Development of large-volume ¹³⁰TeO₂ bolometers for the CROSS 2β decay search experiment

We report on the development of thermal detectors based on large-size tellurium dioxide crystals (45x45x45 mm), containing tellurium enriched in ¹³⁰Te to ~91%, for the CROSS double-beta decay experiment. A powder used for the crystals growth was additionally purified by the directional solidification method, resulting in the reduction of the concentration of impurities by a factor 10, to a few ppm of the total concentration of residual elements (the main impurity is Fe). The purest part of the ingot (the first ~200 mm, ~80% of the total length of the cylindrical part of the ingot) was determined by scanning segregation profiles of impurities (both on the surface and in center of the ingot) and used for the ¹³⁰TeO₂ powder production with no evidence of re-contamination. The crystal growth was verified with precursors produced from powder with natural Te isotopic composition, and two small-size (20x20x10 mm) samples were tested at a sea-level laboratory showing high bolometric and spectrometric performance together with acceptable ²¹⁰Po content (below 10 mBq/kg). This growth method was then applied for the production of six large cubic ¹³⁰TeO₂ crystals for the CROSS experiment and 4 of them were taken randomly to be characterized at the Canfranc underground laboratory, in the CROSS-dedicated low-background cryogenic facility. Two ¹³⁰TeO₂ samples were coated with a thin metal film in form of Al layer (on 4 sides) or AlPd grid (on a single side) to investigate the possibility to tag surface events by pulse-shape discrimination. Similarly to the small natural precursors, large-volume ¹³⁰TeO₂ bolometers show high performance and even better internal purity (²¹⁰Po activity ~ 1 mBq/kg, while activities of ²²⁸Th and ²²⁶Ra are below 0.01 mBq/kg), satisfying requirements for the CROSS and, potentially, next-generation experiments.