COULOMB EXCITATION OF LIGHT NEUTRON-RICH NUCLEI

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The Coulomb excitation of a very neutron-rich nucleus ^<32>Mg to its 2^+ state was studied using an unstable nuclear beam of ^<32>Mg at 49 MeV/u with a ^<208>Pb target. The beam was generated by projectile fragmentation reaction induced by 94 MeV/u ^<40>Ar ions bombarding a ^9Be target, and separated by the RIPS facility, a system with essentially two dipole magnets with an energy loss degradeer, of RIKEN (the Institute of Physical and Chemical Research). A stuck of 64 NaI(Tl) scintillators were settled around the target to detect gamma-rays from the 2^+ state of ^<32>Mg in coincidence with inelastically scattered ^<32>Mg nuclei detected by silicon counter telescope consisting of five/400 mum thick silicon detectors. The silicon telescope provided particle identification to select the inelastic scattering. The relevant inelastic scattering cross section was obtained by integrating the gamma-ray yield with corrections due to the Doppler shift of the photon energy and the detection efficiencies of the gamma-ray detector and the silicon counter telescope.The B(E2) value of 426(〕SY.+-.〔)34 e^2fm^2 was extracted by a coupled channel analysis. The value is in good agreement with a recent shell model prediction and also with a global phenomenological expression of the N_pN_n scheme. The major ingredient of these models is an obliteration of the N=20 shell closure. Therefore the presenct result confirms a large deformation of ^<32>Mg suggested by the low excitation energy of the 2^+ state, and points to the vanishing of the N=20 shell gap. The success of the present study opens up a new profound field of research to investigate a large number of unknown 0^+-2^+ transitions in unstable nuclei.