Test of statistical model cross section calculations for $\alpha$-induced reactions on ${}^{107}\mathrm{Ag}$ at energies of astrophysical interest

Background: Astrophysical reaction rates, which are mostly derived from theoretical cross sections, are necessary input to nuclear reaction network simulations for studying the origin of $p$ nuclei. Past experiments have found a considerable difference between theoretical and experimental cross sections in some cases, especially for ($\alpha ,\gamma$) reactions at low energy. Therefore, it is important to experimentally test theoretical cross section predictions at low, astrophysically relevant energies.

Purpose: The aim is to measure reaction cross sections of ${}^{107}\mathrm{Ag}\left(\alpha ,\gamma \right){}^{111}\mathrm{In}$ and ${}^{107}\mathrm{Ag}\left(\alpha ,n\right){}^{110}\mathrm{In}$ at low energies in order to extend the experimental database for astrophysical reactions involving $\alpha$ particles towards lower mass numbers. Reaction rate predictions are very sensitive to the optical model parameters and this introduces a large uncertainty into theoretical rates involving $\alpha$ particles at low energy. We have also used Hauser-Feshbach statistical model calculations to study the origin of possible discrepancies between prediction and data.

Method: An activation technique has been used to measure the reaction cross sections at effective center of mass energies between 7.79 MeV and 12.50 MeV. Isomeric and ground state cross sections of the ($\alpha ,n$) reaction were determined separately.

Results: The measured cross sections were found to be lower than theoretical predictions for the ($\alpha ,\gamma$) reaction. Varying the calculated averaged widths in the Hauser-Feshbach model, it became evident that the data for the ($\alpha ,\gamma$) and ($\alpha ,n$) reactions can only be simultaneously reproduced when rescaling the ratio of $\gamma$ to neutron width and using an energy-dependent imaginary part in the optical $\alpha$ + ${}^{107}\mathrm{Ag}$ potential.

Conclusions: The new data extend the range of measured charged-particle cross sections for astrophysical applications to lower mass numbers and lower energies. The modifications in the model predictions required to reproduce the present data are fully consistent with what was found in previous investigations. Thus, our results confirm the previously suggested energy-dependent modification of the optical $\alpha$+nucleus potential.

Figure 1

A schematic drawing of the target chamber used for the irradiations.

Figure 2

(a) Low and (b) high energy parts of the $\gamma$ spectrum taken after an 8.7 h irradiation of a target with a 10 MeV $\alpha$ beam. The $\gamma$ lines used for analysis are indicated on the spectrum. The other peaks are from either laboratory background or the other $\gamma$ transitions (1 Channel = 0.204 keV).

Figure 3

Decay of the isotopes produced on natural silver targets by $\alpha$ irradiation.

Figure 4

Measured cross section of ${}^{107}\mathrm{Ag}(\alpha ,\gamma$) compared to theory using the smaragd code [64] (see text for details). Previous results from Baglin [51] are also included in the figure.

Figure 5

Measured cross section of ${}^{107}\mathrm{Ag}(\alpha ,n$) compared to theory using the smaragd code [64] (see text for details). Previous results from Stelson [52] are also included in the figure.

Figure 6

Sensitivity of the ${}^{107}\mathrm{Ag}\left(\alpha ,\gamma \right){}^{111}\mathrm{In}$ reaction cross sections to variations in various averaged reaction widths as function of energy [56]. The cross sections are insensitive to a variation of the proton width across the shown energy range.

Figure 7

Same as Fig. 6 but for ${}^{107}\mathrm{Ag}\left(\alpha ,n\right){}^{110}\mathrm{In}$.