V.B. Brudanin, V.G. Egorov, R. Hodák, F. Mamedov, E. Rukhadze, I. Štekl et al.: The Low-Background HPGE Γ-Spectrometer OBELIX for the Investigation of the Double Beta Decay to Excited States. IOSR Journal of Applied Physics 09(01), 22-29 (2017). doi:10.9790/4861-0901012229