The total number of protons and neutrons determine the nuclide. Its fission product yield decreases as the mass of the fissile nuclide increases. See Isotope#Notation for an explanation of the notation used for different nuclide or isotope types. Lead-204 is entirely a primordial nuclide and is not a radiogenic nuclide. The chance is dependent on the nuclide as well as neutron energy. The daughter nuclide of a decay event may also be unstable (radioactive). The resulting second daughter nuclide may also be radioactive. Argon produced directly by stellar nucleosynthesis, is dominated by the alpha-process nuclide . K has the longest known half life for any positron-emitter nuclide. Calcium-48 is the lightest nuclide to undergo double beta decay. Argon produced directly, by stellar nucleosynthesis, is dominated by the alpha-process nuclide . Iodine-131 has a half-life of 8.02 days, decaying into a stable nuclide. This was primarily related to the contradicting decay data for the known nuclide rutherfordium-261. All are radioactive and the most stable nuclide, Es, has a half-life of 471.7 days. Nickel-48, discovered in 1999, is the most proton-rich nuclide known beyond helium-3.