A radiation detector includes a plurality of pixels configured to detect radiation, and at least one of the plurality of pixels includes a radiation absorbing layer configured to convert photons incident on the radiation absorbing layer into a first electrical signal, and a photon processor including a plurality of storages configured to count and store the number of the photons based on the first electrical signal. At least one of the plurality of storages is configured to compare the first electrical signal with a first reference value to obtain a second electrical signal, and count and store the number of the photons based on a third electrical signal that is obtained based on a comparison of the second electrical signal with a second reference value.