Michael R. Kusner

A cost effective means of extending the lifetime of plastic scintillators in portal monitors

Large plastic scintillators (5000 to >35000 cc) deployed as portal monitors and subjected to environmental extremes have been shown to suffer performance degradation due to water vapor penetration of the base material. Low temperatures can cause the absorbed vapor to condense and manifest itself as a fog within the scintillator that can dissipate as the material warms. Repeated cold cycles may lead to a permanent residual haze. Both polyvinyltoluene and polystyrene based scintillators are susceptible to this effect. The presence of internal haze causes scattering of scintillation light and results in reduced detector sensitivity. An obvious, but expensive solution would be to hermetically seal the plastic as one would a hygroscopic scintillating crystal. In this study we examine the performance of 12 plastic scintillation detectors packaged in various configurations and exposed to >60 days storage at 55C and 85% relative humidity. The detectors were cycled to 25C, 0C and −30C. Pulse height spectra were acquired continuously during the experiment. Count rate information along with the pulse height spectra are used to gauge the effectiveness of the various packaging configurations in preventing water vapor penetration and subsequent fogging at low temperatures.