Evidence for Pygmy and Giant Dipole Resonances inSn130andSn132

Abstract

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.

Self-consistent calculations of the strength function and radiative neutron capture cross section for stable and unstable tin isotopes

“…[8], along with SLy4 Skyrme forces, we probed BSk5 [50] and SkM* [51] for 132 Sn. In all the three cases we obtained very similar mean-energy values and widths (i.e., 14.3 and 2.9 MeV, respectively, for the SLy4 force), while the experimental data are 16.1 ± 0.7 and 4.7 ± 2.1 MeV [49]. It has to be noted that other theoretical approaches (see Fig.…”

“…Although our results are not fit by a Lorentzian form and only moments are compared, it can be seen that we have a reasonable agreement for both the mean energy and the width of the E1 resonance. The 132 Sn and 130 Sn are the only unstable tin isotopes which were probed to study the GDR and PDR [49]. Although the SLy4 forces works quite well for stable nuclei, there is no proof that it is good for unstable nuclei.…”

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

“…The EMPM strength collected by the lowlying states up to ∼ 11 MeV exhausts ∼ 5.8% of the TRK sum rule. This value is within the error of the measured fraction 4(3)% [2].…”

“…Both TDA and EMPM calculations yield a small peak in the cross section around ∼ 10 MeV, fairly close in position and height to the one at ∼ 9.8 MeV observed experimentally [2]. The EMPM strength collected by the lowlying states up to ∼ 11 MeV exhausts ∼ 5.8% of the TRK sum rule.…”

“…An appreciable dipole strength just above the neutron decay threshold was extracted through Coulomb scattering in inverse kinematic in tin isotopes around the unstable 132 Sn [2]. 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].…”