ILLUMINANT AND RADIATION DETECTOR

Abstract

An illuminant has a short fluorescence lifetime, high transparency, and high light yield and a radiation detector uses the illuminant. The illuminant is appropriate for a radiation detector for detecting gamma-rays, X-rays, -rays, and neutron rays, and has high radiation resistance, a short fluorescence decay time and high emission intensity. The illuminant has a garnet structure using emission from the 4f5d level of Ce.sup.3+, and includes a garnet illuminant prepared by co-doping of at least one type of monovalent or divalent cation at a molar ratio of 7000 ppm or less with respect to all cations, to an illuminant having a garnet structure represented by general formula Ce.sub.xRE.sub.3xM.sub.5+yO.sub.12+3y/2 (where 0.0001x0.3, 0y0.5 or 0y0.5, M is one type or two or more types selected from Al, Lu, Ga, and Sc, and RE is one type or two or more types selected from La, Pr, Gd, Tb, Yb, Y, and Lu).

Claims

1. An illuminant, including a garnet illuminant prepared by co-doping of at least one type of monovalent or divalent cation at a molar ratio of 7000 ppm or less with respect to all cations, to an illuminant having a garnet structure represented by general formula Ce.sub.xRE.sub.3xM.sub.5+yO.sub.12+3y/2 (where 0.0001x0.3, 0y0.5 or 0y0.5, M is one type or two or more types selected from Al, Lu, Ga, and Sc, and RE is one type or two or more types selected from La, Pr, Gd, Tb, Yb, Y, and Lu).

2. An illuminant, including a garnet illuminant prepared by co-doping of Li at a molar ratio of 7000 ppm or less with respect to all cations, to an illuminant having a garnet structure represented by general formula Ce.sub.xRE.sub.3xM.sub.5+yO.sub.12+3y/2 (where, 0.0001x0.3, 0y0.5 or 0y0.5, M is one type or two or more types selected from Al, Lu, Ga, and Sc, and RE is one type or two or more types selected from La, Pr, Gd, Tb, Yb, Y, and Lu).

3. An illuminant, including a garnet illuminant prepared by co-doping of Mg at a molar ratio of 7000 ppm or less with respect to all cations, to an illuminant having a garnet structure represented by general formula Ce.sub.xRE.sub.3xM.sub.5+yO.sub.12+3y/2 (where 0.0001x0.3, 0<y0.5 or 0<y0.5, and RE is one type or two or more types selected from Y and Lu).

4. An illuminant, including a garnet illuminant prepared by co-doping of Li or Mg at a molar ratio of 7000 ppm or less with respect to all cations, to an illuminant having a garnet structure represented by general formula Ce.sub.xGd.sub.3x(Ga.sub.zAl.sub.1z).sub.5+yO.sub.12+3y/2 (where 0.0001x0.3, 0<y0.5 or 0<y0.5, 0.49z0.7).

5. The illuminant according to claim 1, comprising a transparent body that is obtained by heating a raw material at 1000 C. or higher, has a light yield of 20000 photons/MeV or more and a time resolution of 300 ps or less, contains a 0.5% or less phosphorescence component, and has a diffuse transmittance of 80% or more.

6. An illuminant, including a garnet illuminant prepared by co-doping of at least one type of monovalent or divalent cation at a molar ratio of 7000 ppm or less with respect to all cations, to an illuminant having a garnet structure represented by general formula Gd.sub.3x-zCe.sub.xRE.sub.zM.sub.5O.sub.12 (where 0.0001x0.1, 0z<3, M is one type or two or more types selected from Al, Lu, Ga, and Sc, and RE is one type or two or more types selected from La, Pr, Tb, Yb, Y, and Lu).

7. An illuminant, including a garnet illuminant prepared by co-doping of at least one type of monovalent or divalent cation at a molar ratio of 7000 ppm or less with respect to all cations, to an illuminant having a garnet structure represented by general formula Gd.sub.3x-zCe.sub.xRE.sub.zM.sub.5+yO.sub.12+3y/2 (where 0.0001x0.1, 0<y<0.5 or 0<y<0.5, 0z<3, M is one type or two or more types selected from Al, Lu, Ga, and Sc, and RE is one type or two or more types selected from La, Pr, Tb, Yb, Y, and Lu).

8. The illuminant according to claim 6, which is co-doped with Mg as said cation.

9. The illuminant according to claim 6, comprising a transparent body that is obtained by heating a raw material at 1000 C. or higher, and has a light yield of 40000 photons/MeV or more and a time resolution of 240 ps or less.

10. An illuminant, having a garnet structure represented by general formula Ce.sub.xRE.sub.3xM.sub.5+yO.sub.12+3y/2 (where 0.0001x0.3, 0<y0.5 or 0<y0.5, M is one type or two or more types selected from Al, Lu, Ga, and Sc, and RE is one type or two or more types selected from La, Pr, Gd, Tb, Yb, Y, and Lu).

11. An illuminant, which is produced by, after production of the illuminant according to claim 1, annealing the illuminant at 1000 C. or higher in an atmosphere containing oxygen or an inert gas atmosphere.

12. The illuminant according to claim 1, which has etch pits on a surface as a result of etching treatment involving immersion in an etchant containing phosphoric acid, and has a nonglossy surface having normal incidence reflectivity of 8.5% or less.

13. The illuminant according to claim 1, which is a single crystal.

14. A radiation detector, having an illuminant that absorbs radiation such as -rays, X-rays, -rays, and neutron rays and high energy photons and emits light, and a photoreceiver that detects the emission from the illuminant, wherein said illuminant is the illuminant according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] FIG. 1 is a graph showing voltage pulse signals obtained using a digital oscilloscope when an illuminant of an embodiment of the present invention, Gd.sub.2.985Ce.sub.0.0015Ga.sub.3Al.sub.2O.sub.12 prepared by co-doping of Li at 1500 ppm, and a crystal not-co-doped with Li were irradiated with a .sup.137Cs gamma-ray.

[0045] FIG. 2 is a graph showing voltage pulse signals obtained using a digital oscilloscope when an illuminant of an embodiment of the present invention, Gd.sub.2.985Ce.sub.0.0015Ga.sub.3Al.sub.2O.sub.12 prepared by co-doping of Mg at 1500 ppm and a crystal not-co-doped with Mg were irradiated with a .sup.137Cs gamma-ray.

[0046] FIG. 3 shows (a) an optical microscopic photograph of the surface of a crystal after etching treatment of an illuminant of an embodiment of the present invention, Gd.sub.2.985Ce.sub.0.0015Ga.sub.3Al.sub.2O.sub.12 prepared by co-doping of Mg at 300 ppm, and (b) an optical microscopic photograph of the surface of the crystal before etching treatment.

MODES FOR CARRYING OUT THE INVENTION

[0047] The embodiments of the present invention are described as follows.

[0048] The illuminants of the embodiments of the present invention include a garnet illuminant prepared by co-doping of at least one type of monovalent or divalent cation at a molar ratio of 7000 ppm or less with respect to all cations, to an illuminant having a garnet structure represented by general formula Ce.sub.xRE.sub.3xM.sub.5+yO.sub.12+3y/2 (where 0.0001x0.3, 0y0.5 or 0y0.5, M is one type or two or more types selected from Al, Lu, Ga, and Sc, and RE is one type or two or more types selected from La, Pr, Gd, Tb, Yb, Y, and Lu).

[0049] The illuminants of the embodiments of the present invention have short fluorescence decay times, short rise times of emission, high emission intensity, high radiation resistance, high light yields, and few phosphorescence components.

[0050] Among the illuminants of the embodiments of the present invention, the first illuminant of the embodiments of the present invention is an illuminant having a garnet structure in which emission from the 4f5d level of Ce.sup.3+ is used, and includes a garnet illuminant prepared by co-doping of Li at a molar ratio of 7000 ppm or less with respect to all cations, to a illuminant having a garnet structure represented by general formula Ce.sub.xRE.sub.3xM.sub.5+yO.sub.12+3y/2 (where 0.0001x0.3, 0y0.5 or 0y0.5, M is one type or two or more types selected from Al, Lu, Ga, and Sc, and RE is one type or two or more types selected from La, Pr, Gd, Tb, Yb, Y, and Lu).

[0051] Furthermore, the second illuminant of the embodiments of the present invention is an illuminant having a garnet structure in which emission from the 4f5d level of Ce.sup.3+ is used, and has a garnet structure represented by general formula Ce.sub.xRE.sub.3xM.sub.5+yO.sub.12+3y/2 (where 0.0001x0.3, 0<y0.5 or 0<y0.5, M is one type or two or more types selected from Al, Lu, Ga, Sc, and RE is one type or two or more types selected from La, Pr, Gd, Tb, Yb, Y, and Lu). In this case, in particular, compared to phosphor with y=0 or 1, the illuminant has a short fluorescence decay time, a short rise time of emission, high emission intensity, high radiation resistance, a high light yield, and few phosphorescence components.

[0052] Furthermore, the third illuminant of the embodiments of the present invention is an illuminant having a garnet structure in which emission from the 4f5d level of Ce.sup.3+ is used, and includes a garnet illuminant prepared by co-doping of at least one type of monovalent or divalent cation at a molar ratio of 7000 ppm or less with respect to all cations, to an illuminant having a garnet structure represented by general formula Gd.sub.3x-zCe.sub.xRE.sub.zM.sub.5O.sub.12 (where 0.0001x0.1, 0z<3, M is one type or two or more types selected from Al, Lu, Ga, and Sc, and RE is one type or two or more types selected from La, Pr, Tb, Yb, Y, and Lu).

[0053] Furthermore, the fourth illuminant of the embodiments of the present invention is an illuminant having a garnet structure in which emission from the 4f5d level of Ce.sup.3+ is used, and includes a garnet illuminant prepared by co-doping of at least one type of monovalent or divalent cation at a molar ratio of 7000 ppm or less with respect to all cations, to an illuminant having a garnet structure represented by general formula Gd.sub.3x-zCe.sub.xRE.sub.zM.sub.5+yO.sub.12+3y/2 (where 0.0001x0.1, 0<y<0.5 or 0<y<0.5, 0z<3, M is one type or two or more types selected from Al, Lu, Ga, and Sc, and RE is one type or two or more types selected from La, Pr, Tb, Yb, Y, and Lu).

[0054] The illuminants of the embodiments of the present invention are produced by a method for producing single crystals using a micro-pulling down method, for example. In addition, a micro-pulling down method is described as a method for producing the illuminants of the embodiments of the present invention, but the production method is not limited thereto.

[0055] The micro-pulling down method is performed using a controlled atmosphere micro-pulling down apparatus using high-frequency induction heating. The micro-pulling down apparatus is composed of single crystal production equipment provided with a crucible, a seed holder for holding a seed that is brought into contact with melt flowing out from pores provided on the bottom of the crucible, a moving mechanism for moving the seed holder downward, a moving speed control device of the moving mechanism, and a means for induction heating for heating the crucible. With the use of such single crystal production equipment, a single crystal can be prepared by forming solid-liquid interface immediately below the crucible and then moving the seed crystal downward.

[0056] The crucible is composed of carbon, platinum, iridium, rhodium, rhenium, tungsten, molybdenum or an alloy thereof, wherein an after heater, that is a heating element made of carbon, platinum, iridium, rhodium, rhenium, tungsten, molybdenum, or an alloy thereof is arranged on the periphery of the bottom of the crucible. The crucible and the after heater allow the adjustment of a heating value by power conditioning of a means for induction heating, so as to enable the control of the temperature and the distribution of a solid-liquid interface region of the melt that is pulled out from pores provided on the bottom of the crucible.

[0057] The chamber material is SUS and the material for the aperture is quartz. The micro-pulling down apparatus enables controlled atmosphere, and thus is provided with a rotary pump and is configured to be able to regulate the degree of vacuum at 110.sup.3 Torr or less before gas substitution. The apparatus is also configured so that Ar, N.sub.2, H.sub.2, and O.sub.2 gases, for example, can be introduced into the chamber at a flow rate precisely adjusted by an accompanying gas flow meter.

[0058] The use of this apparatus involves introducing raw materials weighed and mixed to a target composition at the time of melt formation into the crucible, evacuating the furnace via high-vacuum evacuation, introducing an Ar gas or a mixed gas of an Ar gas and an O.sub.2 gas into the furnace, so as to create an inert gas atmosphere or an atmosphere with low oxygen partial pressure within the furnace, gradually applying high-frequency power to the means for induction heating, so as to heat the crucible, and then completely melting raw materials within the crucible. In addition, raw materials are preferably composed of at least a 99.99% (4N or more) high-purity raw material, and contain impurities other than those of the target composition in concentrations as low as possible (e.g., 1 ppm or less).

[0059] After melting raw materials, a seed crystal is gradually ascended at a predetermined rate, so that the tip is brought into contact with pores at the lower end of the crucible for the seed crystal to be sufficiently fitted to the pores. Subsequently, a pulling down shaft of a seed holder is descended while adjusting the temperature of the melt, thereby growing the crystal. In addition, a seed crystal that is preferably used herein has a structure and a composition equivalent to or similar to those of an object for crystal growth, but is not limited thereto. Moreover, a seed crystal with clear orientation is preferably used herein.

[0060] The time when the prepared raw materials are all crystallized and the melt disappears is regarded as the completion of crystal growth. In addition, an instrument for continuous charge of raw materials may be incorporated in order to keep the uniform composition and create a long size.

[0061] A radiation detector of the embodiments of the present invention is composed in combination of the illuminants of the embodiments of the present invention comprising a scintillator crystal and a photoreceiver. The radiation detectors of the embodiments of the present invention can also be used as radiation detectors for radiation inspecting apparatuses.

[0062] Examples of such a radiation inspecting apparatus include detectors for resource survey, detectors for high energy physics, environmental radioactivity detectors, gamma cameras, and medical image processing apparatuses. Examples of medical image processing apparatuses are appropriate for applications including a positron emission tomograph (PET), X-ray CT, and SPECT, for example. Preferred embodiments of PET include two-dimensional PET, three-dimensional PET, Time-of-Flight (TOF) PET, and depth of interaction (DOT) PET. Moreover, these PETs may be used in combination.

[0063] In the radiation detectors of the embodiments of the present invention, as a photoreceiver, a position sensitive photomultiplier tube (PS-PMT), a silicon photomultiplier (Si-PM) photodiode (PD), or an avalanche-photodiode (APD) can be used, for example.

[0064] Examples of the illuminants of the embodiments of the present invention are described in detail as follows with reference to drawings, but are not intended to limit the present invention. In addition, in the following examples, Ce and monovalent or divalent cations to be used for co-doping were specified with concentrations in crystal or concentrations in a melt (preparation), however, there was a relationship in each example such that the ratio of a concentration in a crystal to a concentration upon preparation is about 1:1-100.

[0065] Moreover, in each example, time resolution was measured as follows. First, a transparent illuminant in each example was processed and polished into a size of 33 mm. Two illuminants of which were adhered using an optical adhesive to two Si-PMs arranged about 5 cm away and facing each other, and then surfaces other than adhesion surfaces were covered with Teflon (registered trademark) tape. Next, a .sup.22N a gamma-ray source was installed at the center between the two scintillator single crystals (illuminants), so that each illuminant was irradiated with 511 keV gamma-rays simultaneously emitted from the .sup.22N a gamma-ray source at an angle of about 180 as a result of -ray decay. Fluorescence of each illuminant resulting from gamma-ray irradiation was measured by a coincidence method using a digital oscilloscope, thereby measuring a time resolution.

Example 1

[0066] According to a micro-pulling down method, garnet scintillator single crystals having a composition of Gd.sub.2.985Ce.sub.0.015Ga.sub.3Al.sub.2O.sub.12 were prepared by co-doping of Li at 300, 1500, and 3000 ppm. These single crystals had a diameter of 3 mm and a length of 15 mm, and were yellowish transparent. Emission from the 4f5d level of Ce.sup.3+ was confirmed at wavelengths in the vicinity of 520 nm. The diffuse transmittance at 520 nm was 91% per cm.

Example 2

[0067] According to the micro-pulling down method, garnet scintillator single crystals having a composition of Lu.sub.2.985Ce.sub.0.015Al.sub.5O.sub.12 were prepared by co-doping of Li at 300, 1500, and 3000 ppm. These single crystals had a diameter of about 3 mm and a length of about 15 mm, and were yellowish transparent. Emission from the 4f5d level of Ce.sup.3+ was confirmed at wavelengths in the vicinity of 480 nm. The diffuse transmittance at 480 nm was 90% per cm.

Example 3

[0068] According to the micro-pulling down method, garnet scintillator single crystals having a composition of Y.sub.2.985Ce.sub.0.015Al.sub.5O.sub.12 were prepared by co-doping of Li at 300, 1500, and 3000 ppm. These single crystals had a diameter of about 3 mm and a length of about 15 mm, and were yellowish transparent. Emission from the 4f5d level of Ce.sup.3+ was confirmed at wavelengths in the vicinity of 480 nm. The diffuse transmittance at 480 nm was 90% per cm.

Comparative Example 1

[0069] According to the micro-pulling down method, a non-co-doped garnet scintillator single crystal having a composition of Gd.sub.2.985Ce.sub.0.015Ga.sub.3Al.sub.2O.sub.12 was prepared. The thus obtained single crystal had a diameter of about 3 mm and a length of about 15 mm, and was yellowish transparent. Emission from the 4f5d level of Ce.sup.3+ was confirmed at wavelengths in the vicinity of 520 nm. The diffuse transmittance at 520 nm was 92% per cm.

Comparative Example 2

[0070] According to the micro-pulling down method, a garnet scintillator single crystal having a composition of Gd.sub.2.985Ce.sub.0.015Ga.sub.3Al.sub.2O.sub.12 was prepared by co-doping of Li at 20000 ppm. The thus obtained single crystal had a diameter of about 3 mm and a length of about 15 mm, and was yellowish transparent. Emission from the 4f5d level of Ce.sup.3+ was confirmed at wavelengths in the vicinity of 520 nm. The diffuse transmittance at 520 nm was 70% per cm. The emission intensity decreased by 40% compared to the non-co-doped crystal of Comparative example 1.

Comparative Example 3

[0071] According to the micro-pulling down method, a non-co-doped garnet scintillator single crystal having a composition of Lu.sub.2.985Ce.sub.0.015Al.sub.5O.sub.12 was prepared. This single crystal had a diameter of about 3 mm and a length of about 15 mm, and was yellowish transparent. Emission from the 4f5d level of Ce.sup.3+ was confirmed at wavelengths in the vicinity of 480 nm. The diffuse transmittance at 480 nm was 90% per cm.

Comparative Example 4

[0072] According to the micro-pulling down method, a non-co-doped garnet scintillator single crystal having a composition of Y.sub.2.985Ce.sub.0.015Al.sub.5O.sub.12 was prepared. This single crystal had a diameter of about 3 mm and a length of about 15 mm, and was yellowish transparent. Emission from the 4f5d level of Ce.sup.3+ was confirmed at wavelengths in the vicinity of 480 nm. The diffuse transmittance at 480 nm was 90% per cm.

Example 4

[0073] According to the micro-pulling down method, a garnet scintillator single crystal having a composition of Gd.sub.2.985Ce.sub.0.015Ga.sub.3.15Al.sub.2.1O.sub.12.375 was prepared by co-doping of Li at 300 ppm. This single crystal had a diameter of about 3 mm and a length of about 15 mm, and was yellowish transparent. Emission from the 4f5d level of Ce.sup.3+ was confirmed at wavelengths in the vicinity of 520 nm. The diffuse transmittance at 520 nm was 92% per cm.

Example 5

[0074] According to the micro-pulling down method, a garnet scintillator single crystal having a composition of Gd.sub.2.985Ce.sub.0.015Ga.sub.2.85Al.sub.1.9O.sub.11.625 was prepared by co-doping of Li at 300 ppm. This single crystal had a diameter of about 3 mm and a length of about 15 mm, and was yellowish transparent. Emission from the 4f5d level of Ce.sup.3+ was confirmed at wavelengths in the vicinity of 520 nm. The diffuse transmittance at 520 nm was 91% per cm.

Example 6

[0075] According to the micro-pulling down method, a garnet scintillator single crystal having a composition of Gd.sub.2.985Ce.sub.0.015Ga.sub.3.15Al.sub.2.1O.sub.12.375 was prepared. This single crystal had a diameter of about 3 mm and a length of about 15 mm, and was yellowish transparent. Emission from the 4f5d level of Ce.sup.3+ was confirmed at wavelengths in the vicinity of 520 nm. The diffuse transmittance at 520 nm was 91% per cm.

Example 7

[0076] According to the micro-pulling down method, a garnet scintillator single crystal having a composition of Gd.sub.2.985Ce.sub.0.015Ga.sub.2.85Al.sub.1.9O.sub.11.625 was prepared. This single crystal had a diameter of about 3 mm and a length of about 15 mm, and was yellowish transparent. Emission from the 4f5d level of Ce.sup.3+ was confirmed at wavelengths in the vicinity of 520 nm. The diffuse transmittance at 520 nm was 90% per cm.

Example 8

[0077] Gd.sub.2.985Ce.sub.0.015Ga.sub.3Al.sub.2O.sub.12 co-doped with Li at 300 ppm of those in Example 1 was subjected to 24 hours of annealing in an argon atmosphere containing 3% oxygen within a temperature range of 1700 C.

Example 9

[0078] Gd.sub.2.985Ce.sub.0.015Ga.sub.3.15Al.sub.2.1O.sub.12.375 in Example 6 was subjected to 24 hours of annealing in an argon atmosphere containing 3% oxygen within a temperature range of 1700 C.

Example 10

[0079] LU.sub.2.985Ce.sub.0.015Al.sub.5O.sub.12 co-doped with Li at 300 ppm of those in Example 2 was subjected to 24 hours of annealing in air within a temperature range of 1200 C.

Comparative Example 5

[0080] Gd.sub.2.985Ce.sub.0.015Ga.sub.3.15Al.sub.2.1O.sub.12.375 co-doped with Li at 300 ppm in Example 4 was subjected to 48 hours of annealing in an argon atmosphere containing 3% hydrogen within a temperature range of 1000 C.

Example 11

[0081] According to the micro-pulling down method, a garnet scintillator single crystal having a composition of Lu.sub.2.985Ce.sub.0.015Al.sub.5.2O.sub.12.3 was prepared by co-doping of Mg at 300 ppm. This single crystal had a diameter of about 3 mm and a length of about 15 mm, and was yellowish transparent. Emission from the 4f5d level of Ce.sup.3+ was confirmed at wavelengths in the vicinity of 480 nm. The diffuse transmittance at 480 nm was 90% per cm.

Example 12

[0082] According to the micro-pulling down method, a garnet scintillator single crystal having a composition of Y.sub.2.985Ce.sub.0.015Al.sub.5.2O.sub.12.3 was prepared by co-doping of Mg at 300 ppm. This single crystal had a diameter of about 3 mm and a length of about 15 mm, and was yellowish transparent. Emission from the 4f5d level of Ce.sup.3+ was confirmed at wavelengths in the vicinity of 480 nm. The diffuse transmittance at 480 nm was 91% per cm.

Example 13

[0083] According to the micro-pulling down method, a garnet scintillator single crystal having a composition of Lu.sub.2.985Ce.sub.0.015Al.sub.4.8O.sub.11.7 was prepared by co-doping of Mg at 300 ppm. This single crystal had a diameter of about 3 mm and a length of about 15 mm, and was yellowish transparent. Emission from the 4f5d level of Ce.sup.3+ was confirmed at wavelengths in the vicinity of 480 nm. The diffuse transmittance at 480 nm was 90% per cm.

Example 14

[0084] According to the micro-pulling down method, a garnet scintillator single crystal having a composition of Y.sub.2.985Ce.sub.0.015Al.sub.4.8O.sub.11.7 was prepared by co-doping of Mg at 300 ppm. This single crystal had a diameter of about 3 mm and a length of about 15 mm, and was yellowish transparent. Emission from the 4f5d level of Ce.sup.3+ was confirmed at wavelengths in the vicinity of 480 nm. The diffuse transmittance at 480 nm was 91% per cm.

[0085] Table 1 shows the results of evaluating crystals obtained in Examples 1 to 14 and Comparative examples 1 to 5 for emission intensity, rise time of emission, fluorescence lifetime, and time resolution. Light yields were each evaluated by processing and polishing the scintillator single crystals of examples and comparative examples to a size of 31 mm, adhering each crystal piece to a photomultiplier tube using an optical adhesive, covering the top surface of which with Teflon (Trademark) tape, irradiating the surface with a .sup.137Cs gamma-ray, and then analyzing photoelectric absorption peaks of the thus obtained energy spectra.

TABLE-US-00001 TABLE 1 Co- Peak Emission Rise doping emission intensity time of Fluorescence Time level wavelength ratio for non- emission lifetime resolution Host crystal composition (ppm) (nm) co-doped crystal (ns) (ns) (ps) Example 1 Gd.sub.2.985Ce.sub.0.015Ga.sub.3Al.sub.2O.sub.12 Li 300 520 1.30 1.9 50(90)% 240 140(10)% Li 1500 520 1.21 1.8 43(100)% 210 Li 3000 520 1.11 1.7 42(100)% 190 Example 2 Lu.sub.2.985Ce.sub.0.015Al.sub.5O.sub.12 Li 300 480 1.60 0.9 38(85)% 230 141(15)% Li 1500 480 1.50 0.8 35(100)% 220 Li 3000 480 1.30 0.7 32(100)% 210 Example 3 Y.sub.2.985Ce.sub.0.015Al.sub.5O.sub.12 Li 300 480 1.50 1.0 36(86)% 240 155(14)% Li 1500 480 1.44 0.9 36(100)% 230 Li 3000 480 1.11 0.9 31(100)% 220 Comparative Gd.sub.2.985Ce.sub.0.015Ga.sub.3Al.sub.2O.sub.12 0 520 1.00 2.3 64(75)% 400 example 1 248(25)% Comparative Gd.sub.2.985Ce.sub.0.015Ga.sub.3Al.sub.2O.sub.12 Li 20000 520 0.60 1.2 40(95)% 540 example 2 88(5)% Comparative Lu.sub.2.985Ce.sub.0.015Al.sub.5O.sub.12 0 480 1.00 1.2 50(70)% 350 example 3 254(30)% Comparative Y.sub.2.985Ce.sub.0.015Al.sub.5O.sub.12 0 480 1.00 1.2 45(70)% 330 example 4 233(30)% Example 4 Gd.sub.2.985Ce.sub.0.015Ga.sub.3.15Al.sub.2.1O.sub.12.375 Li 300 520 1.36 2.0 39(95)% 170 99(5)% Example 5 Gd.sub.2.985Ce.sub.0.015Ga.sub.2.85Al.sub.1.9O.sub.11.625 Li 300 520 1.34 2.0 40(97)% 175 110(3)% Example 6 Gd.sub.2.985Ce.sub.0.015Ga.sub.3.15Al.sub.2.1O.sub.12.375 0 520 1.12 1.8 83(85)% 390 199(15)% Example 7 Gd.sub.2.985Ce.sub.0.015Ga.sub.2.85Al.sub.1.9O.sub.11.625 0 520 1.11 1.9 85(88)% 385 160(12)% Example 8 Gd.sub.2.985Ce.sub.0.015Ga.sub.3Al.sub.2O.sub.12 Li 300 520 1.34 1.7 47(90)% 235 133(10)% Example 9 Gd.sub.2.985Ce.sub.0.015Ga.sub.3.15Al.sub.2.1O.sub.12.375 0 520 1.08 1.9 86(83)% 350 188(17)% Example 10 Lu.sub.2.985Ce.sub.0.015Al.sub.5O.sub.12 Li 300 480 1.09 1.0 44(79)% 290 154(21)% Comparative Gd.sub.2.985Ce.sub.0.015Ga.sub.3.15Al.sub.2.1O.sub.12.375 Li 300 520 0.40 3.3 114(65)% 670 example 5 1248(35)% Example 11 Lu.sub.2.985Ce.sub.0.015Al.sub.5.2O.sub.12.3 Mg 300 480 1.32 0.8 32(100)% 240 Example 12 Y.sub.2.985Ce.sub.0.015Al.sub.5.2O.sub.12.3 Mg 300 480 1.28 0.7 31(100)% 230 Example 13 Lu.sub.2.985Ce.sub.0.015Al.sub.4.78 O.sub.11.7 Mg 300 480 1.24 0.8 33(100)% 230 Example 14 Y.sub.2.985Ce.sub.0.015Al.sub.4.8 O.sub.11.7 Mg 300 480 1.22 0.7 32(100)% 230

[0086] FIG. 1 shows voltage pulse signals obtained using a digital oscilloscope by processing and polishing Gd.sub.2.985Ce.sub.0.015Ga.sub.3Al.sub.2O.sub.12 co-doped with Li at 1500 ppm of those in Example 1 and the non-co-doped scintillator single crystal (not co-doped with Li) having the same composition in Comparative example 1 to a size of 31 mm, adhering a crystal piece to a photomultiplier tube using an optical adhesive, covering the top surface with Teflon (Registered trademark) tape, irradiating the top surface with a .sup.137Cs gamma-ray. The thus obtained voltage pulse signals were analyzed and evaluated for emission intensity, rise time of emission, and fluorescence lifetime. As shown in FIG. 1 and Table 1, as a result of co-doping of Li at 1500 ppm, the emission intensity increased by 21%, and the rise time was shortened by 22% from 2.3 ns (not-co-doped) to 1.8 ns (co-doped at 1500 ppm), compared to the non-co-doped crystal (Comparative example 1). Furthermore, the fluorescence lifetime was shortened by 33% from 64 ns (not-co-doped) to 43 ns (co-doped at 1500 ppm), and the components having long fluorescence lifetimes existing without co-doping decreased. Moreover, X-ray irradiation was performed under conditions of CuK, 40 mA, and 40 mV, the maximum emission intensity was compared with emission intensity after 1 ms, and then the content of a phosphorescence component was measured. As a result, the content of the phosphorescence component decreased from 1% (not-co-doped) to 0.1% (co-doped at 1500 ppm).

[0087] Two scintillator single crystals, the single crystal having a composition of Gd.sub.2.985Ce.sub.0.015Ga.sub.3Al.sub.2O.sub.12 prepared by co-doping of Li at 1500 ppm of those in Example 1 and the non-co-doped single crystal of Comparative example 1 having the same composition (not-co-doped with Mg), were measured for time resolution by the above coincidence method. As shown in Table 1, as a result of co-doping of Li at 1500 ppm, the time resolution was improved such that it was shortened from 400 ps (Comparative example 1) to 210 ps (Example 1), compared to the non-co-doped crystal.

[0088] Furthermore, as shown in Table 1, when Examples 1 to 3 were compared with Comparative examples 1, 3, and 4, it was confirmed that co-doping of Li resulted in increased emission intensity, shortened rise time of emission and shortened fluorescence lifetime, and the components having long fluorescence lifetimes decreased.

[0089] Single crystals of Examples 6 and 7 wherein the value of y was represented by 0<y<0.5 or 0<y<0.5 in general formula Ce.sub.xRE.sub.3xM.sub.5+yO.sub.12+3y/2 and a single crystal having Gd.sub.2.985Ce.sub.0.015Ga.sub.3Al.sub.2O.sub.12 (y=0) of Comparative example 1 were irradiated with X-rays under conditions of CuK, 40 mA, and 40 mV. Maximum emission intensity was compared with emission intensity after 1 ms, and then the content of a phosphorescence component was measured. As a result, the content of the phosphorescence component decreased from 1.8% (comparative example 1) to 0.2% (Example 6) and 0.2% (Example 7). It was confirmed that specifying the value of y to be 0<y<0.5 or 0<y<0.5 in general formula Ce.sub.xRE.sub.3xM.sub.5+yO.sub.12+3y/2 resulted in the decreased content of the phosphorescence component.

[0090] A scintillator single crystal having Gd.sub.2.985Ce.sub.0.015Ga.sub.3Al.sub.2O.sub.12 co-doped with Li at 1500 ppm of those in Example 1 and a scintillator single crystal of Comparative example 1 having the same composition not-co-doped with Li were processed and polished into a size of 31 mm, and then irradiated with X-rays corresponding to 600 Gy generated under conditions of CuK, 40 mA, and 40 mV. Next, the rate of increase in absorption coefficient at 520 nm was measured before and after X-ray irradiation. The rate of increase in absorption coefficient of non-co-doped crystal (Comparative example 1) was 50%, and the rate of increase in absorption coefficient of the crystal (Example 1) co-doped with Li at 1500 ppm was 1.0%. It was confirmed that co-doping of Li improved radiation resistance.

[0091] Scintillator single crystals of Examples 6 and 7 wherein the value of y was represented by 0<y<0.5 or 0<y<0.5 in general formula Ce.sub.xRE.sub.3xM.sub.5+yO.sub.12+3y/2 and a scintillator single crystal having Gd.sub.2.985Ce.sub.0.015Ga.sub.3Al.sub.2O.sub.12 (y=0) of Comparative example 1 were processed and polished into a size of (31 mm, and then irradiated with X-rays corresponding to 600Gy generated under conditions of CuK, 40 mA, and 40 mV. When the rate of increase in absorption coefficient was measured at 520 nm before and after X-ray irradiation, the rate of increase in absorption coefficient decreased from 50% (Comparative example 1) to 1.2% (Example 6) and 1.5% (Example 7). It was confirmed that specifying the value of y to be 0<y<0.5 or 0<y<0.5 in general formula Ce.sub.xRE.sub.3xM.sub.5+yO.sub.12+3y/2 improved radiation resistance.

[0092] Mg-co-doped scintillator single crystals of Examples 12 and 14 wherein the value of y was represented by 0<y<0.5 or 0<y<0.5 in general formula Ce.sub.xRE.sub.3xM.sub.5+yO.sub.12+3y/2 and a scintillator single crystal having Y.sub.2.985Ce.sub.0.015Al.sub.5O.sub.12 (y=0) of Comparative example 4 were processed and polished into a size of 31 mm, and then irradiated with X-rays corresponding to 600 Gy generated under conditions of CuK, 40 mA, and 40 mV. When the rate of increase in absorption coefficient was measured at 520 nm before and after X-ray irradiation, the rate of increase in absorption coefficient decreased from 55% (Comparative example 4) to 0.5% (Example 6) and 0.8% (Example 7). It was confirmed that specifying the value of y to be 0<y<0.5 or 0<y<0.5 in general formula Ce.sub.xRE.sub.3xM.sub.5+yO.sub.12+3y/2 improved radiation resistance.

[0093] Mg-co-doped scintillator single crystals of Examples 11 and 13 wherein the value of y was represented by 0<y<0.5 or 0<y<0.5 in general formula Ce.sub.xRE.sub.3xM.sub.5+yO.sub.12+3y/2 and a scintillator single crystal having Lu.sub.2.985Ce.sub.0.015Al.sub.5O.sub.12 (y=0) of Comparative example 3 were processed and polished into a size of 31 mm, and then irradiated with X-rays corresponding to 600 Gy generated under conditions of CuK, 40 mA, and 40 mV. When the rate of increase in absorption coefficient was measured at 520 nm before and after X-ray irradiation, the rate of increase in absorption coefficient decreased from 55% (Comparative example 4) to 0.8% (Example 6) and 0.9% (Example 7). It was confirmed that specifying the value of y to be 0<y<0.5 or 0<y<0.5 in general formula Ce.sub.xRE.sub.3xM.sub.5+yO.sub.12+3y/2 improved radiation resistance.

[0094] Furthermore, as shown in Table 1, light yields, time resolutions, and emission intensity in Examples 6 and 7 wherein the value of y was represented by 0<y<0.5 or 0<y<0.5 in general formula Ce.sub.xRE.sub.3xM.sub.5+yO.sub.12+3y/2, were confirmed to be more improved than those in Comparative example 1 (y=0). Moreover, fluorescence lifetimes in Examples 6 and 7 were confirmed to be shortened and the long-life components were confirmed to decrease, compared to Comparative example 1. This is probably because the anti-site phenomenon involving partial replacement by a rare-earth element in the 6-coordinated site or Al and Ga in the 8-coordinated site was reduced by specifying the value of y to be 0<y<0.5 or 0<y<0.5 in general formula Ce.sub.xRE.sub.3xM.sub.5+yO.sub.12+3y/2, and the defect level resulting from anti-sites decreased, so that Ce.sup.3+ 4f5d emission was accelerated. Furthermore, such decreased defect level was considered to cause a decrease in absorption associated with the defect level upon radiation, and improve radiation resistance.

[0095] Furthermore, as shown in Table 1, the emission intensity was confirmed to increase, the rise time of emission and the fluorescence lifetime were confirmed to be shorter, and the components having long fluorescence lifetimes were confirmed to decrease after annealing in an atmosphere containing oxygen in Example 9 and Example 10, compared to the results obtained before annealing (Example 6 and Example 2, respectively).

Example 15

[0096] According to the micro-pulling down method, garnet scintillator single crystals having a composition of Gd.sub.2.985Ce.sub.0.015Ga.sub.3Al.sub.2O.sub.12 were prepared by co-doping of Mg at 300, 1500, and 3000 ppm. These single crystals' had a diameter of about 3 mm and a length of about 15 mm, and were yellowish transparent. Emission from the 4f5d level of Ce.sup.3+ was confirmed at wavelengths in the vicinity of 520 nm. The diffuse transmittance at 520 nm was 91% per cm.

Example 16

[0097] According to the micro-pulling down method, garnet scintillator single crystals having a composition of Gd.sub.2.985Ce.sub.0.015Ga.sub.3Al.sub.2O.sub.12 were prepared by co-doping of Ca at 300, 1500, and 3000 ppm. The thus obtained single crystals had a diameter of about 3 mm and a length of about 15 mm, and were yellowish transparent. Emission from the 4f5d level of Ce.sup.3+ was confirmed at wavelengths in the vicinity of 520 nm. The diffuse transmittance at 520 nm was 91% per cm.

Example 17

[0098] According to the micro-pulling down method, a garnet scintillator single crystal having a composition of Gd.sub.2.985Ce.sub.0.015Ga.sub.3Al.sub.2O.sub.12 was prepared by co-doping of K at 300 ppm. The thus obtained single crystal had a diameter of about 3 mm and a length of about 15 mm, and was transparent. Emission from the 4f5d level of Ce.sup.3+ was confirmed at wavelengths in the vicinity of 520 nm. The diffuse transmittance at 520 nm was 90% per cm.

Example 18

[0099] According to the micro-pulling down method, a garnet scintillator single crystal having a composition of Gd.sub.2.985Ce.sub.0.015Ga.sub.3Al.sub.2O.sub.12 was prepared by co-doping of Na at 300 ppm. The thus obtained single crystal had a diameter of about 3 mm and a length of about 15 mm, and was yellowish transparent. Emission from the 4f5d level of Ce.sup.3+ was confirmed at wavelengths in the vicinity of 520 nm. The diffuse transmittance at 520 nm was 92% per cm.

Example 19

[0100] According to the micro-pulling down method, garnet scintillator single crystals having a composition of Lu.sub.2.885Gd.sub.0.1Ce.sub.0.015Al.sub.5O.sub.12 were prepared by co-doping of Mg at 300, 1500, and 3000 ppm. These single crystals had a diameter of about 3 mm and a length of about 15 mm, and were yellowish transparent. Emission from the 4f5d level of Ce.sup.3+ was confirmed at wavelengths in the vicinity of 480 nm. The diffuse transmittance at 480 nm was 89% per cm.

Comparative Example 6

[0101] According to the micro-pulling down method, a non-co-doped garnet scintillator single crystal having a composition of Gd.sub.2.985Ce.sub.0.015Ga.sub.3Al.sub.2O.sub.12 was prepared. The thus obtained single crystal had a diameter of about 3 mm and a length of about 15 mm, and was yellowish transparent. Emission from the 4f5d level of Ce.sup.3+ was confirmed at wavelengths in the vicinity of 520 nm. The diffuse transmittance at 520 nm was 92% per cm.

Comparative Example 7

[0102] According to the micro-pulling down method, a garnet scintillator single crystal having a composition of Gd.sub.2.985Ce.sub.0.0015Ga.sub.3Al.sub.2O.sub.12 was each prepared by co-doping of Ca at 7500 ppm. The thus obtained single crystal had a diameter of about 3 mm and a length of about 15 mm, and was yellowish transparent. Emission from the 4f5d level of Ce.sup.3+ was confirmed at wavelengths in the vicinity of 520 nm. The diffuse transmittance at 520 nm was 92% per cm. Compared to the non-co-doped crystal in Comparative example 1, the emission intensity decreased by 40%.

Comparative Example 8

[0103] According to the micro-pulling down method, a non-co-doped garnet scintillator single crystal having a composition of Lu.sub.2.885Gd.sub.0.1Ce.sub.0.015Al.sub.5O.sub.12 was prepared. This single crystal had a diameter of about 3 mm and a length of about 15 mm, and was yellowish transparent. Emission from the 4f5d level of Ce.sup.3+ was confirmed at wavelengths in the vicinity of 480 nm. The diffuse transmittance at 480 nm was 90% per cm.

Example 20

[0104] According to the micro-pulling down method, a garnet scintillator single crystal having a composition of Gd.sub.2.985Ce.sub.0.015Ga.sub.3.15Al.sub.2.1O.sub.12.375 was prepared by co-doping of Mg at 300 ppm. This single crystal had a diameter of about 3 mm and a length of about 15 mm, and was yellowish transparent. Emission from the 4f5d level of Ce.sup.3+ was confirmed at wavelengths in the vicinity of 520 nm. The diffuse transmittance at 520 nm was 92% per cm.

Example 21

[0105] According to the micro-pulling down method, a garnet scintillator single crystal having a composition of Gd.sub.2.985Ce.sub.0.015Ga.sub.2.85Al.sub.1.9O.sub.11.625 was prepared by co-doping of Mg at 300 ppm. This single crystal had a diameter of about 3 mm and a length of about 15 mm, and was yellowish transparent. Emission from the 4f5d level of Ce.sup.3+ was confirmed at wavelengths in the vicinity of 520 nm. The diffuse transmittance at 520 nm was 91% per cm.

Example 22

[0106] Gd.sub.2.985Ce.sub.0.015Ga.sub.3Al.sub.2O.sub.12 co-doped with Mg at 300 ppm of those in Example 15 was subjected to 24 hours of annealing in an argon atmosphere containing 3% oxygen within a temperature range of 1600 C.

Example 23

[0107] Lu.sub.2.885Gd.sub.0.1Ce.sub.0.015Al.sub.5O.sub.12 co-doped with Mg at 300 ppm of those in Example 19 was subjected to 24 hours of annealing in air within a temperature range of 1200 C.

Comparative Example 9

[0108] Gd.sub.2.985Ce.sub.0.015Ga.sub.3Al.sub.2O.sub.12 co-doped with Mg at 300 ppm of those in Example 15 was subjected to 24 hours of annealing in an argon atmosphere containing 3% hydrogen within a temperature range of 1000 C.

[0109] Table 2 shows the results of evaluating crystals obtained in Examples 15 to 23 and Comparative examples 6 to 9 for emission intensity, light yield, rise time of emission, fluorescence lifetime, and time resolution. Light yields were evaluated by processing and polishing the scintillator single crystals of examples and comparative examples to a size of 31 mm, adhering each crystal piece to a photomultiplier tube using an optical adhesive, covering the top surface of which with Teflon (Trademark) tape, irradiating the surface with a .sup.137Cs gamma-ray, and then analyzing a photoelectric absorption peak of the thus obtained energy spectrum.

TABLE-US-00002 TABLE 2 Co- Peak Emission Light Rise doping emission intensity yield time of Fluorescence level/ wavelength/ ratio for non- Photons/ emission/ lifetime/ Time Host crystal composition ppm nm co-doped crystal MeV ns ns resolution Example 15 Gd.sub.2.985Ce.sub.0.015Ga.sub.3Al.sub.2O.sub.12 Mg 300 520 1.25 48000 2.0 40(95)% 180 120(5)% Mg 1500 520 1.29 46000 1.8 39(100)% 170 Mg 3000 520 1.32 42000 1.8 39(100)% 160 Example 16 Gd.sub.2.985Ce.sub.0.015Ga.sub.3Al.sub.2O.sub.12 Ca 300 520 1.10 36000 1.9 41(87)% 180 135(13)% Ca 1500 520 1.20 32000 1.8 39(100)% 190 Ca 3000 520 1.20 30000 1.7 37(100)% 200 Example 17 Gd.sub.2.985Ce.sub.0.015Ga.sub.3Al.sub.2O.sub.12 K300 520 1.05 46000 2.1 55(86)% 240 225(14)% Example 18 Gd.sub.2.985Ce.sub.0.015Ga.sub.3Al.sub.2O.sub.12 Na 300 520 1.05 46000 2.1 54(85)% 240 198(15)% Comparative Gd.sub.2.985Ce.sub.0.015Ga.sub.3Al.sub.2O.sub.12 0 520 1.00 50000 2.3 60(80)% 400 example 6 254(20)% Comparative Gd.sub.2.985Ce.sub.0.015Ga.sub.3Al.sub.2O.sub.12 Ca 7500 520 0.60 20000 1.8 40(100)% 250 example 7 Example 19 Lu.sub.2.885Gd.sub.0.1Ce.sub.0.015Al.sub.5O.sub.12 Mg 300 480 2.30 30000 2.5 46(96)% 220 115(4)% Mg 1500 480 4.90 28000 2.3 48(100)% 230 Mg 3000 480 4.50 25000 2.2 47(100)% 220 Comparative Lu.sub.2.885Gd.sub.0.1Ce.sub.0.015Al.sub.5O.sub.12 0 480 1.00 22000 4.4 58(72)% 240 example 8 334(28)% Example 20 Gd.sub.2.985Ce.sub.0.015Ga.sub.3.15Al.sub.2.1O.sub.12.375 Mg 300 520 1.31 50000 2.0 39(95)% 170 99(5)% Example 21 Gd.sub.2.985Ce.sub.0.015Ga.sub.2.85Al.sub.1.9O.sub.11.625 Mg 300 520 1.29 49000 2.0 40(97)% 175 110(3)% Example 22 Gd.sub.2.985Ce.sub.0.015Ga.sub.3Al.sub.2O.sub.12 Mg 300 520 1.35 50000 1.9 40(96)% 175 102(5)% Example 23 Lu.sub.2.885Gd.sub.0.1Ce.sub.0.015Al.sub.5O.sub.12 Mg 300 480 3.30 32000 1.8 43(97)% 215 110(3)% Comparative Gd.sub.2.985Ce.sub.0.015Ga.sub.3Al.sub.2O.sub.12 Mg 300 520 0.60 24000 2.9 70(60)% 580 example 9 2500(40)%

[0110] FIG. 2 shows voltage pulse signals obtained using a digital oscilloscope by processing and polishing Gd.sub.2.985Ce.sub.0.0015Ga.sub.3Al.sub.2O.sub.12 co-doped with Mg at 1500 ppm of those in Example 15 and the non-co-doped scintillator single crystal (not co-doped with Mg) having the same composition in Comparative example 6 to a size of 31 mm, adhering a crystal piece to a photomultiplier tube using an optical adhesive, covering the top surface with Teflon (Registered trademark) tape, irradiating the top surface with a .sup.137Cs gamma-ray. The thus obtained voltage pulse signals were analyzed and evaluated for emission intensity, rise time of emission, and fluorescence lifetime. As shown in FIG. 2 and Table 2, co-doping of Mg at 1500 ppm resulted in emission intensity increased by 30%, and rise time shortened by 22% from 2.3 ns (not-co-doped) to 1.8 ns (co-doped at 1500 ppm), compared to the non-co-doped crystal (Comparative example 6). Furthermore, the fluorescence lifetime was shortened by 35% from 60 ns (not-co-doped) to 39 ns (co-doped at 1500 ppm), and the components having long fluorescence lifetimes existing without co-doping decreased.

[0111] Two scintillator single crystals, the single crystal having a composition of Gd.sub.2.985Ce.sub.0.0015Ga.sub.3Al.sub.2O.sub.12 prepared by co-doping of Mg at 1500 ppm of those in Example 15 and the non-co-doped single crystal having the same composition (not-co-doped with Mg) in Comparative example 6, were measured for time resolution by the above coincidence method. As shown in Table 2, co-doping of Mg at 1500 ppm resulted in improved time resolution, and the time resolution shortened from 400 ps (Comparative example 6) to 170 ps (Example 15), compared to the non-co-doped crystal.

[0112] Furthermore, as shown in Table 2, when Examples 15 and 17 were compared with Comparative examples 6 and 8, co-doping of a monovalent alkali metal ion or a divalent alkaline-earth metal ion was confirmed to increase the emission intensity, to lower the rise time of emission and to shorten the fluorescence lifetime, and to decrease the components having long fluorescence lifetimes.

[0113] Furthermore, as shown in Table 2, when Example 20 and Example 21 wherein the value of y was represented by 0<y<0.5 or 0<y<0.5 in general formula Gd.sub.3x-zCe.sub.xRE.sub.zM.sub.5+yO.sub.12+3y/2, were compared with Example 15 (y=0), it was confirmed that the light yield, the time resolution, and the emission intensity were improved, the fluorescence lifetime was shortened, and the long-life components decreased. Furthermore, it was confirmed that the emission intensity increased, the rise time of emission and the fluorescence lifetime were shortened, and the components having long fluorescence lifetimes decreased after annealing in an atmosphere containing oxygen in Example 22 and Example 23, compared to the results obtained before annealing (Example 15 and Example 19, respectively).

Example 24

[0114] First, commercially available orthophosphoric acid (H.sub.3PO.sub.4) was mixed with sulfuric acid (H.sub.2SO.sub.4) at 5%-95% (capacity), and then the solution was heated to 200 C. It was considered that orthophosphoric acid was mainly altered to pyrophosphoric acid (H.sub.4P.sub.2O.sub.7) by heating. Subsequently, the temperature of the solution after heating was maintained at an appropriate temperature range of 150 C.-350 C., so that an etchant was prepared. Gd.sub.2.985Ce.sub.0.0015Ga.sub.3Al.sub.2O.sub.12 co-doped with Mg at 300 ppm of those in Example 15 was cut using a diamond cutter with a peripheral cutting edge into a size of 33 mm.sup.3 and then immersed in the above etchant for etching treatment. FIG. 3 shows optical microscopic photographs showing a mirror surface before etching and the same after etching, respectively. As shown in FIG. 3, it was confirmed that etch pits appeared on the surface as a result of etching treatment, and a non-glossy surface was obtained.

Comparative Example 10

[0115] Gd.sub.2.985Ce.sub.0.015Ga.sub.3Al.sub.2O.sub.12 co-doped with Mg at 300 ppm of those in Example 15 was cut using a diamond cutter with a peripheral cutting edge into a size of 33 mm.sup.3, and then mirror polishing was performed by a mechanical polishing method.

Comparative Example 11

[0116] Gd.sub.2.985Ce.sub.0.015 Ga.sub.3Al.sub.2O.sub.12 co-doped with Mg at 300 ppm of those in Example 15 was cut using a diamond cutter with a peripheral cutting edge into a size of 33 mm.sup.3.

[0117] Table 3 shows the results of measuring the crystals of Example 24, Comparative example 10 and Comparative example 11 for scintillator performance, and evaluating the crystals for emission intensity ratio, light yield, time resolution, and normal incidence reflectivity. As shown in Table 3, the single crystal (Example 24) subjected to etching treatment was confirmed to exhibit scintillation characteristics equivalent to or better than those of the single crystals (Comparative examples 10 and 11) obtained by a conventional machining method.

TABLE-US-00003 TABLE 3 Emission Normal intensity Light yield Time incidence ratio Photon/MeV resolution/ps reflectivity/% Comparative 1.00 42000 230 7.1 0.1 example 10 Comparative 0.7 30000 280 0.1 0.1 example 11 Example 24 1.25 52000 190 0.5 0.1