Design and testing of a unique active compton-suppressed LaBr3(Ce) detector system for improved sensitivity assays of TRU in remote-handled TRU wastes

The US Department of Energy\´s transuranic (TRU) waste inventory includes about 4,500 m³ of remote-handled TRU (RH-TRU) wastes composed of a variety of containerized waste forms having a contact surface dose rate that exceeds 2 mSv/hr (200 mrem/hr) containing waste materials with a total TRU concentration greater than 3700 Bq/g (100 nCi/g). As part of a research project to investigate the use of active Compton-suppressed room-temperature gamma-ray detectors for direct non-destructive quantification of the TRU content of these RH-TRU wastes, we have designed and purchased a unique detector system using a LaBr₃(Ce) primary detector and a Nal(Tl) suppression mantle. The LaBr₃(Ce) primary detector is a cylindrical unit ~25 mm in diameter by 76 mm long viewed by a 38 mm diameter photomultiplier. The Nal(Tl) suppression mantle (secondary detector) is 175 mm by 175 mm with a center well that accommodates the primary detector. An important feature of this arrangement is the lack of any "can" between the primary and secondary detectors. These primary and secondary detectors are optically isolated by a thin layer (.003") of aluminized Kapton. This arrangement virtually eliminates the "dead" material between the primary and secondary detectors, a feature that preliminary modeling indicated would substantially improve the Compton suppression capability of this device. This paper presents both the expected performance of this unit determined from modeling with MCNPX, and the performance measured in our laboratory with radioactive sources.