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

Abstract The radioactivity of 180m Ta 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 180 Ta of 180 mg. The sample was measured for 68 days and the resulting lower bound for the half-life of 180m Ta was 2.0×10 16  y, which is a factor of 2.8 higher than the previous highest value.