Shell Quenching in Neutron Rich be and B Isotopes

Experimental study of β-decay measurement of the neutron drip line nucleus ¹⁴Be is presented. Its β-decay scheme is determined by measuring β-rays, delayed neutrons and γ-rays in triple coincidence. The decay scheme of ¹⁴Be associated with single neutron emission is unambiguously determined. A new level at 1.28 MeV in the daughter nucleus, ¹⁴B, is found. The deduced level scheme of ¹⁴B indicates the lowering of the 2s_1/2 single neutron orbital with respect to the 1p _1/2 orbital in the N=9 isotones. The shell gap energy at N=8 in neutron rich beryllium and boron isotopes is discussed.