Nature of Moisture-Induced fogging defects in scintillator plastic
[1];
- Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sandia National Lab. (SNL-CA), Livermore, CA (United States)
- US Department of Homeland Security (DHS), Washington DC (United States)
We report polyvinyl toluene (PVT) scintillator plastic may degrade in the field due to inward water diffusion at elevated temperatures, which exceeds the saturation limit at lower temperatures, and after long periods of time (e.g., many years), leads to the formation of disk-like defects that attenuate scintillation light, leading to detector degradation. In this work, using fractography and high-magnification optical and electron microscopy to characterize the water-induced defects, a model of the fogging process is hypothesized as follows: excess water present at low temperatures diffuses to spheroids to minimize contact with the hydrophobic polymer. Through the hydrophobic effect, the entropy of water increases by forming nanoclusters which minimizes the contact between the water and the PVT. As the water nanoclusters grow, they break and fold the polymer into densely-packed crystalline regions creating more space for water within the spheroids. The polymer outside the spheroid resists the shrinkage which builds up tension within the spheroid. Once the tensile stress exceeds the yield strength of the plastic, the spheroid is torn in half resulting in a defect. Excess water then drains into the cavities along the disk thereby further increasing its entropy. Slower cooling (over 1 day) leads to larger spheroids and hence, larger “permanent” defects. Freezing causes some defects to further grow due to the expansion of water to ice. These findings imply that the remaining lifetime of scintillator plastic in the field could be predicted using temperature and humidity data thereby mitigating security risks of degraded radiation portal monitors.
- Research Organization:
- Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States); Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- USDOE National Nuclear Security Administration (NNSA); US Department of Homeland Security (DHS)
- Grant/Contract Number:
- AC05-00OR22725; IAA HSHQDN-16-X-00051; HSHQDCN-17-X-00035; AC52-07NA27344
- OSTI ID:
- 1607288
- Alternate ID(s):
- OSTI ID: 1825854; OSTI ID: 1871780
- Report Number(s):
- LLNL-JRNL-823753; LLNL-JRNL-820291; TRN: US2104617
- Journal Information:
- Nuclear Instruments and Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment, Vol. 954, Issue C; ISSN 0168-9002
- Publisher:
- ElsevierCopyright Statement
- Country of Publication:
- United States
- Language:
- English
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