Radiation detector
BACKGROUND OF THE INVENTION 
     This invention relates to a radiation detector for X-rays, γ-rays, etc., and particularly to a radiation detector for use in X-ray CT (computed tomography), a position camera, etc. 
     X-ray CT includes various types and requires an array of 20 to 1,000 X-ray detecting elements. 
     Heretofore, a gas chamber filled with xenon, a combination of BGO single crystal (bismuth germanate) and a photomultiplier, and a combination of CsI : T1 single crystal or CdWO 4  single crystal and a photodiode have been used for the radiation detector for use in X-ray CT, etc. The xenon chamber requries a thick window owing to the sealing of a high pressure xenon gas in the detector, and its X-ray absorbance is as low as about 50% because two collimators must be provided per element. In the case of BGO, the luminescence efficiency is low, e.g. about 1%, and thus must be used together with a photomultiplier. That is, the photomultiplier and its accessory high voltage power source are requries and consequently the number of elements in an array is limited. 
     The CsI : T1 single crystal has a high efficiency, but is deliquescent and has an after-glow (phenomenon of luminescence after the X-ray has been turned off). Thus, it still has a practical problem. 
     The CdWO 4  crystal has a low luminescence efficiency, and has such problems as easy cleavage at cutting and a toxicity. 
     The common drawback of the foregoing single crystal scintillator is a fluctuation of luminescence properties in the single crystal. Generally, a single crystal is made to grow from a melting solution, and lattice defects are liable to develop in the crystal during the growth process, and an after-glow will often appear. An activator is often added to the scintillator, but it is difficult to distribute it uniformly in the crystal, and thus the crystal has uneven luminescence. These problems mean that it is very difficult to make the characteristics of the individual detectors uniform. 
     To solve these problems, some of the present inventors have already proposed a radiation detector using phosphor particles as scintillator [Japanese patent applications Kokai (Laid-open) Nos. 56-151,376 and 57-70,476]. To obtain a sectional image in a radiation detector for X-ray CT, the radiation detector is usually 1 to 3 mm wide and about 20 mm long. Thus, the number of phosphor particles in one radiation detector is, for example, about 300,000, though dependent on their particle sizes. The characteristics of the individual phosphor particles may by slightly different from one another, but a fluctuation of the characteristics as a scintillator can be reduced to 1/square root of number of phosphor particles, that is, about 0.01% by using thoroughly mixed phosphor particles as one scintillator. A satisfactory result can be obtained thereby. 
     This type of radiation will be described, referring to FIG. 1. After transmission through a cover 5 composed of an aluminum film and a light-scattering layer 4, incoming X-ray 1 makes a scintillator particle layer 2 (phosphor particles solidified by polystyrene) emit light. The emitted light passes through a space 7 and a secondary radiation-preventing layer 8 (Pb glass) and reaches a photodiode 3, where the emitted light is converted to an electric current. To efficiently lead the light emitted from the scintillator particle layer to the photodiode, a vessel 6 is entirely coated with a reflecting film. This type of radiation detector can have about 2-fold output power, as compared with a detector based on a combination of a single crystal scintillator and a photodiode, and is very desirable as a detector for a head scanner, but still has some problem, when applied as a detector for a whole body scanner, because the image processing system for the head scanner is different from that for the whole body scanner. 
     In FIG. 1, the luminescence of the scintillator layer 2 is uniform, but the emitted light undergoes reflections on the reflecting film surrounding the space 7 many times until the light reaches the photodiode 3. Therefore, if the reflecting film is uneven in the reflectivity, no accurate information of the incoming X-ray can be obtained. When many such detectors are used as elements in an array, there are fluctuations in the characteristis among the individual elements. As a result, the image produced by such an array will have a ring-pattern unevenness or artifact. 
     To solve these problems, a radiation detector of such a structure as shown in FIG. 2 is preferable. That is, a silicon photodiode 3 is provided at the bottom of a vessel 6 made from brass, and a light-reflecting aluminum layer is formed on the inside surface of the container 6. Pb glass 8 is laid on the silicon photodiode 3 in the vessel 6, and a scintillator layer 2 is formed thereon. The Pb glass is provided to cut the fluorescent X-ray emitted from the scintillator. 
     In the case of a scintillator particle layer in the structure as shown in FIG. 2, the particle layer itself is optically opaque, and it is difficult to efficiently lead the emitted light to the silicon photodiode. Thus, even if a highly efficient Gd 2  O 2  S: Pr, Ce, F scintillator is used, a signal output of a level only substantially equal to that of a detector based on a combination of single crystal scintillator CdWO 4  and silicon photodiode can be obtained. This is another problem. To increase the output level, a transparent signal crystal scintillator of rare earth oxysulfide should be used, but only single crystal having a size of a few square millimeter can be obtained according to the process disclosed in J. Appl. Phys. 42, 3049 (1971). 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a radiation detector having a scintillator of high light output level. 
     This and other objects of the present invention can be attained by a radiation detector, which comprises a scintillator capable of emitting light by radiation and a photodetector for detecting the light emitted from the scintillator, the scintillator being composed of a sintered body prepared by sintering rare earth oxysulfide as a starting material containing a densification additive by hot isostactic pressing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 and 2 are cross-sectional view of radiation detectors for explaining the present invention. 
     FIGS. 3a and 3b show schematic structures of the present scintillator material and comparative material, respectively. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The rare earth oxysulfide for use in the present invention includes phosphors known from the Japanese patent application Kokai (Laid-open) No. 56-151,376, etc. for example, phosphors represented by the general formula: 
     
         (Ln.sub.1-x-y M.sub.x Ce.sub.y).sub.2 O.sub.2 S:(X) 
    
     wherein Ln is at least one element selected from the group consisting of Gd, La and Y; M is at least one element selected from the group consisting of Eu, Pr and Tb; X is at least one element selected from the group consisting of F and Cl; x is a value in a range of 3×10 -6  ≦×≦0.2; y is a value in a range of 10 -6  ≦Y≦5×10 -3  ; and X is in an amount ranging from 5 to 1,000 ppm by weight. 
     The reasons why the values of x and y in said ranges and the amount of F and Cl in said range are preferable are disclosed in said Japanese patent application Kokai (Laid-open) No. 56-151,376. The preferable element represented by M is Pr, and the preferable element represented by X is F. More preferable amount of F is in a range of 2 to 300 ppm by weight. Other phosphors than those mentioned above can be used. 
     It is preferable to add, as the densification additive, 0.001 to 10% by weight of at least one member selected from the group consisting of Li 2  GeF 6 , (NH  4 ) 2  GeF 6 , Na 2  GeF 6 , K 2  GeF 6 , NaPF 6 , KPF 6 , NH 4  PF 6 , Na 3  AlF 6 , K 2  SiF 6 , Li 2  SiF 6 .2H 2  O, Na 2  SiF 6 , LiF, NaHF 2 , KHF 2 , NH 4  HF 2 , Na 2  TiF 6 , K 2  TiF 6 , K 2  ZrF 6 , (NH 4 ) 2  ZrF 6 , MgSiF 6 , SrSiF 6 , Li 2  B 4  O 7 , LiBF 4 , NaBF 4  and KBF 4 , on the basis of the rare earth oxysulfide. 
     Even the addition of 0.001% by weight of the densification additive on the basis of the rare earth oxysulfide is effective. Too much addition does not increase the light output of the scintillator material and is also economically disadvantageous. Thus, addition of not more than 10% by weight of the densification additive is preferable, but the present invention is practicable in excess of 10% by weight. 
     It is preferable to produce a sintered body through hot isostatic pressing by filling powdered scintillator and a densification additive in a metal container, vacuum sealing the container and subjecting the container to hot isostatic pressing at 800° to 1,700° C. under a pressure of 500 to 2,000 atmospheres. When the pressure is lower under these conditions, the hot isostatic pressing must be carried out at a higher temperature. More preferable hot isostatic pressing conditions are such that the amount of the densification additive to be added is 0.01 to 4% by weight, the temperature is 1,100° to 1,500° C., and the pressure is 800 to 1,800 atmospheres. Materials for the metal vessel for the hot isostatic pressing are preferably metals easily deformable (being easily softened) at an elevated temperature, for example, pure iron, stainless steel, nickel, platinum, etc. 
     It is preferable to anneal the sample taken out of the metal container after the hot isostatic pressing in an inert gas. The annealing temperature is preferably 500° to 1,500° C., more preferably 900° to 1,300° C. Increase in the light output by annealing somewhat depends on the species of the densification additive, and may be 30-50% increased when annealed in an Ar gas at 1,200° C. for 30 minutes, as compared with the light output before the annealing. Since the half width in the peak of X-ray difffaction pattern of a sample subjected to hot isostatic pressing and successive annealing is smaller than that of a sample subjected only to the hot isostatic annealing, it seems that the increase in light output by annealing is due to promotion of crystallization. 
     Since the hot isostatic pressing is carried out after the addition of the densification additive in the present invention, the resulting sintered body has a much higher density than that of the sintered body obtained without any addition of the densification additive, and a scintillator material of high density, i.e. packing density (ratio of an experimental density to the theoretical value) of 96% or higher can be obtained. 
     Furthermore, the shapes of crystal grains observed in a cross-section of the sintered body according to the present invention differs from those obtained without the addition of the densification additive. That is, the sintered body obtained according to the present invention characteristically contains much more crystal grains having a columnar appearance. The presence of columnar particles can be observed in the cross-section of the sintered body by a microscope. It has been found that the distinguished characteristics as desired in the present invention can be obtained, if the columnar particles are contained in an area ratio of 10% or more in the cross-section by observation of a microscopic picture. FIG. 3a shows the shapes of crystal grains when no densification additive is added, and FIG. 3b shows shapes of crystal grains when the densification additive is added. 
     EXAMPLE 1 
     0.087 g of Li 2  GeF 6  (corresponding to 0.1% by weight on the basis of powdered scintillator) was added to 87 g of powdered scintillator (Gd 0 .999 Pr 0 .001 Ce 6 ×10 -6) 2  O 2  S: (F), and the mixture was filled in a pure iron container, 52 mm in diameter, 37 mm long and 1 mm thick (available capacity: 21.9 cm 3 ). The container was vaccum sealed while degasifying the container with heating. Then, the container was placed in a hot isostatic press and subjected to hot isostatic pressing at 1,300° C. under 1,500 atmospheres for 1.5 hour, and the sintered body was taken out of the container after cooling, and then processed into a sheet, 1 mm thick. The sheet was annealed in an inert gas at 1,200° C for 30 minutes. The thus obtained sintered body sheet was assembled into a detector as shown in FIG. 2, and was subjected to measurement of light output under irradiation of X-ray (tube voltage 120 kV, 100 mA). It was found that the relative light output of the detector using the sintered body sheet was 1.6 on the basis of the light output of a detector using a sintered body sheet containing no densification additive as unity. 
     EXAMPLE 2 
     0.00087 g of Li 2  GeF 6  (corresponding to 0.001% by weight on the basis of powdered scintillator) was added to 87 g of powdered scintillator (Gd 0 .999 Pr 0 .001 Ce 6 ×10 -6) 2  O 2  S:(F), and the mixture was then treated in the same manner as in Example 1. The thus obtained sintered body sheet was assembled into a detector as in FIG. 2 and subjected to measurement of light output under irradiation of X-ray (tube voltage 120 kV, 100 mA). It was found that the relative light output of the detector using the sintered body sheet of this Example was 1.1 on the basis of the light output of a detector likewise prepared without the densification additive as unity. 
     EXAMPLE 3 
     8.7 g of Li 2  GeF 6  (corresponding to 10% by weight on the basis of powdered scintillator) was added to 87 g of powdered scintillator (Gd 0 .999 Pr 0 .001 Ce 6 ×10 -6) 2  O 2  S:(F), and then the mixture was treated in the same manner as in Example 1. The thus obtained sintered body sheet was assembled into a detector as in FIG. 2 and subjected to measurement of light output under irradiation of X-ray shown in FIG. 2, and was subjected to measurement of light output under irradiation of X-ray (tube voltage 120 kV, 100 mA). It was found that the relative light output of the detector using the sintered body sheet was 1.2 on the basis of the light output of a detector using a sintered body sheet containing no densification additive as unity. 
     EXAMPLES 4-26 
     0.087 g each of various densification additives (corresponding to 0.1% by weight on the basis of powdered scintillator) was added to 87 g of powdered scintillator (Gd 0 .999 Pr 0 .001 Ce 6 ×10 -6) 2  O 2  S:(F), and the mixtures were then treated in the same manner as in Example 1. Each of the thus obtained sintered body sheet was assembled into a detector as in FIG. 2 and subjected to measurement of light output under irradiation of X-ray (tube voltage 120 kV, 100 mA). The relative light outputs of the detectors using the sintered body sheets are shown in Table 1 on the basis of the light outputs of detectors using sintered body sheets containing no densification additives as unity. 
     
                       TABLE 1                                                     
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Example                    Relative                                       
No.         Densification additive                                        
                           light output                                   
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4           (NH.sub.4).sub.2 GeF.sub.6                                    
                           1.6                                            
5           Na.sub.2 GeF.sub.6                                            
                           1.5                                            
6           K.sub.2 GeF.sub.6                                             
                           1.5                                            
7           NaPF.sub.6     1.4                                            
8           KPF.sub.6      1.4                                            
9           NH.sub.4 PF.sub.6                                             
                           1.5                                            
10          Na.sub.3 AlF.sub.6                                            
                           1.5                                            
11          K.sub.2 SiF.sub.6                                             
                           1.3                                            
12          Li.sub.2 SiF.sub.6.2H.sub.2 O                                 
                           1.5                                            
13          Na.sub.2 SiF.sub.6                                            
                           1.3                                            
14          LiF            1.6                                            
15          NaHF.sub.2     1.5                                            
16          KHF.sub.2      1.4                                            
17          NH.sub.4 HF.sub.2                                             
                           1.4                                            
18          Na.sub.2 TiF.sub.6                                            
                           1.3                                            
19          K.sub.2 TiF.sub.6                                             
                           1.4                                            
20          K.sub.2 ZrF.sub.6                                             
                           1.3                                            
21          (NH.sub.4).sub.2 ZrF.sub.6                                    
                           1.4                                            
22          MgSiF.sub.6    1.4                                            
23          SrSiF.sub.6    1.4                                            
24          LiBF.sub.4     1.5                                            
25          NaBF.sub.4     1.4                                            
26          KBF.sub.4      1.3                                            
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     EXAMPLES 27-35 AND COMPARATIVE EXAMPLE 1 
     Powdered scitillators of (Gd 0 .997 Pr 0 .003 Ce6×10-6) 2  O 2  S containing 90 ppm of F and various densification additives shown in Table 2 were vacuum sealed in stainless steel containers, and then the containers were subjected to hot isostatic pressing in an argon gas at 1,300° C. under 1,000 atmospheres for 3 hours. Packing density, cross-sectional area ratio of columnar microcrystals in any cross-section and relative light output of the thus obtained sintered bodies on the basis of sintered bodies containing no densification additives likewise obtained by hot isostatic pressing are shown in Table 2. 
     
                       TABLE 2                                                     
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                           Cross-                                         
                           sectional                                      
                           area                                           
Densification              ratio of                                       
additive          *Packing columnar Relative                              
Example          Amount   density                                         
                                 micro- light                             
No.    Formula   (wt %)   (%)    crystal                                  
                                        output                            
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27     LiF       0.08     99.8   43     1.2                               
28     LiB.sub.4 O.sub.7                                                  
                 0.10     99.8   37     1.3                               
29     Na.sub.3 AlF.sub.6                                                 
                 0.09     99.6   25     1.2                               
30     NaPF.sub.6                                                         
                 0.12     99.2   33     1.4                               
31     Li.sub.2 GeF.sub.6                                                 
                 0.10     99.8   35     1.4                               
32     NaBF.sub.4                                                         
                 0.20     98.9   61     1.1                               
33     LiBF.sub.4                                                         
                 0.07     98.8   42     1.2                               
34     (NH.sub.4)GeF.sub.5                                                
                 0.10     99.1   51     1.2                               
35     MgSiF.sub.6                                                        
                 0.15     99.3   38     1.3                               
Comp.  None      0        95.5   3.1    1.0                               
Ex. 1                                                                     
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 *Ratio of an experimental density to the theoretical value               
 
    
     EXAMPLES 36-45 AND COMPARATIVE EXAMPLES 2 
     Powdered scintillators of (Gd 0 .997 Pr 0 .003 Ce 6 ×10 -6) 2  O 2  S containing 90 ppm of F and various densification additives shown in Table 3 were vacuum sealed in stainless steel containers, and then the containers were subjected to hot isostatic pressing in an argon gas at 1,300° C. under 1,500 atmospheres for 1/.5 hours. Packing density, cross-sectional area ratio of columnar, microcrystals in any cross-section and relative light output of the thus obtained sintered bodies on the basis of sintered bodies containing no densification additives likewise obtained by hot isostatic pressing are shown in Table 3. 
     
                       TABLE 3                                                     
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                          Cross-                                          
                          sectional                                       
                          area                                            
Densification             ratio of                                        
addition         Packing  columnar  Relative                              
Example         Amount   density                                          
                                micro-  light                             
No.    Formula  (wt %)   (%)    crystal output                            
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36     LiF      0.0005   97.7   2.1     1.0                               
37     &#34;        0.001    97.2   14      1.1                               
38     &#34;        0.1      98.9   50      1.6                               
39     &#34;        10       97.1   73      1.1                               
40     &#34;        12       95.9   72      0.9                               
41     Li.sub.2 GeF.sub.6                                                 
                0.0005   95.8   3.4     1.0                               
42     &#34;        0.001    97.1   11      1.1                               
43     &#34;        0.1      99.8   64      1.6                               
44     &#34;        10       96.3   75      1.2                               
45     &#34;        21       95.6   73      0.8                               
Comp.  None     --       95.8   2.1     1.0                               
Ex. 2                                                                     
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     EXAMPLES 46-56 
     Powered scintillators of rare earth oxysulfides shown in Table 4 containing densification additives shown in Table 5 were vacuum sealed in stainless steel containers, and then the containers were subjected to hot isostatic pressing in an argon gas at 1,300° C. under 1,250 atmospheres for 2 hours. 
     
                       TABLE 4                                                     
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No.      Composition of scintillators                                     
______________________________________                                    
A        (Gd.sub.0.99799 Tb.sub.0.002 Ce.sub.1×10.spsb.-5).sub.2    
         O.sub.2 S:100 ppmF                                               
B        (La.sub.0.99799 Pr.sub.0.002 Ce.sub.1×10.spsb.-5).sub.2    
         O.sub.2 S:100 ppmF                                               
C        (La.sub.0.99799 Tb.sub.0.002 Ce.sub.1×10.spsb.-5).sub.2    
         O.sub.2 S:90 ppm F                                               
D        (Gd.sub.0.99799 Tb.sub.0.002 Ce.sub.1×10.spsb.-5).sub.2    
         O.sub.2 S:100 ppmF                                               
E        (La.sub.0.99799 Pr.sub.0.002 Ce.sub.1×10.spsb.-5).sub.2    
         O.sub.2 S:100 ppmF                                               
F        (La.sub.0.99799 Tb.sub.0.002 Ce.sub.1×10.spsb.-5).sub.2    
         O.sub.2 S:90 ppmF                                                
G        (Gd.sub.0.96 Eu.sub.0.03 Tb.sub.0.01).sub.2 O.sub.2 S            
H        (La.sub.0.96 Eu.sub.0.03 Pr.sub.0.01).sub.2 O.sub.2 S            
I        (La.sub.0.96 Eu.sub.0.03 Tb.sub.0.01).sub.2 O.sub.2 S            
J        (Y.sub.0.96 Eu.sub.0.03 Pr.sub.0.01).sub.2 O.sub.2 S             
K        (Y.sub.0.96 Eu.sub.0.03 Tb.sub.0.01).sub.2 O.sub.2 S             
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     Packing density, cross-sectional area ratio of columnar, microcrystals in any cross-section and relative light output of the thus obtained sintered bodies, on the basis of sintered bodies containing no densitification additive likewise obtained by hot isostatic pressing are shown in Table 5. 
     
                       TABLE 5                                                     
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                                  Cross-                                  
                                  sectional                               
                                  area                                    
             Densification        ratio of                                
                                         Rela-                            
Exam-        additive      Packing                                        
                                  columnar                                
                                         tive                             
ple                   Amount density                                      
                                    micro- light                          
No.   *No.   Formula  (wt %) (%)    crystal                               
                                           output                         
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46    A      LiF      0.1    98.8   54     1.2                            
47    B      LiF      0.2    98.5   45     1.1                            
48    C      Li.sub.2 GeF.sub.6                                           
                      0.2    97.9   49     1.3                            
49    D      LiF      0.1    99.5   38     1.1                            
50    E      LiF      0.2    98.9   51     1.4                            
51    F      Li.sub.2 GeF.sub.6                                           
                      0.1    97.8   45     1.3                            
52    G      Li.sub.2 GeF.sub.6                                           
                      0.1    99.3   38     1.1                            
53    H      NaBF.sub.4                                                   
                      0.2    98.9   42     1.2                            
54    I      NaBF.sub.4                                                   
                      0.1    98.6   52     1.2                            
55    J      NaBF.sub.4                                                   
                      0.3    98.9   46     1.1                            
56    K      NaBF.sub.4                                                   
                      0.2    99.1   61     1.3                            
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 *No. shows the composition of scintillators in Table 4.                  
 
    
     As described in detail above, the present invention provides a sintered body of rear earth oxysulfidie having a higher light output than that of the conventional sintered body of rare earth oxysulfide, and has the following effects: 
     (1) A scintillator of high light output for radiation defection can be obtained. 
     (2) The thickness of a scintillator can be made larger owing to the higher light output of the scintillator, unnecessitating the radiation-absorbing material, such so Pb glass, or making the film thinner. This can lead to cost reduction. 
     (3) The S/N (signal to noise) ratio can be made larger or the angular aperture of element can be made smaller, whereby the spatial resolution and density resolution can be increased over the detection limit of the conventional radiation detector.