Abstract
Polyvinyl toluene (PVT) plastic is often used to make gamma ray sensitive scintillators and is used in many important applications. For example, PVT is used to scan recycled steel going into a plant to be processed. It is also used in personnel portals to scan employees, and is used at the borders of many countries to scan cargo and cars passing through. In recent years, it was discovered that PVT can become degraded when it absorbs water and then is subjected to daily temperature swings, as is evidence by a reduction in scintillation light. Water absorption by PVT was notionally associated with temporary fogging (i.e., a rapid decrease in opacity which was reversible over time) and permanent fogging (i.e., permanent crack-like defects) and was for some time suspected to be the leading cause of the fogging due to circumstantial evidence. However, definitive proof was not established and in particular the specific mechanisms by which water entered the plastic and ultimately created temporary and permanent point-like defects was not fully understood. This paper is an overview of an effort initiated to reveal the fundamental root cause and also the dynamics of the fogging and degradation processes. This understanding is important not only to solve degradation problems for PVT in fielded systems, but also (1) to provide direction for future procurements for new equipment, and (2) to provide direction for future R&D efforts into advanced plastics with enhanced spectroscopic capabilities or dual particle gamma/neutron capabilities. Unexpected outcomes of this investigation included (1) the capability to predict the onset of fogging based on models using weather data alone, and (2) new modified formulations of PVT and PS which are fog-resistant. This paper gives an overview of the water uptake and associated mechanisms for various compositions, characterization of resulting temporary and permanent defects, predictive modeling of fogging, and possible solutions including encapsulation, heaters and new formulations. While this paper is an overview of the root cause analysis and solution, five accompanying papers (Lance et al., 2018, Myllenbeck and Feng, 2018, Payne et al., 2018, Zaitseva et al. 2018, Kouzes et al., 2018) from the same conference (SORMA 2018) delve further into the details and also expand upon the scope of what will be reported herein.
Original language | English |
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Article number | 161703 |
Journal | Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment |
Volume | 954 |
DOIs | |
State | Published - Feb 21 2020 |
Funding
This work was supported by the U.S. Department of Homeland Security, Domestic Nuclear Detection Office , under competitively awarded Inter-Agency Agreements (IAAs) HSHQDN-16-X-00051 and HSHQDN-17-X-00035 . This support does not constitute an expressed or implied endorsement on the part of the Government. This work was supported by the U.S. Department of Homeland Security, Domestic Nuclear Detection Office, under competitively awarded Inter-Agency Agreements (IAAs) HSHQDN-16-X-00051 and HSHQDN-17-X-00035. This support does not constitute an expressed or implied endorsement on the part of the Government. The authors wish to offer their gratitude to many additional individuals at Lawrence Livermore National Laboratory, Sandia National Laboratory, Pacific Northwest National Laboratory, and Oak Ridge National Laboratory, who participated in experiments and activities that provided insights into the nature of fogging for scintillator plastics. The authors also wish to thank Eljen Technologies, Inc. Ludlum Measurements, Inc. and Saint Gobain for participating in discussions about plastics.
Funders | Funder number |
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Eljen Technologies, Inc. | |
U.S. Department of Homeland Security | |
Oak Ridge National Laboratory | |
Sandia National Laboratories | |
Pacific Northwest National Laboratory | |
Domestic Nuclear Detection Office | HSHQDN-17-X-00035, HSHQDN-16-X-00051 |
Keywords
- Defects
- Degradation
- Fogging
- Modeling
- Polyvinyl toluene (PVT)
- Scintillator
- Water