Motivations

One of the remaining open questions in stellar nucleosynthesis is an accurate description of the synthesis of the heavy elements, i.e. above the region of iron. Two neutron-induced processes, s- and r-processes, are the main contributors to the creation of these nuclei. There is however a group of 35 proton-rich nuclei which cannot be created by these two processes and thus have very low isotopic abundances compared to their more neutron rich neighbours:

74Se, 78Kr, 84Sr, 92Mo, 94Mo, 96Ru, 98Ru, 102Pd, 106Cd, 108Cd, 112Sn, 113In, 114Sn, 115Sn, 120Te,
124Xe, 126Xe, 130Ba, 132Ba, 136Ce, 138La, 138Ce, 144Sm, 152Gd, 156Dy, 158Dy, 162Er, 164Er, 168Yb,
174Hf, 180Ta, 180W, 184Os, 190Pt, 196Hg.

The term p-process is commonly used to describe the production of these rare stable isotopes although it is not yet clear whether it represents a single nucleosynthetic mechanism or a combination of several independent ones.

The most favored scenario for the p-process is believed to take place in the O/Ne layers of massive stars during their pre-supernova phases or during their explosion as supernovae type II, at temperatures between 1.8-3.3 GK. Photodisintegration reactions take place on preexisting seed nuclei and move the reaction flow toward the neutron-deficient side of the valley of β-stability. These reactions are initially dominated by (γ,n) reactions on neutron rich isotopes, creating new isotopes along the same elemental chain. At some point along this chain the (γ,p) and/or (γ,α) reaction(s) become faster than the otherwise dominating (γ,n) and the reaction flow moves into a different elemental chain. These points are called branching points and in many cases their location is critical for defining the final p-nuclei abundances.

Our main goal is investigation of the p-process reactions, especially around the branching points. In particular, we measure (p,γ) and (α,γ) reaction cross sections, implementing two techniques: activation measurements and summing technique. Especially with the summing technique we are planning on exploring p-process reaction path involving radioactive nuclei. For this purpose we developed a NaI(Tl) gamma detector (SuN) that will be used at NSCL with reaccelerated beams from the ReA3 accelerator. This will provide a unique opportunity for investigation of (p,γ) and (α,γ) reactions in inverse kinematics using radioactive beams.

In parallel to the experimental work, we are performing astrophysical network calculations that focus on production of p-nuclei. We use the libnucnet code developed at Clemson University. With this code we can investigate the sensitivity of the abundance of p-nuclei to the production rates we obtain from our experiments.