Double beta decay

Double beta decay can occur near the bottom part of the mass valley where a given nucleus $(A,Z)$ has an adjacent nucleus $(A,Z+1)$ with higher mass while the nucleus $(A,Z+2)$ has a lower mass corresponding to an energy difference $\Delta E$. Double beta decay does exist as a second order decay process. The lifetimes estimated are typically of the order $T\approx 10^{20}$ y. Double beta decay carries a number of very intriguing aspects related to neutrino properties. Double beta decay has been observed both by geochemical methods and with counters.