Evaluating the Response of Polyvinyl Toluene Scintillators used in Portal

The goal of this research work was to determine if there exists, and to what extent, variability in the pulse-height spectrum, and resolution produced by ɤ-photon induced scintillation events in polyvinyl toluene scintillators. Scintillator panels of the same type as those used in portal detector units currently operated by the Department of Homeland Security, made of polyvinyl toluene and measuring 12 x 15 x 2 in were purchased from three vendors. Each vendor supplied two scintillators, one wrapped by the vendor with aluminum foil coated mylar, and two unwrapped. The scintillators where exposed to an ≈ 8µCi source collimated 137Cs source which decays with a 662 keV ɤ-ray 85% of the time. A Hamamatsu R329-02 photomultiplier tube was optically mated to the 12 x 2 in surface of the scintillator. The pulse height spectrum was recorded with the source placed at different positions across the surface of the scintillator. Analysis of the pulse height spectrum was performed to determine efficiency and resolution differences across the surface of the scintillation panels. The resolution was found to have values that ranged from 23% at the corner furthest from the PMT to 33% an inch away from the PMT. The absolute efficiency ranged from 0.02 to 0.027 % for the majority of the measurements. The location and height of the Compton edge had noticeable trends, with the height being the greatest at the center and dropping off on the sides, but the peak location having a maximum in the corner along the axis of the PMT