Elastic α -scattering experiments on Sr 88 to constrain the α -induced reaction cross sections for weak- r -process studies

Background: Low-mass neutron-rich isotopes between Fe and Ag are predicted to be synthesized in neutrino-driven winds emitted after the collapse of a massive star. The modeling of this nucleosynthesis scenario, called weak r -process or α -process, requires large nuclear reaction network calculations. The necessary reaction cross sections are usually taken from the statistical model which uses global optical potentials. Several global α + nucleus optical potential ( α -OMP) parametrizations exist and deviations between their predicted cross sections exceed even an order of magnitude at astrophysical energies. Purpose: In the relevant mass range of the weak r -process, cross-section calculations performed with the Atomki-V2 potential generally reproduce experimental data within ± 50 % . However, recently the Sr 88 ( α , n ) Zr 91 cross section was measured and it was found that, unlike other available cross section data, it is lower than the calculations performed with recent OMPs, including the Atomki-V2 calculations, by a factor of 3. Thus, to better understand the source of the discrepancy, the present work aims to determine the Sr 88 total reaction cross section by measuring Sr 88 ( α , α ) Sr 88 elastic scattering angular distributions. Methods: Between ϑ lab . = 20 ∘ – 167 . 5 ∘ complete elastic α scattering angular distributions were measured at E lab . = 13.50 , 16.21 , and 19.47 MeV energies, with precision better than 7 % for each angle. Results: The total reaction cross section ( σ reac ) was determined from the experimental data using three independent methods. First, the standard approach of fitting the parameters of the complex α -nucleus optical potential was applied. Second, a model-independent phase shift fit was used. Finally, σ reac values were also derived using a Bayesian analysis. The excellent agreement of the derived σ reac from the three fundamentally different methods shows that the measured angular distributions constrain the total reaction cross sections well. Conclusions: There is serious tension between the very-well-constrained experimental σ reac values from elastic scattering and those obtained from the measured ( α , n ) data. The present results from elastic scattering are robust, agree well with the prediction of recent α -OMPs, and fit into the general systematics of cross sections. Thus, an independent verification of the Sr 88 ( α , n ) Zr 91 cross section data is clearly needed.