Enhanced neutron pair transfer and collective excitations in the system $\chem{^{206}Pb}$ + $\chem{^{118}Sn}$ at barrier energies

I. Peter; W. von Oertzen; S. Thummerer; H.G. Bohlen; B. Gebauer; J. Gerl; M. Kaspar; I. Kozhoukharov; T. Kröll; M. Rejmund; H.J. Wollersheim; I.J. Thompson

doi:doi:10.1140/epja/i2002-10124-7

2024 Impact factor 2.8

Hadrons and Nuclei

Eur. Phys. J. A 16, 509-525 (2003)
DOI: 10.1140/epja/i2002-10124-7

Enhanced neutron pair transfer and collective excitations in the system $\chem{^{206}Pb}$ + $\chem{^{118}Sn}$ at barrier energies

I. Peter¹, W. von Oertzen^{2, 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

oertzen@hmi.de

(Received: 27 August 2002 / Revised version: 21 October 2002 / Published online: 25 March 2003)

Abstract
At energies below the Coulomb barrier, neutron transfer and Coulomb excitation have been measured in a very heavy asymmetric nuclear system, in $\chem{^{206}Pb}$ + $\chem{^{118}Sn}$ . These are semi-magic nuclei showing super-fluid properties. Particle- $\gamma$ 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 $\gamma$ -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 $\gamma$ -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 $\chem{^{120}Sn}$ and $\chem{^{116}Sn}$ , while the 2n transition remains in the ground state for the ^20NPb nuclei. Large enhancements up to $EF \simeq 10^3$ 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.