Deep-underground search for the decay of 180m-Ta with an ultra-low-background HPGe detector

Abstract

${}^{180m}$Ta is the longest-lived metastable state presently known. Its decay has not been observed yet. In this work, we report a new result on the decay of \mTa obtained with a $2015.12$-g tantalum sample measured for $527.7$ d with an ultra-low background HPGe detector in the STELLA laboratory of the Laboratori Nazionali del Gran Sasso (LNGS), in Italy. Before the measurement, the sample has been stored deep-underground for ten years, resulting in subdominant background contributions from cosmogenically activated ${}^{182}$Ta. We observe no signal in the regions of interest and set half-life limits on the process for the two channels EC and ${\beta }^{-}$: $T_{1/2,\mathrm{EC}}>1.6\times 10^{18}$ yr and $T_{1/2,{\beta }^{-}}>1.1\times 10^{18}$ yr ($90$\% C.\,I.), respectively. We also set the limit on the $\gamma$ de-excitation / IC channel: $T_{1/2,\mathrm{IC}}>4.1\times 10^{15}$ yr ($90$\% C.\,I.). These are, as of now, the most stringent bounds on the decay of ${}^{180m}$Ta worldwide. Finally, we test the hypothetical scenarios of de-excitation of ${}^{180m}$Ta by cosmological Dark Matter and constrain new parameter space for strongly-interacting dark-matter particle with mass up to $10^5$ GeV.

9 pages, 7 figures, 4 tables

References in corpus (4)

• \textsc{MaGe} - a {\sc Geant4}-based Monte Carlo Application Framework for Low-background Germanium Experiments
• Survival of Nature's Rarest Isotope 180Ta under Stellar Conditions
• A search for rare and induced nuclear decays in hafnium
• Dark Matter Search with the Nuclear Isomer Ta-180m