Positron emission versus electron capture
The emission of a positron and the capture of an electron are twin reactions which both result in the diminution of the number of protons by 1 (from Z to Z-1) and the production of a neutrino.The positron observed in the final stage of the beta decay (top) is a new particle requiring the 0.511 MeV of its rest mass energy to be created. There is no such energy threshold in the case of electron capture (bottom). In both cases, practically all the enegy released is carried by the light particles.
IN2P3