Constraining the γ-Process Using HECTOR

How to accurately model and predict the observed abundances of the 35 stable p-nuclei remains an open question in the field of nuclear astrophysics. The γ-process, an explosive astrophysical scenario, is thought to be the primary mechanism in creating the p-nuclei. However, current γ-process models remain insufficient in describing the observed p-nuclei abundances, with disagreements in some cases by two orders of magnitude.

There are still several significant sources of uncertainty when modeling the γ- process. One of these is the uncertainty of the nuclear reactions rates relevant to the γ-process. This is especially the case when the (γ,n) and (γ,p) reaction rates for an isotope are comparable in magnitude at temperatures relevant to the γ-process.
These are known as branching points, and knowing at which temperature the (γ,n) and (γ,p) reaction rates become equal for an isotope is crucial for accurately modeling the reaction flow of the γ-process.

In a recent sensitivity study, ¹¹¹In has been identified as a potential (γ,p)/(γ,n) branching point within the γ-process. In order to identify if ¹¹¹In is a branching point, the ¹⁰²Pd(p,γ)¹⁰³Ag,¹⁰⁸Cd(p,γ)¹⁰⁹In, and ¹¹⁰Cd(p,γ)¹¹¹In cross sections were measured at the University of Notre Dame Nuclear Science Laboratory. The measurements were performed with the High EffiCiency TOtal absorption spectrometeR (HECTOR), using the γ-summing technique.

As HECTOR was a fairly new detector at the time of this writing, the development and optimization of the detector is discussed, to include the development of a statistical method used to estimate the summing efficiency of the detector. In order to validate the analytical techniques presented in this thesis, a commissioning experiment was performed at the Compact Accelerator System for Performing Astrophysical Research (CASPAR) laboratory through the measurement of several resonance strengths in the ²⁷Al(p,γ)²⁸Si reaction.

After the (p,γ) cross sections at the NSL were performed, the cross-section data was used to constrain parameters within the Hauser-Feshbach formalism that could then deduce the inverse (γ,p) and (γ,n) reaction rates of interest. The results of this research support the conclusion that ¹¹¹In is indeed a (γ,p)/(γ,n) branching-point, with a branch-point temperature at 2.71 ±0.05 GK, well within the temperature window of the γ-process. This may have an impact on the predicted abundances of the p-nuclei and warrants further investigation. For future works, cross-section measurements to identify other potential γ-process branching points such as ⁸⁵Rb and ¹²⁹Cs points are desirable. Continuing to constrain the nuclear input in this manner will provide further insight into the astrophysical conditions necessary to produce the p-nuclei.