Double beta decay was first observed in a laboratory in 1987 by the group of Michael Moe at UC Irvine in Se. Since then, many experiments have observed ordinary double beta decay in other isotopes. None of those experiments have produced positive results for the neutrinoless process, raising the half-life lower bound to approximately 10 years. 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 Ba observed in 2001, Kr observed in 2013, and Xe observed in 2019), and all have a mean lifetime over 10 yr (table below).