Half-lives of double β^--decay with two neutrinos

Nuclear double β^--decay with two neutrinos is an important decay mode for some unstable nuclei. Based on the available experimental data of nuclear double β^--decay, we propose that there is a law between the logarithm of double β^--decay half-lives and the reciprocal of the decay energy. The physics behind the law is discussed and it is found that this is associated with the universal properties of the weak interaction. This double β^--decay law is similar to the famous Geiger-Nuttall law of α-decay. The law is applied to predictions of the nuclear double β^--decay half-lives for six even-even nuclei from Z = 84 to Z = 98 and we found that ²³²Th is very interesting for future experiments. The branching ratios between double β^--decay and α-decay are also estimated for the six even-even nuclei and this is useful for future experimental search of new emitters of double β^--decay. (authors)