Root cause analysis and solutions for plastic gamma detector degradation in challenging environments—An overview

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.