  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.
