a radiation detector in which a plurality of charge generation units configured to generate a charge corresponding to energy or the number of particles of incident radiation and a plurality of reading units configured to output a digital value based on the charge generated by each of the plurality of charge generation units are mutually stacked and two-dimensionally disposed; and

a circuit board on which a plurality of radiation detectors is disposed,

wherein the plurality of reading units are configured to transfer digital data to another adjacent radiation detector in response to a control signal from an outside source,

wherein each of the plurality of reading units is associated with a respective charge generation unit of the plurality of charge generation units, and is configured to output a digital value based on charge generated by said associated charge generation unit,

wherein the charge generation units are pixels,

wherein the plurality of reading units are configured to sequentially transfer the digital value to another adjacent reading unit in response to a control signal from an outside source, and

wherein the plurality of reading units are configured to transfer the digital value via all reading units included in the one radiation detector.