Underground search for the decay of ${}^{180}\mathrm{Ta}$${}^m$

Tantalum-180m is a very rare primordial isotope and is not in its nuclear ground state. The radioactivity of ${}^{180}\mathrm{Ta}$${}^m$ has not yet been observed. Previous attempts to measure the half-life of ${}^{180}\mathrm{Ta}$${}^m$ have been performed using various detectors located above ground. In this work a 606 g Ta disk of natural isotopic composition was measured for 170 d in the 225 m deep underground laboratory HADES. The new lower bound for the half-life is $1.7\times {10}^{16}$ y for electron capture decay and $1.2\times {10}^{16}$ y for ${\beta }^{-}$ decay. This gives a total lower bound for the half-life of $7.1\times {10}^{15}$ y, which is a factor of 6 higher than the previous lower bound.

Figure 1

The decay scheme of ${}^{180}\mathrm{Ta}$${}^m$.

Figure 2

The part of the $\gamma$-ray spectrum collected for 170 d that encompasses the energy regions with the $\gamma$ rays expected from the decay of ${}^{180}\mathrm{Ta}$${}^m$. The spectrum has been re-binned to enhance visibility. The peaks are all from background of the naturally occurring decay chains and the radionuclides being the major contributors to those peaks are indicated.