Characterization of light production and transport in tellurium dioxide crystals

Simultaneous measurement of phonon and light signatures is an effective way to reduce the backgrounds and increase the sensitivity of CUPID, a next-generation bolometric neutrinoless double-beta decay (0 nu beta beta) experiment. Light emission in tellurium dioxide (TeO2) crystals, one of the candidate materials for CUPID, is dominated by faint Cherenkov radiation, and the high refractive index of TeO2 complicates light collection. Positive identification of 0 nu beta beta events therefore requires high-sensitivity light detectors and careful optimization of light transport. A detailed microphysical understanding of the optical properties of TeO2 crystals is essential for such optimization. We present a set of quantitative measurements of light production and transport in a cubic TeO2 crystal, verified with a complete optical model and calibrated against a UVT acrylic standard. We measure the optical surface properties of the crystal, and set stringent limits on the amount of room-temperature scintillation in TeO2 for beta and alpha particles of 5.3 and 8 photons/MeV, respectively, at 90% confidence. The techniques described here can be used to optimize and verify the particle identification capabilities of CUPID.