JP3851547B2 - Scintillator material, manufacturing method thereof, and radiation detection apparatus using the material - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a scintillator material used for detection of radiation such as X-rays and γ-rays, a manufacturing method thereof, and a radiation detection apparatus using the same.
[0002]
[Prior art]
Conventionally, scintillator materials (photofunctional materials that convert radiation into ultraviolet light and visible light emission) have been researched and developed over many years, centering on alkali halide crystals (CsI, NaI, etc.), and have been widely put into practical use. Among them, X-ray imaging apparatuses are attracting attention in medical examinations using X-ray projection and non-destructive examinations such as IC substrates and circuit boards. As a scintillator material for the apparatus, a material in which CsI is used as a base and impurities are added to increase the scintillation efficiency has been put into practical use. Generally, CsI: Na doped with NaI (sodium iodide) or TlI (thallium iodide), CsI: Tl.
[0003]
Reasons for using CsI as a matrix include (1) high radiation absorption efficiency, (2) little radiation damage, and (3) thin film preparation by vacuum deposition or the like.
[0004]
However, CsI: Na is excellent in luminous efficiency, but has a disadvantage that scintillation efficiency is lowered due to deliquescence in the atmosphere.
[0005]
CsI: Tl does not cause degradation of scintillation characteristics in the air in a short time, but Tl (thallium) is a very toxic substance and has environmental problems. Yes.
[0006]
[Problems to be solved by the invention]
The present invention provides a scintillator material that is based on CsI, which is an excellent material as a scintillator material, and does not decrease the scintillation efficiency in the atmosphere like CsI: Na, and has no toxicity, and a method for producing the scintillator material. is there.
[0007]
[Means for Solving the Problems]
The present invention relates to a scintillator material composed of a CsI-CuI mixed crystal, preferably a scintillator material composed of a CsI-CuI mixed crystal having a CuI concentration of 5 to 50 mol% with respect to CsI, and more preferably a CuI concentration of 10 to CsI. Provided is a scintillator material comprising a CsI-CuI mixed crystal of -30 mol%.
[0008]
Further, the present invention is to prepare a CsI-CuI mixture by adding CuI to CsI, and to form a CsI-CuI mixed crystal on the substrate using the CsI-CuI mixture as a supply source. A method for producing a scintillator material is provided.
[0009]
In the present invention, as an extremely practical scintillator material, a CsI thin film and a CuI thin film are alternately formed on a substrate, and a CsI-CuI mixed crystal is formed on the boundary surface between the CsI thin film and the CuI thin film. A scintillator material is provided.
[0010]
Further, according to the present invention, as a practical method for producing a scintillator material, CsI and CuI are used as independent supply sources, and CsI and CuI are selectively supplied from the respective CsI supply source and CuI supply source. Provides a method for producing a scintillator material, characterized in that CsI thin films and CuI thin films are alternately formed on a substrate.
[0011]
Furthermore, the present invention provides a radiation detection apparatus comprising a scintillator material made of a mixed crystal of CsI-CuI and a fluorescence detection element that detects fluorescence emitted from the scintillator material.
[0012]
The present invention also provides a scintillator material in which a CsI thin film and a CuI thin film are alternately formed on a substrate, and a CsI-CuI mixed crystal is formed on the interface between the CsI thin film and the CuI thin film, and the scintillator material. Provided is a radiation detection apparatus including a fluorescence detection element for detecting emitted fluorescence.
[0013]
The present invention also provides a scintillator material comprising a CsBr—CuBr mixed crystal or a CsCl—CuCl mixed crystal.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, typical embodiments of the present invention will be described in detail with reference to the drawings.
[0015]
As described above, since CsI is a very excellent scintillator material, it is based on CsI, is the same group of I-VII, has good chemical consistency, and is relatively stable in the atmosphere, not a poison. A scintillator material was prepared using certain copper halide CuI as an additive. As a method for preparing the thin film sample, a vacuum deposition method using a normal deposition source heating method was adopted because of the advantage of simplicity. The following two types of thin film structures were produced. One is a CsI: Cu thin film using a mixture of CuI at a predetermined concentration with respect to CsI as an evaporation source, and the other is CsI in which thin films are alternately deposited using CsI and CuI as independent evaporation sources. / CuI multilayer film. In the CsI / CuI multilayer film, it was found that CsI and CuI were mixed by atomic diffusion at the interface, that is, a mixed crystal of CsI-CuI was generated.
[0016]
In order to develop a scintillator material, it is essential to evaluate both the structural characteristics and scintillation characteristics of the material. Regarding the structural characteristics, crystallinity evaluation by X-ray structural analysis (θ-2θ method) and surface structure observation by an atomic force microscope were performed. For the scintillation characteristics, the X-ray excitation emission spectrum and response time were evaluated. The results of the present invention were obtained by combining these evaluation results.
[0017]
First, an example of a CsI: Cu thin film will be described. As the substrate, quartz glass was used for optical measurement. However, since the bonds between constituent atoms of weakly ionic I-VII materials are weak, it is a thin film substrate that causes problems in the production of covalently bonded semiconductor thin films. Material dependence is negligible. The substrate temperature was set to around 50 ° C., and the deposition rate was set to 0.3 to 1 nm / sec. These production conditions are common to all the examples.
[0018]
FIG. 1 shows X-ray excitation emission spectra of CsI: Cu thin films (film thickness: 1 μm) at different CuI addition concentrations. Each spectrum is normalized by the maximum emission intensity. The X-ray source used is an X-ray tube having a copper target. At an addition concentration of 0.1 mol%, 4.0 eV emission due to self-bound excitons inherent in the base CsI crystal is observed as main emission. Here, the self-bound exciton means a state where an exciton in which electrons and holes are combined is localized by lattice strain produced by itself, and 4.0 eV emission is the origin of scintillation in a pure CsI crystal. is there. Emission due to CuI having a peak at 2.8 eV was observed at an addition concentration of 1 mol% or more. At 10 mol%, the intensity of 4.0 eV emission intrinsic to CsI of the base is negligibly low compared to 2.8 eV emission. Become.
[0019]
FIG. 2 shows the dependence of 2.8 eV emission on the CuI addition concentration. The light emission efficiency is particularly high in the concentration range of 5 to 50 mol%, and it is maximum at 10 to 30 mol%, which is equivalent to CsI: Na. Specifically, it is about 90% with respect to the efficiency of a CsI: Na thin film produced using a commercially available CsI: Na scintillator crystal as a deposition source. The above results clearly show the effectiveness and optimum conditions of CuI addition for the scintillation function.
[0020]
FIG. 3 shows the time decay characteristics of 2.8 eV emission in a CsI: Cu thin film (CuI concentration 10 mol%: film thickness 1 μm). In the measurement, a nitrogen pulse laser (wavelength 337 nm, pulse width 300 ps) was used as an excitation source. The excitation process is a two-photon absorption process of a nitrogen pulse laser, the substantial excitation wavelength corresponds to 168 nm, and electron excitation with sufficiently high energy occurs. The observed emission lifetime (corresponding to the time when the emission intensity falls to 1 / e) is 1.0 μs, which is the same as the Na emission lifetime of 0.63 μs in CsI: Na, and 1.0 μs of the Tl emission lifetime in CsI: Tl. The scintillation response time is comparable.
[0021]
Next, the change with time of the CsI: Cu thin film (CuI concentration 10 mol%: film thickness 1 μm) in X-ray excitation luminescence intensity in the air was measured. As shown in FIG. 4, the light emission intensity did not change at all even when left for 4 weeks. This result clearly shows the stability of scintillation upon addition of CuI. However, in the case of a CsI: Cu thin film, the thin film tends to become cloudy. White turbidity is expected to reduce image sharpness in some applications, such as X-ray imaging applications, because it enhances light scattering in thin films and reduces light transmission. The solution to this problem will be described later.
[0022]
As described above, when CuI is added, a concentration of about 10 to 30 mol% is required to obtain the maximum luminous efficiency. In crystal physics, this concentration is not a doping premised on a dilute system, but falls within the category of mixed crystals. In addition, the addition concentration of NaI and TlI in conventional CsI: Na and CsI: Tl is usually 1 mol% or less. The height of the CuI addition concentration in the CsI: Cu thin film was evaluated from the X-ray structural analysis.
[0023]
FIG. 5 shows X-ray diffraction patterns of CsI: Cu thin films (film thickness: 1 μm) at different CuI addition concentrations. At a CuI concentration of 5 mol% or more, peaks different from (110) diffraction of CsI as a base material clearly appear, and at 10 mol% or more, they are main peaks. The appearance of diffraction peaks other than CsI means that a substance different from CsI is formed, and can be attributed to the diffraction pattern of Cs ₃ Cu ₂ I ₅ [(CsI) _3- (CuI) ₂ mixed crystal]. That is, it can be said that the 2.8 eV emission generated by the addition of CuI originates from a CsI-CuI mixed crystal. So far, the scintillation function of CsI-CuI mixed crystals has not been known, and this function has been demonstrated for the first time by the present invention.
[0024]
Although the CsI-CuI mixed crystal thin film has excellent scintillation characteristics as described above, it has a drawback that the thin film becomes cloudy. As an attempt to solve this, a CsI / CuI multilayer film was produced and evaluated for characteristics. The point of interest is in the prediction that a CsI-CuI mixed crystal is formed by atomic diffusion at the CsI / CuI interface.
[0025]
FIG. 6 shows an X-ray diffraction pattern of a CsI (50 nm) / CuI (dnm) multilayer film in which the CsI layer thickness is fixed to 50 nm and the CuI layer thickness is changed from 1 to 30 nm. The total film thickness of each sample is set to 1 μm.
[0026]
6 and the X-ray diffraction pattern of the CsI: Cu thin film (FIG. 5), it can be confirmed that a CsI-CuI mixed crystal is formed in the CsI / CuI multilayer film, which proves the initial prediction. It was something to do.
[0027]
FIG. 7 shows an X-ray excitation emission spectrum of a CsI (50 nm) / CuI (10 nm) multilayer film (total film thickness 1 μm) and a CsI: Cu thin film (CuI concentration 10 mol%: film thickness 1 μm). In the multilayer film, the same 2.8 eV emission as the CsI: Cu thin film is observed. The CuI concentration of the CsI: Cu thin film compared in FIG. 7 is 10 mol% at which the luminous efficiency is the highest (see FIG. 2), and the luminous efficiency of the CsI (50 nm) / CuI (10 nm) multilayer film is higher than that. (About 150%).
[0028]
FIG. 8 shows the CuI layer thickness dependence of the X-ray excitation luminescence intensity of the CsI (50 nm) / CuI (dnm) multilayer film (total film thickness 1 μm). The luminous efficiency is maximized when the CuI layer thickness is 10 to 20 nm.
[0029]
Although it has been described above that white turbidity is a problem in the CsI: Cu thin film, the CsI / CuI multilayer film has very high transparency. FIG. 9 shows a transmission spectrum of a CsI (50 nm) / CuI (10 nm) multilayer film (total film thickness 1 μm) and a CsI: Cu thin film (CuI concentration 10 mol%: film thickness 1 μm). It clearly demonstrates the superiority. The reason why the transmittance sharply decreases to zero near 4 eV is that the fundamental absorption edge of the CsI-CuI mixed crystal is at 4.1 eV. The vibration structure seen in the transmission spectrum of the multilayer film is due to Fabry-Perot interference unique to a highly transparent thin film structure [multiple interference effect due to reflection of the front and back surfaces of the thin film (interface between the substrate and the thin film)]. In the energy region of scintillation luminescence, the maximum transmittance is almost 100%, indicating ideal transparency. In consideration of sufficiently absorbing radiation, the number of CsI and CuI multilayer films is preferably large, and several hundred layers are suitable for practical use.
[0030]
It is well known that the transparency of a substance depends on light scattering due to surface irregularities. Therefore, in order to evaluate the surface structure of the CsI / CuI multilayer film in detail, measurement was performed with an atomic force microscope (AFM). FIG. 10 shows the surface roughness shape of the CsI (50 nm) / CuI (10 nm) multilayer film obtained from the AFM measurement. It can be confirmed that the unevenness is about several nm, and it is clear that the surface is extremely flat. In the CsI: Cu thin film, irregularities of about 50 nm were confirmed.
[0031]
In the above embodiment, a CsI-CuI mixed crystal thin film or a CsI / CuI multilayer film is described. However, a CsI-CuI mixed crystal is effective even for a bulk crystal. Next, FIG. 11 shows an embodiment in which CuBr or CuCl is applied instead of CuI.
[0032]
FIG. 11 shows X-ray excitation light emission when a CsBr (50 nm) / CuBr (10 nm) multilayer film (total film thickness 1 μm) and a CsCl (50 nm) / CuCl (10 nm) multilayer film (total film thickness 1 μm) are used as samples. The spectrum is shown. Although the strength is about 1/10 that of the CsI / CuI multilayer film, these materials can also be used as new and useful scintillation materials.
[0033]
FIG. 12 is a schematic cross-sectional view schematically showing a radiation detection apparatus according to an embodiment of the present invention, in which 1 is a scintillator material, and 2 is a substrate (for example, quartz glass) on which the scintillator material is formed. ) 3 is an optical fiber, 4 is a fluorescence detection element (for example, CCD), and 5 is a frame. For example, the incident radiation 6 is converted into fluorescence by the scintillator material 1, and the fluorescence is detected via the optical fiber. It is an electrical output at.
[0034]
【The invention's effect】
As described above, the present invention provides a scintillator material composed of a mixed crystal of CsI-CuI, and scintillation efficiency and response time are equivalent to those of conventional CsI: Na or CsI: Tl. Thus, a new material that does not have a decrease in scintillation efficiency in the atmosphere and has no concern about toxicity such as CsI: Tl was provided.
[0035]
In addition, a so-called multilayer thin film in which CsI thin films and CuI thin films are alternately formed has improved transparency and surface flatness, which were not sufficient with a single layer film or bulk crystal, and can further contribute to an improvement in efficiency.
[Brief description of the drawings]
FIG. 1 shows X-ray excited emission spectra of CsI: Cu thin films at different CuI doping concentrations.
FIG. 2 is a graph showing the CuI concentration dependence of the 2.8 eV emission intensity of a CsI: Cu thin film.
FIG. 3 is a diagram showing time decay characteristics of 2.8 eV emission of CsI: Cu thin film (10 mol%).
FIG. 4 is a graph showing the change over time in the air atmosphere of the X-ray excited luminescence intensity of a CsI: Cu thin film (10 mol%).
FIG. 5 shows X-ray diffraction patterns of CsI: Cu thin films at different CuI doping concentrations.
FIG. 6 is a diagram showing an X-ray diffraction pattern of a CsI (50 nm) / CuI (dnm) multilayer film.
FIG. 7 is a diagram showing an X-ray excitation emission spectrum of a CsI (50 nm) / CuI (10 nm) multilayer film and a CsI: Cu thin film (10 mol%).
FIG. 8 is a graph showing the CuI layer thickness (d) dependence of the X-ray excited luminescence intensity of a CsI (50 nm) / CuI (dnm) multilayer film.
FIG. 9 is a diagram showing transmission spectra of a CsI (50 nm) / CuI (10 nm) multilayer film and a CuI: Cu thin film (10 mol%).
FIG. 10 is a diagram showing surface roughness characteristics of a CsI (50 nm) / CuI (10 nm) multilayer film obtained from atomic force microscope (AFM) measurement.
FIG. 11 is a diagram showing X-ray excitation emission spectra of CsBr (50 nm) / CuBr (10 nm) and CsCl (50 nm) / CuCl (10 nm) multilayer films.
FIG. 12 is a schematic sectional view of a radiation detection apparatus according to an embodiment of the present invention.

Claims (9)

A scintillator material comprising a CsI (cesium iodide) -CuI (copper iodide) mixed crystal. A scintillator material which is a CsI-CuI mixed crystal and has a CuI concentration of 5 to 50 mol% with respect to CsI. A scintillator material which is a CsI-CuI mixed crystal and has a CuI concentration of 10 to 30 mol% with respect to CsI. 2. A CsI—CuI mixture is prepared by adding CuI to CsI, and a CsI—CuI mixed crystal is formed on a substrate using the CsI—CuI mixture as a supply source. A method for producing a scintillator material according to any one of items 2 and 3. A scintillator material in which CsI thin films and CuI thin films are alternately formed on a substrate, and a CsI-CuI mixed crystal is formed on the boundary surface between the CsI thin films and the CuI thin films. CsI and CuI are used as independent sources, and CsI and CuI thin films are alternately formed on the substrate by selectively supplying CsI and CuI from the respective CsI and CuI sources. A method for manufacturing a scintillator material. A radiation detection apparatus comprising a scintillator material and a fluorescence detection element for detecting fluorescence emitted from the scintillator material, wherein the scintillator material is the scintillator material according to any one of claims 1, 2, and 3. A CsI thin film and a CuI thin film are alternately formed on a substrate, and a scintillator material in which a mixed crystal of CsI-CuI is formed on the interface between the CsI thin film and the CuI thin film, and fluorescence emitted from the scintillator material. A radiation detection device comprising a fluorescence detection element for detection. A scintillator material comprising a CsBr (cesium bromide) -CuBr (copper bromide) mixed crystal or a CsCl (cesium chloride) -CuCl (copper chloride) mixed crystal.

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