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The performance of multi pixel photon counters (MPPC) of 3 mm 2 3 mm size, with 14400 and 3600 pixels, were studied by means of the signal from a laser light pulser and using the 3 mm 2 3 mm 2 20 mm LSO pixel scintillator. Special attention was paid to measure number of fired pixels, generated by the light of pulser and that of the LSO crystal, using a direct method of a comparison of the light peak position in the pulse height spectrum with that of the single photoelectron. The tests of the LSO crystal showed 1550 6 80 fired pixels per MeV in the MPPC with 14400 pixels assuring a good linearity of the response up to about 1 MeV energy of gamma rays absorbed in the LSO crystal. Energy resolution of 14.8% for 662 keV gamma rays from 137 Cs source and a time resolution of about 850 ps for 511 keV annihilation quanta were limited by a rather low number of the fired pixels compared to the number of photoelectrons in photomultipliers.Index Terms-Detector linearity, multi pixel photon counters (MPPC), scintillation detection.