The photoelectron yield, expressed as a number of photoelectrons per MeV (phe/MeV) for each γ-
peak, was measured by Bertolaccini method [9,10]. In this method the number of photoelectrons is
measured by comparing the position of a full energy peak of γ-rays detected in the crystals with that of the single photoelectron peak from the photocathode, which determines the gain of PMT.