LAUSR

Abstract180mTa, which is the second excited state of 180Ta at Ex = 77 keV, is naturally occurring and is the only known stable isomer. The half-life of 180mTa is considered to be an important parameter for nuclear synthesis models for heavy elements. However, the decay of 180mTa has never been observed even though several groups tried to measure it. We will search for gamma transitions from 180mTa decays in a tantalum sample by using an array of fourteen HPGe detectors recently installed in an underground laboratory at Yangyang, Korea. In preparation for the measurement, Monte-Carlo simulation studies were conducted to optimize the tantalum sample configuration. Based on the simulation study, we decided on a configuration composed of a 2 mm thick disk with diameter of 200 mm and six 2 mm thick rectangular plates with dimension of 158 × 195 mm2. The finalized tantalum sample configuration gives 2.0 and 7.5 coincidence events per year for the EC and the β-decay of 180mTa, respectively. In this study, we used T1/2 =2.0 × 1017 years and T1/2 = 5.8 × 1016 years for the EC and β-decay which are the present best lower-limits as reported by B. Lehnert et al.