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Co-doping Lu2SiO5:Ce (LSO:Ce) with Ca divalent cations changes the scintillation properties of the crystal. In the present work an influence of Ca2+ co-doping on energy resolution, light output and non-proportionality was investigated for samples with 0, 0.1, 0.2, 0.3, and 0.4 atomic percent Ca with respect to Lu. A substantial improvement of energy resolution in the co-doped crystals was found and higher light output by about 10% was observed. The best energy resolution of 7.35 plusmn 0.15% was measured for LSO with 0.2% Ca. Contrary to our expectations, the change in the measured energy resolution of Ca2+ co-doped LSO samples is not reflected in the non-proportional characteristic of the studied crystals as the non-proportionality curves are independent of Ca concentration. Possible explanations of the underlying mechanism of improving the energy resolution include afterglow suppression via Ca co-doping. Earlier studies showed that calcium co-doping significantly reduces the trap population, hence the decay time of LSO is shortened and the afterglow is substantially quenched. In the current work, the integrated afterglow intensities as well as the afterglow effective decay times correlate with the concentration of Ca2+. Since the afterglow was measured about 30 ms after the crystal was irradiated by a strong X-ray source, the integral intensity does not include the faster components of afterglow. Hence, the correlation between afterglow intensity and energy resolution treated in this work is very preliminary.