JP2010286316A - Optical detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical detector which detects light such as ultraviolet rays, infrared rays, and visible rays with high sensitivity and has excellent positional resolution and counting rate characteristics. <P>SOLUTION: The optical detector 101 includes an optoelectric conversion section 2 for converting incident light into electrons and emitting them, a pixel type electrode 6 which is placed at a position facing the optoelectric conversion section 2 to amplify the electrons through gas within a chamber 7 and detect them, and a minus electrode 9 which is in contact with the periphery of the optoelectric conversion section 2 to be negative potential with respect to the pixel type electrode 6. The optical detector forms an electric field diverging from the optoelectric conversion section to the pixel type electrode, thereby allowing the electrons generated by the optoelectric conversion section to arrive at the pixel type electrode through similarly expansion with their positional information maintained. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a photodetector that detects light by converting light into electrons.

  Radiation utilization technologies are diverse, including medical fields such as positron emission tomography and X-ray CT, industrial fields such as various nondestructive inspections, and security fields such as radiation monitors and personal belongings inspections.

  The radiological image detector is an elemental technology that occupies an important position in radiation utilization technology. With the development of radiation utilization technology, more advanced performance is achieved in terms of detection sensitivity, position resolution with respect to the incident position of radiation, or count rate characteristics. Is required. In addition, with the spread of radiation utilization technology, it is also required to reduce the cost of radiation image detectors and increase the area of sensitive areas.

  In order to meet the demand for the radiation image detector, a particle beam image detector using gas amplification by a pixel-type electrode has been developed (see Patent Document 1). The particle beam image detector detects the electrons generated by the incident particle beam ionizing gas molecules with the pixel electrode, and has excellent position resolution and count rate characteristics, and can easily enlarge the sensitive area. Moreover, it has the advantage that it can be manufactured at low cost. However, since the gas used has a small atomic weight, there is a problem that it has poor stopping power for photons having high energy such as hard X-rays and gamma rays, and therefore has poor detection sensitivity for the photons.

  In view of such a problem, a method has been disclosed in which incident radiation is converted into ultraviolet rays using a scintillator having a large atomic weight and is detected by a photodetector that detects the ultraviolet rays (see Patent Document 2). This photodetector has a gas amplification type detector having position resolution. Similar attempts to convert radiation into light and detect the light have been made by others (see Non-Patent Document 1). However, since the photodetector as described above detects electrons generated by ionizing gas molecules by light such as ultraviolet rays, the range of ultraviolet rays has an extent corresponding to the thickness of the gas layer, so that the position resolution and The counting rate characteristic is low, and it is a problem to improve this. Moreover, the gas ionized by light is chemically unstable, and the gas molecules themselves deteriorate or the gas molecules tend to adhere to the electrodes of the detector. Patent Document 2).

  On the other hand, when detecting ultraviolet rays generated from the scintillator by radiation, it is also attempted to apply a photodetector that converts ultraviolet rays to electrons using a photoelectric conversion unit and detects the electrons by a gas amplification type detector ( Non-Patent Document 3). This photodetector is considered to be able to avoid the above-described problems related to the degradation of the position resolution and count rate characteristics and the stability of the operation to some extent. However, as described above, the photoelectric conversion unit generates a small amount of electrons with the extremely weak ultraviolet rays generated by the scintillator, and the gas amplification type detector has an insufficient amplification factor to detect minute electrons. The photodetector has a problem that it is difficult to detect minute light with high sensitivity.

Japanese Patent No. 3354551 JP 2008-202977 A

P. Schotanus, et al. , “Detection of LaF3: Nd3 + Scintillation Light in a Photosensitive Multiwire Chamber” Nuclear Instruments and Methods in Physics9, 1987. J. et al. Va'vra, “Wire Aging of Hydrocarbon Cases with TMAE Additions” IEEE Transactions on Nuclear Science, NS-34, 486-490 (1987). J. et al. van der Marel, et al. , “A LaF3: Nd (10%) Scintillation Detector with Microgap Gas Chamber Read-out for the Detection of γ-rays”, Nuclear Instruments and Reis.

  Accordingly, the present invention provides a photodetector that can detect light such as ultraviolet rays, infrared rays, or visible light with high sensitivity and has excellent position resolution and count rate characteristics in order to solve the above-described problems. With the goal.

  The present inventors have made various studies on a method for detecting extremely weak ultraviolet rays generated from the above-described scintillator with high sensitivity. As a result, by using a photodetector composed of a photoelectric conversion unit and a pixel type electrode, ultraviolet rays generated from the scintillator are converted into electrons in the photoelectric conversion unit, and by detecting these electrons using the pixel type electrode, We found that radiation can be detected with high sensitivity. And it succeeded in acquiring the image of light with the photodetector which combined a photoelectric conversion part and a pixel type electrode, and came to complete this invention.

  Specifically, the photodetector according to the present invention is a photodetector attached to the chamber, and converts the incident light into electrons and emits it at a position opposite to the photoelectric conversion unit. A pixel-type electrode that is arranged and detects electrons by amplifying electrons with the gas in the chamber; and a negative electrode that contacts the periphery of the photoelectric conversion unit and has a negative potential with respect to the pixel-type electrode. The electric field spreads from the photoelectric conversion unit toward the pixel electrode.

  In the photodetector according to the present invention, the photoelectric conversion unit generates electrons from light. For this reason, this photodetector can efficiently convert incident light into electrons. In addition, this photodetector forms an electric field that spreads from the photoelectric converter to the pixel electrode, and extends the similarity while maintaining the position information of the electrons generated by the photoelectric converter to reach the pixel electrode. Can be made. The position resolution of the photodetector is improved by similar expansion of the electron position information. Furthermore, since this photodetector can amplify and detect electrons that have arrived at the pixel-type electrode, it has high sensitivity and excellent count rate characteristics.

  Therefore, the present invention can detect light such as ultraviolet rays, infrared rays, or visible light with high sensitivity, and can provide a photodetector having excellent position resolution and count rate characteristics.

  The photodetector according to the present invention is characterized in that an outer shape of the pixel-type electrode is larger than an outer shape of the minus electrode as viewed from a direction of light incident on the photoelectric conversion unit. This configuration can form an electric field that spreads from the photoelectric conversion portion toward the pixel-type electrode.

  The photodetector according to the present invention preferably further includes at least one gas electron amplifier that is disposed between the photoelectric conversion unit and the pixel-type electrode and amplifies electrons with the gas in the chamber. The gas electron amplifier can accelerate the electrons generated by the photoelectric conversion unit as primary electrons and generate a large number of secondary electrons by the electron avalanche effect. For this reason, this photodetector can detect radiation with higher sensitivity.

  The photodetector according to the present invention is characterized in that an outer shape of the gas electronic amplifier is larger than an outer shape of the minus electrode when viewed from a direction of light incident on the photoelectric conversion unit. This configuration can form an electric field that spreads from the photoelectric conversion unit toward the gas electronic amplifier. The position resolution of the photodetector is improved by expanding the position information of the primary electrons in a similar manner until the secondary electrons are generated by the gas electron amplifier.

  The photodetector according to the present invention further includes a ring electrode between the photoelectric conversion unit and the pixel-type electrode, or between the photoelectric conversion unit and the gas electronic amplifier closest to the photoelectric conversion unit. It is preferable. Even when the distance from the photoelectric conversion unit to the pixel-type electrode or the gas electronic amplifier is long, the ring electrode can prevent the electric field from being disturbed.

  The ring-shaped electrode of the photodetector according to the present invention can be set to a potential that spreads electrons emitted from the photoelectric conversion unit toward the pixel-type electrode. The spread of the electric field can be adjusted, and the magnification of the similarity expansion of the position information of the primary electrons can be controlled.

  The present invention can detect light such as ultraviolet light, infrared light, or visible light with high sensitivity, and can provide a photodetector having excellent position resolution and count rate characteristics. Further, when the gas electron amplifier is used, there is a problem that it is necessary to consider that an error occurs in the position information because the primary electrons diffuse to a certain range before reaching the pixel type electrode. However, in the photodetector according to the present invention, even if an error in the position information of the primary electrons occurs in the gas electron amplifier, the position information is enlarged similarly, so that the error becomes relatively small. For this reason, the photodetector according to the present invention can use a gas electronic amplifier without reducing the position resolution.

  In addition, by combining the photodetector according to the present invention and the scintillator, it is possible to detect weak ultraviolet light, infrared light, visible light, or the like generated by being converted from radiation by the scintillator with high sensitivity. A radiological image detector having excellent counting rate characteristics can be provided. In addition, the photodetector of the present invention can be suitably used in fields such as medical, industrial, and security because the sensitive area can be easily enlarged and manufactured at low cost.

It is a schematic diagram which shows the photodetector which concerns on this invention. It is a schematic diagram which shows the photodetector which concerns on this invention. It is a schematic diagram which shows an example of the gas electronic amplifier used by this invention. It is a schematic diagram which shows the slit used in a present Example. It is a schematic diagram which shows the slit used in a present Example. It is a two-dimensional image which the photodetector of a present Example outputs. It is a two-dimensional image which the photodetector of a present Example outputs. It is a two-dimensional image which the photodetector of a present Example outputs. It is a schematic diagram which shows the photodetector of a present Example. It is a figure explaining the electric field dependence of electron diffusion.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In the present specification and drawings, the same reference numerals denote the same components.

[Embodiment 1]
FIG. 1 is a schematic diagram illustrating a photodetector 101 according to the present embodiment. The photodetector 101 is a photoelectric conversion unit 2 that converts incident light into electrons and emits it, and a pixel type that is arranged at a position opposite to the photoelectric conversion unit 2 and amplifies and detects electrons with the gas in the chamber 7. The electrode 6 includes a negative electrode 9 that contacts the periphery of the photoelectric conversion unit 2 and has a negative potential with respect to the pixel-type electrode 6. A scintillator 1 that converts radiation into light is connected to the photodetector 101 in FIG. 1 and operates as a radiation image detector.

(Scintillator)
The scintillator 1 connected to the light detector 101 may be a scintillator that generates infrared light, visible light, or ultraviolet light upon incidence of radiation. In the present embodiment, the case where the scintillator 1 generates ultraviolet rays from radiation will be described. The scintillator 1 is particularly preferably a scintillator that generates vacuum ultraviolet rays having a wavelength of 200 nm or less among ultraviolet rays in view of photoelectric conversion efficiency from ultraviolet rays to electrons in the photoelectric conversion unit 2.

  By selecting a scintillator to be used according to the type of radiation to be detected, radiation can be detected without being limited to radiation to be detected such as X-rays, α rays, β rays, γ rays, or neutron rays. It is preferable to use a scintillator having a large atomic weight because high energy photons such as hard X-rays or γ-rays can be efficiently detected among the radiations.

Further, in order to emit the ultraviolet rays generated by the incidence of radiation without being absorbed by the scintillator itself, it is preferable to use a scintillator that hardly absorbs the ultraviolet rays. Examples of such scintillators that hardly absorb ultraviolet rays include metal fluorides, metal oxides such as Al ₂ O ₃ , YAlO ₃ , Lu ₃ Al ₅ O ₁₂ , metal phosphorous oxides such as LuPO ₄ and YPO ₄ , or one Scintillators made of a part of metal borate and the like.

  Note that the form of the scintillator is not particularly limited, and crystals, glass, ceramics, or the like can be used as appropriate. However, it is preferable to use crystals in view of the conversion efficiency from radiation to ultraviolet rays.

  A metal fluoride can be suitably used as the scintillator that generates vacuum ultraviolet rays. Since vacuum ultraviolet rays have the property of being absorbed by many materials, there is a problem that the scintillator itself absorbs vacuum ultraviolet rays emitted by the incidence of radiation, but metal fluoride is exceptionally difficult to absorb vacuum ultraviolet rays. Therefore, it can be suitably used as the scintillator 1.

  The type of the metal fluoride is not particularly limited, and any conventionally known metal fluoride can be arbitrarily used as a scintillator that generates vacuum ultraviolet rays. Specifically, lithium fluoride, magnesium fluoride, calcium fluoride, scandium fluoride, titanium fluoride, chromium fluoride, manganese fluoride, iron fluoride, cobalt fluoride, nickel fluoride, copper fluoride, fluorine Zinc fluoride, gallium fluoride, germanium fluoride, aluminum fluoride, strontium fluoride, yttrium fluoride, zirconium fluoride, barium fluoride, lanthanum fluoride, cerium fluoride, praseodymium fluoride, neodymium fluoride, europium fluoride And metal fluorides composed of at least one of gadolinium fluoride, terbium fluoride, erbium fluoride, thulium fluoride, ytterbium fluoride, lutesium fluoride, hafnium fluoride, tantalum fluoride, lead fluoride, etc. .

  Moreover, it is preferable to use as the scintillator 1 a compound containing an emission center element that causes a radiation transition in the ultraviolet region.

  The luminescent center element is not particularly limited as long as it emits ultraviolet light by the radiative transition, but the luminescent center element exhibits 5d-4f transition light emission due to electron transition from the 5d level to the 4f level, and has a light emission lifetime. Since it is short and has high-speed response, it is particularly preferable. As the luminescent center element exhibiting such 5d-4f transition emission, Pr, Nd, Er, Tm and the like can be suitably used.

  In the scintillator containing the luminescence center element, the content when the luminescence center element is contained varies depending on the type of scintillator or the kind of luminescence center element, but is preferably in the range of 0.005 to 20 wt%. . By setting the added amount to 0.005 wt% or more, the intensity of light emission of the scintillator can be increased. On the other hand, by setting the amount to 20 wt% or less, the attenuation of light emission of the scintillator resulting from concentration quenching can be suppressed. it can.

  If the preferable aspect of the scintillator containing the said luminescent center element is illustrated, the crystal | crystallization which consists of the said metal fluoride, a metal oxide, or a metal phosphate containing the said luminescent center element will be mentioned.

  In the present invention, it is preferable to use a scintillator having a high density and a large effective atomic number in order to increase detection sensitivity for high energy photons such as hard X-rays or γ-rays.

  In the present invention, the effective atomic number is an index defined by the following formula [1], which affects the stopping power against hard X-rays or γ-rays. The larger the effective atomic number, the harder X-rays or The stopping power against γ rays is increased, and therefore the sensitivity of the scintillator to hard X rays or γ rays is improved.

Effective atomic number = (ΣW _i Z _i ⁴ ) ^1/4 [1]
(W _i and Z _i represent the mass fraction and atomic number of the i-th element among the elements constituting the scintillator, respectively.)

  Although the shape of the scintillator 1 is not particularly limited, it is preferable that the scintillator 1 has a light exit surface 1-out facing the photodetector 101, and the light exit surface 1-out is optically polished. By having the light exit surface 1-out, the ultraviolet rays generated by the scintillator 1 can be efficiently incident on the photodetector 101.

  The shape of the light exit surface 1-out is not limited, and a shape according to the application such as a square with a side length of several mm to several hundreds mm square or a circle with a diameter of several mm to several hundred mm is appropriately used. It can be selected and used.

  Further, the thickness of the scintillator 1 in the radiation incident direction varies depending on the type and energy of the radiation to be detected, but is generally several hundred μm to several hundred mm.

  Further, it is preferable that the ultraviolet reflection film 10 made of aluminum, Teflon (registered trademark), or the like is provided on the surface that does not face the photodetector 101 so that the ultraviolet rays generated in the scintillator 1 can be prevented from being dissipated. Moreover, the position resolution of the photodetector 101 can be particularly enhanced by using a large number of scintillators provided with the ultraviolet reflecting film 10 in an array.

  The manufacturing method of a scintillator is not specifically limited, You may manufacture the scintillator 1 by a well-known manufacturing method.

  The photodetector 101 includes a photoelectric conversion unit 2 and a pixel electrode 6. Hereinafter, a specific example will be described in detail.

(Photoelectric converter)
The photoelectric conversion unit 2 emits electrons from the place where the light is received. The material of the photoelectric conversion unit 2 differs depending on the type of light received. For example, when receiving ultraviolet rays, cesium iodide (CsI), cesium telluride (CsTe), and the like can be exemplified. Among these, it is preferable to use cesium iodide in view of photoelectric conversion efficiency and chemical stability when ultraviolet rays are converted to electrons. In the case of a radiation image detector, the photoelectric conversion unit 2 converts ultraviolet rays generated from the scintillator 1 into electrons. In the case of receiving infrared rays, Ag—O—Cs, GaAs, and InGaAs can be exemplified. In the case of receiving visible light, K—Na—Sb and Cs—Na—K—Sb can be exemplified.

  The photoelectric conversion unit 2 is preferably a thin film in order to efficiently extract electrons converted from light. The photoelectric conversion unit 2 is formed on the inner surface of the light incident window 8 as will be described later.

(Pixel electrode)
The pixel-type electrode 6 includes an anode strip formed on the back surface of the double-sided substrate, a cylindrical anode electrode that is implanted in the anode strip and has an upper end surface exposed on the surface of the double-sided substrate, and the cylindrical anode. A strip-like cathode electrode in which holes are formed around the upper end surface of the electrode, the anode strip preferably has a width of 200 μm to 400 μm, and the anode strips are arranged at intervals of 400 μm, It is particularly preferable that holes having a diameter of 200 to 300 [mu] m are formed in the strip-like cathode electrode, and the cylindrical anode electrode has a diameter of 40 to 60 [mu] m and a height of 50 to 150 [mu] m.

  By applying a predetermined applied voltage between the cylindrical anode electrode of the pixel electrode 6 and the strip-like cathode electrode, a strong electric field is generated in the vicinity of the cylindrical anode electrode. Electrons accelerated by the electric field cause an electron avalanche and are amplified and detected from the cylindrical anode electrode. In this process, cationized gas molecules quickly drift to the surrounding strip-like cathode electrode.

  Therefore, since electric charges that can be observed on the electric circuit are generated in both the cylindrical anode electrode and the strip-like cathode electrode, by observing which strip of the anode / cathode has caused this amplification phenomenon, the incident occurs. Know the position of the particle beam. As a signal processing circuit for reading out signals and obtaining a two-dimensional image, conventionally known ones can be used without limitation.

  In this embodiment, the suitable range of the applied voltage is generally 400V to 800V, although it varies depending on the type of detection gas. Since the pixel-type electrode 6 uses a pixel as an anode, a high electric field is easily generated and the amplification factor is large. Therefore, the amplification factor obtained at the applied voltage reaches several thousands to several tens of thousands.

Further, since the pixel type electrode 6 has a very short drift distance of the cationized gas molecules, the dead time is shorter than that of other gas amplification type detectors, and about 5 × 10 ⁶ count / (sec · mm ^2). High count rate characteristics exceeding

  Furthermore, since the pixel-type electrode 6 can be manufactured by using a printed circuit board manufacturing technique, a pixel having a large area can be provided at a low cost.

  When viewed from the direction of the radiation incident on the scintillator 1, the outer shape of the pixel-type electrode 6 is made larger than the outer shape of the minus electrode 9 described later. The pixel electrode 6 is installed in parallel to the light incident window 8.

(Chamber)
The photoelectric conversion unit 2 and the pixel-type electrode 6 are installed from the side close to the opening in a chamber 7 having an opening for incident light from the outside, and the opening is sealed with a light incident window 8. ing.

When the photodetector 101 is used in combination with the scintillator 1 for a radiation image detector, the light incident window 8 is preferably made of LiF, MgF ₂ , or CaF ₂ that has high transparency to ultraviolet rays.

The chamber 7 is filled with a predetermined detection gas. As the detection gas, a combination of a rare gas and a quencher gas is generally used. Examples of the rare gas include He, Ne, Ar, and Xe. In addition, examples of the quencher gas include CO ₂ , CH ₄ , C ₂ H ₆ , and CF ₄ . Further, the mixing amount of the quencher gas in the rare gas is preferably 1 to 30%. Besides the rare gas, a gas mainly composed of CF ₄ having a small electron diffusion can also be used as the detection gas.

  As shown in FIG. 1, when the photoelectric conversion unit 2 is formed on the inner surface of the light incident window 8, in order to efficiently supply electrons to the photoelectric conversion unit 2, and between the photoelectric conversion unit 2 and the pixel type electrode 6. In order to apply such an electric field, a negative electrode 9 made of a metal layer is provided on the outer periphery of the photoelectric conversion unit 2.

(Radiation image detector)
Hereinafter, the radiation image detector in which the photodetector 101 and the scintillator 1 are combined will be described in more detail.

  As shown in FIG. 1, the ultraviolet reflecting film 10 is applied to a surface other than the light exit surface 1-out of the scintillator 1, and the light exit surface 1-out and the light incident window 8 are placed in close contact with each other. It is preferable to fill the grease 11 between the light emitting surface 1-out and the light incident window 8. By filling the grease, the ultraviolet light that has reached the ultraviolet emission surface 1-out from the inside of the scintillator 1 can be led out to the outside without being reflected by the ultraviolet emission surface, and the incidence efficiency to the photodetector 101 can be increased. . As the grease, it is preferable to use a fluorinated grease having a high refractive index and high transparency to ultraviolet rays. As such a fluorinated grease, for example, Krytox manufactured by DuPont can be suitably used.

  Further, when the thickness of the scintillator 1 in the radiation incident direction is large and the position resolution is lowered due to the spread of ultraviolet rays inside, the scintillator 1 has a small ultraviolet emitting surface, and an ultraviolet reflecting film is applied to a surface other than the ultraviolet emitting surface. By arranging a large number of the scintillators, the spread of ultraviolet rays can be suppressed.

  As another aspect when the radiation image detector is configured using the scintillator and the photodetector, the chamber opening may be directly sealed with the scintillator instead of the light incident window. According to this aspect, it is possible to avoid a decrease in position resolution due to the spread of ultraviolet rays in the light incident window, and it is possible to simplify the structure, which is preferable.

(Operation of photodetector)
First, a predetermined voltage is applied between the negative electrode 9 and the pixel electrode 6 with the negative electrode 9 as a negative potential. By applying this voltage, an electric field E is formed in the chamber 7. Since the outer shape of the pixel electrode 6 is larger than the outer shape of the minus electrode 9, the electric field E has a shape that spreads from the photoelectric conversion unit 2 toward the pixel electrode 6. For example, the potential of the negative electrode 9 can be -1300V, the potential of the strip-like cathode electrode of the pixel electrode 6 can be 0V, and the potential of the cylindrical anode electrode of the pixel electrode 6 can be + 480V. In addition, when the distance from the photoelectric conversion part 2 to the pixel type electrode 6 is long, it is good to arrange | position the ring-shaped electrode 16 demonstrated in Embodiment 2 between the photoelectric conversion part 2 and the pixel type electrode 6. FIG. Disturbance of the electric field E can be prevented.

  The photoelectric conversion unit 2 converts light incident from the outside into electrons 3 and emits them into the chamber 7. For example, in the case of a radiation image detector in which the photodetector 101 and the scintillator 1 are combined, the scintillator 1 converts incident radiation into ultraviolet rays, and the photoelectric conversion unit 2 converts the ultraviolet rays into electrons 3 and enters the chamber 7. discharge. The electrons 3 emitted into the chamber 7 move toward the pixel-type electrode 6 in accordance with the electric field E formed in the chamber 7. Here, since the formed electric field E extends toward the pixel type electrode 6, the electrons 3 are similarly expanded while maintaining the positional information on the photoelectric conversion unit 2 from which the electrons 3 are emitted. Reach 6 If a voltage is applied to the ring-shaped electrode 16, the electric field E can be expanded, and the expansion rate of the similarity expansion can be adjusted for the position information of the electrons 3.

  A signal processing circuit for reading signals and obtaining a two-dimensional image is connected to the pixel-type electrode 6. The pixel-type electrode 6 detects electrons as described above and outputs a signal for a two-dimensional image. When reading a signal from the pixel-type electrode 6 and obtaining a two-dimensional image, the position resolution can be particularly improved by using an anger-type signal processing circuit based on anger logic. Anger logic is a method for specifying the incident position of radiation by obtaining the position of the center of gravity of the scintillation light when the scintillation light generated by the incidence of the radiation is detected with a spatial spread. .

  The anger type signal processing circuit includes a readout circuit for reading out the signal intensity at each pixel of the pixel type electrode, a coincidence circuit for discriminating scintillation light generated by the incidence of individual radiation, and a readout from each pixel. The center of gravity calculation circuit is used to obtain the position of the center of gravity of the scintillation light from the intensity of the output signal.

  In the anger type signal processing circuit, only the signal generated by the incidence of a single radiation among the signals obtained from the readout circuit is discriminated by the coincidence counting circuit. Subsequently, the incident position of the radiation is specified by using the discriminated signal as a target and obtaining a weighted average of the intensity of the signal by a gravity center calculation circuit. According to such an anger type signal processing circuit, the position resolution can be improved to about 100 μm.

  In the case where the scintillator 1 is not provided and the photodetector 101 is used alone, the photodetector 101 detects an image of light projected on the photoelectric conversion unit 2.

[Embodiment 2]
FIG. 2 is a schematic diagram illustrating the photodetector 102 according to the present embodiment. In the photodetector 102, at least one gas electronic amplifier and a ring-shaped electrode 16 are further added to the photodetector 101. The gas element amplifier is disposed between the photoelectric conversion unit 2 and the pixel electrode 6 and amplifies electrons with the gas in the chamber 7. In the photodetector 102 of FIG. 2, there are two gas electronic amplifiers, which are a gas electronic amplifier 4-1 and a gas electronic amplifier 4-2 from the light incident window 8 side.

(Gas electronic amplifier)
The gas electron amplifier 4-1 amplifies the electrons 3 (primary electrons) generated from the photoelectric conversion unit 2. The gas electronic amplifier (4-1, 4-2) will be described in detail with reference to FIG. The gas electronic amplifier (4-1, 4-2) includes a plate-like multilayer body constituted by a plate-like insulating layer 12 and a planar metal layer 13 coated on both surfaces of the plate-like insulating layer 12, and the plate And is formed by a through-hole 14 having an inner wall perpendicular to the plane of the metal layer 13. In the gas electron amplifier (4-1, 4-2), by applying a predetermined voltage between the metal layers 13 and generating an electric field inside the through hole 14, primary electrons entering the inside of the through hole 14. 3 is accelerated, an avalanche effect is generated, and amplified to a large number of secondary electrons 5 while maintaining position information.

  The material of the plate-like insulating layer 12 is preferably polyimide, liquid crystal polymer, or the like in view of workability and mechanical strength.

  Moreover, since the discharge between the metal layers 13 on the front surface and the back surface can be suppressed as the thickness of the plate-like insulating layer 12 (Di in FIG. 3) is thicker, it is higher by applying a higher applied voltage. Although an amplification factor can be obtained, when it is extremely thick, processing when providing the through hole 14 becomes difficult. Therefore, the thickness of the plate-like insulating layer 12 is preferably 50 μm to 300 μm.

  Although the material and thickness (Dm in FIG. 3) of the metal layer 13 are not particularly limited, for example, the metal layer 13 having a material of copper, aluminum, or gold and a thickness of about 5 to 10 μm can be preferably used. .

  In the present invention, the diameter of the through-hole 14 (d in FIG. 3) is not particularly limited, and a desired diameter is selected in view of the strength of the electric field generated inside the through-hole 14 and the ease of processing. Can be used. A specific example of such a diameter is generally 50 to 100 μm.

  The through holes 14 are preferably provided on the entire surface of the plate-like multilayer body at a predetermined pitch (P in FIG. 3) in order to improve the uniformity of the generated electric field. The pitch depends on the material and thickness of the plate-like insulating layer 12 and the diameter of the through hole 14, but is generally about twice the diameter of the through hole 14. Moreover, when providing the through-hole 14, as shown in FIG. 3, it is preferable to set it as the arrangement | positioning which arranged the equilateral triangle. By adopting such an arrangement, the aperture ratio of the through hole 14 with respect to the area of the plate-like multilayer body can be increased, so that a high amplification factor can be obtained and ion feedback described later can be suppressed.

  In the operation of the gas electronic amplifier (4-1, 4-2), the higher the applied voltage, the higher the gain, but when the applied voltage is extremely high, the gas electronic amplifier (4-1, 4- Discharge occurs between the metal layers 13 on the front and back sides of 2), making stable operation difficult. Although the suitable range of an applied voltage changes with thickness of the plate-shaped insulating layer 12, it is generally 200V-1000V, Comprising: The amplification factor obtained in this applied voltage is generally several dozen-several thousand.

  When viewed from the direction of radiation incident on the scintillator 1, the outer shape of the gas electronic amplifier (4-1, 4-2) is made larger than the outer shape of the minus electrode 9. Gas electronic amplifiers (4-1 and 4-2) are also installed in parallel to the light incident window 8. In view of the amplification factor and the operational stability, it is preferable to install a plurality of gas electronic amplifiers 4, and it is particularly preferable to install about two.

  The photodetector 102 further amplifies and detects the secondary electrons 5 amplified by the gas electron amplifiers (4-1 and 4-2) using the pixel type electrode 6. That is, by amplifying electrons with each of the plurality of gas electron amplifiers (4-1, 4-2) and the pixel-type electrode 6, the electrons are amplified step by step, and the resulting overall gain is increased. Can greatly increase.

  Further, by using a plurality of gas electronic amplifiers, ion feedback can be effectively suppressed, and operational stability can be improved. Ion feedback is a phenomenon in which cationic gas molecules generated secondary by the electron avalanche effect are accumulated and the electric field E is distorted. When such ion feedback occurs, the amplification factor and count rate characteristics become unstable. As a result, the stability of the operation is hindered.

(Ring electrode)
The ring-shaped electrode 16 is disposed between the photoelectric conversion unit 2 and the gas electronic amplifier 4-1 closest to the photoelectric conversion unit 2. The ring electrode 16 is made of a metal such as copper or aluminum. When the distance between the photoelectric conversion unit 2 and the gas electronic amplifier 4-1 is long, the electric field E may be disturbed. Disturbance of the electric field E can be prevented by inserting the ring electrode 16 between the photoelectric conversion unit 2 and the gas electronic amplifier 4-1.

(Radiation image detector)
As described with reference to FIG. 1, a radiation image detector can be obtained by combining the photodetector 102 and the scintillator 1.

(Operation of photodetector)
First, a predetermined voltage is applied to each of the negative electrode 9, the gas electronic amplifier (4-1, 4-2), and the pixel type electrode 6. As described with reference to FIG. 1, an electric field E that spreads from the photoelectric conversion unit 2 toward the pixel electrode 6 is formed in the chamber 7 by applying this voltage. In the case of FIG. 2, due to the size of the negative electrode 9, the gas electronic amplifier (4-1, 4-2), and the pixel type electrode 6, the photoelectric conversion unit 2 is directed to the gas electronic amplifier 4-1 in the chamber 7. An electric field E that spreads out is formed.

Electrons in the detection gas diffuse to a certain range as they move. FIG. 10 is a diagram simulating the electric field dependence of electron diffusion when the detection gas is CF ₄ or Ar—C ₂ H ₆ . From this result, if the electron moving distance is 1 cm and the electric field is 0.5 kV / cm, the electrons diffuse about 480 μm when the detection gas is Ar—C ₂ H ₆ and about 120 μm when the detection gas is CF _4. I understand that. In the case of a photodetector in which the electron moving distance from the photoelectric conversion unit 2 to the pixel type electrode 6 is 1 cm and the resolution of the pixel type electrode 6 is 100 μm, the position information of the electrons can be detected using Ar—C ₂ H ₆ . If it is 4.8 times or more, and the detection gas is CF ₄ , it is less likely to be affected by diffusion by enlarging it to 1.2 times or more.

  For example, the potential of the negative electrode 9 is −1300 V, the potential of the electrode layer 13 on the photoelectric conversion unit 2 side of the gas electronic amplifier 4-1 is −910 V, and the potential of the electrode layer 13 on the pixel electrode 6 side of the gas electronic amplifier 4-1 is The potential is −630 V, the potential of the electrode layer 13 on the photoelectric conversion unit 2 side of the gas electronic amplifier 4-2 is −380 V, the potential of the electrode layer 13 on the pixel electrode 6 side of the gas electronic amplifier 4-2 is −100 V, and the pixel The potential of the strip-like cathode electrode of the mold electrode 6 can be 0V, and the potential of the cylindrical anode electrode of the pixel electrode 6 can be + 480V. Since the voltage of the ring-shaped electrode 16 is not applied, the potential is determined by the insertion location.

  As shown in FIG. 1, the photoelectric conversion unit 2 converts ultraviolet rays into electrons and emits them as primary electrons 3 into the chamber 7. The primary electrons 3 emitted into the chamber 7 move in the direction of the pixel electrode 6 according to the electric field E formed in the chamber 7. The electric field E formed in the chamber 7 of FIG. 2 spreads toward the gas electron amplifier 4-1, so that the primary electrons 3 retain the positional information on the photoelectric conversion unit 2 from which they have been emitted. Similar expansion is performed to reach the gas electronic amplifier 4-1.

  As described above, the primary electrons 3 are amplified to a large amount of secondary electrons 5 by the gas electron amplifier 4-1 and the gas electron amplifier 4-2. The secondary electrons 5 reach the pixel electrode 6 according to the electric field E. As described with reference to FIG. 1, the pixel-type electrode 6 outputs a signal, and is two-dimensionally imaged by the signal processing circuit.

  In addition, the electric field E can be actively expanded by applying a voltage to the ring-shaped electrode 16, and the expansion rate of the similar expansion can be adjusted for the positional information of the primary electrons 3.

  When the scintillator 1 is not provided and the photodetector 102 is used alone, the photodetector 102 detects an image of the light projected on the photoelectric conversion unit 2.

  FIG. 9 illustrates an embodiment of the present invention in detail. In this embodiment, the radiation image detector is a combination of the scintillator 1 and the photodetector 103. The present invention is not limited in any way by these examples.

(Scintillator)
In this example, the scintillator used a lanthanum fluoride crystal containing neodymium as the luminescent center element. The scintillator was processed into a 20 mm square cube using a wire saw equipped with a diamond wire, and then the entire surface was optically polished. One surface of the optically polished surface was used as an ultraviolet emitting surface, and the other surface was provided with an ultraviolet reflecting film made of Teflon (registered trademark). This scintillator was confirmed to convert incident radiation (X-rays) into ultraviolet light having a wavelength of 173 nm.

(Photodetector)
The photodetector 103 used for the radiation image detector was manufactured by the following method.

  As shown in FIG. 9, two gas electronic amplifiers (4-1, 4-2) and a pixel-type electrode 6 are installed in parallel in a chamber 7 having an opening from the side close to the opening, The opening was sealed with a light incident window 8.

In this embodiment, MgF ₂ having a diameter of 70 mm and a thickness of 5 mm is used for the light incident window 8, and a thin film of cesium iodide is provided on the inner surface of the light incident window 8 as the photoelectric conversion unit 2. A negative electrode 9 made of an aluminum layer and a copper layer and having an outer diameter of 70 mm and an inner diameter of 50 mm was provided on the outer periphery of the cesium fluoride thin film.

  The distance G1 ′ between the light incident window 8 and the gas electronic amplifier 4-1 is 2.5 mm, the distance G2 between the gas electronic amplifier 4-1 and the gas electronic amplifier 4-2 is 2 mm, and the distance between the gas electronic amplifier 4-2 and the gas electronic amplifier 4-2. The distance G3 from the pixel type electrode 6 was 2 mm.

  In this embodiment, the gas electronic amplifier (4-1, 4-2) is formed by depositing copper with a thickness of 5 μm as the metal layer 13 on both sides of the polyimide plate-like insulating layer 12 having a thickness of 50 μm. A plate-like multilayer body was used in which columnar through holes 14 having a diameter of 70 μm were provided on the entire surface of the plate-like multilayer body in an arrangement in which equilateral triangles were arranged at a pitch of 140 μm. The size of the gas electronic amplifier (4-1, 4-2) is 100 mm × 100 mm.

  The pixel-type electrode 6 uses a polyimide substrate having a thickness of 100 μm, an anode strip having a width of 300 μm is provided on the back surface of the substrate, a cylindrical anode that is implanted in the anode strip and exposed on the surface of the substrate. The electrodes were arranged at intervals of 400 μm, and a strip-like cathode electrode in which a hole having a diameter of 260 μm was formed around the upper end surface of the cylindrical anode electrode was used. The diameter of the columnar anode electrode was 50 μm at the portion embedded in the substrate and 70 μm at the portion exposed on the surface of the substrate. The height of the cylindrical anode electrode was 110 μm, and the upper end portion 10 μm was exposed on the surface. The size of the pixel-type electrode 6 is 100 mm × 100 mm.

  A high voltage power source for applying a voltage is connected to the negative electrode 9, both surfaces of the gas electronic amplifier (4-1, 4-2), and the anode electrode and the cathode electrode of the pixel electrode 6. A signal processing circuit for reading signals and obtaining a two-dimensional image was connected to the anode electrode and the cathode electrode.

The chamber 7 was filled with Ar mixed with 10% C ₂ H ₆ as a detection gas.

The voltage described in FIG. 2 was applied from the high voltage power source to the negative electrode 9, both surfaces of the gas electronic amplifier (4-1, 4-2), and the anode electrode and the cathode electrode of the pixel electrode 6. At the applied voltage, the overall gain obtained by the gas electronic amplifier (4-1, 4-2) and the pixel-type electrode 6 reaches 6.7 × 10 ⁵ , and even at such a high gain, the gas electronic amplifier It was confirmed that the discharge at the front and back sides of (4-1, 4-2) and the discharge at the pixel-type electrode 6 did not occur and operated stably for a long time.

(Radiation image detector)
The light emission surface 1-out of the scintillator 1 and the light incident window 8 of the photodetector 102 were placed in close contact as shown in FIG. 9 to obtain a radiation image detector. The space between the light exit surface 1-out and the light incident window 8 was filled with DuPont Krytox as fluorine-based grease.

In order to evaluate the resolution of this photodetector, a slit shown in FIGS. 4 and 5 is inserted between the scintillator 1 and the light incident window 8, and a ²⁴¹ Am isotope having a radioactivity of 2.6 MBq is used as a radiation source. Irradiated with radiation. In the slit A in FIG. 4, two 2 mm × 15 mm slits are arranged at intervals of 10 mm. In the slit B of FIG. 5, four slits of 2 mm × 15 mm are arranged at intervals of 5 mm.

  Using a signal processing circuit connected to the pixel type electrode 6, signals output from the respective anode electrodes of the pixel type electrode were obtained to construct a two-dimensional image. The results are shown in FIGS. 6 to 8, (a) is a diagram showing a two-dimensional image obtained by detecting electrons that have reached the pixel-type electrode 6. (B) is the figure which plotted the quantity of the electron which reached | attained the pixel-type electrode 6 on the X-axis or the Y-axis. FIG. 6 shows the result of inserting the slit A. Images of two slits were obtained, indicating that the ultraviolet rays that passed through the two slits of the slit A were converted into electrons by the photoelectric conversion unit 2 and could be detected by the pixel electrode 6. Further, the image was enlarged by the spread of the electrode E, and the image interval between the two slits was about 16 mm, which was about 1.6 times.

  FIG. 7 shows the result of arranging the slit A by rotating the slit direction by 90 degrees. The image interval between the two slits is about 16 mm, and is enlarged by about 1.6 times even if the slit direction is changed. This indicates that the image can be enlarged at the same magnification in both the X direction and the Y direction. FIG. 8 shows the result of inserting the slit B. The image interval between the four slits is about 8 mm, and can be enlarged 1.6 times even with a fine pitch.

  As a result of the above, the photodetector of the present example was able to image the light position information uniformly and vertically by 1.6 times. Furthermore, the photodetector of this embodiment can resolve a pattern of about 1 mm.

  The photodetector according to the present invention can be suitably used in medical fields such as positron tomography and X-ray CT, industrial fields such as various nondestructive inspections, and security fields such as radiation monitors and personal belongings inspections.

DESCRIPTION OF SYMBOLS 1 Scintillator 1-out Ultraviolet emission surface 2 Photoelectric conversion part 3 Electron, primary electron 4-1, 4-2 Gas electron amplifier 5 Secondary electron 6 Pixel type electrode 7 Chamber 8 Light incident window 9 Negative electrode 10 Ultraviolet reflective film 11 Grease 12 Plate-like insulating layer 13 Metal layer 14 Through-hole 16 Ring-shaped electrode E Electric field

Claims (7)

A photodetector mounted in the chamber,
A photoelectric conversion unit that converts incident light into electrons and emits it;
A pixel-type electrode that is disposed at a position opposite to the photoelectric conversion unit and detects electrons by amplifying electrons with the gas in the chamber;
A negative electrode in contact with the periphery of the photoelectric conversion unit and having a negative potential with respect to the pixel-type electrode;
With
The photodetector, wherein an electric field spreads from the photoelectric conversion unit toward the pixel-type electrode.   2. The photodetector according to claim 1, wherein an outer shape of the pixel-type electrode is larger than an outer shape of the negative electrode when viewed from a direction of light incident on the photoelectric conversion unit.   The photodetection according to claim 1, further comprising at least one gas electron amplifier disposed between the photoelectric conversion unit and the pixel-type electrode and amplifying electrons with the gas in the chamber. vessel.   The photodetector according to claim 3, wherein an outer shape of the gas electronic amplifier is larger than an outer shape of the minus electrode when viewed from a direction of light incident on the photoelectric conversion unit.   The photodetector according to claim 1, further comprising a ring electrode between the photoelectric conversion unit and the pixel-type electrode.   The photodetector according to claim 3, further comprising a ring electrode between the photoelectric conversion unit and the gas electronic amplifier closest to the photoelectric conversion unit.   The photodetector according to claim 5, wherein the ring electrode has a potential that spreads electrons emitted from the photoelectric conversion unit toward the pixel electrode.

Images