 Geochemical experiments continued through the 1990s, producing positive results for several isotopes.Double beta decay is the rarest known kind of radioactive decay; as of 2019 it has been observed in only 14 isotopes (including double electron capture in observed in 2001, observed in 2013, and observed in 2019), and all have a mean lifetime over 1018 yr (table below).In a typical double beta decay, two neutrons in the nucleus are converted to protons, and two electrons and two electron antineutrinos are emitted.