Half-life measurement of $^{180m}$Ta decay with ulta-low background HPGe detector at Kamioka Underground Laboratory

 Among various nuclear isomers, $^{180m}$Ta is a nuclide with a long lifetime that its decay has not been detected. The main decay processes are considered to be $eta^{-}$ decay and the electron capture (EC). The most stringent half-life limit for $^{180m}$Ta decays is $1.5 imes 10^{19}$ years. In this research, we analyzed the data measured for about 500 days using an ultra-low background(BG) HPGe semiconductor detector installed in the Kamioka Underground Laboratory, with a natural Ta sample placed. The half-life of $^{180m}$Ta was evaluated by identifying $gamma$-rays emitted from the excited states of $^{180}$W ($eta^{-}$ decay) and $^{180}$Hf (EC).

 The search for rare decay events[s6]  of $^{180m}$Ta requires a long-term measurement, for which the detector gain often changes. The gain changes deteriorate the detector energy resolution. To overcome this problem, a constant pulse height artificial signal with 1Hz was fed into the pre-amplifier of the HPGe detector during the measurement to make correction of the gain variation.

 We also tried to separate environmental BG $gamma$-rays from $gamma$-rays emitted by $^{180m}$ Ta decays by analyzing the waveform.

 The obtained energy spectrum and the sensitivity of the $^{180m}$Ta half-life measurement will be presented.