 The shortest @-@ lived known isotope of beryllium is 13Be which decays through neutron emission . It has a half @-@ life of 2 @.@ 7 × 10 − 21 s . 6Be is also very short @-@ lived with a half @-@ life of 5 @.@ 0 × 10 − 21 s . The exotic isotopes 11Be and 14Be are known to exhibit a nuclear halo . This phenomenon can be understood as the nuclei of 11Be and 14Be have , respectively , 1 and 4 neutrons orbiting substantially outside the classical Fermi ' waterdrop ' model of the nucleus . 
