The present invention identifies α decay and other events included in the emission of an LaBr3 scintillator and only collects γ ray events. An LaBr3 scintillation detector is provided with an LaBr3 scintillator 10, a photomultiplier tube 12, an oscilloscope 14, and a computer 18. The computer 18 detects a peak value Vp and a total charge amount Qtotal of a voltage waveform signal and calculates an error propagation expression function for a ratio of the peak value Vp to the total charge amount Qtotal. This error propagation expression function is used as a threshold function for identifying and removing α decay events. The α decay events are identified from the peak value Vp and total charge amount Qtotal, which are measurement values that can be measured in real time.