E1 and M1 strength functions at low energy

¹ Helmholtz-Zentrum Dresden-Rossendorf, 01328 Dresden, Germany
² Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
³ Technische Universität Dresden, 01062 Dresden, Germany
⁴ Lawrence Livermore National Laboratory, Livermore, California 94550, USA
⁵ Triangle Universities Nuclear Laboratory and Duke University, Durham, North Carolina 27708, USA

^a e-mail: r.schwengner@hzdr.de

Published online: 13 September 2017

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 ⁷⁴Ge 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.