Low-lying neutron intruder state in 13B and the fading of the N = 8 shell closure.

A lifetime measurement of the excited states in the neutron-rich isotope (13)B has been performed using the (7)Li((7)Li,p)(13)B reaction. An anomalously long mean lifetime of 1.3(3) ps was found for the excited state at 3.53 MeV, giving the upper limits of the transition strengths to the ground state: B(M1) = 7.2 x 10;{-4} Weisskopf unit (W.u.) and B(E2) = 0.81 W.u.. The hindered transition strengths indicate significant intruder configurations for the excited state, coexisting with the normal ground state. The data are well explained by recent shell-model calculations, which suggest J;{pi} = 3/2;{-} for the 3.53-MeV state with the dominant intruder (nu2p2h) configuration.

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.