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

Abstract

Abstract For the first time the de-excitation cross section of the isomeric-state nuclide 180 m Ta 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 h with photon fluxes ranging from 0 . 3 × 1 0 7 γ /( cm 2 × s) to 3 . 2 × 1 0 7 γ /( cm 2 × s), the 180 m Ta de-excitation cross section was obtained from the yield of the K α 1 and K α 2 X rays of 180 m Hf at 55.79 keV and 54.61 keV, respectively, produced after electron capture on the unstable 180 m Ta ground state. The cross-section values found vary between ( 2 . 0 ± 0 . 7 ) mb and ( 4 . 7 ± 1 . 5 ) mb, providing a very large cross section for the de-excitation of 180 m Ta. 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. It was found that even a very small contamination of the mono-energetic photon beam by bremsstrahlung photons with E γ > 7 . 6 MeV can provide a sizable yield of 180 m Hf X rays via the reaction 181 Ta ( γ , n ) 180 Ta, if a natural tantalum target is used, which contain four orders of magnitude more 181 Ta nuclei than 180 m Ta nuclei available for the 180 m Ta ( γ , γ ′ ) 180 Ta reaction of interest.