Abstract
Inelastic proton scattering at energies of a few 100 MeV and forward angles including 0∘ provides a novel method to measure gamma strength functions (GSF) in nuclei in an energy range of about 5–23 MeV. The experiments provide not only the E1 but also the M1 part of the GSF. The latter is poorly known in heavy nuclei. A case study of 208 Pb indicates that the systematics proposed for the M1-GSF in RIPL-3 needs to be substantially revised. Comparison with gamma decay data (e.g. from the Oslo method) allows to test the generalised Brink-Axel (BA) hypothesis in the energy region of the pygmy dipole resonance (PDR) crucial for the modelling of (n,γ) and (γ,n) reactions in astrophysical reaction networks. A fluctuation analysis of the high-resolution data also provides a direct measure of level densities in the energy region well above the neutron threshold, where hardly any experimental information is available.
Highlights
Gamma strength functions describe the average gamma decay behaviour of a nucleus as a function of excitation energy
The gamma strength functions (GSF) derived from the ( p, p ) data is systematically higher in the pygmy dipole resonance (PDR) region they seem still compatible within error bars in the peak region around the neutron threshold
The main aim of this work was to determine the E1, M1 and total GSF of 208Pb for tests of models recommended in the RIPL-3 data base as well as to study the BA hypothesis by comparison with decay data obtained with the Oslo method
Summary
Gamma strength functions describe the average gamma decay behaviour of a nucleus as a function of excitation energy. Many applications imply an environment of finite temperature, notably in stellar scenarios [4], and reactions on initially excited states become relevant. Their contributions to the reaction rates are usually estimated applying the generalised Brink-Axel hypothesis [5, 6] which states that the GSF is independent of the properties of the initial and final states. Recent work utilising compound nucleus gamma decay with the so-called Oslo method [8] has demonstrated independence of the GSF from excitation energies and spins of initial and final states in accordance with the BA hypothesis once the level densities are sufficiently high to suppress large intensity fluctuations [9]. There are a number of experimental results which seem to violate the BA hypothesis in the low-energy region [10,11,12]
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