Evolution of shell Structure and Collectivity in Neutron-Rich Nuclei

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The present research aims at studying characteristic excited states in neutron-rich nuclei by using secondary reactions of radioactive ion (RI) beams. The followings are summary of the results:(a) Experimental performance in measurement of alpha-induced reactions and their decay process : Angular distribution of the differential cross sections deduced by inverse-kinematics measurement of alpha inelastic scattering provides information on the transferred angular momentum. The angular distribution of the decay process provides the spin and the alignment of the excited state, which is well reproduced by direct reaction theory such as DWBA. The nucleon stripping reactions from the alpha particle at several tens of MeV have been studied as powerful tool for the investigation of the single-particle states in exotic nuclei.(b) Cluster states in ^<12>Be : Alpha inelastic scattering on ^<12>Be followed by the H^<6>e+H^<6>e and the H^<4>e+H^<8>e decay has been measured. The angular distributions … More of the differential cross sections and the decay process have been analyzed by an extended multipole decomposition method. The deduced excitation spectrum for each spin shows resonance structures which are candidates of exotic cluster states.(c) Disappearance of N=8 magicity around ^<12>Be : The lifetime and branching ratio of the second 0^<+> state in ^<12>Be have been determined. Relatively large EO and E2 transition probabilities indicate a strong mixing of single-particle particle configurations in a largely deformed mean field. The large deformation is supported by a low-lying proton intruder state with l/2^<+> in ^<13>B populated by the proton-transfer reaction to ^<12>Be.(d) Proton shell structure in neutron-rich Oxygen isotopes : Proton single particle states in ^<23>F have been studied by the proton transfer reaction to ^<22>O. Evolution of the spin-orbit splitting as a function of the neutron occupation number has been demonstrated.(e) High spin states in ^<48>Ca region by fusion reactions of RI beams : Fusion reaction of the ^<46>Ar+^<9>Be system has been measured at 3-7 MeV per nucleon by the secondary RI beam of ^<46>Ar. High spin states in ^<49-51>Ti have been studied by in-beam gamma-ray spectroscopy using the GRAPE array consisting of position-sensitive Ge detectors. New high-spin states have been found showing a shell gap around N=28.(f) Research and Development of GRAPE : Automatic measurement system for characterizing the GRAPE has been constructed. Digital signal processing for 3-dimensional position determination has been developed. Less