Constraining the Astrophysical γ Process: Cross Section Measurements of (p,γ) Reactions in Inverse Kinematics

One of the most fundamental queries in nuclear astrophysics is understanding the mechanisms through which the elements are forged in the stars. For the vast majority of the elements heavier than iron, stellar nucleosynthesis is largely governed by the slow and rapid neutron capture processes. However, a relatively small group of naturally occurring, neutron-deficient isotopes, located in the region between ⁷⁴Se and ¹⁹⁶Hg, the so called p nuclei, cannot be formed by either of those processes. These ~30 stable nuclei are believed to be formed in the so called γ process from the "burning" of preexisting r and s process seeds at stellar environments of sufficiently high temperature, where a sequence of photodisintegration reactions can occur.

Networks of nuclear reactions are simulated under appropriate astrophysical conditions in order to reproduce the p nuclei abundances that are observed in nature. However, as experimental cross sections of γ process reactions are almost entirely unknown, the related reaction rates are based entirely on Hauser-Feshbach (HF) theoretical calculations and therefore carry large uncertainties. For this purpose the accurate cross section measurement of photodisintegration reactions within the astrophysically relevant Gamow window is of crucial importance.

In this talk two such experiments will be presented, namely the total cross section measurement of the ⁸²Kr(p,γ)⁸³Rb and ⁷³As(p,γ)⁷⁴Se reactions. Specifically the latter reaction is found to be of significant importance to the final abundance of the lightest p-nucleus, ⁷⁴Se, as the inverse reaction is its main destruction mechanism. The experiments took place at Michigan State University using the ReA facility. The ⁸²Kr and ⁷³As beams were directed onto a hydrogen gas cell located in the center of the Summing NaI(Tl) (SuN) detector and the obtained spectra were analyzed using the γ-summing technique. In addition to the total cross section measurement of the particular reaction, statistical properties of the compound nucleus (nuclear level density and γ-ray strength function) can also be extracted. Results from the two experiments along with their comparison to standard statistical model calculations using the NON-SMOKER and TALYS codes will be presented.