JP2013001577A - Metal fluoride crystal, light emitting element, and scintillator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a scintillator which exhibits light emission with high luminance on radiation irradiation, and to provide a metal fluoride crystal suitable for the scintillator.SOLUTION: With respect to a metal fluoride crystal having a composition of LiCaMgGaF(0.1≤x≤0.9) and containing a lanthanoid as an activating agent, situations to exhibit light emission with high luminance on radiation irradiation are found out. The metal fluoride crystal is suitably used in an application as a light emitting element, and in particular suitable for a scintillator including the application for neutron detection.

Description

  The present invention relates to a novel metal fluoride crystal and a light emitting device comprising the crystal. Specifically, the present invention relates to a metal fluoride crystal useful as a scintillator.

Metal fluoride crystals are metal compound crystals that are expected to be used in various ways. For example, single crystals such as calcium fluoride and magnesium fluoride have high transmittance, low refractive index, and low dispersion over a wide wavelength band from vacuum ultraviolet to infrared, and excellent chemical stability. Therefore, it is used for window materials, lenses, prisms and the like as optical materials in a wide range. In particular, a window of an apparatus such as a stepper (reduction projection type exposure apparatus) using an ArF laser (193 nm) or an F ₂ laser (157 nm) light source, which is being developed as a next-generation short wavelength light source in photolithography technology. Expectation is expected as a material, a light source lens, an illumination lens, and a projection lens.

  A single crystal of lithium calcium gallium fluoride, which is a metal fluoride crystal, has been studied for use in lasers and has been shown to be useful (Non-Patent Document 1).

  Metal fluoride crystals are also being developed as scintillator crystals (Patent Documents 1 and 2). A scintillator refers to a substance that absorbs radiation and emits fluorescence when irradiated. One of the characteristics required for the scintillator is high luminance of light emission. This is because it affects the various performances of the radiation detector using the scintillator, for example, the detection efficiency for radiation. The detection efficiency is the ratio of the number of radiation counted by the detector to the number of radiation emitted from the radiation source and incident on the detector. Metal fluoride crystals are being developed as scintillators for detecting neutrons, among other scintillators. This is because fluoride has a small atomic number and has little interaction with radiation that can be emitted with neutrons such as X-rays and γ-rays.

JP 2010-280543 A JP 2010-181373 A

Jenssen, HansP; Cassanho, Arlete “Fluoride laser crystals old and new” Proc. of SPIE Vol. 6100, 61000 W (2006).

  Therefore, the inventors tried to apply lithium calcium gallium fluoride, which is known to be useful as a laser crystal as described above, as a metal fluoride for scintillator applications. However, according to the study by the present inventors, the lithium calcium gallium fluoride crystal containing Ce as an activator was irradiated with radiation, and the characteristics as a scintillator were examined. Was not obtained, and it became clear that the scintillator did not have sufficient performance.

  Accordingly, an object of the present invention is to provide a metal fluoride crystal having lithium calcium gallium fluoride as a basic structure and capable of obtaining sufficient light emission as a scintillator.

The present inventors have intensively studied to solve the above problems. Among them, it is thought that the ionic radius of the calcium site, which is a constituent element of lithium calcium calcium gallium fluoride, has a great influence on the luminance of light emission during irradiation, and attempts to replace calcium with magnesium. It was.
As a result, it has been found that the use of a mother crystal in which calcium of lithium calcium gallium fluoride is replaced with magnesium improves the light emission characteristics during irradiation, and the present invention has been completed.

That is, the present invention is a metal fluoride crystal having a composition of LiCa _(1-x) Mg _x GaF ₆ (0.1 ≦ x ≦ 0.9) and containing a lanthanoid element. Furthermore, a light-emitting element and a scintillator made of the metal fluoride crystal.

In the metal fluoride crystal, the ⁶ Li isotope ratio is preferably 50% or more, and the metal fluoride crystal is particularly preferably used as a scintillator for neutron detection.

  According to the present invention, a metal fluoride crystal that does not have sufficient performance as a scintillator and has excellent light emitting characteristics and can be suitably used for various light emitting devices despite being a crystal having lithium calcium gallium fluoride as a basic structure. Can provide. Among them, it is possible to provide a neutron detection scintillator with high detection efficiency for radiation, particularly neutrons.

This figure is a schematic diagram of the unidirectional solidification method. This figure is a schematic diagram of an emission spectrum measuring apparatus at the time of α-ray irradiation. This figure is an α-ray excited emission spectrum of Example 1 and Comparative Example 1 during radiation irradiation. This figure is an α-ray excited emission spectrum of Example 2 and Comparative Example 1 at the time of radiation irradiation. This figure is an α-ray excited emission spectrum of Example 3 and Comparative Example 1 at the time of radiation irradiation.

  The metal fluoride crystal of the present invention has a composition in which lithium calcium gallium fluoride crystal, which is a metal fluoride containing lithium, calcium, and gallium, is a basic constituent, and a part of calcium is substituted with magnesium. It is a crystal containing a lanthanoid as an activator in the body.

The metal fluoride crystal of the present invention is a metal fluoride crystal whose mother crystal has a composition of the chemical formula LiCa _(1-x) Mg _x GaF ₆ (0.1 ≦ x ≦ 0.9), and is a colorless transparent crystal And belongs to a hexagonal crystal. X in the chemical formula is a ratio of magnesium when the total number of calcium and magnesium is 1, and the ratio X is 0.1 or more and 0.9 or less. The crystals have good chemical stability, and in normal use, no performance deterioration is observed in a short period of time. Mechanical strength and workability are also good, and it can be used after being processed into a desired shape.

  When X is less than 0.1, only a slight effect of increasing the luminance of light emission in the present invention is obtained. On the other hand, when X is larger than 0.9, the content of the activator in the obtained metal fluoride is reduced to 0.001 mol% or less, and the luminance of light emission is lowered. X is preferably 0.7 or less. When the ratio of magnesium is 0.7 or less, the obtained metal fluoride is single-crystallized and the transmittance is improved, so that the luminance of the obtained light emission is improved. More preferably, 0.4 ≦ X ≦ 0.6. When the ratio of magnesium is within this range, crystallization can be easily performed, and the effect of increasing the luminance of light emission is great.

  The metal fluoride crystal of the present invention contains a lanthanoid element. The lanthanoid element contained in the metal fluoride crystal of the present invention functions as an activator, and the activator exhibits light emission to exhibit characteristics as a scintillator. Examples of lanthanoids used as activators include lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium, and these lanthanoid elements are used without limitation. can do. Moreover, what is necessary is just to contain the at least 1 sort (s) of element chosen from this lanthanoid element.

  When cerium is used as the activator, it is preferable in that a scintillator having a high counting rate can be obtained because decay of fluorescence is fast. The counting rate is the number of radiation counted per unit time. Moreover, when europium is used as the activator, it is preferable in that a scintillator with high luminance and high detection efficiency can be obtained.

In the metal fluoride crystal of the present invention, the higher the content of the lanthanoid element as an activator for LiCa _(1-x) Mg _x GaF ₆ (0.1 ≦ x ≦ 0.9), the higher the luminance. Obtainable. However, when the content is too high, the transparency of the crystal decreases due to precipitation of excess lanthanoid elements, and the crystal itself absorbs light emitted by the lanthanoid elements. it is preferable _{LiCa (1-x) Mg x} GaF 6 (0.1 ≦ x ≦ 0.9) , based on a 0.001 mole%. When the content is 0.001 mol% or more, high-luminance light emission can be obtained, and when the content is 20 mol% or less, the lanthanoid element-doped LiCa _{(1-x )} Mg _x GaF ₆ (0.1 ≦ x ≦ 0.9) can be obtained.

In the metal fluoride crystal of the present invention, the ⁶ Li isotope ratio in the Li element contained in the crystal is preferably 50% or more. ^By setting the ⁶ Li isotope ratio to 50% or more, the probability of the neutron capture reaction in which the ⁶ Li isotope captures neutrons to generate α particles and tritium is increased, and the detection efficiency for neutrons is improved. The ⁶ Li isotope ratio is preferably 50% or more, and most preferably 90% or more. On the other hand, in view of the costs associated with isotope enrichment, the ⁶ Li isotope ratio is preferably 99% or less.

The metal fluoride of the present invention is single crystal or polycrystal, and the single crystal has higher light transmittance,
Even a large-sized solid sample is suitable because it can be easily taken out without attenuating light emission from the inside.

  The metal fluoride crystals of the present invention can be used for various optical elements such as laser crystals and scintillators. In particular, it can be suitably used for scintillators, more preferably neutron detection scintillators.

  The method for producing the metal fluoride crystal of the present invention is not particularly limited. The Bridgman method in which the crucible is lowered and the melt is solidified in one direction from below to below, the crystal is formed while maintaining the solid-liquid interface at a fixed position. Examples include the Czochralski method of pulling up while solidifying in one direction, or the micro-pulling down method of pulling down while crystallizing in one direction while keeping the solid-liquid interface at a fixed position.

  Hereinafter, the manufacturing method will be described in detail with reference to 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.

  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 materials preferably each have a purity of 99.99% or more. By using such a raw material, the purity of the crystal can be increased and the luminance of light emission can be improved. As the raw material, a mixed raw material obtained by mixing each raw material in advance may be filled in a crucible. However, a raw material that is once sintered or melted and solidified after mixing may be used as the raw material. Since lithium fluoride and gallium fluoride used as raw materials are highly volatile elements, it is better to charge in excess of about 0.1 to 10% from the theoretical composition in advance.

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 even by 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 an 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 crystal growth.

  When producing a metal fluoride crystal by the micro pull-down method, the wettability of the raw material melt with respect to the crucible is poor, and the melt may not easily exude from the hole at the bottom of the crucible. In such a case, a 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, and the raw material melt is attached to the metal wire, and then the raw material melt is applied to the metal wire. The crystal can be grown by pulling it down together. For example, the high-frequency output is adjusted, and the metal wire is inserted into the hole at the bottom of the crucible while the temperature of the raw material is gradually raised from the melting point of the metal fluoride, and is drawn out. This operation is repeated until the raw material melt is drawn together with the metal wire, and crystal growth is started. 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. However, 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, a crystal can be obtained by continuously pulling it down at a constant pulling rate. The pulling speed is not particularly limited, but is preferably in the range of 0.5 to 10 mm / hr.

  In the production of the metal fluoride crystal of the present invention, an annealing operation may be performed after the production of the crystal for the purpose of removing crystal defects due to the thermal strain.

  The obtained metal fluoride crystal has good workability and can be easily processed into a desired shape. For 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.

  Metal fluoride crystals can be processed into various light-emitting elements by processing into a desired shape, and can be suitably used for scintillators including those for photolithography, laser, and neutron detection.

  When the scintillator for neutron detection is used, the shape may be any shape, and it may be an array in which a plurality of metal fluoride crystals having a plate shape, a block shape, or a square column shape are arranged. The neutron scintillator can be combined with a photodetector such as a photomultiplier tube to form a neutron detector. That is, by converting scintillation light emitted from a neutron detection scintillator by neutron irradiation into an electrical signal by a photomultiplier tube, the presence and intensity of neutrons can be grasped as an electrical signal. In addition, the structure and manufacturing method of a neutron detector are not restrict | limited at all, A conventionally well-known structure and method are employable.

  Specifically, for example, the neutron detection scintillator of the present invention is bonded to the photocathode of the photomultiplier tube with optical grease or the like, a high voltage is applied to the photomultiplier tube, and the photomultiplier tube outputs the photomultiplier tube. A method of measuring an electrical signal is available. For the purpose of analyzing the intensity of the neutron beam and the like using the electrical signal output from the photomultiplier tube, an amplifier, a multi-wave height analyzer, etc. may be provided after the photomultiplier tube.

  The neutron detection scintillator comprising the metal fluoride crystal of the present invention can be combined with a position sensitive photodetector to form a neutron imaging device. As such a position-sensitive photodetector, a position-sensitive photomultiplier tube can be suitably used. Specific examples include XP85012 manufactured by PHOTONIS.

  Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not limited to these examples. In addition, not all combinations of features described in the embodiments are essential to the solution means of the present invention.

Example 1
Examples of the present invention will be specifically described below, but the present invention is not limited to these examples. In addition, not all combinations of features described in the embodiments are essential to the solution means of the present invention.

A crystal of LiCa _0.5 Mg _0.5 GaF ₆ metal fluoride crystal containing cerium as an activator was manufactured using the crystal manufacturing apparatus shown in FIG. As raw materials, lithium fluoride having a purity of 99.99% ( ⁶ Li isotope ratio: 95%), calcium fluoride, magnesium fluoride, gallium fluoride, and cerium fluoride 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 in the bottom of the crucible is a circle having a diameter of 2.0 mm and a length of 0.5 mm. It was columnar.

For the purpose of preparing LiCa _0.5 Mg _0.5 GaF ₆ metal fluoride crystal containing cerium as an activator, lithium fluoride 0.38 g, calcium fluoride 0.59 g, magnesium fluoride 0.47 g Then, 2.02 g of gallium fluoride and 60 mg of cerium fluoride were weighed and mixed well, and then filled in the crucible 5. 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 the crucible.
Subsequently, the inside of the chamber 6 was evacuated to 5.0 × 10 ⁻⁴ Pa using an evacuation apparatus including an oil rotary pump and an oil diffusion pump. At the same time, heating was performed using the high-frequency coil 7 so that the temperature inside the crucible during evacuation was 570K.

  Argon 95 vol% -tetrafluoromethane 5 vol% mixed gas is introduced into the chamber 6 and the high frequency coil 7 is used to measure the temperature at the bottom of the crucible and adjust the output of the high frequency heating coil so that the heating temperature becomes 790K. did. The pressure in the chamber 6 after the mixed gas replacement was atmospheric pressure, and heating was continued for 30 minutes in this state. Next, evacuation was performed while continuing heating by the high-frequency heating coil, and argon gas was introduced into the chamber 6 to perform gas replacement. The pressure in the chamber 6 after argon gas replacement was atmospheric pressure. The same operation was performed twice.

  The raw material was heated and melted using the high frequency heating coil 7. Next, while adjusting the high frequency output and gradually raising the temperature of the raw material melt, a metal wire made of a W-Re alloy provided at the tip of the pulling rod 8 is inserted into the hole at the bottom of the crucible 5 and pulled down. The raw material melt was drawn out from the hole. The high frequency output was fixed so that the temperature at this time was maintained, the raw material melt was pulled down, and crystal growth was started. The metal fluoride single crystal containing 0.010 mol% of cerium having a diameter of 2.1 mm and a length of 20 mm was finally obtained by continuously pulling down at a rate of 1 mm / hr for 20 hours. The cerium content was determined by absorptiometry.

The obtained crystal is cut into a length of about 15 mm by a blade saw equipped with a diamond cutting grindstone, and the side surface is ground and processed into a shape having a length of 10 mm, a width of 2 mm, and a thickness of 1 mm. A sample was used. The obtained sample was optically polished using a 1 μm diamond slurry. The obtained sample was placed at the position 9 in FIG. 2, ²⁴¹ Am was placed at the position 10 in FIG. 2, and the surface of the sample having a width of 2 mm was irradiated with α rays at ²⁴¹ Am. Luminescence obtained from the sample was measured with a CCD camera at position 11 in FIG. 2, and a spectrum was recorded.

Example 2
For the purpose of preparing LiCa _0.75 Mg _0.25 GaF ₆ metal fluoride crystal containing cerium as an activator, lithium fluoride 0.37 g, calcium fluoride 0.88 g, magnesium fluoride as raw materials Crystal growth was performed in the same manner as in Example 1 except that 0.23 g, 1.99 g of gallium fluoride, and 59 mg of cerium fluoride were weighed, and finally cerium containing cerium having a diameter of 2.1 mm and a length of 20 mm was obtained. A metal fluoride single crystal containing 0.012 mol% was obtained. The cerium content was determined by absorptiometry. In the same manner as in Example 1, the emission spectrum at the time of α-ray irradiation was measured. The results are shown in FIG.

Example 3
For the purpose of preparing LiCa _0.25 Mg _0.75 GaF ₆ metal fluoride crystals containing cerium as an activator, lithium fluoride 0.39 g, calcium fluoride 0.30 g, magnesium fluoride as raw materials Crystal growth was performed in the same manner as in Example 1 except that 0.72 g, gallium fluoride 2.06 g, and cerium fluoride 61 mg were weighed, and finally cerium containing cerium having a diameter of 2.1 mm and a length of 20 mm was added. A metal fluoride single crystal containing 0.007 mol% was obtained. The cerium content was determined by absorptiometry. In the same manner as in Example 1, the emission spectrum at the time of α-ray irradiation was measured. The results are shown in FIG.

Comparative Example 1
For the purpose of preparing LiCaGaF ₆ metal fluoride crystals containing cerium as an activator, as raw materials, 0.37 g of lithium fluoride, 1.15 g of calcium fluoride, 1.96 g of gallium fluoride, 58 mg of cerium fluoride The crystal growth was carried out in the same manner as in Example 1 except for weighing each of the above, and finally a metal fluoride single crystal containing 0.015 mol% of cerium containing cerium having a diameter of 2.1 mm and a length of 20 mm was obtained. It was. The cerium content was determined by absorptiometry. In the same manner as in Example 1, the emission spectrum at the time of α-ray irradiation was measured. The results are shown in FIGS.
FIG. 3 shows that the amount of luminescence is increased by substituting a part of calcium with magnesium.

3 to 4, Examples 1 to 3 having a structure in which a part of calcium of lithium calcium gallium fluoride doped with cerium is replaced with magnesium are the same as in Comparative Example 1 in which a part of calcium is not replaced with magnesium. By comparison, it can be seen that the obtained metal fluoride crystal has high luminance and is useful as a scintillator. At the same time, these results indicate that LiCa _(1-x) Mg _x GaF ₆ , which is a novel metal fluoride crystal, is useful as a mother crystal of a scintillator used by containing a lanthanoid element such as cerium. Yes.

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 crystal 10 ²⁴¹ Am
11 CCD camera

Claims (5)

A metal fluoride crystal having a composition of LiCa _(1-x) Mg _x GaF ₆ (0.1 ≦ x ≦ 0.9) and containing a lanthanoid element. The metal fluoride crystal according to claim 1, wherein the ⁶ Li isotope ratio is 50% or more.   A light emitting device comprising the metal fluoride crystal according to claim 1. A scintillator comprising the metal fluoride crystal according to claim 1. A metal fluoride crystal having a composition of LiCa _(1-x) Mg _x GaF ₆ (0.1 ≦ x ≦ 0.9).

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