 The isotopes of cobalt range in atomic weight from 50 u ( <unk> ) to 73 u ( <unk> ) . The primary decay mode for isotopes with atomic mass unit values less than that of the most abundant stable isotope , 59Co , is electron capture and the primary mode of decay for those of greater than 59 atomic mass units is beta decay . The primary decay products before 59Co are element 26 ( iron ) isotopes and the primary products after are element 28 ( nickel ) isotopes . 
