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