Postdoctoral position: "Cross section measurements for the p-process nucleosynthesis" - IP2I (UMR5822) et LGL-TPE (UMR5276)

Research project

In the Universe chemical composition, several elements beyond iron are observed to have stable isotopes situated on the proton-rich side of the stability valley: these isotopes are called p-nuclei. Their specificity is that they cannot have been produced by neutron-capture processes, contrary to the majority of nuclei heavier than iron. The nucleosynthesis mechanism that leads to the formation of p-nuclei is called the p-process. According to the dominant astrophysical scenario, it takes place during core-collapse supernovae, and mainly involves the photodisintegration of a seed population of nuclei originating from the neutron rich side of the stability valley. First, several neutrons are expelled by (γ,n) reactions, but the final abundance of the different p-nuclei crucially depends on (γ,p) and (γ,α) reactions that occur in the end. They are an essential input for the nucleosynthesis calculations that aim to test different astrophysical models to reproduce the observed abundance of p-nuclei.

Globally, the p-process involves a network around 2000 nuclei and 20000 reactions, most of which cannot be measured experimentally. Nucleosynthesis calculations have to rely on theoretical calculations of cross sections based on the Hauser-Feshbach statistical model. The nuclear statistical parameters that allow to perform such calculations are the optical model potentials, the level density and the gamma strength, which are themselves described by different models. Experimental data are much needed to better constrain these models. Especially, it is repeatedly mentioned in the literature that the lack of experimental knowledge on the alpha-nucleus optical potential at energy of astrophysical interest (namely far below the Coulomb barrier) hinders the progress in the nucleosynthesis modeling. Our project is focused on the measurement of cross sections that are crucial to improve the knowledge of astrophysical reaction rates.

We have selected several key reactions for this purpose. The main objective of our project is to perform alpha and proton capture experiments at the NFS-SPIRAL2 facility, whose high-intensity alpha and proton beams in the relevant energy range offers a unique opportunity to access the very low cross sections that are needed in the astrophysical context. These measurements will involve two types of techniques: if the lifetime of the reaction product allows, the well-known activation technique can be used ; else, in-beam techniques have to be employed to count the gamma rays produced instantaneously after the reaction. We plan to perform such experiments around 2023. Meanwhile, preparatory experiments should be performed at medium-scale facilities, to better control the classical experimental issues before facing the new challenges associated with very high intensity. Furthermore, the knowledge of alpha-nucleus optical potentials also need to be increased by complementary alpha-scattering measurements: the exploration of isotopic and deformation effects should be particularly enlightening, and several such measurements will take place in the next years, for instance at the Orsay Split-Pole facility.