Simulation Study for the Half-Life Measurement of Ta-180m Using HPGe Detectors

Abstract

Ta-180m, which is the second excited state of Ta-180 at E-x = 77 keV, is naturally occurring and is the only known stable isomer. The half-life of Ta-180m is considered to be an important parameter for nuclear synthesis models for heavy elements. However, the decay of Ta-180m has never been observed even though several groups tried to measure it. We will search for gamma transitions from Ta-180m 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 x 195 mm(2). The finalized tantalum sample configuration gives 2.0 and 7.5 coincidence events per year for the EC and the beta-decay of Ta-180m, respectively. In this study, we used T-1/2 =2.0 x 10(17) years and T-1/2 = 5.8 x 10(16) years for the EC and beta-decay which are the present best lower-limits as reported by B. Lehnert et al. c 2019 The Korean Physical Society