Double-beta decay investigation with highly pure enriched ... formula ...Se for the LUCIFER experiment

摘要

The LUCIFER project aims at deploying the first array of enriched scintillating bolometers for the investigation of neutrinoless double-beta decay of ⁸²Se. The matrix which embeds the source is an array of ZnSe crystals, where enriched ⁸²Se is used as decay isotope. The radiopurity of the initial components employed for manufacturing crystals, that can be operated as bolometers, is crucial for achieving a null background level in the region of interest for double-beta decay investigations. In this work, we evaluated the radioactive content in 2.5 kg of 96.3 % enriched ⁸²Se metal, measured with a high-purity germanium detector at the Gran Sasso deep underground laboratory. The limits on internal contaminations of primordial decay chain elements of ²³²Th, ²³⁸U and ²³⁵U are respectively:   61,   110 and   74 μBq/kg at 90 % C.L. The extremely low-background conditions in which the measurement was carried out and the high radiopurity of the ⁸²Se allowed us to establish the most stringent lower limits on the half-lives of the double-beta decay of ⁸²Se to 01+, 22+ and 21+ excited states of ⁸²Kr of 3.4 id="IEq19"> · 10 id="IEq20">²², 1.3 id="IEq21"> · 10 id="IEq22">²² and 1.0 id="IEq23"> · 10 id="IEq24">²² y, respectively, with a 90 % C.L.