1. Field of the Invention
The present invention relates to a radiation image detector that generates charges (electric charges) by irradiation with an electromagnetic wave for recording, the electromagnetic wave carrying a radiation image, and that records the radiation image by accumulating the charges.
2. Description of the Related Art
These days, in the field of radiography using X-rays (radiation) for medical diagnosis or the like, various kinds of X-ray radiography apparatuses have been proposed and used practically. The X-ray radiography apparatuses use radiation image detectors that include semiconductors as the main parts thereof and detect X-rays that have passed through subjects. Accordingly, image signals representing X-ray images related to the subjects are obtained.
Further, as the radiation image detectors that are used in the X-ray radiography apparatuses, various kinds of radiation image detectors have been proposed. For example, if the radiation image detectors are classified based on the charge generation process for converting X-rays into charges, there are a radiation image detector using an indirect conversion method, a radiation image detector using a direct conversion method and the like. The radiation image detector using the indirect conversion method obtains signal charges by detecting, at a photoconductive layer, fluorescence output from a phosphor by irradiation with X-rays, temporarily accumulates the signal charges in a charge accumulation portion, converts the accumulated charges into image signals and outputs the image signals. The radiation image detector using the direct conversion method temporarily accumulates signal charges that have been generated in a photoconductive layer by irradiation with X-rays in a charge accumulation portion, converts the accumulated charges into image signals and outputs the image signals.
Meanwhile, if the radiation image detectors are classified based on the charge readout process for reading out the accumulated charges from the outside thereof, there are a radiation image detector using an optical readout method, a radiation image detector using an electrical readout method, as disclosed in Japanese Unexamined Patent Publication No. 8 (1996)-106869, and the like. In the optical readout method, charges are read out from the radiation image detector by irradiating the radiation image detector with readout light (an electromagnetic wave for readout). In the electrical readout method, charges are read out from the radiation image detector by scan-driving a switching device, such as a TFT (thin film transistor), a CCD (charge coupled device) or a CMOS (complementary metal oxide semiconductor) sensor, connected to the charge accumulation portion.
Further, the applicant of the priority application of the present application proposed a solid-state detector using an improved direct conversion method in U.S. Pat. No. 6,121,620 and the like. The solid-state detector using the improved direct conversion method uses the direct conversion method and the optical readout method. In the solid-state detector using the improved direct conversion method, a photoconductive layer for recording, a charge transfer layer and a photoconductive layer for readout are deposited one on another in this order. The photoconductive layer for recording exhibits photoconductivity by irradiation with recording light (X-rays, fluorescence generated by irradiation with X-rays or the like). The charge transfer layer substantially acts as an insulator with respect to charges that have the same polarity with the polarity of latent image charges. Further, the charge transfer layer substantially acts as a conductor with respect to transfer charges that have a polarity opposite to the polarity of the latent image charges. The photoconductive layer for readout exhibits photoconductivity by irradiation with an electromagnetic wave for readout. In the solid-state detector using the improved direct conversion method, signal charges (latent image charges) that carry image information are accumulated at the interface (charge accumulation portion) between the photoconductive layer for recording and the charge transfer layer.
Here, the photoconductive layer of the radiation image detector as described above is made of a-Se or the like, for example. When the temperature exceeds a glass transition temperature, a-Se or the like is crystallized. Further, when humidity is high, condensation of vapor occurs, thereby crystallizing a-Se.