10.16 cm) diameter by 2” (5.08 cm) deep cylindrical stilbene cells were used to measure nuclear materials at LLNL. This effort includes selecting optimal gates to digitize the photomultiplier tube wave forms, tuning pulse shape discrimination and data acquisition electronics. An integrated data processing software was developed to analyze list mode neutron multiplicity data. We also studied mitigation strategies for detrimental effects that are inherently present in all fast neutron counting systems (multiple scattering crosstalk, gamma-ray rejection, shielding).LLNL developed a theoretical model for correcting the multiple scatter crosstalk in fast detector arrays, which is implemented in the transport modeling. Applied to the Feynman moment reconstruction theory, this correction method is more robust and efficient than the ad-hoc arbitrary rejection of adjacent events in the list mode data within a time window that is widely used. The findings of the simulated performance study will be presented.The modeling effort benefited from the experimental work to verify the list mode generation functionality in MCNP as applied to the stilbene array. Furthermore, the current assumption using the Hansen-Richter quench function value for generating the stilbene light output spectrum by post- processing MCNP simulations is being benchmarked and will be presented.This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344, and was sponsored by the U.S. Department of Energy, National Nuclear Security Administration, Office of Nonproliferation and Arms Control.