Search for the decay of nature's rarest isotope $^{\rm {180m}}$Ta

TL;DR

This study searches for the decay of the rare isotope $^{180m}$Ta using ultra low background gamma spectrometry, setting new lower half-life limits and finding no decay signals, thus extending previous bounds significantly.

Contribution

It provides the first search for $^{180m}$Ta decay with improved lower half-life limits using an ultra low background setup and Bayesian analysis.

Findings

No decay signal observed for $^{180m}$Ta.

Lower half-life limits set at $5.8\times10^{16}$ yr and $2.0\times10^{17}$ yr.

Total half-life longer than $4.5\times10^{16}$ yr.

Abstract

$^{\rm {180m}}$Ta is the rarest naturally occurring quasi-stable isotope and the longest lived metastable state which is known. Its possible decay via the $\beta^-$ or the electron capture channel has never been observed. This article presents a search for the decay of $^{\rm {180m}}$Ta with an ultra low background Sandwich HPGe gamma spectrometry setup in the HADES underground laboratory. No signal is observed and improved lower partial half-life limits are set with a Bayesian analysis to $5.8\cdot10^{16}$ yr for the $\beta^-$ channel and $2.0\cdot10^{17}$ yr for the electron capture channel (90% credibility). The total half-life of $^{\rm {180m}}$Ta is longer than $4.5\cdot10^{16}$ yr. This is more than a factor of two improvement compared to previous searches.