Gamma-rays detection efficiency evaluation of low background HPGe detector for the half-life measurement of $^{180m}$Ta

The nuclide $^{180m}$Ta is considered to be the longest lived nuclear isomer. It undergoes decay processes such as beta decay, electron capture, and deexcitation, but its half-life has not yet been measured. The latest lower limit of the half-life is $ imes$ 10$^{19}$ years. Using an ultra-low background HPGe detector installed in the Kamioka Underground Laboratory, $gamma$-ray emitted from $^{180m}$Ta decay was measured by placing natural Tantalum sample(850 g) for about 500 days. In this research, we evaluated the $gamma$-ray detection efficiency of the HPGe detector to determine the half-life from the $gamma$-ray counts, and reproduced the detection efficiency by Monte Carlo simulation (MC).

It is particularly important to evaluate the detection efficiency in the energy range of 93 keV to 351 keV, where the $gamma$-ray energy from $^{180m}$Ta decay is observed. Detection efficiencies are measured for 81 keV-- 356 keV using $^{133}$Ba calibration source. The Ta sample was placed to cover the entire HPGe detector. Since the detection efficiency of such low energy $gamma$-ray has position dependence, measurements were taken at the head and sides of the HPGe detector irradiating a collimated $gamma$-ray source of ¹³³Ba in 2mm steps. The obtained results of the position dependence of $gamma$-ray detection efficiency and the reproduction by MC will be presented.