Dual energy analysis using phoswich scintillation detectors for low-level in-vivo counting

Electronics developed for a whole body counting system analyze the pulse heights of signals from both the sodium iodide (NaI(Tl)) and cesium iodide (CsI(Na)) crystals of a phoswich scintillation detector. Pulse shape discrimination is used to determine the origin of the scintillation, NaI or CsI, while rejecting coincident Compton-scattered events and photomultiplier tube noise. Both energy spectra are accumulated simultaneously in separate memory quadrants of a multichannel analyzer. The system allows for low-energy (5 keV - 125 keV) x-ray analysis from the NaI(Tl) crystal and high-energy (100 keV - 2.5 MeV) x-ray or γ-ray analysis from the CsI(Na) crystal. The discrimination system is better than 95% efficient over both energy ranges, with a low-energy (12-24keV) shielded background count of less than 2 cpm for a single 5-in.-diam phoswich detector. This system thus extends the detector's normal energy range by more than an order of magnitude.