  Photomultipliers are commonly used in commercial PET scanner as devices which
convert light produced in scintillator by gamma quanta from positron-electron
annihilation into electrical signal. For proper analysis of obtained electrical
signal, a photomultiplier gain curve must be known, since gain can be
significantly different even between photomultipliers of the same model. In
this article we describe single photoelectron method used for photomultipliers
calibration applied for J-PET scanner, a novel PET detector being developed at
the Jagiellonian University. Description of calibration method, an example of
calibration curve and gain of few R4998 Hamamatsu photomultipliers are
presented.
