Enhanced neutron pair transfer and collective excitations in the system + at barrier energiesI. Peter1, W. von Oertzen2, 3, S. Thummerer2, H.G. Bohlen2, B. Gebauer2, J. Gerl1, M. Kaspar1, I. Kozhoukharov1, T. Kröll4, M. Rejmund1, H.J. Wollersheim1 and I.J. Thompson5
1 Gesellschaft für Schwerionenforschung Darmstadt, Planckstraße 1, 64291 Darmstadt, Germany
2 Hahn-Meitner-Institut Berlin, Glienicker Straße 100, 14109 Berlin, Germany
3 Fachbereich Physik, Freie Universität Berlin, Germany
4 INFN, Sezione di Padova,Via Marzolo 8, 35131 Padova, Italy
5 Department of Physics, University of Surrey, Guildford GU2 7XH, UK
(Received: 27 August 2002 / Revised version: 21 October 2002 / Published online: 25 March 2003)
At energies below the Coulomb barrier, neutron transfer and Coulomb excitation have been measured in a very heavy asymmetric nuclear system, in + . These are semi-magic nuclei showing super-fluid properties. Particle- coincidence techniques using 5 Euroball Cluster detectors (EB), combined in a set-up with the Heidelberg-Darmstadt NaI Crystal Ball (CB), have been used. Position-sensitive detectors allowed the observation of scattering processes covering angles from 110 up to 150 degrees. The fragments are identified via the known -decays of the lowest excited states using the high resolution of EB. Using the unique feature of the set-up with the CB, transfer to well-defined final channels with known quantum numbers is selected using the high-efficiency multiplicity filter of the CB with no second -ray, i.e. without feeding. The data are analysed using the semi-classical approach and transfer probabilities are obtained. Coulomb excitation has been analysed using known transition probabilities. The enhancement is deduced for the two-neutron transfer populating the low-lying super-fluid 2 + states in and , while the 2n transition remains in the ground state for the 20NPb nuclei. Large enhancements up to are observed. This is the first observation of neutron pair transfer enhancement for a heavy nuclear binary system with super-fluid properties with experimentally separated levels. The calculations with microscopic 2-neutron wave functions, with configuration mixing over six shell model configurations and using the coupled reaction channels approach, reproduce well the observed probabilities and the enhancement.
24.10.Eq - Coupled-channel and distorted-wave models.
25.70.Hi - Transfer reactions.
© Società Italiana di Fisica, Springer-Verlag 2003