Structure of the neutron-rich isotope $^{13}$B with $N$~=~8 studied via lifetime measurements with low-energy fusion reactions

We report recent experimental studies on the structure of the neutron-rich isotope $^{13}$B with $N$~=~8 performed at the FN Tandem facility of the University of Cologne [1]. The lifetime measurements of the excited states in $^{13}$B were performed by the Doppler-shift attenuation method with the $^{7}$Li($^{7}$Li,$p$)$^{13}$B reaction at a beam energy of 5.4~MeV. To select the reaction channel unambiguously, and hence reduce the background considerably, the particle-$\gamma$ coincidence was employed. An anomalously long mean lifetime of 1.3(3)~ps was found for the excited state at 3.53~MeV in $^{13}$B. The hindered transition strengths between the ground and 3.53-MeV states clearly indicate significant intruder configurations for the excited 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$2$p$2$h$) configuration, pointing to the fading effects of the $N$~=~8 shell closure. The occurrence of the intruder configurations in the $N$~=~8 isotones will be discussed. \\[0pt] [1]~H.~Iwasaki {\it et al.}, Phys. Rev. Lett. {\bf 102}, 202502 (2009).