Development of an optical sensor for measuring opacity changes in polyvinyl toluene scintillators
Introduction
Radiation portal monitors (RPMs) are an important screening tool for vehicles and cargo at borders. Plastic scintillator material, such as polyvinyl toluene (PVT), is commonly used in RPMs for the detection of gamma rays from radioactive material, primarily due to the efficiency per unit cost and availability in large sizes compared to other detection materials (Kouzes, 2004). More recently, it has been shown that PVT in some of these RPMs experience internal fogging when subjected to larger temperature and humidity changes between seasons (Cameron et al., 2015) which reduces the light collected by the photomultiplier tubes (PMT) over time. Cameron et al. (1961) noted that the PVT fogging is due to prolonged and repeated exposure to high heat and humidity allowing water permeation into the plastic, followed by cooling that causes micro-fractures. This influx of water leads to the formation of disk-like defects that scatter and attenuate scintillation light (Lance et al., 2019). Fig. 1 illustrates a side-by-side comparison of two 15 cm × 7.6 cm x 3.8 cm plastic scintillator samples employed in this research. The “clear” brick is from an older detection system and was not exposed to an extreme environment. The severely fogged PVT block was subjected to 7 days of high heat and humidity followed by 24 h of freezing.
This fogging has appeared in some RPMs deployed in environments where large seasonal changes in temperature and humidity occur. In this research, an Opacity Monitoring System (OMS) was developed to monitor opacity changes for PVT scintillators operating in RPMs. The device uses light-intensity measurements to relate the onset of fogging with a decrease in light transmission through the plastic. To examine the evolution of the PVT fogging process as a function of temperature and humidity, a prototype OMS was coupled to a PVT detector and the package was subjected to repeated heating and freezing cycles in an environmental chamber.
Section snippets
OMS description
The OMS utilized transmitted light intensity as an indication of opacity changes due to fogging in PVT. The OMS consisted of an Adafruit Industries TSL2561 Digital Light Sensor based on Texas Advanced Optoelectronic Solutions optical sensor (OS), light emitting diode (LED) array, and an Arduino Mega microcontroller for data capture and transmission. The Arduino supplied power to an array of different colored LEDs through 10 Ω resistors and regulated 1-s light bursts for each at a controlled
Results
It was a concern that LED brightness and/or OS sensitivity could be affected by temperature. To investigate this possibility the OMS was tested with only air between the LED array and OS. Fig. 5 displays the light intensity readings normalized to 50 °C for the LED array between the temperatures of −22 °C–50 °C. A 1σ uncertainty is included for each point. Table 2 lists the average light intensity reading normalized to 50 °C with 1σ uncertainty. With temperatures varying between approximately
Conclusions
Recent studies have indicated that PVT-based radiation detectors can undergo a fogging phenomenon when exposed to certain wide-ranging environment fluctuations over time. This fogging could lead to the absorption of a significant amount of scintillation light, potentially reducing the performance of these detectors when used in the field. An Opacity Monitoring System (OMS) that consisted of a multi-colored LED array and optical sensor was developed as a proof-of concept system to observe this
CRediT authorship contribution statement
C.M. Marianno: Conceptualization, Methodology, Formal analysis, Resources, Data curation, Writing - original draft, Writing - review & editing, Visualization, Supervision, Project administration, Funding acquisition. E.A. Ordonez: Methodology, Software, Investigation, Writing - original draft, Writing - review & editing, Visualization. J.W. King: Methodology, Investigation, Writing - review & editing. R. Suh: Conceptualization, Methodology, Investigation, Data curation, Writing - original
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This project was funded by the National Nuclear Security Administration through National Technology & Engineering Solutions of Sandia, LLC. Award number DE-AC04-94-AL85000. National Technology & Engineering Solutions of Sandia, LLC,is a multi-mission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract
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