First experimental constraint on the 191Os(n, γ) reaction rate relevant to s-process nucleosynthesis

Abstract

The first experimentally constrained Maxwellian-averaged cross section for the 191Os(n, γ)192Os reaction relevant to s-process nucleosynthesis has been calculated by means of the nuclear level density and γ-strength function of 192Os. By performing an experiment at the Oslo Cyclotron Laboratory (OCL) using an ion beam of 30 MeV α-particles, particle-coincidence data from the 192Os(α, α' γ)192Os reaction was obtained. The Oslo method was utilized on the resulting matrix of γ-ray spectra versus excitation energy to simultaneously extract the nuclear level density and the γ-strength function of 192Os. Due to the lack of neutron-resonance data for the unstable isotope 191Os, the required normalization parameters have been estimated by means of data from isotopes in the same mass region. % to obtain the absolute value and slope of the level density and γ-strength function. To calculate the Maxwellian-averaged cross section of the 191Os(n, γ)192Os reaction, the decomposed E1 and M strengths of the γ-strength function and the level density of 192Os were used as input in the TALYS reaction code. The systematic uncertainties introduced by the normalization procedure of the level density and γ-strength function were investigated and propagated to the calculated Maxwellian-averaged cross section. Quite large uncertainties were deduced in the final level density and γ-strength function of 192Os. The errors propagated by the Oslo method software and the uncertainties introduced by the normalization parameters are found to be of approximately the same order of magnitude. The final result of the Maxwellian-averaged cross section, ⟨σ⟩n,γ = 1134 ± 354 mb, is in very good agreement with the theoretical estimate provided by the KADoNiS project. To study the impact of the present result on s-process nucleosynthesis in this mass region, the obtained cross section will, in the near future, be included in a reaction network taking branch points into account.