In beta decay, positron emission, how come the condition for decay is: M_p > M_d + 2m_e Thats: atomic mass of parent > "daughter + twice the mass of an electron. I'm sure there is some simple way of showing it, but I cant seem to find it!! Also, is the most stable isobar on an atomic mass parabola the most stable one? It's a question my lecturer posed to us, and I have been thinking about it a while. Thanks!