Measurement of the ${}^{115}\mathrm{In} (\gamma ,{\gamma }^{\prime }){}^{115m}\mathrm{In}$ inelastic scattering cross section in the 1.8 to 3.7 MeV energy range with monoenergetic photon beams

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 ($\mathrm{HI}\gamma \mathrm{S}$) facility with natural In targets and incident photon fluxes ranging from $0.7\times {10}^7\gamma /\left({\mathrm{cm}}^2\text{s}\right)$ to $3.2\times {10}^7\gamma /\left({\mathrm{cm}}^2\text{s}\right)$. The inelastic scattering cross section was obtained from the yield of the 336.24 keV deexcitation $\gamma$ rays. The cross-section values varied between $<0.05$ and 28.7 $\mu \mathrm{b}$ 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 $E1 \gamma$-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.

Figure 1

Partial level scheme of ${}^{115}\mathrm{In}$ [9]. The present status of the spin and parities is shown on the left-hand side of the partial level scheme, while corresponding energies in keV are shown on the right-hand side. The energy separation between the ground state and the isomeric state at 336.24 keV is not to scale.

Figure 2

Photon energy spectrum at $E_{\gamma }=2.59$ MeV obtained from a HPGe detector pulse-height spectrum, as described in the text.

Figure 3

Schematic diagram (not to scale) of the experimental setup consisting of photon beam collimator, scintillator paddle, and In activation foil. The NaI detector of known efficiency is used for cross-calibrating the scintillator paddle rate. The HPGe detector is used for photon beam energy measurements. Both detectors are rotated out of the photon beam during the activation measurements.

Figure 4

Measured cross-section data for the reaction ${}^{115}\mathrm{In}(\gamma ,{\gamma }^{\prime }){}^{115m}\mathrm{In}$ in comparison to different talys-1.9 [21, 22] evaluations. The model parameters are detailed in the text.

Figure 5

The ${}^{115}\mathrm{In}(\gamma ,n){}^{114}\mathrm{In}$ cross section up to 25 MeV. The evaluated cross section of Varlamov et al. [31] (black curve) is compared to talys-1.9 using the default values (red curve) and using the Gogny D1M HFB+QRPA model for the level density, $E1$, and $M1$ strength.