${}^{180m}\mathrm{Ta}$ is the rarest naturally occurring quasistable 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 ${}^{180m}\mathrm{Ta}$ with an ultralow 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 $5.8\times {10}^{16}\mathrm{yr}$ for the ${\beta }^{-}$ channel and $2.0\times {10}^{17}\mathrm{yr}$ for the electron capture channel (90 % credibility). The total half-life of ${}^{180m}\mathrm{Ta}$ is longer than $4.5\times {10}^{16}\mathrm{yr}$. This is more than a factor of two improvement compared to previous searches.

• Received 13 September 2016

DOI:https://doi.org/10.1103/PhysRevC.95.044306