Talk:Double electron capture

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[edit] Conditions

Double electron capture is a decay mode of atomic nucleus. For a nuclide (A, Z) with number of nucleons A and atomic number Z, double electron capture is only possible if the mass of the nuclide of (A, Z-2) is lower. How is this ever possible? If you count the mass of a neutron and proton as equal, the mass would remain the same. If you use the real mass then the proton has a slightly smaller mass then (A, Z-2) would have a somewhat higher mass. Can someone explain this to me? --84.246.55.33 12:22, 6 February 2007 (UTC)

The mass of a nucleus is the mass of the protons, plus the mass of the neutrons, minus the mass defect. The whole is less than the sum of its parts. This mass defect is just the binding energy (divided by -c^2) that it would take to pull the nucleus apart. Since selenium-78 has higher binding energy than krypton-78, it has less mass. You can look up the binding energy (or, for that matter, the primary decay modes) of any nuclide at Wikipedia, or various other sources. --69.107.81.199 17:59, 17 March 2007 (UTC)