Gamma Strength Functions and Level Densities from High-Resolution Proton Scattering under 0°

Peter Von Neumann-Cosel,Atsushi Tamii,Sergej Bassauer,Dirk Martin,Yang Sun

doi:10.1051/epjconf/201817806002

Peter Von Neumann-Cosel, Atsushi Tamii + Show 3 more

Open Access

https://doi.org/10.1051/epjconf/201817806002

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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 – 20 MeV. The experiments provide not only the E1 but also the M1 part of the GSF. The latter is poorly known in heavy nuclei. 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. From the two test cases studied, 208Pb remains inconclusive in the energy region of the PDR because of large Porter-Thomas fluctuations due to the small level density (LD), while the BA hypothesis seems to hold in case of 96Mo. A fluctuation analysis of the high-resolution data also provides a direct measure of the LD in the energy region of the isovector giant dipole resonance (IVGDR) well above the neutron threshold, where hardly any experimental information is available. This permits an independent test of the decomposition of GSF and LD in Oslo-type experiments.

Highlights

The gamma strength function describes the average γ decay behavior of a nucleus
Particle threshold it is governed by the isovector giant dipole resonance (IVGDR) but at lower excitation energies one observes In nuclei with neutron excess the formation of a pygmy dipole resonance (PDR) [4] sitting on the low-energy tail of the IVGDR
While no conclusion can be drawn for 208Pb, the extracted gamma strength function 96Mo agrees with results of compound nucleus γ decay experiments [22, 23] indicating that the BA hypothesis holds in the energy region of the PDR, in contrast to results from the (γ, γ ) reaction [24] and the claims of Ref. [27]

Summary

Introduction

The gamma strength function describes the average γ decay behavior of a nucleus. Their knowledge is required for applications of statistical nuclear theory in astrophysics [1], reactor design [2], and waste transmutation [3]. The GSF is dominated by E1 radiation with smaller contributions from M1 strength, while higher multipoles contribute little Particle threshold it is governed by the IVGDR but at lower excitation energies one observes In nuclei with neutron excess the formation of a PDR [4] sitting on the low-energy tail of the IVGDR. A new method for the measurement of complete E1 strength distributions – and thereby the E1 part of the GSFs – in nuclei from about 5 to 20 MeV has been developed using relativistic Coulomb excitation in polarized inelastic proton scattering at energies of a few hundred MeV and scattering angles close to 0◦ [29,30,31,32,33] These data allow the dipole polarizability to be determined which provides important constraints on the neutron skin of nuclei and the poorly known parameters of the symmetry energy [34]. This allows an important test of the model-dependent decomposition of LD and GSF in the Oslo method [14]

Experiments and and multipole decomposition

Gamma strength functions of 96Mo and 208Pb

Level densities

Concluding Remarks