Underground search for the decay of 180Tam

Tantalum-180m is a very rare primordial isotope and is not in its nuclear ground state. The radioactivity of 180Tam has not yet been observed. Previous attempts to measure the half-life of 180Tam 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 1.7x1016 y for electron capture decay and 1.2x1016 y for β- decay. This gives a total lower bound for the half-life of 7.1x1015 y, which is a factor of 6 higher than the previous lower bound