Search for the radioactivity of 180mTa using an underground HPGe sandwich spectrometer

The radioactivity of 180mTa has never been detected. The present attempt to detect it was carried out using a newly developed HPGe sandwich spectrometer installed 500 m water equivalent underground in the HADES laboratory. The sample consisted of 6 discs of tantalum of natural isotopic composition with a total mass of 1500 g and a total mass for 180Ta of 180 mg. The sample was measured for 68 days and the resulting lower bound for the half-life of 180mTa was 2.0×1016 y, which is a factor of 2.8 higher than the previous highest value.