Abstract
Recent measurements of the nuclear level densities and γ-ray strength functions below the neutron thresholds in 180,181,182Ta are used as input in the nuclear reaction code TALYS. These experimental average quantities are utilized in the calculations of the 179,180,181Ta radiative neutron capture cross sections. From the latter, astrophysical Maxwellian-averaged (n,γ) cross sections (MACS) and reaction rates are extracted, which in turn are used in large astrophysical network calculations to probe the production mechanism of 180Ta. These calculations are performed for two scenarios, the s-process production of 180,181Ta in Asymptotic Giant Branch (AGB) stars and the p-process nucleosynthesis of Tam180 in Type-II supernovae. Based on the results from this work, the s-process in stellar evolution is considered negligible in the production of Tam180 whereas 181Ta is partially produced by AGB stars. The new measurements strongly constrain the production and destruction rates of Tam180 at p-process temperatures and confirm significant production of nature's rarest stable isotope Tam180 by the p-process.
Highlights
A small number of naturally occurring neutron-deficient nuclides with Z ≥ 34, referred to as p-nuclei, cannot be produced by neutron-capture processes [1]
It is not the intention to give a complete review of the 180Ta nucleosynthesis in different astrophysical sites, but rather to re-estimate, in light of the newly determined 179Ta(n, γ )180Tags,m, 180Tags,m(n, γ )181Ta and 181Ta(n, γ )182Ta rates, the 180Ta and 181Ta yields originating from the s-process in Asymptotic Giant Branch (AGB) stars and the p-process in massive stars exploding as Type-II supernovae
The 180,181,182Ta nuclear level density (NLD) and γ -ray strength function (γ SF) obtained experimentally [14] are used as input parameters in the TALYS reaction code to calculate the 179,180,181Ta radiative (n, γ ) cross sections
Summary
A small number of naturally occurring neutron-deficient nuclides with Z ≥ 34, referred to as p-nuclei, cannot be produced by neutron-capture processes [1]. For 138La and 180Ta, for instance, the p-process is not sufficient to explain their observed solar abundances and additional processes are invoked. While the situation for 180Ta is not clear, it was shown that for 138La the photodisintegration of seed nuclei is not efficient enough but that neutrino (ν) captures during the p-process in Type-II supernovae may be responsible for its galactic enrichment [2,3,4]. The significance of individual processes cannot be clearly determined, as a result of the uncertainties on the reaction rates for 180Ta due to the unavailability of experimental data, e.g. the radiative neutron capture rates of 179,180Ta isotopes or other nuclear ingredients needed to constrain these rates, such as the nuclear level density (NLD) and γ -ray strength function (γ SF) [3]. The p-process nucleosynthesis for 180Ta is studied on the basis of the Type-II supernova (SNII) model
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