Complete dipole response in [sup 208]Pb from high-resolution polarized proton scattering at 0°

Abstract

At the Research Center for Nuclear Physics, Osaka, Japan, the 208Pb(p,p´) reaction was measured at Ep=295 MeV and scattering angles Θlab= 0° - 10°. A high energy resolution of the order of ΔE/E ≈ 8x10^-5 was achieved, corresponding to ΔE=25-30 keV (FWHM). Cross sections were extracted by a multipole decomposition analysis of the angular distributions. Dominant contributions at very forward angles originate from E1 excitation due to Coulomb projectile-target interaction and spin M1 transitions caused by the spin-isospin part of the proton-nucleus interaction. A separation of these contributions was performed with two independent methods, viz. a multipole decomposition of the angular distributions and utilizing polarization transfer observables. Excellent agreement between both techniques is achieved within errors bars. The B(E1) strength distribution was extracted in the energy range between 5 and 20 MeV. Below the neutron separation energy (Sn = 7.367 MeV) it shows excellent agreement with available (γ,γ´) data and in the region of the giant dipole resonance with photoabsorption experiments. The shape of the angular distributions indicates a structural change of E1 strength below and above 8.2 MeV. The centroid energy and summed B(E1) strength of the PDR are extracted and amount to Ec=7.43(2) MeV and ΣB(E1)=1.54(16) e2fm2, respectively. Previously unobserved strength is found in the region above neutron threshold up to 8.2 MeV. The deduced E1 polarizability in the energy range from 5 to 19 MeV is αD = 18.7(13)fm3/e2. Averaging over all available data a highly precise value of αD = 18.9(5) fm3/e2 can be extracted. As the strong correlations predicted by microscopical models, this puts important constraints on the neutron skin thickness in 208Pb and the density dependence of the symmetry energy. The fine structure of the giant dipole resonance was analyzed with wavelet methods. Characteristic scales at E=100 keV, 340 keV, 520 keV, 1 MeV, and 2.1 MeV can be found. A comparison with microscopic calculations including the coupling to 2p-2h states suggests Landau damping as the dominant mechanism contributing to the decay width. Level densities of 1- states were extracted with a fluctuation analysis in the giant dipole resonance region. All phenomenological and microscopic models fail to describe the level densities in 208Pb, except for a version of the back-shifted Fermi gas model allowing for additional phenomenological parameters in local mass regions.