JP2012185025A - Radiographic image detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a novel radiation image detector capable of detecting radiation such as hard X-rays and γ-rays with high sensitivity and having excellent position resolution and count rate characteristics.
Radiation that combines a scintillator that converts incident radiation into ultraviolet rays, and a gas amplification type ultraviolet image detector that converts ultraviolet rays into electrons and amplifies and detects the electrons using a gas electron avalanche phenomenon. An image detector, wherein the scintillator is a LuF ₃ crystal containing at least one element selected from Nd, Er and Tm.
[Selection] Figure 1

Description

  The present invention relates to a novel radiation image detector. The radiation image detector 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.

  Radiation utilization technologies are diverse, including medical fields such as positron emission tomography and X-ray CT, industrial fields such as various non-destructive 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 gas amplification type radiation image detector has been developed (see Non-Patent Document 1). Such a gas amplification type radiological image detector detects electrons with a multi-wire type wire or a fine electrode after amplifying electrons generated by incident radiation ionizing gas molecules by a gas electron avalanche phenomenon under a high electric field. This has the advantage of excellent position resolution and count rate characteristics with respect to the incident position.

  In addition, among the gas amplification type radiological image detectors, a gas amplification type radiographic image using a pixel type electrode that operates stably for a long period of time and can easily and inexpensively produce a detector having a large sensitive area. A detector is disclosed (see Patent Document 1).

  Although the gas amplification type radiological image detector has various advantages, since it has a poor gas stopping power for photons having high energy such as hard X-rays and gamma rays, these photons are detected. In this case, there is a problem that the detection sensitivity is low.

  In view of such a problem, the present inventors have already used a scintillator having a sufficient stopping power for photons having high energy to convert incident radiation into ultraviolet rays, and the ultraviolet rays have positional resolution. A method of detecting by a gas amplification type detector has been proposed (see Patent Document 2), and attempts to detect radiation by a similar method have been made by others (see Non-Patent Document 2).

  Further, the present inventors have already provided a radiographic image comprising a scintillator for converting incident radiation into ultraviolet rays, and a gas amplification type ultraviolet image detector comprising a photoelectric conversion substance, a gas electronic amplifier, and a pixel type electrode. A detector has been proposed (see Patent Document 3).

Japanese Patent No. 3354551 International Publication No. 2008/099971 Pamphlet International Publication No. 2010/113682 Pamphlet

R. “Fourme,“ Position-sensitive gas detectors: MWPCs and the next given descendants ”, Nuclear Instruments and Methods in Physics Research, A 972, 1192. P. Schotanus, et al. , “Detection of LaF3: Nd3 + Scintillation Light in a Photosensitive Multiwire Chamber” Nuclear Instruments and Methods in Physics9, 1987.

  The present invention is a radiation image detector configured by combining a scintillator that converts incident radiation into ultraviolet rays and a gas amplification type ultraviolet image detector that converts ultraviolet rays into electrons and amplifies and detects the electrons. An object of the present invention is to provide a radiation image detector capable of detecting radiation such as hard X-rays and γ-rays with high sensitivity.

The present inventors have made various studies on scintillators that convert incident radiation into ultraviolet rays. As a result, it was found that a high light emission amount can be obtained by using a LuF ₃ crystal containing at least one element selected from Nd, Er and Tm as a scintillator.

In addition, the present inventors have found that the detection efficiency for radiation can be dramatically improved by detecting ultraviolet rays generated from the scintillator made of the LuF ₃ crystal using a gas amplification type ultraviolet image detector. The invention has been completed.

That is, according to the present invention, a scintillator that converts incident radiation into ultraviolet rays and a radiation image detector comprising a gas amplification type ultraviolet image detector, wherein the scintillator is selected from at least Nd, Er, and Tm. A radiation image detector is provided that is a LuF ₃ crystal containing one element.

  In the invention of the radiation image detector, the gas amplification type ultraviolet image detector is preferably composed of a photoelectric conversion substance, a gas electronic amplifier, and a pixel type electrode.

  According to the present invention, the amount of light emitted from the scintillator that converts incident radiation into ultraviolet light is improved, and the ultraviolet light generated from the scintillator can be detected with high sensitivity by the gas amplification type ultraviolet image detector. A radiation image detector having excellent rate characteristics can be provided. In addition, the radiation image detector of the present invention is extremely valuable in fields such as medical, industrial, and security because the sensitive area can be easily enlarged and manufactured at low cost.

This figure is a schematic diagram of the radiation image detector of the present invention. This figure is a schematic diagram of the radiation image detector of the present invention. This figure is a schematic diagram of the radiation image detector of the present invention. This figure is a schematic diagram of the radiation image detector of the present invention. This figure is a schematic diagram of a gas electronic amplifier used in the present invention. This figure is the schematic of the manufacturing apparatus by a pulling-up method. This figure is a radiation image obtained using ⁵⁷ Co in Example 1. This figure is a radiographic image obtained in Example 1 using ²⁴¹ Am. This figure shows the wave height distribution obtained in Example 1 and Comparative Example 1. This figure is a radiographic image obtained using ⁵⁷ Co in Example 2. This figure is a radiographic image obtained by using ²⁴¹ Am in Example 2.

〔Operating principle〕
The radiation image detector of the present invention operates by the following mechanism. First, incident radiation is converted into ultraviolet rays by a scintillator. Next, the ultraviolet rays are converted into primary electrons by a photoelectric conversion substance or an ionizing gas having a low ionization potential. The primary electrons are amplified by a gas electron avalanche phenomenon under a high electric field, and the amplified electrons are detected by a multi-wire wire or a fine electrode. By processing a signal based on the detected electrons with an external circuit, the radiation incident position can be specified, and a radiation image can be obtained. Hereinafter, the radiation image detector of the present invention will be described in more detail.

[Scintillator]
The radiation image detector of the present invention is characterized by using a LuF ₃ crystal (hereinafter also referred to as LuF) containing at least one element selected from Nd, Er and Tm as a scintillator.

  The scintillator is characterized in that it generates ultraviolet light with a high light emission amount and has a light emission wavelength in the vacuum ultraviolet region of 200 nm or less. By using such a scintillator that generates ultraviolet rays in the vacuum ultraviolet region, it is possible to increase the photoelectric conversion efficiency from ultraviolet rays to electrons in the photoelectric conversion substance or ionizing gas.

  Moreover, since the scintillator emits a large amount of light, the ultraviolet light generated from the scintillator can be efficiently detected by a gas amplification type ultraviolet image detector at the subsequent stage. As a result, the detection efficiency with respect to radiation can be dramatically improved, and the time required to acquire a radiation image can be greatly shortened.

  In the present invention, at least one element selected from Nd, Er, and Tm contained in the scintillator (hereinafter referred to as an emission center element) generates vacuum ultraviolet rays by 5d-4f transition light emission, and therefore is preferably used in the present invention. Is done. Among the luminescent center elements, Nd is particularly preferable because it has a short emission lifetime and high-speed response.

  The content of the luminescent center element varies depending on the type of the luminescent center element, but is preferably in the range of 0.01 to 20 mol%, preferably in the range of 0.02 to 10 mol% with respect to lutetium fluoride. Is particularly preferred. By setting the content of the luminescent center element to 0.01 mol% or more, more preferably 0.02 mol% or more, the probability of light emission through the luminescent center element is increased, and thus high emission intensity can be obtained. Further, by setting the content of the luminescent center element to 20 mol% or less, more preferably 10 mol% or less, it is possible to avoid a decrease in light emission due to concentration quenching.

  In the present invention, the radiation to be detected is not particularly limited, and any radiation such as X-ray, α-ray, β-ray, or γ-ray can be detected. However, the scintillator of the present invention has an effective atomic number and density. These are about 65 to 66 and about 8.3 g / ml, respectively, and are sufficiently large compared to the conventionally known scintillators, so that high-energy photons such as hard X-rays and γ-rays are detected particularly efficiently in radiation. it can.

  In the present invention, the effective atomic number is an index defined by the following formula [1] and affects the stopping power against hard X-rays and γ-rays. As the effective atomic number is larger, the stopping power against hard X-rays and γ-rays increases, and as a result, the sensitivity of the scintillator to hard X-rays and γ-rays improves.

Effective atomic number = (ΣW _i Z _i ⁴ ) ^1/4 [1]
(In the formula, 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 is not particularly limited, it has an ultraviolet emission surface (hereinafter also simply referred to as an ultraviolet emission surface) facing the gas amplification type ultraviolet image detector described later, and the ultraviolet emission surface is optically polished. It is preferable. By having such an ultraviolet emission surface, ultraviolet rays generated by the scintillator can be efficiently incident on the gas amplification type ultraviolet image detector.

  The shape of the ultraviolet light exit surface is not limited, and a shape according to the application such as a square having a side length of several mm to several hundreds mm square and a circle having a diameter of several mm to several hundred mm can be appropriately selected.

  The greater the thickness of the scintillator in the radiation incident direction, the higher the radiation absorption efficiency. However, if it is too thick, the scintillator itself absorbs the UV light generated by the scintillator and the UV light may be attenuated. There is a possibility that the position resolution is lowered due to the spread of ultraviolet rays. Therefore, it is preferable to select an appropriate thickness according to the type and energy of radiation to be detected. For example, when a hard X-ray of about 100 keV is to be detected, an absorption efficiency of about 80% can be achieved with a scintillator having a thickness of 1 mm, so that the thickness is preferably about 1 to 5 mm.

  In addition, it is preferable to apply an ultraviolet reflecting film made of aluminum, Teflon (registered trademark), or the like on a surface not facing the gas amplification type ultraviolet image detector in terms of preventing the dissipation of ultraviolet rays generated by the scintillator. Furthermore, the positional resolution of the radiation image detector can be remarkably increased by using a large number of scintillators provided with such an ultraviolet reflecting film.

  LuF may be in a single crystal or polycrystal form, but it is preferable to use a single crystal from the viewpoint of conversion efficiency from radiation to ultraviolet light.

  The method for producing the LuF single crystal is not particularly limited, but an alkali metal fluoride is added to the raw material mixture when the raw material mixture is melted to obtain a raw material melt and the LuF single crystal is grown from the raw material melt. The method is preferred. Hereinafter, a method for producing such a LuF single crystal will be described.

  Lutetium fluoride has a freezing point of about 1180 ° C. and a phase transition point at about 950 ° C., which is lower than the freezing point. Therefore, when a LuF single crystal is grown from a raw material melt consisting of lutetium fluoride and a fluoride of at least one element selected from Nd, Er and Tm, after LuF is crystallized from the raw material melt, In the process of cooling to room temperature, a phase transformation occurs from a hexagonal crystal structure to an orthorhombic crystal structure, resulting in numerous cracks in the crystal. Such cracks significantly deteriorate the transparency of the crystal, making it difficult to use as a scintillator.

  The alkali metal fluoride is added to lower the freezing point of the raw material melt below the phase transition point of the lutetium fluoride. By adding the alkali metal fluoride, an orthorhombic crystal structure is added. Thus, it is possible to directly grow the LuF single crystal from the raw material melt, and therefore it is possible to avoid the occurrence of cracks due to the phase transformation.

    In the method for producing a LuF single crystal, the type of the alkali metal fluoride is not particularly limited, but lithium fluoride (LiF) is most preferable. Since LiF has a high effect of lowering the freezing point of the raw material melt and is difficult to be mixed into the LuF single crystal, it can be suitably used in a method for producing a LuF single crystal.

  Moreover, although the addition amount of the said alkali metal fluoride is not specifically limited, It is preferable to set it as 25-75 mol% with respect to lutetium fluoride, and it is especially preferable to set it as 30-60 mol%. By setting the addition amount of the alkali metal fluoride to 25 mol% or more, more preferably 30 mol% or more, the freezing point of the raw material melt can be made sufficiently lower than the phase transition point of lutetium fluoride. Crystal cracks can be avoided. Moreover, mixing the alkali metal fluoride into the LuF single crystal can be suppressed by setting the addition amount of the alkali metal fluoride to 75 mol% or less, more preferably 60 mol% or less.

In growing the LuF single crystal from the raw material melt, it is preferable to grow the LuF single crystal by a pulling method. According to the pulling method, a large LuF ₃ single crystal having a diameter of several tens of mm can be manufactured at low cost.

Furthermore, it is preferable that the growth rate of the single crystal at the time of growing the LuF single crystal by the pulling method is not more than v _max represented by the following formula.
v _max = α · R ^1/2 / d ^1/3
(Where α is 0.0062, R represents the number of revolutions (rpm) of the single crystal relative to the raw material melt, and d represents the average diameter (mm) of the single crystal.)
In the above formula, α is a constant determined empirically and has a dimension of mm ^4/3 · min ^−1/2 .

From the above formula, v _max is obtained as the length (mm / min) of single crystal growth per minute, but in the following description, it is a unit of crystal growth rate generally used by those skilled in the art. A value converted into a length (mm / hr) of growth of a single crystal per hour is used.

In the manufacturing method using the pulling method, when the growth rate of the single crystal is high, the alkali metal fluoride may be mixed into the LuF single crystal and the LuF single crystal may become cloudy. By setting it to v _max or less, white turbidity of the LuF single crystal due to the mixing of the alkali metal fluoride can be avoided. The lower limit of the growth rate of the single crystal is not particularly limited, but is preferably set to 0.1 mm / hr or more in view of manufacturing efficiency.

Note that, as described above, the equation for defining _vmax is an equation that has been empirically obtained through the study of the present inventors, and is described as follows.

  In the above formula, R represents the rotational speed (rpm) of the single crystal relative to the raw material melt, and is an index of the stirring effect of the raw material melt in the vicinity of the interface between the LuF single crystal and the raw material melt (hereinafter referred to as the solid-liquid interface). is there. In the vicinity of the solid-liquid interface, the excess alkali metal fluoride accompanying the growth of the LuF single crystal is concentrated in the raw material melt, but by increasing the R, the excess alkali metal fluoride is reduced. It is possible to diffuse quickly and to prevent the alkali metal fluoride from being mixed into the LuF single crystal.

  As described above, as R is increased, the alkali metal fluoride can be prevented from being mixed into the LuF single crystal. However, when R is excessively increased, the shape of the LuF single crystal is distorted. Since there is a possibility that the LuF single crystal may be cracked due to, R is preferably 20 rpm or less. Further, since the distortion of the single crystal due to the excessive R tends to become more prominent as the diameter of the single crystal is larger, when the diameter of the single crystal is 30 mm or more, R may be 15 rpm or less. preferable.

  In the above formula, d represents the average diameter (mm) of the LuF single crystal. The larger the d, the greater the amount of alkali metal fluoride that becomes surplus when the single crystal grows in unit length and is concentrated in the raw material melt. Therefore, in order to suppress the mixing of the alkali metal fluoride into the LuF single crystal, it is necessary to reduce the growth rate of the single crystal as d increases.

  In the method for producing a single crystal of LuF, the d is not particularly limited. However, the larger the d, the larger the volume of the single crystal produced per unit time and the more efficient the production. It is preferable.

  Hereinafter, a general method for producing a LuF single crystal by a pulling method will be specifically described with reference to FIG.

First, a crucible 1 is filled with a mixed raw material obtained by mixing a predetermined amount of fluoride of at least one element selected from lutetium fluoride, Nd, Er, and Tm and an alkali metal fluoride. In the method for producing a LuF single crystal, the purity of the raw material is not particularly limited, but it is preferably 99.99% or more. By using such a raw material, the purity of the LuF ₃ single crystal can be increased, and characteristics such as light emission intensity of the scintillator are improved. Such raw materials may be powdery or granular raw materials, or may be used after being sintered or melted and solidified in advance.

Next, it is set in a growing apparatus equipped with a crucible chamber filled with the above raw materials. After evacuating the interior of the chamber to 1.0 × 10 ⁻³ Pa or less using a vacuum exhaust device, an inert gas such as high-purity argon is introduced into the chamber to perform gas replacement. The pressure in the chamber after gas replacement is not particularly limited, but atmospheric pressure is common. By this gas replacement operation, moisture adhering to the raw material or the chamber can be removed, and deterioration of the characteristics of the LuF single crystal derived from such moisture can be prevented.

  In order to avoid adverse effects due to moisture that cannot be removed even by the gas replacement operation, it is preferable to remove moisture using a scavenger that is highly reactive with moisture. As such a scavenger, a gas scavenger such as tetrafluoromethane can be suitably used. In addition, when using a gas scavenger, the method of mixing in the said inert gas and introducing in a chamber is suitable.

  After performing the gas replacement operation, the raw material is heated and melted with a heater, and the seed crystal placed at the tip of the pulling rod is brought into contact with the molten raw material melt.

  In the method for producing a LuF single crystal, the heating method is not particularly limited, and for example, an induction heating method using a high-frequency coil and a heater, a resistance heating method using a carbon heater, or the like can be appropriately used.

  Next, the seed crystal is pulled up while rotating to start growing a single crystal. Immediately after the start of single crystal growth, the diameter of the single crystal is enlarged at a certain rate and adjusted to a desired diameter.

  When the diameter of the single crystal is enlarged, it is preferable to perform a necking operation for expanding the diameter after once reducing the diameter for the purpose of reducing the dislocation density of the single crystal.

After expanding the diameter of the single crystal to a predetermined value, the single crystal is continuously pulled at a constant pulling speed while rotating the single crystal at a constant speed. In a series of operations of starting the growth of a single crystal in contact with the raw material melt and continuously pulling up after expanding the diameter of the single crystal to a predetermined value, the number of rotations of the single crystal relative to the raw material melt And the average diameters of the single crystals are R and d in the above formula, respectively, and the pulling rate of the single crystal relative to the raw material melt corresponds to the crystal growth rate. In the method for producing a single crystal of LuF, it is important that the crystal growth rate is not more than v _max defined by the above formula.

  In this series of operations, it is preferable to use a crystal diameter control device comprising a load cell provided on the upper part of the pulling rod and a circuit for feeding back a signal from the load cell to the heater output. According to the crystal diameter control device, it becomes easy to stably produce a single crystal having a desired shape.

  The crystal can be obtained by raising the output of the heater when it is pulled up to a predetermined length, separating the crystal from the raw material melt, and then slowly cooling it.

  The content of the luminescent center element in the LuF single crystal can be adjusted to a desired value by adjusting the amount of the luminescent center element fluoride added to the raw material mixture. In the process of manufacturing the LuF single crystal, segregation may occur. Even when such segregation occurs, the segregation coefficient is obtained in advance, and the emission center element added to the raw material mixture is added to the segregation coefficient. By adjusting the amount of fluoride, a LuF single crystal containing a desired amount of the luminescent center element can be obtained.

  In the manufacturing method of a LuF single crystal, when the LuF single crystal is manufactured using the pulling method, an annealing operation is performed after the manufacturing of the crystal for the purpose of removing crystal defects caused by defects of fluorine atoms or thermal strain. Also good.

  The obtained LuF single crystal has good workability and can be easily processed into a desired shape. In processing, a known cutting machine such as a blade saw or wire saw, a grinding machine, or a polishing machine can be used without any limitation.

[Gas amplification type UV image detector]
The gas amplification type ultraviolet image detector included in the radiation image detector of the present invention includes a photoelectric conversion unit that converts ultraviolet rays generated from a scintillator into primary electrons and a detection unit that amplifies and detects the primary electrons.

  The photoelectric conversion unit appropriately selects a photoelectric conversion material that emits electrons by an external photoelectric effect when ultraviolet light is incident, or an ionizing gas that has a low ionization potential and ionizes by the action of ultraviolet rays to emit electrons. Can be used.

  Specific examples of the photoelectric conversion material include cesium iodide (CsI), cesium telluride (CsTe), and the like. On the other hand, specific examples of the ionizing gas include tetrakisdimethylaminoethylene, trimethylamine, triethylamine, acetone, and benzene.

  Among the photoelectric conversion units exemplified above, it is preferable to use a photoelectric conversion material from the viewpoint of photoelectric conversion efficiency when converting ultraviolet rays into electrons, and chemical stability, and it is preferable to use cesium iodide as the photoelectric conversion material. Particularly preferred.

  On the other hand, as the detection unit, the Multi-wire Proportional Chamber described in Non-Patent Document 1, Micro-Stripe Gas Counter, Micro-Mesh-Gasture Structure, Computers A Trous or Micro-Gap Chamber described in Micro-Gap Chamber, A pixel-type electrode or the like can be preferably used. Among these, a pixel electrode is preferable because it can operate stably for a long period of time and can easily and inexpensively manufacture a detector having a large sensitive area. Furthermore, the combined use of the pixel-type electrode and the gas electronic amplifier makes it possible to construct a detection unit having a high amplification factor and excellent long-term stability of operation, which is particularly preferable.

  Hereinafter, a gas amplification type ultraviolet image detector composed of a photoelectric conversion material, a gas electronic amplifier, and a pixel type electrode, which is a preferred embodiment of the present invention, will be specifically described with reference to FIG. First, incident radiation is converted into ultraviolet rays by the scintillator 1. Next, the generated ultraviolet light is converted into primary electrons 3 by the photoelectric conversion substance 2. The primary electrons 3 are amplified by the gas electron amplifier 4 using the amplification action caused by the gas electron avalanche phenomenon under a high electric field to obtain the secondary electrons 5, and then the secondary electrons 5 are further amplified by the pixel electrode 6. Detect while.

  The photoelectric conversion material is preferably a thin film in order to efficiently extract primary electrons converted from ultraviolet rays. Further, it is preferably formed on the inner surface of the ultraviolet incident window as will be described later or on the surface of the gas electronic amplifier that faces the ultraviolet incident window.

  Next, primary electrons generated from the photoelectric conversion material are amplified by a gas electron amplifier. The gas electronic amplifier was developed by Sauli in 1997 and is known as Gas Electron Multiplier (GEM). In the present invention, as the gas electronic amplifier, for example, the technique described in Japanese Patent Application Laid-Open No. 2006-302844 or Japanese Patent Application Laid-Open No. 2007-234485 can be suitably used. Hereinafter, the gas electronic amplifier used in the present invention will be described in detail with reference to FIG.

  The gas electronic amplifier is provided with a plate-like multilayer body composed of a resin-made plate-like insulating layer 12 and a planar metal layer 13 coated on both sides of the plate-like insulating layer, and the plate-like multilayer body. Further, the through hole 14 has an inner wall perpendicular to the plane of the metal layer. In the gas electronic amplifier, by applying a predetermined applied voltage to the metal layer and generating an electric field inside the through hole, primary electrons that have penetrated into the through hole structure are accelerated, causing an electron avalanche phenomenon. Amplified into a large number of secondary electrons while retaining the position information. The material of the plate-like insulating layer is preferably polyimide or liquid crystal polymer in view of processability and mechanical strength.

As the thickness of the plate-like insulating layer (D _{i in} FIG. 5) is larger, the discharge between the front and back metal layers can be suppressed. Therefore, a higher applied voltage is applied to increase the amplification factor. Obtainable. However, when it is extremely thick, it is difficult to process the through hole. Therefore, the thickness of the plate-like insulating layer is preferably 50 μm to 300 μm. The material and thickness of the metal layer (D _{m in} FIG. 5) are not particularly limited. For example, a metal layer having a thickness of about 5 μm is suitable for the material being copper, aluminum, or gold.

  The diameter of the through hole (d in FIG. 5) is not particularly limited, and is appropriately selected in consideration of the strength of the electric field generated in the through hole and the ease of processing. A specific example of such a diameter is generally 50 to 100 μm. The through holes are preferably provided at a predetermined pitch (P in FIG. 5) on the entire surface of the plate-like multilayer body 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 and the diameter of the through hole, but is generally about twice the diameter of the through hole. Moreover, when providing a through-hole, it is preferable to set it as the arrangement | positioning which arranged the equilateral triangle, as shown in FIG. By adopting such an arrangement, the aperture ratio of the through holes 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, a higher amplification factor can be obtained as the applied voltage is higher. However, if the applied voltage is extremely high, a discharge occurs between the metal layers on the front and back of the gas electronic amplifier, and stable operation is difficult. Become. Although the suitable range of the applied voltage varies depending on the thickness of the plate-like insulating layer, it is generally 200 V to 1000 V, and the gain obtained at the applied voltage is generally several tens to several thousand.

  The secondary electrons amplified by the gas electron amplifier are further amplified and detected using the pixel type electrode. Since the pixel-type electrode is disclosed in detail in the above-mentioned Patent Document 1, it may be manufactured according to the technique disclosed therein. Specifically, the pixel-type electrode includes an anode strip formed on the back surface of the double-sided substrate, and a cylindrical anode electrode that is implanted in the anode strip and whose upper end surface is exposed on the surface of the double-sided substrate; A strip-like cathode electrode having a hole formed around the upper end surface of the cylindrical anode electrode. The anode strip preferably has a width of 200 μm to 400 μm. Further, the anode strip is arranged at intervals of 400 μm, holes having a diameter of 200 to 300 μm are formed at regular intervals in the strip-like cathode electrode, A shape having a diameter of 40 to 60 μm and a height of 50 to 150 μm is particularly preferable.

  A strong electric field is generated in the vicinity of the cylindrical anode electrode by applying a predetermined applied voltage between the cylindrical anode electrode of the pixel electrode and the strip cathode electrode. Secondary 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.

Although the suitable range of the applied voltage of a pixel electrode changes with kinds of detection gas, it is generally 400V-800V. Since the pixel-type electrode 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. In addition, since the pixel type electrode 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 Further, since the pixel-type electrode can be manufactured by using a printed circuit board manufacturing technique, a large-area electrode can be provided at low cost.

  Hereinafter, a preferred embodiment when a gas amplification type ultraviolet image detector is configured using the photoelectric conversion substance, the gas electronic amplifier, and the pixel type electrode will be described in detail with reference to FIGS.

A photoelectric conversion substance 2, a gas electronic amplifier 4, and a pixel-type electrode 6 are installed in a chamber 7 having an opening for incident ultraviolet light generated from the scintillator 1, from the side close to the opening. It is sealed with a window 8. As a material for the ultraviolet light incident window, it is preferable to use lithium fluoride (LiF), magnesium fluoride (MgF ₂ ), or calcium fluoride (CaF ₂ ) having high transparency to ultraviolet light.

A predetermined detection gas is filled in the chamber. As the detection gas, a combination of a rare gas and a quencher gas is generally used. Examples of the rare gas include helium (He), neon (Ne), argon (Ar), and xenon (Xe). Examples of the quencher gas include carbon dioxide (CO ₂ ), methane (CH ₄ ), ethane (C ₂ H ₆ ), and tetrafluoromethane (CF ₄ ). The mixing amount of the quencher gas in the rare gas is preferably 5 to 30%.

  The gas pressure when the detection gas is filled in the chamber is not particularly limited. However, the lower the gas pressure, the higher the amplification factor of the gas amplification type ultraviolet image detector can be achieved. Since an amplification factor can be obtained, it is preferably 1 atm or less. The lower limit of the gas pressure is not particularly limited, but is preferably 0.05 atm or more, and particularly preferably 0.2 atm or more. By setting the gas pressure to 0.05 atm or more, it is possible to avoid malfunctions due to discharge in the detection unit, and it is possible to improve the uniformity of the amplification factor.

  The photoelectric conversion material is preferably a thin film in order to efficiently extract primary electrons converted from ultraviolet rays. The thin film is preferably formed on the inner surface of the ultraviolet light incident window as shown in FIG. 1 or on the surface facing the ultraviolet light incident window of the gas electronic amplifier as shown in FIG. When a thin film of photoelectric conversion material is formed on the inner surface of an ultraviolet incident window, in order to efficiently supply electrons to the thin film and to provide a uniform electric field between the thin film and the gas electron amplifier, It is preferable to provide the electrode 9 which consists of a metal layer in an outer peripheral part. When a thin film of photoelectric conversion material is formed on the surface of the gas electronic amplifier facing the ultraviolet light incident window, the metal layer is made of gold and metal in order to avoid reaction between the metal layer of the gas electronic amplifier and the photoelectric conversion material. It is preferable to do. Furthermore, in view of the ease and production cost when laminating to the plate-like insulating layer, the metal layer is a multilayer metal layer laminated in the order of copper, nickel and gold from the side close to the plate-like insulating layer. Is most preferred.

  The gas electronic amplifier and the pixel type electrode are respectively installed in parallel to the ultraviolet light incident window. From the viewpoint of amplification factor and operational stability, a plurality of gas electronic amplifiers are used, and are preferably installed in parallel with the ultraviolet incident window, and about two or three are particularly preferable. By amplifying the electrons with each of the plurality of gas electron amplifiers and the pixel type electrodes, the electrons are amplified in stages, and the overall gain obtained as a result can be greatly increased. 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 phenomenon are accumulated and the electric field is distorted. When such ion feedback occurs, the amplification factor and count rate characteristics become unstable. This will hinder the stability of operation.

The length of the gap (G _{1 in} FIG. 1) between the ultraviolet light incident window and the first stage gas electronic amplifier, the length of the gap between each gas electronic amplifier (G _{2 in} FIG. 1), and the last stage gas electronic amplifier As the length of the gap between the pixel electrode and the pixel electrode (G _{3 in} FIG. 1) is shorter, the count rate characteristics and the position resolution are improved. However, when the length is extremely short, it is difficult to install them so that they do not contact each other. It becomes. Accordingly, the preferred lengths of G ₁ , G ₂ , and G ₃ are both about 1 mm to 20 mm.

The magnitude of the electric field generated in G ₁ , G ₂ , and G ₃ is not particularly limited, and can be appropriately selected in view of the intended amplification factor, the effect of suppressing ion feedback, and the charge collection efficiency. . If the preferable range of the magnitude | size of the said electric field is specifically illustrated, it will generally be 0.3-10 kV / cm. By setting the magnitude of the electric field, a high amplification factor and suppression of the ion feedback can be achieved at the same time.

According to the study by the present inventors, by combining two gas electronic amplifiers and pixel type electrodes and optimizing the applied voltage applied to the gas electronic amplifier and pixel type electrodes, the gas electronic amplifier and pixel type electrodes are used. As an overall amplification factor, an amplification factor exceeding 1 × 10 ⁵ can be stably obtained, and an image can be formed by weak ultraviolet rays generated from the scintillator.

[Radiation image detector]
In the radiation image detector of the present invention, a high-voltage power supply for applying a voltage is connected to each of the photoelectric conversion substance, the gas electronic amplifier, and the pixel type electrode, and signal readout and two-dimensional image are applied to the pixel type electrode. Is connected to a signal processing circuit. In addition, when reading a signal from a pixel-type electrode 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.

  Hereinafter, the suitable aspect at the time of comprising the radiographic image detector of this invention using the said scintillator and a gas amplification type | mold ultraviolet image detector is demonstrated in detail using FIGS. 1-4.

  As shown in FIG. 1, an ultraviolet reflecting film 10 is applied to a surface other than the ultraviolet light emitting surface of the scintillator, and the ultraviolet light emitting surface of the scintillator and the ultraviolet light incident window of the gas amplification type ultraviolet image detector are placed in close contact with each other. The grease 11 is filled between the ultraviolet light emitting surface and the ultraviolet light incident window. By filling the grease, the ultraviolet rays that have reached the ultraviolet emission surface from the inside of the scintillator can be led out to the outside without being reflected by the ultraviolet emission surface, and the incidence efficiency to the gas amplification type ultraviolet image detector can be increased. As the grease, it is preferable to use a fluorine-based grease having a high refractive index and high transparency to ultraviolet rays. For example, “Crytox” manufactured by DuPont can be suitably used.

  When the scintillator is thick in the radiation incident direction and the position resolution is lowered by the spread of ultraviolet rays in the scintillator, it has a small ultraviolet emitting surface as shown in FIG. By arranging a large number of scintillators provided with a film, it is possible to suppress the spread of ultraviolet rays.

  As another aspect of the radiological image detector of the present invention, as shown in FIG. 4, the opening of the chamber may be sealed with a scintillator instead of the ultraviolet incident window. Such an embodiment is preferable because it is possible to avoid a decrease in position resolution due to the spread of ultraviolet rays in the ultraviolet incident window and to simplify the structure.

In this embodiment, the LuF of the present invention exhibits a special effect. That is, in this aspect, a thin film of photoelectric conversion material is formed on the surface of the scintillator, and an electric field is formed between the thin film and the gas electronic amplifier. When such an electric field is formed, a negative high voltage may be applied to the thin film. According to the study by the present inventors, for example, when a LaF ₃ crystal containing Nd described in Non-Patent Document 2 is used as the scintillator, there is a problem that the crystal is broken by application of the high voltage. . On the other hand, since LuF does not break a crystal even when a high voltage is applied, LuF can also be suitably used in this mode.

  Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not limited to these examples. In addition, not all combinations of features described in the embodiments are essential to the solution means of the present invention.

Example 1
<Manufacture of scintillator and measurement of light emission characteristics>
In this example, a LuF ₃ crystal containing Nd was manufactured using a manufacturing apparatus shown in FIG. First, a crucible 16 was filled with a mixed raw material obtained by mixing 2850 g of lutetium fluoride, 12.3 g of neodymium fluoride, and 143 g of lithium fluoride. That is, in this example, 45 mol% of LiF was added as an alkali metal fluoride to the raw material mixture in the production of the single crystal. In addition, the purity of each said raw material used the raw material of 99.99% or more Then, the crucible 16, the heater 17, the heat insulating material 18, and the stage 19 which filled the said raw material were set as shown in FIG. The crucible 16, the heater 17, the heat insulating material 18, and the stage 19 were made of high purity carbon.

The inside of the chamber 20 was evacuated to 1.0 × 10 ⁻³ Pa or less using a vacuum evacuation apparatus, and then a gas mixture was introduced by introducing a tetrafluoromethane-argon mixed gas to the atmospheric pressure. .

  After performing the gas replacement operation, the raw material was heated and melted by induction heating with the high-frequency coil 21 and the heater 17, and the seed crystal placed at the tip of the pulling rod 22 was brought into contact with the melt of the melted raw material.

  Next, the seed crystal was pulled up while rotating to start growing a single crystal. Immediately after the start of single crystal growth, a necking operation was performed in which the diameter was once reduced and then increased, and then the diameter of the single crystal was increased at a certain rate and adjusted to a predetermined diameter. After increasing the diameter of the single crystal to a predetermined value, the single crystal was continuously rotated at a constant pulling speed while rotating the single crystal at a constant speed.

In this example, the rotation speed (R) of the single crystal relative to the raw material melt and the average diameter (d) of the single crystal were 15 rpm and 30 mm, respectively. Since the v _max calculated from R and d is 0.46 mm / hr, in this example, the crystal growth rate was set to 0.4 mm / hr which is equal to or lower than v _max .

  This series of operations was performed using a crystal diameter control device comprising a load cell provided on the upper part of the pulling rod and a circuit for feeding back a signal from the load cell to the heater output.

  When a single crystal having a predetermined diameter was pulled up to a length of 40 mm, the output of the heater was increased, the crystal was separated from the raw material melt, and then gradually cooled to obtain a transparent single crystal free from white turbidity and cracks.

  A part of the obtained single crystal was pulverized into powder and subjected to powder X-ray diffraction measurement. From the result of analyzing the diffraction pattern obtained by the powder X-ray diffraction method, it was found that all the single crystals of this example consisted only of lutetium fluoride type crystals.

  Moreover, as a result of preparing a solution by an alkali melting method using a part of the single crystal and measuring the content of Nd using inductively coupled plasma mass spectrometry, the content of Nd in the LuF single crystal of this example was determined. All were 0.05 mol% with respect to lutetium fluoride.

  The LuF single crystal was cut with a wire saw equipped with a diamond wire, and then optically polished on the entire surface to obtain a scintillator having a shape of 10 mm × 10 mm × 1 mm. One surface of 10 mm × 10 mm of the scintillator was used as an ultraviolet emission surface.

  With respect to this scintillator, the wavelength of ultraviolet rays emitted by converting incident radiation was measured by the following method.

  A scintillator was irradiated with X-rays using an encapsulated X-ray tube targeting tungsten. The tube voltage and tube current when generating X-rays from the enclosed X-ray tube were set to 60 kV and 40 mA, respectively. The ultraviolet rays generated from the ultraviolet emission surface of the scintillator were collected by a condenser mirror, monochromatic by a spectroscope, and the intensity of each wavelength was recorded to obtain the spectrum of the ultraviolet rays generated from the scintillator. As a result of the measurement, it was confirmed that the scintillator of this example converts incident radiation into vacuum ultraviolet light having a wavelength of 178 nm.

<Production of gas amplification type UV image detector>
A gas amplification type ultraviolet image detector which is a component of the radiation image detector of the present invention was produced by the following method.

  As shown in FIG. 1, two gas electronic amplifiers and a pixel type electrode were installed in parallel in a chamber having an opening from the side close to the opening, and the opening was sealed with an ultraviolet incident window. The distance between the ultraviolet incident window and the first stage gas electronic amplifier was 10 mm, the distance between the first stage gas electronic amplifier and the rear stage gas electronic amplifier was 2 mm, and the distance between the rear stage gas electronic amplifier and the pixel-type electrode was 2 mm.

  The gas electronic amplifier has a plate-like multilayer body by depositing copper with a thickness of 5 μm as a metal layer on both sides of a polyimide plate-like insulating layer, and a cylindrical shape having a diameter of 70 μm on the entire surface of the plate-like multilayer body. The through-holes provided in an arrangement in which equilateral triangles are arranged at a pitch of 140 μm were used. Note that the thicknesses of the plate-like insulating layers of the first-stage gas electronic amplifier and the latter-stage gas electronic amplifier were 100 μm and 50 μm, respectively.

  The pixel electrode uses a polyimide substrate having a thickness of 100 μm, and an anode strip having a width of 300 μm is provided on the back surface of the substrate, and cylindrical anode electrodes that are implanted in the anode strip and exposed on the surface of the substrate are spaced by 400 μm. The strip-shaped cathode electrode provided with a hole having a diameter of 260 μm around the upper end surface of the cylindrical anode electrode was used. The diameter of the cylindrical 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 columnar anode electrode was 110 μm, and the upper end 10 μm was exposed on the surface.

MgF ₂ having a diameter of 54 mm and a thickness of 3 mm is used for the ultraviolet incident window, and a thin film of cesium iodide is provided as a photoelectric conversion material on the inner surface of the ultraviolet incident window, and further, the outer periphery of the cesium iodide thin film is provided. An electrode made of a nickel layer was provided. An applied voltage is applied to the nickel layer electrode on the outer periphery of the cesium iodide thin film, both sides of the first stage gas electronic amplifier, both sides of the subsequent stage gas electronic amplifier, and the anode and cathode electrodes of the pixel electrode. A signal processing circuit for reading signals and obtaining a two-dimensional image was connected to the anode electrode and the cathode electrode of the pixel electrode.

The chamber was filled with Ar mixed with 10% C ₂ H ₆ as a detection gas to obtain a gas amplification type ultraviolet image detector which is a component of the present invention. The gas pressure when the detection gas was filled in the chamber was 1 atm.

  In the gas amplification type ultraviolet image detector, −1350 V is applied to an electrode made of a nickel layer provided on the outer peripheral portion of the cesium iodide thin film, and each of the first-stage gas electronic amplifier and the latter-stage gas electronic amplifier is subjected to both sides. 400 V and 300 V were applied between the metal layers, and 400 V was applied between the anode electrode and the cathode electrode of the pixel electrode. The electric field between the ultraviolet light entrance window and the first stage gas electronic amplifier is 0.25 kV / cm, the electric field between the first stage gas electronic amplifier and the rear stage gas electronic amplifier is 1.0 kV / cm, The applied voltage was adjusted so that the electric field between the pixel-type electrodes was 3.0 kV / cm.

Under the applied voltage, the total gain obtained by the two gas electronic amplifiers and the pixel type electrode reaches 2.0 × 10 ⁵ , and even at such a high gain, It was confirmed that no discharge occurred in the pixel-type electrode and the pixel-type electrode operated stably for a long time.

<Production and evaluation of radiation image detector>
The ultraviolet ray exit surface of the scintillator produced by the above-described method and the ultraviolet ray incident window of the gas amplification type ultraviolet image detector were placed in close contact as shown in FIG. 1 to obtain the radiation image detector of the present invention. In addition, “Crytox” manufactured by DuPont was filled as a fluorine-based grease between the ultraviolet emitting surface and the ultraviolet incident window.

In order to evaluate the performance of the radiation image detector, a ⁵⁷ Co isotope having a radioactivity of 7.7 × 10 ⁵ Bq and a ²⁴¹ Am isotope having a radioactivity of 8 × 10 ³ Bq are used as a radiation source. The response of the radiological image detector to the resulting radiation was evaluated. The isotopes are radiation sources that emit 122 keV γ rays and 5.5 MeV α rays, respectively. A radiation source was installed close to the scintillator, and radiation generated from the radiation source was applied to the scintillator proximity surface. Using a signal processing circuit connected to the pixel type electrode, a signal output from each anode electrode of the pixel type electrode was obtained to construct a two-dimensional image. The measurement time when acquiring the two-dimensional image was 61 seconds and 136 seconds, respectively.

Images obtained using ⁵⁷ Co and ²⁴¹ Am as radiation sources are shown in FIGS. 7 and 8, respectively. In addition, the broken line part in each figure shows the position which injected the radiation. As shown in FIGS. 7 and 8, the incident position of radiation can be captured as an image, and it was confirmed that the radiation image detector of the present invention has sufficient sensitivity and excellent position resolution.

Further, the pulse height distribution was measured by the following method using ⁵⁷ Co as a radiation source. First, for each event in which radiation was incident, the amount of charge detected at each anode electrode of the pixel electrode was integrated to obtain the total amount of charge. Next, the number of primary electrons was calculated by dividing the total charge amount by the total amplification factor (2.0 × 10 ⁵ ) obtained by the gas electron amplifier and the pixel type electrode. A wave height distribution was obtained by making a histogram of the frequency of events that caused the number of primary electrons. The obtained wave height distribution is shown in FIG. In the wave height distribution, a count having a primary electron number of 1 or more represents the number of effective events when acquiring a radiographic image.

Comparative Example 1
A radiation image detector was produced in the same manner as in Example 1 except that a LaF ₃ crystal containing Nd was used as the scintillator instead of the LuF ₃ crystal containing Nd.

In order to evaluate the performance of the radiation image detector, an attempt was made to acquire a two-dimensional image using ⁵⁷ Co and ²⁴¹ Am as a radiation source in the same manner as in Example 1. Note that the measurement time for acquiring the two-dimensional image was 61 seconds and 136 seconds, respectively, as in Example 1. As a result, the LaF ₃ crystal containing Nd of this comparative example has a smaller amount of light emission than the LuF ₃ crystal containing Nd of Example 1, so that the signal output from the radiation image detector of this comparative example The counting rate was low, and no image showing the radiation incident position was obtained during the measurement time.

Further, ⁵⁷ Co was used as a radiation source, and the wave height distribution measurement was performed in the same manner as in Example 1. The obtained wave height distribution is shown in FIG. From FIG. 9, it can be seen that the use of LuF having a high light emission amount increases the number of primary electrons obtained, and as a result, the number of effective events when acquiring a radiographic image can be significantly increased.

Example 2
<Preparation of scintillator>
In this example, the same scintillator as in Example 1 was used as the scintillator.

<Production of gas amplification type UV image detector>
A gas amplification type ultraviolet image detector which is a component of the radiation image detector of the present invention was produced by the following method.

As shown in FIG. 2, two gas electronic amplifiers and a pixel type electrode were installed in parallel in a chamber having an opening from the side close to the opening, and the opening was sealed with an ultraviolet incident window. The distance between the ultraviolet incident window and the first stage gas electronic amplifier was 10 mm, the distance between the first stage gas electronic amplifier and the rear stage gas electronic amplifier was 2 mm, and the distance between the rear stage gas electronic amplifier and the pixel-type electrode was 2 mm.

  The same gas electronic amplifier as that in Example 1 was used. As shown in FIG. 2, a thin film of cesium iodide was provided as a photoelectric conversion material on the surface of the first stage gas electronic amplifier facing the ultraviolet light incident window.

The pixel type electrode was the same as in Example 1, and MgF ₂ having a diameter of 54 mm and a thickness of 3 mm was used for the ultraviolet incident window.

The chamber was filled with Ar mixed with 10% C ₂ H ₆ as a detection gas to obtain a gas amplification type ultraviolet image detector which is a component of the present invention. The gas pressure when the detection gas was filled in the chamber was 1 atm.

  In the gas amplification type ultraviolet image detector, instead of the electrode made of the nickel layer provided on the outer peripheral portion of the cesium iodide thin film in the first embodiment, a copper ring 12 is provided on the outer peripheral portion of the ultraviolet incident window as shown in FIG. Was provided. A voltage was applied to each of the copper ring, the two gas electronic amplifiers, and the pixel-type electrode in the same manner as in Example 1.

Under the applied voltage, the total gain obtained by the two gas electronic amplifiers and the pixel type electrode reaches 2.0 × 10 ⁵ , and even at such a high gain, It was confirmed that no discharge occurred in the pixel-type electrode and the pixel-type electrode operated stably for a long time.

<Production and evaluation of radiation image detector>
In the same manner as in Example 1, the radiographic image detector of the present invention was produced, and in order to evaluate the performance of the radiographic image detector, ⁵⁷ Co and ²⁴¹ Am were used as the radiation source in the same manner as in Example 1. An attempt was made to acquire a two-dimensional image. The measurement time when acquiring the two-dimensional image was 68 seconds and 139 seconds, respectively.

Images obtained using ⁵⁷ Co and ²⁴¹ Am as radiation sources are shown in FIGS. 10 and 11, respectively. In addition, the broken line part in each figure shows the position which injected the radiation. As shown in FIGS. 10 and 11, the incident position of radiation can be captured as an image, and it was confirmed that the radiation image detector of the present invention has sufficient sensitivity and excellent position resolution.

Comparative Example 2
A radiographic image detector was produced in the same manner as in Example 2 except that a LaF ₃ crystal containing Nd was used instead of the LuF ₃ crystal containing Nd as the scintillator.

In order to evaluate the performance of the radiation image detector, an attempt was made to acquire a two-dimensional image using ⁵⁷ Co and ²⁴¹ Am as a radiation source in the same manner as in Example 2. Note that the measurement time for acquiring the two-dimensional image was set to 68 seconds and 139 seconds, respectively, as in Example 2. As a result, the LaF ₃ crystal containing Nd of this comparative example has a smaller amount of light emission than the LuF ₃ crystal containing Nd of Example 2, so that the signal output from the radiation image detector of this comparative example The counting rate was low, and no image showing the radiation incident position was obtained during the measurement time.

About the radiographic image detectors obtained in Examples 1 and 2 and Comparative Examples 1 and 2, ⁵⁷ Co isotope and ²⁴¹ Am isotope were used as radiation sources, and radiation generated from the radiation sources installed in the vicinity of the scintillator was used as scintillators. Table 1 shows the counting rate when irradiated. The count rate is the number of times radiation is detected per unit time, and represents the detection efficiency for radiation. From Table 1, it can be seen that according to the radiation image detector of the present invention, the counting rate, that is, the detection efficiency with respect to radiation can be remarkably increased, and the radiation image can be acquired in a short time.

DESCRIPTION OF SYMBOLS 1 Scintillator 2 Photoelectric conversion substance 3 Primary electron 4 Gas electron amplifier 5 Secondary electron 6 Pixel type electrode 7 Chamber 8 Ultraviolet incident window 9 Electrode 10 Ultraviolet reflective film 11 Grease 12 Copper ring 13 Plate-like insulating layer 14 Metal layer 15 Through-hole 16 Crucible 17 Heater 18 Heat insulating material 19 Stage 20 Chamber 21 High frequency coil 22 Pulling rod

Claims (4)

A radiation image detector comprising a scintillator that converts incident radiation into ultraviolet light and a gas amplification type ultraviolet image detector, wherein the scintillator contains at least one element selected from Nd, Er, and Tm. A radiation image detector comprising _three crystals. 2. The radiation image detector according to claim 1, wherein the gas amplification type ultraviolet image detector includes a photoelectric conversion substance, a gas electronic amplifier, and a pixel type electrode. The radiation image detector according to claim 2, wherein the photoelectric conversion substance is cesium iodide or cesium telluride. The radiographic image detector according to claim 2 or 3, wherein there are two or three gas electronic amplifiers.