Search for the decay ofTam180

A sample of ${\mathrm{Ta}}_2$ ${\mathrm{O}}_5$ containing 151 mg of Ta enriched to 5.47% in ${}_{}{}^{180}{}_{}{}^{}\mathrm{Ta}_{}^{\mathrm{m}}{}_{}{}^{}$ was counted in a well-type high-purity Ge detector. This was enclosed in a large NaI(Tl) detector operated in anticoincidence to reduce the cosmic-ray continuum background. Several runs of ∼100 days each were made under different conditions to search for γ-ray singles or summing peaks that would follow the electron capture decay of ${}_{}{}^{180}{}_{}{}^{}\mathrm{Ta}_{}^{\mathrm{m}}{}_{}{}^{}$ to ${}_{}{}^{180}{}_{}{}^{}\mathrm{Hf}$ or the ${\beta }^{\mathrm{-}}$ decay to ${}_{}{}^{180}{}_{}{}^{}\mathrm{W}$. Lower limits derived were $T_{1/2}$>3.0×${10}^{15}$ yr, log$f_0$t>22.9 for electron capture decay and $T_{1/2}$>1.9×${10}^{15}$ yr, log$f_0$t>21.3 for ${\beta }^{\mathrm{-}}$ decay. The total half-life of ${}_{}{}^{180}{}_{}{}^{}\mathrm{Ta}_{}^{\mathrm{m}}{}_{}{}^{}$ is therefore $T_{1/2}$>1.2×${10}^{15}$yr.