Electromagnetic strength functions from photon scattering

Gamma-ray strength functions are an important ingredient for the calculation of reaction cross sections within the statistical model. Photon scattering from nuclei is a specific tool to study dipole strength functions below the neutron-separation energy as predominantly states with spin J = 1 are excited from the ground state in an even-even nucleus. We present photon-scattering experiments using bremsstrahlung at the γELBE facility of Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and using quasi-monoenergetic, polarized γ rays at the HIγS facility of the Triangle Universities Nuclear Laboratory (TUNL) in Durham. To deduce the photoabsorption cross sections at high excitation energy and high level density, unresolved strength in the quasi-continuum of nuclear states and branching ratios of the ground-state transitions have to be taken into account. In the analysis of the spectra measured by using bremsstrahlung at γELBE, we include the continuum and 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.
Photoabsorption cross sections deduced in this way are presented for selected nuclides. Strength in the energy region of the so-called pygmy dipole resonance (PDR) is considered in nuclei around mass 80 and in xenon isotopes.