Nuclear symmetry energy and neutron skins derived from pygmy dipole resonances

Abstract

By exploiting Coulomb dissociation of high-energy radioactive beams of the neutron-rich nuclei [129][130][131][132]134 Sb, their dipole-strength distributions have been measured. A sizable fraction of "pygmy" dipole strength, energetically located below the giant dipole resonance, is observed in all of these nuclei. A comparison with available pygmy resonance data in stable nuclei ( 208 Pb and N = 82 isotones) indicates a trend of strength increasing with the proton-to-neutron asymmetry. On theoretical grounds, employing the RQRPA approach, a one-to-one correlation is found between the pygmy strength and parameters describing the density dependence of the nuclear symmetry energy, and in turn with the thicknesses of the neutron skins. On this basis, by using the experimental pygmy strength, parameters of the nuclear symmetry energy (a 4 = 32.0 ± 1.8 MeV and p o = 2.3 ± 0.8 MeV/fm 3 ) are deduced as well as neutron-skin thicknesses R n − R p of 0.24 ± 0.04 fm for 132 Sn and of 0.18 ± 0.035 fm for 208 Pb, both doubly magic nuclei. Astrophysical implications with regard to neutron stars are briefly addressed.The neutron root-mean-square (rms) radii of nuclei are fundamental quantities which are difficult to measure in a model-free way [1] and are, therefore, known only for few cases and with relatively poor accuracy [2][3][4]. This fact is particularly cumbersome since neutron rms radii belong to the few laboratory data that can be used to constrain the isospin-asymmetric part of the equation of state of nuclear matter [5][6][7], which in turn is closely related, e.g., to the radii of such exotic objects as neutron stars. Neutron skins in heavy nuclei and the crust of neutron stars are both built from neutron-rich nuclear matter and one-to-one correlations were drawn between neutron-skin thicknesses in nuclei [8][9][10] and specific properties of neutron stars. In a recent paper, Piekarewicz [11] pointed out that the experimentally observed "pygmy" dipole (E1) strength [12] might play an equivalent role as the neutron rms radius in constraining the nuclear symmetry energy. Excess neutrons forming the skin give rise to pygmy dipole transitions at excitation energies below the giant dipole resonance; to which extent such transitions represent a collective vibration of excess neutrons against an isospinsymmetric core is theoretically under discussion yet [13][14][15][16].Experimental evidence for pygmy dipole resonances (PDR) is still rather scarce. In an earlier paper [12], we reported on low-lying E1 strength observed in the exotic nuclei 130,132 Sn exhausting a few percent of the energy-weighted ThomasReiche-Kuhn (TRK) sum rule. Stable N = 82 isotones and 208 Pb investigated in (γ, γ ) reactions [17-19] display a concentration of dipole strength below the neutron-separation threshold, absorbing, however, a much smaller fraction of the TRK sum rule.In the first part of this Rapid Communication we present new experimental data for the unstable isotopes 129,131 Sn and 133,134 Sb obtained from the same measurement as in...

Neutron skin thickness from the measured electric dipole polarizability inNi68,Sn120, andRoca-Maza1, Viñas2, Centelles3 et al. 2015Phys. Rev. CSelf Cite238322563View full textAdd to dashboardCite

“…In addition to the identification of the prominent Giant Dipole Resonance (GDR), the electric dipole response of neutron-rich nuclei displays a smaller concentration of strength at lower energies, that is commonly referred to as the Pygmy Dipole Strength (PDS) [5]. Data on the PDS have been used in the past to constrain the symmetry energy and to obtain * xavier.roca.maza@mi.infn.it information on the neutron skin thickness of neutronrich nuclei [6][7][8][9][10][11]. In one of the earliest applications of uncertainty quantification to the domain of energy density functionals (EDFs), Reinhard and Nazarewicz carried out a covariance analysis to correlate the neutron skin thickness of 208 Pb to the properties of both finite nuclei and infinite nuclear matter [1].…”

Experimental results on the Pygmy Dipole Resonance

“…So far the E1 response in medium-heavy to heavy nuclei has been studied via Coulomb excitation in inverse kinematics in the region of 130,132 Sn [4,21] and in 68 Ni [22]. Both experiments were sensitive to excitation energies above the corresponding particle thresholds.…”

“…In addition it has gained interest in the last decade due to its possible connections to the neutron skin of atomic nuclei and properties of nuclear matter [3][4][5][6][7] as well as reaction rates in the synthesis of heavy elements [8][9][10]. However, the microscopic structure of the PDR, its collectivity and the robustness of the connections to other properties mentioned above are a matter of ongoing discussions.…”

Nuclei far from stability

“…It was also found that this low energy strength increases with the N/Z ratio. The problem of how the E1strength evolves for nuclei far from stability in the neutron rich side is presently one of the interesting topics in nuclear structure since it provides information on the neutron skin and on the nuclear equation of state for asymmetric nuclear matter, relevant for the study of neutron stars [29,30]. In addition, the strength of the PDR is presently attracting a lot of attention as the dipole strength distribution affects reaction rates in astrophysical scenarios where photodisintegration reactions are important, i.e., in hot stars and stellar explosions [31].…”