Anomaly of N = 8 shell closure in neutron-rich Be and B isotopes studied via delayed neutron emitting ¹⁴Be β decay

We investigated the β decay of the neutron drip line nucleus ¹⁴Be by means of double and triple coincidence measurements of β rays, delayed neutrons, and γ rays. The decay sequences were established and new values of the half-life and branching ratios were obtained. The decay scheme obtained is characterized by a single dominant branch leading to a 1⁺ state in ¹⁴B with a branching ratio of (91±9)%, which subsequently emits a delayed neutron with an energy of 288±8 keV. The excitation energy and width of the 1⁺ state were determined to be 1.28±0.02 MeV and 49±2 keV, respectively. The excitation energy and width of this level as well as the log ft value were treated in terms of a simple shell model and found to be strongly related to the N = 8 shell gap quenching in ^13,14B and ¹²Be.