The ⁹⁵Zr(n, γ)⁹⁶Zr Cross Section from the Surrogate Ratio Method and Its Effect on s-process Nucleosynthesis

Abstract

The 95 Zr(n,γ) 96 Zr reaction cross section is crucial in the modelling of s-process nucleosynthesis in asymptotic giant branch stars because it controls the operation of the branching point at the unstable 95 Zr and the subsequent production of 96 Zr. We have carried out the measurement of the 94 Zr( 18 O, 16 O) and 90 Zr( 18 O, 16 O) reactions and obtained the γ-decay probability ratio of 96 Zr* and 92 Zr* to determine the 95 Zr(n,γ) 96 Zr reaction cross sections with the surrogate ratio method. Our deduced maxwellian-averaged cross section of 66±16 mb at 30 keV is close to the value recommended by Bao et al. (2000), but 30% and more than a factor of two larger than the values proposed by Toukan & Käppeler (1990) and Lugaro et al. (2014), respectively, and routinely used in s-process models. We tested the new rate in stellar models with masses between 2 and 6 M ⊙ and metallicities 0.014 and 0.03. The largest changes -up 80% variations in 96 Zr -are seen in models of mass 3-4 M ⊙ , where the 22 Ne neutron source is mildly activated. The new rate can still provide a match to data from meteoritic stardust silicon carbide grains, provided the maximum mass of the parent stars is below 4 M ⊙ , for a metallicity of 0.03.

The Radioactive Nuclei $^{\textbf{26}}$Al and $^{\textbf{60}}$Fe in the Cosmos and in the Solar System

“…In such an experiment, one can measure the 56,58 Fe(2n,γ) 58,60 Fe rate to obtain the ratio between 57 Fe(n,γ) 58 Fe, which is well known, because 57 Fe is a stable isotope, and 59 Fe(n,γ) 60 Fe. This approach has been proven as feasible in determination of the key s-process reaction 95 Zr(n,γ) 96 Zr (Yan et al, 2017), and suggests a precision of 30% or better. Another surrogate approach is through the 59 Fe(d,p) 60 Fe reaction with inverse kinematics.…”

The RADIOSTAR Project

“…Updated investigations will still be needed on the impact of nuclear reaction rate uncertainties on the production of each SLR nucleus in CCSNe, even for the best studied 26 Al and 60 Fe. For instance, new experimental constraints have allowed us to significantly reduce the impact of the uncertainty of the 59 Fe(n,γ) 60 Fe cross section [69], one of the main nuclear inputs for the production of 60 Fe [32]. Massive star winds and the production of 107 Pd We are currently calculating the production and ejection of 36 Cl and 41 Ca in binary systems.…”

“…On top of the efforts related to the modelling of the stellar sources and galactic evolution listed above, improvements in the nuclear and meteoritic experimental data would strongly help us to constrain the problem of the origin of SLR nuclei in the ESS. In terms of nuclear physics inputs, several advances have been made, also with the contribution of the RADIOSTAR project (see [69,77] and Laird et al 2022, submitted to J.Phys.G.). Main inputs are still required in terms of charged-particle reactions, for example, on the crucial 22 Ne(α,n) 25 Mg reaction that produces the neutrons needed to make 182 Hf in AGB stars.…”