JP2000241553A - Radiation detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce light crosstalk between scintillator elements and to improve the resolution of a CT device by allowing a light separator to project by a specific amount from the light emission surface of an emission part. SOLUTION: The thickness of an optical adhesive layer 3 sticking a silicon photodiode SPD 4 to an emission surface 23 ranges from approximately 1 to 10 μm. A light separator 22 projects from the emission surface 23 of the lower surface of a scintillator element 21 of an emission part 2, namely from a surface facing to the SPD 4, into the optical adhesive layer 3 by 0.1-5 μm. Further, the amount of projection of the light separator 22 is preferably 0.1-2 μm. By allowing the light separator 22 to project from the emission surface 23 of the lower surface of the emission part 2 into the optical adhesive layer 3, a light crosstalk can be reduced, in which visible light discharged from the scintillator element is received by an adjacent photodiode element 41 through the optical adhesive layer 3 in the light crosstalk.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a radiation detector, and more particularly to a radiation detector used in a computed tomography (CT) apparatus using radiation such as X-rays and γ-rays.

[0002]

2. Description of the Related Art In a radiation CT apparatus, a large number of radiation detectors are arranged side by side at a position symmetrical to a radiation source (for example, an X-ray tube) with respect to an object to be photographed, and the radiation intensity at the position of each detector is measured. The internal structure of the object is observed. Since the radiation detectors arranged next to each other correspond to each pixel, the radiation detectors are made as small as possible and the interval between adjacent radiation detectors is reduced to increase the resolution and resolution.

The radiation detector has a structure in which a plurality of scintillator elements and a photodiode element are stacked, and the scintillator element is opened to the radiation source side to receive radiation such as X-rays by the scintillator element. I have. The scintillator elements are CdWO ₄ , Bi ₄ Ge ₃ O ₁₂ , Gd ₂ O
₂ S: Made of Pr (Ce, F) or the like, and emits visible light when radiation is incident. This visible light is incident on a photodiode element provided opposite to the back of the scintillator element to convert it into an electric signal. When the radiation incident on a certain scintillator element passes through the scintillator element and re-enters the adjacent scintillator element, the resolution is reduced.Therefore, a radiation shielding plate is provided between the scintillator elements so that the radiation does not pass. . Further, the visible light generated by the scintillator element is generated in the direction of all solid angles, but needs to be guided to a photodiode element provided below the scintillator element. Therefore, the scintillator element has a structure in which the periphery is covered with a material having good light reflectivity except for the surface facing the photodiode element.

As a shielding plate, a metal plate such as Mo, W, Pb, a metal foil, or a metal film is used. Further, in order to attach a material having good light reflectivity to the scintillator surface, a metal such as aluminum is vapor-deposited or sputtered to a thickness of 0.1 to 5 μm.
m thick or a white paint such as titanium oxide (Ti
O ₂ ), zinc white (ZnO), lead white (PbO), zinc sulfide (ZnS), and the like. Titanium oxide is zinc white,
Compared to lead white and zinc sulfide, they have high chemical stability against acids and alkalis and have high light reflectance, so they are often used as light reflectors. The present inventors have already invented that, among titanium oxides, rutile-type titanium oxide can be used as a stable reflector because of its excellent light resistance. Is described as a light reflecting material. Such a white paint light reflecting material provided on a radiation shielding plate of Mo, W, Pb or the like is inserted between adjacent scintillator elements. In this specification, the radiation shield provided between the scintillator elements and the light reflecting material (or light reflecting adhesive layer) are collectively referred to as an optical separator.

[0005] SPD is adhered to the lower surface of a light emitting portion having a plurality of scintillator elements with an adhesive, and a light detecting diode element of the SPD is provided at a position facing each scintillator element. It is necessary to transmit the visible light generated from the scintillator element emitted by the radiation irradiation to the light detection diode element with as little loss as possible. Optical adhesives made of good materials are used.
Since the bonding surface between the lower surface of the light emitting portion and the SPD has some irregularities, the thickness of the optical adhesive layer varies from about 1 to 10 μm.

In one block of the radiation detector,
A plurality of rectangular scintillator elements are provided between the scintillator elements via a light separator on one SPD in which channels are patterned. Conventional 1
In order to increase the resolution, 24 scintillator elements were added to the SPD in order to increase the resolution.
24 channels / block provided side by side. In order to further increase the resolution, a device which is divided at right angles to the channel direction and formed into a lattice has begun to be put into practical use. Since the scintillator elements are arranged in a lattice, the thickness of the optical separator must be reduced as much as possible. Accordingly, the thickness of the optical separator has been gradually reduced, and the thickness of the optical separator has been reduced from 200 to 250 μm to 50 to 100 μm.

[0007] Since the thickness of the optical separator has become thinner as described above, light generated by the scintillator element travels through the optical adhesive layer and is detected by the light detecting diode element facing the adjacent scintillator element. It has become more and more. When the light intensity detected by the adjacent light-detecting diode element is compared with the light intensity detected by the light-detecting diode element facing under the scintillator element that has received the light, it may reach about 10% or more. . By reducing the width of the photodiode element, such optical crosstalk can be reduced. However, if the width of the photodiode element is reduced, the output also decreases and the detection sensitivity deteriorates. Was.

Various measures have been proposed to reduce such optical crosstalk. For example, according to the method disclosed in Japanese Patent Application Laid-Open No. 2-17489, a scintillator wafer and an SPD are bonded to each other with an optical adhesive.
The scintillator wafer is divided into a plurality of scintillator elements, and an X-ray shielding heavy metal plate is inserted into the groove. Further, in the one proposed in Japanese Patent Application Laid-Open No. Hei 3-206992, a special structure of an SPD in which a light emitting portion in which a plurality of scintillator elements are arranged via an optical separator therebetween is joined by an optical adhesive is shown. ing. In this device, a groove is formed in a glass substrate, a photodiode element is buried in the groove, and an optical adhesive is applied on the SPD in the groove and joined to the light emitting portion. By doing so, the glass portion protruding and remaining between the photodiode elements forms a shielding plate continuously with the optical separator between the scintillator elements.

[0009]

It is considered that these known techniques can reduce optical crosstalk generated along the optical adhesive layer between the light emitting portion and the SPD. However, forming a groove in the scintillator wafer with a dicer and inserting a shielding plate in the groove not only requires processing, but also damages the photodiode element because the processing is also performed on the SPD. . Further, when a groove for embedding a photodiode element is provided in a glass substrate, it is difficult to accurately control the depth of the groove.
In addition, since it is necessary to prepare a photodiode element which is individually separated, it is troublesome from the viewpoint of production.

Accordingly, an object of the present invention is to provide a radiation detector which is suitable for use in a radiation CT apparatus or the like and which reduces optical crosstalk between scintillator elements, and which is easy to manufacture.

[0011]

A radiation detector according to the present invention has a light emitting portion in which a plurality of scintillator elements are arranged in a plane with an optical separator interposed therebetween, and optically bonded to a light emitting surface on a lower surface of the light emitting portion. A silicon photodiode (SPD) attached with an agent, wherein the photodiode element of the SPD is provided at a position facing the scintillator element under each of the scintillator elements, wherein the optical separator emits light. It is characterized in that it protrudes from the light emitting surface of the portion by 0.1 to 5 μm.

Further, in the radiation detector of the present invention, it is preferable that a radiation shielding plate also protrudes from a portion of the light emitting portion that protrudes from the light emitting surface.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A radiation detector according to the present invention and a radiation CT apparatus using the same will be described below in detail with reference to the drawings. Here, FIG. 1 is a cross-sectional view of the radiation detector of the present invention, FIG. 2 is a view for explaining a manufacturing process of the radiation detector of the present invention, and FIG. 3 is a lower surface of a light emitting unit used in the radiation detector of the present invention. FIG. 4 is an explanatory diagram of a radiation CT apparatus.

As shown in the sectional view of FIG. 1, the radiation detector of the present invention comprises a light emitting section 2 in which a plurality of rectangular parallelepiped scintillator elements 21 are arranged in a plane with an optical separator 22 interposed therebetween. It has a silicon photodiode (SPD) 4 attached to the light emitting surface 23 on the lower surface of the light emitting unit 2 (here, the lower surface of the light emitting unit is referred to as “light emitting surface”) with the optical adhesive layer 3. This SPD 4 is connected to each scintillator element 21.
The photodiode element 41 is provided at a position facing the lower surface of the scintillator element under the light emitting element. Photodiode element 41
These electrodes are connected to a radiation detection circuit so that the amount of light received by the photodiode element 41 can be measured. S
Since the configurations and functions of the PD and the photodiode element are already well known, description thereof will be omitted.

The scintillator element 21 is composed of CdWO ₄ , Bi
_{It is made of} 4Ge ₃ O ₁₂ , Gd ₂ O ₂ S: Pr (Ce, F) or the like, and emits visible light by emitting light at a portion receiving radiation, for example, X-rays. Adjacent scintillator elements 2
1 is a radiation shielding plate 221 made of a metal having good radiation shielding performance such as Mo, W, and Pb.
Is provided with an adhesive layer 222 on the surface thereof. The adhesive layer 222 bonds the scintillator element 21 and the radiation shielding plate 221 and contains rutile-type titanium oxide powder, and has a light reflectance of 90% or more, preferably 9%.
It is preferable that the content be 5% or more so as to function also as a light reflecting material. The upper surface and the end surface of the light emitting section 2 are cast with a light reflecting material layer 5.

The upper surface of the light emitting section 2, that is, the upper surface of the scintillator element 21 faces the direction facing the radiation source, and the radiation enters the scintillator element 21 from that surface. The visible light generated by the radiation incident on the scintillator element 21 exits in the direction of the full solid angle, but is reflected by the adhesive layer 222 of the optical separator 22 and the light reflecting material layer 5 attached to the upper surface and the end surface of the light emitting section 2. Then, the light is guided to and detected by the photodiode element 41 attached to the light emitting surface 23 on the lower surface of the scintillator element 21. Further, the radiation transmitted through the scintillator element 21 is substantially absorbed by the radiation shielding plate 221 of the optical separator 22 provided on the side surface of the element, so that the influence on the adjacent scintillator element is small.

The thickness of the optical adhesive layer 3 for attaching the SPD 4 and the light emitting surface 23 is about 1 to 10 μm. From the light emitting surface 23 on the lower surface of the scintillator element 21 of the light emitting section 2, that is, the surface facing the SPD, the light separator 22 is 0.1 to
It protrudes into the 5 μm optical adhesive layer 3. The projection may include the radiation shielding plate 221 therein, or only the radiation shielding plate 221 without the adhesive layer 222 may project, but the effect remains unchanged.

The length of the optical separator 22 protruding into the optical adhesive layer 3 from the light emitting surface 23 on the lower surface of the scintillator element 21 of the light emitting section 2, that is, the surface facing the SPD 4, is 0.1 to 2 μm. Is more preferable.
The projection of the optical separator 22 having such a length can be obtained without deterioration of the scintillator element 21 during processing as described later.

By causing the optical separator 22 to protrude into the optical adhesive layer 3 from the light emitting surface 23 on the lower surface of the light emitting section 2, visible light emitted from a certain scintillator element causes the optical adhesive layer 3 to pass through. The so-called optical crosstalk that is transmitted and received by the adjacent photodiode element 41 can be reduced. As will be described later, when the protrusion amount of the optical separator 22 is 0.1 μm, optical crosstalk is reduced by 4 μm.
%, And the optical crosstalk can be reduced to 1.0% with a protrusion amount of 1.5 μm or more.

The radiation detector 1 of the present invention can be manufactured by the manufacturing steps shown in FIG. First, FIG.
The scintillator wafer 210 and the radiation shielding plate 221 are overlapped with each other as described in (1), and are bonded therebetween via an adhesive layer 222 containing rutile-type titanium oxide powder. The scintillator wafers 210 are stacked so that the required number of scintillator elements is obtained (b). After heating at a temperature at which the adhesive resin cures for several hours to solidify the adhesive layer 222,
The light emitting part 2 is formed as shown in FIG. This is placed on the adhesive sheet 6, and a mold 7 is placed on the adhesive sheet 6 so as to surround the light emitting portion.
The mixture of 0.5 to 3 is poured into the mold 7 and the periphery of the light emitting section 2 is cast with the mixture (which becomes the light reflecting material layer 5) (e). This is heated for several hours at a temperature at which the epoxy resin cures in the air to solidify the resin. Next, the adhesive sheet 6 and the mold 7 are removed, and machining is performed. At this time,
The lower surface of the light emitting section 2, the surface showing the processing roughness in FIG. 2 (f), is lapped with # 4000 or coarser abrasive grains. By lapping using the abrasive grains in this manner, a step is generated between the scintillator element and the optical separator as shown in the cross-sectional view of FIG. 1, and the optical separator projects from the scintillator element. This is S
The PD 4 is attached to the PD 4 using the optical adhesive layer 3 and assembled (g) to obtain the radiation detector 1. The manufacture of the lattice-shaped scintillator element can be realized by the same steps after (a) by using the assembly obtained in (d) above as the scintillator wafer 210 of (a).

The above description made with reference to FIG. 2F will be described in more detail. FIG. 3A is a cross-sectional view of the light emitting unit 2 before lapping is performed.
1 and optical separators 22 are alternately arranged. This lower surface, that is, the surface showing the processing roughness, is where lapping is performed. When this surface is lapped, the portion of the scintillator element 21 is polished more quickly as shown in the sectional view of FIG. 3B, and the portion of the optical separator 22 projects from the light emitting surface 23.

The size of this projection is almost determined by the number of abrasive grains used for lapping. As shown in Table 1, the use of finer abrasive grains reduces the amount of projection.
It can be seen that small projections, that is, coarse abrasive grains may be used to increase the amount of protrusion.

[0023]

[Table 1]

As described above, the optical crosstalk ratio was measured using a radiation detector having a light emitting portion in which the optical separator was projected from the scintillator element. Here, the thickness of the optical adhesive layer was 10 μm, and the thickness of the optical separator was about 100 μm assuming that a light reflective adhesive layer containing rutile-type titanium oxide powder was attached to the surface of a Mo thin plate as an optical separator. . Gd is used as a scintillator element.
A radiation detector made of ₂ O ₂ S: Pr (Ce, F) and having 16 channels / block was used. Table 2 shows the optical crosstalk ratio when the protrusion amount of the optical separator is changed to 0.05 to 2.5 μm.

[0025]

[Table 2]

As is clear from Table 2, the optical crosstalk is reduced by projecting the optical separator from the lower surface of the scintillator element by 0.1 μm or more. It can be seen that when the protrusion amount is 1.5 μm or more, the optical crosstalk is as extremely small as 1%. However, even if the protrusion amount is further increased, the optical crosstalk rate does not decrease, so that 1.5 to 2.5 μm seems to be the best. In order to increase the protrusion amount of the optical separator, it is necessary to use coarse abrasive grains during lapping. However, if coarse abrasive grains are used, Gd ₂ O ₂ S: Pr is used as a scintillator material.
When (Ce, F) is used, processing deterioration becomes severe, which leads to a decrease in light emission output.
m or less.

By controlling the amount of protrusion of the optical separator,
The control of the adhesive thickness of the optical adhesive became easy, and the presence of the protruding portion increased the adhesive area, thereby improving the adhesive strength.

As shown in FIG. 4, a radiation CT apparatus using a radiation detector has a subject 81 at the center of the CT apparatus 8.
Can be provided. Subject 81
A radiation source (for example, an X-ray tube) 82 is arranged so as to be able to move around the object, and the radiation detectors 1 are arranged side by side at a position facing the radiation source 82 with respect to the object to be imaged 81. The fan-shaped radiation 83 emitted from the radiation source 82 is absorbed by each part of the object 81 to produce a shadow of the object 81 on the radiation detector 1. Thus, the brightness of the shadow of the object 81 can be obtained. The radiation source 82 and the radiation detector 1
, The same measurement is performed while rotating, and the measured values are combined and reconstructed into an image to obtain a CT image. By using a radiation detector with a small optical crosstalk rate as in the present invention, the resolution of the detector is increased, so that a radiation CT apparatus with extremely high sensitivity can be obtained.

[0029]

As described above, the radiation detector according to the present invention reduces optical crosstalk between scintillator elements, and has a structure that is easy to manufacture. A radiation CT apparatus using this radiation detector can have a high resolution.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a radiation detector according to the present invention.

FIG. 2 is a diagram illustrating a manufacturing process of the radiation detector of the present invention.

FIG. 3 is a diagram illustrating polishing of a lower surface of a light emitting unit used in the radiation detector of the present invention.

FIG. 4 is an explanatory diagram of a radiation CT apparatus.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Radiation detector 2 Light emitting part 21 Scintillator element 210 Scintillator wafer 22 Optical separator 221 Radiation shielding plate 222 Adhesive layer 23 Light emitting surface 3 Optical adhesive layer 4 Silicon photodiode (SPD) 41 Photodiode element 5 Light reflective material layer 6 Adhesive sheet 7 Formwork 8 Radiation CT device 81 Object to be photographed 82 Radiation source 83 Radiation

Claims (2)

[Claims] 1. A light emitting unit in which a plurality of scintillator elements are arranged in a plane with an optical separator therebetween, and a silicon photodiode (SPD) attached to a light emitting surface on a lower surface of the light emitting unit with an optical adhesive. Wherein the photodiode element of the SPD is a radiation detector provided at a position facing the scintillator element below each of the scintillator elements, wherein the optical separator is 0.1 to 0.1 mm from the light emitting surface of the light emitting unit. 5 μm
A radiation detector characterized by being protruding. 2. The radiation detector according to claim 1, wherein a radiation shielding plate also protrudes from a portion of the light emitting unit that protrudes from the light emitting surface.