PURPOSE: To improve the sensitivity of a radiation detector by providing a collimator which passes only radiation in a specific direction in radiation emitted by an RI(radioisotope) distributed in a sample.
CONSTITUTION: The radiation detector 1 consists of the collimator 2 provided on a gamma-ray incidence side and a storage type phosphor sheet 3 provided nearby behind the collimator 2. A gamma-ray 13 is made incident on an imaging plate 3 through the collimator 2 from the RI 12 distributed in the sample 11. Consequently, the incident of the gamma-ray is stopped and an exciting light irradiating device 4 is put in operation with an operation signal from a controller 10 to scan for example, a visible light beam on the imaging plate 3. Consequently, the gamma-ray is stored on the imaging plate 3 to generate stimulated fluorescent light corresponding to the energy. Then an image signal which is stored in a frame memory 7 and based upon the imaging plate 3 is converted into a video signal. This video signal is displayed on a display device 9 finally.