Measurement of the In115 (γ,γ′)In115m

Abstract

For the first time the excitation cross section of 115 In to its first excited state at 336.24 keV has been measured with mono-energetic photon beams at 15 energies between 1.8 and 3.7 MeV. The measurements were performed at the HIγS facility with natural In targets and incident photon fluxes ranging from 0.7 × 10 7 γ/(cm 2 s) to 3.2 × 10 7 γ/(cm 2 s). The inelastic scattering cross section was obtained from the yield of the 336.24 keV de-excitation γ rays. The cross-section values varied between < 0.05 µb and 28.7 µb at 1.8 MeV and 3.7 MeV, respectively. Compared to standard photon inelastic scattering cross-section data in this energy range of medium-mass nuclei, these extremely small values are not completely unexpected, given the nature of the low-lying nuclear levels of 115 In. Model calculations, however, overestimate the measured cross-section data by approximately one order of magnitude. Using different choices for the E1 γ-ray strength function, the data can be well described. The data are important for improving our knowledge of the level scheme of 115 In, especially spin and parity assignments of already known states.