JP2007008983A - Inorganic scintillator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inorganic scintillator having a high fluorescence output. <P>SOLUTION: The inorganic scintillator is an inorganic scintillator comprising a crystal capable of undergoing scintillation by a radiation, represented by formula (1): Lu<SB>2x</SB>Gd<SB>2(1-x)-y</SB>SiO<SB>5</SB>:Ce<SB>y</SB>(wherein x and y are in the ranges: 0.05≤x≤0.4, and 0.001≤y≤0.02), and containing cerium as a luminescence center. The inorganic scintillator can be obtained by the growing step of growing a crystal in an atmosphere of an oxygen concentration of 300 to 500 ppm. vol. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an inorganic scintillator.

  In the positron emission computed tomography (hereinafter referred to as “PET”), one of the most important factors for improving the performance of the entire apparatus is what kind of characteristic or specification the scintillator is adopted. Become. As the insurance application of PET diagnosis is progressing mainly in the United States and business expansion is progressing, in order to obtain a high-performance PET device, material search for excellent scintillators, development of development technology for practical application, etc. have been vigorously advanced. Yes.

  In recent years, attention has been focused on TOF (Time Of Flight) type PET as the performance of PET has improved. In TOF type PET, the scintillator is required to have high temporal resolution and fluorescence output, fast rise time, and short fluorescence decay time.

  In the field of high energy physics, a scintillator capable of efficiently detecting a small amount of high energy particles is required for use in experiments such as detecting a small amount of high energy particles from space.

Examples of the scintillator include a single crystal scintillator disclosed in Patent Document 1. This single crystal scintillator has a general formula Gd _{2- (x + y)} Ln _x Ce _y SiO ₅ (where Ln is at least one selected from the group consisting of Sc, Tb, Dy, Ho, Er, Tm, Yb and Lu) Represents an element, and x is a value of 0.03 to 1.9 and y is a value of 0.001 to 0.2).
Japanese Examined Patent Publication No. 7-78215

  As described above, the items indicating the characteristics of the scintillator include fluorescence output, energy resolution, fluorescence decay time, and the like, and further improvement of these characteristics is desired.

  However, conventional scintillators such as those described in Patent Document 1 are still not sufficient to satisfy the above requirements.

  Therefore, the present invention has an object to provide an inorganic scintillator having a sufficiently high fluorescence output, among the above requirements.

  According to the study by the present inventors, it has been clarified that when a scintillator crystal is grown in a nitrogen atmosphere as described in Patent Document 1, the resulting crystal is colored to reduce the fluorescence output. On the other hand, even if the crystal is grown in an air atmosphere containing a large amount of oxygen, the fluorescence output is lowered due to the coloring of the crystal.

  The present invention is an inorganic scintillator made of a crystal capable of causing scintillation by radiation, containing cerium as a luminescent center, and growing a crystal in an atmosphere having an oxygen concentration of 300 volume ppm to 500 volume ppm An inorganic scintillator that is obtained through the process is provided.

The following can be considered as one of the factors that the inorganic scintillator has a sufficiently high fluorescence output. Cerium (Ce), which is the emission center, exists as Ce ^{3+ in} the crystal, while Ce ⁴⁺ is generally considered to absorb Ce ³⁺ emission. When the concentration of oxygen in the atmosphere containing Ce is very low and less than 300 ppm by volume, it is assumed that the emission of Ce ³⁺ is hindered due to an increase in the number of oxygen defects in the crystal. Is done. On the other hand, when the oxygen concentration in the atmosphere at the time of crystal growth containing Ce is extremely high and exceeds 500 ppm by volume, the ratio of Ce ⁴⁺ is increased, and it is considered that the light emission of Ce ³⁺ is absorbed. Therefore, when the oxygen concentration in the atmosphere at the time of crystal growth is 300 ppm by volume to 500 ppm by volume, inhibition of emission of Ce ^{3+ due} to oxygen defects or Ce ⁴⁺ is sufficiently suppressed. The inorganic scintillator of the present invention obtained through the growth is presumed to have a sufficient fluorescence output.

The inorganic scintillator of the present invention has a chemical composition represented by the following general formula (1) and a condition represented by the following formulas (2) and (3) from the viewpoint of obtaining further excellent scintillation characteristics. It is preferable to satisfy the requirements at the same time.
_{Lu 2x Gd 2 (1-x} ) -y SiO 5: Ce y (1)
0.05 ≦ x ≦ 0.4 (2)
0.001 ≦ y ≦ 0.02 (3)

In the general formula (1), “: Ce” indicates that cerium can function as a light emission center. The chemical composition represented by the general formula (1) can also be expressed as Ce _y Gd _{2 (1-x) -y} Lu _2x SiO ₅ .

  The inorganic scintillator of the present invention is preferably a single crystal because excellent scintillation characteristics can be obtained more reliably.

  According to the present invention, an inorganic scintillator having a sufficiently high fluorescence output can be provided.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings as necessary. In the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted. Further, the positional relationship such as up, down, left and right is based on the positional relationship shown in the drawings unless otherwise specified. Further, the dimensional ratios in the drawings are not limited to the illustrated ratios.

[Scintillator]
The inorganic scintillator according to the present embodiment (hereinafter simply referred to as “scintillator”) contains cerium as a light emission center and is obtained through a growth process in which crystals are grown in an atmosphere having an oxygen concentration of 300 volume ppm to 500 volume ppm. Is.

In this scintillator, a sufficiently high fluorescence output can be obtained as compared with the conventional scintillator. When a scintillator crystal is grown in an atmosphere having an oxygen concentration of less than 300 ppm by volume, it is considered that oxygen defects in the crystal are greatly increased because the oxygen in the atmosphere is very small. In addition, when a scintillator crystal is grown in an atmosphere where the oxygen concentration exceeds 500 ppm by volume, it is considered that tetravalent Ce (Ce ⁴⁺ ) in the crystal becomes very large due to the oxidizing action by oxygen in the atmosphere. These oxygen vacancies and Ce ⁴⁺ are both presumed to inhibit the light-emitting action of trivalent Ce (Ce ³⁺ ) that functions as a luminescence center. The scintillator according to the present embodiment is considered to have a sufficiently high fluorescence output because the content ratio of the Ce ³⁺ light-emitting action is sufficiently small.

  From the same viewpoint, the oxygen concentration in the atmosphere is preferably 350 volume ppm to 450 volume ppm.

  The scintillator is a scintillator capable of causing scintillation by radiation. Examples of the radiation include particle beams (for example, α rays, β rays, γ rays, X rays, etc.) having sufficient energy to ionize atoms or molecules.

  The scintillator may be single crystal or polycrystal as long as it is in a crystal state where excellent scintillation characteristics can be obtained. The scintillator crystal is preferably a single crystal from the viewpoint of obtaining better scintillation characteristics.

The base material of the scintillator (hereinafter referred to as “base material”) preferably contains a rare earth element and a metal oxide. Examples of rare earth elements include yttrium (Y), lutetium (Lu), gadolinium (Gd), and the like. Examples of the metal element contained in the metal oxide include tantalum (Ta), aluminum (Al), silicon (Si), and the like. In this specification, unless otherwise specified, “metal element” is a concept including Si, and “rare earth element” does not include Ce. Examples of such a base material include YTaO ₄ , Y ₃ Al ₅ O ₁₂ , Lu ₂ SiO ₅ , Y ₂ SiO ₅ , Gd ₂ SiO _{5 and the} like.

Specifically, the base material is preferably a rare earth element silicate (hereinafter referred to as “rare earth silicate”). Examples of the rare earth silicate include Lu ₂ SiO ₅ , Y ₂ SiO ₅ , Gd ₂ SiO _{5 and the} like. By using rare earth silicate as a base material, the inorganic scintillator of this embodiment can have a large fluorescence output.

Among these materials, the base material is Gd ₂ SiO ₅ (gadlium silicate), Lu ₂ SiO5 (lutetium silicate) or Y ₂ SiO5 whose rare earth element is Gd, Lu or Y, or a mixture of two or more of these rare earth elements. It is preferable that When the rare earth element is Gd or Y, there is an advantage that a single crystal having a large size can be easily formed. Further, when the rare earth element is Lu, there is an advantage that the density is increased and the fluorescence output is increased. These can be appropriately selected depending on the application and required characteristics.

The scintillator of this embodiment is preferably composed of Gd, Lu, Si, Ce and O, but has a chemical composition represented by the following general formula (1), and the following formulas (2) and ( It is more preferable that the conditions represented by 3) are satisfied at the same time. Although the cause has not been clarified, such a scintillator of Gd and Lu system is more effective in the above-described oxygen concentration, and can obtain further excellent scintillation characteristics.
_{Lu 2x Gd 2 (1-x} ) -y SiO 5: Ce y (1)
0.05 ≦ x ≦ 0.4 (2)
0.001 ≦ y ≦ 0.02 (3)

Furthermore, it is more preferable that the scintillator has a chemical composition represented by the general formula (1) and simultaneously satisfies the conditions represented by the following formulas (4) and (5). In this case, particularly excellent scintillation characteristics can be obtained.
0.2 ≦ x ≦ 0.3 (4)
0.015 ≦ y ≦ 0.02 (5)

  The scintillator of this embodiment may have oxygen defects to some extent. In addition, although the oxygen defect is not shown in the said General formula (1), the scintillator which has a chemical composition represented by General formula (1) may have an oxygen defect.

[Method of manufacturing scintillator]
Next, a preferred embodiment of the manufacturing method according to the scintillator of the present invention will be described.

  The manufacturing method of the scintillator of this embodiment is a manufacturing method of a scintillator having cerium as a light emission center, a melting step for obtaining a molten liquid in which the raw material of the scintillator is in a molten state based on a melting method, and a seed crystal in the molten liquid. A cooling and solidifying step of growing a crystal along a predetermined crystal plane of the seed crystal to obtain a single crystal ingot by immersing at least a part and cooling and solidifying the melt in which the seed crystal is immersed, and a single crystal ingot And a cutting step of cutting into a desired shape and size.

  From the viewpoint of obtaining the scintillator of the present embodiment more reliably, the melting method in the melting step is preferably the Czochralski method. Further, in this case, it is preferable to perform operations in the melting step and the cooling and solidifying step using the pulling device 10 having the configuration shown in FIG.

  FIG. 1 is a schematic cross-sectional view showing an example of a basic configuration of a manufacturing apparatus for manufacturing the scintillator of this embodiment.

  A pulling apparatus 10 shown in FIG. 1 has a high-frequency induction heating furnace (two-zone heating growth furnace) 14. This high frequency induction heating furnace 14 is for continuously performing the operations in the melting step and the cooling and solidification step described above.

  The high-frequency induction heating furnace 14 is a bottomed container having a cylindrical side wall having fire resistance, and the shape of the bottomed container itself is the same as that used for manufacturing a single crystal based on the known Czochralski method. A high frequency induction coil 15 is wound around the outer surface of the bottom of the high frequency induction heating furnace 14. A crucible 17 (for example, an Ir (iridium) crucible) is disposed on the bottom surface inside the high-frequency induction heating furnace 14. This crucible 17 also serves as a high-frequency induction heater. When a scintillator raw material is put into the crucible 17 and high frequency induction is applied to the high frequency induction coil 15, the crucible 17 is heated to obtain a melt 18 (melt) made of the constituent material of the scintillator.

  A heater 13 (resistance heater) is further disposed on the upper inner wall surface of the high-frequency induction heating furnace 14 that does not contact the melt 18. This heater can control the heating output of the high frequency induction coil 15 independently.

  In the center of the bottom of the high frequency induction heating furnace 14, an opening (not shown) penetrating from the inside of the high frequency induction heating furnace 14 to the outside is provided. A crucible support bar 16 is inserted from the outside of the high-frequency induction heating furnace 14 through this opening, and the tip of the crucible support bar 16 is connected to the bottom of the crucible 17. By rotating the crucible support rod 16, the crucible 17 can be rotated in the high-frequency induction heating furnace 14. A space between the opening and the crucible support rod 16 is sealed with packing or the like.

  Next, a more specific manufacturing method using the pulling device 10 will be described.

  First, in the melting step, a raw material of a scintillator single crystal is put into the crucible 17 and high frequency induction is applied to the high frequency induction coil 15 to obtain a melt 18 (melt) made of a scintillator constituent material. As the raw material for the single crystal, for example, a rare earth element constituting the single crystal or a single oxide of Si can be used.

  Next, by cooling and solidifying the melt in the cooling and solidifying step, a scintillator single crystal ingot 1 having a circular cross section and having a neck portion 1a, a cone portion 1b and a straight body portion 1d is obtained. More specifically, the work proceeds in two steps, a growth step and a cooling step described later.

  First, in the growth step, a single crystal of an inorganic scintillator is immersed from the upper part of the high-frequency induction heating furnace 14 by immersing the pulling rod 12 with the seed crystal 2 fixed to the lower end in the melt 18 and then pulling the pulling rod 12 up. Ingot 1 is formed. At this time, in the growing process, the heating output of the heater 13 is adjusted, and the single crystal ingot 1 of the inorganic scintillator pulled up from the melt 18 is grown until the cross section has a predetermined diameter.

In order to obtain a single crystal belonging to a specific space group more reliably, the seed crystal serving as the nucleus of the single crystal ingot 1 is preferably a single crystal belonging to the specific space group. Moreover, in order to obtain a desired scintillator more reliably, it is preferable that the seed crystal has the same chemical composition as the base material of the scintillator. Specifically, when the base material is a rare earth silicate, the seed crystal is preferably a rare earth silicate single crystal, and when the base material is gadolinium silicate, the seed crystal is a gadolinium silicate single crystal. Is preferred. Further, if the base metal is _{Lu 2x Gd 2 (1-x} ) -y SiO 5 , it is preferable that the seed crystal is _{Lu 2x Gd 2 (1-x} ) -y SiO 5 single crystal.

  In the growing step, the gas phase (atmosphere) in the high frequency induction heating furnace 14 has an oxygen concentration of 300 volume ppm to 500 volume ppm, and more preferably 350 volume ppm to 450 volume ppm. When the inside of the high-frequency induction heating furnace 14 is in such an atmosphere, coloring can be sufficiently suppressed, and a scintillator that exhibits sufficient fluorescence output can be formed. If the gas other than oxygen contained in the atmosphere in the high-frequency induction heating furnace 14 is an inert gas such as nitrogen gas or a rare gas such as Ar or He, a scintillator having desired scintillation characteristics can be obtained more easily. It is preferable because it is possible.

  Next, in the cooling step, the heating output of the heater is adjusted to cool the grown single crystal ingot (not shown) obtained after the growing step. The atmosphere in the high frequency induction heating furnace 14 in the cooling process is not particularly limited, but from the same viewpoint as in the growing process, the oxygen concentration is preferably 300 volume ppm to 500 volume ppm, and 350 volume ppm to 450 volume ppm. More preferably.

  Next, in the cutting step, the scintillator single crystal ingot 1 is cut into a desired shape and size to obtain a scintillator single crystal.

  As mentioned above, although preferred embodiment of this invention was described in detail, this invention is not limited to the said embodiment. In another embodiment, the scintillator may be an amorphous or quasicrystal instead of a crystal, or a mixture of two types of crystal, quasicrystal and amorphous. However, in these states, the scintillation characteristics are low as compared with the case where the scintillator is a crystal. Moreover, when the scintillator of this invention is a polycrystal, it can obtain by manufacturing methods, such as a sol-gel method similarly to the conventional polycrystal scintillator.

  EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on an Example and a comparative example, this invention is not limited to a following example.

Example 1
Have the same shape shown in FIG. 1, a diameter 110 mm, height 110 mm, in the Ir crucible thickness _{3mm, Lu 2x Gd 2 (1} -x) -y SiO 5: Ce y (x = 0 .2, y = 0.02) as raw materials, gadolinium oxide (Gd ₂ O ₃ , purity 99.99 mass%) 3600.64 g, lutetium oxide (Lu ₂ O ₃ , purity 99.99 mass%) 1000.64 g , 755.44 g of silicon dioxide (SiO ₂ , purity 99.99% by mass) and 43.28 g of cerium oxide (CeO ₂ , purity 99.99% by mass) were respectively added to obtain 5400 g of a mixture thereof. Next, it was heated to 1950 ° C. or higher in a high frequency induction heating furnace and melted to obtain a melt.

  Next, the tip of the pulling rod with the seed crystal fixed to the tip was placed in the melt and seeded. As the seed crystal, a single crystal made of a metal oxide containing Lu, Gd, Ce, and Si obtained by a normal crystal growth method was used.

Subsequently, the growth of the single crystal was started, and the single crystal ingot was pulled up at a pulling rate of 1.5 to 2 mm / h to form a neck portion. Thereafter, the cone portion (straight barrel portion) was pulled up, and the straight barrel portion was started to be pulled up when the diameter reached 60 mmφ. After growing the straight body part, the single crystal ingot was cut off from the melt, and cooling was started. After cooling, the obtained single crystal was taken out. From the start of single crystal growth to the end of cooling, the atmosphere in the high-frequency induction heating furnace is a mixed atmosphere of oxygen and nitrogen, and the oxygen concentration is measured using a zirconia sensor (trade name “ECOAZ-CG O ₂ ANALYZER” manufactured by Token Co., Ltd.). ) And was 430 volume ppm. The obtained single crystal ingot had a crystal mass of about 3500 g, a cone part length of about 40 mm, and a straight body part length of about 170 mm.

  The obtained single crystal was first cut between the cone part and the straight body part, and then the straight body part was cut into round pieces at intervals of 20 mm. Next, a plurality of substantially rectangular parallelepiped samples (single crystal of scintillator) having a size of 4 mm × 6 mm × 20 mm were cut out from each of the straight body portions cut into rings. An inner peripheral cutting machine was used for cutting, and the blade of the inner peripheral cutting machine used was electrodeposited with # 325-400 natural diamond.

Of the plurality of samples, three are extracted, and one of the surfaces (hereinafter referred to as “radiation incident surface”) having a size of 4 mm × 6 mm among the six surfaces of the samples (substantially rectangular parallelepiped). The remaining five surfaces were coated with polytetrafluoroethylene (PTFE) tape as a reflector. Next, in each of the samples, the radiation incident surface not covered with the PTFE tape was made to face the photomultiplier surface (photoelectric conversion surface) of a photomultiplier tube (H1949, trade name) manufactured by Hamamatsu Photonics. The inorganic scintillators of Examples 1 to 12 were fixed with optical grease. These samples were irradiated with 662 KeV radiation using ¹³⁷ Cs, and the energy spectrum was measured using a Tektronix digital oscilloscope (TDS3052, trade name) to derive the fluorescence output. The results are shown in Table 1. The fluorescence output is the average value of the extracted samples.

(Comparative Examples 1-4)
The same procedure as in Example 1 was performed except that the oxygen concentration in the atmosphere in the high-frequency induction heating furnace was changed as shown in Table 1 from the start of single crystal growth to the end of cooling. The results of fluorescence output are shown in Table 1. For the scintillator of Comparative Example 2, two samples were used.

  The scintillator of the present invention is suitably used as, for example, a scintillator mounted on PET, a scintillator for underground exploration such as petroleum exploration, a scintillator for high energy physics research, and the like.

It is a schematic cross section which shows an example of the basic composition of the scintillator manufacturing apparatus used suitably for manufacture of the scintillator of this embodiment. DESCRIPTION OF SYMBOLS 1 ... Single crystal ingot, 2 ... Seed crystal, 10 ... Lifting device, 12 ... Lifting rod, 13 ... Resistance heater, 14 ... High frequency induction heating furnace (2 zone heating growth furnace), 15 ... High frequency induction coil, 16 ... Crucible Support rod, 17 ... crucible, 18 ... melt (melt).

Claims (3)

An inorganic scintillator made of crystals capable of causing scintillation by radiation,
Containing cerium as the luminescent center,
An inorganic scintillator that is obtained through a growth step of growing crystals in an atmosphere having an oxygen concentration of 300 volume ppm to 500 volume ppm. The inorganic scintillator according to claim 1, which has a chemical composition represented by the following general formula (1) and satisfies the conditions represented by the following formulas (2) and (3) simultaneously.
_{Lu 2x Gd 2 (1-x} ) -y SiO 5: Ce y (1)
0.05 ≦ x ≦ 0.4 (2)
0.001 ≦ y ≦ 0.02 (3) The inorganic scintillator according to claim 1 or 2, wherein the crystal is a single crystal.

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