Characterizing Polyvinyl Toluene Scintillator's State of Health Using Light Readings from a Photomultiplier Tube

Abstract

Polyvinyl toluene (PVT) based detectors are used in radiation portal monitors (RPM) to detect the illicit trafficking of nuclear materials. These detectors have been observed to internally fog after being subjected to environments with large temperature and humidity fluctuations, potentially decreasing the overall effectiveness of the RPMs. As temperature decreases, PVT fogging is induced by the formation of water-filled voids within the plastic. An Opacity Monitoring System (OMS) was originally developed to measure and track changes in PVT opacity in-situ. This was accomplished by employing an array of different colored light emitting diodes (LED) and optical sensors (OS) to measure light transmission through the detector. Changes in PVT opacity were tracked by intermittently flashing each LED and recording the amount of transmitted light observed by the OS. This method, however, required the aforementioned equipment to be adhered onto the detector and produced a separate data stream from the RPM. An alternative method to track opacity changes was conducted for this research. Here, four OMS/PVT systems were placed in an environmental chamber (EC) at Pacific Northwest National Laboratory (PNNL) and RPM count rates were monitored throughout 360 hours of temperature and humidity cycles ranging from -20°C to 50°C and 40% to 100% relative humidity (RH), respectively. The LED-induced RPM count rates were observed to change in response to temperature fluctuations in the environmental chamber. This aided in establishing a correlation between recorded temperature and count rate, thus proving that RPM electronics can be used to track the onset of fogging within the detector. Furthermore, a mathematical model establishing the relationship between the onset of fogging and detector temperature was developed to aid RPM operators to predict PVT the onset of fogging on site.