$^{130}$Ba isotope: double $e$-capture. Double beta-decay of $^{238}$U, $^{130}$Te. Geochemical and radiochemical methods of studying

One of the rarest nuclei decays known today is double $\beta$-decay. Double $\beta$-decay is studied using radiochemical and geochemical methods. It was discovered first for $^{130}$Te isotope and is confirmed by experiment for 9 more isotopes, the heaviest of wich is $^{238}$U. Double $e$-capture was discovered for $^{130}$Ba isotope. Of particular interest is the searching of neutrinoless double $e$-capture, as its observation allows to consider neutrino as a majorana particle.