The dominance of the $\nu (0d_{5/2}{)}^2$ configuration in the $N=8$ shell in ${}^{12}$Be from the breakup reaction on a proton target at intermediate energy

Abstract

The momentum distribution of ${}^{11}$Be fragments produced by the breakup of ${}^{12}$Be interacting with a proton target at 700.5 MeV/$u$ energy has been measured at GSI Darmstadt. To obtain the structure information on the anomaly of the $N=8$ neutron shell, the momentum distribution of ${}^{11}$Be fragments from the one-neutron knockout ${}^{12}$Be(p,pn) reaction, measured in inverse kinematics, has been analysed in the distorted wave impulse approximation (DWIA) based on a quasi-free scattering scenario. The DWIA analysis shows a surprisingly strong contribution of the neutron $0d_{5/2}$ orbital in ${}^{12}$Be to the transverse momentum distribution of the ${}^{11}$Be fragments. The single-neutron $0d_{5/2}$ spectroscopic factor deduced from the present knock-out data is 1.39(10), which is significantly larger than that deduced recently from data of ${}^{12}$Be breakup on a carbon target. This result provides a strong experimental evidence for the dominance of the neutron $\nu (0d_{5/2}{)}^2$ configuration in the ground state of ${}^{12}$Be.

Comments: Accepted for publication in Physics Letters B