Nature of Moisture-Induced fogging defects in scintillator plastic

Michael J. Lance, Natalia P. Zaitseva, Stephen A. Payne, Richard T. Kouzes, Nicholas R. Myllenbeck, Alan Janos

Research output: Contribution to journalReview articlepeer-review

9 Scopus citations

Abstract

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. 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.

Funding

This work was supported by the U.S. Department of Homeland Security, Domestic Nuclear Detection Office , under competitively awarded IAA HSHQDN-16-X-00051 and HSHQDCN-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 ORNL, LLNL, SNL, and PNNL who participated in experiments and activities that provided insight into the nature of fogging for scintillator plastics in RPMs. The authors also wish to acknowledge useful discussions regarding fractography with Andrew Wereszczak (ORNL) and the technical support of Randy Parten, Shirley Waters, Joshua Seylar and Sabrina Schwerzler (ORNL) and the technical reviews of Eric Sword, Andrew Wereszczak and Edgar Lara-Curzio (ORNL). Notice: This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan ( http://energy.gov/downloads/doe-public-access-plan ). This work was supported by the U.S. Department of Homeland Security, Domestic Nuclear Detection Office, under competitively awarded IAA HSHQDN-16-X-00051 and HSHQDCN-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 ORNL, LLNL, SNL, and PNNL who participated in experiments and activities that provided insight into the nature of fogging for scintillator plastics in RPMs. The authors also wish to acknowledge useful discussions regarding fractography with Andrew Wereszczak (ORNL) and the technical support of Randy Parten, Shirley Waters, Joshua Seylar and Sabrina Schwerzler (ORNL) and the technical reviews of Eric Sword, Andrew Wereszczak and Edgar Lara-Curzio (ORNL). Notice: This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

Keywords

  • Defects
  • Fogging
  • Fractography
  • Polyvinyl toluene (PVT)
  • Scintillator
  • Water

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