Study of $^{180}$Ta$^{\mathrm{m}}$ Lifetime by using HPGe Detector at Kamioka Underground Observatory

For all nuclear isotopes that exist on the Earth, $^{180}$Ta$^{\mathrm{m}}$ has known as the only isotope that naturally occurring in metastable state with remarkable properties of long-lived isomeric state (T$_{1/2}$ \textgreater 1.2 $\times$ 10$^{15}$ y) and short-lived ground state (T$_{1/2} = $ 8.1 h). Many researchers have great concern about this isotope because of its interesting nuclear properties and puzzling nucleosynthesis mechanics. The half-life of $^{180}$Ta$^{\mathrm{m}}$ is yet to be confirmed up until now. Since July 2013, we have developed a new ultra-low background gamma spectroscopy with HPGe detector at the Kamioka Underground Observatory (2700 m.w.e.) and started the observation of the decay of $^{180}$Ta$^{\mathrm{m}}$. Compare with other worldwide researches, Kamioka stands a good chance to conclude or impose a longer limit on the half-life of $^{\mathrm{180}}$Ta$^{\mathrm{m}}$ in the order of 10$^{17}$ years. With effort on background reduction such as shield design and pulse shape discrimination analysis, we have successfully improved the signal-to-noise ratio of the measurement. The result will be presented for the first time in this meeting.