Photo-nuclear cross sections on ^197Au

A method was developed for measuring photonuclear reactions concurrently at several discrete photon beam energies on a stack of different target materials via a single irradiation. Concentric ring targets of the materials (in order from front to back targets: Au, TiO_2, Zn, Os, and Au) were irradiated at the High Intensity Gamma-ray Source (HIγS). As a proof of principle, we report the result of the cross section measurements from the front Au target. The excitation functions of the ^197Au(γ,n)^196Au and ^197Au(γ,3n)^194Au reactions were determined in the incident photon energy range of 13-31 MeV using quasi-monoenergetic photon beams provided at HIγS. The cross sections of the combined ground state (2^-) and short-lived first isomeric state (m1, 5^+), and of the second isomeric state (m2, 12^-) in the ^196Au production are obtained separately by subtracting the γ rays from the internal conversion of the second isomeric state. The excitation function of the second isomeric state via the photon-induced reaction ^197Au(γ,n)^196m2Au was measured for the first time. By using the activation method rather than direct neutron counting, the exclusive cross sections for the (γ,n) and (γ,3n) reactions were determined. Comparing the yields from the front and back gold targets validates our ability to simulate the effect of photon scattering in the target stack and provides a method for assessing the systematic uncertainty of our technique.