T. Kishida

Anomalously hindered E2 strength B(E2;2+(1)-->0+) in 16C.

The electric quadrupole transition from the first 2(+) state to the ground 0(+) state in 16C is studied through measurement of the lifetime by a recoil shadow method applied to inelastically scattered radioactive 16C nuclei. The measured mean lifetime is 77+/-14(stat)+/-19(syst) ps. The central value of mean lifetime corresponds to a B(E2;2+(1)-->0(+)) value of 0.63e(2) fm(4), or 0.26 Weisskopf units. The transition strength is found to be anomalously small compared to the empirically predicted value.

Application of Doppler-shift attenuation method to the de-excitation γ rays from the in-flight 12Be beam

The B(E2; ↓) value of the neutron‐rich nucleus 12Be has been determined through the measurement of the lifetime of the first 2+ state using a modified Doppler‐shift attenuation method (DSAM). The first excited state was produced by the inelastic scattering of the intermediate energy 12Be beam off a thick gold target, which played two roles, to excite the beam and to decrease the velocity. The lifetime has been determined to be 2.5±0.7 (stat) ±0.3 (syst) ps, which gives a B(E2) value of 5.0±1.3±0.5 in Weisskopf units. The result shows a large quadrupole strength in the ground state transition. The B(E2) value together with the deformation length measured by proton inelastic scattering yields a neutron quadrupole matrix element two times larger than those for 14C and 16O.

Evolution of the N = 20 shell gap

Stucture of N = 17,18 nuclei has been investigated via proton inelastic scattering and neutron removal reactions up to the neutron dripline in inverse kinematics by use of radioctive ion beams provided by the RIKEN isotope separator, RIPS. Low energy excited states have been found in all the 26,27F and 27Ne nuclei, which are out of the neutron sd shell configuration space and are considered as intruder states arisng from cross shell excitations. Monte-Carlo shell model calculations with the sdpf-m interaction predict low energy negative parity excited states from neutron cross shell excitation at N = 17,18, which is in agreement with our experimental results. Observation of intruder states even at N = 17, especially the p3/2 state in 27Ne at 765 keV is a clear indication of a vanishing N = 20 shell gap at Z = 8 as predicted by the sdpf-m shell model calculations.