Scintillator, radiation detector, radiation imaging system, and method for manufacturing scintillator

Abstract

To provide a scintillator for radiation detection, which can suppress deterioration in luminance due to long-time radiation exposure while suppressing burn-in on itself.SOLUTION: A scintillator includes: a plurality of columnar crystals which is arranged on a substrate so as to convert radiation into light; and protective films covering the surfaces of the plurality of columnar crystals. The plurality of columnar crystals contains an activator agent. The protective films contain silica. Decrease in an amount of luminescence after irradiation of the scintillator with the radiation of 1000 gray (Gy) is 35% or less compared to before the irradiation with the radiation.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a scintillator, a radiation detector, a radiation imaging system, and a method for manufacturing a scintillator. [Background technology]

[0002] Some flat panel detectors (FPDs) used for X-ray photography in medical settings receive X-rays that pass through a subject with a radiation-detecting scintillator, and then detect the light emitted by the scintillator with a photodetector. A scintillator is made up of columnar crystals of an alkali metal halide, such as cesium iodide, formed on a substrate. Air gaps are formed between each columnar crystal. For example, due to the ratio of the refractive index of cesium iodide (approximately 1.8) to that of air (1.0), light undergoes repeated total reflection within the columnar crystals containing cesium iodide, which has a high refractive index, and can be effectively guided to the photodetector.

[0003] However, since the luminescence output of cesium iodide alone is small, an element called an activator, such as thallium, which can replace part of the cesium cation, is sometimes added. Activators can be introduced into the crystal by co-evaporating a source containing thallium, such as thallium iodide, during the evaporation of the alkali metal halide.

[0004] On the other hand, it is known that long-term irradiation of a scintillator causes coloration of the scintillator itself, resulting in a decrease in the amount of light emitted, i.e., brightness. This coloration is called color center formation. Patent Document 1 describes a technology for suppressing color center formation by using a low concentration of an activator. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-161408 Summary of the Invention [Problem to be solved by the invention]

[0006] However, lowering the concentration of the activator can sometimes cause burn-in in the scintillator. An object of the present invention is to provide a scintillator for radiation detection that can suppress burn-in in the scintillator while also suppressing a decrease in luminance due to long-term radiation exposure. [Means for solving the problem]

[0007] In order to solve the above-mentioned problems, the present invention provides a scintillator comprising a plurality of columnar crystals disposed on a substrate and converting radiation into light, and a protective film covering surfaces of the plurality of columnar crystals, wherein the plurality of columnar crystals contain an activator, and the protective film contains silica; the scintillator is heat-treated before irradiation; The scintillator is characterized in that the decrease in the amount of luminescence after irradiation with 1000 gray (Gy) of radiation is 35% or less compared to before irradiation. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a scintillator for radiation detection that can suppress burn-in of the scintillator and also suppress a decrease in brightness due to long-term radiation exposure. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a graph showing the relationship between the cumulative radiation exposure dose and the rate of decrease in the amount of luminescence in Example 1 according to the present invention and Comparative Example 1. [Figure 2] FIG. 2 is a cross-sectional view showing a state in which a group of columnar crystals according to the present invention is covered with a protective film. [Figure 3] 1 is a diagram showing a radiation detector according to the present invention; [Figure 4] 10 is a graph showing the relationship between wavelength and reflectance before irradiation with radiation in Example 5 according to the present invention and Comparative Example 3. [Figure 5] 10 is a graph showing the relationship between wavelength and reflectance after irradiation with 1000 Gy of radiation in Example 5 according to the present invention and Comparative Example 3. [Figure 6] FIG. 1 is a diagram illustrating an example of the configuration of a radiation imaging system. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the scope of the invention claimed. Although multiple features are described in the embodiments, not all of these multiple features are necessarily essential to the invention, and multiple features may be combined arbitrarily. Furthermore, in the accompanying drawings, the same reference numerals are used to designate the same or similar components, and redundant explanations will be omitted.

[0011] According to this embodiment, it is possible to reduce the decrease in the amount of light emitted by the scintillator relative to the cumulative dose of radiation. Fig. 1 is a graph comparing the amounts of light emitted by a scintillator according to Example 1, to which the present invention is applied (described later), with a scintillator according to Comparative Example 1, to which the present invention is not applied. The decrease in the amount of light emitted can be calculated as the rate of decrease using the following formula. In this graph, the amount of light emitted before radiation exposure corresponds to the state before irradiation of the scintillator with radiation for durability evaluation, and the amount of light emitted after radiation exposure corresponds to the state after irradiation with radiation for durability evaluation.

[0012] (Formula 1) Decrease rate of luminescence intensity = 1-(luminescence intensity after irradiation / luminescence intensity before irradiation) The radiation irradiation conditions for durability evaluation were a tube voltage of 130 kV, an additional aluminum filter 1 mm thick, and an exposure dose rate of 0.1 Gy (gray) / min. The luminescence intensity measurement conditions conformed to RQA5. Figure 1 plots the rate of decrease in the luminescence intensity at cumulative exposure doses of 100 Gy and 1000 Gy. Although the graph shows each measurement point connected by a straight line, it is believed that the actual curve is a smooth curve. After 100 Gy irradiation, Example 1 showed a rate of decrease in the luminescence intensity that was approximately half that of Comparative Example 1. Even after 1000 Gy irradiation, Example 1 showed a smaller rate of decrease in the luminescence intensity than Comparative Example 1. The scintillator according to the present invention will now be described in detail.

[0013] (Scintillator) The base material of the scintillator used in the present invention can be selected from alkali metal halide compounds capable of forming columnar crystal groups, such as cesium iodide. To impart sufficient luminescence to the base material, thallium (Tl) can be used as an activator, for example. If the concentration of thallium used as an activator is low, a type of burn-in phenomenon known as bright burn may occur in the scintillator. It is known that bright burn can be suppressed by increasing the concentration of the activator. To reduce bright burn, the concentration of the thallium activator should be 0.25 mol% or higher. The scintillator of the present invention can be manufactured by a common vacuum film-forming method, such as vapor deposition.

[0014] According to one embodiment of the present invention, as shown in Figures 2a and 2b, the entire crystal separation region of a scintillator consisting of a collection of columnar crystals formed on a substrate is continuously covered with a protective film described below, and the initial layer of the scintillator is partially covered with the protective film.

[0015] Here, the initial layer of the scintillator refers to a layer of fine crystal nuclei formed on the substrate at the initial stage of film formation by selecting the substrate temperature, pressure, and film formation rate in the vapor deposition process of columnar crystals using, for example, cesium iodide as a base material. <100> This refers to the region where the film thickness is between 10 μm and a few tens of μm, where the columnar crystals are growing preferentially along a certain orientation. In this region, the orientation and size of the columnar crystals have not yet been fully determined.

[0016] The crystal separation region of the scintillator is the region where the crystal orientation of the columnar crystals changes as film formation progresses from the initial layer. <100> This refers to a region with a thickness of several tens of micrometers or more where the columnar crystals grow into larger, more oriented columnar crystals. In this region, the columnar crystals grow larger and become separated, with gaps formed and maintained between the crystals. It is preferable to use a liquid source to form a protective film in the gaps of the crystal separation region, as this allows the liquid to seep in by capillary action and form a continuous protective film.

[0017] The liquid precursor for forming the protective film partially penetrates into the voids in the regions where the outlines of the columnar crystals in the initial layer are unclear, forming a discontinuous protective film. If many of the voids in the initial layer are filled with the protective film, the light emitted by the scintillator and guided through the scintillator is scattered and diffused, resulting in a decrease in spatial resolution, which is undesirable. By forming the protective film in the crystal separation region continuously and the protective film in the initial layer discontinuously, it may be possible to achieve both spatial resolution and moisture resistance of the scintillator.

[0018] (protective film) The liquid source material for the protective film used in the present invention contains, for example, a polysilazane-based inorganic polymer, such as perhydropolysilazane, which is a silica conversion material containing silicon, nitrogen, and hydrogen, and its concentration is adjusted with an organic solvent. The liquid material can be produced by using a liquid containing various catalysts added as appropriate. Depending on the type of raw material selected, heating may be performed during the hydrolysis reaction or the conversion reaction to silica glass. However, it is more convenient to select a raw material in which the above reactions occur at room temperature. If the concentration of the silica conversion material in the liquid source material for the protective film is less than 0.5 wt%, the resulting protective film will lack continuity and have insufficient moisture resistance, which is undesirable. Furthermore, if the concentration of the silica conversion material in the liquid source material for the protective film is greater than 2 wt%, the voids in the columnar crystals may be filled with the resulting protective film, or the organic solvent may dry out and silica conversion may occur outside the columnar crystals of the scintillator before the liquid source penetrates the scintillator. This is undesirable because it can cause cracks to form when the applied protective film dries, or the scintillator to lift or peel off from the substrate. If the concentration of the silica conversion material is set to 0.5 wt% or more and 2 wt% or less, it may be possible to achieve both spatial resolution and moisture resistance of the scintillator.

[0019] The protective film applicable to the present invention is characterized by being formed using the capillary action of a liquid raw material. Therefore, as shown in Figure 2a, the coverage area of ​​the protective film gradually changes from a discontinuous state to a continuous state from the initial layer toward the crystal separation region. The protective film is characterized by being continuously formed in the crystal separation region, where the separation between the columnar crystals is sufficiently advanced and the gaps between the columnar crystals are clear, and the gaps between the protective film covering the columnar crystals can be maintained even when the side walls of the columnar crystals are covered with the protective film. This makes it possible to improve the moisture resistance of the scintillator without compromising the spatial resolution.

[0020] (Method for forming protective film) As a method for forming the protective film, by appropriately selecting a method using a liquid such as spin coating, spray coating, dip coating, flow coating, or bar coating, it becomes possible to easily obtain a coating of the desired thickness on the columnar crystals compared to, for example, vapor phase growth methods. Furthermore, by using a liquid source, it becomes easy to supply a large amount of source material, especially to the recesses on the surface of the columnar crystal group, and therefore it is also possible to reduce the surface roughness of the columnar crystal surface irregularities.

[0021] For example, if a spin coating method is used, the applied liquid raw material remains at the tip of the columnar crystal for a long time due to the centrifugal force caused by rotation, which is preferable because it facilitates the formation of a protective film on the tip of the columnar crystal. Also, it is possible to minimize the formation of a protective film between the columnar crystals.

[0022] In addition, spray coating allows for a uniform amount of raw material solution to be applied to a large substrate, while dip coating allows for a large amount of raw material solution to be applied to both sides of the substrate at once. Furthermore, dip coating also allows for the formation of a protective film between the columnar crystals to be suppressed by holding the tips of the columnar crystals facing vertically downward during application and drying of the raw material solution. If necessary, the chemical reaction can be promoted by appropriately raising the temperature or heating the material during raw material supply or application, or even after film formation, or by humidifying the material to a degree that prevents the columnar crystals from deliquescing.

[0023] If there are areas of abnormal crystal growth with large irregularities during the deposition of the scintillator, it is advisable to form a very thin film of the scintillator by a conventional vapor deposition method using a metal alkoxide or the like and then perform a coating process before the protective film treatment of the present invention. Measures to prevent deterioration of properties during the next process or the period required for storage due to the coating process may be taken, and the protective film formation of the present invention may be performed after reducing the scintillator surface roughness (planarization process). Besides the coating process, methods such as applying pressure with a flat plate or roller or removing areas of abnormal crystal growth can also be used as means for the planarization process, and the means are not limited as long as the scintillator surface roughness can be reduced.

[0024] The order of forming the protective film is as follows: after the film is formed, a very thin protective film is formed by coating the scintillator with a metal alkoxide such as ethyl silicate as in the prior art. This reduces the deterioration of properties during the time until the next processing step is completed and during storage, i.e., moisture resistance can be maintained. After that, if necessary, a flattening process can be performed on the abnormal crystal growth portion of the scintillator film, and then the protective film formation of the present invention can be performed.

[0025] When spray coating is used as a means for forming the protective film, the protective film of the present invention can be easily formed over a large area by controlling the amount of coating on the scintillator surface by controlling the wet film thickness.

[0026] Wet film thickness is the film thickness immediately after coating, and can be expressed by the following equation 2, where W is the wet film thickness, G is the discharge amount per unit time, L is the total nozzle movement distance, S is the nozzle scanning speed, d is the liquid raw material density, and A is the coating area.

[0027] (Formula 2) TIFF0007822406000001.tif1948

[0028] For example, if the discharge rate per unit time is 7.2 g / min, the application time is 2 minutes, the density of the liquid material is 0.8 g / cc, and the application area is 30 cm square, the wet film thickness will be 200 μm. The applied liquid material quickly penetrates the scintillator, and as the organic solvent dries, silica conversion occurs, forming a protective film.

[0029] (protective film thickness) Energy-dispersive X-ray fluorescence spectroscopy (EDX) analysis confirmed the presence of silicon (Si) in the crystal separation region and initial layer of the scintillator on which the protective film was formed. The amount of Si was highest at the tips of the columnar crystals in the crystal separation region, followed by the middle, and some was also present in the initial layer. The protective film formed on the scintillator is thought to form a regular tetrahedron structure (height 0.216 nm) with four oxygen (O) atoms coordinated to silicon (Si) (Si-O bond length 0.162 nm). Analysis suggests that the number of molecular layers is approximately 15 to 30. This translates to a thickness of several nanometers, and the gaps between the protective film covering the scintillator crystals are largely maintained, maintaining gaps between the crystals. The thickness of the protective film is also thought to provide sufficient moisture resistance.

[0030] (Protective film coverage) The coverage of the protective film of the present invention in the film thickness direction of the scintillator is described below. For example, when columnar crystals are formed on a substrate by a method such as vapor deposition, columnar crystals growing from fine crystal nuclei gradually undergo selection and fusion to form a group of columnar crystals, so the column diameter of the columnar crystals increases as the film thickness increases. Meanwhile, the spacing between the columnar crystals is extremely small for fine crystal nuclei, but as the film thickness increases, the separation between the columns progresses and, at a certain film thickness, the spacing is maintained at a roughly constant distance. If the protective film of the present invention is formed in the initial deposition region consisting of fine crystal nuclei, the protective film fills the gaps between the columnar crystals, eliminating the gaps, causing scattering of the guided light and reducing spatial resolution.

[0031] (Heat treatment) In this embodiment, a heat treatment is performed after the above-described protective film is formed on the scintillator surface. The main parameters of the heat treatment are temperature and time. Since the heating is performed after the protective film is formed, it can be performed in an air atmosphere. Even if the heat treatment is performed in air without controlling the atmospheric gas, there is no effect on the spatial resolution due to deliquescence. Therefore, it is possible to use inexpensive commercially available clean ovens, hot plates, electric furnaces, etc.

[0032] The heat treatment of this embodiment makes it possible to suppress the decrease in the amount of light emitted by radiation exposure, and this is thought to be due to the following reasons. First, it is thought that forming a protective film makes it difficult for halide elements, which are constituent elements, to be desorbed from the surface of the scintillator and to react with elements adsorbed on the surface. It is thought that carrying out heat treatment under such conditions homogenizes the crystal arrangement inside the scintillator, resulting in a structure in which defects are less likely to be formed. It is presumed that the energy of the heat treatment correlates with the homogenization of the crystal arrangement, and that this affects the rate of decrease in the amount of light emitted depending on the conditions of the heat treatment.

[0033] (evaluation) The size and changes in the diameter of the columnar crystals in the manufactured scintillator can be evaluated by observing their shape, for example, with a scanning electron microscope (SEM), etc. In addition, the chemical composition of the deposited film can be evaluated, for example, with X-ray fluorescence analysis or inductively coupled plasma analysis, and the crystallinity can be evaluated, for example, with X-ray diffraction analysis.

[0034] The formation state of the protective film can be evaluated by, for example, element mapping evaluation using energy dispersive X-ray fluorescence spectroscopy (EDX) or morphological observation using a transmission electron microscope (TEM).The reflectance of the scintillator can be evaluated using a spectrophotometer.

[0035] Spatial resolution characteristics can be quantitatively compared by measuring the modulation transfer function (MTF). Detective quantum efficiency (DQE) and the amount of light emitted by scintillators can be evaluated using various light-receiving elements, such as charge-coupled devices (CCDs) and complementary metal-oxide semiconductors (CMOSs), as well as photodetectors such as cameras.

[0036] Furthermore, the X-ray irradiation device used to evaluate radiation durability can be an X-ray generator capable of irradiating at a dose rate of about 0.1 Gy per minute.

[0037] (Radiation detector) A radiation detector using the scintillator of the present invention may have a configuration in which, as shown in FIG. 3a, a plurality of columnar crystals 310 are formed on a photosensor-equipped substrate 305 on which photoelectric conversion elements are arranged, and the substrate is combined with a reflective layer 303 via an adhesive layer 301. This configuration is sometimes called a direct type. Alternatively, as shown in FIG. 3b, a plurality of columnar crystals 310 are formed on a substrate 304 on which a reflective layer 303 is arranged, and the substrate is combined with a photosensor-equipped substrate 302 via an adhesive layer 301. This configuration is sometimes called an indirect type. In addition, to adjust the spatial resolution and sensitivity, optical elements or materials such as a fiber optic plate (FOP) or a light-absorbing filter may be added between the photosensor-equipped substrate 302 on which photoelectric conversion elements are arranged and the scintillator.

[0038] The scintillator and radiation detector described above can be applied to a radiation detection device, a radiation imaging device, and a radiation imaging system that detect radiation. X-rays are typically used as the radiation, but alpha rays, beta rays, etc. may also be used.

[0039] 6 shows a radiation imaging system, which is an example of a usage mode of the radiation detector according to this embodiment. Radiation 611 generated by a radiation source 610 passes through a chest 621 of a subject 620, such as a patient, and enters a radiation detection device 630. The radiation 611 that enters the device 630 contains information about the inside of the patient 620, and the device 630 acquires electrical information corresponding to the radiation 611. This electrical information is converted into a digital signal by an image processor 640 equipped with a signal processing unit, and then undergoes predetermined signal processing.

[0040] A user such as a doctor can view a radiographic image corresponding to this electrical information, for example, on a display 650 in a control room. The user can transfer the radiographic image or its data to a remote location using a predetermined communication means 660, and view the radiographic image on a display 651 in a doctor's room, which is another location. The user can also record the radiographic image or its data on a predetermined recording medium, for example, by using a film processor 670 to record it on film 671.

[0041] Although some preferred examples have been shown above, the present invention is not limited to these, and some modifications may be made without departing from the spirit of the present invention. Furthermore, the individual terms used in this specification are merely used for the purpose of explaining the present invention, and it goes without saying that the present invention is not limited to the strict meaning of the terms, and may also include equivalents thereof.

[0042] Examples of scintillators to which the present invention is applied and comparative examples of scintillators will be described below.

[0043] Example 1 In this example, a scintillator with a columnar crystal structure was formed by vapor deposition. A material supply source filled with cesium iodide as the base material (base material) to be deposited, a material supply source filled with thallium iodide as the activator material to be deposited, and a substrate on which the crystals would be formed were placed in a vacuum deposition apparatus. The substrate consisted of a glass substrate with a 100 nm thick aluminum reflective layer and a 50 nm thick silicon dioxide layer. After evacuating the deposition apparatus to a pressure of 0.01 Pa or less, current was gradually passed through each material supply source to heat it. Once the set temperature was reached, film formation began by opening the shutter between the substrate and the material supply source while rotating the substrate. The substrate temperature was gradually increased from 80°C to 160°C. While monitoring the film formation, the shutter was closed to terminate film formation when the desired film thickness (200 μm) was reached. As a result, columnar crystals primarily composed of cesium iodide were formed on the substrate.

[0044] After the substrate and material supply source were cooled to room temperature, the deposited film was quickly contacted with ethyl silicate using a vapor deposition method and covered with a protective film. Four substrates were simultaneously deposited using this process. The concentration of thallium element, an activator, on one of the substrates was determined using an X-ray fluorescence analyzer, and was found to be 0.34 mol%.

[0045] Next, one of the substrates with the deposited film was set in a spray coater, and a protective film was formed using a solution raw material containing 1 wt% perhydropolysilazane in a dibutyl ether solvent, with the discharge amount and scanning speed individually set to achieve a wet film thickness of 100 μm.

[0046] The protective film was then dried in an environment of 25°C and 50% humidity, and then heat-treated in a clean oven at 230°C for 1 hour in open air. After heating, the film was allowed to cool naturally in the clean oven.

[0047] The surface of the scintillator deposition film thus created was attached to a CMOS photodetector via an FOP (Fiber Optic Plate), and radiation conforming to the international standard radiation quality RQA5 was irradiated from the substrate side to capture an image and determine the amount of light emitted. The exposure dose was set to 4.8 μGy, and the output value (LSB) of the CMOS photodetector was taken as the amount of light emitted.

[0048] Next, the substrate with the deposited film was removed from the CMOS photodetector and a radiation durability evaluation was carried out. The conditions were a tube voltage of 130 kV, an Al-added filter 1 mm thick, and an exposure dose rate of 0.1 Gy / min. After 100 Gy of exposure, the sample was again placed in close contact with the CMOS photodetector and the luminescence intensity was measured. The measurement conditions were the same as the previous measurement, with an RQA5 and 4.8 μGy. The result was that the luminescence intensity had decreased by 11% compared to before the X-ray exposure.

[0049] Similarly, as an additional radiation durability evaluation, the luminescence intensity was measured after an additional 900 Gy irradiation, and the result was that the luminescence intensity was reduced by 27% compared to before the X-ray durability test.

[0050] <Comparative Example 1> Comparative Example 1 is the same as Example 1 up to the point where a scintillator with a columnar crystal structure is formed using a vacuum deposition apparatus with cesium iodide as a base material and thallium iodide as an activator. However, a protective film of perhydropolysilazane is not subsequently formed. Furthermore, the heat treatment performed in Example 1 is not performed. In this state, the sample will deliquesce during the radiation durability evaluation described below, so a 10 μm film of parylene was formed to prepare a sample for Comparative Example 1.

[0051] The surface of the deposited film thus created was attached to a CMOS photodetector via an FOP (Fiber Optic Plate), and radiation conforming to the international standard radiation quality RQA5 was irradiated from the substrate side to capture an image and determine the amount of light emitted. The exposure dose was set to 4.8 μGy, and the output value (LSB) of the CMOS photodetector was taken as the amount of light emitted.

[0052] Next, the substrate with the deposited film was removed from the CMOS photodetector and a radiation durability evaluation was carried out. The conditions were the same as in Example 1: tube voltage 130 kV, Al additional filter 1 mm, and exposure dose rate 0.1 Gy / min. After 100 Gy irradiation, the sample was again placed in close contact with the CMOS photodetector and the luminescence intensity was measured. The measurement conditions were the same as in the previous measurement, with RQA5 and 4.8 μGy. As a result, the luminescence intensity was reduced by 21% compared to before X-ray exposure. This value is approximately twice as large as the 11% reduction in Example 1.

[0053] Similarly, as an additional radiation durability evaluation, the luminescence amount was measured after an additional 900 Gy irradiation. As a result, the luminescence amount was reduced by 38% compared to before the X-ray durability test. This value is larger than the 27% reduction in Example 1.

[0054] <Example 2> The heat treatment conditions in this Example 2 are different from those in the previously described Example 1. The heat treatment conditions were 210° C. in open air for 3 hours, and after heating, the sample was allowed to cool naturally in a clean oven.

[0055] The surface of the scintillator vapor deposition film thus prepared was attached to a CMOS photodetector via a FOP (Fiber Optic Plate), and a radiation durability evaluation was performed in the same manner as in Example 1 and Comparative Example 1. As a result, after 100 Gy irradiation, the amount of light emitted was reduced by 12% compared to before X-ray durability. Furthermore, after a further 900 Gy irradiation, the amount of light emitted was reduced by 35% compared to before radiation durability.

[0056] It can be seen that the rate of decrease in the amount of light emitted is also smaller in this Example 2 than in Comparative Example 1. Table 1 shows a list of the results of Examples 1 and 2 and Comparative Example 1 above.

[0057] (Table 1) TIFF0007822406000002.tif2278

[0058] Example 3 In Example 3, a substrate was prepared on which a vapor-deposited film containing 0.25 mol% of Tl element as an activator was formed. One of the substrates with the vapor-deposited film was set in a spray coater, and then a protective film was formed using a solution containing 1 wt% perhydropolysilazane in a dibutyl ether solvent, with the discharge amount and scanning speed individually adjusted to achieve a wet film thickness of 100 μm.

[0059] The protective film was then dried in an environment of 25°C and 50% humidity, and then heat-treated in a clean oven. The heat-treatment conditions were 230°C in open air for 1 hour, and the film was then allowed to cool naturally in the clean oven. The heat-treatment conditions were the same as in Example 1.

[0060] The surface of the deposited film thus prepared was attached to a CMOS photodetector via a fiber optic plate (FOP), and a radiation durability evaluation was performed in the same manner as in the above-mentioned Examples and Comparative Examples. The results showed that after 100 Gy irradiation, the amount of light emitted was reduced by 10% compared to before X-ray durability. Furthermore, after a further 900 Gy irradiation, the amount of light emitted was reduced by 28% compared to before radiation durability.

[0061] Example 4 The heat treatment conditions in Example 4 were different from those in Example 3. The heat treatment conditions were 230°C in open air for 0.5 hours, and after heating, the film was allowed to cool naturally in a clean oven. The surface of the vapor-deposited film thus produced was brought into close contact with a CMOS photodetector via a fiber optic plate (FOP), and a radiation durability evaluation was carried out in the same manner as in the above-mentioned Examples and Comparative Examples.

[0062] As a result, after 100 Gy irradiation, the amount of luminescence was reduced by 11% compared to before X-ray exposure, and after a further 900 Gy irradiation, the amount of luminescence was reduced by 34% compared to before radiation exposure.

[0063] <Comparative Example 2> In Comparative Example 2, unlike Examples 3 and 4, a protective film was not formed using perhydropolysilazane. Furthermore, no heat treatment was performed. Since deliquescence would occur during the radiation durability evaluation described below in this state, a 10 μm parylene film was formed to prepare a sample for Comparative Example 2.

[0064] The surface of the deposited film thus prepared was attached to a CMOS photodetector via a fiber optic plate (FOP), and radiation durability evaluation was performed in the same manner as in the above-mentioned Examples and Comparative Examples. The results showed that after 100 Gy irradiation, the amount of light emitted was reduced by 22% compared to before X-ray durability. Furthermore, after a further 900 Gy irradiation, the amount of light emitted was reduced by 41% compared to before radiation durability.

[0065] The results of Examples 3 and 4 and Comparative Example 2 are listed in Table 2. Table 2 shows that the time of heat treatment affects the rate of decrease in the amount of luminescence as a condition. After irradiation with 100 Gy, the difference in the amount of luminescence was small, but after irradiation with 1000 Gy, the rate of decrease in the amount of luminescence was smaller with a longer heat treatment time.

[0066] (Table 2) TIFF0007822406000003.tif2277

[0067] <Example 5> In Example 5, a vapor-deposited film containing 0.71 mol% of Tl, an activator, was prepared. One of the substrates with the vapor-deposited film was then placed in a spray coater. A solution containing 1 wt% perhydropolysilazane in dibutyl ether solvent was used to form a protective film with a wet film thickness of 100 μm, adjusting the discharge rate and scanning speed accordingly. The protective film was then dried in an environment of 25°C and 50% humidity, and then heat-treated in a clean oven. The heat treatment conditions were 250°C in open air for 1 hour, and the film was then allowed to cool naturally in the clean oven.

[0068] The surface of the deposited film thus prepared was attached to a CMOS photodetector via a fiber optic plate (FOP), and a radiation durability evaluation was performed in the same manner as in the above-mentioned Examples and Comparative Examples. The results showed that after 100 Gy irradiation, the amount of light emitted was reduced by 18% compared to before X-ray durability. Furthermore, after a further 900 Gy irradiation, the amount of light emitted was reduced by 30% compared to before radiation durability.

[0069] Example 6 This Example 6 differs from Example 5 in the heat treatment conditions. The heat treatment conditions were 200°C in open air for 1 hour, followed by natural cooling in a clean oven. The surface of the deposited film thus prepared was attached to a CMOS photodetector via a fiber optic plate (FOP), and a radiation durability evaluation was performed in the same manner as in the above-mentioned Examples and Comparative Examples. As a result, after 100 Gy irradiation, the light emission intensity was reduced by 22% compared to before X-ray durability. Furthermore, after a further 900 Gy irradiation, the light emission intensity was reduced by 35% compared to before radiation durability.

[0070] <Comparative Example 3> In Comparative Example 3, unlike Examples 5 and 6, a protective film was not formed using perhydropolysilazane. Furthermore, no heat treatment was performed. In this state, the sample would deliquesce during the radiation resistance evaluation described below, so a 10 μm parylene film was formed to prepare the sample for Comparative Example 3. The surface of the vapor-deposited film thus produced was brought into close contact with a CMOS photodetector via a fiber optic plate (FOP), and a radiation resistance evaluation was performed in the same manner as in the above-described Examples and Comparative Examples.

[0071] As a result, after 100 Gy irradiation, the amount of luminescence was reduced by 29% compared to before X-ray exposure. Furthermore, after a further 900 Gy irradiation, the amount of luminescence was reduced by 45% compared to before radiation exposure. A list of the results of Examples 5, 6, and Comparative Example 3 is shown in Table 3 of FIG. 9. It can be seen from Table 3 that temperature, as a heat treatment condition, affects the rate of reduction in the amount of luminescence. After 100 Gy irradiation, the difference was small, but after 1000 Gy irradiation, the rate of reduction in the amount of luminescence was smaller when the heat treatment temperature was higher.

[0072] (Table 3) TIFF0007822406000004.tif2377

[0073] For the samples of Example 5 and Comparative Example 3, the reflectance was also measured before radiation durability and after cumulative irradiation of 1000 Gy. The results of measuring the reflectance before radiation durability are shown in FIG. 4. The results of measuring the reflectance after cumulative irradiation of 1000 Gy are shown in FIG. 5. In the graph of FIG. 4, the horizontal axis represents wavelength and the vertical axis represents reflectance. Compared to Comparative Example 3, the sample of Example 5 has a higher reflectance in the wavelength range of approximately 450 nm or less, and a lower reflectance above 450 nm.

[0074] From the graph in Figure 5, it can be seen that even after a cumulative irradiation of 1000 Gy, the sample of Example 5 tends to have higher reflectance at approximately 450 nm or less compared to Comparative Example 3. Furthermore, at wavelengths above 450 nm, the difference between Example 5 and Comparative Example 3 is small. In particular, in Comparative Example 3, a large decrease in reflectance is observed near wavelengths of 520 nm and 560 nm, but in Example 5, this decrease is small. This corresponds to the suppression of the rate of decrease in the amount of light emission mentioned above. The above results demonstrate that this example also has improved radiation durability.

[0075] <Summary of Examples> Comparing the scintillators described in Examples 1 to 6 with the comparative example, the decrease in luminance between the scintillators after irradiating with 1000 Gy of radiation by heat treatment and the scintillators before radiation exposure was 35% or less. The heat treatment conditions for Examples 1 to 6 were a temperature of 200°C or higher. Examples 1, 3, and 5 showed better luminance decreases of 30% or less, but the heat treatment temperature for these Examples 1, 3, and 5 was 230°C or higher and the heat treatment time was 1 hour or longer. In contrast, the comparative example showed a luminance decrease of 38% to 45%. From the above, it was found that a heat treatment temperature of 200°C or higher could keep the luminance decrease to 35% or less. Preferably, a heat treatment temperature of 230°C or higher and a heat treatment time of 1 hour or longer could keep the luminance decrease to 30% or less.

[0076] (Other embodiments) The disclosure of the present specification includes the following scintillator, radiation detector, radiation imaging device, and method for manufacturing a scintillator.

[0077] (Item 1) a plurality of columnar crystals disposed on a substrate for converting radiation into light; a protective film covering the surfaces of the plurality of columnar crystals, the plurality of columnar crystals contain an activator; the protective film contains silica; A scintillator characterized in that the amount of luminescence after irradiation of the scintillator with 1000 gray (Gy) of radiation decreases by 35% or less compared to before irradiation. (Item 2) 2. The scintillator according to item 1, wherein there are gaps between the protective films covering the plurality of columnar crystals. (Item 3) 3. The scintillator according to item 1 or 2, wherein the plurality of columnar crystals are composed mainly of cesium iodide. (Item 4) 4. The scintillator according to any one of items 1 to 3, wherein the decrease in the amount of luminescence after irradiation with 1000 Gy of radiation is 30% or less compared to before irradiation. (Item 5) 5. The scintillator according to any one of items 1 to 4, wherein the activator is thallium and the concentration of the activator is 0.25 mol % or more. (Item 6) a plurality of columnar crystals disposed on a substrate, the columnar crystals converting radiation into light; a protective film covering the surfaces of the plurality of columnar crystals, the plurality of columnar crystals contain an activator; the protective film contains silica; The scintillator is characterized in that the scintillator is heat-treated. (Item 7) 7. The scintillator according to item 6, wherein there are gaps between the protective films covering the plurality of columnar crystals. (Item 8) 8. The scintillator according to item 6 or 7, wherein the plurality of columnar crystals are composed mainly of cesium iodide. (Item 9) Item 10. The scintillator according to any one of items 1 to 8, a photoelectric conversion element that converts light from the scintillator into an electric charge. (Item 10) Item 9. The radiation detector according to item 9, a signal processing unit that processes a signal from the radiation detector. (Item 11) forming a plurality of columnar crystals containing an activator on a substrate; forming a protective film containing silica on the surfaces of the plurality of columnar crystals; and performing a heat treatment at 200° C. or higher after forming the protective film. (Item 12) Item 12. The method for manufacturing a scintillator according to item 11, wherein gaps are formed between the plurality of columnar crystals with the protective film formed. (Item 13) 13. The method for manufacturing a scintillator according to item 11 or 12, wherein the plurality of columnar crystals are composed mainly of cesium iodide. (Item 14) 14. The method for manufacturing a scintillator according to any one of items 11 to 13, further comprising a planarization step of forming a film that flattens the surfaces of the plurality of columnar crystals, wherein the protective film is formed after the planarization step. (Item 15) 15. The method for producing a scintillator according to any one of items 11 to 14, wherein the concentration of the silica conversion material contained in the liquid raw material used to form the protective film is 0.5% by weight or more and 2% by weight or less. (Item 16) 16. The method for producing a scintillator according to any one of items 11 to 15, wherein the silica conversion material contained in the liquid raw material used to form the protective film contains a polysilazane-based inorganic polymer. (Item 17) 17. The method for producing a scintillator according to any one of items 11 to 16, wherein the temperature of the heat treatment is 230° C. or higher, and the time of the heat treatment is 1 hour or longer.

[0078] The invention is not limited to the above-described embodiments, and various changes and modifications can be made without departing from the spirit and scope of the invention. Accordingly, the following claims are appended to apprise the public of the scope of the invention. [Explanation of symbols]

[0079] 100: substrate, 101: columnar crystal, 102: initial layer, 104: protective film

Claims

1. a plurality of columnar crystals disposed on a substrate for converting radiation into light; a protective film covering the surfaces of the plurality of columnar crystals, the plurality of columnar crystals contain an activator; the protective film contains silica; the scintillator is heat-treated before irradiation; A scintillator characterized in that the reduction in the amount of luminescence after irradiation of 1000 gray (Gy) of radiation to said scintillator is 35% or less compared to before irradiation.

2. 2. The scintillator according to claim 1, wherein there are gaps between the protective films covering the plurality of columnar crystals.

3. 2. The scintillator according to claim 1, wherein the plurality of columnar crystals are composed mainly of cesium iodide.

4. 2. The scintillator according to claim 1, wherein the decrease in the amount of luminescence after irradiation with 1000 Gy of radiation is 30% or less compared to before irradiation.

5. 2. The scintillator according to claim 1, wherein the activator is thallium, and the concentration of the activator is 0.25 mol % or more.

6. A scintillator according to any one of claims 1 to 5; a photoelectric conversion element that converts light from the scintillator into an electric charge.

7. The radiation detector according to claim 6 ; a signal processing unit that processes a signal from the radiation detector.

8. forming a plurality of columnar crystals containing an activator on a substrate; forming a protective film containing silica on the surfaces of the plurality of columnar crystals; a step of performing a heat treatment at 230°C or higher for 1 hour or longer before the radiation exposure and after the formation of the protective film, With the protective film formed, gaps are formed between the plurality of columnar crystals, and the plurality of columnar crystals are made of cesium iodide as a main component. A method for manufacturing a scintillator comprising:

9. 9. The method for manufacturing a scintillator according to claim 8, further comprising a planarization step of forming a film that flattens the surfaces of the plurality of columnar crystals, wherein the protective film is formed after the planarization step.

10. 9. The method for producing a scintillator according to claim 8, wherein the concentration of the silica conversion material contained in the liquid raw material used to form the protective film is 0.5% by weight or more and 2% by weight or less.

11. 9. The method for producing a scintillator according to claim 8, wherein the silica conversion material contained in the liquid raw material used to form the protective film contains a polysilazane inorganic polymer.

Images