According to an embodiment, a signal processor includes an integrator, a differentiator, a zero cross detector, a pile-up detector, an event interval detector, a counter, and a creator. The integrator is configured to calculate charge of current from a photoelectric converter for an incident radiation. The differentiator is configured to calculate a differential value of the current. The zero cross detector is configured to detect a zero cross of the differential value. The pile-up detector is configured to detect pile-up of the current based on the zero cross. The event interval detector is configured to detect, based on the zero cross and pile-up, an event interval of the radiation entering. The counter is configured to count, based on the charge and pile-up, the respective numbers of events according to the charge and the event interval. The creator is configured to create histograms for the numbers of events.