JP2012121781A - Fluoride single crystal, vacuum ultraviolet light emitting element, and scintillator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fluoride single crystal which has an effective atomic number and a density equal to a BaLuFsingle crystal, and can be efficiently produced in accordance with a melt growth method without incurring a phase transformation from anorthorhombic crystal structure to a monoclinic crystal structure.SOLUTION: A fluoride single crystal is provided which is expressed by the chemical formula Ba(MYLu)F(wherein M is at least one kind of element selected from among Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb; x is in a range of 0 to 0.5, y is in a range of 0 to 0.8, excluding the case where both x and y are 0) and has a monoclinic crystal structure. The average ion radius of elements M, Y and Lu is from 98.5 to 102.5 pm.

Description

  The present invention relates to a novel fluoride single crystal, and a vacuum ultraviolet light emitting device and a scintillator comprising the fluoride single crystal. The vacuum ultraviolet light-emitting device can be suitably used as a vacuum ultraviolet light-emitting device used for photolithography, semiconductor or liquid crystal substrate cleaning, sterilization, next-generation large-capacity optical disk, medical treatment (ophthalmic treatment, DNA cutting), and the like. The scintillator of the present invention can be suitably used as a scintillator for radiation detectors used for cancer diagnosis by PET or X-ray CT.

  High-intensity ultraviolet light-emitting devices are materials that support advanced technologies in the semiconductor field, information field, medical field, etc. In recent years, in order to respond to many demands such as an increase in recording density on recording media, a shorter wavelength is required. Development of ultraviolet light-emitting elements that emit light at room temperature is underway. As an ultraviolet light emitting element that emits light at a short wavelength, an LED having an emission wavelength of about 360 nm using an ultraviolet light emitting material such as GaN is commercially available.

  Although a vacuum ultraviolet light emitting material having a shorter wavelength of 200 nm or less can be suitably used for photolithography, semiconductor or liquid crystal substrate cleaning, sterilization, etc. as a vacuum ultraviolet light emitting element, development is desired. It is not easy to obtain a vacuum ultraviolet light emitting element, and only a few examples are known at present.

  A factor that makes it difficult to develop a vacuum ultraviolet light emitting material is that vacuum ultraviolet rays are absorbed by many substances, so that substances that do not cause self-absorption are limited.

  Furthermore, the emission characteristics in the vacuum ultraviolet region are easily affected by impurities in the material, and even a material having an emission energy level in the vacuum ultraviolet region has a long wavelength based on a lower energy level. In many cases, the desired vacuum ultraviolet light emission cannot be obtained due to reasons such as light emission being dominant or loss due to non-radiative transition being significant.

  Therefore, it is extremely difficult to predict the light emission characteristics in the vacuum ultraviolet region in advance, and this is a big barrier in the development of vacuum ultraviolet light emitting elements.

  On the other hand, a single crystal that emits light when irradiated with radiation can be used as a scintillator.

  A radiation detector used for cancer diagnosis by PET or X-ray CT is composed of a material called a scintillator that emits light when irradiated with a weak light detector such as a photomultiplier tube or a semiconductor light receiving element. .

As the scintillator, high detection sensitivity for effective atomic number and what density high γ-rays and X-rays, is suitable as a scintillator satisfying such requirements, Balu ₂ F ₈ disclosed in Non-Patent Document 1 Can be mentioned.

J. et al. C. van't Spijker et al. , “Luminescence and scintillation properties of BaY2F8: Ce3 +, BaLu2F8 and BaLu2F8: Ce3 +” Journal of Luminescence 85 (1999) 11-19.

As a method for efficiently producing a single crystal, there is a melt growth method. In the study by the present inventors, when the BaLu ₂ F ₈ is produced by a conventionally known melt growth method, BaLu _{2 is used.} F ₈ crystallizes from the melt and then undergoes a phase transformation from the orthorhombic crystal structure to the monoclinic crystal structure in the process of cooling to room temperature, thus causing a problem of cracking in the crystal.

  In the production of a crystal that causes such a phase transformation, a method of crystallizing at a temperature at which the phase transformation occurs, that is, a phase transition point or lower is effective. For example, a solution growth method that crystallizes from a raw material solution below a phase transition point or the like. It was general. However, such a solution growth method generally takes enormous time to produce crystals, and thus it has been difficult to produce crystals efficiently.

The present invention has been made in view of the above problems, and has an effective atomic number and density equivalent to those of a BaLu ₂ F ₈ single crystal, and from an orthorhombic crystal structure to a monoclinic crystal structure. An object of the present invention is to provide a fluoride single crystal that can be efficiently produced by a melt growth method without causing phase transformation.

When manufacturing BaLu ₂ F ₈ type single crystal by the melt growth method, the present inventors do not cause a phase transformation from an orthorhombic crystal structure to a monoclinic crystal structure, and are free from cracks. Various methods for obtaining a BaLu ₂ F ₈ type single crystal having a crystal type crystal structure were studied. As a result, a part of Lu is at least one element selected from Y and / or Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb (hereinafter referred to as M). ) And adjusting the average ionic radii of the Lu, Y, and M to a predetermined range, the phase transformation can be avoided, and thus a crack-free BaLu ₂ F ₈ type single crystal is removed from the melt. I found that I could grow directly.

Further, the inventors have found that such BaLu ₂ F ₈ type single crystal can be suitably used as a vacuum ultraviolet light emitting device and a scintillator, and have completed the present invention.

That is, the present invention has the formula _{Ba (M x Y y Lu 1} -x-y) 2 F 8 ( although, M is Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb represents at least one element selected from Yb, x is in the range of 0 to 0.5, y is in the range of 0 to 0.8, and x and y are both 0. A fluoride single crystal having a monoclinic crystal structure and having an average ionic radius of the elements M, Y and Lu of 98.5 to 102.5 pm, and The vacuum ultraviolet light emitting element and scintillator made of the single crystal.

  According to the fluoride single crystal of the present invention, high-luminance light emission in the vacuum ultraviolet region can be obtained, and photolithography, semiconductor and liquid crystal substrate cleaning, sterilization, next-generation large-capacity optical discs, and medical treatment (ophthalmic treatment, DNA For example, it can be suitably used for cutting). Moreover, since the fluoride single crystal of the present invention has a good response to radiation, it can be suitably used as a scintillator.

This figure is a schematic view of an apparatus for producing a crystal by the micro pull-down method. This figure is a schematic diagram of an X-ray excitation emission spectrum measuring apparatus. This figure is an X-ray excitation emission spectrum of Example 1. This figure is the X-ray excitation emission spectrum of Examples 2-4. This figure is an X-ray excitation emission spectrum of Examples 2, 5 and 6.

Hereinafter, the fluoride single crystal of the present invention will be described in detail. Fluoride single crystal of the present invention has the formula _{Ba (M x Y y Lu 1} -x-y) 2 F 8 ( although, M is Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er Represents at least one element selected from Tm and Yb, x is in the range of 0 to 0.5, y is in the range of 0 to 0.8, and x and y are both 0. ) And having a monoclinic crystal structure, the average ionic radius of M, Y and Lu is 98.5 to 102.5 pm.

The fluoride single crystal of the present invention has a monoclinic system, BaLu ₂ F ₈ belonging to the space group C2 / m as a basic structure, and a part of Lu of the BaLu ₂ F ₈ is substituted with M and Y. . In the chemical formulas, x and y represent substitution ratios when Lu is substituted with M and Y, respectively, and when x or y is 0, Lu is not substituted with M or Y.

Such a fluoride single crystal having a basic structure of BaLu ₂ F ₈ has a large band gap and does not absorb vacuum ultraviolet rays, and therefore can be suitably used as a vacuum ultraviolet light emitting device. In the present invention, vacuum ultraviolet means a wavelength region of 200 nm or less.

Further, the density and effective atomic number of the fluoride single crystal of the present invention vary depending on the kind of M and the values of x and y, but are large as about 5.4 to 7.0 g / cm ³ and about 51 to 62, respectively. . Therefore, the detection sensitivity to γ rays and X rays is high, and it can be suitably used as a scintillator.

  In the present invention, the effective atomic number is an index defined by the following formula.

Effective atomic number = (ΣW _i Z _i ⁴ ) ^1/4
(Wherein W _i and Z _i are the mass fraction and atomic number of the i-th element among the elements constituting the fluoride single crystal, respectively)

  The fluoride single crystal of the present invention exhibits core-valence emission (hereinafter also referred to as CVL) based on electronic transition between the valence band and the inner core level of barium, and the emission wavelength of the CVL is 180 to 210 nm. Therefore, it can be suitably used as a vacuum ultraviolet light emitting device. In order to increase the emission intensity of CVL, it is preferable to reduce x, and most preferably 0.

  In the fluoride single crystal of the present invention, when x is larger than 0, that is, a part of Lu is selected from M (Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb. In addition to the CVL, light emission based on the M can be obtained, so that it can be particularly preferably used as a scintillator.

  In the present invention, the upper limit of x is 0.5. When x exceeds 0.5, a problem arises in that the crystal becomes cloudy due to the precipitation of different crystal phases. In addition, when using multiple types of M, let x be the total sum of the ratios of substitution when Lu is replaced by each M.

  In order to increase the light emission intensity of light emission based on M, x is preferably 0.0005 or more, although it varies depending on the type of M. On the other hand, in order to avoid a decrease in emission intensity due to concentration quenching, x is preferably 0.1 or less.

  Since the emission wavelength of the scintillator formed by substituting part of Lu with M varies from the vacuum ultraviolet to the infrared region depending on the type of M, a photomultiplier tube or a semiconductor used for constituting a radiation detector Elements can be appropriately selected and used in accordance with the wavelength sensitivity characteristics of a weak light detector such as a light receiving element.

  In addition, when M is at least one element selected from Nd, Er, and Tm, light is emitted in the vacuum ultraviolet region by the action of M in addition to the CVL. Therefore, the emission intensity in the vacuum ultraviolet region is high. Therefore, it can be used particularly suitably as a vacuum ultraviolet light emitting device.

  In addition to the characteristics of density and effective atomic number, considering the point of emission in the vacuum ultraviolet region, for example, a scintillator and a gas counter such as a radiation detector described in Japanese Patent Application Laid-Open No. 2008-202977 It can apply especially suitably in the radiation detector which combined.

  Further, the scintillator in which M is at least one element selected from Ce, Pr and Nd has a short fluorescence lifetime, and therefore can be suitably used in a radiation detector that requires time resolution and high count rate characteristics.

Fluoride single crystal of the present invention, M, average ionic radius of Y and Lu _(hereinafter referred to as _{r avg)} is the largest, characterized in that the 98.5~102.5pm a. In the present invention, the r _avg is calculated by the following equation.

r _avg = xr _M + yr _Y + (1−xy) r _Lu
(Where r _M , r _Y and r _Lu are the ionic radii of M, Y and Lu, respectively)

When a plurality of types of M are used, the element fraction and the ionic radius of each M are set to x ₁ , x ₂ ,..., X _i and r _M1 , r _{M2 ,.} as a seek? x _i r _Mi, can be calculated _{r avg} by using instead the? x _i r _Mi to xr _M in formula.

In the calculation of the _{r avg,} ionic radii of the elements underlying the calculations, the previously reported values (R.D. Shannon, "Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides" Acta Crystallographica Section A 32 ( 1976) 751.) was adopted. Table 1 shows the ionic radius of each element.

According to the study by the present inventors, when the r _avg is less than 98.5 pm, a phase transformation from an orthorhombic crystal structure to a monoclinic crystal structure occurs, and is produced by a melt growth method. Cracks occur in the single crystal. On the other hand, when r _avg exceeds 102.5 pm, a polycrystal in which a plurality of types of crystal phases are mixed is obtained, and there is a problem that a transparent single crystal composed of a single crystal phase cannot be obtained. In order to produce a transparent single crystal composed of a single crystal phase without causing phase transformation, it is particularly preferable that r _avg is 99.0 to 102.0 pm.

  In the present invention, in order to adjust the average ionic radius to the above range, a part of Lu is replaced with Y. y represents the ratio of such substitution, and the upper limit is 0.8. When y exceeds 0.8, the density and effective atomic number of the resulting fluoride single crystal are lowered, so that the detection sensitivity to γ rays and X rays is lowered. In order to improve the detection sensitivity for γ rays and X rays, y is particularly preferably 0.6 or less.

When r _avg falls within the intended range by replacing part of Lu with M, it is not necessary to replace part of Lu with Y. Therefore, y may be 0.

  The fluoride single crystal of the present invention is a colorless or slightly colored transparent solid, has good chemical stability, and performance degradation in a short period is not observed in normal use. Furthermore, mechanical strength and workability are also good, and it is easy to process and use it in a desired shape.

  In the present invention, the production method of the fluoride single crystal is not particularly limited, but it can be produced by a conventionally known melt growth method represented by the Czochralski method or the micro pulling down method.

  The micro pulling-down method is a method for producing a crystal by drawing a raw material melt from a hole provided in the bottom of the crucible 5 using an apparatus as shown in FIG.

  Hereinafter, a general method for producing the fluoride single crystal of the present invention by the micro pull-down method will be described.

  First, a predetermined amount of raw material is filled into a crucible 5 having a hole at the bottom. Although the shape of the hole provided in the crucible bottom is not particularly limited, it is preferably a cylindrical shape having a diameter of 0.5 to 4 mm and a length of 0 to 2 mm.

  In the present invention, the raw material is not particularly limited, but it is preferable to use a mixed raw material in which fluoride powders each having a purity of 99.99% or more are mixed. By using such a mixed raw material, the purity of the crystal can be increased, and characteristics such as emission intensity are improved. The mixed raw material may be used after being sintered or melted and solidified after mixing.

Next, the crucible 5 filled with the raw materials, the after heater 1, the heater 2, the heat insulating material 3, and the stage 4 are set as shown in FIG. After evacuating the inside of the chamber 6 to 1.0 × 10 ⁻³ Pa or less using a vacuum evacuation apparatus, an inert gas such as high-purity argon is introduced into the chamber 6 to perform gas replacement. The pressure in the chamber after gas replacement is not particularly limited, but atmospheric pressure is common.

  By the gas replacement operation, moisture attached to the raw material or the chamber can be removed, and deterioration of crystals derived from the moisture can be prevented. In order to avoid the influence of moisture that cannot be removed by the gas replacement operation, it is preferable to use a solid scavenger such as zinc fluoride or a gas scavenger such as tetrafluoromethane. When using a solid scavenger, a method of mixing in the raw material in advance is preferable, and when using a gas scavenger, a method of mixing with the above inert gas and introducing it into the chamber is preferable.

  After performing the gas replacement operation, the raw material is heated and melted by the high-frequency coil 7, and the melted raw material melt is drawn out from the hole at the bottom of the crucible to start manufacturing the crystal.

  Here, the metal wire is provided at the tip of the pull-down rod, the metal wire is inserted into the crucible through the hole at the bottom of the crucible, the raw material melt is attached to the metal wire, and then the raw material melt is pulled down together with the metal wire. Makes it possible to produce crystals. The material of the metal wire can be used without limitation as long as it is a material that does not substantially react with the raw material melt, but a material excellent in corrosion resistance at high temperatures such as a W-Re alloy is preferable.

  After pulling out the raw material melt with the metal wire, the crystal can be obtained by continuously pulling it down at a constant pulling rate.

  The pulling speed is not particularly limited, but if it is too fast, the crystallinity tends to be poor, and if it is too slow, the crystallinity is improved, but the time required for production becomes enormous. The range of hr is preferable.

  In the production of a fluoride single crystal, an annealing operation may be performed after the production of the crystal for the purpose of removing crystal defects caused by thermal strain.

  The fluoride single crystal of the present invention can be used after being processed into a desired shape. In processing, a known cutting machine such as a blade saw or wire saw, a grinding machine, or a polishing machine can be used without any limitation.

The vacuum ultraviolet light emitting device comprising the fluoride single crystal of the present invention can be made into a vacuum ultraviolet light generator by combining with an appropriate excitation source such as an electron beam or an F ₂ laser. Such a vacuum ultraviolet light generator is suitably used in fields such as photolithography, sterilization, next-generation large-capacity optical disks, and medicine (ophthalmic treatment, DNA cutting).

  Moreover, the scintillator comprising the fluoride single crystal of the present invention can be suitably used as a radiation detector in combination with a weak light detector such as a photomultiplier tube or a semiconductor light receiving element.

  The scintillator comprising the fluoride single crystal of the present invention is not limited to radiation to be detected, and can be used for detection of radiation such as X-rays, α-rays, β-rays, or γ-rays. Since each density is large, the maximum effect is exhibited in the detection of high energy photons such as hard X-rays or γ-rays.

  Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not limited to these examples.

Examples 1-6, Comparative Examples 1-3
(Crystal production)
Fluoride crystals of Examples 1 to 6 and Comparative Examples 1 to 3 were manufactured using the crystal manufacturing apparatus shown in FIG.

As raw materials, BaF ₂ , NdF ₃ , ErF ₃ , TmF ₃ , YF ₃ and LuF ₃ having a purity of 99.99% were used. The after heater 1, the heater 2, the heat insulating material 3, the stage 4, and the crucible 5 are made of high-purity carbon, and the shape of the hole provided at the bottom of the crucible is a circle having a diameter of 2.2 mm and a length of 0.5 mm. It was columnar.

  First, each raw material was weighed as shown in Table 2, mixed well to prepare a mixed raw material, and the mixed raw material was filled in the crucible 5. Table 2 also shows the type of M used, the values of x and y, and the average ionic radius, effective atomic number and density under such conditions.

The crucible 5 filled with the raw material was set on the upper part of the after heater 1, and the heater 2 and the heat insulating material 3 were sequentially set around it. Next, the inside of the chamber 6 is evacuated to 9.0 × 10 ⁻⁴ Pa using an evacuation apparatus including an oil rotary pump and an oil diffusion pump, and then a 90% argon-tetrafluoromethane 10% mixed gas is added to the chamber. The gas was replaced by introducing the gas into 6.

  A high frequency current is applied to the high frequency coil 7, the raw material is heated to about 1000 degrees by induction heating and melted, and a W-Re wire provided at the tip of the pulling rod 8 is inserted into the hole at the bottom of the crucible 5, The raw material melt was pulled down from the hole, and crystallization was started. While adjusting the output of the high frequency, it was continuously pulled down at a speed of 3 mm / hr to obtain a fluoride crystal. The crystal had a diameter of 2.2 mm and a length of 40 mm. The fluoride single crystals of the present invention produced in Examples 1 to 6 were good quality single crystals without white turbidity and cracks, whereas cracks occurred in the crystals of Comparative Examples 1 to 3.

(Identification of crystal phase)
The crystal obtained by the production of the crystal was identified by the following method.

Part of the obtained crystal was pulverized into powder, and powder X-ray diffraction measurement was performed. As a measuring device, D8 DISCOVER manufactured by Bruker AXS was used. From the result of analyzing the diffraction pattern by the powder X-ray diffraction method, the fluoride crystals of Examples 1 to 6 and Comparative Examples 1 to 3 are monoclinic, BaLu ₂ F ₈ type crystal structure belonging to the space group C2 / m. It was found to have

(Evaluation of luminous characteristics)
The obtained crystals of Examples 1 to 6 were cut into a length of about 10 mm with a wire saw, the side surfaces were ground and processed into a shape having a length of 10 mm, a width of about 2 mm, and a thickness of 1 mm. Both surfaces of a surface having a width of 10 mm and a width of about 2 mm were mirror-polished to obtain a vacuum ultraviolet light emitting device of the present invention.

  About the vacuum ultraviolet light emitting element of this invention, the vacuum ultraviolet light emission characteristic under X-ray excitation was evaluated using the apparatus shown in FIG. The vacuum ultraviolet light emitting element 9 of the present invention was set at a predetermined position in the apparatus, and the entire interior of the apparatus was replaced with nitrogen gas. The X-ray tube 10 (Rigaku tungsten target, tube voltage 60 kV, tube current 35 mA) was irradiated to the vacuum ultraviolet light emitting element 9. The light emitted from the vacuum ultraviolet light emitting element 9 is dispersed with an emission spectrometer 11 (manufactured by Spectrometer, KV201 type extreme ultraviolet spectrometer), the emission intensity at each wavelength is recorded with a CCD detector 12, and an X-ray excitation emission spectrum is obtained. Obtained.

  The obtained X-ray excitation emission spectrum is shown in FIGS. FIG. 3 shows that the fluoride single crystal of Example 1 emits light in the vacuum ultraviolet region by CVL, and is useful as a vacuum ultraviolet light emitting device. Moreover, from FIG. 4, the fluoride single crystals of Example 2, Example 3, and Example 4 to which Nd, Er, and Tm are added as M, respectively, exhibit vacuum ultraviolet emission derived from M in addition to the CVL. Thus, it can be seen that it is particularly useful as a vacuum ultraviolet light emitting device. FIG. 5 is a diagram showing comparison when Nd is used as M and y is changed. From FIG. 5, it is understood that light emission with higher luminance can be obtained as the value of y is smaller.

Moreover, since all the fluoride single crystals of Examples 1 to 6 of the present invention emit light upon incidence of X-rays, it is understood that they are useful as scintillators.

DESCRIPTION OF SYMBOLS 1 After heater 2 Heater 3 Heat insulating material 4 Stage 5 Crucible 6 Chamber 7 High frequency coil 8 Pulling rod 9 Fluoride single crystal 10 X-ray tube 11 Emission spectrometer 12 CCD detector

Claims (4)

Formula _{Ba (M x Y y Lu 1} -x-y) 2 F 8 ( provided that at least 1 M is the Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, selected from Tm and Yb And x is in the range of 0 to 0.5, y is in the range of 0 to 0.8, and x and y are both 0). A fluoride single crystal having a type crystal structure, and having an average ionic radius of the elements M, Y and Lu of 98.5 to 102.5 pm. The fluoride single crystal according to claim 1, wherein M is at least one element selected from Nd, Er and Tm. A vacuum ultraviolet light emitting device comprising the fluoride single crystal according to claim 2. A scintillator comprising the fluoride single crystal according to claim 1.

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