PURPOSE: To enhance the sensing efficiency by irradiating the inside of a com pound semiconductor of a radiation sensor with infrared rays having a greater energy than the trap level existing in the crystal of this substance.

CONSTITUTION: When an X-ray sensor 1 is in operation, infrared rays from a laser diode 4 are cast into its CdTe base board 1a, and thereby the carrier trapped by a defect etc. is released and collected quickly to an electrode 1b or 1c. That is, if the carrier generated in crystal is trapped by crystalline defect etc., the carrier drops to a deep level as shown in Fig. 2-a When infrared rays having a larger energy than the trap level at this time are cast into the crystals, the trapped carrier is excited with its light energy h2, and the carrier C is returned to a transmission belt as shown in Fig. 2-b and collected to the electrode upon traveling in the CdTe crystal. According to this constitution where infrared rays are cast into the CdTe base board 1a, all carrier generated is collected to the electrode, and thereby the sensing efficiency of X-ray sensor will be enhanced.