For β- minus decay Q = m_x - m_y Whereas for β+ Q = m_x - m_y - 2m_e Where the masses are atomic masses, x and y are the parent/daughter. The SEMF can be used to find a parabolic curve showing the mass excesses for an A isobar for particular values of Z, and thus show at which Z the atom has the least mass. However a textbook of mine says; "The mass-energy difference between two adjacent isobars is the energy available for a radioactive transition from the heavier to the lighter one". However, is this strictly true? The Q value for Beta minus has a -2m_e term, so it's not a simple case of the differences in atomic masses being the energy available. Which suggests to me that for a mass excess/Z diagram for an A isobar, even if (on the proton rich side [to the right of the minimum of mass excess]) the atom with Z protons is JUST above the atom with Z-1 protons (to which it would decay to), this doesn't necessarily mean the decay might happen, because the -1MeV arising from the 2m_e term might mean the Q value is less than zero for beta minus decay. So although it looks like the decay process should happen on a mass excess/Z diagram, it shouldn't. Is that right?