Active-Well Counting Using New “PSD Plastic” Detector

Present quantification of bulk uranium materials for international safeguards and domestic materials control and accounting relies on active neutron coincidence count- ing systems, such as the Active Well Coincidence Counter (AWCC) and the Uranium Neutron Coincidence Collar (UNCL), that use moderated He-3 proportional counters along with necessarily low-intensity 241Am(Li) neutron sources. Scintillation-based fast-neutron detectors are a potentially superior technology to the existing AWCC and UNCL designs due to their spectroscopic capability and their inherently short neutron coincidence times that largely eliminate random coincidences and enable interrogation by stronger sources. One of the past impediments to the investigation and adoption of scintillation counters for the purpose of quantifying bulk uranium was the com- mercial availability of scintillators having the necessary neutron-gamma pulse-shape discrimination properties only as flammable liquids. Recently, Eljen EJ-299-34 “PSD- plastic” scintillator with pulse-shape discrimination capabilities has become available. The present work is the first assessment of an array of PSD-plastic detectors for the purposes of quantifying bulk uranium. The detector panel used in the present work was originally built as the focal plane for a fast-neutron imager, but it was repur- posed for the present investigation by construction of a stand to support the inner well of an AWCC immediately in front of the detector panel. The detector panel and data acquisition of this system are particularly well suited for performing active-well fast-neutron counting of LEU and HEU samples because the active detector volume is solid, the 241Am(Li) interrogating neutrons are largely below the detector threshold, and the segmented construction of the detector modules allows optimal separation of inter-detector scattering using the kinematics of neutron scattering. The results from a series of measurements of a suite of uranium standards as well as the prospects for the performance of a purpose-built device will be presented.