The history of the predictions and the discovery of the double beta decay; early experiments. An overview of the methods of direct detection of double beta decay

History of the study of the double beta decay began in 1934 and is not finished yet. The physics of this process is closely connected with neutrino physics. The first experiments of the search for double beta decay were unsuccessful. Nowadays $2\beta 2\nu$ double beta decay was observed, but even modern experiments using a variety of detection methods could not confirm the existence of neutrinoless double beta decay, the possibility of which was theoretically demonstrated in 1937.