WO2012032816A1

WO2012032816A1 - Radiological image detector

Info

Publication number: WO2012032816A1
Application number: PCT/JP2011/062215
Authority: WO
Inventors: 福田　健太郎; 澄人石津; 範明河口; 敏尚須山; 吉川　彰; 健之柳田; 有為横田; 秀利窪; 俊介黒澤; 幸次郎谷上; 達谷森; 洋之関谷
Original assignee: 株式会社トクヤマ; 国立大学法人東北大学; 国立大学法人京都大学; 国立大学法人東京大学
Priority date: 2010-09-10
Filing date: 2011-05-27
Publication date: 2012-03-15
Also published as: JP2012058154A

Abstract

[Problem] The purpose of the present invention is to provide a novel radiological image detector, which can detect radiation, such as hard X-rays and γ-rays, with high sensitivity, and which has excellent position resolution and count rate characteristics. [Solution] Provided is a radiological image detector having combined therein a scintillator, which converts inputted radiation into ultraviolet, and a gas-multiplication ultraviolet image detector, which converts ultraviolet into electrons, multiplies the electrons using gas electron avalanche phenomenon, and detects the electrons. The radiological image detector is characterized in that the scintillator is formed of a LiLuF₄ crystal containing at least one kind of emission center element selected from among a group composed of Nd, Er and Tm, and that the gas-multiplication ultraviolet image detector is configured of a photoelectric converter, such as cesium iodide and cesium telluride that convert ultraviolet into electrons, a gas electron multiplier, which multiplies electrons using gas electron avalanche phenomenon, and a pixelated electrode having a multiplication function and a detection function.

Description

Radiation image detector

The present invention relates to a novel radiation image detector. The radiation image detector is suitable for 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. Can be used for

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 radiation image detector is an elemental technology that occupies an important position in radiation utilization technology. With the development of radiation utilization technology, detection sensitivity, position resolution with respect to the incident position of radiation, or count rate characteristics, More advanced performance 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 pixelated electrode has been developed (see Patent Document 1). The particle beam image detector detects electrons generated by ionization of gas molecules by the incident particle beam with a pixel-type electrode, has excellent position resolution and count rate characteristics, and can easily enlarge the sensitive area. And it has the advantage that it can be manufactured at low cost. However, since the gas used has a small atomic weight, it has poor stopping power against photons with high energy such as hard X-rays and gamma rays, and therefore the detection sensitivity for these photons is low. There was a problem of being low.

In view of such problems, the present inventors have already converted incident radiation into ultraviolet rays using a scintillator made of a chemical substance having a large atomic weight, and the ultraviolet rays are converted by a gas amplification detector having positional resolution. A detection method was proposed (see Patent Document 2). In addition, attempts to detect radiation by the same method have been made by others (see Non-Patent Document 1). However, in these methods, since the ultraviolet rays generated from the scintillator detect electrons generated by ionizing gas molecules, the range of the ultraviolet rays has a spread corresponding to the thickness of the gas layer. As a result, when used as a radiographic image detector, there is a problem that the position resolution and the count rate characteristics are degraded. In addition, it is necessary to use chemically unstable gas molecules, which may cause problems such as deterioration of gas molecules themselves or adhesion of gas molecules to detector electrodes, making it difficult to operate stably over a long period of time. (See Non-Patent Document 2).

On the other hand, when detecting ultraviolet rays generated from a scintillator by radiation, a method has been attempted in which ultraviolet rays are converted into electrons using a photoelectric converter and the electrons are detected by a gas amplification type detector. (See Patent Document 3). According to such a method, although it is considered that the problems relating to the degradation of the position resolution and the count rate characteristics and the stability of the operation can be avoided, the ultraviolet rays generated from the scintillator are extremely weak, and electrons are detected as gas amplification type detectors. As a result, the amplification factor during amplification could not be sufficiently increased. As a result, extremely weak ultraviolet rays generated from the scintillator cannot be detected with high sensitivity, and no attempt has been made to produce a device capable of detecting a radiation image by a method using such a photoelectric conversion substance. there were.

Japanese Patent No. 3354551 JP 2008-202977 A

The present invention can detect radiation such as hard X-rays and γ-rays with high sensitivity, has excellent position resolution and count rate characteristics, and converts the incident radiation into ultraviolet rays, and converts ultraviolet rays into electrons. Another object of the present invention is to provide a radiation image detector configured by combining with a novel gas amplification type ultraviolet image detector that amplifies and detects such electrons.

The present inventors have made various studies on scintillators that convert incident radiation into ultraviolet rays. As a result, by using a lithium lutetium fluoride (LiLuF ₄ ) crystal containing at least one element selected from neodymium (Nd), erbium (Er) and thulium (Tm) as a scintillator, high light emission I found that light yield was obtained.
Further, the present inventors paid attention to the gas amplification type ultraviolet image detector constituting the radiation image detector and conducted various studies on the method for detecting the ultraviolet ray generated from the scintillator with high sensitivity.
As a result, UV light generated from the scintillator is converted into electrons with a photoelectric conversion material using a gas amplification type ultraviolet image detector composed of a photoelectric conversion material, a gas electron multiplier (GEM), and a pixel-type electrode. Then, the present inventors have found that the radiation can be detected with high sensitivity by amplifying the electrons with a gas electron amplifier and then detecting them using a pixel-type electrode. In addition, the present invention has been completed by successfully obtaining a radiographic image by a radiographic image detector comprising a combination of the scintillator and a gas amplification type ultraviolet image detector.
That is, according to the present invention,
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 is provided, which is a lithium lutetium fluoride (LiLuF ₄ ) crystal, and the gas amplification type ultraviolet image detector includes a photoelectric conversion material, a gas electronic amplifier, and a pixel type electrode.
In the invention of the radiation image detector,
(1) The lithium lutetium fluoride (LiLuF ₄ ) crystal is a single crystal. (2) The photoelectric conversion material is a thin film of the photoelectric conversion material. (3) The photoelectric conversion material is cesium iodide or tellurium. (4) It is composed of a two-stage or three-stage gas electronic amplifier. (5) It is preferable that the radiation to be detected is hard X-rays or γ-rays.

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 a radiation image obtained by using ⁶⁰ Co in Example 1. 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 is a radiographic image obtained in Example 1 using ²⁴¹ Am. This figure is a radiographic image obtained in Example 2. This figure is a radiographic image obtained in Example 2. This figure is a radiographic image obtained in Example 2.

〔Operating principle〕
The operation principle of the radiation image detector of the present invention will be described with reference to FIG. First, incident radiation is converted into ultraviolet rays by the scintillator 1. Next, the generated ultraviolet rays are converted into primary electrons 3 by the photoelectric conversion material 2. The primary electrons 3 are amplified by a gas electron amplifier 4 using an amplification action by a gas electron avalanche phenomenon in a high electric field to obtain secondary electrons 5, and then secondary electrons 5 are obtained. Is detected while further amplified by the pixel-type electrode 6. By processing a signal based on electrons detected by the pixel-type electrode 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 scintillator that is a constituent element of the radiation image detector of the present invention is a LiLuF ₄ crystal (hereinafter also referred to as an LLF crystal) containing at least one element selected from Nd, Er, and Tm. 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 material described later.

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 10 wt%. By setting the added amount to 0.01 wt% or more, the intensity of light emitted from the scintillator can be increased. On the other hand, by setting the amount to 10 wt% or less, attenuation of light emitted from the scintillator due to concentration quenching can be suppressed. .
The LLF crystal may be either a single crystal or a polycrystal, but it is preferable to use a single crystal from the viewpoint of conversion efficiency from radiation to ultraviolet light.
In the present invention, the radiation to be detected is not particularly limited, and any radiation such as X-rays, α-rays, β-rays, γ-rays, or neutrons can be detected, but the scintillator of the present invention has an effective atomic number. Since the (effective atomic number) and density are high, high energy photons such as hard X-rays and γ-rays can be detected particularly efficiently.

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. The effective atomic number of the LLF crystal used in the present invention varies depending on the kind and content of the luminescent center element, but is about 63 to 64, which is sufficiently larger than that of conventionally known scintillators.
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 emitting surface (hereinafter also simply referred to as an ultraviolet emitting surface) facing the gas amplification ultraviolet image detector described later, and the ultraviolet emitting surface is optically polished (optical polishing). ) Is preferably applied. 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 thickness of the scintillator with respect to 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.
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.

The production method of the scintillator is not particularly limited, but it is preferably produced by a melt growth method such as the Czochralski method or the Bridgman method. By producing by the melt growth method, an LLF crystal excellent in quality such as transparency can be produced, and a large crystal having a diameter of several inches can be produced at a low cost. In the production of the LLF crystal, an annealing operation may be performed after the production of the crystal in order to remove crystal defects caused by thermal strain or the like. The obtained LLF crystal has good processability and is processed into a desired shape and used as a scintillator. In processing, a known cutting machine such as a blade saw or a wire saw, a grinding machine, or a polishing machine can be used without any limitation.

The gas amplification type ultraviolet image detector included in the radiation image detector of the present invention is basically composed of a photoelectric conversion substance, a gas electronic amplifier, and a pixel type electrode. Hereinafter, the gas amplification type ultraviolet image detector will be specifically described.

[Photoelectric conversion material]
The photoelectric conversion material functions to convert ultraviolet rays generated from the scintillator into primary electrons. The type is not particularly limited as long as it has this function. Specific examples include cesium iodide (CsI), cesium telluride (CsTe), and the like. Among these, cesium iodide is preferable from the viewpoints of photoelectric conversion efficiency when converting ultraviolet light into electrons, and chemical stability.
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.

[Gas electronic amplifier]
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 JP-A-2006-302844 or JP-A-2007-234485 can be preferably 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, the higher the applied voltage, the higher the amplification factor is obtained. Become. A suitable range of the applied voltage varies depending on the thickness of the plate-like insulating layer, but is generally 200 V to 1000 V, and an amplification factor obtained at the applied voltage is generally several tens to several thousand.

[Bixel electrode]
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 strips are arranged at intervals of 400 μm, and 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-like 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.

A suitable range of the voltage applied to the pixel electrode varies depending on the type of detection gas, but is generally 400V to 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, the pixel electrode has a very short distance for the cationized gas molecules to drift, and therefore has a dead time shorter than that of other gas amplification detectors, and is about 5 × 10 ⁶ count / (sec. -It has a high count rate characteristic exceeding mm ² ). 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.

[Gas amplification type UV image detector]
Hereinafter, a preferred embodiment when a gas amplification type ultraviolet image detector is constructed 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 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 installed in parallel to the ultraviolet incident window. From the viewpoint of amplification factor and operational stability, the gas electronic amplifier is used in a plurality of stages, and is preferably installed in parallel to the ultraviolet incident window, and is preferably installed in two or three stages. By amplifying electrons in each of the multi-stage gas electron amplifier and the pixel type electrode, the electrons are amplified step by step, and the overall gain obtained as a result can be greatly increased. In addition, 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 all 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. . Specifically, a preferable range of the magnitude of the electric field is generally 0.3 to 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, a gas electronic amplifier and a pixel type electrode are combined by combining a two-stage gas electronic amplifier and a pixel type electrode, and optimizing the applied voltage applied to the gas electronic amplifier and the pixel type electrode. 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, and a coincidence counter for discriminate scintillation light generated by the incidence of individual radiation. ), And a gravity center calculation circuit for obtaining the gravity center position of the scintillation light from the intensity of the signal read from each pixel. In the anger type signal processing circuit, only signals generated by the incidence of a single radiation among the signals obtained from the readout circuit are 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 following, preferred embodiments of the radiation image detector of the present invention using the scintillator and the gas amplification type ultraviolet image detector will be described in detail with reference to FIGS.
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 installed closely, preferably 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, a fluorine-based grease having a high refractive index and high transparency to ultraviolet rays is preferably used. For example, “DuPont“ Krytox ”manufactured by DuPont, etc. can be suitably used.

When the thickness of the scintillator with respect to the radiation incident direction is large and the position resolution is lowered due to 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 LLF crystal 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 3 is used as the scintillator, there is a problem that the crystal is broken by application of the high voltage. . On the other hand, when the LLF crystal was used, such a problem did not occur and it was possible to operate stably for a long time.

Hereinafter, the present invention will be described in detail 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
<Preparation of scintillator>
In this example, the scintillator used LiLuF ₄ crystal containing Nd as the luminescent center element. The LiLuF ₄ crystal containing Nd was manufactured using a crystal manufacturing apparatus by the Czochralski method. As raw materials, LiF, LuF ₃ and NdF ₃ having a purity of 99.99% or more were used. First, 300 g of LiF, 2700 g of LuF ₃ and 23 g of NdF ₃ were weighed, mixed well, and filled into a crucible.
Next, the crucible filled with the above raw materials was set in the chamber of the crystal production apparatus, and the inside of the chamber was evacuated to 1.0 × 10 ⁻³ Pa or less using a vacuum evacuation apparatus, and then high purity tetrafluoromethane A gas mixture was introduced by introducing a mixed gas consisting of argon and argon into the chamber. The pressure in the chamber after gas replacement was atmospheric pressure. After the gas replacement operation, the raw material was heated and melted with a heater, and a seed crystal was brought into contact with the melt of the molten raw material. Next, the seed crystal was pulled up while rotating to start crystal growth.

While pulling up the crystal, the crystal diameter was increased at a certain rate and the crystal diameter was adjusted to 55 mm. After expanding the crystal diameter to 55 mm, the pulling rate was maintained at 3 mm / hr, and the pulling was continued continuously until the length of the crystal reached about 100 mm. Next, the output of the heater was increased, the crystal was separated from the raw material melt, and then slowly cooled to obtain a LiLuF4 crystal containing Nd. The crystal had a diameter of 60 mm and a length of about 100 mm, and was a high-quality crystal without white turbidity or cracks. The content of Nd was 0.23 wt% as a result of measurement using inductively-coupled plasma mass spectrometry (ICP-MS).

The obtained crystal was processed into a 15 mm square cube shape with a wire saw equipped with a diamond wire, and then the entire surface was optically polished to obtain a scintillator. 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). An opening of 5 mm × 5 mm was provided in the central portion of the ultraviolet reflecting film provided on the opposite surface of the ultraviolet emitting surface to form a radiation incident port.
With respect to this scintillator, the wavelength of ultraviolet rays emitted by converting incident radiation was measured by the following method.
The scintillator was irradiated with X-rays using a sealed 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 production example converts incident radiation into vacuum ultraviolet light having a wavelength of 183 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, in a chamber having an opening, two stages of gas electronic amplifiers and pixel electrodes were installed in parallel from the side close to the opening, and the opening was sealed with an ultraviolet incident window. The distance between the ultraviolet light incident window and the first stage gas electronic amplifier was 9 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 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 5 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 high-voltage power supply was connected, and a signal processing circuit for reading signals and obtaining a two-dimensional image was connected to the anode and cathode electrodes of the pixel-type 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.

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 both the first stage gas electronic amplifier and the second stage gas electronic amplifier are arranged on 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 incident 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 second stage gas electronic amplifier is 1.25 kV / cm, The applied voltage was adjusted so that the electric field between the pixel-type electrodes was 3.00 kV / cm.
Under the applied voltage, the total amplification factor obtained by the two-stage gas electronic amplifier and the pixel-type electrode reaches 2.3 × 10 ^{5. Even} at such a high amplification factor, 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 radiological image detector, a ⁶⁰ Co isotope having a radioactivity of 0.65 MBq, a ⁵⁷ Co isotope having a radioactivity of 0.41 MBq, and a ²⁴¹ Am isotope having a radioactivity of 3.1 MBq A radiation source was evaluated, and the response of the radiation image detector to radiation generated from the radiation source was evaluated. The isotopes are radiation sources that emit 1173 keV and 1333 keV γ rays, 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. Images obtained using ⁶⁰ Co, ⁵⁷ Co and ²⁴¹ Am as radiation sources are shown in FIGS. 6, 7 and 8 and 9, respectively. In addition, the broken line part in each figure has shown the position which installed the scintillator, and FIG. 9 is the image acquired by rotating only the scintillator 45 degrees in the same structure. As shown in FIGS. 6 to 9, the shape of the scintillator can be captured as an image, and it was confirmed that the radiographic image detector of the present invention has sufficient sensitivity and excellent position resolution.

Example 2
<Preparation of scintillator>
The LiLuF ₄ crystal containing Nd as the luminescent center element produced in Example 1 was processed into a disk shape having a diameter of 54 mm and a thickness of 5 mm, and then optically polished on both sides to obtain a scintillator. In addition, the optically polished surface of the scintillator was used as an ultraviolet emission surface.
<Production of gas amplification type UV image detector>
A gas amplification type ultraviolet image detector was produced by the following method.
In this example, as shown in FIG. 4, a thin film of cesium iodide was provided as a photoelectric conversion material on the ultraviolet emission surface of the scintillator, and an electrode made of a nickel layer was provided on the outer periphery of the thin film of cesium iodide. . The cesium iodide thin film was provided in a region having a diameter of 34 mm in the central portion of the ultraviolet emitting surface of the scintillator.
As shown in FIG. 4, in a chamber having an opening, a two-stage gas electronic amplifier and a pixel type electrode were installed in parallel from the side close to the opening, and the opening was sealed with the scintillator. That is, in this example, the opening of the chamber was sealed with a scintillator instead of the ultraviolet incident window. By adopting such a structure, the position resolution due to the spread of ultraviolet rays at the ultraviolet incident window is avoided, and at the same time, the structure of the radiation image detector is simplified.
The distance between the ultraviolet emission surface of the scintillator provided with the cesium iodide thin film and the first stage gas electronic amplifier is 9 mm, the distance between the first stage gas electronic amplifier and the second stage gas electronic amplifier is 2 mm, the second stage gas electronic amplifier and the pixel. The distance from the mold electrode was 2 mm.
In addition, the same thing as Example 1 was used for the gas electronic amplifier and the pixel type electrode.
An applied voltage is applied to the electrode made of a nickel layer provided on the outer peripheral portion 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 electrode and the cathode electrode of the pixel type electrode. A high-voltage power source for application was connected, and 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.

<Production and evaluation of radiation image detector>
Evaluation of the performance of the radiation image detector was performed by using a ⁶⁰ Co isotope having a radioactivity of 0.65 MBq as a radiation source and evaluating the response of the radiation image detector to radiation generated from the radiation source. The ⁶⁰ Co isotope is a radiation source that emits γ rays of 1173 keV and 1333 keV. Radiation generated from the radiation source was irradiated to a specific part of the scintillator using a lead collimator. The locations irradiated with radiation were the center location of the scintillator, the location shifted 5 mm to the left from the center, and the location offset 5 mm to the right from the center. The irradiated range was 5 mm in diameter.
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 obtained images are shown in FIGS. 10, 11 and 12, respectively.
As a result, as shown in FIGS. 10 to 12, it was possible to capture a portion irradiated with radiation as an image, and it was confirmed that the radiation image detector of the present invention has sufficient sensitivity and excellent position resolution.

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 Plate-like insulating layer 13 Metal layer 14 Through-hole

Claims

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, characterized in that it is a lithium lutetium fluoride (LiLuF 4 ) crystal, and the gas-amplified ultraviolet image detector comprises a photoelectric conversion substance, a gas electronic amplifier, and a pixel-type electrode.
The radiation image detector according to claim 1, wherein the lithium lutetium fluoride (LiLuF 4 ) crystal is a single crystal.
The radiation image detector according to claim 1, wherein the photoelectric conversion substance is a thin film of a photoelectric conversion substance.
2. The radiation image detector according to claim 1, wherein the photoelectric conversion substance is cesium iodide or cesium telluride.
2. The radiation image detector according to claim 1, comprising a two-stage or three-stage gas electronic amplifier.
The radiation image detector according to claim 1, wherein the radiation is hard X-ray or γ-ray.