T-2, Nuclear and Particle Physics, Astrophysics and Cosmology

Neutron captures during the freeze-out of the r-process

Khalil Farouqi
Mainz University, Germany

In order to study possible neutron-capture effects during an r-process, it is necessary to perform fully dynamical simulations. We have performed such calculations within the model of an adiabatically expanding high-entropy bubble of a SN II, using temperature-dependent reaction rates including the NON-SMOKER neutron-capture rates of Rauscher et al. [1]. In agreement with earlier results (see, e.g. [2]), we find that due to the (n,gamma)-(gamma,n) equilibrium established at the onset of the r- process, only late-time neutron captures during the freeze-out phase are important. Since the uncertainties in the theoretical neutron capture rates are large, we have scaled the 'standard' NON-SMOKER rates up and down by a two orders of magnitude, respectively. At low entropy, typical e.g. for the formation of the A~130 solar- system r-abundance peak, the freeze-out is fast. Therefore, even a change of all neutron-capture rates in a range of 4 orders of magnitude will not alter the final abundances significantly. However, the freeze-out at higher entropies, e.g. for conditions that form the A~195 abundance peak, is lower and can alter the final abundances of heavy nuclei. For example, the abundance 'trough' before the 3rd peak obtained when assuming an instantaneous freeze-out can be filled with such late captures of 'primary' neutrons. Furthermore, we have also studied the effects from late re-captures of neutrons emitted after $\beta$-decay of short-lived istopes during the decay back to stability. Similar to the effects observed for the late non-equilibrium capture of the 'primary' neutrons, also the delayed neutrons have no effect on the final abundances in the A~130 peak area, and lead only to minor alterations of the abundances in the heavy rare-earth and the A~195 peak regions.


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