Enhancement of Light Yield and Stability of Radio-Pure Tetraphenyl-Butadiene Based Coatings for VUV Light Detection in Cryogenic Environments

TL;DR

This study enhances VUV light detection in cryogenic environments by optimizing Tetraphenyl-butadiene coatings on reflectors, achieving higher light yield and demonstrating long-term stability and low radioactivity suitable for low-background experiments.

Contribution

It introduces a new optimized coating method with Tetraphenyl-butadiene on Tetratex, improving light yield and stability for cryogenic VUV detection applications.

Findings

3.6 times higher light yield with optimized coating

Stable performance over 100 days in liquid argon

Low radioactive impurities suitable for low-background experiments

Abstract

The detection of VUV scintillation light, e.g. in (liquid) argon detectors, commonly includes a reflector with a fluorescent coating, converting UV photons to visible light. The light yield of these detectors depends directly on the conversion efficiency. Several coating/reflector combinations were produced using VM2000, a specular reflecting multi layer polymer, and Tetratex, a diffuse reflecting PTFE fabric, as reflector foils. The light yield of these coatings was optimised and has been measured in a dedicated liquid argon setup built at the University of Zurich. It employs a small, 1.3 kg LAr cell viewed by a 3-inch, low radioactivity PMT of type R11065-10 from Hamamatsu. The cryogenic stability of these coatings was additionally studied. The optimum reflector/coating combination was found to be Tetratex dip coated with Tetraphenyl-butadiene with a thickness of 0.9 mg/cm$^2$ resulting in a 3.6 times higher light yield compared to uncoated VM2000. Its performance was stable in long term measurements, ran up to 100 days, in liquid argon. This coated reflector was further investigated concerning radioactive impurities found to be suitable for current and upcoming low-background experiments. Therefore it is used for the liquid argon veto in Phase II of the GERDA neutrinoless double beta decay experiment.