Characterizing State Of Health Of Polyvinyl Toluene Scintillators Using Light Readings From Photomultiplier Tube

Characterizing State of Health of Polyvinyl Toluene Scintillators Using Light Readings from Photomultiplier TubePolyvinyl toluene (PVT) based detectors are used in radiation portal monitors (RPM) to detect the illicit trafficking of nuclear materials. PVT detectors have been observed to internally fog after being subjected to environments with large temperature and humidity fluctuations, potentially decreasing the effectiveness of the equipment. 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): red, white, blue, green, and yellow, and optical sensors (OS) to measure light transmission through the detector. PVT opacity changes were tracked by intermittently flashing each LED and recording the amount of transmitted light observed by the OS. This method, however, requires the aforementioned equipment to be adhered onto the detector along with 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 rate was monitored for 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. It was noted that the RPM count rate increased by at least a factor of 1.5 at temperatures near -20 ℃ up to a factor of 4 when compared to LED-induced count rates recorded at 50 ℃. This increase in count rate is due to visible light being preferentially scattered in the “fogged” PVT as opposed to being absorbed. This is similar to the effect of headlights appearing brighter to drivers in fog. With these results it was concluded that RPM response to the LEDs could be used to determine when fogging occurs in the PVT.