Abstract:
Disclosed is an electromagnetic wave information detection apparatus, including a photoelectric converter including first and second electrode layers; a charge generation layer that generates positive and negative charges by irradiation of an electromagnetic wave; and a charge transport layer; an electric potential imparting unit that imparts electric potentials to the first and second electrode layers; a detection unit; and a control unit controlling the electric potential imparting unit and the detection unit such that the electric potentials of the first and second electrode layers are equalized during a predetermined period of time between a process of imparting detection electric potentials to the first and second electrode layers to detect information carried by an electromagnetic wave of a previous irradiation and a process of imparting the detection electric potentials to the first and second electrode layers to detect information carried by an electromagnetic wave of a subsequent irradiation.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority under 35 USC 119 from Japanese Patent Application No. 2010-84375 filed on Mar. 31, 2010, the disclosure of which is incorporated by reference herein. 
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to an electromagnetic wave information detection apparatus and an electromagnetic wave information detection method, and more particularly, to an indirect conversion type radiation imaging apparatus and a method which obtain an image by converting radiation into visible light. 
     2. Related Art 
     J. Non-Cry. Sol. 299-302 (2002) 1240-1244 discloses an indirect conversion type imaging element that uses a scintillator (GOS: gadolinium oxysulfide) which converts radiation into visible light, and an organic photoelectric conversion film (OPC) which serves as a photoelectric conversion film converting visible light into an electrical signal and is a laminate of a charge generation layer (benzimidazole) and a hole transport layer (polycarbonate and TPD (N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine)). However, the literature does not disclose anything about the presence or absence of an afterimage generation or an elimination method of the afterimage when such an imaging element is used. 
     U.S. Pat. No. 6,171,643 discloses a direct conversion type radiation imaging element that does not use a scintillator but uses a multi-layer structure containing amorphous selenium and a method of driving the radiation imaging element. The multi-layer structure has a p-layer (As 2 Se 3  layer), an i-layer (amorphous selenium thick layer), and an n-layer (amorphous selenium layer with doped alkali metal). Sensitivity may be stabilized by short-circuiting between upper and lower electrodes for a short period of time to neutralize space charges in a detector after an X-ray irradiation. This advantage is achieved by a layer (As 2 Se 3  layer) with lower resistance than that of an amorphous selenium layer that performs generating charges and transporting the charges. Thus, it is considered that the As 2 Se 3  layer is essential, and the advantage is said to be achieved because the As 2 Se 3  layer has low resistance since the As 2 Se 3  layer has more free holes than the amorphous selenium layer. A junction between the As 2 Se 3  layer and a substrate electrode is said to perform the role of “charge injection”, thereby accelerating the neutralization of negative space charges. However, the As 2 Se 3  layer has problems in that, for example, it has low heat resistance, evaporation cost is high, and toxicity is significant. U.S. Pat. No. 6,171,643 relates to a direct conversion type radiation imaging element that does not use scintillator but uses amorphous selenium, and does not disclose either the presence or absence of an afterimage generation in an imaging element including a photoelectric conversion film having a charge generation layer and a charge transport layer nor an elimination method of the afterimage. 
     The present inventors have found that sensitivity varies in an imaging element including a photoelectric conversion film having a charge generation layer and a charge transport layer when the imaging element continuously captures images in many times or the imaging element captures an image with a large dosage. 
     SUMMARY 
     A main object of the present invention is to provide an electromagnetic wave information detection apparatus and an electromagnetic wave information detection method capable of suppressing variations in sensitivity. 
     According to a first aspect of the present invention, there is provided an electromagnetic wave information detection apparatus, including: 
     a photoelectric converter including: a first electrode layer; a charge generation layer that generates positive charge and negative charge upon being irradiated with an electromagnetic wave carrying information and that is electrically connected to the first electrode layer; a charge transport layer that transports only either the positive charge or the negative charge generated in the charge generation layer; and a second electrode layer that is electrically connected to the charge transport layer; 
     an electric potential imparting unit that imparts respective electric potentials to the first and second electrode layers; 
     a detection unit that is connected to the photoelectric converter and is configured to detect the information, which is carried by the electromagnetic wave and photoelectrically converted by the photoelectric converter; and 
     a control unit that controls the electric potential imparting unit and the detection unit such that the electric potential imparting unit equalizes the electric potentials of the first and second electrode layers during a predetermined period of time between a process in which the electric potential imparting unit imparts respective detection electric potentials to the first and second electrode layers and the detection unit detects information carried by an electromagnetic wave of a previous irradiation and a process in which the electric potential imparting unit imparts the respective detection electric potentials to the first and second electrode layers and the detection unit detects information carried by an electromagnetic wave of a subsequent irradiation. 
     According to a second aspect of the present invention, there is provide an electromagnetic wave information detection method, including: 
     detecting information, which is carried by an electromagnetic wave of an irradiation and is photoelectrically converted by a photoelectric converter, by imparting respective detection electric potentials to first and second electrode layers of the photoelectric converter, which includes: the first electrode layer; a charge generation layer that generates positive charge and negative charge upon being irradiated with an electromagnetic wave carrying information and that is electrically connected to the first electrode layer; a charge transport layer that transports only either the positive charge or the negative charge generated in the charge generation layer; and the second electrode layer, which is electrically connected to the charge transport layer; 
     thereafter equalizing electric potentials of the first and second electrode layers during a predetermined period of time; and 
     thereafter detecting information of an electromagnetic wave of a subsequent irradiation by imparting the respective detection electric potentials to the first and second electrode layers of the photoelectric converter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An exemplary embodiment of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a schematic longitudinal sectional view for explaining a radiation dosage monitor and a method of monitoring a radiation dosage according to a first preferred exemplary embodiment of the invention; 
         FIG. 2  is a diagram illustrating a relationship between an X-ray irradiation time and a photocurrent density in the radiation dosage monitor according to the first preferred exemplary embodiment of the invention; 
         FIG. 3  is a diagram illustrating a relationship between an X-ray irradiation time and a photocurrent density in the radiation dosage monitor according to a Comparative Example 1; 
         FIG. 4  is a schematic longitudinal sectional view for explaining an indirect conversion type radiation imaging apparatus and an imaging method using the same according to a second preferred exemplary embodiment of the invention; and 
         FIG. 5  is a schematic longitudinal sectional view for explaining a pixel unit of the indirect conversion type radiation imaging apparatus according to the second preferred exemplary embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, exemplary embodiments of the invention will be described with reference to the accompanying drawings. 
     (First Exemplary Embodiment) 
     Referring to  FIG. 1 , a radiation dosage monitor  100  according to a first preferred exemplary embodiment includes a sensor unit  110  and a control device  140 . The sensor unit  110  includes a flexible substrate  112 , an electrode layer  114  disposed on the flexible substrate  112 , a polymer undercoat layer  116  disposed on the electrode layer  114 , a charge generation layer  118  disposed on the polymer undercoat layer  116 , a charge transport layer  120  disposed on the charge generation layer  118 , a bias electrode layer  122  disposed on the charge transport layer  120 , and a fluorescent layer  126  attached onto the bias electrode layer  122  with an adhesive layer  124  interposed therebetween. 
     The flexible substrate  112  is made of polyethylene naphthalate (PEN). The electrode layer  114  is made of Indium Tin Oxide (ITO). The polymer undercoat layer  116  includes those obtained by application and drying of alcohol-soluble polyamide, and has function of strongly joining the electrode layer  114  and the charge generation layer  118 . The charge generation layer  118  includes a charge generator that generates holes having a positive charge and electrons having a negative charge by being irradiated with electromagnetic wave, and a polymer binder. A dibromoantoanthrone pigment is used as the charge generator, and a polyvinyl butyral resin is used as the polymer binder. The charge transport layer  120  includes a charge transport agent that transports only holes, and a polymer binder. An example of the charge transport agent is N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine An example of the polymer binder is polycarbonate. The bias electrode layer  122  is made of indium-zinc oxide (IZO). The adhesion layer  124  includes an acrylic double-sided adhesion tape. The fluorescent layer  126  includes KODAK Lanex Regular (Ga 2 O 2 S: Tb) that is a fluorescent sheet manufactured by KODAK Company. 
     The sensor unit  110  is manufactured as described below. First, the flexible substrate  112  is prepared in a form in which an ITO film that serves as the electrode layer  114  is formed in a thickness of, for example, 100 nm by sputter deposition of the ITO film, onto polyethylene naphthalate (PEN) having a thickness of, for example, 0.1 mm. 
     Next, alcohol-soluble polyamide was applied onto the electrode layer  114  made of ITO to be 0.1 μm in thickness by spin coating, and dried to form the polymer undercoat layer  116 . 
     Next, the dibromoantoanthrone pigment and the polyvinyl butyral resin were added, in the mass ratio of 50:50, to cyclohexanone and dispersed. The obtained dispersion was applied by spin coating onto the polymer undercoat layer  116 , to form the charge generation layer  118  having a thickness of 0.5 μm. 
     Next, 3.5 g of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine and 6.5 g of polycarbonate (molecular weight: about 35,000 to 40,000) as the charge transport agent were dissolved in 35 g of methylene chloride, and the obtained solution was applied onto the charge generation layer  118  by bar coating to form the charge transport layer  120 . The charge transport layer  120  was dried at 135° C. for 1 hour, and measured for the film thickness. The measured thickness was 2 μm. 
     Next, IZO was film-formed to have a thickness of 100 nm on the charge transport layer  120  to form the bias electrode layer  122 . 
     Next, the sensor unit  110  of the radiation dosage monitor  100  was manufactured by preparing KODAK Lanex Regular (Ga 2 O 2 S: Tb 380 μm), which is a fluorescent sheet manufactured by KODAK Company and attaching the fluorescent sheet to the bias electrode layer  122  with the use of the adhesive layer  124  (with a thickness of 20 μm) of an acrylic double-sided adhesion tape interposed therebetween to form the fluorescent sheet  116  which serves as an X-ray scintillator. 
     The control device  140  includes a bias voltage application circuit  142 , a detection circuit  144 , and a control unit  146 . One end of the bias voltage application circuit  142  is connected to the bias electrode layer  122 , the other end of the bias voltage application circuit  142  is connected to one end of the detection circuit  144 , and the other end of the detection circuit  144  is connected to the electrode layer  114  and a ground potential  130 . The bias voltage application circuit  142  and the detection circuit  144  are connected to the control unit  146  to be controlled by the control unit  146 . 
     Next, a method of measuring a radiation dosage using the radiation dosage monitor  100  will be described. 
     The bias voltage application circuit  142  and the detection circuit  144  were controlled by the control unit  146  to apply a bias voltage of −20 V to the bias electrode layer  122 , and an X-ray with a dosage of 400 mR (80 kVp) was irradiated by ten shots at an interval of 30 seconds. A photocurrent density for each shot was detected by the detection circuit  144  and the detected photocurrent densities were compared with each other. The voltage application time of each shot was 20 seconds. Before each shot, the bias electrode layer  122  is applied with a ground voltage and the electric potential of the bias electrode layer  122  is set to be the same (short-circuiting) as that of the electrode layer  114 . In such a manner, a sensitivity stabilization process was performed. A bias voltage short-circuiting time before detection of the radiation dosage in each shot was set to 10 seconds. As a consequence, as shown in  FIG. 2 , variations in sensitivity of the photocurrent by X-ray irradiation were within ±1%. In this exemplary embodiment, the bias voltage short-circuiting time was set to 10 seconds. However, when the bias voltage short-circuiting time is 1 second or more, the variations in sensitivity can be sufficiently suppressed. Moreover, the variations in sensitivity can be reliably suppressed when the bias voltage short-circuiting time is 10 seconds or more. 
     Next, as a Comparative Example 1, the bias voltage application circuit  142  and the detection circuit  144  were controlled by the control unit  146  to apply a bias voltage of −20 V to the bias electrode layer  122 , and an X-ray with a dosage of 400 mR (80 kVp) was irradiated by ten shots at an interval of 30 seconds. A photocurrent density for each shot was detected by the detection circuit  144  and the detected photocurrent densities were compared with each other. The voltage application time of each shot was 30 seconds. In the Comparative Example 1, the bias voltage of −20 V was kept to be applied to the bias electrode layer  122  for ten shots and no sensitivity stabilization process described above was performed. As a consequence, as shown in  FIG. 3 , variations in sensitivity of the photocurrent by X-ray irradiation were ±10% or more. 
     (Second Exemplary Embodiment) 
     Referring to  FIG. 4 , an indirection conversion type radiation imaging apparatus  200  according to a second preferred exemplary embodiment of the present invention includes an imaging unit  210 , a control device  260 , and a radiation generation device  270 . 
     The imaging unit  210  includes a radiation detection unit  220 , a gate line driver  242 , a signal processing unit  244 , a bias voltage application circuit  246 , and a reference electric potential imparting circuit  248 . 
     The control device  260  are connected to the gate line driver  242 , the signal processing unit  244 , the bias voltage application circuit  246 , and the reference electric potential imparting circuit  248 . The control device  260  controls the gate line driver  242 , the signal processing unit  244 , the bias voltage application circuit  246 , and the reference electric potential imparting circuit  248 . 
     In the radiation detection unit  220 , plural pixel units  300  are disposed on a substrate  222  in a matrix. Each of the pixel units  300  includes a radiation charge conversion unit  304  which converts radiation into charges, a storage capacitor  330  which stores the charges converted by the radiation charge conversion unit  304 , and a thin film transistor (TFT)  320  which reads the charges stored in the storage capacitor  330 . 
     Plural gate wiring lines  226  are arranged in a given direction (row direction) and turns on/off the thin film transistors  320  of the respective pixel units  222 . Plural signal wiring lines  228  are arranged in a direction (column direction) intersecting the gate wiring lines  226  to read the stored charges from the storage capacitors  330  via the turned-on thin film transistors  320 . The gate wiring lines  226  and the signal wiring lines  228  are disposed on the substrate  222 . 
     The gate wiring lines  226  are connected to the gate line driver  242  and the signal wiring lines  228  are connected to the signal processing unit  244 . While the pixel units  300  are being irradiated with radiation, the thin film transistors  320  of the pixel units  300  are all turned off in response to signals supplied from the gate line driver  242  via the gate wiring lines  226  under the control of the control device  260 , and thus the charges corresponding to the charges that are photoelectrically converted from the radiation by the radiation charge conversion units  304  are stored in the storage capacitors  330  of the pixel units  300 . After the pixel units  300  are irradiated with the radiation, the thin film transistors  320  of the pixel units  300  are turned on sequentially one row after another in response to signals supplied from the gate line driver  242  via the gate wiring lines  226  under the control of the control device  260 . The charges stored in the storage capacitor  330  of the pixel unit  300  in which the thin film transistor  320  is turned on is transmitted as an analog electric signal and is input to the signal processing unit  244  via the signal wiring line  228 . Thus, the charges stored in the storage capacitors  330  of the pixel units  300  are read sequentially by a row. 
     Each of the signal wiring lines  228  is also connected to the reference electric potential imparting circuit  248 . Changeover switches (not shown) switching between the signal processing unit  244  and the reference electric potential imparting circuit  248  for each signal wiring line  228  are respectively provided in the signal processing unit  244  and the reference electric potential imparting circuit  248 . The changeover switches perform switching under the control of the control device  260  so that the respective signal wiring lines  228  are connected to either the signal processing unit  244  or the reference electric potential imparting circuit  248 . 
     A bias electrode  346  (see  FIG. 5 ) described below is disposed on the entire surface of the substrate  222 . The bias electrode  346  (see  FIG. 5 ) is connected to the bias voltage application circuit  246 . Under the control of the control device  260 , the bias voltage application circuit  246  applies a predetermined bias voltage or a reference electric potential such as a ground potential to the bias electrode  346  (see  FIG. 5 ). An electrode  332  (storage capacitor lower electrode  332 , see  FIG. 5 ) of the storage capacitor opposite to the radiation charge conversion unit  304  is connected to a ground potential  302 . 
     Next, each pixel unit  300  of the radiation detection unit  220  will be described with reference to  FIG. 5 . 
     The storage capacitors  330  and the thin film transistors  320  are disposed on one surface of the flexible substrate  310 . The storage capacitor  330  includes a storage capacitor upper electrode  334 , a storage capacitor lower electrode  332 , and a dielectric layer  314  (the dielectric layer  314  also functions as an insulation layer) interposed therebetween. The thin film transistor  320  includes a drain electrode  324 , a source electrode  326  connected to the storage capacitor upper electrode  334 , an active layer (channel layer)  328  located between the source electrode  326  and the drain electrode  324 , a protective layer  329  formed to cover the active layer  328 , and a gate electrode  322  located opposite to the active layer  328  with the dielectric layer  314  functioning as an insulation layer interposed therebetween. 
     An inter-layer insulation layer  338  is disposed on the storage capacitors  330  and the thin film transistors  320 . A contact hole  339  is disposed in the inter-layer insulation layer  338  on the storage capacitor upper electrode  334 . A charge collection electrode (lower electrode)  336  is disposed on the inter-layer insulation layer  338 . The charge collection electrode  336  is a pixel electrode. The charge collection electrode  336  is connected to the storage capacitor upper electrode  334  through the contact hole  339 . 
     A polymer undercoat layer  312  is disposed on the charge collection electrode  336 . A charge generation layer  344 , a charge transport layer  342 , and a bias electrode  346  are disposed on the polymer undercoat layer  312  in this order. The charge generation layer  344  and the charge transport layer  342  form an organic photoelectric conversion layer  340 . A fluorescent layer  350  is disposed on the bias electrode  346  with an inter-layer insulation layer  316  interposed therebetween. 
     The flexible substrate  310  is formed by attaching polyethylene naphthalate (PEN) and thin glass to each other. The gate electrode  322  and the storage capacitor lower electrode  332  are made of Mo. The dielectric layer  314  is made of silicon dioxide. The source electrode  326 , the drain electrode  324 , and the storage capacitor upper electrode  334  are made of IZO. The active layer  328  is made of IGZO (In—Ga—Zn-Oxide). The protective layer  329  is made of amorphous Ga 2 O 3 . The inter-layer insulation layer  338  is made of acrylic resin. The charge collection electrode  336  is made of IZO. The polymer undercoat layer  116  is obtained by application and drying of alcohol-soluble polyamide, and has a function of strongly joining the charge collection electrode  336  and the charge generation layer  344 . 
     The charge generating layer  344  includes of a charge generating agent that generates holes having a positive charge and electrons having a negative charge when it is irradiated with an electromagnetic wave, and a polymer binder. A dibromoantoanthrone pigment is used as the charge generator, and a polyvinyl butyral resin is used as the polymer binder. The charge transport layer  342  includes a charge transport agent that transports only holes, and a polymer binder. As the charge transport agent, N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine is used. As the polymer binder, polycarbonate is used. The bias electrode  346  is made of IZO. 
     The inter-layer insulation layer  316  includes an acrylic double-sided adhesion tape. The fluorescent layer  350  includes KODAK Lanex Regular (Ga 2 O 2 S: Tb) that is a fluorescent sheet manufactured by KODAK Company. The fluorescent layer  350  converts radiation, which has been transmitted through an inspection object of an imaging target to be examined when irradiating the inspection object with radiation, into visible light. 
     The radiation detection unit  220  was manufactured in the following way. First, Mo was deposited into a film with a thickness of 40 nm by sputtering on the flexible substrate  310  formed by attaching thin glass with a thickness of 0.15 mm and PEN with a thickness of 0.1 mm to each other, and subjected to patterning through photolithography and wet etching, to form the gate electrode  322  and the storage capacitor lower electrode  332 . 
     On the resultant structure, silicon dioxide was deposited into a film with a thickness of 200 nm by sputtering to form the dielectric layer  314  that also functions as an insulation layer. 
     Next, IZO was deposited into a film with a thickness of 200 nm by sputtering without introducing oxygen during deposition, and subjected to patterning through photolithography and wet etching, to form the source electrode  326 , the drain electrode  324 , and the storage capacitor upper electrode  334 . The edges of the source electrode  326  and the drain electrode  324  facing each other formed a tapered angle of 25°. 
     Next, IGZO was deposited into a film with a thickness of 10 nm by sputtering, and subjected to patterning through photolithography and wet etching, to form the active layer  328 . 
     Next, amorphous Ga 2 O 3  was deposited into a film with a thickness of  10  rim by sputtering, and only an area covering the active layer  328  was remained to form the protective layer  329 . 
     Next, the inter-layer insulation film  338  made of acrylic resin was applied and the contact hole  339  was formed in the inter-layer insulation layer  338  on the storage capacitor upper electrode  334 . 
     Next, IZO was deposited into a film with a thickness of 40 nm and then patterned to form the charge collection electrode (lower electrode)  336 . The charge collection electrode  336  was connected to the storage capacitor upper electrode  334  through the contact hole  339 . 
     Next, alcohol-soluble polyamide was applied by spin coating onto the charge collection electrode  336  in a thickness of 0.1 μm, and dried to form the polymer undercoat layer  312 . 
     Next, the dibromoantoanthrone pigment and the polyvinyl butyral resin were added in the mass ratio of 50:50 to cyclohexanone, and dispersed. The obtained dispersion was applied by spin coating onto the polymer undercoat layer  312 , to form the charge generation layer  344  of 0.5 μm thickness. 
     Next, 3.5 g of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine and 6.5 g of polycarbonate (molecular weight: about 35,000 to 40,000) as a charge transport agent were dissolved in 35 g of methylene chloride, and the obtained solution was applied onto the charge generation layer  344  by bar coating to form the charge transport layer  342 . The charge transport layer  342  was dried at 135° C. for 1 hour, and measured for the film thickness. The measured thickness was 2 μm. 
     Next, IZO was deposited into a film with a thickness of 100 nm on the charge transport layer  342  to form the bias electrode layer  346 . 
     Next, KODAK Lanex Regular (Ga 2 O 2 S: Tb 380 μm) being a fluorescent sheet manufactured by KODAK Company was prepared, and the fluorescent sheet and the bias electrode layer  346  were attached to each other with the adhesive layer  124  (with a thickness of 20 μm) of an acrylic double-sided adhesion tape interposed therebetween, to manufacture the radiation detection unit  220  of the imaging unit  210  of the indirect conversion type imaging apparatus  200 . 
     In the indirect conversion type imaging apparatus  200  according to this exemplary embodiment, the radiation that has transmitted through a subject to be inspected is incident onto the fluorescent layer  350  and is converted into visible light that is an electromagnetic wave with a wavelength longer than that of the radiation by the fluorescent layer  350 . The converted visible light is converted into holes having a positive charge and electrons having a negative charge in the organic photoelectric conversion layer  340 . The converted holes in the organic photoelectric conversion layer  340  migrate through the charge transport layer  342  due to an electric potential difference between the bias electrode  346  and the charge collection electrode  336 , and were collected to the bias electrode  346 . The converted electrons in the organic photoelectric conversion layer  340  are stored in the storage capacitor  330  including the storage capacitor upper electrode  334  connected to the charge collection electrode  336 , the storage capacitor lower electrode  332 , and the dielectric layer  314  therebetween. The storage capacitor upper electrode  334  and the drain electrode  324  are connected to each other. Therefore, when an ON-voltage is applied to the gate electrode  322  to turn on the thin film transistor  320 , the charge stored in the storage capacitor  330  is transmitted from the drain electrode  324  through the signal wiring line  228  (see  FIG. 4 ) and is input to the signal processing unit  244  (see  FIG. 4 ). After the charges are input to the signal processing unit  244  (see  FIG. 4 ), the charges are appropriately A/D-converted and stored in an image memory (not shown). Radiation image information stored in the image memory (not shown) or the like is displayed as a visible image on a display unit of a display apparatus (not shown). 
     Next, an imaging method using the indirect conversion type radiation imaging apparatus  200  will be described. 
     The radiation generation device  270 , the gate line driver  242 , the signal processing unit  244 , the bias voltage application circuit  246 , and the reference electric potential imparting circuit  248  were controlled by the control unit  260 , thereby a bias voltage of −20 V was applied to the bias electrode  346  during a bias application time of 20 seconds, and in this condition the radiation generation device  270  applied an X ray with a radiation dosage of 400 mR (80 kVp) by ten shots at an interval of 30 seconds. The X ray sensitivity for each shot was read from a captured image and the X ray sensitivities for all shots were compared. 
     Before each shot, the source electrodes  326  (connected to the charge collection electrodes  336  which are pixel electrodes) and the drain electrodes  324  of the thin film transistors  320  are electrically conducted by applying the ON-voltage to the gates of the thin film transistors  320  by the gate line driver  242  under the control of the control device  260 . In this state, the sensitivity stabilization process was performed by switching the changeover switches (not shown) in the signal processing unit  244  and the reference electric potential imparting circuit  248  under the control of the control device  260  to change the connection of the signal wiring lines  228  from the signal processing unit  244  to the reference electric potential imparting circuit  248 , imparting a ground potential to the respective signal wiring lines  228  by the reference electric potential imparting circuit  248 , and simultaneously imparting the ground potential to the bias electrodes  346  by the bias voltage application circuit  246  so that the bias electrodes  346  and the charge collection electrodes  336  have the same electric potential (that is, the bias electrodes  346  and the charge collection electrodes  336  are short-circuited). A bias voltage short-circuiting time before each shot was set to 10 seconds. As a consequence, variations in sensitivity of the X-ray current were within ±1% and no afterimage did occur. In this exemplary embodiment, the bias voltage short-circuiting time was set to 10 seconds. However, when the bias voltage short-circuiting time is 1 second or more, the variations in sensitivity can be sufficiently suppressed. Moreover, the variations in sensitivity can be reliably suppressed when the bias voltage short-circuiting time is 10 seconds or more. 
     Next, in a Comparative Example 2, the radiation generation device  270 , the gate line driver  242 , the signal processing unit  244 , the bias voltage application circuit  246 , and the reference electric potential imparting circuit  248  were controlled by the control unit  260 , thereby a bias voltage of −20 V was applied to the bias electrode  346 , and in this condition, the radiation generation device  270  applied an X ray with a radiation dosage of 400 mR (80 kVp) by ten shots at an interval of 30 seconds. The X-ray sensitivity for each shot was read from a captured image, and the X ray sensitivities for all shots were compared. The constant bias voltage of −20 V was kept to be applied to the bias electrodes  346  without performing the sensitivity stabilization process. As a consequence, variations in sensitivity of the X-ray current were 5% or more and afterimage occurred. 
     In this exemplary embodiment, all the gates of the thin film transistors  320  were maintained to be in the ON state while the bias electrodes  346  and the charge collection electrodes  336  were set to have the same electric potential. However, it is preferable to alternately repeat the ON state and the OFF state of the gates in terms of no variation in the minimum threshold value of the thin film transistors  320 . When the bias electrodes  346  and the charge collection electrodes  336  are once set to have the same electric potential, the charge collection electrodes  336  changes to a floating state from the same electric potential state even though the gates of the thin film transistors  320  are turned off. Therefore, there is almost no influence on the advantage of suppressing the variations in sensitivity. Therefore, when the bias voltage short-circuiting time including the period during which the gates are turned off is 1 second or more, the variations in sensitivity can be sufficiently suppressed. Moreover, the variations in sensitivity can be reliably suppressed when the bias voltage short-circuiting time including the period during which the gates are turned off is 10 seconds or more. 
     It is general that several frames can be scanned for 1 second. Therefore, even when all the gates of the thin film transistors  320  are not simultaneously turned on, the bias electrodes  346  and all the charge collection electrodes  336  come to have the same electric potential if the gates of the thin film transistors  320  are turned on sequentially one line after another in the initial frame. Therefore, even if there is a period in which the gates of the thin film transistors  320  are subsequently turned off, the charge collection electrodes  336  just change to the floating state from the same electric potential state. Therefore, the advantage of suppressed variations in sensitivity is hardly influenced. 
     Although the gadolinium sulfur compound (GOS: Tb) was used as the fluorescent sheet  126  in the first exemplary embodiment or as the fluorescent layer  350  of the second exemplary embodiment, cesium iodide (CsI: Tl) is preferably used. 
     The fluorescent sheet  126  in the first exemplary embodiment and the fluorescent layer  350  in the second exemplary embodiment are used as the scintillator converting radiation into visible light, however, if such scintillator is not used, the radiation dosage monitor  100  according to the first exemplary embodiment can be used as a visible right dosage monitor and the indirect conversion type radiation imaging apparatus  200  according to the second exemplary embodiment can be used as a visible light imaging apparatus. 
     In the first and second exemplary embodiments, the sensitivity may be stabilized when irradiation is performed with the X-ray dosage of 400 mR (80 KVp) by ten shots at an interval of 30 seconds. However, the sensitivity may be stabilized even in the case in which the apparatus is used for 100 shots at an interval of 30 seconds if an X ray is irradiated with a small dosage (for example, 30 mR or less, for imaging of a chest) for successive imaging for a long time as well as in the case in which an X ray is irradiated with a large dosage as in the first and second exemplary embodiments, for example, for imaging of a lumber spine. 
     The thickness of the charge generation layer  118  according to the first exemplary embodiment is 0.5 μm and the thickness of the charge generation layer  344  according to the second exemplary embodiment is 0.5 μm. However, thickness of the charge generation layer is preferably in the range from 0.5 μm to 3 μm. 
     The thickness of the charge transport layer  120  according to the first exemplary embodiment is 2 μm and the thickness of the charge transport layer  342  according to the second exemplary embodiment is 2 μm. However, the thickness of the charge transport layer is preferably in the range from 0.5 μm to 7 μm. When the charge transport layer is thin in the range from 0.5 μm to 7 μm, the short-circuiting time of the sensitivity stabilization process may preferably be shortened. The charge transport layer is a transparent layer with respect to an electromagnetic wave which generates charges in the charge generation layer. No charges are generated in the charge transport layer by the electromagnetic wave that generates charge in the charge generation layer. 
     In the first and second exemplary embodiments, during the short-circuiting time of the sensitivity stabilization process, the charge generation layer  118 , the charge generation layer  344 , the charge transport layer  120 , and the charge transport layer  342  are not irradiated with erasing light. Thus, according to the first and second exemplary embodiments, the sensitivity may be stabilized even without irradiation of erasing light. 
     Each of the charge generation layer  118  and the charge generation layer  344  in the first and second exemplary embodiments contains a charge generator and a polymer binder as components. As the charge generator, for example, phthalocyanine dyes or phthalocyanine pigments such as metal phthalocyanine and metal-free phthalocyanine, naphthalocyanine dyes or naphthalocyanine pigments, indigo dyes, quinacridone dyes, anthraquinone dyes, antoanthrone dyes such as dibromoantoanthrone, perylene dyes, azo dyes such as monoazo dyes, bisazo dyes and trisazo dyes, and cyanine dyes are preferably used. 
     An example of the polymer binder that is used in the charge generation layer  118  and the charge generation layer  344  includes polyvinyl butyral. 
     The ratio of the charge generator to the polymer binder in the mass ratio of the former:the latter is preferably selected from a range of 80:20 to 20:80, in terms of the balance between sensitivity stability and temporal stability. The ratio is more preferably selected from a range of 45:55 to 25:75. 
     A charge generator is dispersed in a solution in which a polymer binder is dissolved in a solvent to prepare a dispersion, and the dispersion is subjected to spin-coating and drying (also called baking) to evaporate the solvent to form the charge generation layer  118  and the charge generation layer  344 . 
     Each of the charge transport layer  120  and the charge transport layer  342  of the first and second exemplary embodiments contains a charge transport agent and a polymer binder as components. 
     As the charge transport agent, those known as a hole transport substance is preferably used. For example, examples of the charge transport agent include a triarylamine compound, a benzidine compound, a pyrazoline compound, a styrylamine compound, a hydrazone compound, a triphenylmethane compound, a carbazole compound, a polysilane compound, a thiophene compound, a phthalocyanine compound, a cyanine compound, a merocyanine compound, an oxonol compound, a polyamine compound, an indole compound, a pyrrole compound, a pyrazole compound, a polyarylene compound, a condensed aromatic carbocyclic compound (a naphthalene derivative, anthracene derivative, a phenanthrene derivative, a tetracene derivative, a pyrene derivative, a perylene derivative, a fluoranthene derivative), and a metal complex that has nitrogen-containing heterocyclic compound as a ligand. However, the charge transport agent is not limited thereto, and if it is an organic compound that has less ionization potential than that of an organic compound used as a charge generator, such organic compound may be used as a charge transport agent. 
     Examples of the polymer binder that is used in the charge transport layer  42  include polycarbonate, polyvinyl butyral, a homopolymer of acrylic acid ester or a copolymer of acrylic acid with other copolymeric monomers, a homopolymer of methacrylic acid ester or a copolymer of methacrylic acid with other copolymeric monomers, a homopolymer of styrene or a copolymer of styrene with other copolymeric monomers, for example, a copolymer with acrylonitrile, and polysulfone. 
     The charge transport layer is formed by preparing a solution in which a charge transport agent and a polymer binder are dissolved in a solvent, and applying the solution onto the charge generation layer by, for example, dip coating and spin coating, and baking the charge generation layer to evaporate the solvent. 
     Various exemplary embodiments of the invention have hitherto been described, however the invention is not limited the exemplary embodiments. Accordingly, the scope of the invention is limited only by the appended claims.