부산대학교 도서관

$^{180m}$Ta, which is the second excited state of $^{180}$Ta at E$_x$ = 77 keV, is naturally occurring and is the only known stable isomer. The half-life of $^{180m}$Ta is considered to be an important parameter for nuclear synthesis models for heavy elements. However, the decay of $^{180m}$Ta has never been observed even though several groups tried to measure it. We will search for gamma transitions from $^{180m}$Ta 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 $\times$ 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 $^{180m}$Ta, respectively. In this study, we used T$_{1/2}$ = 2.0~$\times$~10$^{17}$ years and T$_{1/2}$ = 5.8~$\times$~10$^{16}$ years for the EC and $\beta$-decay which are the present best lower-limits as reported by B. Lehnert \textit{et al.}