text: If the mass difference between the mother and daughter atoms is more than two masses of an electron (1.022 MeV), the energy released in the process is enough to allow another mode of decay, called electron capture with positron emission. It occurs along with double electron capture, their branching ratio depending on nuclear properties.
  When the mass difference is more than four electron masses (2.044 MeV), the third mode, called double positron decay, is allowed. Only six naturally occurring nuclides (Kr, Ru, Cd, Xe, Ba, and Ce) plus the non-primordial Gd and Dy are energetically allowed to decay via these three modes simultaneously.