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

ABSTRACT

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.