Patent Number: 
Section: claims

1. A radiation detector, comprising:a scintillator layer having multiple scintillator crystals arranged in two dimensions; anda light receiving element including a plurality of pixels arranged in two dimensions for detecting scintillation light from the scintillator layer, wherein:the multiple scintillator crystals each comprise two crystal phases, which are:(i) a first crystal phase including a plurality of columnar crystals having unidirectionality and having a refractive index n1, and(ii) a second crystal phase, which is between the plurality of columnar crystals and which has a refractive index n2; anda material having a refractive index n3 is disposed between adjacent said scintillator crystals, and the refractive indices n1, n2 and n3 satisfy a relationship of one of n1≦n3≦n2 and n2≦n3≦n1. 2. The radiation detector according to claim 1, wherein a crystal phase having a higher refractive index of the first crystal phase and the second crystal phase functions as a scintillator. 3. The radiation detector according to claim 2, wherein the first crystal phase is a crystal phase comprising NaCl as a main component, and the second crystal phase is a crystal phase comprising CsI as a main component. 4. The radiation detector according to claim 1, wherein the scintillator crystal has a phase separation structure. 5. The radiation detector according to claim 4, wherein the phase separation structure of the multiple scintillator crystal comprises a structure in which the first crystal phase is formed in the second crystal phase. 6. The radiation detector according to claim 5, wherein any one of the first crystal phase and the second crystal phase having a higher refractive index comprises a scintillator material which emits light when being excited by radiation. 7. The radiation detector according to claim 4, wherein a material forming the first crystal phase and a material forming the second crystal phase are capable of forming a eutectic structure. 8. The radiation detector according to claim 4, wherein a material forming the first crystal phase and a material forming the second crystal phase are a eutectic composition. 9. The radiation detector according to claim 1, wherein the scintillator crystal has a phase separation structure including the two crystal phases, one crystal phase having a refractive index lower than a refractive index of another crystal phase, and has a first principal surface and a second principal surface which are not located on a same surface, the another crystal phase being exposed on part of the first principal surface and on part of the second principal surface, the part of the another crystal phase exposed on the first principal surface and the part of the another crystal phase exposed on the second principal surface being connected to each other. 10. The radiation detector according to claim 9, wherein the one crystal phase as a lower refractive index crystal phase of the two crystal phases also has a part exposed on the first principal surface and a part exposed on the second principal surface, and the exposed parts are connected to each other. 11. The radiation detector according to claim 9, wherein the one crystal phase as a lower refractive index crystal phase of the two crystal phases is located within the another crystal phase as a higher refractive index crystal phase. 12. The radiation detector according to claim 1, wherein the scintillator crystal has a waveguiding function. 13. The radiation detector according to claim 1, wherein the scintillator crystals are arranged in two dimensions by tiling multiple scintillator crystals processed to have a predetermined shape. 14. The radiation detector according to claim 1, wherein the material having the refractive index n3 is selected from a group consisting of an epoxy resin, a melamine resin, a polystyrene resin, a vinylidene chloride resin, and a polycarbonate resin. 15. The radiation detector according to claim 1, wherein the scintillator crystal comprises NaCl—CsI phase separation structure scintillator crystal. 16. The radiation detector according to claim 1, wherein the light receiving element includes a plurality of detection regions for each of which illuminance is obtained, and wherein a width of the each of the plurality of detection regions is smaller than a clearance between the multiple scintillator crystals. 17. The radiation detector according to claim 16, wherein at least one of the plurality of detection regions does not overlap any of scintillators in a direction perpendicular to a light receiving surface and is covered with the material having the refractive index n3. 18. The radiation detector according to claim 1,wherein the scintillator has a first surface and a second surface, which are not located on a same plane,wherein the first surface and the second surface each have a portion to which the second crystal phase is exposed, andwherein a portion exposed to the first surface and a portion exposed to the second surface of the second crystal phase are connected. 19. The radiation detector according to claim 1, wherein the plurality of columnar crystals extend in a direction perpendicular to a light receiving surface of the light receiving element. 20. The radiation detector according to claim 1, wherein the second crystal phase is filled between the plurality of columnar crystals.