Quality control of PEN wavelength shifters for DarkSide-20k veto

Abstract: Efficient Wavelength Shifters (WLS) are crucial for Liquid Argon (LAr) dark matter detectors. As they grow larger in volume, the scalability of WLS becomes an important concern. Tetraphenyl butadiene (TPB), the most common WLS in use, requires to be deposited with vacuum evaporation, impractical for detectors with very large surface area due to its high cost and energy requirements. The neutron veto of the DarkSide-20k detector will utilize nearly 200 m^2 of polyethylene naphthalate (PEN) wavelength shifter, available in the form of large format polymeric foils. In order to assess the quality of PEN sheets in the DarkSide-20k production batch, multiple samples will be tested at cryogenic temperatures. For this purpose, a new Argon Gas Setup (ArGSet) has been recently commissioned. In this setup, we exploit the Argon scintillation light (128 nm) as excitation for measuring the wavelength shifting efficiency of the samples. In this work, we will present the results of the first measurements performed at cryogenic temperature with this setup.