PNNL

Abstract

Abstract– Phoswich detectors (two scintillators attached to the same photomultiplier-tube) have been used in the past to measure either betas or gammas separately but were not used to measure beta-gamma coincidence signatures. These coincidence signatures are very important for the detection of many fission products and are exploited to detect four radioxenon isotopes using the Automated Radioxenon Sampler/Analyzer (ARSA) [1]. Previous PNNL work with a phoswich detector used a commercially available, thin disk of scintillating CaF2(Eu) and a 2” thick NaI(Tl) crystal in a phoswich arrangement. Studies with this detector measured the beta-gamma coincidence signatures from 133Xe, 214Pb and 214Bi [2]. This scintillator combination worked but was not a good match in scintillation light decay times, 940-ns for CaF2(Eu) and 230 ns for NaI(Tl). Additionally, a 6 mm thick quartz window was placed between the NaI(Tl) and the CaF2 to ensure a hermetic seal for the NaI(Tl) crystal . This dead layer significantly reduced the detection probability of the low energy x-rays and gammas that are part of the coincidence signatures for 214Pb, 214Bi and the radioxenons. Further research showed that Yttrium aluminum perovskit (YAP) and bismuth germanate (BGO) have very good scintillation light characteristics and no hermetic seal requirements. The 27-ns scintillation light decay time of YAP and the 300-ns decay time for BGO are a good match between fast and slow light output. The scintillation light output was measured using XIATM digital signal processing readout electronics, and the fast (YAP) and slow (BGO) light components allowed discrimination between the beta and gamma contributions of the radioactive decays. In this paper we discuss the experimental setup and results obtained with this new phoswich detector and the applications beyond radioxenon gas measurements.