JP2013028671A - Scintillator crystal having phase separation structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a scintillator crystal having a unidirectional phase separation structure, which has an optical waveguide function of eliminating the need for forming a partition wall for preventing crosstalk in a scintillator used for detecting radiation such as an X-ray CT apparatus.SOLUTION: The scintillator crystal has the phase separation structure comprising: a first crystal phase comprising unidirectional columnar crystal; and a second crystal phase that covers the side surface of the first crystal phase. One of the first and second crystal phases contains an oxide, and the other thereof contains a fluoride. The phase containing the oxide emits light by means of the radiation.

Description

  The present invention relates to a scintillator that is a material that emits light by radiation, and a radiation detector using the scintillator.

  In an X-ray CT (Computed Tomography) apparatus used in a medical field or the like, X-rays passing through a subject are received by a scintillator, and light emitted from the scintillator is detected by a photodetector. The detectors are arranged as a two-dimensional array, and each scintillator is separated by a partition so as not to cause light crosstalk. Then, it has been desired that the partition wall be formed as thin as possible from the viewpoint of not contributing to X-ray detection and degrading the spatial resolution. For example, in Patent Document 1, a scintillator array is formed by bonding a large number of scintillator crystals with an adhesive, and then the adhesive is etched, and the resulting voids are filled with titanium oxide powder as a partition material. ing. In this case, it is shown that the thickness of the partition can be reduced to about 1 μm.

JP 2008-145335 A

  Even if the partition wall can be formed thin, its existence cannot be eliminated. In addition, in the manufacturing process, there is a problem that a lot of labor is required from cutting of the scintillator to pasting for forming a partition wall. The present invention has been made to eliminate such problems, and has fundamentally improved that a partition wall serving as a scattering surface or a reflection surface has been required since the scintillator itself has no function of guiding light. Therefore, it is an object to provide an optical waveguide function to the scintillator itself.

  The scintillator crystal of the present invention has a phase separation structure composed of a first crystal phase forming a columnar crystal having unidirectionality and a second crystal phase filling a side surface of the first crystal phase. One of the one crystal phase and the second crystal phase contains an oxide, the other contains a fluoride, and the phase containing the oxide emits light by radiation.

  The scintillator crystal of the present invention has a phase separation structure composed of two phases having unidirectionality, and can impart a function of guiding light to the crystal itself. Therefore, the manufacturing process of forming a partition wall from the conventional cutting of the scintillator is unnecessary, and the radiation detector can be manufactured only by arranging the scintillator crystal of the present invention on the photodetector.

It is a schematic diagram showing the scintillator crystal body of this invention. It is a schematic diagram showing an example of the apparatus which produces the scintillator crystal body of this invention. It is a transmission optical microscope image of the scintillator crystal body of this invention. It is a figure which shows the difference in the phase-separation structure by a composition. It is a schematic diagram showing the equilibrium ZnO-ZnF ₂ system. It is a figure showing the outline | summary of the radiation detector of this invention.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings. There are various forms (various configurations and various materials) for carrying out the present invention, but what is common to all the embodiments is that it has two crystal phases, A scintillator crystal having a phase separation structure comprising two phases of a crystal phase and another crystal phase having a higher refractive index than one crystal phase includes a first main surface and a second main surface that are not located on the same plane. The main surface has a portion where the other crystal phase is exposed, and the portion exposed on the first main surface of the other crystal phase is connected to the portion exposed on the second main surface. Thereby, the light in the high refractive index crystal phase is totally reflected by the low refractive index crystal phase located around the high refractive index phase, and as a result, travels while being guided in the high refractive index crystal. At this time, the crystal phase having a high refractive index is exposed to the first main surface and the second main surface, and since this exposed portion is connected, the waveguide (optical guiding) is the first main surface. To the surface or the second main surface. In other words, the light generated in the scintillator crystal is directed to the first main surface or the second main surface while being confined in the other crystal phase having a higher refractive index (that is, the light does not spread). It can be said that it progresses. Thus, in all the embodiments of the present invention, the scintillator crystal itself has a waveguide function (optical guiding function). Here, for example, the first main surface 61 is a surface facing the photodetector 64, and the second main surface 62 is a surface on which radiation such as X-rays is incident (see FIG. 6). ). As a result, the light generated in the scintillator crystal body 65 can be guided (optical guiding) toward the photodetector 64, and a scintillator crystal body with excellent light utilization efficiency can be provided and used. It is possible to provide a high-intensity, high-resolution radiation detector. Reference numeral 63 represents a substrate.

  In each embodiment described below, one crystal phase that is a low refractive index phase also has a portion exposed to the first main surface and the second main surface, and these exposed portions are connected. The configuration is preferable. As a result, the light in the other crystal phase, which is a high refractive index phase, can be more reliably guided (optical guiding) to the first main surface or the second main surface without spreading. Become.

  In addition, a configuration in which one crystal phase that is a low refractive index phase is located in the other crystal phase that is a high refractive index phase is preferable. As a result, a sufficient waveguide function (optical guiding function) can be obtained while suppressing the proportion of one crystal phase that is a low refractive index phase in the scintillator crystal.

[Structure of scintillator crystal]
FIG. 1 shows a schematic structure of the scintillator crystal of the present invention.
The scintillator crystal body having the phase separation structure of the present invention includes a first crystal phase 11 having a number of unidirectional columnar crystals and a second crystal phase 12 filling the side surfaces of the first crystal phase 11. It is composed of phases. The shape of the columnar crystals constituting the first crystal phase 11 is not limited to a circle, and may be constituted by a plurality of crystal planes and may be polygonal. The diameter 13 of the columnar crystals is preferably in the range of 50 nm to 30 μm. Further, the period 14 between adjacent columnar crystals is preferably in the range of 500 nm to 50 μm. However, when the scintillator crystal of the present invention is combined with a detector or a detector array, it is possible to combine those having a structure size such that a large number of columnar crystals are arranged on the light receiving region of the photodetector. preferable. For example, when the light-receiving region is square and one side is 20 μm, it has a structure size such that the columnar crystal has a diameter of 5 μm and a period of 8 μm. Therefore, it is preferable to combine those having a small structure size in accordance with the size of the light receiving region without being limited by the range of the structure size. Further, the size range of the structure is determined by the selection of the material system and the conditions at the time of manufacturing, and the tendency will be described later.

  Further, the thickness 15 of the scintillator crystal can be adjusted to an arbitrary thickness although it depends on the manufacturing method. Substantially, it is preferable to continue straight across the thickness direction 16 of the columnar crystal, but it includes parts that are interrupted, branched or fused, the diameter fluctuates, or is bent rather than straight. It does not exclude cases such as. By appropriately controlling the direction of the solidification interface, it is possible to bend the columnar crystals. FIG. 1 is a schematic diagram to the last, and is not limited to this.

  It is preferable that either the first crystal phase or the second crystal phase is an oxide and the other is a fluoride. This is because fluoride has a relatively low refractive index relative to an oxide, and when the oxide functions as a scintillator, the refractive index ratio (low refractive index / high refractive index) becomes small, This is because the most preferable configuration from the viewpoint can be obtained. The refractive index of the oxide is mainly 1.65 or more, and the fluoride is mainly less than 1.65.

Any oxide can be used as long as it functions as a scintillator. Further, regarding fluorides, those having a eutectic relationship in combination with oxides can be applied. In particular, oxide scintillators include Gd ₂ SiO ₅ , Gd ₂ Si ₂ O ₇ , Lu ₂ SiO ₅ , Lu ₂ Si ₂ O ₇ , Y ₂ SiO ₅ , Y ₂ Si ₂ O ₇ , (Lu, Y) ₂ SiO ₅ , Bi ₄ Si ₃ O ₁₂ , Bi ₄ Ge ₃ O ₁₂ , LuAlO ₃ , YAlO ₃ , GdAlO ₃ , Y ₃ Al ₅ O ₁₂ , Lu ₃ Al ₅ O ₁₂ , Lu ₃ Al ₅ O ₁₂ , Y ₃ Ga ₅ O ₁₂ , Gd ₃ Al ₃ Ga ₂ O ₁₂ , Gd ₃ Sc ₂ Al ₃ O ₁₂ , Lu ₃ Al ₅ Sc ₅ O ₁₂ , CdWO ₄ , ZnWO ₄ , PbWO ₄ , CaWO ₄ , SrWO ₄ , BaWO ₄ , GdBO _{3, LuBO 3, YBO 3,} LaB 3 O 6, Li 6 Gd (BO 3) 3, Li 6 Y (BO 3) 3, Lu 2 O 3, KLuP 2 O 7, KYP 2 O 7, NaLuP 2 O 7 , LuPO ₄ , NaGd (PO ₃ ) ₄ , SrHfO ₃ , La ₂ HfO ₇ , ZnO and the like are preferably used. In addition, for the purpose of improving the characteristics of the scintillator, it is also preferable to add an emission center as appropriate. In many oxides, rare earth Ce, Pr, etc. are often added, but in producing the phase separation structure of the present invention, a trace amount of luminescent center does not affect, so any element can be added. It is. As the fluoride, LiF, NaF, KF, RbF , CsF, MgF 2, CaF 2, SrF 2, BaF 2, ZnF 2, CdF 2, PbF 2, AlF 3, BiF 3, ScF 3, YF 3, LaF ₃ , CeF ₃ , PrF ₃ , NdF ₃ , SmF ₃ , GdF ₃ , TbF ₃ , YbF ₃ , LuF _{3 and the} like are preferably used. It is possible to obtain the desired phase separation structure of the present invention by unidirectionally solidifying the mixture in the eutectic composition with a combination of eutectic relationships.

For example, in the combination of ZnO and ZnF ₂ , the eutectic composition is 25:75 mol%, and the eutectic temperature is about 857 ° C. These values are the result of our intensive studies using DTA (Differential Thermal Analysis). In order to obtain a good separation structure as shown in the schematic diagram of FIG. This composition corresponds to the eutectic point. However, it should not be deviated from the above composition at all, and the range of ± 4 mol% with respect to the composition is preferably an allowable range. More preferably, it is ± 2 mol%. The reason for limiting the range in the vicinity of these compositions is that each phase has a eutectic relationship in the formation of the structure, and a high-quality structure as shown in FIG. 1 is obtained by performing unidirectional solidification in the vicinity of the eutectic composition. Because you can. In other composition ranges, that is, when deviating by 4 mol% or more, one phase is precipitated first, which becomes a factor disturbing the structure from the viewpoint of structure formation. Similarly, the eutectic temperature described is not limited to the above-mentioned temperature because of the measurement error and the like.

  Materials other than those described above may be added to the first and second crystal phases. In particular, the addition to the material constituting the first crystal phase 11 is not limited to the first crystal phase 11 in the additive composition. A material that dissolves and does not dissolve in the second crystal phase 12 is preferable. Further, the material added to the material constituting the second crystal phase 12 is preferably a material that does not dissolve in the first crystal phase 11 and does not dissolve in the second crystal phase 12 in the additive composition. . If there is no hindrance to the structure formation, a material that dissolves in both may be added. In the case where 1 mol% or more is added instead of a very small amount of addition as described below, the purpose is to control the lattice constant, the band gap, and the emission color.

In the ZnO-ZnF ₂ phase separation structure of an example of the present invention, the scintillator material ZnO constitutes a first crystal phase and becomes a columnar shape, and ZnF ₂ constitutes a second crystal phase and surrounds the periphery of ZnO. Located in. By irradiating this structure with radiation, ZnO is excited and can emit light. It is also preferable that the second crystal phase has a high refractive index, not only when the first crystal phase has a high refractive index as in the ZnO-ZnF ₂ system. Furthermore, it is preferable that at least one crystal phase emits light, but both may emit light. However, since light emission on the low refractive index side has no advantage from the viewpoint of waveguide, it is preferable not to emit light depending on how it is used. Furthermore, it is most preferable to create a situation in which light emission on the low refractive index side excites the high refractive index side and the high refractive index side emits light again. For example, when Gd is added to ZnF ₂ , it exhibits ultraviolet light emission, which matches the excitation band of ZnO, so that the above situation can be achieved. In this way, both materials contribute to radiation absorption, and since only the high refractive index side is responsible for wave guiding, it is possible to achieve both a high light receiving amount and high resolution.

An important characteristic of the unidirectional scintillator of the present invention is that it guides light. For the ZnO—ZnF ₂ material system, the refractive index is 2.0 for ZnO and 1.5 for ZnF ₂ . Since the refractive index ratio is 0.75 and sufficiently smaller than 1, light is effectively guided. However, the refractive index is not exact because it has wavelength dependency and changes due to additives, and it is important that the refractive index ratio is not 1 between the constituent materials.

  According to Snell's law, light emission on the high refractive index side should cause total reflection above the critical angle that is the interface with the low refractive index side, and reflection and refraction should occur at narrower angles. Therefore, the scintillator crystal of the present invention shows that there is a situation where light does not spread due to total reflection in a high refractive index medium. That is, refraction and reflection are repeated, and the high refractive index medium is relatively confined and propagated.

  Thus, the scintillator crystal of the present invention is characterized in that it guides light in a direction parallel to the columnar crystal and does not guide in the vertical direction due to scattering or reflection. Therefore, crosstalk of light can be suppressed without providing a partition wall in the single crystal group as in the prior art.

[Production of scintillator crystals]
The scintillator crystal of the present invention can be produced by any method as long as it is a method in which a desired material system is melt-solidified with an optimal composition and unidirectionality. In particular, it is required to control the temperature gradient so as to flatten the solid-liquid interface, and a temperature gradient of about 30 ° C./mm or more is required.

  As shown in FIG. 2, in the Bridgman method in an apparatus for creating a scintillator crystal, a sample sealed in a platinum tube or the like is placed in a vertical shape, and the position of the solidification interface is moved by moving the heater or the sample at a constant speed. Thus, it is possible to produce the scintillator crystal of the present invention. In particular, as shown in FIG. 2 (A), the apparatus is composed of a heater unit 21 comparable to the length of the sample 23 and a water cooling unit 22 for realizing 30 ° C./mm at the solid-liquid interface. Further, as shown in FIG. 2 (B), the water cooling unit 22 may be provided above and below, and the heater unit 21 may correspond to only a partial region of the sample 23. Furthermore, the manufacturing method which takes an equivalent means may be sufficient.

  In addition, as in the Czochralski method, a scintillator crystal can be similarly produced by pulling a crystal from a melt. In this case, since it is not solidification in the sample container in the Bridgman method, it can be said that it is more preferable in that a solid-liquid interface can be formed without being affected by the wall surface of the container. Further, it can also be produced by a floating zone method.

  In particular, in the Bridgman method, the solidification rate must be set so that the solid-liquid interface of the sample is as flat as possible, but since heat exchange is mainly from the side of the sample, it depends on the diameter of the sample. That is, if the diameter of the sample is large, it takes time for heat to enter and exit, and if the solidification rate is not slowed, the solid-liquid interface is considerably curved, and the columnar crystals that are the first crystal phase 11 are bent in most regions of the sample. Will be formed. This is because the growth direction of the columnar crystals is almost perpendicular to the solid-liquid interface. Furthermore, if the solidification rate is higher than the sample size, the solid-liquid interface is not flat but cannot be kept smooth, resulting in a situation where micro undulations occur and dendrites are generated. It is important to avoid this too. Therefore, it is preferable that the temperature gradient at the solid-liquid interface is sufficiently increased and at the same time, the speed is 850 mm / hour or less. More preferably, it is 500 mm / hour or less, and further 300 mm / hour or less.

Further, the diameter of the first crystal phase 11 of the scintillator crystal body and the period thereof depend on the solidification rate, and particularly the period of the columnar crystals has the following correlation. If the period is λ and the solidification rate is v, λ ² · v = constant. Therefore, if there is a desired structural period, the solidification rate is inevitably limited. On the other hand, as a limitation on the manufacturing method, there is a solidification rate at which the solid-liquid interface can be controlled flat and smooth, so the range of the period λ is in the range of 500 nm to 50 μm. Correspondingly, the diameter of the columnar crystals is in the range of 50 nm to 30 μm.

  Here, the diameter of the columnar crystal may not be circular, and if it is indefinite, the shortest diameter is included in the above range. Moreover, it is preferable that the ratio of the longest diameter to the shortest diameter is 10 or less as an average value of many columnar crystals. Above this ratio, a lamellar structure is appropriate. However, even if only some of the columnar crystals have a value of 10 or more, it is acceptable if the average value is below. Further, in terms of manufacturing conditions, the closer the molar ratio of the two-phase material system is to 1: 1, the easier it is to adopt a lamellar structure. Therefore, it is preferable to select the manufacturing conditions and additive materials so as not to constitute the lamellar structure.

Finally, the preparation composition of the raw material of the sample to be produced will be described. The composition ratio of the above scintillator crystal is, for example, 25:75 mol% in a combination of ZnO and ZnF ₂ , but the charged composition may deviate from ± 4 mol% or more. In other words, in the case of the Bridgman method, if the entire sample is solidified in one direction, the material that deviates from the eutectic composition in the initial stage of solidification will precipitate first, and remain. This is because the melt has a eutectic composition. Further, in the Czochralski method, since a deviation from the eutectic composition is pulled up at the initial stage of pulling, it is preferable to pull it up once with a dummy and then pull it up again after the melt has the eutectic composition. What is necessary is just to cut away an unnecessary part after crystal body preparation.

[Use of scintillator crystals]
The scintillator crystal of the present invention can be used as a radiation detector for medical use, industrial use, high energy physics use, and space use in combination with a photodetector. In particular, since it has a light guiding function without providing a partition wall or the like, it is preferably applied to a situation where light needs to be guided in a specific direction toward the detector. It is also effective for use in an X-ray CT apparatus that requires barrier rib formation and for the replacement of CsI needle crystals in an X-ray flat panel detector (FPD). In this case, it is also possible to adjust the emission wavelength of the scintillator by adding multiple materials to the base material or adding the emission center so as to match the light receiving sensitivity characteristic of the detector.

  Furthermore, it is also preferable that the scintillator crystal of the present invention is bonded or disposed through one or more protective layers or a film or layer having a function of antireflection, etc., in addition to being directly provided on the photodetector.

[Example 1]
This example relates to the production of the scintillator crystal of the present invention.

ZnO is selected as the oxide and ZnF ₂ is selected as the fluoride.

ZnO and ZnF ₂ powders were mixed at a eutectic composition of 25:75 (mol ratio). A platinum tube having an inner diameter of φ5 mm was filled, and locally heated by the apparatus shown in FIG. 2, and the tube was moved downward to perform unidirectional solidification. The temperature of the heating part was set to about 865 ° C. The sample thus prepared was cut out and the structure was observed with an optical microscope. As a result, as shown in FIG. 3, it was confirmed that in the structure perpendicular to the solidification direction, a desired structure of columnar crystal ZnO and ZnF ₂ surrounding it was obtained. It was also confirmed that the obtained crystal exhibited light emission having a peak in the vicinity of 390 nm by radiation excitation. Since this is the same wavelength range as the light emission specific to ZnO, it is clear that ZnO columnar crystals in the crystal emit light.

[Example 2]
This example relates to the composition of the scintillator crystal of the present invention.
Regarding the composition of the scintillator crystal of the present invention, the ZnO—ZnF ₂ (25 mol%) system, ZnO—ZnF ₂ (23 mol%), and ZnO—ZnF ₂ (10 mol%) 3 indicated by the arrows in the phase diagram shown in FIG. Two types of samples are manufactured using the same apparatus as in Example 1 shown in FIG. 2 (A), and when the heater is narrow and locally melted as shown in FIG. 2 (B). A total of 6 types were prepared.

When solidification is started after the entire sample is melted at the initial stage in the same manner as in Example 1, the sample of any composition has a good structure as shown in the optical microscope image of FIG. It turns out that things can be obtained. However, in the case of ZnO—ZnF ₂ (10 mol%), a crystal accompanied with precipitation of ZnF ₂ dendritic crystals as shown in FIG. It was observed that a good region such as 4 (A) was formed. In the case of other ZnO-ZnF ₂ (25 mol%) and ZnO-ZnF ₂ (23 mol%), the initial solidification region of the sample is not clearly formed as in the case of 10 mol%, and there is no significant difference. It was. However, the difference of 2 mol% due to the difference in composition is not that there is no difference at all due to the influence of impurities of the raw material. Thus, when the entire sample is melted, the material that deviates from the eutectic composition in the initial solidification region of the sample is precipitated at a temperature higher than the eutectic temperature, so the remaining melt portion becomes the eutectic composition, It is considered that a good structure is formed thereafter.

Also, FIG. 2 in the case where solidified while melting only a local area of a sample as in (B), ZnF ₂ as ZnO-ZnF ₂ FIG even in a system at any place of the sample (10mol%) 4 (B) There are some dendritic crystals, and some ZnO columnar crystals are formed in the gaps, but it cannot be said to be the target structure. However, in the case of ZnO—ZnF ₂ (25 mol%) or ZnO—ZnF ₂ (23 mol%), ZnO columnar crystals are formed throughout as shown in FIG. 4 (A), and a good sample is obtained. be able to. However, although there is an influence of impurities, the sample of ZnO-ZnF ₂ (23 mol%) appears to have more structural disorder than the case of ZnO-ZnF ₂ (25 mol%). Therefore, when locally melting and solidifying, it is not possible to go through the process toward the optimal composition in the sample that occurs when the whole is melted, and the composition and structure are sensitively affected. I understand.

  Therefore, in the scintillator crystal of the present invention, a slight fluctuation of the composition of about 2 mol% does not greatly affect the structure formation, but a large deviation of 15 mol% affects the structure formation, so the optimum composition is close to the eutectic composition. It is clear that Also, if the solidification is controlled after melting the entire sample in the manufacturing method, even if the composition deviates, the material of the deviation preferentially precipitates in the initial solidification region of the sample, and the remaining eutectic composition. It is important that it has been found that good regions can be obtained from the melt.

[Example 3]
This example relates to the optical waveguide of the scintillator crystal of the present invention.
The observation result of the sample piece produced in Example 1 with a transmission optical microscope is shown in FIG. Obviously, the columnar crystals of ZnO appear brightest, indicating that light is guided through the ZnO side, which has a high refractive index. However, in the image of FIG. 3, it may appear intermittently dark if the condition of incidence on the ZnO columnar crystal is not adjusted due to the incidence of light on the sample. This is related to the condition of incidence of light on the sample, and does not impose such a restriction on the light that is excited by radiation and emitted inside, so that it does not cause any problem on the wave guide.
Therefore, it was shown that the scintillator crystal of the present invention has the property of reliably guiding only in the columnar crystal direction.

[Example 4]
This example relates to radiation detection using the scintillator crystal of the present invention.
When a scintillator crystal cut out to a thickness of 1 mm is placed on a photodetector and irradiated with X-rays, when a single crystal without a partition is irradiated, light diffuses and propagates in the crystal plane. On the other hand, it can be confirmed from the output of the detector that the spread is suppressed.
Furthermore, at the junction of the scintillator crystal and the photodetector, it is confirmed that the output of the detector increases when it is connected as a protective phase of one or more layers so that there is no space in the resin. It is possible to adopt a layer configuration in consideration of light extraction to the vessel.

  The scintillator crystal of the present invention emits light by radiation and has the property of guiding the emitted light. Therefore, it is possible to detect radiation by using it in combination with a photodetector without forming a conventional partition wall. It is useful as a vessel. In particular, it can be used for measuring devices for medical, industrial, high energy physics, and space using radiation such as X-rays.

11 First crystal phase 12 Second crystal phase 13 Columnar crystal diameter 14 Period between adjacent columnar crystals 15 Thickness of scintillator crystal 16 Columnar crystal thickness direction 21 Heater unit 22 Water-cooled unit 23 Sample 61 First main surface 62 second main surface 63 substrate 64 photodetector 65 scintillator crystal

Claims (5)

  A phase separation structure composed of a first crystal phase forming a columnar crystal having unidirectionality and a second crystal phase filling a side surface of the first crystal phase, and the first crystal phase and the first crystal phase A scintillator crystal, wherein one of the two crystal phases contains an oxide, the other contains a fluoride, and the phase containing the oxide emits light.   The scintillator crystal according to claim 1, wherein the composition constituting the scintillator crystal is a composition at a eutectic point.   The scintillator crystal according to claim 1, wherein the oxide is ZnO. The scintillator crystal according to claim 3, wherein the fluoride is ZnF ₂ .   A radiation detector, wherein the scintillator crystal according to any one of claims 1 to 4 is arranged on a photodetector directly or via one or more protective layers.