Isospin Character of the Pygmy Dipole Resonance inSn124

Abstract

The pygmy dipole resonance has been studied in the proton-magic nucleus 124 Sn with the (α, α ′ γ) coincidence method at Eα = 136 MeV. The comparison with results of photon-scattering experiments reveals a splitting into two components with different structure: one group of states which is excited in (α, α ′ γ) as well as in (γ, γ ′ ) reactions and a group of states at higher energies which is only excited in (γ, γ ′ ) reactions. Calculations with the self-consistent relativistic quasiparticle timeblocking approximation and the quasi-particle phonon model are in qualitative agreement with the experimental results and predict a low-lying isoscalar component dominated by neutron-skin oscillations and a higher-lying more isovector component on the tail of the giant dipole resonance.PACS numbers: 24.30.Cz, Collective phenomena are a common feature of strongly interacting many-body quantum systems directly linked to the relevant effective interactions. Atomic nuclei also show collective behavior. One example is given by the giant resonances, which have been investigated intensively using different experimental methods, see e.g., [1]. The isovector electric giant dipole resonance (IVGDR) has been the first giant resonance to be observed in atomic nuclei. Ever since it has been of particular interest, because collective E1 response is related to symmetry breaking between neutrons and protons. In recent years, the so-called pygmy dipole resonance (PDR) [2][3][4], a concentration of electric dipole strength energetically below the IVGDR, has been studied intensively in various nuclei. Within most modern microscopic nuclear structure models, this new excitation mode is related to the oscillation of a neutron skin against a symmetric proton-neutron core with isospin T = 0; for an overview see the recent review by Paar et al. [5]. Consequently, one expects an increase of the PDR strength approaching isotopes with extreme neutron-to-proton ratios. Experiments on radioactive neutron-rich nuclei seem to support this assumption [6][7][8][9][10][11]. If this picture holds, the strength of the PDR is related to the thickness of the neutron skin and the density dependence of the symmetry energy of nuclear matter [7,12]. The PDR thus permits experimental access to these properties. However, more consistent systematic investigations and especially more constraints on the structure of the PDR are mandatory, such as the experiments presented in this Letter, in order to confirm this picture.Up to now only experiments on stable nuclei allow more detailed investigations of the PDR which yield additional observables in order to understand the underlying structure of this new excitation mode. In nuclear resonance fluorescence (NRF) experiments the systematics of the PDR as well as its fragmentation and fine-structure can be studied [3,4,[13][14][15][16][17][18][19] up to the particle threshold. The mean excitation energy and the summed transition strength B(E1)↑ (of up to 1% of the isovector energy weighted sum rule) show a smooth variat...

Experimental results on the Pygmy Dipole Resonance

“…This observation is a clear indication for a change of structure in the low-lying E1 strength. The same pattern has been observed in 94 Mo [26], 124 Sn [27,28], and 138 Ba [29]. Figure 2 provides an overview of the data, showing the comparison of the results of the corresponding (α, α γ) and (γ, γ ) experiments for each nucleus.…”

“…The comparison to microscopic QPM and RQTBA calculations shows a qualitative agreement using the proper isoscalar (IS) E1 operator to compare to the (α, α γ) data and the EM operator for the NRF results [27]. Based on this agreement conclusions on the nature of the E1 strength can be drawn using the calculated transition densities of the corresponding excitations.…”

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

“…Combined (γ, γ ) and (α, α γ) experiments on the open shell 124 Sn [11,12,28] found that the low-lying dipole strength splits into an upper sector composed of isovector transitions promoted by γ's and a lower region of levels populated by both α's and γ's. Only these low-lying peaks form therefore the PDR.…”

“…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].…”

Study of the isospin character of 1-states using hadronic probes at intermediate energies

“…Aiming at isospin characters of these 1 − states, it has been demonstrated that the complementary studies of real-photon scattering and high-resolution (α, α γ) coincidence experiments at 136 MeV allow separating isovector and isoscalar dipole response which is important for a deeper understanding of the underlying mechanisms generating the dipole strength [2]. Systematic studies using these complementary probes revealed a splitting into low-lying isospin-mixed E1 excitations and higher-lying dominantly isovector E1 excitations in neutron-magic, proton-magic, and non-magic nuclei (namely 140 Ce, 138 Ba, 124 Sn, and 94 Mo) [2][3][4][5]. This experimentally observed common feature of the low-lying E1 response is reproduced by several theoretical calculations (see, e.g., Refs.…”