Development of an optical sensor for measuring opacity changes in polyvinyl toluene scintillators

https://doi.org/10.1016/j.radphyschem.2020.109189Get rights and content

Highlights

  • Describes environmental induced fogging that occurs in polyvinyl toluene (PVT) detectors.

  • An in-situ device that utilized diferent colored LEDs was created to indicate the degree of fogging as a function of temperature.

  • A significant reduction in light intensity occurred in at temperatures below 10 °C for all colors.

  • Blue light provided the “brightest response”, but was affected more by scattering.

  • Green light had the largest reduction in transmitted light and was less susceptible to scattering due to internal fogging.

Abstract

Polyvinyl toluene (PVT) based detectors are used in radiation portal monitors (RPMs) throughout the world to detect the trafficking of illicit nuclear material. PVT scintillators, which are normally optically clear, have been observed to suffer internal fogging throughout their volume due to prolonged exposure to varying environmental conditions. These changes could lead to reduced performance for RPMs that utilize plastic scintillators. In this research, a proof of concept system, consisting of different color light emitting diodes (LEDs) and a single optical sensor (OS), were used to examine the change in light transmission through a PVT scintillator. This Optical Monitoring System (OMS), coupled with an environmentally exposed PVT detector, was tested in an environmental chamber where it was subjected to changes in temperature and humidity ranging from 55 °C at 100% relative humidity to −20 °C at 40% relative humidity. At temperatures below 10 °C, light transmission was reduced by 81% ± 8% for blue LEDs, 84% ± 5% for yellow LEDs, and 49% ± 4% for green LEDs. Similar reductions in detected light were not recorded when the OMS was tested with only air between the LEDs and OS. Therefore, the significant reductions in transmitted light were attributed to changes occurring within the PVT scintillator. These results indicate that a significant reduction in PVT opacity occurs due to wide environmental changes. A device such as the OMS could be used to track these changes and provide users an early indication that a portal monitor is suffering from reduced performance.

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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