Abstract:
This invention relates to an X-ray image sensor formed by fitting and integrating a fiber plate equipped with a phosphor layer to and with a solid state imaging device, and to an X-ray image sensor which uses a material containing chromium (Cr) adapting to the spectral sensitivity characteristics of a silicon type solid state imaging device as the material of the phosphor layer.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to an X-ray image sensor used for industrial and remedial purposes. 
     In a conventional X-ray image sensor, a fiber optic plate is fitted onto a solid state imaging device, a phosphor layer having a sufficient thickness above 1 millimeter is disposed on the fiber optic plate to prevent the X-rays from transmitting through the fiber optic plate. A phosphor that has the center of its light emission range near 500 nm is used as the material of the phosphor layer. 
     As described above, the conventional X-ray image sensor comprises the combination of the phosphor having a light emission center near 500 nm with the solid state imaging device that has silicon as a principal material and has a broad spectral sensitivity characteristics with 800 nm as the center. Therefore, a loss of from 30 to 40% occurs due to deviation of the phosphors light emission range and the spectral sensitivity characteristics of the imaging device and this problem is left yet to be solved. 
     BRIEF SUMMARY OF THE INVENTION 
     To solve the problem described above, the X-ray image sensor of the present invention comprises a solid state imaging device, a fiber optic plate and a phosphor layer containing Cr, and at least one member selected from the group consisting of Gd 3  Ga 5  O 12  (hereinafter called &#34;GGG&#34;), Gd 3  Sc 2  Ga 3  O  12  (hereinafter called &#34;GSGG&#34;), Gd 3  Sc 2  Al 3  O 12  (hereinafter called &#34;GSAG&#34;), Gd 3  Ga 2  Al 3  O 12  (hereinafter called &#34;GGAG&#34;) and La 3  Lu 2  Ga 3  O 12  (hereinafter called &#34;LLGG&#34;) is used as the base material of the phosphor layer. 
     The Cr-containing phosphor layer used in the present invention has light emission near 750 nm as the center and this range is substantially in conformity with the broad spectral sensitivity characteristics of the solid state imaging device which is made of silicon as its principal material and having its center at 800 nm. A converted optical image, derived by the present invention does not undergo the loss of from 30 to 40% due to the deviation of the phosphors light emission range from the spectral sensitivity characteristics of the solid state imaging device. 
     Cerium oxide is preferably added to the core of the fiber optic plate in order to prevent damage by the X-rays passing through the phosphor layer. 
     Further, at least one metal oxide selected from the group consisting of lead oxide, barium oxide and lanthanum oxide is preferably added to the fiber optic plate in order to prevent the X-rays passing through the fiber optic plate from being incident to the solid state imaging device. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a sectional view showing an X-ray image sensor in accordance with one embodiment of the present invention; 
     FIG. 2 is a plan view showing the surface condition of a phosphor layer in the first embodiment; 
     FIG. 3 is a sectional view showing the sectional condition of the phosphor layer in the first embodiment; and 
     FIG. 4 is a sectional view of the X-ray image sensor in accordance with another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments of the present invention will be explained with reference to the drawings. 
     First Embodiment 
     As the first embodiment, an embodiment which uses a radiation shielding material for the fiber optic plate will be explained with reference to the drawings 
     FIG. 1 is a sectional view showing the X-ray image sensor in accordance with the first embodiment of the present invention. Its structure is as follows. A fiber optic plate (102) is tightly bonded to an image input surface (104) of a solid state imaging device by an optical adhesive (103). Further, a phosphor layer (101) is formed on the fiber optic plate (102) to form an integrated X-ray image sensor. 
     In this X-ray image sensor, the following treatment is conducted for the fiber optic plate (102) and the phosphor layer (101). 
     In order to prevent damage due to the X-rays passing through the phosphor layer (101), 0.2 wt % of cerium oxide is added to the core of the fiber optic plate (102). Furthermore, lead oxide is added to the fiber optic plate (102) to shield the X-rays and thus to prevent the X-rays from passing through the fiber optic plate (102) from entering the solid state imaging device. 
     The phosphor layer (101) is formed in a uniform thickness of 20±3 microns by adding a binder to a powder of a GGG crystal to which 4 wt % of chromium is doped. Square portions (201) and groove portions (202) are formed in grid form on the surface of the phosphor layer (101) as shown in FIG. 2 in order to improve resolution and to suppress bleeding of the image due to expansion of emitted rays of light. Furthermore, the surface of the phosphor layer (101) is covered with a 0.01˜0.1 μ-thick aluminum metal back (301) as shown in FIG. 3 so as to limit the drop of brightness due to dissipation of the emitted rays of light. 
     In the X-ray image sensor completed by the structure described above, sensitivity of the visible range can be improved by 30% in comparison with the X-ray image sensors that use the conventional phosphor and resolution of 25 microns can be obtained. 
     Second Embodiment 
     In the second embodiment, the X-ray image sensor is produced in the same way as in the first embodiment by adding a binder to a powder of the GSAG crystal to which 1 wt % of chromium is doped as the phosphor material and then forming a 10 μ-thick phosphor layer (101). 
     Here, barium oxide is added to the fiber optic plate (102) for shielding the X-rays and 0.5 wt % of cerium oxide is added to the core. 
     In this X-ray image sensor, sensitivity of the visible range can be improved by 20% in comparison with X-ray image sensors that use the conventional phosphor and resolution of 10 microns can be obtained. 
     Third Embodiment 
     In the third embodiment, the X-ray image sensor is produced in the same way as in the first embodiment by adding a binder to a powder of the GGAG crystal to which 0.5 wt % of chromium is doped as the phosphor material and then forming a 10 μ-thick phosphor layer (101). 
     Here, lanthanum oxide is added to the fiber optic plate (102) for shielding the X-rays and 0.8 wt % of cerium oxide is added to the core. 
     In this X-ray image sensor, sensitivity of the visible range can be improved by 15% in comparison with the X-ray image sensors that use the conventional phosphor and resolution of 10 microns can be obtained. 
     Fourth Embodiment 
     In the fourth embodiment, the X-ray image sensor is produced in the same way as in the first embodiment by adding a binder to a powder of the LLGG crystal to which 1 wt % of chromium is doped as the phosphor material and then forming a 20 μ-thick phosphor layer (101). 
     Here, lead oxide and barium oxide are added to the fiber optic plate (102) for shielding the X-rays and 0.4 wt % of cerium oxide is added to the core. 
     In this X-ray image sensor, sensitivity of the visible range can be improved by 25% in comparison with the X-ray image sensors that use the conventional phosphor and resolution of 25 microns can be obtained. 
     Fifth Embodiment 
     The fifth embodiment of the present invention will be described with reference to FIG. 4. 
     The structure is substantially the same as that of the first embodiment. However, though 0.5 wt % of cerium oxide is added to the fiber optic plate (401), the X-ray shielding material such as lead oxide is not added. Therefore, this embodiment employs the structure wherein the X-ray reception surface of the phosphor layer (101) and the visible ray reception surface of the solid state imaging device are deviated from each other, in order to prevent the transmitting X-rays from being incident to the input surface of the solid state imaging device. The material of the phosphor is the GSGG crystal to which 2 wt % of chromium is doped, a binder is added to this powder and a 30 μ-thick phosphor layer (101) is formed to produce the X-ray image sensor. 
     In this X-ray image sensor, sensitivity of the visible range can be improved by 27% in comparison with the X-ray image sensors that use the conventional phosphor and resolution of 30 microns can be obtained. 
     In accordance with the present invention, stable imaging with resolution of 10˜30 microns and efficient imaging without loss can be made by use of the phosphor having a light emission range which matches the spectral sensitivity characteristics of the solid state imaging device. Furthermore, since the present invention uses cerium oxide for the fiber optic plate, it can eliminate X-ray coloration damage of the optical system and can accomplish the X-ray image sensor having much more improved life than conventional X-ray image sensors.