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Final Activity Report Summary - GLOBAL ALPHA POTENTI (Global alpha potentials for p-process nucleosynthesis)

The nuclear reaction networks relevant to p-process nucleosynthesis involve alpha-nucleus reactions at low energies near the Coulomb barrier. Very few experimental data exist for this kind of reactions and all abundance calculations rely on theoretical predictions of cross sections at large. These are sensitive to the alpha optical potential at low energies. Our work is a systematic effort combining experimental studies and theoretical work to obtain an improved and updated global alpha-nucleus optical model potential (OP) for p-process nucleosynthesis.

From the experimental point of view, a series of measurements of radiative alpha-capture cross sections was carried out at the 4MV Dynamitron Tandem accelerator of the Institut fuer Experimentalphysik III, University of Bochum, Germany, on Ge-72, Zr-91,92, Mo-92, Pd-104, Sn-116,118 targets. One major problem in deriving the experimental cross section was determining the efficiency of the detector. A new methodology was developed to extract an effective energy-dependent and photon-multiplicity-dependent efficiency. The resulting efficiencies were subsequently applied to derive cross sections for the reactions studied. The new data were included in an extended and updated cross-section database that was used for re-evaluating the parameters of the global semi-microscopic alpha OP of Demetriou, Grama and Goriely, Nucl. Phys. A 707, 142 (2002).

The alpha OP was implemented in a software package including a graphical interface built around the nuclear reaction code TALYS. TALYS uses the implemented alpha OP to calculate the alpha elastic-scattering cross sections (optical model) and the alpha-nucleus reaction cross sections (statistical Hauser-Feshbach model). The implementation of the semi-microscopic alpha OP in TALYS required extensive software development including the incorporation of the SCAT2 subroutine to calculate transmission coefficients and reaction cross sections. The latter was necessary since the default subroutine call ed by TALYS is programmed to work for a certain analytical forms of the potentials only. The resulting computer code system was then applied to an extensive search for the optimal parameters of the imaginary part of the dispersive semi-microscopic alpha OP of Demetriou et al (see above reference)]. The parameters were adjusted to reproduce all the elastic scattering and reaction data in the newly updated cross section database. The optimisation procedure lead to modified values for the diffuseness, depth, surface vs volume contribution and damping of the former term of the imaginary potential. Moreover, a new approach to the calculation of the dispersive relation was adopted that is more appropriate for nucleus-nucleus interactions.

The most pronounced difference of the new OP with respect to the previous one is the weaker surface absorption compared to volume absorption. The results and conclusions of this work were presented in two international conferences and are also in preparation for submission for publication in international peer-reviewed journals.

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