Dipole response of Rb 87 and its impact on the Rb 86 (n,γ) Rb 87 cross section
Background: Detailed information on the low-lying dipole response in atomic nuclei along isotonic or isotopic chains is well suited to systematically investigate the structure and evolution of the pygmy dipole resonance (PDR). Moreover, the dipole strength below and around the neutron separation energy Sn has impact on statistical model calculations for nucleosynthesis processes. Purpose: The photon strength function (PSF) of Rb87, which is directly connected to the photoabsorption cross section, is a crucial input for statistical model calculations constraining the Maxwellian-averaged cross section (MACS) of the neutron capture of the unstable s-process branching-point nucleus Rb86. Within this work, the photoabsorption cross section is investigated. Methods: The photoabsorption cross section of the N=50 nucleus Rb87 was determined from photon-scattering experiments via the nuclear resonance fluorescence (NRF) technique. Bremsstrahlung beams at the γELBE facility in conjunction with monoenergetic photon beams at the HIγS facility were used to determine the integrated cross sections Is of isolated states as well as the averaged cross section as function of the excitation energy. Decays to the ground state were disentangled from decays to first low-lying excited states. Statistical and experimental approaches for the γ-decay properties at various excitation energies were applied. The linearly polarized photon beams at HIγS provide information on the ratio of electric and magnetic type of radiation. Results: Within this work, more than 200 ground-state decays and associated levels in Rb87 were identified. Moreover, transitions below the sensitivity limit of the state-by-state analysis were taken into account via a statistical approach from the bremsstrahlung data as well as model-independently from the HIγS data. The photoabsorption cross sections at various excitation energies were determined. The dipole response between 6 and 10 MeV of Rb87 is in agreement with assuming contributions of electric multipolarity, only. Conclusions: The photoabsorption cross section of Rb87 does not contradict with the trend of decreasing E1 strength with increasing proton number along the N=50 isotonic chain but might also be associated with a constant trend. The experimental γ decay at various excitation energies of the HIγS data supports the statistical approach but does not provide a stringent proof due to the limited sensitivity in the decay channels. The additional E1 strength observed in the present experiments significantly enhances the MACSs compared only to recent microscopic HFB+QRPA (Hartree-Fock-Bogoliubov plus quasiparticle random-phase approximation) calculations using the D1M interaction. Moreover, theoretical estimations provided by the KADoNiS project could be significantly improved.