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 B-13 has been performed using the Li-7(Li-7,p)B-13 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)=7.2x10(-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 (nu 2p2h) configuration.