Dipole excitations via isoscalar probes: The splitting of the pygmy dipole resonance in124Sn

Abstract

Inelastic α-scattering excitation cross sections are calculated for electric dipole excitations in 124 Sn based on the transition densities obtained from the relativistic quasiparticle time-blocking approximation (RQTBA) in the framework of a semiclassical model. The calculation provides the missing link to directly compare the results from the microscopic RQTBA calculations to recent experimental data measured via the (α, α ′ γ) reaction, which show a structural splitting of the lowlying E1 strength often denoted as pygmy dipole resonance (PDR). The experimentally observed splitting is reproduced by the cross section calculations, which allows to draw conclusion on the structure of the PDR.The study of nuclei with neutron excess have been pursued in the last years with focus to the properties of collective states. Special attention has been devoted to the electric dipole strength at low excitation energy, the so called Pygmy Dipole Resonance (PDR), both in theory [1] as well as experimentally [2]. While the PDR represents an interesting new nuclear phenomenon itself, the connection of the (low-lying) E1 strength to the neutron skin of atomic nuclei [3][4][5][6][7][8][9][10][11] and to isovector parameters in the equation of state of nuclear matter [8,[12][13][14] as well as its possible importance for reaction rates in astrophysical scenarios [15,16] has further increased the interest in the PDR region. Different microscopic models predict the presence of the low-lying PDR states below the well-know (isovector) electric giant dipole resonance (IVGDR), see [1,2] and references therein. Most calculations show similar results for the transition densities of the low-lying E1 strength, which often is used to "define" the PDR states: The neutron and proton transition densities are in phase inside the nucleus and at the surface only the neutron part considerably contributes. The investigation of the PDR by means of isovector (IV) as well as isoscalar (IS) probes provides information on this structure of the involved transition densities, which show a strong mixing of isospin character. While for isovector probes (i.e. photons) data exist for various nuclei obtained mainly in nuclear resonance fluorescence (NRF) and Coulomb excitation (for a recent overview see [2]), only recently data on the PDR using an isoscalar probe became available [17][18][19][20][21]. Applying the (α, α ′ γ) coincidence method in combination with high-resolution γ-ray spectroscopy [22] allowed to * Electronic address: lanza@ct.infn.it study the PDR for several semi magic stable nuclei in αscattering experiments. The comparison to results from NRF revealed a surprising splitting of the PDR strength below the particle threshold into two groups [17,19]: the lower lying group of states is excited by both isoscalar and isovector probes while the states at higher energy are excited by photons only. A first qualitative comparison to calculations within the relativistic quasiparticle time-blocking approximation (RQTBA) as well as quasiparticle phonon mo...

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“…Furthermore, experiments with complementary probes like α-particles at intermediate energy, indicate an isospin splitting of the low-lying 1 − states [21]. Similar to the experimental findings, theoretical models show that at lower energies the E1 strength is predominantly of isoscalar character which gradually becomes more isovector with increasing excitation energy toward the IVGDR [18,19,[22][23][24][25][26].…”

Nuclear response theory with multiphonon coupling in a covariant framework

“…In the lowenergy region below the neutron threshold of mediummass even-even nuclei this term is responsible solely for the appearing strength. In the relativistic framework, the latter was confirmed and extensively studied [49,61], and verified by comparison to experimental data [50,51,[62][63][64]. However, a comparison with high-resolution experiments on the dipole strength below the neutron threshold has revealed that, although the total strength and some gross features of the strength are reproduced well, the fine features are sensitive to truncation of the configuration space by 2q⊗phonon configurations and further extensions of the method are needed.…”

Proton pygmy resonances: Predictions for N = 20 isotones

“…First we note that a skin oscillation would respond strongly in both channels, IS and IV. This is in analogy to the strong IS response expected of neutron-skin modes [33][34][35]. In Fig.…”