ELECTROMAGNETIC DISSOCIATION OF LI-11 AND SOFT DIPOLE MODE

The electromagnetic dissociation in the (11Li, 9Li) projectile fragmentation has been studied using a formalism developed by Winther and Alder. Two types of excitations of 11Li contribute to the process: One is the excitation of the 9Li core in 11Li to its first excited state and the other the excitations of the soft dipole mode where the relative motion between the 9Li core and two valence neutrons plays a prime role. The cluster-orbital shell model for the 9Li+n+n system has been worked out to describe those excited states which are relevant to the soft dipole mode. The electric dipole strength obtained at very low excitation energy, which exceeds that every observed, corresponds to about 40% of the non-energy-weighted sum rule and to about 10% of the energy-weighted sum rule. The predicted electromagnetic dissociation cross section for a Pb target is in good agreement with experiment, and the present theory strongly supports the existence of a soft dipole mode at low excitation energy of extremely neutron-rich nuclei. © 1990.