JP2012001662A - ZnO SCINTILLATOR AND METHOD FOR PRODUCING THE SAME, RADIATION DETECTOR, RADIATION INSPECTION APPARATUS, OR α-RAY CAMERA USING THE ZnO SCINTILLATOR - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a scintillator that has high amount of luminescence and high energy resolution and is useful for discrimination of α-ray sources.SOLUTION: An impurity-doped ZnO single crystal scintillator that has high amount of luminescence and high energy resolution is obtained by producing a Cu-doped or Cd-doped single crystal using a method of liquid phase epitaxy in which a ZnO single crystal is grown by bringing a substrate into direct contact with mixed melt of solutes, ZnO and CuO or CdO, and a solvent. The radiation detector, a radiation measurement device, and an α-ray camera including the impurity-doped ZnO single crystal scintillator are provided.

Description

  The present invention relates to a ZnO single crystal used as a scintillator in a scintillation detector.

  There is a scintillation detector as a device for measuring radiation. A typical scintillation detector configuration is shown in FIG. In FIG. 1, when radiation enters the scintillation detector 100, fluorescence corresponding to the incident radiation is generated from the scintillator crystal 110, and this light is detected by a photomultiplier tube or a semiconductor detector 120 to detect the radiation. Can do.

  In facilities that handle plutonium, such as MOX fuel manufacturing facilities and reprocessing facilities, pollution control is performed mainly by α-ray measurement. Since the influence of radon progeny, which is a natural radionuclide, cannot be ignored in α-ray measurements, the influence is reduced by wave height discrimination using a surface barrier semiconductor detector as necessary.

  The semiconductor detector is a detector that essentially operates in the same manner as an ionization chamber, and uses semiconductor electrons and holes instead of electrons and ions of the gas detector. Since it has superior energy resolution compared to other charged particle detectors, it is used for energy discrimination between natural radionuclides and radon progeny nuclides in air. However, a silicon semiconductor detector in which germanium or Li is diffused needs to be kept at a low temperature during measurement, and must be continuously cooled with liquid nitrogen or the like. A surface-barrier silicon detector with an electrode with a high-purity silicon surface oxidized and thin gold deposited on the surface of the oxide film can be used at room temperature, but performance degradation due to radiation irradiation is significant. However, it has drawbacks such as being vulnerable to noise and being expensive.

  Therefore, conventionally, a ZnS (Ag) scintillator has been used for α-ray measurement. Since the scintillator is relatively inexpensive, it is most widely used for alpha ray radiation management. The ZnS (Ag) scintillator is a ceramic in which ZnS is doped with Ag, and generates scintillation light when irradiated with α-rays (Non-Patent Document 1; “New Nuclear Handbook”) 3.4 Measurement of Radiation 73-78 ). In the conventional detector, the radioactivity of the α-ray detection nuclide contained in the measurement object is measured by counting the pulse signals obtained by amplifying the scintillation light.

  However, the ZnS (Ag) scintillation detector has a problem in that α-rays from natural radioactive elements (radon progeny) affect the counting at a time when static electricity is likely to occur or in a place with poor ventilation. ZnS (Ag) scintillators have the advantage of a large amount of α-ray excited scintillation emission, but have low energy resolution, so whether the α-ray source is derived from a radioactive element or from radon progeny in the air Could not be discriminated.

  As a technique for solving the above-mentioned problems, the thickness of the α-ray emission nuclide to be separated is greater than the range of the α-ray emission nuclide, and all α-ray energy absorption occurs in the scintillator layer, and the generated scintillation light is shielded by the scintillator itself. ZnS (Ag) scintillator has been proposed in which the thickness is negligible (Patent Document 1; Japanese Patent Laid-Open No. 2006-258755). By using the present invention, the discrimination performance of radioactive elements and radon progeny nuclides in the air is improved, but there is an overlap in the peak value spectra of both, making it difficult to sufficiently discriminate.

JP 2006-258755 A US Patent Application Publication No. 2006/219928 International Publication No. 2007/095475 Pamphlet International Publication No. 2005/114256 Pamphlet International Publication No. 2007/100146 Pamphlet

“New Nuclear Handbook” 3.4 Measurement of Radiation Pages 73-78 Nuclear Instruments and Methods in Physics Research A505 (2003) 111-117 Nuclear Instruments and Methods in Physics Research A505 (2003) 82-84 New Energy and Industrial Technology Development Organization 2005 Research Results Report “Development of Ultra-High Speed Scintillator Single Crystal Materials for Practical TOF Detectors” High Energy News Vol. 21 No. 2 41-50 2002 Appl. Phys. Lett. 78 1237 (2001)

  A problem to be solved by the present invention is a scintillator that emits scintillation light when excited by incident α-rays, and has an α-ray source discrimination performance exceeding ZnS (Ag), and radiation provided with the scintillator It is to provide a detector, a radiation inspection apparatus, and an α-ray camera.

  If a scintillator with high α-ray source discrimination performance that can replace ZnS (Ag) scintillator can be developed, it will be possible to discriminate between radioactive materials and radon progeny in the air, making it easier to contaminate radioactive materials in radioactive material handling facilities. It can be carried out. The α-ray source discrimination performance can be judged by the overlap between the wave height spectrum by the radioactive material source and the wave height spectrum by the radon progeny nuclide source in the air in the peak value spectrum measurement. In order to reduce the overlap of wave height distributions based on both radiation sources, it is necessary to increase the energy resolution of the scintillator used. The energy resolution can be defined by the number of peak channels and its half-value width in wave height distribution measurement. That is, energy resolution ΔE (%) = 2.35 × σ / Peak−Channel × 100. Here, σ is the half-value width (Ch) at the time of Gaussian fitting of the peak value spectrum, and Peak-Channel is the number of peak channels (Ch) of the peak value spectrum. Therefore, in order to increase the energy resolution, it is necessary to increase the amount of light emission and reduce the half width at the time of α-ray excitation.

  The ZnS (Ag) ceramic scintillator has a very large amount of light emission by α-ray irradiation, and it is not easy to develop an α-ray scintillator that exceeds the ZnS (Ag) light emission amount. Therefore, as a result of intensive studies, the researchers focused on the half-value width in peak value spectrum measurement. The ZnS (Ag) scintillator material is ceramic. In the case of a polycrystal, there are cases where the emission intensity varies depending on the orientation of the microcrystals and the spatial resolution depends on the particle size. As a result, the half value width in the wave value spectrum measurement was increased, and the energy resolution was lowered. For scintillator light emission with high spatial resolution and high energy resolution, the scintillator is preferably a single crystal.

  As a scintillator material that emits light by irradiation with ionizing radiation, an exciton light emitting scintillator of a wide band gap compound semiconductor has been proposed by Derenzo et al. (Non-Patent Document 2; Nuclear Instruments and Methods in Physics Research A505 (2003) 111-117, Patent Document 2: US Patent Application Publication No. 2006/219928).

  If a wide gap type compound semiconductor such as ZnO or CdS is used as a scintillator, the fluorescence lifetime is short, the emission wavelength is 370 to 380 nm, and a general-purpose PMT or semiconductor detector can be used. According to this document, the fluorescence lifetime due to X-ray excitation is as short as 0.11 to 0.82 nsec. However, the scintillator materials described in these documents are polycrystalline. In the case of a polycrystal, there are cases where the emission intensity varies depending on the orientation of the microcrystals and the spatial resolution depends on the particle size. For scintillator light emission with high spatial resolution and high energy resolution, the scintillator is preferably a single crystal.

  In addition, an exciton emission scintillator application of a ZnO single crystal doped with about 1 ppm to 10 mol% of a group III and a lanthanoid has been proposed (Patent Document 3; International Publication No. 2007/094785, Non-Patent Document 3; Nuclear Instruments). and Methods in Physics Research A505 (2003) 82-84). According to this document, the fluorescence lifetime of an In-doped ZnO single crystal is described as 0.6 nsec. However, the scintillator materials described in these documents are manufactured by the “High pressure direct melting technique” method. In this method, high pressure and high temperature are required, the crystal growth cost is high, and a single crystal portion and a polycrystalline portion are mixed, so that the same defects as the polycrystalline body exist.

  As a technique for solving the problems of the polycrystalline body, a scintillator using exciton emission of a wide band gap ZnO single crystal has been proposed (Patent Document 4; International Publication No. 2005/114256 pamphlet, non-patent document). 4; New Energy and Industrial Technology Development Organization, 2005 Research Results Report “Development of ultra-high-speed scintillator single crystal materials for practical TOF detectors”).

In this report, a hydrothermal synthesis method using a Pt internal autoclave is adopted as a ZnO single crystal growth method. When this growth method is used, a large group III-doped ZnO single crystal having high crystallinity can be grown. However, the exciton light emitting scintillator using ZnO or the like has a problem that the light emission amount is small due to self-absorption. That is, in the exciton emission wavelength region, the crystal itself becomes an absorber, and thus the amount of scintillator emission is small particularly in a transmission type scintillator device. Therefore, although the energy resolution of the ZnO-based exciton light-emitting scintillator is high, the amount of emitted light is only about 10 to 15% of Bi ₄ Ge ₃ O ₁₂ (BGO), which is a general scintillator (Non-patent Document 5). High energy news Vol.21 No.2 41-50 2002).

As a method for improving the low light emission amount based on the self-absorption of the single crystal scintillator, the present inventors have filed a patent application in which the name of the invention is “laminated ZnO single crystal scintillator and manufacturing method thereof” Application 2009-135438). By using the method of the same application, the solute ZnO and the solvents PbO and Bi ₂ O ₃ or PbF ₂ and PbO are mixed and melted, and then the substrate is brought into direct contact with the obtained melt. ZnO-based laminated single crystals having different band gaps can be grown. A stacked body of ZnO-based semiconductors having different band gaps is manufactured, and the amount of light emission can be greatly increased by forming a layer having a small band gap to a thickness that allows ionizing radiation such as α rays and electron beams to enter. However, even when the same method is used, the amount of light emitted from α-rays remains at a BGO ratio of about 135%. As described above, the α-ray source discriminating scintillator requires high energy resolution and further increases in light emission amount.

In addition, the present inventors have applied for a patent on a method for growing a ZnO single crystal by a solution growth method (Patent Document 5; International Publication No. 2007/100146 pamphlet). By using the method of the same application, the solute ZnO and the solvents PbO and Bi ₂ O ₃ or PbF ₂ and PbO are mixed and melted, and then the substrate is brought into direct contact with the obtained melt. A non-doped ZnO single crystal can be grown on the substrate. In addition, when the method of the same application is used, a ZnO single crystal containing impurities in an amount of 1 mol% or less can also be produced. The α-ray source discrimination scintillator requires high energy resolution but does not require a fast decay time constant. Therefore, it has been found that doping with transition metal impurities such as Cu and Cd during ZnO single crystal growth increases visible light emission, and as a result, increases energy resolution in wave height distribution measurement and increases α-ray source discrimination performance. The present invention has been reached.

  According to a preferred embodiment of the present invention, it is possible to provide a method for producing an impurity-doped ZnO single crystal having a large light emission amount and high energy resolution, and a radiation detector, a radiation inspection apparatus, and an α-ray camera using the crystal. Can be provided.

It is a block diagram of a typical scintillation detector. It is a schematic block diagram of the conventional general LPE growth furnace. It is a schematic block diagram of the LPE growth furnace used in an Example. It is a figure which shows the BGO specific light emission amount for every CuO preparation composition. It is a figure which shows the energy resolution for every CuO preparation composition. It is a result figure of the peak value spectrum measurement of Cu dope ZnO single crystal. It is a result figure of the peak value spectrum measurement of ZnS (Ag) ceramics. 2 is an α-ray irradiation emission spectrum diagram of Example 3 and Comparative Example 1. FIG. It is a figure which shows the BGO specific light emission amount for every CdO preparation composition. It is a figure which shows the energy resolution for every CdO preparation composition. It is a figure of the alpha ray irradiation emission spectrum of Example 9. FIG.

Hereinafter, the present invention will be described in detail. A compound semiconductor such as ZnO emits exciton emission corresponding to the band gap when irradiated with ionizing radiation such as α-rays or electron beams. The band gap of ZnO alone is about 3.30 eV. Exciton emission itself is almost absorbed by ZnO itself and hardly transmitted to the opposite side of the surface irradiated with ionizing radiation. Therefore, the amount of light emitted from the scintillator is basically only the scintillation light in which the long wavelength component of the exciton light is transmitted to the opposite side of the irradiated surface, and the light amount of the exciton light emission type scintillator is relatively small. It will be. The inventors have filed a patent application in which the title of the invention is “multilayered ZnO-based single crystal scintillator and manufacturing method thereof” (Japanese Patent Application No. 2009-135438). By using the method of the same application, the solute ZnO and the solvents PbO and Bi ₂ O ₃ or PbF ₂ and PbO are mixed and melted, and then the substrate is brought into direct contact with the obtained melt. ZnO-based laminated single crystals having different band gaps can be grown. Manufactures ZnO-based semiconductor stacks with different band gaps, and reduces the amount of self-absorption of scintillation light and increases the amount of light emission by making layers with a small band gap thick enough to allow ionizing radiation such as alpha rays and electron beams to penetrate. However, the light emission amount itself remains at a BGO ratio of about 135%.

  On the other hand, when ZnO is doped with impurities, light emission based on crystal defects occurs. Electrons excited by radiation irradiation are trapped in a defect level and then emitted by recombination. Defect-derived light emission is longer-wavelength light emission than exciton light emission because the recombination energy is small. Further, the light emitted from this defect has a high emission intensity but a long decay time constant. The α-ray source discriminating scintillator does not need to be rapidly attenuated, and is preferably a single crystal having a high light emission amount and high energy resolution.

  Although not limited to this principle, in the present invention, it is considered that the discrimination performance of α-rays can be improved by doping ZnO with impurities to increase light emission derived from defects.

<First Embodiment>
The first embodiment is a method of manufacturing a ZnO scintillator by liquid phase epitaxial growth (LPE growth) of a ZnO single crystal by bringing a substrate into direct contact with a mixture / melt of ZnO and CuO as solutes and a solvent. The present invention relates to a method for producing a Cu-doped ZnO scintillator, characterized by mixing 0.01 mol% to 1.0 mol% of CuO with respect to ZnO.

  According to the technology for which the inventors have applied for a patent, a ZnO single crystal containing impurities in an amount of 1 mol% or less can be produced by using a liquid phase epitaxial growth method. As a result of intensive studies by the present inventors, it has been found that when CuO is added at 0.01 mol% to 1.0 mol% with respect to ZnO and the liquid phase epitaxial growth method is performed, the amount of light emitted upon irradiation with α rays exceeds that of BGO.

  According to the Cu-doped ZnO single crystal scintillator obtained by the manufacturing method according to the present embodiment, it is possible to increase not only exciton light emission but also light emission in the visible region when excited by ionizing radiation such as α rays and electron beams. Therefore, it is possible to increase the energy resolution of scintillation light. Further, the Cu-doped ZnO single crystal scintillator obtained in this embodiment is useful for α-ray source discrimination. As the ionizing radiation irradiated to the Cu-doped ZnO single crystal scintillator, α rays are suitable, but it is also useful for increasing the amount of light emitted when γ rays, electron rays, X rays and neutron rays are irradiated.

<Second Embodiment>
The second embodiment is a method for producing a ZnO scintillator by liquid phase epitaxial growth of a Cu-doped ZnO single crystal by bringing a substrate into direct contact with a mixture / melt of ZnO and CuO as a solute and a solvent. Solvents are PbF ₂ and PbO, and the mixing ratio of solute and solvent is solute converted to ZnO only: solvent = 2 to 20 mol%: 98 to 80 mol%, and the mixing ratio of PbF ₂ and PbO as the solvent is PbF _2: PbO = 20~80mol%: a 80~20mol%, a method of manufacturing a Cu-doped ZnO single crystal scintillator mentioned above.

According to this embodiment, PbO and PbF ₂ form a eutectic system, and LPE growth at a temperature lower than the melting point of PbO or PbF ₂ alone becomes possible, so that evaporation of the solvent can be suppressed. Accordingly, stable crystal growth can be performed with little composition variation, furnace material consumption can be suppressed, and the growth furnace does not have to be a closed system, so that low-cost production is possible.

  Further, since the LPE method close to thermal equilibrium growth is used as the crystal growth method, a high-quality ZnO single crystal can be manufactured. The Cu-doped ZnO single crystal scintillator obtained in this embodiment is useful for α-ray source discrimination.

As the solvent composition, preferably _PbF 2: PbO = _20~80mol%: a 80～20Mol%, more preferably _PbF 2: PbO = _30~70mol%: a 70～30Mol%, more preferably _PbF 2: PbO = 40 to 60 mol%: 60 to 40 mol%. Within this range, can be suppressed evaporation amount of PbF ₂ and PbO as the solvent, so that the variation in solute concentration is reduced, the impurity Dopuno ZnO single crystal can be stably grown by the LPE method. Furthermore, since the furnace material consumption can be suppressed and the growth furnace does not have to be a closed system, a ZnO single crystal can be grown at a low cost.

The mixing ratio of ZnO as a solute and PbF ₂ and PbO as a solvent is preferably solute: solvent = 2 to 20 mol%: 98 to 80 mol% in terms of ZnO alone. More preferably, the solute concentration converted to ZnO alone is 5 mol% to 10 mol%. If the solute concentration converted to ZnO alone is less than 5 mol%, the effective growth rate is slow, and if it exceeds 20 mol%, the temperature at which the solute component is dissolved increases and the amount of solvent evaporation may increase. The “solute converted to ZnO alone” (mol%) is [ZnO (mol)] × 100 / ([ZnO (mol)] + [PbO (mol)] + [PbF ₂ (mol)]). expressed.

<Third Embodiment>
The third embodiment is a method for producing a ZnO scintillator by liquid phase epitaxial growth of a Cu-doped ZnO single crystal by directly contacting a substrate with a mixture / melt of ZnO and CuO as a solute and a solvent. Solvents are PbO and Bi ₂ O ₃ , and the mixing ratio of the solute and the solvent is solute in terms of only ZnO: solvent = 5-30 mol%: 95-70 mol%, and the solvents PbO and Bi ₂ O ₃ the mixing ratio of _{PbO: Bi 2 O 3 = 0.1~95mol} %: a 99.9~5mol%, a method of manufacturing a Cu-doped ZnO single crystal scintillator mentioned above.

According to this embodiment, the LPE method close to thermal equilibrium growth is used as the crystal growth method, and PbO and Bi ₂ O ₃ that are fluxes composed of elements having a large ion radius that are difficult to be incorporated into the ZnO crystal are used. As a result, a high-quality ZnO single crystal with few impurities mixed into the crystal can be produced. In particular, a high-quality ZnO single crystal with reduced contamination of fluorine impurities can be produced. The ZnO single crystal scintillator obtained in this embodiment can be applied to detectors such as α rays and electron beams.

As the solvent _{composition, PbO: Bi 2 O 3 =} 0.1~95mol%: 99.9~5mol% is preferred. More _{preferably, PbO: Bi 2 O 3 =} 30~90mol%: a 70~10mol%, particularly _{preferably, PbO: Bi 2 O 3 =} 60~80mol%: a 40~20mol%. In the case of PbO or Bi ₂ O ₃ single solvent, the liquid phase growth temperature becomes high, and therefore a mixed solvent of PbO and Bi ₂ O ₃ having the above mixing ratio is preferable.

The mixing ratio of ZnO as the solute and PbO and Bi ₂ O ₃ as the solvent is preferably solute: solvent = 5-30 mol%: 95-70 mol% converted to ZnO alone. More preferably, the solute concentration converted to ZnO alone is 5 mol% to 10 mol%. If the solute concentration converted only to ZnO is less than 5 mol%, the growth rate is slow, and if it exceeds 30 mol%, the growth temperature may be high. The “solute converted to ZnO” (mol%) means [ZnO (mol)] × 100 / ([ZnO (mol)] + [PbO (mol)] + [Bi ₂ O ₃ (mol)] ).

<Fourth Embodiment>
The fourth embodiment is a method for producing a ZnO scintillator by liquid phase epitaxial growth of a ZnO single crystal by bringing a substrate into direct contact with a mixture / melt of ZnO and CdO, which are solutes, and a solvent. The present invention relates to a method for producing a Cd-doped ZnO scintillator, wherein 7 to 50 mol% of CdO is mixed.

Here, the effect of doping Cd with respect to ZnO will be described. According to the technology for which the present inventors have applied for a patent, the band gap can be controlled from 3.00 to 3.30 eV by forming a mixed crystal of Zn and Cd (Non-Patent Document 6; Appl. Phys. Lett. 78 1237 (2001)). A laminated body of two or more layers of ZnO-based mixed crystals (Zn _1-x Cd _x ) O having different band gaps is formed, and ionizing radiation is irradiated from the surface (irradiation surface) of the layer having a smaller band gap. By setting the thickness of the irradiated (Zn _1-x Cd _x ) O layer to 5 μm to 50 μm where ionizing radiation can penetrate, the ionizing radiation is absorbed by the layer having a small band gap and converted to scintillation light. The second and subsequent layers counted from the layer irradiated with ionizing radiation have a band gap larger than that of the layer (first layer) where scintillation light is generated, so that more scintillation light can be transmitted. The light emission amount at this time can be increased to 135% in terms of the BGO ratio. As described in Non-Patent Document 6, the equilibrium solid solution amount of CdO with respect to ZnO is 0 ≦ y ≦ 0.07. Therefore, the effect of charging CdO from 0 to 7 mol% with respect to ZnO is a reduction in self-absorption. On the other hand, when CdO exceeding 7 mol% is charged, (Zn _1-x Cd _x ) O can no longer be formed as a solid solution and is taken in as interstitial Cd. As a result, the amount of impurities increases and visible light emission increases.

  According to the Cd-doped ZnO single crystal scintillator obtained by the manufacturing method according to the present embodiment, it is possible to increase not only exciton light emission but also light emission in the visible region when excited by ionizing radiation such as α rays and electron beams. Therefore, it is possible to increase the energy resolution of scintillation light. The Cd-doped ZnO single crystal scintillator obtained in this embodiment is useful for α-ray source discrimination. In addition, as ionizing radiation with which the Cd-doped ZnO single crystal scintillator is irradiated, α rays are suitable, but it is also useful for increasing the amount of light emitted upon irradiation with γ rays, electron beams, X rays and neutron rays.

<Fifth Embodiment>
The fifth embodiment is a method for producing a ZnO scintillator by liquid phase epitaxial growth of a Cd-doped ZnO single crystal by bringing a substrate into direct contact with a mixture / melt of a solute ZnO and CdO and a solvent. Solvents are PbF ₂ and PbO, and the mixing ratio of solute and solvent is solute converted to ZnO only: solvent = 2 to 20 mol%: 98 to 80 mol%, and the mixing ratio of PbF ₂ and PbO as the solvent is PbF _2: PbO = 20~80mol%: a 80~20mol%, a method of manufacturing a Cd-doped ZnO single crystal scintillator mentioned above.

According to this embodiment, PbO and PbF ₂ form a eutectic system, and LPE growth at a temperature lower than the melting point of PbO or PbF ₂ alone becomes possible, so that evaporation of the solvent can be suppressed. Accordingly, stable crystal growth can be performed with little composition variation, furnace material consumption can be suppressed, and the growth furnace does not have to be a closed system, so that low-cost production is possible.

  Further, since the LPE method close to thermal equilibrium growth is used as the crystal growth method, a high-quality ZnO single crystal can be manufactured. The Cu-doped ZnO single crystal scintillator obtained in this embodiment is useful for α-ray source discrimination.

As the solvent composition, preferably _PbF 2: PbO = _20~80mol%: a 80～20Mol%, more preferably _PbF 2: PbO = _30~70mol%: a 70～30Mol%, more preferably _PbF 2: PbO = 40 to 60 mol%: 60 to 40 mol%. Within this range, it is possible to suppress evaporation of PbF ₂ and PbO as the solvent, so that the variation in solute concentration is reduced, the impurity Dopuno ZnO single crystal can be stably grown by the LPE method. Furthermore, since the furnace material consumption can be suppressed and the growth furnace does not have to be a closed system, a ZnO single crystal can be grown at a low cost.

The mixing ratio of ZnO as a solute and PbF ₂ and PbO as a solvent is preferably solute: solvent = 2 to 20 mol%: 98 to 80 mol% in terms of ZnO alone. More preferably, the solute concentration converted to ZnO alone is 5 mol% to 10 mol%. If the solute concentration converted to ZnO alone is less than 5 mol%, the effective growth rate is slow, and if it exceeds 20 mol%, the temperature at which the solute component is dissolved increases and the amount of solvent evaporation may increase. The “solute converted to ZnO alone” (mol%) is [ZnO (mol)] × 100 / ([ZnO (mol)] + [PbO (mol)] + [PbF ₂ (mol)]). expressed.

<Sixth Embodiment>
The sixth embodiment is a method for producing a ZnO scintillator by liquid phase epitaxial growth of a Cd-doped ZnO single crystal by directly contacting a substrate with a mixture / melt of ZnO and CuO as a solute and a solvent. Solvents are PbO and Bi ₂ O ₃ , and the mixing ratio of the solute and the solvent is solute in terms of only ZnO: solvent = 5-30 mol%: 95-70 mol%, and the solvents PbO and Bi ₂ O ₃ Is a PdO: Bi ₂ O ₃ = 0.1 to 95 mol%: 99.9 to 5 mol%, and relates to a method for producing the above-mentioned Cd-doped ZnO single crystal scintillator.

According to this embodiment, the LPE method close to thermal equilibrium growth is used as the crystal growth method, and PbO and Bi ₂ O ₃ that are fluxes composed of elements having a large ion radius that are difficult to be incorporated into the ZnO crystal are used. As a result, a high-quality ZnO single crystal with few impurities mixed into the crystal can be produced. In particular, a high-quality ZnO single crystal with reduced contamination of fluorine impurities can be produced. The ZnO single crystal scintillator obtained in this embodiment can be applied to high-speed detectors such as α rays and electron beams.

As the solvent _{composition, PbO: Bi 2 O 3 =} 0.1~95mol%: 99.9~5mol% is preferred. More _{preferably, PbO: Bi 2 O 3 =} 30~90mol%: a 70~10mol%, particularly _{preferably, PbO: Bi 2 O 3 =} 60~80mol%: a 40~20mol%. In the case of PbO or Bi ₂ O ₃ single solvent, the liquid phase growth temperature becomes high, and therefore a mixed solvent of PbO and Bi ₂ O ₃ having the above mixing ratio is preferable.

The mixing ratio of ZnO as the solute and PbO and Bi ₂ O ₃ as the solvent is preferably solute: solvent = 5-30 mol%: 95-70 mol% converted to ZnO alone. More preferably, the solute concentration converted to ZnO alone is 5 mol% to 10 mol%. If the solute concentration converted only to ZnO is less than 5 mol%, the growth rate is slow, and if it exceeds 30 mol%, the growth temperature may be high. The “solute converted to ZnO” (mol%) means [ZnO (mol)] × 100 / ([ZnO (mol)] + [PbO (mol)] + [Bi ₂ O ₃ (mol)] ).

<Seventh and Eighth Embodiments>
The seventh embodiment relates to a radiation detector including the above-described Cu or Cd-doped ZnO single crystal scintillator. As a configuration example of the radiation detector, the above-described Cu or Cd-doped ZnO single crystal scintillator is provided, and a fluorescent unit that emits light from the single crystal scintillator according to incident radiation; The example which comprises the photoelectric conversion means which receives light and converts an optical output into an electrical output is mentioned. In addition, the eighth embodiment relates to a radiation inspection apparatus including the radiation detector. As the photoelectric conversion means, for example, a photomultiplier tube or an avalanche photodiode is used.

<Ninth Embodiment>
The ninth embodiment relates to an α-ray camera including the above-described Cu or Cd-doped ZnO single crystal scintillator. As an example of the configuration of the α-ray camera, the above-described Cu or Cd-doped ZnO single crystal scintillator is provided, and a fluorescent means for emitting the single crystal scintillator in response to incident radiation; Examples include using a position sensitive photomultiplier tube or an avalanche photodiode as a photoelectron conversion means for receiving light and converting the light output into an electric output.

<Crystal growth method>
As a method for growing a non-doped ZnO single crystal or an impurity-doped ZnO single crystal, a vapor phase growth method and a liquid phase growth method have been used roughly. As the vapor phase growth method, chemical vapor transport method (see Japanese Patent Application Laid-Open No. 2004-131301), molecular beam epitaxy or metal organic vapor phase growth method (see Japanese Patent Application Laid-Open No. 2004-84001), sublimation method (Japanese Patent Application Laid-Open No. Hei 5). -70286, etc.) have been used, but there were many dislocations and defects, and the crystal quality was insufficient.

  On the other hand, the liquid phase growth method has an advantage that it is easier to manufacture high quality crystals than the vapor phase growth method because crystal growth proceeds in principle in thermal equilibrium. However, since ZnO has a high melting point of about 1975 ° C. and easily evaporates, it has been difficult to grow a ZnO single crystal using the Czochralski method employed in silicon single crystals and the like. Therefore, as a method for growing the ZnO single crystal, the target substance is dissolved in a suitable solvent, the mixed solution is cooled to a saturated state, and the target substance is grown from the melt. A flux method, a floating zone method, a top seed solution growth (TSSG) method, a solution pulling method, a liquid phase epitaxial growth method (LPE growth method), and the like have been used.

  As the ZnO single crystal growth method in the present invention, a liquid phase epitaxial growth method, a flux method, a TSSG method, a solution pulling method, and the like can be used. Among these various methods, as a method of growing a ZnO-based single crystal, growth at a relatively low cost and a large area is possible. In particular, considering application to a scintillator, a layer structure according to function is formed. An easy LPE growth method is suitable. In particular, a method of growing an impurity-doped ZnO single crystal by the LPE method (liquid phase homoepitaxial growth) using a ZnO single crystal produced by a hydrothermal synthesis method with high crystallinity and capable of easily obtaining a large-area substrate. Method) is preferred.

<LPE growth furnace>
A typical LPE growth furnace is shown in FIG. In the LPE growth furnace, a platinum crucible 4 for melting the raw material and storing it as a melt is placed on a crucible base 9 made of mullite (alumina + silica). Outside the platinum crucible 4 and on the side thereof, three-stage side heaters (upper heater 1, central heater 2, lower heater 3) for heating and melting the raw material in the platinum crucible 4 are provided. . The outputs of the heaters are controlled independently, and the heating amount for the melt is adjusted independently. A mullite-made core tube 11 is provided between the heater and the inner wall of the manufacturing furnace, and a mullite-made furnace lid 12 is provided above the core tube 11. A pulling mechanism is provided above the platinum crucible 4. A pulling shaft 5 made of alumina is fixed to the pulling mechanism, and a substrate holder 6 and a substrate 7 fixed by the holder are provided at the tip thereof. A mechanism for rotating the shaft is provided on the upper portion of the pull-up shaft 5.

  Conventionally, alumina and mullite have been exclusively used for the crucible base 9, the furnace core tube 11, the pull-up shaft 5 and the furnace lid 12 in the members constituting the LPE furnace. Therefore, in the temperature range of 700 to 1100 ° C., which is the LPE growth temperature and the raw material melting temperature, the Al component is volatilized from the alumina and mullite furnace materials and dissolved in the solvent, and this is mixed in the ZnO single crystal thin film. it is conceivable that.

In the present invention, it is important to control the doping amount of impurities. It is desirable to control the doping amount of impurities by the charged composition. Therefore, mixing the Al impurities into the LPE-grown ZnO single crystal thin film can be reduced by making the furnace material constituting the LPE furnace a non-Al material. As a non-Al-based furnace material, a ZnO furnace material is optimal, but considering that it is generally not commercially available, MgO is suitable as a material that does not function as a carrier even when mixed in a ZnO thin film. Considering the SIMS analysis result that the Si impurity concentration in the LPE film does not increase even when a mullite furnace material composed of alumina and silica is used, a quartz furnace material is also preferable. In addition, calcium, silica, ZrO ₂ and zircon (ZrO ₂ + SiO ₂ ), SiC, Si ₃ N _{4 and the} like can be used.

  As described above, in a preferred embodiment, a non-doped ZnO single crystal or a group III and lanthanoid doped ZnO single crystal is manufactured using an LPE growth furnace composed of MgO and / or quartz as a non-Al furnace material. And a crucible base for placing the crucible on the crucible, a core tube provided so as to surround the outer periphery of the crucible base, a furnace lid provided on the top of the core tube for opening and closing the furnace, and A mode in which a pulling shaft for moving the substrate up and down is provided, and these members are independently made of MgO or quartz is also preferable.

<Other requirements>
In the embodiment, the _third component is added to the solvent for the purpose of controlling the LPE growth temperature and adjusting the solvent viscosity within a range in which the ZnO solubility, the evaporation amount of PbF ₂ and PbO, or the evaporation amount of PbO and Bi ₂ O ₃ do not change greatly. 1 type, or 2 or more types can be added. For example, examples of the third component include B ₂ O ₃ , P ₂ O ₅ , V ₂ O ₅ , MoO ₃ , WO ₃ , SiO ₂ , MgO, and BaO.

The substrate that can be used is not particularly limited as long as it has a crystal structure similar to ZnO and the grown thin film and the substrate do not react with each other, and a substrate having a close lattice constant is preferably used. For example, _{sapphire, LiGaO 2, LiAlO 2, LiNbO} 3, LiTaO 3, ZnO or the like can be mentioned. Considering that the target single crystal is ZnO, ZnO having a high lattice matching degree between the substrate and the grown crystal is optimal.

  Hereinafter, as a method for growing an impurity-doped ZnO single crystal according to an embodiment of the present invention, a method of forming these ZnO single crystal films on a ZnO single crystal substrate by an LPE method will be described. The present invention is not limited to the following examples.

<Outline of operating conditions of LPE growth furnace>
The block diagram of the LPE growth furnace used in the Example is shown in FIG. In the single crystal manufacturing furnace, a platinum crucible 4 for melting a raw material and storing it as a melt is provided on a crucible base 9 'made of MgO. Outside the platinum crucible 4 and on the side thereof, three-stage side heaters (upper heater 1, central heater 2, lower heater 3) for heating and melting the raw material in the platinum crucible 4 are provided. . The outputs of the heaters are controlled independently, and the heating amount for the melt is adjusted independently. A quartz core tube 11 ′ is provided between the heater and the inner wall of the manufacturing furnace, and an MgO furnace lid 12 ′ for opening and closing the inside of the furnace is provided above the core tube 11 ′. A pulling mechanism is provided above the platinum crucible 4. A pulling shaft 5 'made of quartz is fixed to the pulling mechanism, and a substrate holder 6 and a substrate 7 fixed by the holder are provided at the tip thereof. A mechanism for rotating the pull-up shaft 5 'is provided on the upper portion of the pull-up shaft 5' made of quartz. Below the platinum crucible 4, a thermocouple 10 for melting the raw material in the crucible is provided.

  In order to melt the raw material in the platinum crucible, the temperature of the production furnace is increased until the raw material is melted. The temperature is preferably raised to 650 to 1000 ° C, more preferably 700 to 900 ° C, and the mixture is allowed to stand for 2 to 3 hours to stabilize the raw material melt. At this time, an offset is applied to the three-stage heater, and the bottom of the crucible is adjusted to be several degrees higher than the melt surface. Preferably, −100 ° C. ≦ H1 offset ≦ 0 ° C., 0 ° C. ≦ H3 offset ≦ 100 ° C., more preferably −50 ° C. ≦ H1 offset ≦ 0 ° C., 0 ° C. ≦ H3 offset ≦ 50 ° C.

  After adjusting the bottom temperature of the crucible to a seeding temperature of 700 to 900 ° C. and stabilizing the temperature of the melt, the substrate is brought to the melt surface by lowering the pulling shaft while rotating the substrate at 5 to 120 rpm. Wetted in contact with. After the substrate is adjusted to the melt, a temperature drop is started at a constant temperature or 0.025 to 5.0 ° C./hr to grow a desired non-doped or doped ZnO single crystal on the surface of the substrate. Also during the growth, the substrate is rotated at 5 to 300 rpm by the rotation of the pulling shaft, and is reversely rotated every predetermined time.

  After crystal growth over about 24 to 36 hours, the substrate is separated from the melt, and the melt component is separated by rotating the pulling shaft at a high speed of about 200 to 300 rpm. Then, it cools to room temperature over 1 to 24 hours, and obtains the target ZnO single crystal thin film. Further, depending on the growth film thickness, it is possible to grow while pulling up the pulling shaft made of quartz with time or continuously.

<Examples 1-6, Comparative Examples 1-3>
In this example and comparative example, a Cu-doped ZnO single crystal was produced by a liquid phase epitaxial growth method using PbO and Bi ₂ O ₃ as solvents.

Raw materials were charged in a platinum crucible having an inner diameter of 75 mmφ, a height of 75 mmh, and a thickness of 1 mm according to the formulation shown in Table 1 below. The solvent composition is PbO: Bi ₂ O ₃ = 66.67 mol%: 33.33 mol%, and the solute concentration converted to ZnO alone is about 7.00 mol%. The CuO composition is non-doped with ZnO (Comparative Example 1) or 0.01 mol% to 1.1 mol% (Examples 1 to 6 and Comparative Example 2 respectively). In addition, it is compared with BGO (Bi ₄ Ge ₃ O ₁₂ ).

  The crucible charged with the raw material was placed in the furnace shown in FIG. 3 and melted at a crucible bottom temperature of about 900 ° C. After that, after holding at the same temperature for 3 hours, the temperature was lowered until the crucible bottom temperature reached about 800 ° C., and then a ZnO single crystal substrate having a size of 10 mm × 10 mm × 0.5 mmt in the + C plane orientation grown by the hydrothermal synthesis method And was grown at the same temperature for 24 hours while rotating the pulling shaft made of quartz at 60 rpm. At this time, the rotation direction of the shaft was reversed every 5 minutes. Then, the quartz pull-up shaft was lifted to separate it from the melt, and the shaft was rotated at 200 rpm to shake off the melt components to obtain a colorless and transparent non-doped or Cu-doped ZnO single crystal thin film.

  Table 2 shows the results of growth time, LPE film thickness, and growth rate of the obtained non-doped or Cu-doped ZnO single crystal thin film. The thickness of these obtained ZnO films was 100 to 142 μm. The LPE growth method had a relatively high growth rate, and the growth rate was 4.2 to 5.9 μm / hr.

  Further, Table 2 shows the ratio and energy resolution when the rocking curve half-value width (evaluation of crystallinity) of the (002) plane after polishing the obtained ZnO film, and the BGO emission amount is 100. In the polishing, the obtained ZnO film was lapped and polished to flatten the surface.

As the scintillator characteristics, peak value spectrum measurement was performed. ²⁴¹ Am was used as the α-ray source. PMT (R7600: manufactured by Hamamatsu Photonics) was used as a light receiving element, and five surfaces of the polished LPE-grown ZnO single crystal were covered with Teflon (registered trademark) tape so that scintillation light could be obtained from only one surface. At that time, a 2 mm square window was left for passing α-rays. A PMT was installed on the surface where the scintillation light was emitted, and a high voltage was applied to the PMT to amplify the signal and read out. The crest value was amplified by pre-amp, the waveform was adjusted by Pulse shape amp, the signal was obtained as a signal through a multi channel analyzer, and a crest spectrum was obtained. Gaussian fitting was applied to the same wave height distribution, the amount of luminescence was calculated from the number of peak channels of the Gaussian curve, and the energy resolution was calculated from the number of peak channels and the half width of the Gaussian curve.

  As can be seen from the results of Examples 1 to 6, when a Cu-doped ZnO film was grown using the LPE method close to thermal equilibrium growth, the (002) plane rocking curve half width was 19 to 42 arcsec. Since the half width of the ZnO films according to Examples 1 to 6 is not significantly different from the half width (20 arcsec) of the hydrothermal synthesis substrate used as the substrate, the ZnO films according to Examples 1 to 6 have high crystallinity. It became clear to have.

  4 and 5 show the BGO specific emission amount and energy resolution for each CuO preparation composition. As can be seen from the figure, the Cu-doped ZnO single crystal grown at a CuO charge composition of 0.01 mol% to 1.0 mol% showed a light emission amount exceeding BGO, and was found to have the same energy resolution as BGO. .

  Using the Cu-doped ZnO single crystal scintillator produced in Example 3, the α-ray source discrimination performance was compared with that of a ZnS (Ag) ceramic scintillator. The radon progeny nuclide source was an air sampler and collected on filter paper. FIG. 6 shows the result of the peak value spectrum measurement of the Cu-doped ZnO single crystal, and FIG. 7 shows the result of the peak value spectrum measurement of the ZnS (Ag) ceramics.

  When Cu-doped ZnO single crystals are used, the crest value spectrum of the radioactive radiation Am source and the crest spectrum of the radon progeny nuclide source in the air are clearly separated, and the discrimination performance of the α source is high. I understand. On the other hand, when a ZnS (Ag) ceramic scintillator is used, there is an overlap between the peak value spectrum of the Am source and the peak value spectrum of the radon progeny nuclide source, and the counts of both sources cannot be clearly distinguished.

  Further, the Gaussian fitting was applied to the peak spectrum in FIG. 6 and ΔE of the Cu-doped ZnO single crystal obtained from the number of peak channels and the half-value width obtained from the fitting was 43% for the Am source and the radon progeny nuclide line. The source was 39%. On the other hand, ΔE of ZnS (Ag) ceramics obtained from the peak value spectrum in FIG. 7 was 114% for the Am source and 109% for the radon progeny source. From this result, it became clear that the ZnO single crystal has higher energy resolution than the ZnS (Ag) ceramics.

  The α-ray irradiation emission spectra of Example 3 and Comparative Example 1 are shown in FIG. In the non-doped ZnO single crystal (Comparative Example 1), only the long wavelength component of the band edge emission is transmitted to the side opposite to the α-irradiated surface, but in Example 3 doped with Cu as an impurity, the long wavelength of the band edge emission is long. Although the component decreases, visible light emission having a peak at 520 nm occurs. This visible light emission contributes to an increase in α-ray irradiation scintillation light.

<Examples 7 to 11 and Comparative Example 4>
In this example and comparative example, a Cd-doped ZnO single crystal was produced by a liquid phase epitaxial growth method using PbO and Bi ₂ O ₃ as solvents.

Raw materials were charged in a platinum crucible having an inner diameter of 75 mmφ, a height of 75 mmh, and a thickness of 1 mm according to the formulation shown in Table 3 below. The solvent composition is PbO: Bi ₂ O ₃ = 66.67 mol%: 33.33 mol%, and the solute concentration converted to ZnO alone is about 7.00 mol%. The CdO composition is non-doped with ZnO (Comparative Example 1 described above), or 7.3 mol% to 52 mol% (Examples 7 to 11 and Comparative Example 4 respectively).

  The crucible charged with the raw material was placed in the furnace shown in FIG. 3 and melted at a crucible bottom temperature of about 900 ° C. After that, after holding at the same temperature for 3 hours, the temperature was lowered until the crucible bottom temperature reached about 800 ° C., and then a ZnO single crystal substrate having a size of 10 mm × 10 mm × 0.5 mmt in the + C plane orientation grown by the hydrothermal synthesis method And was grown at the same temperature for 24 hours while rotating the pulling shaft made of quartz at 90 rpm. At this time, the rotation direction of the shaft was reversed every 5 minutes. Thereafter, the quartz pull-up shaft was raised to separate it from the melt, and the shaft was rotated at 200 rpm to shake off the melt components, thereby obtaining a colorless and transparent Cd-doped ZnO single crystal thin film.

  Table 4 shows the results of growth time, LPE film thickness, and growth rate of the obtained non-doped or Cd-doped ZnO single crystal thin film. The thicknesses of these obtained ZnO films were 131 to 155 μm. The LPE growth method had a relatively high growth rate, and the growth rate was 5.5 to 6.5 μm / hr. In Comparative Example 4 in which the CdO charge composition exceeded 50 mol%, CdO did not dissolve and LPE could not be grown.

  Table 4 shows the ratio and energy resolution when the rocking curve half-value width (evaluation of crystallinity) of the (002) plane after polishing the obtained ZnO film and the amount of BGO light emission are 100. In the polishing, the obtained ZnO film was lapped and polished to flatten the surface.

As the scintillator characteristics, α-ray excited scintillator light emission measurement was performed. ²⁴¹ Am was used as the α-ray source. PMT (R7600: manufactured by Hamamatsu Photonics) was used as a light receiving element, and five surfaces of the polished LPE-grown ZnO single crystal were covered with Teflon (registered trademark) tape so that scintillation light could be obtained from only one surface. At that time, a 2 mm square window was left for passing α-rays. A PMT was installed on the surface where the scintillation light was emitted, and a high voltage was applied to the PMT to amplify the signal and read out. The wave height value was amplified by pre-amp, the waveform was adjusted by Pulse shape amp, and the wave height distribution was obtained by obtaining as a signal to a computer via multi channel analyzer. Gaussian fitting was applied to the same wave height distribution, the amount of luminescence was calculated from the number of peak channels of the Gaussian curve, and the energy resolution was calculated from the number of peak channels and the half width of the Gaussian curve.

  As can be seen from the results of Examples 7 to 11, when the Cd-doped ZnO film was grown using the LPE method close to thermal equilibrium growth, the (002) plane rocking curve half width was 36 to 63 arcsec. Since the half width of the ZnO films according to Examples 7 to 11 is not significantly different from the half width (20 arcsec) of the hydrothermal synthesis substrate used as the substrate, the ZnO films according to Examples 7 to 11 have high crystallinity. It became clear.

  9 and 10 show the BGO specific emission amount and energy resolution for each CdO charged composition. As can be seen from the figure, the Cd-doped ZnO single crystal grown with a CdO charge composition of 7.3 mol% to 50 mol% showed a light emission amount exceeding BGO, and was found to have the same energy resolution as BGO. Incidentally, the patent application (Japanese Patent Application No. 2009) with the title of the invention “Laminated ZnO-based single crystal scintillator and method for producing the same” is shown in FIG. -135438).

  As an example, the α-ray irradiation emission spectrum of Example 9 is shown in FIG. In Example 9 in which Cd is doped as an impurity, the long wavelength component of the band edge emission is reduced, but visible light emission having a peak at 530 nm is generated. This visible light emission contributes to an increase in α-ray irradiation scintillation light.

<Examples 12 to 17, Comparative Examples 5 and 6>
In this example and comparative example, a Cu-doped ZnO single crystal was prepared by a liquid phase epitaxial growth method using PbO and PbF ₂ as solvents.

Raw materials were charged in a platinum crucible having an inner diameter of 75 mmφ, a height of 75 mmh, and a thickness of 1 mm according to the formulation shown in Table 5 below. The solvent composition is PbO: PbF ₂ = 50.00 mol%: 50.00 mol%, and the solute concentration converted to ZnO alone is about 5.00 mol%. The CuO composition is non-doped with ZnO (Comparative Example 5) or 0.01 mol% to 1.1 mol% (Examples 12 to 17 and Comparative Example 6 respectively).

  The crucible charged with the raw material was placed in the furnace shown in FIG. 3, and a Cu-doped ZnO single crystal was obtained in the same manner as in Example 1.

  Table 6 shows the results of the growth time, LPE film thickness, and growth rate of the obtained non-doped or Cu-doped ZnO single crystal thin film. The thicknesses of these obtained ZnO films were 108 to 148 μm. The LPE growth method had a relatively high growth rate, and the growth rate was 3.0 to 4.1 μm / hr.

  Further, Table 6 shows the ratio and energy resolution when the rocking curve half-value width (evaluation of crystallinity) of the (002) plane after polishing the obtained ZnO film and the BGO emission amount as 100 are shown. In the polishing, the obtained ZnO film was lapped and polished to flatten the surface.

As the scintillator characteristics, peak value spectrum measurement was performed. ²⁴¹ Am was used as the α-ray source. PMT (R7600: manufactured by Hamamatsu Photonics) was used as a light receiving element, and five surfaces of the polished LPE-grown ZnO single crystal were covered with Teflon (registered trademark) tape so that scintillation light could be obtained from only one surface. At that time, a 2 mm square window was left for passing α-rays. A PMT was installed on the surface where the scintillation light was emitted, and a high voltage was applied to the PMT to amplify the signal and read out. The crest value was amplified by pre-amp, the waveform was adjusted by Pulse shape amp, the signal was obtained as a signal through a multi channel analyzer, and a crest spectrum was obtained. Gaussian fitting was applied to the same wave height distribution, the amount of luminescence was calculated from the number of peak channels of the Gaussian curve, and the energy resolution was calculated from the number of peak channels and the half width of the Gaussian curve.

  As can be seen from the results of Examples 12 to 17, when a Cu-doped ZnO film was grown using the LPE method close to thermal equilibrium growth, the (002) plane rocking curve half width was 35 to 49 arcsec. Since the half width of the ZnO films according to Examples 12 to 17 is not significantly different from the half width (20 arcsec) of the hydrothermal synthesis substrate used as the substrate, the ZnO films according to Examples 12 to 17 have high crystallinity. It became clear to have.

  Table 6 shows the BGO specific emission amount and energy resolution for each CuO preparation composition. As can be seen from the table, the Cu-doped ZnO single crystal grown at a CuO charge composition of 0.01 mol% to 1.0 mol% showed a light emission amount exceeding BGO and was found to have the same energy resolution as BGO. .

<Examples 18 to 22, Comparative Example 7>
In this example and comparative example, a Cd-doped ZnO single crystal was prepared by a liquid phase epitaxial growth method using PbO and PbF ₂ as solvents.

Raw materials were charged in a platinum crucible having an inner diameter of 75 mmφ, a height of 75 mmh, and a thickness of 1 mm according to the formulation shown in Table 7 below. The solvent composition is PbO: PbF ₂ = 50.00 mol%: 50.00 mol%, and the solute concentration converted to ZnO alone is about 5.00 mol%. The CdO composition is 7.31 mol% to 53 mol% with respect to ZnO (Examples 18 to 22 and Comparative Example 7 respectively).

  The crucible charged with the raw material was placed in the furnace shown in FIG. 3, and a Cd-doped ZnO single crystal was obtained in the same manner as in Example 7.

  Table 8 shows the results of growth time, LPE film thickness, and growth rate of the obtained non-doped or Cd-doped ZnO single crystal thin film. The thickness of these obtained ZnO films was 112 to 144 μm. The LPE growth method had a relatively high growth rate, and the growth rate was 3.1 to 4.0 μm / hr. In Comparative Example 7 where the CdO charge composition exceeded 50 mol%, CdO did not dissolve and LPE could not be grown.

  Table 8 shows the ratio and energy resolution when the rocking curve half-value width (evaluation of crystallinity) of the (002) plane after polishing the obtained ZnO film and the BGO emission amount as 100. In the polishing, the obtained ZnO film was lapped and polished to flatten the surface.

As the scintillator characteristics, α-ray excited scintillator light emission measurement was performed. ²⁴¹ Am was used as the α-ray source. PMT (R7600: manufactured by Hamamatsu Photonics) was used as a light receiving element, and five surfaces of the polished LPE-grown ZnO single crystal were covered with Teflon (registered trademark) tape so that scintillation light could be obtained from only one surface. At that time, a 2 mm square window was left for passing α-rays. A PMT was installed on the surface where the scintillation light was emitted, and a high voltage was applied to the PMT to amplify the signal and read out. The wave height value was amplified by pre-amp, the waveform was adjusted by Pulse shape amp, and the wave height distribution was obtained by obtaining as a signal to a computer via multi channel analyzer. Gaussian fitting was applied to the same wave height distribution, the amount of luminescence was calculated from the number of peak channels of the Gaussian curve, and the energy resolution was calculated from the number of peak channels and the half width of the Gaussian curve.

  As can be seen from the results of Examples 18 to 22, when the Cd-doped ZnO film was grown using the LPE method close to thermal equilibrium growth, the (002) plane rocking curve half width was 42 to 69 arcsec. Since the half width of the ZnO films according to Examples 18 to 22 is not significantly different from the half width (20 arcsec) of the hydrothermal synthesis substrate used as the substrate, the ZnO films according to Examples 18 to 22 have high crystallinity. It became clear to have.

  Table 8 shows the BGO specific emission amount and energy resolution for each CdO charged composition. As can be seen from the table, the Cd-doped ZnO single crystal grown with a CdO charge composition of 7 mol% to 50 mol% showed an emission amount exceeding BGO, and was found to have the same energy resolution as BGO.

DESCRIPTION OF SYMBOLS 1 ... Upper stage heater 2 ... Center part heater 3 ... Lower stage heater 4 ... Platinum crucible 5 ... Pulling up shaft 6 ... Substrate holder 7 ... Substrate 9 ... Crucible base 11 ... core tube 12 ... furnace lid

Claims (9)

  In a method for producing a ZnO scintillator by liquid phase epitaxial growth of a ZnO single crystal by bringing a substrate into direct contact with a mixture / melt of ZnO and CuO as a solute and a solvent, 0.01 mol% of CuO with respect to ZnO A method for producing a ZnO scintillator, characterized by mixing ~ 1.0 mol%. The solvent is PbF ₂ and PbO, and the mixing ratio of the solute and the solvent is solute in terms of only ZnO: solvent = 2 to 20 mol%: 98 to 80 mol%, and the mixture of the solvent PbF ₂ and PbO The method for producing a ZnO scintillator according to claim 1, wherein the ratio is PbF ₂ : PbO = 20 to 80 mol%: 80 to 20 mol%. The solvent is PbO and Bi ₂ O ₃ , and the mixing ratio of the solute and the solvent is solute in terms of only ZnO: solvent = 5-30 mol%: 95-70 mol%, and the solvents PbO and Bi ₂ O ₃ and the mixing ratio of _{PbO: Bi 2 O 3 = 0.1~95mol} %: a 99.9~5mol%, the production method of the ZnO scintillator according to claim 1.   In a method for producing a ZnO scintillator by liquid phase epitaxial growth of a ZnO single crystal by bringing a substrate into direct contact with a mixture / melt of ZnO and CdO as a solute and a solvent, 7 to 50 mol of CdO with respect to ZnO A method for producing a ZnO scintillator, characterized by comprising mixing at%. The solvent is PbF ₂ and PbO, and the mixing ratio of the solute and the solvent is solute in terms of only ZnO: solvent = 2 to 20 mol%: 98 to 80 mol%, and the mixture of the solvent PbF ₂ and PbO ratio _PbF 2: PbO = _20~80mol%: a 80~20mol%, the production method of the ZnO scintillator according to claim 4. The solvent is PbO and Bi ₂ O ₃ , and the mixing ratio of the solute and the solvent is solute in terms of only ZnO: solvent = 5-30 mol%: 95-70 mol%, and the solvents PbO and Bi ₂ O ₃ and the mixing ratio of _{PbO: Bi 2 O 3 = 0.1~95mol} %: a 99.9~5mol%, the production method of the ZnO scintillator according to claim 4.   A radiation detector comprising a ZnO scintillator manufactured by the manufacturing method according to claim 1. A radiation inspection apparatus comprising the radiation detector according to claim 7.   An α-ray camera comprising a ZnO scintillator manufactured by the manufacturing method according to claim 1.