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Detectors based on liquid argon (LAr) often require surfaces that can shiftvacuum ultraviolet (VUV) light and reflect the visible shifted light. For theLAr instrumentation of the LEGEND-200 neutrinoless double beta decayexperiment, several square meters of wavelength-shifting reflectors (WLSR) wereprepared: the reflector Tetratex (TTX) was in-situ evaporated with thewavelength shifter tetraphenyl butadiene (TPB). For even larger detectors, TPBevaporation will be more challenging and plastic films of polyethylenenaphthalate (PEN) are considered as an option to ease scalability.  In this work, we first characterized the absorption (and reflectivity) ofPEN, TPB (and TTX) films in response to visible light. We then measured TPB andPEN coupled to TTX in a LAr setup equipped with a VUV sensitive photomultipliertube. The effective light yield in the setup was first measured using anabsorbing reference sample, and the VUV reflectivity of TTX quantified. Thecharacterization and simulation of the setup along with the measurements andmodelling of the optical parameters of TPB, PEN and TTX allowed to estimate thequantum efficiency (QE) of TPB and PEN in LAr (at 87K) for the first time:these were found to be above 67% and 49%, respectively (at 90% CL). Theseresults provide relevant input for the optical simulations of experiments thatuse TPB in LAr, such as LEGEND-200, and for experiments that plan to use TPB orPEN to shift VUV scintillation light.

european physical journal. c, particles and fields

R&D of Wavelength-Shifting Reflectors and Characterization of the  Quantum Efficiency of Tetraphenyl Butadiene and Polyethylene Naphthalate in  Liquid Argon