In nuclear physics, Ronen's golden rule for cluster radioactivity is that the most favorable parents for heavy ion emission (cluster radioactivity) are those that emit clusters which have atomic mass A z which is given by A z = A z − − 2 + 4 + B z / 2 . The atomic number Z is even, A 0 = 0 , B z / 2 = 0 for Z / 2 - odd and B z / 2 = 2 for Z / 2 - even. The daughter nuclei is preferably magic, close to the double magic Pb.