1. An apparatus for capturing a radiation image of a subject, the subject comprising at least two materials, the apparatus comprising: a radiation irradiating unit configured to irradiate multi-energy radiation comprising at least two energy bands to a calibration model comprising a plurality of thicknesses of each of the at least two materials; an attenuation-coefficient estimating unit configured to estimate attenuation coefficients for each of the at least two materials for each of the at least two energy bands based on values obtained by passing the multi-energy radiation through the calibration model; and an energy-band determining unit configured to determine an optimal combination of at least two energy bands to be included in multi-energy radiation to be irradiated to the subject from a plurality of different combinations of at least two energy bands based on the attenuation coefficients estimated by the attenuation-coefficient estimating unit and the values obtained by passing the multi-energy radiation through the calibration model.