Development of large-volume ${}^{130}$TeO${}_2$ bolometers for the CROSS $2{\hat{I}}^2$ decay search experiment

Abstract

We report on the development of thermal detectors based on large-size tellurium dioxide crystals (45x45x45 mm), containing tellurium enriched in ${}^{130}$Te to about 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, about 80% of the total length of the cylindrical part of the ingot) was determined by scanning segregation profiles of impurities and used for the ${}^{130}$TeO${}_2$ 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 ${}^{210}$Po content (below 10 mBq/kg). This growth method was then applied for the production of six large cubic ${}^{130}$TeO${}_2$ crystals and 4 of them were taken randomly to be characterized at the Canfranc underground laboratory, in the CROSS-dedicated low-background cryogenic facility. Two ${}^{130}$TeO${}_2$ samples were coated with a thin, $O$(100 nm), 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 ${}^{130}$TeO${}_2$ bolometers show high performance and even better internal purity (${}^{210}$Po activity $\sim$ 1 mBq/kg, while activities of ${}^{228}$Th and ${}^{226}$Ra are below 0.01 mBq/kg), satisfying requirements for the CROSS and, potentially, next-generation experiments.

Submitted to JINST; 22 pages, 11 figures, 4 tables