Measurement of the In115 (γ,γ′)In115m inelastic scattering cross section in the 1.8 to 3.7 MeV energy range with monoenergetic photon beams

Abstract

For the first time the excitation cross section of $^{115}\mathrm{In}$ to its first excited state at 336.24 keV has been measured with monoenergetic photon beams at 15 energies between 1.8 and 3.7 MeV. The measurements were performed at the High-Intensity Gamma-ray Source ($\mathrm{HI}\ensuremath{\gamma}\mathrm{S}$) facility with natural In targets and incident photon fluxes ranging from $0.7\ifmmode\times\else\texttimes\fi{}{10}^{7}\ensuremath{\gamma}/({\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}\text{s})$ to $3.2\ifmmode\times\else\texttimes\fi{}{10}^{7}\ensuremath{\gamma}/({\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}\text{s})$. The inelastic scattering cross section was obtained from the yield of the 336.24 keV deexcitation $\ensuremath{\gamma}$ rays. The cross-section values varied between $l0.05$ and 28.7 $\ensuremath{\mu}\mathrm{b}$ at 1.8 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 $^{115}\mathrm{In}$. Model calculations, however, overestimate the measured cross-section data by approximately one order of magnitude. Using different choices for the $E1 \ensuremath{\gamma}$-ray strength function, the data can be well described. The data are important for improving our knowledge of the level scheme of $^{115}\mathrm{In}$, especially spin and parity assignments of already known states.