A new method for calibration was developed that takes the energy deposited by radioisotope sources into account; it was validated with a plastic scintillation plate using a (207)Bi source. The energy deposits of the conversion electrons in the (207)Bi source led to an energy difference of a few keV for the 976 keV monoenergetic peak. Calibration with high accuracy is essential to estimate radiation doses as well as to evaluate detector performance. The method will be available not only for use in scintillation detectors but also for other detectors.