In a probe, two scintillators are arranged in tandem, the front end surfaces of two optical fibers transmitting the light emitted by the scintillators are connected to the rear of the scintillators and two photoelectric converters converting the light to pulse signals are connected to the rear of the optical fibers. A gate delay generator produces a gate signal from one pulse signal. A delay amplifier delays the other pulse signal. An M.C.A. will count up in case the pulse signal output by the delay amplifier is obtained during the gate signal.