Abstract
We report photon-scattering experiments using bremsstrahlung at the γELBE facility of Helmholtz-Zentrum Dresden-Rossendorf and using quasi-monoenergetic, polarized γ beams at the HIγS facility of the Triangle Universities Nuclear Laboratory in Durham. To deduce the photoabsorption cross sections at high excitation energy and high level density, unresolved strength in the quasicontinuum of nuclear states has been taken into account. In the analysis of the spectra measured by using bremsstrahlung at γELBE, we perform simulations of statistical γ-ray cascades using the code γDEX to estimate intensities of inelastic transitions to low-lying excited states. Simulated average branching ratios are compared with model-independent branching ratios obtained from spectra measured by using monoenergetic γ beams at HIγS. E1 strength in the energy region of the pygmy dipole resonance is discussed in nuclei around mass 90 and in xenon isotopes. M1 strength in the region of the spin-flip resonance is also considered for xenon isotopes. The dipole strength function of 74 Ge deduced from γELBE experiments is compared with the one obtained from experiments at the Oslo Cyclotron Laboratory. The low-energy upbend seen in the Oslo data is interpreted as M1 strength on the basis of shell-model calculations.
Highlights
An improved experimental and theoretical description of photonuclear reactions and their inverse, the radiativecapture reactions, is very important for the description of astrophysical processes, such as the synthesis of heavy nuclei in stellar environments
Photon-scattering experiments were carried out using bremsstrahlung at the γ ELBE facility [13] of HelmholtzZentrum Dresden-Rossendorf (HZDR), Germany, and using quasimonoenergetic, highly polarized γ beams at the High-Intensity γ -Ray Source (HIγ S) [14] operated by the Triangle Universities Nuclear Laboratory (TUNL) in Durham, North Carolina, USA
In several nuclides around mass A = 90, we found extra strength considered as a pygmy dipole resonance (PDR) on top of the lowenergy tail of the giant dipole resonance (GDR)
Summary
An improved experimental and theoretical description of photonuclear reactions and their inverse, the radiativecapture reactions, is very important for the description of astrophysical processes, such as the synthesis of heavy nuclei in stellar environments. Gamma-ray strength functions, in particular electric dipole (E1) and magnetic dipole (M1) strength functions, are an important ingredient for the calculation of reaction cross sections within the statistical model. Precise measurements of dipole strength distributions are needed for a correct determination of those cross sections and for an improvement of theoretical descriptions. Strong M1 excitations around 3 MeV observed in various NRF studies are considered as the scissors mode [2].
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