Photo-induced Depopulation of The180tamisomer Via Low-Lying Intermediate States: Structure and Astrophysical Implications

The photo-induced depopulation of the quasistable isomer (t(1/2) greater than or equal to1.2 x10(15) yr) in Ta-180 with angular momentum and parity J(pi) = 9(-) at an excitation energy E-x = 75 keV was studied at the new bremsstrahlung irradiation facility installed at the Stuttgart 4.3 MV DYNAMITRON accelerator in the energy range of bremsstrahlung end point energies between E-0 = 0.8-3.1 MeV. The onset of the isomer depopulation could be observed starting at an end point energy of E-0 approximate to1 MeV, i. e., at an intermediate state of Ta-180 at or below that energy. Higher-lying intermediate states were found at 1.22, 1.43, 1.55, 1.85, 2.16, 2.40, 2.64, and 2.80 MeV. The extracted integrated cross sections show a remarkably strong depopulation of the Ta-180(m) isomer by photoexcitation. The results are compared with previous experiments and recent quasiparticle-phonon model calculations. Implications of the results for a possible nucleosynthesis of Ta-180 in the s process and the neutrino process are discussed.