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

摘要

For the first time the de-excitation cross section of the isomeric-state nuclide Ta-180m 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 x 10(7) gamma/(cm(2) x s) to 3.2 x 10(7) gamma/(cm(2) x s), the Ta-180m de-excitation cross section was obtained from the yield of the K-alpha 1 and K-alpha 2 X rays of Hf-180m at 55.79 keV and 54.61 keV, respectively, produced after electron capture on the unstable Ta-180m 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 Ta-180m. 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-gamma 7.6 MeV can provide a sizable yield of Hf-180m X rays via the reaction Ta-181(gamma, n)Ta-180, if a natural tantalum target is used, which contain four orders of magnitude more Ta-181 nuclei than Ta-180m nuclei available for the Ta-180m(gamma, gamma')Ta-180 reaction of interest.