JP2007163250A - Radiation detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a radiation detector for effecting satisfactory matching of the emission spectrum of a phosphor with the spectral sensitivity of a photoconductive layer. <P>SOLUTION: This radiation detector 1 is made up of a light transforming part comprising a scintillator 3 for transforming radiation thereto applied into visible light of an amount corresponding to the dose of the radiation, a photoconductive layer for therein storing electric charge with the visible light from the transforming part transformed into electric charge, and a solid photo-detector 2 for reading electric charge stored in the photoconductive layer. The scintillator 3 is made of a phosphor with one-half or more of its luminous photons falling within a wavelength range from 500 to 700 nm while the photoconductive layer is made of a-Se doped with Te. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a radiation detector suitable for application to a radiation imaging apparatus such as an X-ray, and particularly relates to a radiation detector using a combination of a scintillator and a photodetector.

2. Description of the Related Art In radiography for medical diagnosis, there is known a radiological image detection apparatus that uses a radiation detector (which mainly includes a semiconductor) that detects radiation and converts it into an electrical signal. As a radiation detector, a direct conversion method in which radiation is directly converted into electric charge and accumulated, and radiation is once converted into light by a scintillator such as CsI: Tl, GOS (Gd ₂ O ₂ S: Tb), and the light There is an indirect conversion method in which the photoconductive layer converts the charges into charges and accumulates them. In addition, from the reading method, reading is performed by a radiation image detector using a semiconductor material that generates a charge when irradiated with light, a so-called optical reading method, and the charge generated by irradiation of radiation is accumulated, and the accumulated charge is stored in a thin film transistor ( A method of reading by turning on and off an electrical switch such as a thin film transistor (TFT) one pixel at a time (hereinafter referred to as a TFT method).

As the indirect conversion type radiation detector, a scintillator using CsI: Tl, GOS (Gd ₂ O ₂ S: Tb) which emits greenish white or green light, and a-Se as a photoconductive layer is known. (Patent Document 1). However, these combinations have a problem of low spectral sensitivity. On the other hand, when a-Se is used as a photoconductive layer, a combination of CsI: Na that emits blue light is known as a scintillator that exhibits good spectral sensitivity (Patent Documents 2 and 3). , CsI: Na has a problem that its moisture resistance is extremely poor because of its high deliquescence.
JP 2000-137080 A JP 2000-346951 A JP 2000-035480 A

  The present invention has been made in view of the above circumstances, and in a radiation detector in which half or more of the emitted photons are in the wavelength range of 500 nm to 700 nm and the photoconductive layer is a combination of a-Se, An object of the present invention is to provide a radiation detector having a good spectral sensitivity matching between a light emission spectrum of a body and a photoconductive layer and having high moisture resistance.

The radiation detector according to the present invention includes a light conversion unit composed of a phosphor layer that converts irradiated radiation into visible light in an amount corresponding to the dose of the radiation, and charges by absorbing visible light from the light conversion unit. In a radiation detector comprising a photoconductive layer for generating and transporting light and an electrode portion for taking out the charge generated in the photoconductive layer, the phosphor layer has a wavelength of 500 nm to 700 nm in which ½ or more of the emitted photons are It is made of a certain phosphor, and the photoconductive layer is made of a-Se doped with Te.
The doping amount of Te is preferably 0.1 to 30 mol% of the a-Se.

As a phosphor in which ½ or more of the emitted photons are in the wavelength range of 500 nm to 700 nm, CsX: Tl (X is either I, Br or Cl, or a mixture thereof. Hereinafter, this description is omitted. ), Or an oxide or oxysulfide phosphor activated with Tb ³⁺ and / or Eu ³⁺ .

  The radiation detector according to the present invention includes a light conversion unit composed of a phosphor layer that converts irradiated radiation into visible light in an amount corresponding to the dose of the radiation, and absorbs visible light from the light conversion unit to charge. In a radiation detector comprising a photoconductive layer that generates and transports light and an electrode part that extracts charges generated in the photoconductive layer, the phosphor layer has a wavelength of 500 nm to 700 nm in which at least half of the emitted photons are Since it consists of fluorescent substance and a photoconductive layer consists of a-Se doped with Te, it can be set as a radiation detector with the very favorable matching of the spectral sensitivity of the light emission spectrum of a fluorescent substance, and a photoconductive layer.

In particular, when the phosphor is made of an oxide or oxysulfide-based phosphor activated with CsX: Tl or Tb ³⁺ and / or Eu ³⁺ , matching between the emission light of the phosphor layer and the detector In addition, the light conversion efficiency of the fluorescent layer can be further improved, radiation can be converted into an image signal without waste, and a radiation detector having both high efficiency and moisture resistance can be obtained.

  Hereinafter, the radiation detector of the present invention will be described with reference to the drawings. FIG. 1 is a partially enlarged view showing a schematic configuration diagram of a radiation detector of the present invention. As shown in FIG. 1, the radiation detector 1 is configured by laminating a solid-state photodetector 2 and a scintillator 3 on a support 21, and the solid-state photodetector 2 includes a photoconductive portion 10 including a photoconductive layer 12. And a thin film transistor layer 20 are formed. The thin film transistor layer 20 is a layer in which a large number of transistors 20a arranged two-dimensionally with a desired pixel pitch are formed. One transistor 20a and the corresponding portion of the photoconductive portion 10 constitute one solid state detection element, that is, the solid state detector 2 is composed of a large number of solid state detection elements arranged two-dimensionally.

The scintillator 3 is a light conversion unit composed of a phosphor layer that converts irradiated radiation into visible light in an amount corresponding to the dose of the radiation, and ½ or more of the emitted photons have a wavelength range of 500 nm to 700 nm. It consists of fluorescent substance in. More preferably, it consists of an oxide or oxysulfide-based phosphor activated by CsX: Tl, or Tb ³⁺ and / or Eu ³⁺ . The scintillator 3 may be composed of a particle dispersion film composed of the phosphor and a binder containing and supporting the phosphor in a dispersed state (coating method), or vaporizing and vaporizing the phosphor or its raw material by a vapor deposition method or the like. It is also possible to use a vapor deposition film (vapor deposition method) formed by depositing on a support. The layer thickness of the scintillator when using the coating method is preferably about 50 to 300 μm, and the layer thickness of the scintillator when using the vapor deposition method is preferably about 100 to 1000 μm.

FIG. 3 shows an enlarged view of one solid state detection element of the solid state detector 2. As described above, the solid state detector 2 includes the photoconductive portion 10 and the thin film transistor layer 20 (hereinafter referred to as the TFT layer 20). As shown in FIG. 3, the structure of each TFT 20a of the TFT layer 20 includes a semiconductor film (amorphous silicon (a-Si layer), polysilicon (p-Si layer), organic semiconductor, amorphous oxide formed on the substrate 21. A source electrode and drain electrodes 23 and 24 and a gate electrode 26 via a gate insulating film 25 are formed with a physical semiconductor film (a-InGaZnO ₄ layer or the like) 22 interposed therebetween. The electrodes 23, 24 and 26 are all made of a conductive material such as metal or transparent oxide.

  As the substrate 21, glass, aluminum, ceramics, or the like is used. However, a resin sheet that retains strength necessary for the support may be used.

  The photoconductive portion 10 is composed of a photoconductive layer 12 that receives light and exhibits conductivity, that is, performs photoelectric conversion, and transparent electrodes 11 and 13 disposed with the photoconductive layer 12 interposed therebetween. Note that the solid state detector 2 of the present embodiment includes a power storage unit 15 that accumulates charges generated in the photoconductive unit 10, and takes out the charges accumulated in the power storage unit 15 by TFT. The power storage unit 15 includes an electrode 13 and an electrode 14 and an insulating layer 25 sandwiched between the electrodes.

  The photoconductive layer 12 is made of a-Se doped with Te. The doping amount of Te is preferably 0.1 to 30 mol%, more preferably 1 to 10 mol% of a-Se. The photoconductive layer can be laminated on the TFT by continuously forming the photoconductive layer by vapor deposition. The layer thickness of the photoconductive layer is preferably about 0.1 to 100 μm. When the photoconductive layer becomes thicker, its own X-ray absorption is also involved in image formation, which can be said to be a hybrid type that forms an image together with X-ray absorption by the phosphor layer.

Subsequently, radiographic imaging using the radiation detector 1 of the present invention will be briefly described.
When X-rays that have passed through the subject are irradiated onto the radiation detector 1, the X-rays irradiated onto the radiation detector 1 are converted into visible light by the scintillator 3. The scintillator 3 emits visible light in an amount corresponding to the absorbed X-ray dose. This visible light is photoelectrically converted in the photoconductive layer 10 and electric charges are accumulated in the power storage unit 15 in accordance with the emission intensity. Thereafter, this electric charge is read out, and an image signal as an electrical signal is output. The output image signal is input to the information processing means and subjected to predetermined image processing and the like. The radiographic image of the subject is reproduced as a visible image.

  In the above embodiment, as the solid state detector, the arrangement in which the photoconductive layer and the TFT layer are arranged in the arrangement in which the photoconductive layer is on the radiation irradiation surface side has been described. It is good also as a structure which has arrange | positioned the layer.

  Next, CsI: Tl phosphor as a phosphor in which ½ or more of the emitted photons are in the wavelength range of 500 nm to 700 nm and CsI: Na phosphor, which has been known to have a better match with a-Se than in the past. A combination of three types of scintillators and three photoconductive layers of a-Se and a-Se doped with 5 mol% of a-Se and Te and a-Se doped with 30 mol% of Te are evaluated for matching. .

  FIG. 3 shows an emission spectrum I (λ) showing the relationship between wavelength λ and relative luminance in two types of scintillators of CsI: Tl phosphor and CsI: Na phosphor (each peak intensity is normalized to 1), 4 is a spectral sensitivity S (λ) indicating the relationship between wavelength λ and relative sensitivity in three types of photoconductive layers of a-Se, a-Se doped with 5 mol% Te, and a-Se doped with 30 mol% Te. (The respective peak intensities are normalized to 1).

Here, the matching factor (MF) is defined as follows.
MF = ∫S _x (λ) I _y (λ) dλ / ∫I _y (λ) dλ
For example, if S _x (λ) = 1, then MF = 1. If there is no overlap between the emission and spectral sensitivities, MF = 0 because S _x (λ) I _y (λ) = 0.

Table 1 and FIG. 5 show the matching factors of the emission spectrum and the spectral sensitivity calculated as described above.

  As is apparent from FIG. 4, the spectral sensitivity of the photoconductive layer made of a-Se is improved by doping Te, and as shown in FIG. 5, in the photoconductive layer doped with Te, a- The matching with the scintillator CsI: Tl is improved over Se, and the matching is better than the CsI: Na phosphor, which is better matched with a-Se than in the past. In general, the MF obtained from the above calculation formula is preferably 0.65 or more.

Here, as an example of CsX: Tl, only matching with CsI: Tl is shown, but even if X is Br, Cl or a mixture thereof, Ln ₂ O ₂ S: Tb (Ln is Gd , Lu, La or Y, or a mixture thereof) can be obtained even in an oxide or oxysulfide-based phosphor activated with Tb ³⁺ and / or Eu ^3+. it can.

  As described above, in the radiation detector of the present invention, the phosphor layer is made of a phosphor in which ½ or more of the emitted photons are in the wavelength range of 500 nm to 700 nm, and the photoconductive layer is doped with Te. Since it consists of Se, it can be set as the radiation detector with the very favorable matching of the spectral sensitivity of the light emission spectrum of a fluorescent substance, and a photoconductive layer.

  Although the TFT type radiation detector has been described above, the combination of the scintillator of the present invention and the photoconductive layer is a so-called light which is read by a radiation image detector using a semiconductor material that generates a charge when irradiated with light. It can also be used as a reading method.

The elements on larger scale which show the schematic block diagram of the radiation detector of this invention Partial enlarged view showing one element of a solid state detector Emission spectrum showing the relationship between phosphor wavelength and relative luminance Spectral sensitivity showing the relationship between wavelength of photoconductive layer and relative sensitivity Graph showing matching of emission spectrum and spectral sensitivity

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Radiation detector 2 Solid state light detector 3 Scintillator
10 Photoconductive section
11 Transparent electrode
12 Photoconductive layer
13,14 electrode
15 Animal power department
20 Thin film transistor layer
20a thin film transistor
21 Board
22 Semiconductor film

Claims (3)

  A light conversion unit comprising a phosphor layer that converts irradiated radiation into visible light in an amount corresponding to the radiation dose, and a photoconductive layer that absorbs visible light from the light conversion unit to generate and transport charges And a radiation detector comprising an electrode portion for taking out the charge generated in the photoconductive layer, wherein the phosphor layer is made of a phosphor in which a half or more of the emitted photons are in a wavelength range of 500 nm to 700 nm, and the light A radiation detector, wherein the conductive layer is made of a-Se doped with Te.   The radiation detector according to claim 1, wherein a doping amount of the Te is 0.1 to 30 mol% of the a-Se. An oxide or oxysulfide system in which the phosphor layer is activated by CsX: Tl (X is any of I, Br, Cl, or a mixture thereof), or Tb ³⁺ and / or Eu ³⁺ The radiation detector according to claim 1, wherein the radiation detector is made of a phosphor.

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