Optical characterization of microcellular PET and crystallized PEN reflectors in liquid argon for LEGEND-1000

Abstract

Efficient collection of liquid argon (LAr) scintillation light is essential for background rejection in the LEGEND experiment. We investigate microcellular polyethylene terephthalate (PET) and crystallized polyethylene naphthalate (PEN) as candidate reflector materials for LEGEND-1000. Their optical responses to 450 nm light and 128 nm LAr scintillation are measured in LAr and complemented by room-temperature reflectance measurements in air. Under scintillation illumination, PET coupled to a PEN foil and crystallized PEN increase the detected photoelectron yield by factors of $2.36\pm 0.11$ and $1.75\pm 0.10$, respectively, relative to a black-metal-velvet reference. A Geant4-based optical model yields reflectivity estimates at $450\mathrm{nm}$ in LAr of $(90\pm 4)\%$ for PET and $(81\pm 5)\%$ for crystallized PEN, compared with $(80\pm 5)\%$ for the Tetratex reference. These results motivate further investigation of both materials for LAr instrumentation, including their radiopurity, long-term cryogenic stability, and performance under matched detector conditions.