Dipole response of neutron-rich Sn isotopes

• Published in: Nuclear physics. A 2007-05, Vol.788 (1), p.145-152
• Main Authors: Klimkiewicz, A., Adrich, P., Boretzky, K., Fallot, M., Aumann, T., Cortina-Gil, D., Datta Pramanik, U., Elze, Th.W., Emling, H., Geissel, H., Hellstroem, M., Jones, K.L., Kratz, J.V., Kulessa, R., Leifels, Y., Nociforo, C., Palit, R., Simon, H., Surowka, G., Sümmerer, K., Typel, S., Walus, W.
• Format: Article
• Language: English
• ISSN: 0375-9474
• DOI: 10.1016/j.nuclphysa.2007.01.075

The neutron-rich isotopes 129–133Sn were studied in a Coulomb excitation experiment at about 500 AMeV using the FRS-LAND setup at GSI. From the exclusive measurement of all projectile-like particles following the excitation and decay of the projectile in a high-Z target, the energy differential cross section can be extracted. At these beam energies dipole transitions are dominating, and within the semi-classical approach the Coulomb excitation cross sections can be transformed into photoabsorption cross sections. In contrast to stable Sn nuclei, a substantial fraction of dipole strength is observed at energies below the giant dipole resonance (GDR). For 130Sn and 132Sn this strength is located in a peak-like structure around 10 MeV excitation energy and exhibits a few percent of the Thomas-Reiche Kuhn (TRK) sum-rule strength. Several calculations predict the appearance of dipole strength at low excitation energies in neutron-rich nuclei. This low-lying strength is often referred to as pygmy dipole resonance (PDR) and, in a macroscopic picture, is discussed in terms of a collective oscillation of excess neutrons versus the core nucleons. Moreover, a sharp rise is observed at the neutron separation threshold around 5 MeV for the odd isotopes. A possible contribution of 'threshold strength', which can be described within the direct-breakup model is discussed. The results for the neutron-rich Sn isotopes are confronted with results on stable nuclei investigated in experiments using real photons.