During radioactive decay, what determines if the emission of a beta particle will be positronic or electronic?
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 Mentor It depends on which of the two final nuclei has a smaller mass than the initial nucleus. The mass difference provides the energy for the electron and antineutrino (or positron and neutrino).
 Blog Entries: 2 For beta-minus decay the following condition must be fulfilled: $$Q \approx [m_{at}(Z,A)-m_{at}(Z+1,A)]c^2 > 0$$ with $$Q$$: Q-value $$m_{at}(Z,A)$$: mass of the atom before beta-minus decay $$m_{at}(Z+1,A)$$: mass of the atom after beta-minus decay For beta-plus decay the following condition must be fulfilled: $$Q \approx [m_{at}(Z,A)-m_{at}(Z-1,A)]c^2-1.022 \rm{MeV} > 0$$ with $$Q$$: Q-value $$m_{at}(Z,A)$$: mass of the atom before beta-plus decay $$m_{at}(Z-1,A)$$: mass of the atom after beta-plus decay $$1.022 \rm{MeV}$$: two times the electron (or positron) mass Thus, for beta-plus decay the mass of the atom before decay must be at least two electron masses greater than after the decay.