Evaluation for $¥gamma$-ray detection efficiency with an activated Tantalum for the half-life measurement of $¥mathrm{^{180m}}$Ta decay

An nuclear isomer, $¥mathrm{^{180m}}$Ta is the rarest isotopes, and the finite half-life has not been measured. The latest result gives a lower limit of $1.5 ¥times 10^{19}$ years.

We measured the half-life of $¥beta^{-}$ decay and electron capture of $¥mathrm{^{180m}}$Ta by analyzing the $¥gamma$-ray energy spectra observed with a ultra-low background(BG) HPGe detector installed in the Kamioka Underground Laboratory.To estimate the half-life of $¥mathrm{^{180m}}$Ta decay from the number of $¥gamma$-ray events, we estimated the $¥gamma$-ray detection efficiency of Ge detector. 

We measured the $¥gamma$-ray energy (93 keV - 351 keV) from the daughter nuclear.We evaluated the detection efficiency at low energies by covering the entire Ge detector with an activated sample of Ta.We used $^{252}$Cf neutron source to activate the Ta sample. Fast neutrons emitted from $¥mathrm{^{252}}$Cf was thermalized with polyethylene moderator and captured by $^{181}$Ta (99.998¥% in natural Ta). By using activated Ta, a uniformly $¥mathrm{^{182}}$Ta which is identical to decaying $¥mathrm{^{180m}}$Ta in natural Ta sample can be produced. The detection efficiency is evaluated by focusing on $¥gamma$-rays with energies of 100.1 keV, 152.4 keV, 222.1 keV, etc. emitted by $^{¥mathrm{182}}$Ta. We will present he results of the evaluation of the detection efficiency.