The dipole strength distribution above the one-neutron separation energy was measured in the unstable 130Sn and the double-magic 132Sn isotopes. The results were deduced from Coulomb dissociation of secondary Sn beams with energies around 500 MeV/nucleon, produced by in-flight fission of a primary 238U beam. In addition to the giant dipole resonance, a resonancelike structure ("pygmy resonance") is observed at a lower excitation energy around 10 MeV exhausting a few percent of the isovector E1 energy-weighted sum rule. The results are discussed in the context of a predicted new dipole mode of excess neutrons oscillating out of phase with the core nucleons.

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...

No abstract