Underground search for the decay ofTa180m

Abstract

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.