Photoexcitation of 180Ta by 60Co and 137Cs gamma rays

The depopulation of the quasistable 180Tam was studied by irradiating natural Ta samples with strong 60Co and 137Cs sources. Decay of the ground state (t1/2=8.1 h) was observed by taking gamma-spectra using a large planar Ge detector in a NaI(Tl) anticoincidence shielding. Clear evidence was found for activation levels lying below 1.4 MeV, and their presence seems to be possible below 0.737 MeV as well, which couple effectively the quasistable isomer and the ground state. Integral cross sections and effective half-lives relevant in stellar environment have been inferred for depopulation of the isomer through mediating levels assumed at various energies. Implications for 180Tam production in the s-process nucleosynthesis are discussed