Exploratory study of the ^180mTa(γ,γ')¹⁸⁰Ta reaction with mono-energetic low-energy photon beams at HIγS

For the first time the de-excitation cross section of the isomeric-state nuclide ^180mTa has been measured with mono-energetic photon beams in the 1.56 MeV to 3.70 MeV energy range. The measurement of this inelastic scattering cross section on nature's rarest isotope was performed at eight energies at the High-Intensity Gamma-Ray Source facility with natural tantalum targets. After irradiation for typically 20 hours with photon fluxes ranging from 0.3 x 10⁷ γ/(cm²×s) to 3.2 x 10⁷ γ/(cm²×s), the ^180mTa de-excitation cross section was obtained from the yield of the K_α1 and K_α2 X rays of ¹⁸⁰Hf at 55.79 keV and 54.61 keV, respectively, produced after electron capture on the unstable ¹⁸⁰Ta ground state. The cross-section values found vary between (1.9 ± 0.9) mb and (4.7 ± 2.4) mb, providing an astonishingly large cross section for the de-excitation of ^180mTa. Extensive tests were performed to check on the reliability of the present data. The effect of bremsstrahlung photons of higher energy than the mono-energetic beam was carefully studied and corrected for.