Production of Activated $^{182}$Ta for the Half-life Measurement of $^{180m}$Ta Decay

Among various nuclear isomers, $^{180m}$Ta is considered to have a

long half-life but it half-life has not been measured. The current

lower limit of the half-life is 1.5 $¥times$ 10$^{19}$ years. Using an ultra-low

background HPGe detector installed in the Kamioka Underground Laboratory,

we measured a natural Tantalum sample (850g) for about 500 days.

The $¥gamma$-rays energy spectrum was analyzed to measure the half-lives of $¥beta$-decay

and electron capture of $^{180m}$Ta. In order to obtain reliable half-life from

the number of observed events, it is necessary to precisely evaluate

the detection efficiency for each $¥gamma$-ray energy. In this study, we experimentally

evaluated the detection efficiency using $¥gamma$-rays emitted from the

$¥beta$-decay of $^{182}$Ta.

A natural Ta sample, which is an identical sample with that of half-life measurement,

was activated by

thermal neutron capture reaction (182Ta sample).

To increase the activity of the 182Ta calibration sample, the neutron emitted

from $^{252}$Cf source was efficiently thermalized in the 10cm thick polyethylene,

and reflected with 20cm thick graphite.

To obtain the accurate efficiency, the uniform activity of 182Ta

sample was required. The specimens were also placed with the 182Ta

sample to investigate the uniformity of activation.

The obtained activity and uniformity of 182Ta sample will be presented.

The accuracy of the $¥gamma$-ray efficiency obtained using the 182Ta sample will be also discussed.