1. An X-ray detector, comprising: a top receiving container in which one or more subjects are disposed; an X-ray detection unit that detects shadow images of the one or more subjects when X-rays are radiated to the one or more subjects, and calculates an X-ray radiation angle of the radiated X-rays based on the shadow images of the one or more subjects; and a bottom receiving container having a receiving space in which the X-ray detection unit is received.