Ultraviolet-induced fluorescence of poly(methyl methacrylate) compared to 1,1,4,4-tetraphenyl-1,3-butadiene down to 4 K

TL;DR

This study investigates the UV-induced fluorescence of acrylic used in particle physics experiments at low temperatures, comparing it to a common wavelength shifter, and finds increased fluorescence yield at 4 K with implications for background signals.

Contribution

It provides the first detailed low-temperature fluorescence characterization of acrylic used in scintillator experiments, comparing it to TPB and analyzing temperature-dependent effects.

Findings

Acrylic fluorescence increases at low temperatures, reaching 0.5% of TPB at 4 K.

Fluorescence time constant of acrylic is less than a few nanoseconds.

Part of acrylic fluorescence is due to additives.

Abstract

Several particle-physics experiments use poly(methyl methacrylate) (a.k.a. PMMA or acrylic) vessels to contain liquid scintillators. Superluminal charged particles emitted from radioactive impurities in or near the acrylic can emit Cherenkov radiation in the ultraviolet (UV) spectra range. If acrylic fluoresces in the visible range due to this UV light, it could be a source of background in experiments where the main signal is visible scintillation light, or UV scintillation light that is absorbed and re-emitted at visible wavelengths by a wavelength shifter. Some of these experiments operate at low temperature. The fluorescence of these materials could change with temperature so we have studied the fluorescence of the acrylic used in the DEAP-3600 experiment down to a temperature of 4 K, and compared it to the common wavelength shifter 1,1,4,4-tetraphenyl-1,3-butadiene (TPB). The light yield and wavelength spectra of these materials were characterized by exciting the sample with 285 nm UV light which acted as a proxy for Cherenkov light in the detector. Spectral measurements indicate at least part of the fluorescence of the acrylic is due to additives. Time-resolved measurements show the light yields of our acrylic sample, TPB sample, and the relative light between both samples, all increase when cooling down. At room temperature, the light yield of our acrylic sample relative to the TPB sample is 0.3 %, while it reaches 0.5 % at 4 K. The main fluorescence time constant of the acrylic is less than a few nanoseconds.