A detector of an embodiment includes a plurality of photomultipliers, a signal line, and identifying circuitry. The photomultipliers each convert light converted from radiation into an electric signal and output the electric signal. The signal line has a first path and a second path through which the electric signal passes and that have different lengths for each of the photomultipliers. The identifying circuitry identifies the photomultiplier that outputs the electric signal by a time difference between the electric signal passing through the first path and the electric signal passing through the second path.