Fluorescence of pyrene-doped polystyrene films from room temperature down to 4 K for wavelength-shifting applications

TL;DR

This study characterizes pyrene-doped polystyrene films' fluorescence properties from room temperature to 4 K, revealing temperature-dependent light yield and decay times for potential use as slow wavelength shifters in particle detectors.

Contribution

It provides detailed measurements of fluorescence yield and decay times of pyrene-polystyrene films across a wide temperature range, highlighting their suitability for background mitigation in liquid argon detectors.

Findings

Light yield increases with cooling.

Decay times slow down at lower temperatures.

Purity affects emission lifetime and monomer/excimer ratio.

Abstract

In liquid argon-based particle detectors, slow wavelength shifters (WLSs) could be used alongside the common, nanosecond scale, WLS tetraphenyl butadiene (TPB) for background mitigation purposes. At room temperature, pyrene has a moderate fluorescence light yield (LY) and a time constant of the order of hundreds of nanoseconds. In this work, four pyrene-doped polystyrene films with various purities and concentrations were characterized in terms of LY and decay time constants in a range of temperature between 4 K and 300 K under ultraviolet excitation. These films were found to have a LY between 35 and 50% of that of evaporated TPB. All light yields increase when cooling down, while the decays slow down. At room temperature, we observed that pyrene purity is strongly correlated with emission lifetime: highest obtainable purity samples were dominated by decays with emission time constants of $\sim$ 250-280 ns, and lower purity samples were dominated by an $\sim$ 80 ns component. One sample was investigated further to better understand the monomer and excimer emissions of pyrene. The excimer-over-monomer intensity ratio decreases when the temperature goes down, with the monomer emission dominating below $\sim$ 87 K.