JPWO2012026585A1 - Fluoride crystal, scintillator for radiation detection and radiation detector - Google Patents

Abstract

PROBLEM TO BE SOLVED To provide a new scintillator suitable for combination with a radiation-resistant vacuum ultraviolet light receiving element and a radiation detector using the scintillator, which can be applied to discriminate between neutron rays and γ rays, Cross-Luminescence and rare earth electrons Provided is a fluoride crystal in which transition light emission coexists. SOLUTION: A fluoride crystal made of KLiLuF5 to which at least one kind of rare earth element is added, a scintillator for radiation detection made of the fluoride crystal, and a radiation detector using the scintillator. [Selection figure] None

Description

  The present invention relates to a novel fluoride crystal. The fluoride crystal can be suitably used as a scintillator for radiation detection used in cancer diagnosis by PET, X-ray CT, or neutron detector.

  A scintillator is a substance that absorbs and emits light when irradiated with radiation such as α-rays, β-rays, γ-rays, X-rays, and neutrons, and is combined with a photodetector such as a photomultiplier tube. It is used for radiation detection and has various application fields such as medical field such as tomography, industrial field such as non-destructive inspection, security field such as personal belongings inspection, and academic field such as high energy physics.

As this scintillator, there are various types of scintillators depending on the type of radiation and purpose of use, and inorganic crystals such as Bi ₄ Ge ₃ O ₁₂ and Gd ₂ SiO ₅ : Ce, organic crystals such as anthracene, and organic phosphors are used. There are polymer bodies such as polystyrene and polyvinyltoluene, liquid scintillators and gas scintillators.

  On the other hand, conventional photo detectors for detecting the light emitted from the scintillator have been highly sensitive to visible light, such as photomultiplier tubes and silicon photo detectors. Recently, however, diamond photo detectors and AlGaN photo detectors have been used. Optical detectors that are sensitive to vacuum ultraviolet light, such as devices, have been developed. In particular, diamond light-receiving elements are expected to be applied to radiation detectors because of their high radiation resistance, but since no suitable scintillator exists at present, they are put to practical use as radiation detectors with scintillators and photodetectors. It has not been.

  Therefore, the development of scintillator crystals that can be used for diamond light-receiving elements, etc. is required. However, since the conventional detectors with high sensitivity to visible light have been the mainstream, research on scintillators is also targeted for scintillators that emit visible light However, there are few candidates for suitable materials.

  As an example of a candidate, as a vacuum ultraviolet light emitting scintillator using rare earth light emission, a lanthanum fluoride crystal added with Nd has been known for more than ten years (Non-patent Document 1), but the effective atomic number is low and γ It has not been put into practical use for reasons such as low line detection efficiency.

As crystals that use light emission associated with energy transition of the core level called Cross-Luminescence, it is known that KCaF ₃ , KMgF ₃ , and BaF ₂ cause relatively high emission intensity vacuum ultraviolet light emission. (Non-Patent Document 2). There is also a report example of LiKLuF ₅ crystals to which different elements are not added (Non-patent Document 3). However, this is a phenomenon peculiar to this light emission mechanism, and since any crystal cannot emit light when excited by neutron rays or alpha rays, its application is limited.

  There are fewer reported examples of vacuum ultraviolet light emitting materials than visible light emitting materials, and the reason why development is difficult is that vacuum ultraviolet rays are absorbed by many substances, so that substances that do not cause self-absorption are limited. Can be mentioned.

  Furthermore, the emission characteristics in the vacuum ultraviolet region are easily affected by impurities in the material, and even if the material has a light emission energy level in the vacuum ultraviolet region, light emission at a longer wavelength based on a lower energy level is dominant. In many cases, the desired vacuum ultraviolet light emission cannot be obtained due to the reason that the loss due to non-radiative transition is significant. Therefore, it is extremely difficult to predict the emission characteristics in the vacuum ultraviolet region in advance, and this is a big barrier in the development of vacuum ultraviolet light emitting materials.

P. SHOTAUS et al. , “DETECTION OF LaF3: Nd3 + SCINTILLATION LIGHT IN A PHOTOSENSITIVE MULTIWIRE CHAMBER” Nuclear Instruments and Methods in Physics16, 1987. C. W. E. Van Eijk et al. , “Cross-Luminescence” Journal of Luminescence 60 & 61 (1994) 936-941 N. Yu. Kirikova et al. , "Cross-luminescence of several complex fluorides excited by synchrotron radiation" Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 359 Issues 1-2, 1 (1995) 351-353, Proceedings of the 10th National Synchrotron Radiation Conference

  As a new scintillator suitable for combination with a radiation-resistant vacuum ultraviolet light receiving element developed in recent years, a scintillator in which Cross-Lumescence and rare earth electron transition emission can coexist, which can be applied to neutron and γ-ray discrimination. The object is to provide crystals.

The present inventors have searched for materials that emit light in the vacuum ultraviolet region, and as a result of various investigations, when a fluoride crystal containing a rare earth element in KLiLuF ₅ is excited by X-rays, cross-luminance vacuum ultraviolet light emission. As a result, it was found that the electron transition emission of rare earth can be obtained in coexistence while maintaining the above, and the present invention has been completed.

That is, the present invention is a radiation detector characterized by using a fluoride crystal of KLiLuF ₅ containing a rare earth element and the fluoride crystal as a scintillator.

  The present invention provides a characteristic material having vacuum ultraviolet emission by Cross-Luminescence that does not occur by excitation by neutron rays or alpha rays, and rare earth electron transition emission that occurs by excitation by many radiations.

The fluoride crystal of the present invention can be suitably used as a scintillator for a vacuum ultraviolet photodetector such as a diamond light receiving element or an AlGaN light receiving element. In addition, due to the electron orbit transition light emission imparted by the addition of rare earth, light emission is exhibited even when excited by neutron rays and alpha rays, and the shape of the emission spectrum of the fluoride crystal of the present invention differs depending on the excitation source. Even when the excitation source is unknown, the type of the excitation source can be discriminated.

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 the fluoride crystals of Example 1 and Comparative Example 1. This figure is the α-ray excited emission spectrum of the fluoride crystal of Example 2. This figure is the α-ray excited emission spectrum of the fluoride crystal of Example 3.

Hereinafter, the fluoride crystal composed of KLiLuF ₅ containing the rare earth of the present invention will be described.

The fluoride crystal of the present invention is a crystal having a composition in which a part of KLiLuF ₅ is replaced with a rare earth element. In the present invention, the rare earth elements are Ce, Eu, Pr, Nd, Er, Tm, Ho, Dy, Tb, Gd, Sm, Yb, La, Y, and Pm.

  The crystal of the present invention can emit light in any state of a single crystal, a polycrystal, or a crystal powder. However, in the case of a single crystal, it is generally high in light transmission and has a large solid sample. However, it is preferable because light emission from the inside is easily taken out without being attenuated.

  Although the production method is not particularly limited, it can be produced by a general melt growth method represented by the Czochralski method or the micro pull-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 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 powders of at least one metal fluoride selected from KF, LiF, LuF ₃ and rare earth fluoride each having a purity of 99.99% or more are mixed. . By using such a high-purity mixed raw material, the purity of the crystal can be increased and characteristics such as emission intensity are improved. LiF for the purpose of improving the detection efficiency for a neutron beam, may be used ⁶ Li concentrated material. The mixed raw material may be used after being sintered or melted and solidified after mixing. For the purpose of detecting neutron beams, the elemental ratio of ⁶ Li isotopes to the total Li elements contained in the raw material or the manufactured scintillator is preferably 50 mol% or more, and preferably 80 mol% or more. More preferably, it is particularly preferably 90 mol% or more.

  The mixing ratio of the raw material powder in the mixed raw material is K: Li: Lu: R = x: 1: 1-y: y (R is at least one kind of rare earth element, x is 1 to 1.5, and y is 0.001. Can be weighed at a ratio of the number of atoms of ~ 0.5). x is more preferably 1 to 1.2, particularly preferably 1 to 1.1, and y is further preferably 0.001 to 0.1, and particularly preferably 0.001 to 0.05. It is. In addition, the element ratio contained in the produced crystal and the mixing ratio of the raw materials are not greatly different.

  Under normal crystal growth conditions, x = 1 may be sufficient, but when heated to a remarkably high temperature compared to the melting point, there may be a difference in volatilization amount of each raw material powder at the time of growth. The value of x needs to be higher than 1 so that much KF is weighed. The value of x is usually 1.5 or less, but when KF is volatilized in a large amount by baking at a high temperature for a long time, it is necessary to make it higher than 1.5.

  As described above, the weighed value of Li element relative to the sum of the weighed values of Lu element and rare earth element is usually the same, but depending on the growth conditions, Li may easily volatilize. Must be weighed.

The value of y is the amount of at least one rare earth element added and affects the vacuum ultraviolet emission characteristics of the produced crystal. When the crystal of the present invention is excited by X-rays, it emits two types of light emission, that is, light emission associated with the energy transition of the core level called Cross-Luminescence of KLiLuF ₅ and light emission caused by electron orbital transition of rare earth elements. As observed, the former emission is reduced by increasing the amount of rare earth element added. In order for two types of light emission to coexist, the value of y is preferably 0.001 to 0.5, and more preferably 0.001 to 0.05.

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 crystal growth.

  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. This makes it possible to grow crystals.

  That is, while adjusting the output of the high frequency and gradually raising the temperature of the raw material, the metal wire is inserted into the hole at the bottom of the crucible and pulled 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, 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 crystal growth becomes enormous. / Hr is preferable.

  In the production of the 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 caused by thermal strain.

  The obtained crystal has good workability and can be easily 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 fluoride crystal of the present invention has good light emission characteristics, and can be excited by radiation such as X-rays, γ-rays, β-rays, α-rays, and neutron rays to emit light.

  The crystal can be processed into a desired shape and incorporated into a radiation detector as a scintillator, and can be suitably used as a radiation detector combined with a vacuum ultraviolet photodetector such as a diamond light receiving element or an AlGaN light receiving element.

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

Examples 1-3, Comparative Example 1
(Production of fluoride crystals)
The crystals of Examples 1 to 3 and Comparative Example 1 were manufactured using the crystal manufacturing apparatus shown in FIG. Hereinafter, although the method produced about Example 1 is explained in full detail, it was produced by the same method also about Examples 2-3 and the comparative example 1 except that the measured value of each raw material differs as shown in Table 1. .

As raw materials, KF, LiF, LuF ₃ and NdF 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 2 mm in diameter and 0.5 mm in length. did.

  First, each raw material was weighed as shown in Table 1, mixed well, and then charged 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. Next, the inside of the chamber 6 is evacuated to 1.0 × 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.

  After the gas replacement, the pressure in the chamber 6 was changed to atmospheric pressure, and then the raw material was heated to about 400 ° C. with the high-frequency coil 7, but no leaching of the raw material melt from the bottom of the crucible 5 was observed. Then, while adjusting the high frequency output and gradually raising the temperature of the raw material melt, the W-Re wire provided at the tip of the pull-down rod 8 was inserted into the hole and pulled down repeatedly. The liquid could be 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 lowered, and crystallization was started. It was continuously pulled down at a speed of 6 mm / hr for 12 hours, and finally a colorless transparent crystal having a diameter of 2 mm and a length of about 70 mm was obtained.

(Evaluation of luminous characteristics)
The obtained crystal is cut into a length of about 10 mm with a wire saw, the side surface is ground and processed into a shape having a length of 10 mm, a width of about 2 mm, and a thickness of 1 mm, and then a surface having a length of 10 mm and a width of about 2 mm. Both surfaces were mirror-polished to prepare a sample for measuring the light emission characteristics.

  The light emission characteristics of the processed crystal due to the excitation of X-rays, which is a kind of room-temperature radiation, were measured as follows using the measuring apparatus shown in FIG.

  The sample 9 of the present invention was set at a predetermined position in the measuring apparatus, and the entire interior of the apparatus was replaced with nitrogen gas. The sample 9 is irradiated with X-rays from the X-ray generator 10 (X-ray generator for RIGAKU SA-HFM3), which is an excitation source, at an output of 60 kV and 35 mA, and emission from the sample 9 is emitted into the emission spectrometer 11 (spectrometer instrument). Manufactured by KV201 type extreme ultraviolet spectrometer). The wavelength of the spectrum by the emission spectrometer 11 was swept in the range of 130 to 210 nm, and the emission intensity at each emission wavelength was recorded by the photomultiplier tube 12.

In the above measurement, the X-ray excited emission spectra of the fluoride crystals of Example 1 and Comparative Example 1 are shown in FIG. From FIG. 3, in addition to the light emission at the wavelength of 140 to 190 nm confirmed in Comparative Example 1 with the crystal of Example 1, light emission at a new wavelength of about 190 nm was confirmed. From these results, in addition to KLiLuF ₅ Cross-Lumescence which cannot be emitted by α-rays and neutrons that have been reported in the past, the crystal of the present invention can be obtained by coexisting light emission due to electron orbital transition of rare earth elements. It was found that it can be used as a scintillator for discriminating radiation species by taking advantage of the two types of luminescence obtained together.

For the purpose of confirming that an electron transition emission of a rare earth other than Nd added in Example 1 is obtained and investigating the emission wavelength, the emission spectrum by α-ray excitation of the fluoride crystals of Examples 2 to 3 is shown. It was measured. As a method, using the apparatus shown in FIG. 2, a ²⁴¹ Am sealed radiation source, which is a 4 MBq radioactive α-ray source, was installed in the vicinity of the measurement sample without irradiating X-rays, and an emission spectrum was obtained.

  The results are shown in FIGS. It can be seen that the fluoride crystal of Example 2 to which Tm was added obtained vacuum ultraviolet light emission at a wavelength of about 170 nm, and the fluoride crystal of Example 3 to which Ce was added obtained ultraviolet light emission at a wavelength of about 350 nm. It can be seen that even when rare earth is added, electron transition emission of rare earth is imparted.

In addition, when using the fluoride crystals of Examples 1 and 2 when the radiation detector is used, it is assumed that light emission is read out by a single vacuum ultraviolet light receiving element, and KLiLuF ₅ Cross-Luminescence and rare earth electron orbit transition light emission. You may distinguish the difference in a radiation type from the difference in a fluorescence lifetime curve. In the case of using the fluoride crystal of Example 3, it is possible to discriminate radiation types from the difference in detection signals of the two types of photodetectors by using a photodetector for visible light in combination. Depending on the configuration of the radiation detector of each configuration, the configuration of the present invention can be selected by selecting the rare earth to be added in this case, depending on the technical aspect and cost of the photodetector. The compound crystal can provide a scintillator material for radiation detection applicable to radiation detectors of different configurations.

DESCRIPTION OF SYMBOLS 1 After heater 2 Heater 3 Heat insulation material 4 Stage 5 Crucible 6 Chamber 7 High frequency coil 8 Pulling rod 9 Sample 10 X-ray generator 11 Emission spectrometer 12 Photomultiplier tube

Claims (4)

A fluoride crystal composed of KLiLuF ₅ containing at least one kind of rare earth element. The fluoride crystal according to claim 1, wherein the rare earth element is any one of Nd, Tm, and Ce. A scintillator for radiation detection, comprising the fluoride crystal according to claim 1. A radiation detector comprising the scintillator according to claim 3.

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