Photodegradation Mechanisms of Tetraphenyl Butadiene Coatings for Liquid Argon Detectors

TL;DR

This study investigates the degradation mechanisms of TPB coatings in liquid argon detectors, identifying benzophenone as a UV-blocking impurity and proposing methods to improve coating longevity using radical inhibitors.

Contribution

It reveals that TPB degradation is driven by free radical-mediated photooxidation and demonstrates a method to enhance coating durability with radical inhibitors.

Findings

Benzophenone detected as a UV-blocking impurity in TPB.

Degradation correlates with benzophenone concentration increase.

Adding radical inhibitors delays degradation and improves performance.

Abstract

We report on studies of degradation mechanisms of tetraphenyl butadiene (TPB) coatings of the type used in neutrino and dark matter liquid argon experiments. Using gas chromatography coupled to mass spectrometry we have detected the ultraviolet-blocking impurity benzophenone (BP). We monitored the drop in performance and increase of benzophenone concentration in TPB plates with exposure to ultraviolet (UV) light, and demonstrate the correlation between these two variables. Based on the presence and initially exponential increase in the concentration of benzophenone observed, we propose that TPB degradation is a free radical-mediated photooxidation reaction, which is subsequently confirmed by displaying delayed degradation using a free radical inhibitor. Finally we show that the performance of wavelength-shifting coatings of the type envisioned for the LBNE experiment can be improved by 10-20%, with significantly delayed UV degradation, by using a 20% admixture of 4-tert-Butylcatechol.