R. Suh, “PVT-PMT System Feedback Design and Feasibility Study”, M.S. Thesis, Nuclear Engineering, Texas A&M University, College Station, TX (2020).
Radiation portal monitors (RPMs) are employed worldwide at border crossings and other entry points as a method to detect the illegal transportation of radioactive material. Scintillating detectors composed of polyvinyl toluene (PVT) plastic are installed within the RPMs for vehicle and cargo screening, seeking gamma ray emissions that indicate the presence of radioactivity. A change in opacity referred to as fogging within PVT has been documented and is attributed to prolonged exposure in temperature and humidity fluctuations. The opacity changes lead to reduced light collection in the photomultiplier tube (PMT) and degradation in PVT over time due to irreparable microfractures. As part of an ongoing effort to combat PVT fogging, the purpose of this study was to develop a Opacity Monitoring System (OMS) capable of observing opacity changes in-situ. The OMS was tested, and the functionality demonstrated. The final design consisted of an array of multiple light emitting diodes (LEDs) as a light source, an optical sensor (OS), and a microcontroller board for data capture and transmission. The OMS was prototyped and tested at Texas A&M University for feasibility on small scale utilizing 0.038m x 0.152m x 0.762m (5 ⅞” x 3″ x 1 ½”) PVT samples, and full scale testing on 0.88m x 0.15m x 0.104m (2.9 ft x 0.5 ft x 0.125 ft) PVT panels in an environmental chamber at Oak Ridge National Laboratories. Light intensity data obtained through the OMS during testing was analyzed and showed clear indication of opacity changes in the PVT detector throughout multiple hours of extreme temperature cycling, thereby demonstrated the capabilities of the design and endurance of the hardware throughout extreme environmental changes.