JP2008103635A - Radiation image detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To raise image quality by enlarging an electric field formed in a photoconductive layer, while suppressing the occurrence of a dark current in a radiation image detector. <P>SOLUTION: An electric field at an edge B of a division electrode 6 is constituted so as not to exceed three times of an electric field at a center section A of the division electrode, more preferably not to exceed 1.5 times thereof, by covering the edge B of the division electrode with an insulation protective film 7. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention provides a photoconductive layer that generates a charge upon irradiation with radiation, a common electrode provided on one side of the photoconductive layer to which a predetermined voltage is applied, and a photoconductive layer provided on the other side. The present invention also relates to a radiation image detector comprising a plurality of divided electrodes that output signals corresponding to the charges generated in the photoconductive layer.

  Today, in the medical field and the like, various radiological image detectors have been proposed in which charges generated by irradiation of radiation transmitted through a subject are temporarily stored in a power storage unit, and the stored charges are converted into electrical signals and output. Yes. Various types of radiation image detectors have been proposed. From the aspect of a charge reading process for reading out accumulated charges to the outside, an optical reading method for reading the detector by irradiating it with reading light, and There is 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 reading method).

  The radiation image detector as described above includes a photoconductive layer that generates charges when irradiated with radiation, a common electrode formed on the side of the photoconductive layer irradiated with radiation, and a common electrode side of the photoconductive layer. A split electrode formed on the opposite side of the electrode, and by applying a negative high voltage to the common electrode and a positive high voltage to the split electrode, an electric field is formed in the photoconductive layer, and the radiation transmitted through the subject The charges generated in the photoconductive layer as a result of irradiation are moved to the divided electrodes under the action of the electric field, and the radiation collected by the divided electrodes is taken out as a radiation detection signal to detect the radiation.

  Here, the charge generated in the photoconductive layer moves to the divided electrodes as the electric field formed in the photoconductive layer increases, and the detected charge amount detected as an image signal increases. On the other hand, when this electric field exceeds a certain threshold value, the dark current increases rapidly, and the generated dark current is detected together with the image signal, leading to degradation of image quality. This threshold varies depending on the material used for the photoconductive layer. For example, in the case of amorphous-Se (a-Se), when the electric field exceeds 25 V / μm to 30 V / μm, the dark current increases rapidly.

  Furthermore, when a positive high voltage is applied to the split electrode, positive charges are concentrated from the central part of the polarization electrode at the end of the split electrode, and the electric field formed at the end of the split electrode is at the central part of the split electrode. Since the electric field is higher than the formed electric field, the phenomenon in which the dark current rapidly increases occurs from the end of the divided electrode.

Therefore, it is necessary to make the electric field at the end of this divided electrode not more than the above threshold value. In Patent Document 1, it is proposed to provide an insulating film at the end of the split electrode in order to prevent dark current generated at the end of the split electrode.
JP 2005-183670 A

  However, in the case where the electric field formed in the photoconductive layer is increased in order to improve the image quality of the radiographic image, it is not sufficient to suppress the dark current simply by providing a protective film as in Patent Document 1. , There is a possibility that image quality will be degraded.

  Accordingly, an object of the present invention is to provide a radiation image detector that can suppress the generation of dark current and improve the image quality by increasing the electric field formed in the photoconductive layer. .

  The radiation image detector of the present invention includes a photoconductive layer that generates charges upon irradiation of radiation, a common electrode provided on one side of the photoconductive layer to which a predetermined voltage is applied, and a photoconductive layer A radiation image detector including a plurality of divided electrodes that output signals corresponding to charges generated in the photoconductive layer provided on the other side, wherein an electric field at an end of the divided electrode is The electric field is configured so as not to exceed three times the electric field of the part.

  In the above apparatus, it is more preferable that the electric field at the end of the divided electrode is configured not to exceed 1.5 times the electric field at the center of the divided electrode.

Further, the electric field at the end of the divided electrode may be configured not to exceed three times the electric field at the center of the divided electrode by covering the end of the divided electrode with an insulating protective film. Moreover, if the radiographic image detector of this invention has the said common electrode, a photoconductive layer, and a division | segmentation electrode in this order, another layer and micro electroconductivity are further between those layers or on the upper and lower sides of those layers. A member may be provided.

  According to the radiation image detector of the present invention, a photoconductive layer that generates charges upon irradiation of radiation, a common electrode provided on one side of the photoconductive layer to which a predetermined voltage is applied, and a photoconductive layer In a radiation image detector comprising a plurality of divided electrodes that output signals corresponding to the charges generated in the photoconductive layer provided on the other side of the layer, the electric field at the ends of the divided electrodes is By configuring so as not to exceed three times the electric field at the center, it is possible to increase the electric field formed in the photoconductive layer while keeping the electric field at the end of the divided electrode below the dark current generation threshold. , Image quality can be improved.

  In the above apparatus, when the electric field at the end of the split electrode is configured not to exceed 1.5 times the electric field at the center of the split electrode, the electric field at the end of the split electrode is kept below the dark current generation threshold. Further, the electric field formed in the photoconductive layer can be further increased, and a radiographic image with better image quality can be obtained.

  Further, by covering the end portion of the divided electrode with an insulating protective film so that the electric field at the end portion of the divided electrode does not exceed three times the electric field at the central portion of the divided electrode, insulating protection The electric field formed in the photoconductive layer can be increased by reducing the electric field at the end of the divided electrode by the film and suppressing the generation of dark current.

  Hereinafter, an embodiment of the radiation image detector of the present invention will be described with reference to the drawings. FIG. 1 is a perspective view showing a schematic configuration of an embodiment of a radiation image detector of the present invention. 2 is a cross-sectional view of the radiation image detector shown in FIG. 1 taken along the line II-II.

  The radiation image detector 1 includes a common electrode 2 that is transparent to radiation, a recording photoconductive layer 3 that is electrically conductive by generating charge pairs upon irradiation with radiation transmitted through the common electrode 2, A charge transport layer 4 that acts as an insulator for the latent image charges generated in the recording photoconductive layer 3 and acts as a conductor for transport charges having a polarity opposite to that of the latent image charges. A photoconductive layer 5 for reading that generates electric charge pairs by receiving irradiation and exhibits conductivity, a plurality of divided electrodes 6 that transmit the read light, and an insulation formed so as to cover the ends of the divided electrodes 6 Protective film 7 is provided. In addition, between the recording photoconductive layer 3 and the charge transport layer 4, a power storage unit 8 that accumulates charges generated in the recording photoconductive layer 3 is formed. The radiation image detector 1 is formed on a light-transmitting substrate (not shown) (for example, a glass substrate).

The common electrode 2 is charged with negative charges when recording radiographic image information, and is formed in a flat plate shape. The common electrode 2 is made of a material that transmits radiation, such as Nesa film (SnO ₂ ), ITO (Indium Tin Oxide), IDIXO (Idemitsu Indium X-metal Oxide) which is an amorphous light-transmitting oxide film; The film is formed to a thickness of 50 to 200 nm using a material such as Co., Ltd.), or is formed to a thickness of 100 nm.

  The recording photoconductive layer 3 generates charge pairs when irradiated with radiation, and is excellent in that, for example, the quantum efficiency is relatively high with respect to radiation and the dark resistance is high. It is formed by depositing a film containing Se as a main component to a thickness of about 500 μm.

The charge transport layer 4 acts as an insulator for charges of one polarity among the charges generated in the recording photoconductive layer 3 and acts as a conductor for charges of the other polarity. It has become. The charge transport layer 4, for example the difference in charge mobility between the mobility of the charge to be the opposite polarity to the charge on the common electrode 2 at the time of recording a radiation image is large (e.g., 10 ² or more, preferably 10 ³ or more For example, poly N-vinylcarbazole (PVK), N, N′-diphenyl-N, N′-bis (3-methylphenyl)-[1,1′-biphenyl] -4,4′- It is formed of an organic compound such as diamine (TPD) or a discotic liquid crystal, a TPD polymer (polycarbonate, polystyrene, PVK) dispersion, or a semiconductor material such as a-Se doped with 10 to 200 ppm of Cl.

  The reading photoconductive layer 5 generates charge pairs upon irradiation with reading light or erasing light. For example, a-Se, Se-Te, Se-As-Te, metal-free phthalocyanine, metal phthalocyanine, It is formed by forming a film to a thickness of about 0.1 to 1 μm using a photoconductive substance mainly composed of at least one of MgPc (Magnesium phtalocyanine), VoPc (phase II of Vanadyl phthalocyanine), CuPc (Cupper phtalocyanine) and the like. Has been.

  The divided electrodes 6 are linear electrodes that read image information recorded in the radiographic image detector 1 as signal charges when the radiographic image detector 1 is irradiated with reading light, and are arranged in stripes. . The divided electrode 6 is made of a material that transmits the reading light L1, and is formed to have a thickness of 0.5 μm to 1 μm using, for example, Al, Au, Cr, ITO (Indium Tin Oxide), IZO (Indium Zinc Oxide), or the like.

As shown in FIG. 2, the protective film 7 suppresses the dark current generated from the end of the divided electrode 6 by reducing the electric field (Eb) at the end B in the direction orthogonal to the longitudinal direction of each divided electrode 6. Therefore, it is formed so as to cover both ends of the divided electrode along the longitudinal direction of the divided electrode 6. Specifically, each divided electrode 6 is charged with a positive charge when a positive high voltage is applied, but is more positive than the central portion A of the divided electrode 6 at the end B of the divided electrode 6 shown in FIG. Therefore, the electric field (Eb) at the end B of the split electrode is higher than the electric field (Ea) at the center A of the split electrode, and dark current is likely to occur. Here, by providing an insulating protective film so as to cover the end B of the divided electrode 6, the electric field (Eb) at the end B in the direction orthogonal to the longitudinal direction of the divided electrode 6 is increased in the longitudinal direction of the divided electrode 6. The electric field (Ea) of the central portion A in the direction perpendicular to the center is less than 3 times, more preferably less than 1.5 times. Various protective materials can be used as long as they are insulating materials. For example, acrylic resin, SiO ₂ , polyimide, PVA (polyvinylalcohol), PVP (polyvinyl pyrrolidone), PAA (Polyacrylic Acid), and novolac resin. , Ta ₂ O ₅ , ZrO ₂ , Al ₂ O _{3 and the} like.

  However, the electric field (Eb) at the end B of the split electrode 6 cannot be made less than three times the electric field (Ea) at the center A of the split electrode simply by providing the protective film 7. For example, by appropriately selecting and providing the protective film thickness D, protective film width W, protective film electrical characteristics (dielectric constant), etc., the electric field (Eb) at the end B of the divided electrode 6 is divided. The electric field (Ea) of the central portion A of the electrode 6 can be less than 3 times, more preferably less than 1.5.

  In Table 1 below, the distance R between the divided electrodes 6 in the radiation image detector 1, the thickness D of the protective film 7, the width W of the protective film 7, and the dielectric constant of the protective film 7 are changed. Table 1 shows the results of examining the ratio of the electric field (Eb) at the end B of the divided electrode 6 to the electric field (Ea) at the center A of the divided electrode 6 (electric field ratio = Eb / Ea).

As a constituent material of the protective film 7, an acrylic resin film is used when the relative dielectric constant is 3, and a Ta ₂ O ₅ composite film is used when the relative dielectric constant is 10. The distance R between the divided electrodes 6 is 5 μm to 20 μm, the width W of the protective film 7 is 0.5 μm to 2 μm, and the thickness D of the protective film 7 is changed in the range of 0.05 μm to 0.2 μm. Ea) was calculated, and the electric field ratio was evaluated with a value of less than 1.5 being ◎, 1.5 to 3 being ○, and 3 or more being ×.

  In Table 1 above, when the relative dielectric constant of the protective film 7 is 3 or 10, the distance R between the divided electrodes 6 is 5 μm to 20 μm, and the width W of the protective film 7 is 0.5 μm to 2 μm, for example, the thickness of the protective film 7 When the thickness D was 0.2 μm or more, the electric field (Eb) at the end B of the divided electrode 6 was less than three times the electric field (Ea) at the center A of the divided electrode 6. When the relative dielectric constant of the protective film 7 is 10, the distance R between the divided electrodes 6 is 5 μm to 10 μm, and the width W of the protective film 7 is 0.5 μm to 2 μm, the thickness D of the protective film 7 is 0.2 μm. In the case above, the electric field (Eb) at the end B of the divided electrode 6 was less than 1.5 times the electric field (Ea) at the center A of the divided electrode 6.

  With the above configuration, in the radiation image detector 1 of this embodiment, when a bias voltage is applied to the common electrode 2 and an electric field is formed between the common electrode 2 and the divided electrode 6, the recording photoconductivity is recorded. When the layer 3 is irradiated with radiation carrying image information, positive and negative charges are generated in the recording photoconductive layer 3, and the negative charges follow the electric field distribution formed by the application of the voltage. Then, it is concentrated on the divided electrode 6 and accumulated as a latent image charge in the power storage unit 8 which is an interface between the recording photoconductive layer 3 and the charge transport layer 4. The amount of latent image charge is substantially proportional to the amount of irradiation radiation, and this amount of latent image charge indicates a radiation image. On the other hand, positive charges generated in the recording photoconductive layer 3 are attracted to the common electrode 2 and are combined with the negative charges injected from the voltage source and disappear.

  Next, when reading the radiation image recorded in the radiation image detector 1 as described above, linear reading light extending in the direction orthogonal to the longitudinal direction of the divided electrode 6 is applied in the longitudinal direction of the divided electrode 6. When the entire surface of the radiation image detector 1 is moved and scanned, positive and negative charge pairs are generated in the reading photoconductive layer 5 corresponding to the scanning position of the reading light, and the positive and negative charges generated in the reading photoconductive layer 5 are generated. The charge is attracted to the latent image charge of the storage unit 8 and is recombined with the latent image charge in the power storage unit 8 and disappears. On the other hand, the negative charge generated in the reading photoconductive layer 5 Combine and disappear. An image signal based on the latent image charge generated corresponding to each pixel at the time of reading can be detected by a current detection amplifier (not shown) connected to the divided electrode 6 and read as a radiation image.

  Next, in the radiation image detector 1, the electric field (Eb) at the end B of the divided electrode 6 is fixed at 25 V / μm, and the ratio of the electric field between the end B and the center A of the divided electrode 6 (Eb / Ea). Table 1 shows the results of calculating the electric field (Ea) of the central portion A of the divided electrode 6 while changing the image quality and evaluating the image quality in each case.

  Here, in each case of Table 1, when a predetermined bias voltage is applied to the common electrode 2 and the divided electrode 6 and an electric field is formed between the common electrode 2 and the divided electrode 6, radiation is generated. The ratio of the electrical signal (bright current) detected in the irradiated state to the electrical signal (dark current) detected in the absence of radiation (hereinafter referred to as the bright / dark current ratio) The image quality is evaluated by comparing the light / dark current ratio in each case with the light / dark current ratio of the comparative example described below.

  As a comparative example, a radiation image detector provided with a second common electrode instead of the divided electrode 6 in the radiation image detector 1 is used. The same bias voltage as in the other cases of Table 1 is applied to the common electrode 2 and the second common electrode of this comparative example, and an electric field is formed between the common electrode 2 and the second common electrode. Sometimes, the ratio (bright / dark current ratio) between the electrical signal (bright current) detected in the state of radiation irradiation and the electrical signal (dark current) detected in the state of no radiation irradiation is obtained.

In each case of Table 1, when the light / dark current ratio is 90 to 100% of the light / dark current ratio of the comparative example, the light / dark current ratio is 50 of the light / dark current ratio of the comparative example. If less than 90%, the bright / dark current ratio was less than 50% of the comparative example bright / dark current ratio.

  As shown in Table 2, the ratio of the electric field (Eb) at the end B of the split electrode 6 to the electric field (Ea) at the center A of the split electrode 6 (electric field ratio = Eb / Ea) is higher than 3. When the ratio of the electric field (Eb) at the end B of the divided electrode 6 to the electric field (Ea) at the center A of the divided electrode 6 (electric field ratio = Eb / Ea) does not exceed 3, the detected charge amount The electric field (Ea) at the central portion A of the divided electrode 6 that controls the above can be made larger than 10 V / μm, and the entire electric field formed in the photoconductive layer can be made larger, resulting in high-quality radiation. An image is obtained. Furthermore, it is clear that if the electric field ratio does not exceed 1.5, it is possible to obtain a radiographic image with better image quality.

  As described above, the radiographic image detector according to the present invention is configured so that the electric field (Eb) at the end B of the divided electrode 6 does not exceed three times the electric field (Ea) at the center A of the divided electrode 6. It is possible to increase the electric field formed in the photoconductive layer while setting the electric field at the end of the divided electrode to be equal to or lower than the dark current generation threshold by preferably configuring not to exceed 1.5 times, Image quality can be improved.

  Further, by covering the end B of the divided electrode 6 with the insulating protective film 7, the electric field (Eb) at the end B of the divided electrode 6 is three times the electric field (Ea) at the center A of the divided electrode 6. More preferably, according to the above embodiment configured not to exceed 1.5 times, the electric field formed in the photoconductive layer can be reduced by suppressing the generation of dark current at the end B of the divided electrode 6. It is possible to increase the image quality and improve the image quality.

 In the above embodiment, the radiographic image detector 1 of the present invention has an electric field (Eb) at the end B of the divided electrode 6 that is three times the electric field (Ea) at the center A of the divided electrode, more preferably 1 As a method of preventing the ratio from exceeding 5 times, a method of providing the protective film 7 at the end A of the divided electrode 6 has been described. The electric field (Eb) at the end B of the divided electrode 6 can be divided by any method such as rounding the extreme end shape, narrowing the gap between the divided electrodes, or combining them. You may comprise so that it may not exceed 3 times of the electric field (Ea) of the center part A of the electrode 6, more preferably 1.5 times.

 In all the divided electrodes 6, the electric field (Eb) at the end B of the divided electrode 6 does not exceed three times, more preferably 1.5 times, the electric field (Ea) at the center A of the divided electrode 6. Although not limited to this, the electric field (Eb) at the end B of the divided electrode 6 is 3 of the electric field (Ea) at the center A of the divided electrode 6 at least in the divided electrode 6 within the image forming range. It is sufficient that it is not to exceed 1.5 times, more preferably 1.5 times.

  Further, the present invention is not limited to the so-called optical reading type radiographic image detector shown in the above embodiment, but can be applied to any type of radiographic image detector such as a TFT reading type.

 For example, the radiographic image detector 11 of the TFT reading system shown in FIG. 3 has a photoconductive layer when a bias voltage is applied to the common electrode 12 and an electric field is formed between the common electrode 12 and the divided electrode 16. When the radiation carrying image information is irradiated to 13, positive and negative charges are generated in the photoconductive layer 13, and the negative charges are divided electrodes along the electric field distribution formed by the application of the voltage. 16 and accumulated in a storage capacitor 34 electrically connected to the divided electrode 16. Thereafter, a signal for turning ON the switching element 18 is sequentially applied via the scanning line 31, the electric charge accumulated in each storage capacitor 34 is taken out via the data line 35, detected by the amplifier 33, and read as a radiation image. Can do. In each pixel portion 15 of this radiographic image detector shown in FIG. 4, the electric field at the end (periphery) of the quadrangular divided electrode 16 indicated by a broken line does not exceed three times the electric field at the center (intersection of diagonal lines). A protective film 17 covering the entire end (periphery) of the divided electrode 16 can be provided.

  In the above-described embodiment, the present invention is applied to a so-called direct conversion radiation oblique line image detector that records radiation images by receiving radiation and converting the radiation directly into electric charges. However, the present invention is not limited to this. For example, the present invention is also applied to a so-called indirect conversion radiation image detector that records radiation images by once converting radiation into visible light and converting the visible light into electric charges. Is possible.

1 is a schematic configuration diagram showing an embodiment of a radiation image detector of the present invention. II-II sectional view of the radiation image detector shown in FIG. Sectional drawing which shows schematic structure of one Embodiment of the radiographic image detector of this invention Layout of the pixel portion of the radiation image detector shown in FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Radiation image detector 2 Common electrode 3 Photoconductive layer for recording (photoconductive layer)
4 Charge Transport Layer 5 Reading Photoconductive Layer 6 Divided Electrode 7 Protective Film 8 Power Storage Unit A Center Part of Divided Electrode B Edge of Divided Electrode R Distance Between Divided Electrodes D Protective Film Thickness W Protective Film Width

Claims (3)

A photoconductive layer that generates a charge upon irradiation with radiation;
A common electrode provided on one side of the photoconductive layer to which a predetermined voltage is applied;
A radiation image detector comprising: a plurality of divided electrodes that output signals corresponding to charges generated in the photoconductive layer provided on the other side of the photoconductive layer;
A radiographic image detector, wherein an electric field at an end portion of the divided electrode is configured not to exceed three times an electric field at a central portion of the divided electrode.   2. The radiation image detector according to claim 1, wherein the electric field at the end of the divided electrode is configured not to exceed 1.5 times the electric field at the center of the divided electrode.   By covering the ends of the divided electrodes with an insulating protective film, the electric field at the ends of the divided electrodes is configured not to exceed three times the electric field at the center of the divided electrodes. The radiation image detector according to claim 1.

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