NI Rukhadze, VB Brudanin, C. Briancon, P. Čermák, OI Kochetov, F. Mamedov, P. Loaiza, F. Piquemal, E. Rukhadze, YA Shitov et al.: A highly efficient HPGE gamma-ray spectrometer for investigating ββ decay to excited states. Bulletin of the Russian Academy of Sciences: Physics 77(4), 379-382 (2013). doi:10.3103/S1062873813040230