Double Beta Decay

The nuclear double beta decay is a rare and second order process involving weak interaction and takes place between two even Z-even isobars differing in charge by two units, where single beta decay is kinematically forbidden



Two Neutrino DBD

In two neutrino DBD, two neutrons undergo beta decay simultaneously giving two elctrons and two anti-electron neutrinos. This is allowed in the Standard Model as it conserves lepton number and typical half lives are > 1017 yrs. Here, the sum energy of the two beta paricles exhibit a continuous beta decay spectrum between 0 and the Q-value of the process. It has already been observed for the 0+ → 0+ transition in 11 nuclei , namely 48Ca, 76Ge, 82Se, 96Zr, 100Mo, 116Cd, 128,130Te, 136Xe, 150Nd and 238U, out of 35 possible candidates.


Neutrinoless DBD

In 1939, W.Furry conjectured another decay mode, namely electron emitting neutrionless double beta decay. Its mechanism is based on the emission of an electron anti-neutrino on the first decay vertex and its absorption in the second vertex. This lepton number violating process is possible only if the neutrino is a Majorana particle. Its experimental signature is the sum-energy peak of of the two elctrons at the Q value of the process. It has not been observed so far and the current best lower limits are 1.07 x 1026 yrs (90% C.L.) in 136Xe and 2.1 x 1025 yrs (90% C.L.) in 76Ge.

Figure : A schematic of the distribution of the sum
of electron energies for the two modes of DBD decay.