Fragmentation and systematics of the pygmy dipole resonance in the stableN=82isotones

Abstract

The low-lying electric dipole (E1) strength in the semi-magic nucleus 136 Xe has been measured which finalizes the systematic survey to investigate the so-called pygmy dipole resonance (PDR) in all stable even N=82 isotones with the method of nuclear resonance fluorescence using real photons in the entrance channel. In all cases, a fragmented resonance-like structure of E1 strength is observed in the energy region 5 MeV to 8 MeV. An analysis of the fragmentation of the strength reveals that the degree of fragmentation decreases towards the proton-deficient isotones while the total integrated strength increases indicating a dependence of the total strength on the neutron-to-proton ratio. The experimental results are compared to microscopic calculations within the quasi-particle phonon model (QPM). The calculation includes complex configurations of up to three phonons and is able to reproduce also the fragmentation of the E1 strength which allows to draw conclusions on the damping of the PDR. Calculations and experimental data are in good agreement in the degree of fragmentation and also in the integrated strength if the sensitivity limit of the experiments is taken into account.

Decay pattern of the Pygmy Dipole Resonance in130Te

“…For more details of the analysis see e.g. [4,5]. However, this kind of experiment is not sensitive to the character of dipole transitions.…”

Dipole response in neutron-rich nuclei within self-consistent approaches using realistic potentials

“…In stable nuclei, dense discrete spectra of dipole states were detected in (γ, γ ) [3][4][5][6][7][8][9] and in (α, α γ) experiments [10][11][12]. Moreover, fairly complete dipole spectra below and above the neutron decay threshold were produced with great accuracy through inelastic proton scattering experiments [13,14].…”

Experimental results on the Pygmy Dipole Resonance

“…Different approaches in the analysis have been used taking into account either only the information of isolated peaks resulting in model-independent results but accepting a limited sensitivity (see e.g. [15]) or including the full spectra to extract the full strength but accepting the need of statistical model assumptions in the analysis and thus model dependent results (see e.g. [16]).…”