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NUSTAR-THEORY-28 Neutron captures and the r-process ∗

By Khalil Farouqi, Johannes Gutenberg, Universität Mainz, Karl-ludwig Kratz, Bernd Pfeiffer, Friedrich-karl Thielemann and Thomas Rauscher


Based on the early description of the electric dipole (E1) strength function of Kadmenskii et al. [1], in a more recent paper Goriely [2] has studied the possible relevance of the existence of a ” pygmy ” dipole resonance (PDR) in the low-energy tail of the giant dipole resonance (GDR) for neutron-capture cross-section (σn,γ) calculations. Under certain conditions, i.e. for nuclei where the PDR lies close to the neutron separation energy (Sn), the author predicts local enhancements of the Hauser-Feshbach cross sections by more than a factor 100. If confi rmed experimentally, this would result in pronounced consequences for the r-abundance distribution. To our knowledge, for the fi rst time results from an experiment aimed on the determination of the GDR and the low-lying E1 PDR on medium-heavy, neutron-rich isotopes have been obtained at the LAND-FRS facility of GSI [3]. With respect to possible astrophysical consequences, the measured position of the PDR in 130 Sn and 132 Sn are of major interest, because some earlier models have predicted considerably lower energies of the PDR in neutron-rich nuclei, in the vicinity of their Sn values. With the known E(PDR)=9.8 MeV and Sn=7.3 MeV in 132 Sn, we now can conclude that at least for this isotope, and presumably for the whole A≃130 mass region of astrophysical interest, there will be no signifi cant enhancement of the σn,γ value due to the PDR. This is in contrast to what one would extract from e.g. Fig. 3 of [2]. Nevertheless, in order to investigate the possible impact of altered σn,γ values of r-process nuclei, we have performed full dynamic network r-process calculations in the model of an adiabatically expanding hot entropy bubble [4, 5]. For the calculations shown below, the seed abundances for the r-process were obtained from an α-rich freeze-out of a charged-particle network. For the subsequent r-process, an almost instantaneous (n,γ) – (γ,n) equilibrium is established at the onset of the r-process. Therefore, only late-time neutron captures are important, which are then expected to mainly modify the abundances around the r-process peaks a

Year: 2010
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