Investigation of Environmental Degradation of Plastic Scintillators

Scintillators are a class of materials that are used as radiation sensors and are especially useful in national security applications. Plastic scintillator detectors are often deployed due to their availability, relatively low cost, and the ability to be cast in large sizes. In recent years it has been discovered that plastics deployed in high-humidity environments will absorb water, especially at high temperatures. When cooled, this water creates defects in the plastic that scatter light and increase maintenance costs, and often require replacement of the detectors. In this research, we have investigated fogging by examining both fresh and field-aged PVT and PS-based scintillators. We have developed protocols for producing temporary and permanent fogging using both liquid water and high-humidity environments combined with temperature cycling. Important properties of fogging have been discovered, such as the correlation between cooling rates and defect size. Temporary defects have also been shown to appear in the same location during multiple cycles, confirming a permanent deformation in the plastic. Optical transmission and photoluminescence measurements revealed that fogging does not affect the scintillation mechanism, but merely increases light scattering in the plastic. PS-based compositions are slightly more fog-resistant but perform worse after aging. New compositions produced and provided by collaborators have been testing using an accelerated aging test. Scintillation measurements were taken on the plastics before, after, and during the test to better understand how aging reduces scintillation performance.