Abstract:
[Problem] To provide a scintillator plate capable of improving the accuracy of radiation detection, and expanding the surface area for practical use while suppressing manufacturing costs, and also provide a radiation measuring apparatus, a radiation imaging apparatus, and a scintillator plate manufacturing method. 
     [Solution] A scintillator plate ( 1 ) includes a scintillator ( 2 ) that generates scintillation light when excited by incident radiation. The scintillator plate ( 1 ) includes a scintillator layer ( 22 ) covered with scintillator powder ( 21 ) having an average particle diameter equal to or greater than the range of the radiation within the scintillator ( 2 ) when the radiation to be measured is either alpha rays, electron beams, or ion beams.

Description:
TECHNICAL FIELD  
       [0001]    The present invention relates to a scintillator plate that is excited by a radiation to emit scintillation light, a radiation measuring apparatus, a radiation imaging apparatus, and a scintillator plate manufacturing method. 
       BACKGROUND ART  
       [0002]    Si semiconductor detectors with extremely high energy resolution are widely used as a device for measuring α rays in a dust monitor for monitoring plutonium contamination. However, the Si semiconductor detectors have a problem that the electrodes are susceptible to corrosion by moisture in the air. In addition, the Si semiconductor detectors have a problem that the detectors tend to pick up electric noise and provide an erroneous output. 
         [0003]    In order to solve the problems of the Si semiconductor detectors described above, Izaki et al., the inventors, have proposed a ZnS(Ag) scintillation detector serving as an α rays measuring apparatus that is filled with a plurality of layers of ZnS(Ag) scintillator particles having a particle diameter of 5 μm having a thickness of 10 mg/cm 2 , in Japanese Patent No. 4061367 (Patent Literature 1). 
         [0004]    There is also a scintillation detector used in a positron emission tomography (PET) apparatus or the like for a nuclear medicine image diagnostic method. 
         [0005]    For example, in Japanese Patent Laid-Open No. 2006-199727, there is proposed a radiation detector that uses a scintillator made of a single crystal cerium-activated lanthanoid silicate as an X-rays detector used in a positron emission tomography apparatus (Patent Literature 2). 
       CITATION LIST  
     Patent Literature  
       [0006]    Patent Literature 1: Japanese Patent No. 4061367 
         [0007]    Patent Literature 2: Japanese patent Laid-Open No. 2006-199727 
       SUMMARY OF INVENTION  
     Technical Problem  
       [0008]    However, the invention described in Patent Literature 1 has a problem that scintillation light emitted as a result of excitation by α rays is repeatedly reflected or diffused by surfaces of the scintillator particles and does not adequately reach the photomultiplier tube or the like that is to receive the scintillation light. Therefore, compared with the Si semiconductor detector, the pulse height discriminating capability for α rays is low and needs to be improved. That is, the invention described in Patent Literature 1 can detect α rays but does not have an energy discriminating capability enough to discriminate α rays having different amounts of energy emitted by a plurality of nuclides, such as plutonium and a radon daughter nuclide. 
         [0009]    In the case of a radiation detector using a scintillator single crystal, such as the invention described in Patent Literature 2, there is a limitation of the size of the single crystal, it is difficult, and unfeasible in terms of cost, to manufacture a thin and wide scintillator plate from a single crystal. In addition, it is difficult to reduce the thickness of the single crystal scintillator while maintaining a large area as described above, there is a problem that the scintillator is susceptible to the effect of γ rays. 
         [0010]    The present invention has been made to solve the problems described above, and an object of the present invention is to provide a scintillator plate that has an improved radiation detection precision and can be practically increased in size while reducing the manufacturing cost, a method of manufacturing the same, and a radiation measuring apparatus and a radiation imaging apparatus using the same. 
       Solution to Problem  
       [0011]    A scintillator plate according to the present invention is a scintillator plate containing a scintillator that is excited by an incident radiation to emit scintillation light, wherein in a case where a measurement target radiation is any of α rays, an electron beam and an ion beam, the scintillator plate has a scintillator layer formed by laying scintillator powders having an average particle diameter equal to or greater than a range of the radiation in the scintillator. 
         [0012]    According to an aspect of the present invention, one or both of surfaces of the scintillator layer can be polished so that the thickness of the scintillator layer is equal to or greater than 20 μm and equal to or smaller than 100 μm. 
         [0013]    A scintillator plate according to the present invention is a scintillator plate containing a scintillator that is excited by an incident radiation to emit scintillation light, wherein in a case where a measurement target radiation is a neutron beam, the scintillator plate has a scintillator layer formed by laying scintillator powders having an average particle diameter equal to or greater than a mean free path of the neutron beam in the scintillator. 
         [0014]    According to an aspect of the present invention, one or both of surfaces of the scintillator layer can be polished so that the thickness of the scintillator layer is equal to or greater than 30 μm and equal to or smaller than 150 μm. 
         [0015]    A scintillator plate according to the present invention is a scintillator plate containing a scintillator that is excited by an incident radiation to emit scintillation light, wherein in a case where a measurement target radiation is X rays, the scintillator plate has a scintillator layer formed by laying scintillator powders having an average particle diameter greater than 300 μm, and one or both of surfaces of the scintillator layer is polished so that the thickness of the scintillator layer is equal to or greater than 300 μm and equal to or smaller than 500 μm. 
         [0016]    According to an aspect of the present invention, the scintillator layer preferably has a single layer structure formed by laying the scintillator powders in a single layer. 
         [0017]    Furthermore, according to an aspect of the present invention, the scintillator layer can be formed by embedding scintillator powders having an average particle diameter equal to or greater than the average particle diameter of the scintillator powders determined by the kind of the radiation in a gap between the scintillator powders adjacent to each other laid in a single layer, and one or both of surfaces of the scintillator layer can be polished. 
         [0018]    Furthermore, according to an aspect of the present invention, the scintillator can be selected from among a GPS scintillator, a ZnS(Ag) scintillator, a YAP scintillator and a CsI scintillator. 
         [0019]    A radiation measuring apparatus according to the present invention comprises: the scintillator plate; a signal converting part that converts scintillation light emitted by the scintillator plate into an electric signal; a radiation discriminating part that discriminates a measurement target radiation and another radiation from each other based on an upper limit value and a lower limit value of the intensity of the electric signal that are set depending on the kind of the measurement target radiation; a counting part that counts discriminated electric signals of the measurement target radiation; and a radiation determining part that determines the presence or absence of the measurement target radiation by comparison between a count from the counting part and a predetermined threshold. 
         [0020]    A radiation imaging apparatus according to the present invention comprises the scintillator plate. 
         [0021]    A scintillator plate manufacturing method according to the present invention comprises: a scintillator layer forming step of forming a scintillator layer by laying scintillator powders having a predetermined average particle diameter on a surface of an adhesive material and bonding the scintillator powders; and a scintillator layer polishing step of polishing one or both of surfaces of the scintillator layer until the scintillator layer has a predetermined thickness. 
         [0022]    According to an aspect of the present invention, in the scintillator layer forming step, a sub-step of removing scintillator powders that are not bonded to the surface of the adhesive material and laying the scintillator powders having the predetermined average particle diameter in gaps between the scintillator powders bonded to the surface of the adhesive material can be repeated at least once. 
         [0023]    Furthermore, according to an aspect of the present invention, before the scintillator layer forming step, the method can further comprise a fine particle removing step of removing fine particles from the scintillator powders to be used by immersing the scintillator powders with the fine particles adhering thereto in a liquid, agitating the liquid and then removing a supernatant containing the fine particles after a lapse of a predetermined time. 
       Advantageous Effect of Invention  
       [0024]    According to the present invention, the radiation detection precision of the scintillator plate can be improved, and the size of the scintillator plate can be practically increased while reducing the manufacturing cost. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0025]    [ FIG. 1 ]  FIG. 1  is an enlarged cross-sectional view of a scintillator plate according to an embodiment of the present invention. 
           [0026]    [ FIG. 2(   a )]  FIG. 2(   a ) is a plan view of a scintillator layer formed in a single layer according to the embodiment. 
           [0027]    [ FIG. 2(   b )]  FIG. 2(   b ) is a cross-sectional view taken along the line A-A in  FIG. 2(   a ). 
           [0028]    [ FIG. 3(   a )]  FIG. 3(   a ) is a plan view of a scintillator layer formed by laying further scintillator powders in gaps between the scintillator powders laid in a single layer according to the embodiment. 
           [0029]    [ FIG. 3(   b )]  FIG. 3(   b ) is a cross-sectional view taken along the line A-A in  FIG. 3(   a ). 
           [0030]    [ FIG. 4 ]  FIG. 4  is an enlarged cross-sectional view of a scintillator layer formed by filling gaps between the scintillator powders laid in a single layer with further scintillator powders having a predetermined average particle diameter. 
           [0031]    [ FIG. 5 ]  FIG. 5  is a graph showing the range of α rays in a GPS scintillator according to the embodiment. 
           [0032]    [ FIG. 6 ]  FIG. 6  is a graph showing a relationship between the thickness of the scintillator layer and the energy absorption of the GPS scintillator according to the embodiment. 
           [0033]    [ FIG. 7 ]  FIG. 7  is a schematic diagram showing steps of a scintillator plate manufacturing method according to the embodiment. 
           [0034]    [ FIG. 8 ]  FIG. 8  is a flowchart for illustrating a sequence of the steps of the scintillator plate manufacturing method according to the embodiment. 
           [0035]    [ FIG. 9 ]  FIG. 9  is a block diagram showing a radiation measuring apparatus according to an embodiment of the present invention. 
           [0036]    [ FIG. 10 ]  FIG. 10  is a block diagram showing a radiation imaging apparatus according to an embodiment of the present invention. 
           [0037]    [ FIG. 11 ]  FIG. 11  shows a picture of a GPS scintillator manufactured in a example 1. 
           [0038]    [ FIG. 12 ]  FIG. 12  is a graph showing results of measurement of α rays using a ZnS scintillator, a single crystal GPS scintillator, and a GPS scintillator plate according to the present invention in the example 1. 
           [0039]    [ FIG. 13 ]  FIG. 13  is a graph showing results of measurement of α rays using GPS scintillator plates having different average particle diameters in the example 1. 
           [0040]    [ FIG. 14 ]  FIG. 14  is a graph showing a relationship between the result of measurement of γ rays using the GPS scintillator and the thickness of the GPS scintillator in the example 1. 
           [0041]    [ FIG. 15 ]  FIG. 15  is a graph showing results of measurement of α rays emitted by americium 241 (241Am) as a radiation source and γ rays emitted by cesium 137 (137Cs) as a radiation source using the GPS scintillator plate according to the present invention in the example 1. 
           [0042]    [ FIG. 16 ]  FIG. 16  is a graph showing relationships between the focused light intensity and the count of α rays measured using scintillator plates in a example 2. 
           [0043]    [ FIG. 17 ]  FIG. 17  is a graph showing relationships between the peak-to-total ratio and the inverse of the resolution determined based on the relationships between the focused light intensity and the count of α rays measured using the scintillator plates in the example 2. 
           [0044]    [ FIG. 18 ]  FIG. 18  is a schematic diagram for illustrating a method of calculating the peak-to-total ratio and the inverse of the resolution in the example 2. 
           [0045]    [ FIG. 19 ]  FIG. 19  is an optical microscopic picture of a polished surface of a test piece (d) in the example 2. 
           [0046]    [ FIG. 20 ]  FIG. 20  is an optical microscopic picture of a polished surface of a test piece (e) in the example 2. 
           [0047]    [ FIG. 21 ]  FIG. 21  is an optical microscopic picture of a polished surface of a test piece (i) in the example 2. 
           [0048]    [ FIG. 22 ]  FIG. 22  is a picture of a radiation imaging apparatus in a example 3. 
           [0049]    [ FIG. 23 ]  FIG. 23  is a three-dimensional graph showing the radioactivity intensity distribution measured by the radiation imaging apparatus in the example 3. 
           [0050]    [ FIG. 24 ]  FIG. 24  is a picture of a measurement sample set in a example 4. 
           [0051]    [ FIG. 25 ]  FIG. 25  shows an output image of a radiation imaging apparatus that shows a neutron distribution measured with a conventional ZnS scintillator in the example 4. 
           [0052]    [ FIG. 26 ]  FIG. 26  shows an output image of the radiation imaging apparatus showing an extracted part of the neutron distribution measured with a conventional Li-6 glass scintillator in the example 4. 
           [0053]    [ FIG. 27 ]  FIG. 27  shows an output image of the radiation imaging apparatus showing an extracted part of the neutron distribution measured with a conventional ZnS scintillator in the example 4. 
           [0054]    [ FIG. 28 ]  FIG. 28  shows an output image of the radiation imaging apparatus showing an extracted part of the neutron distribution measured with the GPS scintillator according to the present invention in the example 4. 
       
    
    
     DESCRIPTION OF EMBODIMENTS  
       [0055]    In the following, a scintillator plate, a radiation measuring apparatus, a radiation imaging apparatus and a scintillator plate manufacturing method according to embodiments of the present invention will be described. 
         [0056]    A scintillator plate according to an embodiment will be first described.  FIG. 1  is an enlarged cross-sectional view of a scintillator plate  1  according to this embodiment. The scintillator plate  1  according to this embodiment comprises scintillator powders  21  of a scintillator  2  that is excited by a radiation, a scintillator layer  22  formed by laying the scintillator powders  21 , and a substrate  3  to which the scintillator layer  22  is fixed. According to this embodiment, a suitable adhesive  4  is used to fix the scintillator powders  21  to the substrate  3  to form the scintillator layer  22 . 
         [0057]    The scintillator  2  is a material that emits scintillation light when it is excited by an incident radiation. For example, the scintillator  2  may be a Gd 2 Si 2 O 7  scintillator (GPS scintillator), a ZnS(Ag) scintillator, a YAP scintillator or a CsI scintillator. 
         [0058]    The GPS scintillator is made of Gd 2 Si 2 O 7  and may further contain Ce added thereto as an active material. In this embodiment, the GPS scintillator is manufactured by crystallization of powders of Gd 2 SiO 3 , CeO 2  and SiO 2  in a top seeded solution growth (TSSG) method. The method of manufacturing the GPS scintillator is not limited to the TSSG method and can be appropriately selected from among a solid phase reaction method, a sol-gel method and the like. 
         [0059]    According to this embodiment, the scintillator  2  is used in a powder form. The average particle diameter d of the scintillator powders  21  is selected to be a predetermined value depending on the kind of the scintillator  2  and the kind of the measurement target radiation, as described below. 
         [0060]    For example, in the case where the measurement target radiation is a charged particle having a charge, which is a directly ionizing radiation that is incident on the scintillator  2  to excite the scintillator  2  and causes the scintillator  2  to emit scintillation light, the average particle diameter d of the scintillator powders  21  is preferably equal to or greater than the range of the particle in the scintillator  2 , in order to increase the efficiency of emission of the scintillation light. The directly ionizing radiation is α rays, an electron beam, or an ion beam, for example. According to the concept of the present invention, electron beams include β rays. The term “range” herein refers to the distance for which a radiation having a charge capable of causing direct ionization, such as α rays, incident on a material can travel in the material. According to this embodiment, the range depends on the combination of the kind of the scintillator  2  and the kind of the radiation. Thus, in the case where the measurement target is the directly ionizing radiation, the average particle diameter d of the scintillator powders  21  is preferably equal to or greater than the range to increase the probability of emission of the scintillation light. In the case where the measurement target radiation is an electron, the projected range, which is in the front direction, is preferably used. 
         [0061]    On the other hand, in the case where the radiation is not the directly ionizing radiation having a charge but an indirectly ionizing radiation, which is incident on the scintillator  2  to collide or otherwise interact with an atom or atomic nucleus in the scintillator  2  to generate a charged particle, which excites the scintillator  2  to emit scintillation light, the average particle diameter d of the scintillator powders  21  is preferably equal to or greater than the mean free path of the indirectly ionizing radiation in the scintillator  2 , in order to increase the efficiency of emission of the scintillation light. The indirectly ionizing radiation is a neutron beam or γ rays, for example. The term “mean free path” herein refers to the average distance for which an indirectly ionizing radiation having no charge, such as a neutron beam, incident on a material travels before interacting or otherwise reacting with the material. According to this embodiment, the means free path depends on the kind of the scintillator  2  and the kind of the radiation. Thus, in the case where the measurement target is the indirectly ionizing radiation, the average particle diameter d of the scintillator powders  21  is preferably equal to or greater than the means free path to increase the probability of emission of the scintillation light. 
         [0062]    The method of selecting the scintillator powders  21  having a predetermined average particle diameter is not particularly limited and can be appropriately selected from among various methods, including a method that involves using a sieve and a method that takes advantage of the difference in the rate of sedimentation due to the difference in the particle diameter. As described later with regard to a fine particle removing step, fine particles can be removed from the scintillator powders  21  by immersing the selected scintillator powders  21  in a liquid, such as a phosphate solution, an alcohol or water, agitating the liquid to diffuse the scintillator powders  21  and the fine particles having adhered to the scintillator powders  21  in the liquid and then discarding the supernatant. 
         [0063]    The scintillator layer  22  is formed by laying the scintillator powders  21  and fixing the scintillator powders  21  with an adhesive or the like. The scintillator layer  22  may be formed by a plurality of beds (layers) of scintillator powders  21  but has to be thinner than a predetermined maximum thickness as described later. In the case where the scintillator layer  22  is formed by a plurality of beds (layers) of scintillator powders  21 , however, the emitted scintillation light can be reflected or diffused by the powders. To avoid this, the scintillator layer  22  is preferably formed by a single layer of scintillator powders as shown in  FIG. 2 . In the present invention, however, the single layer structure of the scintillator layer  22  does not need to be strictly construed and can include a structure in which gaps between adjacent scintillator powders  21  are filled with further scintillator powders, for example, as far as desired effects and advantages are achieved. 
         [0064]    The scintillator layer  22  preferably has a higher filling factor of the scintillator powders  21  in terms of detection precision. Thus, in laying the scintillator powders  21  having a predetermined average particle diameter d, the scintillator powders  21  are preferably compressed to some extent to reduce the gaps between the scintillator powders  21 . As required, as shown in FIG.  3 , further scintillator powders  21  having an average particle diameter equal to greater than the average particle diameter of the scintillator powders  21  determined by the kind of the radiation are preferably embedded in the gaps between the scintillator powders  21  laid in a single layer. In the process of filling the gaps between the scintillator powders, it is preferred to fill not only the gaps between scintillator powders  21  bonded spaced apart from each other as shown in  FIG. 4  but also the gaps between the upper parts of the scintillator powders  21  bonded close to each other. 
         [0065]    Next, the thickness of the scintillator layer  22  will be prescribed. The scintillator layer  22  has to have such a thickness as to allow the measurement target radiation incident on the scintillator layer  22  to excite emission of scintillation light and at the same time suppress excitation of scintillation light by the other radiations than the measurement target radiation, so that the measurement target radiation and the other radiations can be discriminated from each other. In the following, a method of setting the thickness of the scintillator layer for the measurement target radiation will be described with regard to a specific example. 
         [0066]    First, a case where the measurement target radiation is α rays will be described. As shown in  FIG. 5 , in the case where the measurement target radiation is α rays of 5.5 MeV emitted by plutonium-239 (239Pu), and the scintillator  2  is the GPS scintillator, the range is approximately 17 μm. On the other hand, in the case where the measurement target radiation is α rays of 7.7 MeV emitted by a radon daughter nuclide, which exists in nature and emits α rays, the range is approximately 27 μm. Therefore, in the case where the measurement target radiation is α rays emitted by plutonium-238, the thickness of the scintillator powders  21  is preferably equal to or greater than 20 μm taking measurement errors into consideration. 
         [0067]    On the other hand, there are various radiations other than the measurement target radiation, such as β rays and γ rays, in nature. For example, a radon daughter nuclide emits more β rays and γ rays than α rays. When such β rays or γ rays are incident on the scintillator  2 , the scintillator  2  emits scintillation light as in the case where α rays are incident on the scintillator  2 . As can be seen from the result of a example 13 described later (see  FIG. 14 ), when the γ rays are not the measurement target radiation, there is a tendency that the energy contribution of the γ rays in the scintillator  2  increases and the range of the focused light intensities in which the scintillation light is count increases as the thickness of the scintillator  2  increases. β rays also have a similar tendency. Therefore, the scintillator  2  preferably has a small thickness that reduce the energy contribution of the radiation other than the measurement target radiation in the scintillator  2  so that the scintillation light caused by the other radiation can be discriminated from the scintillation light caused by the α rays. With the scintillator plate  1  according to the present invention, the thickness of the scintillator  2  is reduced as far as possible to transmit as much radiations other than the measurement target radiation as possible to minimize the energy contribution of the other radiations in the scintillator  2  and the scintillation light emission, thereby facilitating discrimination between the of the measurement target radiation and the other radiations. 
         [0068]    Therefore, from the viewpoint of the range of the measurement target radiation and the discrimination between the measurement target radiation and the other radiations, in the case where the measurement target radiation is α rays emitted by plutonium, the thickness of the scintillator layer  22  is preferably equal to or greater than 20 μm and equal to or smaller than 100 μm. 
         [0069]    Next, a case where the measurement target radiation is a neutron beam will be described.  FIG. 6  is a graph showing a relationship between the thickness of the scintillator layer  22  and the energy absorption in the case of the GPS scintillator. The “energy absorption” refers to the probability that the radiation incident on the scintillator interacts or otherwise reacts with the scintillator, and the energy absorption value of 1 means that all the radiations react with the scintillator. The mean free path is defined as the thickness at which the energy absorption is (1−1/e), where e denotes the Napierian logarithm. Thus, the mean free path equals to the thickness at which the energy absorption is approximately 0.63, that is, the probability that the radiation reacts with the scintillator  2  is approximately 63%. As shown in  FIG. 6 , the mean free path in the case of the GPS scintillator is approximately 30 μm. 
         [0070]    The thickness of the scintillator layer  22  is preferably equal to or smaller than the thickness that reduces the energy contribution of the other radiations than the measurement target radiation in the scintillator  2  to such an extent that the measurement target radiation can be discriminated from the other radiations, as in the case of the α rays. As shown in  FIG. 6 , the probability that the neutron beam reacts with the scintillator at the depth of approximately 150 μm, which is five times greater than the mean free path, is approximately 99.3%. On the other hand, in the case of the γ rays, which have an energy lower than 59 KeV, the energy absorption increases as the thickness of the scintillator layer  22  increases, but the probability that the γ rays react with the scintillator layer  22  at the depth of approximately 150 μm can be reduced to approximately 50%. Thus, if the thickness of the scintillator layer  22  is equal to or smaller than approximately 150 μm, more of the energy of the neutron beam is converted into scintillation light than the energy of the γ rays. 
         [0071]    Therefore, in the case where the measurement target radiation is the neutron beam, the thickness of the scintillator layer  22  is desirably equal to or greater than 30 μm and equal to or smaller than 150 μm, taking measurement errors into consideration. 
         [0072]    In the case where the measurement target radiation is X rays, taking into consideration that X rays are used in a positron emission tomography apparatus or the like for a nuclear medicine image diagnostic method, the average particle diameter d of the scintillator powders  21  is preferably greater than 300 μm, and the thickness of the scintillator layer  22  is desirably equal to or greater than 300 μm and equal to or smaller than 500 μm. 
         [0073]    Next, the substrate  3  will be described. The substrate  3  to which the scintillator layer  22  is fixed is not particularly limited in terms of material or size. However, the substrate  3  is preferably made of a material that is hard and can be shaped to have a flat surface, such as a transparent glass or chemical synthetic resin that does not prevent transmission of the scintillation light. In the case where the measurement target radiation is the neutron beam, which has a higher transmission than the α rays or the like, various materials can be used for the substrate  3 . For example, an aluminum plate, a silicon plate, an alumina plate or the like can be used, and the substrate  3  may be in close contact with a scintillation light receiving part of a photomultiplier tube  51  described later at the side of the scintillator layer  22 . Furthermore, as required, the substrate  3  may have a reflecting surface on the side of the scintillator layer  22 . The adhesive  4  used to bond the scintillator powders  21  in a layer to form the scintillator layer  22  is not particularly limited but preferably has radiation-proof properties. 
         [0074]    The scintillator plate  1  is not limited to the structure in which the scintillator layer  22  is fixed to the substrate  3 . If the scintillator layer  22  has a predetermined strength, the scintillator layer  22  does not necessarily have to be fixed to the substrate  3  and can be used as the scintillator plate  1  by itself. 
         [0075]    Next, a scintillator plate manufacturing method according to an embodiment of the present invention will be described.  FIG. 7  is a schematic diagram showing steps of the scintillator plate manufacturing method according to this embodiment.  FIG. 8  is a flowchart for illustrating a sequence of the steps. 
         [0076]    As shown in  FIGS. 7 and 8 , the scintillator plate manufacturing method according to this embodiment comprises a fine particle removing step S 1  of removing the fine particles from the scintillator powders  21  to be used, a scintillator layer forming step S 2  of forming the scintillator layer  22  by laying the scintillator powders  21  to be used on a surface of an adhesive material and bonding the scintillator powders  21 , a substrate fixing step S 3  of fixing the scintillator layer to a substrate, an adhesive material removing step S 4  of removing the adhesive material, and a scintillator layer polishing step S 5  of polishing a surface of the scintillator layer until the scintillator layer has a predetermined thickness. In the following, each step will be described in detail. 
         [0077]    The scintillator powders  21  are prepared by pulverizing the single crystal scintillator  2  and selecting particles with a sieve or the like so that a predetermined average particle diameter is achieved. However, selection with the sieve or the like may not be enough to screen out the fine particles of the scintillator  2  adhering to the scintillator powders  21 . Thus, according to this embodiment, those fine particles are removed in the fine particle removing step S 1 . 
         [0078]    The fine particle removing step S 1  involves immersing the scintillator powders  21  to be used in a liquid, such as a phosphate solution, an alcohol or water, and agitating the liquid with an ultrasonic cleaning apparatus or the like to separate the fine particles from the scintillator powders  21  and make the scintillator powders  21  and the fine particles diffuse separately in the liquid. Since the scintillator powders and the fine particles settle out at different rates because of the difference in particle diameter, the fine particles can be removed from the scintillator powders by removing the supernatant after a predetermined time. More specifically, the fine particles have a higher surface-area-to-weight ratio than the scintillator powders  21 , experience a larger buoyant force and a larger resisting force, which is in proportion to the movement speed, than the scintillator powders  21  and therefore settle out at a lower rate than the scintillator powders  21 , so that the fine particles can be removed from the scintillator powders  21 . 
         [0079]    The scintillator layer forming step S 2  is a step of forming the scintillator layer  22  by bonding the scintillator powders  21  having a predetermined average particle diameter by laying the scintillator powders  21  on a surface of an adhesive material, such as an adhesive sheet. Although the adhesive material in the sheet form is used as shown in  FIG. 7  according to this embodiment, the present invention is not limited to thereto, and the scintillator powders  21  may be bonded to each other with an adhesive separately applied thereto. 
         [0080]    In the scintillator layer forming step according to this embodiment, it is preferred that the nonuniformity of the scintillator powders  21  forming the scintillator layer  22  is eliminated to improve the filling factor, and the scintillator powders  21  are laid in a single layer. To this end, after the scintillator powders  21  are laid on the surface of the adhesive material, the surface of the scintillator layer  22  is swept with a brush, or any scintillator powders  21  that are not bonded to the adhesive material are removed by giving an impact thereto, for example. Once the scintillator powders  21  that are not bonded to the adhesive material are removed, the scintillator powders  21  having the predetermined average particle diameter are laid again in the gaps between the previously laid scintillator powders  21  bonded to the adhesive material. According to this embodiment, this step is repeated at least once. 
         [0081]    In the scintillator layer forming step S 2  according to this embodiment, after the scintillator powders  21  are laid on the surface of the adhesive material, the scintillator powders  21  are fixed with the suitable adhesive  4 , such as an epoxy resin, to form the scintillator layer  22 . 
         [0082]    The substrate fixing step S 3  is a step of fixing the scintillator layer  22  to the substrate  3 . According to this embodiment, as shown in  FIG. 7 , the substrate  3  is applied and fixed to the scintillator layer  22  formed on the surface of the adhesive material with the adhesive  4  or the like. In the case where the scintillator plate  1  is formed without the substrate  3 , this step can be omitted. In that case, the adhesive material can serve the function of the substrate  3 , or even the adhesive material can be removed so that the scintillator layer  22  forms the scintillator plate  1  by itself. In the case where the opposite surfaces of the scintillator layer  22  are to be polished, the surfaces can be polished before the substrate  3  is fixed to the scintillator layer  22 , and the substrate  3  can be fixed to the polished surface. 
         [0083]    The adhesive material removing step S 4  is a step of removing the adhesive material bonding to the scintillator layer  22 . If the adhesive material is an adhesive sheet, the adhesive sheet can be simply peeled off. If the adhesive material has no effect on the incidence of the radiation on the scintillator layer  22  or the transmission of the scintillation light, this step can be omitted, and the scintillator layer  22  with the adhesive material can be used as the scintillator plate  1 . 
         [0084]    The scintillator layer polishing step S 5  is a step of polishing the scintillator layer  22  until the scintillator layer  22  has a predetermined thickness. According to this embodiment, the surface of the scintillator layer exposed by removing the adhesive material in the adhesive material removing step S 4  is polished with waterproof abrasive paper. The means for polishing the scintillator layer  22  is not limited to the waterproof abrasive paper, and any means appropriately selected from among a grindstone, a liquid abrasive material and the like can be used. 
         [0085]    In the case where the opposite surfaces of the scintillator layer  22  are to be polished, the surface without the adhesive material can be first polished after the scintillator layer  22  is formed in the scintillator layer forming step S 2 , the polished surface can be fixed to the substrate  3 , and the surface exposed by removing the adhesive material can be then polished. 
         [0086]    Next, a radiation measuring apparatus  5  according to this embodiment will be described. As shown in  FIG. 9 , the radiation measuring apparatus  5  according to this embodiment comprises the scintillator plate  1  described above, a signal converting part  51  that converts the scintillation light emitted by the scintillator plate  1  into an electric signal, an amplifier  52  that amplifies the electric signal, a radiation discriminating part  53  that sets an upper limit value and a lower limit value of the electric signal to discriminate the measurement target radiation and the other radiations from each other, a counting part  54  that counts the discriminated electric pulse signals within the range between the upper limit value and the lower limit value, and a radiation determining part  55  that determines based on the count whether or not the measurement target radiation is incident or not. In the following, each component will be described in detail. 
         [0087]    The signal converting part  51  is provided with the scintillator plate  1  at an end thereof, receives the scintillation light emitted by the scintillator plate  1  excited by an incident radiation and converts the scintillation light into an electric signal. According to this embodiment, a photomultiplier tube  51  is used as the signal converting part  51 . The photomultiplier tube  51  is configured to convert the received scintillation light into an electric pulse signal. 
         [0088]    The amplifier  52  is connected to the photomultiplier tube  51  and arbitrarily amplifies the received electric pulse signal. If the electric pulse signal from the photomultiplier tube  51  is strong enough for discrimination, the amplifier  52  can be omitted. 
         [0089]    The radiation discriminating part  53  is connected to the amplifier  52  and sets an upper limit value and a lower limit value to discriminate the measurement target radiation and the other radiations from each other. According to this embodiment, the radiation discriminating part  53  functions as a filter and discriminates the measurement target radiation from the other radiations by transferring the electric pulse signals corresponding to the scintillation light falling within the range between the upper limit value and the lower limit value to the counting part  54 . 
         [0090]    According to this embodiment, in order to discriminate α rays emitted by plutonium from α rays emitted by a radon daughter nuclide, the upper limit value is set to be slightly greater than the value of the energy of the α rays emitted by plutonium. In addition, in order to discriminate from β rays and γ rays emitted by the radon daughter nuclide present in the measurement environment, for example, the lower limit value is set to be slightly greater than the values of the energy of the β rays and γ rays. 
         [0091]    The counting part  54  is connected to the radiation discriminating part  53  and counts the electric pulse signals discriminated by the radiation discriminating part  53 . According to this embodiment, the counting part  54  is configured to count the electric pulse signals at predetermined intervals of the focused light intensity. 
         [0092]    The radiation determining part  55  is connected to the counting part  54  and determines based on the numerical values obtained from the counting part  54  whether the measurement target radiation is incident or not. More specifically, the radiation determining part  55  is configured to set a threshold, compare the count value with the threshold and determine that the radiation is detected if the count value is greater than the threshold. 
         [0093]    Next, a radiation imaging apparatus  6  according to this embodiment will be described. As shown in  FIG. 10 , the radiation imaging apparatus  6  according to this embodiment, which is used in a positron emission tomography apparatus or the like for a nuclear medicine image diagnostic method, mainly comprises a radiation source device  61  that emits an arbitrary radiation, the scintillator plate  1  described above, a scintillation light distribution measuring device  62  that measures the distribution of the scintillation light emitted by the scintillator plate  1  when the radiation emitted by the radiation source device  61  is incident on the scintillator plate  1 , and an output device  63  that outputs image data on the distribution of the scintillation light received from the scintillation light distribution measuring device  62 . The radiation imaging apparatus  6  can be used not only in the positron emission tomography apparatus but also in an apparatus that measures the distribution of contamination by a radioactive material on a surface of an object, for example. 
         [0094]    The radiation source device  61  radially emits an arbitrary radiation. According to this embodiment, the radiation source device  61  is configured to emit X rays. Note that the apparatus that measures the distribution of contamination by a radioactive material does not have to be provided with the radiation source device  61 . 
         [0095]    The scintillation light distribution measuring device  62  is provided with the scintillator plate  1 , is provided at a position where the scintillation light distribution measuring device  62  can receive the radiation emitted by the radiation source device  61 , and is configured to receive the radiation emitted by the radiation source device  61  and measures the distribution of the radiation. The method of measuring the scintillation light distribution is not particularly limited, and a plurality of receivers for detecting the scintillation light are arranged to measure the scintillation light distribution, for example. 
         [0096]    The output device  63  is connected to the scintillation light distribution measuring device  62  and outputs image data on the received scintillation light distribution. Note that the method of outputting the image data is not particularly limited, and the image data can be displayed on a liquid crystal display or the like or output as a picture. 
         [0097]    Next, an effect of each component according to this embodiment will be described. 
         [0098]    When the measurement target radiation is incident on the scintillator plate  1 , the scintillator plate  1  is excited by the radiation to emit scintillation light. For example, in the case where the measurement target is α rays, which are a directly ionizing radiation, an electrical attraction or repulsion between a charged particle of the α rays and an electron in the scintillator  2  causes excitation of the electron in the scintillator. When recombination of the excited electron then occurs, light is emitted. Since the scintillator layer  22  according to this embodiment has a thickness equal to or greater than the range of the α rays, most of the energy of the incident α rays is converted into scintillation light to be emitted. 
         [0099]    The scintillator layer  22  according to this embodiment is formed by a single layer of scintillator powders  21 . As a result, a reflection or diffusion of the scintillation light in the scintillator layer  22  can be effectively suppressed. Even if the scintillator powders  21  do not have so great particle diameters, gaps between the scintillator powders  21  laid adjacent to each other in a single layer are filled with further scintillator powders  21 , so that the filling factor of the scintillator layer  22  formed by the scintillator powders  21  is improved, and the scintillator layer  22  can achieve effects similar to those of a single crystal. 
         [0100]    The photomultiplier tube  51  in the radiation measuring apparatus  5  according to this embodiment receives the scintillation light emitted by the scintillator plate  1  and converts the scintillation light into an electric signal. The amplifier  52  amplifies the electric pulse signal produced by conversion by the photomultiplier tube  51  to an arbitrary magnitude. 
         [0101]    The radiation discriminating part  53  compares the electric pulse signal received from the amplifier  52  with the upper limit value and the lower limit value to determine whether the radiation is the measurement target radiation or not. The counting part  54  counts the scintillation light filtered by the radiation discriminating part  53  at predetermined intervals of the focused light intensity. The radiation determining part  55  determines that the measurement target radiation is detected if the count value for the scintillation light is greater than a predetermined threshold. The result of the radiation measurement is then output by appropriate output means. 
         [0102]    Next, the radiation imaging apparatus  6  according to this embodiment will be described. The radiation source device  61  irradiates the scintillator plate  1  with an arbitrary radiation. An arbitrary object O whose interior is to be imaged, such as a human body, is disposed between the radiation source device  61  and the scintillator plate  1 . When the radiation transmits through the arbitrary object O, if the radiation passes through parts having different transmittances, such as a bone and flesh, the amount of the radiation reaching the scintillator plate  1  varies. 
         [0103]    The scintillator plate  1  emits a variable amount of scintillation light depending on the amount of the radiation having reached the scintillator plate  1 . The scintillation light distribution measuring device  62  measures the distribution of the scintillation light, and the output device  63  outputs an arbitrary form of data. 
         [0104]    According to this embodiment described above, the following advantages are achieved. 
         [0105]    1. Since the scintillator layer  22  is formed by the scintillator powders  21  having a predetermined average particle diameter determined by the kind of the scintillator  2  and the kind of the radiation and has a thickness falling within a predetermined range, the scintillation light that can be discriminated can be obtained with higher probability. 
         [0106]    2. Since the scintillator powders  21  are laid in a single layer to form the scintillator layer  22 , a reflection or diffusion of the scintillation light that would occur in a plurality of layers of scintillator powders  21  can be suppressed. 
         [0107]    3. Since the scintillator layer  22  can be formed by laying the scintillator powders  21  having a predetermined particle diameter and polishing the scintillator powders  21  until a predetermined thickness is achieved, the manufacturing cost of the scintillator plate  1  can be substantially reduced, the size of the scintillator plate  1  can be easily increased, and the scintillator plate  1  can be manufactured with higher precision. 
         [0108]    4. By using the scintillator plate  1  according to this embodiment, the measurement precision of the radiation measuring apparatus  5  and the radiation imaging apparatus  6  can be improved. 
       EXAMPLE 1  
       [0109]    In a example 1, the scintillator plate  1  according to the present invention was manufactured, and α rays were measured using the scintillator plate  1 . 
       Manufacture of Scintillator Plate  
       [0110]    In the example 1, a test scintillator plate  1  was manufactured from the GPS scintillator. The GPS scintillator prepared by crystal synthesis in the TSSG method was pulverized, and the resulting particles were screened with a sieve with a predetermined mesh size to select particles having uniform diameters. The substrate was a 25-mm-square rectangular glass plate as shown in  FIG. 11 , and the scintillator powders  21  were laid on a central part of the glass plate (indicated by the dotted line in  FIG. 11 ) and fixed with the adhesive  4  made of an epoxy resin. 
       Comparison with Conventional Scintillator  
       [0111]    First, α rays were measured with the conventional ZnS scintillator plate according to the invention described in Patent Literature 1, a scintillator plate made of a single crystal GPS scintillator, and the scintillator plate  1  according to the present invention. The ZnS scintillator plate was manufactured by applying the ZnS scintillator to a glass substrate to a thickness of approximately 10 mg/cm 2 , as with the GPS scintillator plate according to this example 1. The single crystal GPS scintillator used has a thickness of approximately 1.4 mm. In the example 1, three scintillator plates  1 , the scintillator layers  22  of which had thicknesses of approximately 100 μm, approximately 50 μm and approximately 30 μm, were prepared by making the scintillator layer  22  with GPS scintillator powders having an average particle diameter d of approximately 50 μm and polishing one surface of the scintillator layer  22 . 
         [0112]    As an alternative to plutonium, americium 241 (241Am) was used as a radiation source to irradiate each plate with α rays of 5.5 MeV, and the relationship between the focused light intensity and the count was measured. 
         [0113]      FIG. 12  is a graph showing the measurement result. For the ZnS scintillator plate, the count distribution curve was smooth with respect to the focused light intensity and showed no sharp peak. This means that the ZnS scintillator plate can be used to measure α rays, but the energy resolution is low, and therefore there still remains a problem that it is difficult to discriminate different kinds of α rays having different amounts of energy emitted from a plurality of nuclides, such as plutonium and a radon daughter nuclide. 
         [0114]    On the other hand, the single crystal GPS scintillator showed a sharp peak, which means that α rays can be measured with high precision. Similarly, the GPS scintillator plates having the thickness of approximately 100 μm and approximately 50 μm according to the present invention each showed a sharp peak, although the peak is lower than that of the single crystal GPS scintillator. The GPS scintillator plate having the thickness of approximately 30 pm according to the example 1 showed no sharp peak. 
         [0115]    In the example 1, more specific experiments concerning the average particle diameter d of the scintillator powders  21  and the thickness of the scintillator were performed. 
       Investigation on Average Particle Diameter of Scintillator Powders  
       [0116]    First, investigation on the average particle diameter d of the scintillator powders  21  and the measurement precision was performed. Three scintillator plates  1  were prepared with scintillator powders  21  having a particle diameter d equal to or greater than approximately 25 μm and equal to or smaller than approximately 45 μm, scintillator powders  21  having a particle diameter d equal to or greater than approximately 45 μm and equal to or smaller than approximately 75 μm, and scintillator powders  21  having a particle diameter d equal to or greater than approximately 75 μm and equal to or smaller than approximately 106 μm. Americium 241 (241Am) was used as a radiation source to irradiate each scintillator plate  1  with α rays of 5.5 MeV, and the relationship between the focused light intensity and the count was measured. 
         [0117]      FIG. 13  is a graph showing the measurement result. In the case where the particle diameter d of the scintillator powders  21  was equal to or greater than approximately 25 μm and equal to or smaller than approximately 45 μm, a peak of the focused light intensity appeared, which means that α rays were detected. The distribution curve of the focused light intensity was smoother than the cases of the other two scintillator plates, which means that the discrimination precision was inferior. In the case where the particle diameter d of the scintillator powders  21  was equal to or greater than approximately 45 μm and equal to or smaller than approximately 75 μm, a peak of the focused light intensity appeared at greater values, and it can be said that the discrimination precision was improved compared with the case where the particle diameter d of the scintillator powders  21  was equal to or greater than approximately 25 μm and equal to or smaller than approximately 45 μm. In the case where the particle diameter d of the scintillator powders  21  was equal to or greater than approximately 75 μm and equal to or smaller than approximately 106 μm, a peak of the focused light intensity appeared at greater values within a narrower range than in the cases of the other two scintillator plates, and it is sufficiently possible that the discrimination precision was improved. 
         [0118]    In short, it can be expected that the discrimination precision at greater values of the focused light intensity improves as the average particle diameter d of the scintillator powders  21  increases. 
       Investigation on Thickness of Scintillator  
       [0119]    Next, investigation on the thickness of the scintillator  2  was performed for measurement of α rays and discrimination of the α rays from γ rays present in the measurement environment. In the example 1, three scintillator plates  1  were manufactured from the scintillator powders  21  having an average particle diameter d of 50 μm, and the surfaces of the scintillator plates  1  were polished so that the scintillators  2  thereof had different thicknesses, 100 μm, 50 μmm and 30 μm. Cesium (Cs) was used as a radiation source to irradiate each scintillator plate  1  with γ rays of 662 keV, and the relationship between the focused light intensity and the count was measured. For comparison, the measurement was performed for a single crystal GPS scintillator having a thickness of approximately 1.4 mm. 
         [0120]      FIG. 14  is a graph showing the measurement result. As shown in  FIG. 14 , in the case of the single crystal GPS scintillator, the focused light intensity was counted over a wide range. When the focused light intensity is counted over such a wide range, it is difficult to discriminate the target radiation from the other radiations. 
         [0121]    On the other hand, in the case of the scintillator plates  1  manufactured in the example 1, the focused light intensity distribution was concentrated in narrow ranges. In particular, the focused light intensity distribution was concentrated in narrower ranges as the thickness of the scintillator  2  decreased from 100 μm to 50 μm and to 30 μm. In short, it can be seen that the smaller the thickness of the scintillator  2 , the more effectively the effect of the γ rays can be suppressed, and the more easily the α rays can be discriminated. 
       Summary of Average Particle Diameter of Scintillator Powders  
       [0122]    Next, the scintillator plate having a thickness of approximately 50 μm was manufactured from the GPS scintillator powders  21  having an average particle diameter d of 50 μm. Americium 241 (241Am) was used as a radiation source to irradiate the scintillator plate with α rays of 5.5 MeV, cesium (Cs) was used as a radiation source to irradiate the scintillator plate with γ rays of 662 keV, and the relationships between the focused light intensity and the count were measured. 
         [0123]      FIG. 15  is a graph showing the measurement results. The scintillator plate  1  in the example 1 was able to detect the α rays and discriminate the α rays from the γ rays. It is considered that the α rays and the γ rays can be discriminated from each other with reliability if the lower limit value and the upper limit value of the focused light intensity for discrimination are set at approximately 100 and approximately 650, respectively. 
       EXAMPLE 2  
       [0124]    Next, a example 2 will be described. In the example 2, an experiment was performed on the effect, on the detection precision, of the difference between the case where the scintillator layer  22  is formed by a single layer of scintillator powders  21  and the case where the scintillator layer  22  is formed by a plurality of layers scintillator powders  21 . 
       Manufacture of Scintillator Plate  
       [0125]    In the example 2, as in the example 1, test scintillator plates  1  were manufactured from the GPS scintillator. The scintillator powders  21  used for the scintillator plates  1  in the example 2 had the following particle diameters. 
         [0126]    (a) d&lt;5 μm 
         [0127]    (b) 25 μm&lt;d&lt;45 μm 
         [0128]    (c) 45 μm&lt;d&lt;75 μm 
         [0129]    (d) 75 μm&lt;d&lt;106 μm 
         [0130]    (e) 75 μm&lt;d&lt;106 μm (fine particles were removed) 
         [0131]    (f) 180 μm&lt;d&lt;212 μm (fine particles were removed) 
         [0132]    (g) 212 μm&lt;d&lt;250 μm (fine particles were removed) 
         [0133]    (h) 250 μm&lt;d&lt;300 μm (fine particles were removed) 
         [0134]    (i) 500 μm&lt;d&lt;600 μm (fine particles were removed) 
         [0135]    For the test pieces (a) to (d), the scintillator powders  21  were screened with a sieve with a predetermined mesh size to have a uniform particle diameter. For the test pieces (e) to (i), after the scintillator powders  21  were screened with a sieve to have a uniform particle diameter, fine particles of the scintillator  2  adhering to the scintillator powders  21  were removed by immersing the scintillator powders  21  in a phosphate solution, agitating the liquid with an ultrasonic cleaning device to separately diffuse the scintillator powders  21  and the fine particles, and removing the supernatant. In the example 2, the substrate  3  was a 25-mm-square rectangular glass plate, as in the example 1. 
         [0136]    With the scintillator plates  1  in the example 2, the scintillator layer  22  was formed by laying the scintillator powders  21  on the substrate  3  and fixing the scintillator powders  21 . With the test pieces (a) to (h), one surface of the scintillator layer  22  was polished to adjust the thickness of the scintillator layer  22  to approximately 100 μm. With the test piece (i), both surfaces of the scintillator layer  22  were polished to adjust the thickness of the scintillator layer  22  to approximately 100 μm. 
         [0137]    In the example 2, the scintillator layers  22  of the scintillator plates  1  of the test pieces (a) to (c) were formed by laying the scintillator powders  21  in a plurality of layers, and the scintillator layers  22  of the scintillator plates  1  of the test pieces (d) to (i) were formed by laying the scintillator powders  21  in a single layer. 
       Investigation on Scintillator Layers with Single Layer Structure and Scintillator Layers with Multilayer Structure  
       [0138]    In the example 2, as in the example 1, as an alternative to plutonium, americium 241 (241Am) was used as a radiation source to irradiate each plate with α rays of 5.5 MeV, and the relationship between the focused light intensity and the count was measured.  FIG. 16  is a graph showing the measurement results of the scintillator plates of the test pieces (a) to (e) and the measurement result of a scintillator plate of a GPS scintillator single crystal as a comparative example. 
         [0139]    As shown in  FIG. 16 , in the case where the GPS scintillator single crystal was used, a count peak appeared in the vicinity of the focused light intensity of 600. 
         [0140]    On the other hand, with the scintillator plate  1  made of the scintillator powders  21  of the test piece (a), little signals were detected at focused light intensities equal to or higher than 200, and α rays having different amounts of energy emitted by a plurality of nuclides was not able to be discriminated from each other. This is probably because even if the scintillator layer  22  has a thickness equal to or greater than a predetermined value, sufficient scintillator light cannot be emitted if the particle diameter of the scintillator powders  21  is smaller than the range. 
         [0141]    With the scintillator plates  1  made of the scintillator powders  21  of the test pieces (b) to (d), the peak became more recognizable as the particle diameter increases. Comparing the measurement results of the scintillator plates  1  made of the scintillator powders  21  of the test pieces (d) and (e) having the same particle diameter, the peak was more obvious in the case of the test piece (e) from which fine particles were removed. These are probably because, with the test piece (e), the scintillator powders  21  were laid in a single layer, and fine particles were removed from the scintillator powders  21 , so that the scintillator powders  21  were not overlaid one on another, and the scintillation light was more likely to reach the photomultiplier tube  51 , while for the test pieces (b) to (d), the scintillator powders  21  were overlaid one on another, so that a diffuse reflection of the scintillation light occurred, and the amount of scintillation light reaching the photomultiplier tube  51  decreased. 
         [0142]    Next, the measurement results of the scintillator plates  1  made of the scintillator powders  21  of the test pieces (a) to (i) were compared in terms of the inverse of the resolution and the peak-to-total ratio calculated.  FIG. 17  is a graph showing the peak-to-total ratio and the inverse of the resolution based on the measurement results in the example 2. 
         [0143]    In the example 2, as shown in  FIGS. 17 and 18 , the inverse of the resolution was calculated as a ratio between the apparent maximum value of the peak of the count and the apparent half-width at the apparent maximum value of the peak. That is, the greater the inverse of the resolution shown on the longitudinal axis of  FIG. 17 , the sharper the peak becomes, and the higher the energy discriminating capability becomes. 
         [0144]    In the example 2, the peak-to-total ratio means the value of the count at the peak with respect to the count for the entire focused light intensity range. That is, the higher the peak-to-total ratio, the count in the tail part in  FIG. 18  decreases, the count at the peak increases, and the energy discriminating capability becomes higher. 
         [0145]    As shown in  FIG. 17 , in the case of the single crystal scintillator plate, both the inverse of the resolution and the peak-to-total ratio are high, which shows that the energy discriminating capability is high. 
         [0146]    In the cases of the test pieces (b) and (c) in which the scintillator layer had a multilayer structure, the inverse of the resolution and the peak-to-total ratio were low, specifically, the inverse of the resolution was approximately a tenth of the value of the single crystal, the peak-to-total ratio was approximately a fifth of the value of the single crystal, and the energy discriminating capability was low. 
         [0147]    On the other hand, in the cases of the test pieces (d) to (i) in which the scintillator layer had a single layer structure, the inverse of the resolution and the peak-to-total ratio were improved compared with the test pieces (b) and (c). For example, with the test pieces (d) to (i), the inverse of the resolution was approximately 60% of the value of the single crystal, and a sufficient energy discriminating capability was maintained. The efficiency of detection of the peak-to-total ratio tended to improve as the average particle diameter d of the scintillator powders  21  increased. With the test piece (i), the energy discriminating capability was maintained at approximately 70% of that in the case of the single crystal. 
         [0148]    Next, a picture of the scintillator layer  22  of each scintillator plate  1  was taken with an optical microscope, and the difference in precision was investigated based on the images.  FIGS. 19 to 21  are optical microscopic pictures of the scintillator layers  22  of the scintillator plates  1  made of the scintillator powders  21  of the test pieces (d), (e) and (i), respectively. 
         [0149]    As shown in the optical microscopic picture of  FIG. 19 , before the fine particles were removed, the fine particles were randomly fitted in the gaps between the scintillator powders  21 , and a diffuse reflection of light occurred. On the other hand, as shown in  FIG. 20 , in the case of the test piece (e) in which the fine particles were removed, the predetermined scintillator powders  21  were laid substantially in a single layer, and a reflection or diffusion of light was suppressed. 
         [0150]    As shown in  FIG. 21 , in the case where the scintillator layer  22  was made of relatively large scintillator powders  21 , and both surfaces of the scintillator layer  22  were polished, each scintillator powder  21  had a plate-like shape. As a result, it can be considered that the scintillator powders  21  had surfaces parallel with the detection surface of the photomultiplier tube  51 , and a diffuse reflection of the scintillation light was less likely to occur. The gaps between the scintillator powders  21  were reduced, and the result of the image processing showed that the filling factor was as high as approximately 70%, and therefore, it can be said that the scintillator layer  22  had a structure similar to the single crystal. As shown in  FIG. 17 , in the example  2 , the scintillator plate  1  made of the scintillator powders  21  of the test piece (i) achieved a detection efficiency of 70%. 
       Summary of Scintillator Layer with Single Layer Structure and Scintillator Layer with Multilayer Structure  
       [0151]    As can be seen from the above description, the energy discriminating capability of the scintillator powders  21  forming the scintillator layer  22  is improved by suppressing the diffuse reflection of the scintillation light. To this end, it is essential that the scintillator powders are not overlaid one on another, and it can be said that the scintillator powders laid in a single layer have a higher energy discriminating capability than the scintillator powders laid in a plurality of layers. Furthermore, if scintillator powders  21  having a larger average particle diameter are laid in a single layer and polished to a predetermined thickness, the scintillator powders  21  have more surfaces parallel with the detection surface of the photomultiplier tube  51 , and the ratio of the scintillator exposed on the surface of the plate increases. As a result, the energy of the measurement target α rays is imparted to the scintillator  3  without loss. It can be considered that the high energy discriminating capability is achieved a result of that. In addition, it can be seen that the higher the filling factor of the scintillator layer  22  formed by the scintillator powders  21 , the detection efficiency improves. 
       EXAMPLE 3  
       [0152]    In a example 3, the radiation imaging apparatus  6  used as an apparatus that measures the distribution of contamination by a radioactive material on a surface of an object was fabricated, and the radioactivity distribution was measured.  FIG. 22  shows an appearance of the fabricated radiation imaging apparatus  6 . 
         [0153]    As shown in  FIG. 22 , the radiation imaging apparatus  6  in the example 3 comprised the scintillation light distribution measuring device  62  provided with the scintillator plate  1 , and the output device  63  that outputs image data on the distribution. 
         [0154]    The scintillator plate  1  used in the example 3 was a GPS scintillator plate. The scintillation light distribution measuring device  62  used was a multichannel photomultiplier tube (manufactured by Hamamatsu Photonics K.K.), and 64 (8 by 8) measurement points were set in a 5 cm-by-5 cm substantially square detection area. 
         [0155]    In the example 3, an α rays emitting nuclide was appropriately sprayed on the measurement surface, the multichannel photomultiplier tube was set to face the measurement surface, and the distribution of the radiations emitted from the α rays emitting nuclide was measured by measuring the pulse height spectra of the α rays at all the measurement points at the same time. 
         [0156]      FIG. 23  is a diagram showing the measured radioactivity intensity distribution, in which the values between the measurement points were linearly calculated from the radioactivity intensities at the measurement points measured with the scintillation light distribution measuring device  62  and expressed in a 256 by 256 data matrix. The level of the lines in the graph represents the radioactivity intensity. 
         [0157]    As shown in  FIG. 23 , the radioactivity intensity is low at some points and high at other points, and the parts in which there is the α rays emitting nuclide and the parts in which there is not the α rays emitting nuclide can be clearly discriminated. Therefore, even if there is a radiation source that emits a radiation that is not the measurement target, the radiation can be adequately discriminated. 
         [0158]    As described above, in the example 3, the radiation distribution measurement is sufficiently feasible if the scintillator plate  1  according to the present invention is used. This also shows the feasibility of increasing the size of the scintillator plate  1 . 
       EXAMPLE 4  
       [0159]    In a example 4, a neutron beam was measured. The scintillators used were a lithium(Li)-6 scintillator, a ZnS scintillator and the GPS scintillator according to the present invention. Using these scintillators, the measurement was performed with a neutron imaging system incorporating a resistance distributed position detecting photomultiplier tube. Note that the Li-6 glass scintillator is a material known for its high detection precision. 
         [0160]    In the example 4, as shown in  FIG. 24 , an F-character-shaped plate made of cadmium as a measurement sample was set and irradiated with a neutron beam, and the distribution of the neutron beams at the rear of the plate was measured. Note that cadmium has a property to absorb neutrons. 
         [0161]      FIG. 25  shows the result of the measurement using the ZnS scintillator. As shown in  FIG. 25 , the cadmium absorbed neutrons, so that no neutrons were measured and the character “F” clearly appeared at the rear of the cadmium. 
         [0162]    Since the plates had different shapes and sizes, the corresponding areas of the plates extracted from the images of the plates were then compared.  FIG. 26  shows the measurement result in the case where the Li-6 glass scintillator was used,  FIG. 27  shows the measurement result in the case where the ZnS scintillator was used, and  FIG. 28  shows the measurement result in the case where the GPS scintillator was used. In  FIGS. 26 to 28 , the total number of the neutrons in each area was counted. 
         [0163]    The total number of neutrons in the case where the Li-6 glass scintillator was used was 114319, which was the largest of all the cases. On the assumption that the total number in the case where the Li-6 glass scintillator was used was 100, the relative total numbers of neutrons in the cases where the other scintillators were used was determined. Then, the total number in the case where the ZnS scintillator was used was approximately 30, and the total number in the case where the GPS scintillator was used was approximately 81. 
         [0164]    From the above description, it is proved that the scintillator plate  1  according to the present invention can be used to measure the neutron beam. In addition, the scintillator plate  1  according to the present invention is also effective for measurement for radiation imaging. In addition, the GPS scintillator according to the present invention has a higher detection precision than the ZnS scintillator. Furthermore, the GPS scintillator according to the present invention has a high precision that is approximately 81% of that of the Li-6 glass scintillator, which is known for its high detection precision. Thus, the scintillator plate according to the present invention has high practicality not only in terms of cost but also in terms of precision. 
       REFERENCE SIGNS LIST  
       [0165]      1  scintillator plate 
         [0166]      2  scintillator 
         [0167]      3  substrate 
         [0168]      4  adhesive 
         [0169]      5  radiation measuring apparatus 
         [0170]      6  radiation imaging apparatus 
         [0171]      21  scintillator powder 
         [0172]      51  signal converting part (photomultiplier tube) 
         [0173]      52  amplifier 
         [0174]      53  radiation discriminating part 
         [0175]      54  counting part 
         [0176]      55  radiation determining part 
         [0177]      61  radiation source device 
         [0178]      62  scintillation light distribution measuring device 
         [0179]      63  output device