JP2017066245A - Scintillator crystal material, single crystal scintillator, radiation detector, imaging device and non-destructive inspection device - Google Patents

Abstract

To provide a single crystal scintillator having a high light emission amount, easy single crystal growth, no cleavage and no deliquescence, a radiation detector using the same, an imaging device, and a nondestructive inspection device.
A scintillator crystal material is a pyrochlore crystal containing La and Gd, which contains a rare earth metal element emitting 5d-4f transition light as an optical activator, and has a molar fraction M _{La of La} and a molar fraction M _{Gd of Gd.} Satisfies the relationship 0.64 ≧ M _La / M _Gd ≧ 0.15, the emission wavelength of the scintillation light is 200 nm or more and 500 nm or less, and the distribution of the emission wavelength intensity of the scintillation light with respect to the emission wavelength is at least two or more peaks. Indicates. Of highest peak and peak high emission wavelength intensity in the second emission wavelength intensity, the emission wavelength intensity shorter emission wavelength peaks as P _1, the emission wavelength intensity of longer peak emission wavelength was P ₂ In this case, the relationship of P ₁ / P ₂ ≧ 0.97 is satisfied.
[Selection] Figure 1

Description

  The present invention relates to a scintillator crystal material, a single crystal scintillator, a radiation detector, an imaging device, and a nondestructive inspection device.

  Scintillation is an ionizing particle such as alpha, gamma, x-ray, or neutron beam, so-called ionizing radiation is absorbed by the substance and the electrons excite it, and the absorbed energy is converted into visible light, ultraviolet light, or infrared light. It is a relaxation phenomenon that is released. Further, scintillation light refers to light emitted during the scintillation phenomenon.

  A scintillator is a substance that causes a scintillation phenomenon. The scintillator is used as a radiation detector in combination with a photodetector, and is used in various industrial fields.

  Such a radiation detector is, for example, a positron emission tomography (PET) apparatus in the nuclear medicine field, a single photon emission tomography (SPECT) apparatus, a gamma ray as an example in the field of high energy physics. It is widely used in various radiation measuring devices for applications and neutron radiation, and resource exploration devices.

For example, in a PET imaging system, a radioactive substance that decays by β ⁺ is used as a tracer, and the radioactive substance injected into the subject is generated from the interaction between positrons (positrons) generated by β ⁺ decay and electrons in the body 2. A pair of gamma rays (annihilation radiation: 511 keV) is incident on two opposing scintillators and converted to photons that can be detected by a photodetector. The photons emitted from each scintillator are a photo diode (PD), a silicon photomultiplier (Si-Photomultiplier: Si-PM), or a photomultiplier tube (PMT), or other light detection. Can be detected using an instrument.

  The scintillators suitable for these radiation detectors have a large atomic number (high photoelectric absorption ratio) in terms of detection efficiency, a high light emission amount in terms of high energy resolution, and the need for a high-speed response, so that fluorescence It is desired that the lifetime (fluorescence decay time) is short. In addition, in recent systems, multiple scintillators need to be closely arranged in an elongated shape (for example, about 5 mm x 5 mm x 30 mm for PET) to increase the number of layers and increase the resolution. The price is also an important selection factor.

As a scintillator having a high light emission amount, a rare earth halide single crystal scintillator can be cited. In particular, according to Non-Patent Document 1, for example, Ce: LaBr ₃ has a high light emission amount of 61,000 Photon / MeV and an energy resolution of 2.9% under 662 keV gamma irradiation.

  Further, as can be seen in Patent Document 1, in the rare earth bromide crystal, the wavelength with high quantum efficiency of the photodetector with respect to two emission peaks derived from a rare earth metal element that emits 5d-4f transition light as an optical activator. A crystal with high sensitivity to the photodetector is obtained by increasing the ratio of light emission having a wavelength close to the region. As shown in this Patent Document 1, in order to adjust the emission wavelength, it is usually performed by adjusting the addition of a trace amount additive component (dopant).

As a preferable scintillator currently applied to various radiation detectors, there is a pyrochlore oxide-based scintillator Ce: Gd ₂ Si ₂ O ₇ . The scintillator has the advantages that it is chemically stable, has no cleavage or deliquescence, is excellent in workability, and has a high light emission amount. For example, a pyrochlore oxide scintillator described in Non-Patent Document 2 that uses light emission from the Ce ³⁺ 4f-5d level has a short fluorescence lifetime of about 80 ns or less and a high light emission amount.

  In the pyrochlore oxide scintillators described in Patent Document 2 and Non-Patent Document 3, an attempt has been made to obtain a harmonic melt composition by replacing Ce with a site of a rare earth element. This makes it possible to produce a large single crystal by a melt growth method such as the floating zone method, the Czochralski method (pulling method), the micro pulling method, or the Bridgman method.

In patent literature _{2 (Gd 1-x Ce x} ) 2 Si 2 O 7, from the viewpoint of stable crystal growth, the concentration quenching or self-absorption by the excessive addition of dopant Ce, although light emission drops, 0 .1 <x <0.3 is desirable. Still, a large single crystal cannot be obtained, and an example in which a single crystal is taken out from polycrystallized is described.

  Non-Patent Document 4 describes an example of succeeding in taking out a single crystal by a floating zone method by substituting Ce for La to suppress a drastic decrease in light emission amount (concentration quenching) and using a harmonized melting composition. Has been.

  The crystal material used as a scintillator is required to have a high light emission amount, and in order to be widely applied to society, from the viewpoint of cost, it is not cleaved, has no deliquescence, and is easy to handle. Is required. There is also a need for such crystalline materials, and radiation detectors, imaging devices and non-destructive inspection devices using these materials.

However, in general, the LaBr ₃ crystal described in Patent Document 1 is very strong in deliquescence and cannot be exposed to the atmosphere, so that sealing is essential. For this reason, there is a problem that advanced techniques are required for handling and the cost due to processing costs is increased.

  The pyrochlore-type oxide scintillator described in Patent Literature 2 and Non-Patent Literature 4 has a harmonious melt composition by adding Ce, and direct crystal growth without changing the composition from the melt is possible. However, when Ce used as an optical activator is increased, there arises a problem (concentration quenching) that the light emission amount is drastically reduced.

JP 2008-101180 A JP 2009-074039 A

EVD van Loef, P. Dorenbos, CWE van Eijk, KW Kramer and H., U. Gudel. Scintillation properties of LaBr3: Ce3 + crystals: fast, efficient and high-energy-resolution scintillators. Nuclear Instruments & Methods in Physics Research Section A -Accelerators Spectrometers Detectors and Associated Equipment, 486: 254-258, 2002. S. Kawamura, JHKaneko, M. Higuchi, T. Yamaguchi, J. Haruna, Y, Yagi, K. Susa, F. Fujita, A. Homma, S. Nishiyama, H. Ishibashi, K. Kurashige and M. Furusaka , IEEE Nuculear Science Symposium Conference Record, San Diego, USA, 29 October-5 November 2006, pp.1160-1163. Sohan.Kawamura, Junichi H.Kaneko, Mikio Higuchi, Jun Haruna, Shohei Saeki, Fumiyuki Fujita, Akira Homma, Shusuke Nishiyama, Shunsuke Ueda, Kazuhisa Kurashige, Hiroyuki Ishibashi, and Michihiro Furusaka, IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 56, NO. 1, FEBRUARY 2009. Akira Suzuki, Shunsuke Kurosawa, Toetsu Shishido, Jan Pejchal, Yuui Yokota, Yoshisuke Futami, and Akira Yoshikawa, Applied Physics Express 5 (2012) 102601-1-102601-3.

  The present invention has been made in view of the above, a scintillator crystal material having a high light emission amount, a single crystal scintillator having low cleavage and deliquescence using this scintillator crystal material, and a radiation detector using these, An object is to provide an imaging device and a nondestructive inspection device.

In order to solve the above-described problems and achieve the object, the present invention provides a scintillator crystal material that emits scintillation light by irradiation with radiation, wherein the scintillator crystal material is a pyrochlore crystal containing La and Gd, and an optical A rare earth metal element emitting 5d-4f transition light is included as an activator, and the relationship between the molar fraction M _{La of La} and the molar fraction M _Gd of _Gd satisfies 0.64 ≧ M _La / M _Gd ≧ 0.15, The emission wavelength of the scintillation light is 200 nm or more and 500 nm or less, the distribution of emission wavelength intensity of the scintillation light with respect to the emission wavelength shows at least two peaks, the highest peak of the emission wavelength intensity and the second emission. Among the peaks with high wavelength intensity, the emission wavelength intensity of the shorter peak of the emission wavelength is P ₁ When the emission wavelength intensity of the longer peak of the emission wavelength is P ₂ , the relationship of P ₁ / P ₂ ≧ 0.97 is satisfied.

In the scintillator crystal material, the optical activator is Ce, and preferably satisfies 0.64 ≧ M _La / M _Gd ≧ 0.27.

  Moreover, the single crystal scintillator according to the present invention is characterized by comprising the scintillator crystal material according to the present invention.

  A radiation detector according to the present invention includes the scintillator made of the scintillator crystal material or the single crystal scintillator of the present invention, and a photodetector that receives the scintillation light from the scintillator.

  An imaging apparatus according to the present invention includes the radiation detector according to the above invention.

  In addition, a nondestructive inspection apparatus according to the present invention includes the radiation detector according to the above invention.

  According to the present invention, there is provided a scintillator crystal material having a high light emission amount, a single crystal scintillator using the scintillator crystal material and having low deliquescence and low deliquescence, and a radiation detector, an imaging device and a non-destructive inspection device using these Can be provided.

1 is a diagram showing an emission spectrum of Example 1. FIG. FIG. 2 is a diagram showing an emission spectrum of Example 2. FIG. 3 is a graph showing an emission spectrum of Example 3. FIG. 4 is a diagram showing an emission spectrum of Example 4. FIG. 5 is a diagram showing an emission spectrum of Comparative Example 1. 6 is a diagram showing an emission spectrum of Comparative Example 2. FIG. FIG. 7 is a plot of the relationship between the ratio M _La / M _Gd of the ratio of the molar fraction of La and Gd and the relative luminescence amount based on Comparative Example 1 for the results of Examples 1 to 4 and Comparative Example 1. FIG. FIG. 8 plots the relationship between the ratio M _La / M _Gd and the ratio P ₁ / P ₂ of the molar fraction of La and Gd for the results of Example 1, Example 4, Comparative Example 1 and Comparative Example 2. FIG. FIG. 9 is a diagram showing a fluorescence decay curve profile of Example 2.

  Hereinafter, embodiments of a crystal material, a single crystal scintillator, a radiation detector, an imaging device, and a nondestructive inspection device according to the present invention will be described in detail. In addition, the following description illustrates a part of embodiment of this invention, This invention is not limited to these embodiment, As long as a form has the technical idea of this invention And within the scope of the present invention. Each configuration and combination thereof in each embodiment is an example, and the addition, omission, replacement, and other changes of the configuration can be made without departing from the spirit of the present invention.

A crystal material according to an embodiment of the present invention is a scintillator crystal material that emits scintillation light when irradiated with radiation, and the scintillator crystal material is a pyrochlore crystal containing La and Gd, and a 5d-4f transition as an optical activator. Contains rare earth metal elements that emit light. The relationship between the molar fraction M _{La of La} and the molar fraction M _Gd of _Gd satisfies 0.64 ≧ M _La / M _Gd ≧ 0.15. The emission wavelength of the scintillation light is 200 nm or more and 500 nm or less, the distribution of the emission wavelength intensity of the scintillation light with respect to the emission wavelength shows at least two peaks, the peak with the highest emission wavelength intensity and the second highest emission wavelength intensity. Of the peaks, when the emission wavelength intensity of the shorter emission wavelength is P ₁ and the emission wavelength intensity of the longer emission wavelength is P ₂ , the relationship of P ₁ / P ₂ ≧ 0.97 is satisfied. .

  Thereby, the scintillator crystal material according to the present embodiment is a crystal material having a high light emission amount of scintillation light generated by radiation irradiation.

The scintillator crystal material preferably has Ce as the optical activator and satisfies 0.64 ≧ M _La / M _Gd ≧ 0.27.

  Thereby, the scintillator crystal material according to the present embodiment is not only characterized by a high light emission amount of scintillation light generated by irradiation of radiation, but also has a stable crystal structure and a single crystal can be easily obtained. It becomes a crystalline material.

  The crystal material according to the present embodiment can solve the above-described problems, can increase the amount of light emission, and can be applied to production techniques applied to crystal production in the industrial field such as the micro pull-down method and the Czochralski method. it can.

As described above, the distribution of the emission wavelength intensity of the scintillation light with respect to the emission wavelength shows at least two or more peaks, and the emission wavelength shorter than the highest emission wavelength intensity and the second highest emission wavelength intensity peak. When the emission wavelength intensity of the peak is P ₁ and the emission wavelength intensity of the longer emission wavelength is P ₂ , the emission amount can be particularly increased by satisfying the relationship of P ₁ / P ₂ ≧ 0.97. Is currently inferred as follows.

  As for the electron arrangement of the atoms of the rare earth element, the 5s and 5p orbitals are filled with electrons first, and as the atomic number increases, the 4f orbitals are filled with electrons. Generally, when an atom of a rare earth element receives energy and is excited, 4f orbital electrons easily transition to a 5d orbital. This is because the 5d-4f transition is an allowable transition. The electron orbit of the rare earth element varies depending on the number and state of the occupied electrons, and there are many paths that return from the 5d excited state to the 4f ground state. When rare earth metal elements that emit 5d-4f transition light are added as optical activators, energy levels derived from these elements are formed as impurity levels in the band gap of the host crystal.

  The intensity of the emission wavelength, that is, the high frequency, indicates that the process of light emission is likely to occur, and the emission wavelength with the highest intensity is the process of light emission dominant in the light emission phenomenon. Therefore, if the emission wavelength peak with the highest emission wavelength intensity and the emission wavelength peak with the second highest emission wavelength intensity can be adjusted, the emission process, that is, the path of the relaxation process is adjusted. Thereby, it can be expected that electrons related to light emission can be efficiently relaxed, and as a result, the total light emission amount can be increased.

As a result of diligent experiments, the present inventors have determined that the emission wavelength intensity of the shorter peak of the emission wavelength out of the peak with the highest emission wavelength intensity and the second highest emission wavelength intensity is P ₁ , and the emission wavelength It was found that when the emission wavelength intensity of the longer peak of P is P ₂ , satisfying the relationship of P ₁ / P ₂ ≧ 0.97 can increase the overall light emission amount as a result.

In a rare earth metal element emitting 5d-4f transition light, such as Ce, two states with different spins ( ² F _7/2 , ² F _5/2 ) exist in the 4f orbit, and the excited electrons of Ce ³⁺ are constant. Transition to two states at a rate. Since the emission spectrum of the emitted light returns to the ground state, there are two wavelength peaks in the range of 200 nm to 500 nm.

  In addition, in the transition that contributes to light emission at the impurity level formed by the rare earth metal element, the emission energy at the transition from the excited level to the ground level is smaller than the band gap energy of the host crystal.

Therefore, the energy difference of the 5d-4f transition is at most several eV, that is, a wavelength in the ultraviolet region. The lower limit varies depending on the type of element, but when the Gd ₂ Si ₂ O ₇ crystal material is a host crystal and the rare earth metal element emitting 5d-4f transition light is Ce, the actual radiant energy is about 3.3 eV and About 3.5 eV. Therefore, both of the wavelength peaks are in the range of 200 nm to 500 nm.

For example, in Pr ³⁺ , a number of levels such as ³ F ₄ , ³ H ₃ , ³ H ₂ , ³ H ₆ , ³ H ₅ , and ³ H ₄ are shown according to Dieke's Diagram.

Depending on the element added to the crystal in a small amount, the wavelength and light emission amount of the generated scintillation light can be variously selected and selected. For example, when Ce is selected as a dopant, in a pyrochlore crystal containing La and Gd, it is replaced with a site where Gd and La are present, Ce ³⁺ is present, and Ce atoms in a trivalent state are present. Contributes to light emission. The doped Ce ³⁺ electrons transition from the ground state (4f orbit) to the excited state (5d orbit), and then emit extra energy as photons (electromagnetic waves) when transitioning from the excited state to the ground state. .

  In the scintillator crystal material according to the present invention, the composition of the constituent elements of the pyrochlore crystal that is the host crystal is adjusted, and the energy band gap of the pyrochlore crystal is adjusted by changing the crystal lattice spacing. The energy level of the impurity level produced by the dopant is optimized by adjusting the band structure, energy loss due to phonons is reduced, and light can be emitted efficiently. As a result, the total light emission amount can be increased.

  In the pyrochlore crystal, the present inventors have found that the ratio of the peak of the emission wavelength can be variously changed by changing the structure of the host crystal in this way, and the amount of light emission can be optimized by adjusting it. The conclusion was reached.

  Depending on the selected trace additive element, the emission wavelength can be in the vacuum ultraviolet region from 10 nm to 200 nm, and in the infrared region of 700 nm or more, but when used for PET or resource exploration, the emission peak is 200 nm. The thickness is preferably 500 nm or more.

A scintillator crystal material according to an embodiment of the present invention is a scintillator crystal material that emits scintillation light when irradiated with radiation. The scintillator crystal material is a pyrochlore crystal containing La and Gd, and 5d-4f transition light emission as an optical activator. And the molar fraction M _{La of La} and the molar fraction M _Gd of _Gd satisfy 0.64 ≧ M _La / M _Gd ≧ 0.15, and the emission wavelength of scintillation light is 200 nm or more and 500 nm or less Yes, the distribution of the emission intensity of the scintillation light with respect to the emission wavelength shows at least two or more peaks, and the intensity of the peak with the shorter emission wavelength of the highest emission wavelength intensity and the second highest emission wavelength intensity peak Is P _{1 and} the intensity of the longer peak of the emission wavelength is P ₂ , P ₁ / P ₂ ≧ 0.97 should be satisfied, and in addition to the rare earth metal element that emits 5d-4f transition light as an optical activator, other elements such as Sc, a rare earth element, and Ti, V, which are transition elements. , Cr, Mn, Fe, Co, Ni, Cu and Ga, Ge, As, In, Sn, Sb, Tl, Pb, Bi, and the like which are elements that become ns ² ions may be included.

  Specifically, the rare earth metal element emitting 5d-4f transition light is at least one of Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. It is.

A scintillator crystal material according to an embodiment of the present invention is a scintillator crystal material that emits scintillation light when irradiated with radiation. The scintillator crystal material is a pyrochlore crystal containing La and Gd, and 5d-4f as an optical activator. The relationship between the molar fraction M _{La of La} and the molar fraction M _Gd of _Gd contains 0.64 ≧ M _La / M _Gd ≧ 0.15, and the emission wavelength of scintillation light is The emission intensity distribution of the scintillation light with respect to the emission wavelength is 200 nm or more and 500 nm or less, and shows at least two peaks. Of the peak with the highest emission wavelength intensity and the peak with the second highest emission wavelength intensity, the intensity of the shorter peak as P _1, the longer peak intensity of the emission wavelength and P ₂ If _{it may} satisfy the relation of _P 1 / P ₂ ≧ 0.97, may be configured that includes an element other than La and Gd as the host crystals.

Here, when the pyrochlore crystal containing La and Gd which is one of the scintillator crystal materials according to the embodiment of the present invention is (Gd, La) ₂ Si ₂ O ₇ , it can be explained as follows. . That is, when the ratio M _La / M _Gd of the molar fraction M _{La of La} and the molar fraction M _Gd of _Gd is around 0.1, the composition in which the crystal structure becomes the Orthohombic phase (M _La / M _Gd <0.1) And a triclinic phase (M _La / M _Gd > 0.1) boundary exists, and the crystal structure is stabilized by setting M _La / M _Gd > 0.1. That is, even if the host crystal includes an element other than La and Gd, it can be inferred that the crystal structure is stable by setting M _La / M _Gd ≧ 0.15.

When M _La / M _Gd > 0.64, the band structure of the host crystal changes, and the emission energy is small in the transition to two states ( ² F _7/2 , ² F _5/2 ). ^Since the transition to ² F _7/2 increases and the efficiency of light emission decreases, the amount of light emission tends to decrease. In addition, when M _La / M _Gd > 0.79, the light emission amount decreases rapidly.

Preferably, in the scintillator crystal, the optical activator is Ce, and preferably satisfies 0.64 ≧ M _La / M _Gd ≧ 0.27. By using Ce as the optical activator, a scintillator crystal having a high light emission amount and a particularly short fluorescence lifetime can be obtained.

Here, by satisfying 0.64 ≧ M _La / M _Gd ≧ 0.27, the crystal structure is further stabilized, the coincident melting composition is obtained, and the growth of the bulk single crystal becomes very easy.

The term “stable crystal structure” as used herein means that the crystal lattice constant of the Triclinic phase is within a length that does not cause distortion that does not maintain the structure as the Triclinic phase, and Gd that is a peritectic composition. ₂ Si ₂ O ₇ is used in the sense that the total enthalpy of mixing between atoms from positive to negative approaches the melt composition.

  Furthermore, the scintillator crystal material according to the present embodiment can easily grow a single crystal, and can be used as a radiation detector by combining a single crystal scintillator with a photodetector capable of receiving scintillation light. Become. Furthermore, it can be used as a radiation measuring apparatus or a resource exploration apparatus as a nondestructive inspection apparatus provided with these radiation detectors as radiation detectors.

  Next, the radiation detector, imaging apparatus, and nondestructive inspection apparatus according to the present embodiment will be specifically described. The following description exemplifies a part of the embodiments of the present invention, and the present invention is not limited to these embodiments.

  The radiation detector according to the present embodiment includes a scintillator composed of the scintillator crystal or the single crystal scintillator of the present invention, and a photodetector that receives scintillation light from the scintillator. By combining a scintillator and a photodetector as described above, scintillation light is emitted when radiation is incident on the scintillator, converted into an electrical signal and detected, and information on the intensity and energy of the incident radiation is obtained. Can do.

  The imaging apparatus and nondestructive inspection apparatus according to the present embodiment include a radiation source, a radiation control apparatus, a radiation drive control apparatus, a scintillator array in which the radiation detector or the single crystal scintillator is two-dimensionally arranged, and a photodetector. It consists of a radiation detector array in which a two-dimensionally arranged photodetector array or a position detection type photodetector is combined. The radiation source is selected from an X-ray source, a gamma ray source, a neutron source, or a combination thereof.

  A radiation detector or radiation placed on the opposite side of the radiation source centered on the subject, with the subject placed within the imaging range, irradiated with radiation emitted from the radiation source to the subject for a certain period of time by the radiation control device Radiation is detected by a detector array.

  A part of the radiation is absorbed when passing through the subject, and the intensity is weakened. Since the linear attenuation coefficient of the radiation differs depending on the constituent of the transmitted object, the positional information of the constituent, that is, the imaging data depending on the linear attenuation coefficient can be obtained by measuring the radiation intensity for each position. In the imaging apparatus according to the present embodiment, imaging data of the subject can be obtained in this way.

  The imaging apparatus according to the present embodiment further includes a radiation detector array, a tomographic imaging apparatus including a radiation detector ring having an annular structure in which a scintillator is arranged on the inside and a photodetector is arranged on the outside, and the detectors are arranged in an annular shape. ,including. In this case, the subject is placed in the center of the ring, subject projection data is acquired by detecting radiation intensity from each direction of 360 °, image reconstruction is performed from the obtained data, and an internal tomogram of the subject is obtained. Can be obtained. The image reconstruction method is selected according to the application from, for example, a simple back projection method (back projection method), a successive approximation method, an analytical reconstruction method, or the like.

  In addition, regarding the scintillator crystal material and the single crystal thereof according to the present embodiment, the predetermined fluorescence component contained in the scintillation light can have a fluorescence lifetime of 1000 ns or less. As described above, since the fluorescence lifetime is short, the sampling time for fluorescence measurement can be shortened, and the high time resolution, that is, the sampling interval can be reduced. In addition, by realizing high time resolution, the number of samplings per unit time can be increased. Such a crystal material having a short-lived emission can be suitably used as a scintillator for detecting radiation with a high-speed response for PET, SPECT and CT which are imaging devices.

  Moreover, when the peak wavelength of scintillation light is in the range of 200 nm to 500 nm, it can be detected in combination with a semiconductor photodetector such as PD, APD, or Si-PM composed of a silicon semiconductor.

  In particular, when the fluorescence peak wavelength of the fluorescent component is 400 nm or less, it is effective to convert the wavelength to a wavelength of 300 nm to 900 nm, that is, a wavelength in a region where the wavelength sensitivity of the above-described photodetector is sufficient using a wavelength conversion element .

  As the wavelength conversion element, for example, one using a plastic wavelength conversion optical fiber (for example, Y11 (200) MS manufactured by Kuraray Co., Ltd.) can be used.

  Further, the type of photodetector to be combined can be appropriately used according to the fluorescence peak wavelength or the like. For example, PMT or PS-PMT may be used.

  Further, in the crystal material according to the present embodiment, if the fluorescence lifetime of the fluorescent component contained in the scintillation light is 80 ns or less and the fluorescence peak wavelength is in the range of 300 nm to 500 nm, further high resolution and high It is possible to detect scintillation light with sensitivity. Adjustment of the fluorescence lifetime and the fluorescence peak wavelength can be realized by adjusting the composition of the crystal material. For example, the fluorescence lifetime can be shortened by increasing the dopant concentration.

  Further, in the scintillator crystal material according to the present embodiment, the light emission amount of the fluorescent component in the range of the ambient temperature from room temperature to 150 degrees Celsius when the light emission amount of the fluorescent component when the ambient temperature is 0 degrees Celsius is used as a reference. The attenuation ratio from the reference can be less than 20%. Therefore, since the scintillator crystal material according to the present embodiment can reduce the attenuation of light emission even under a high temperature environment, it is very useful as a scintillator crystal material used under a high temperature environment, particularly in an environment exceeding 100 degrees. It is effective for the resource search use used in.

  A method for manufacturing a single crystal crystal material according to the present embodiment will be described below. In any method for producing a single crystal, a general oxide raw material can be used as a starting material, but when used as a single crystal for a scintillator, it has a high purity of 99.99% (4N) or more. It is particularly desirable to use raw materials.

  These starting materials are weighed and mixed so as to have a target composition at the time of melt formation, and used as a crystal growth material. Further, among these starting materials, those starting with as few impurities as possible other than the intended composition are particularly preferred. In particular, it is preferable to use a starting material that contains as little as possible an element that emits light in the vicinity of the scintillation light wavelength of the crystalline material or an element that easily changes its valence.

Crystal growth is preferably performed in an inert gas atmosphere such as Ar or N ₂ . Alternatively, a mixed gas of inert gas and oxygen gas may be used, or a mixed gas of inert gas and hydrogen gas as a reducing atmosphere may be used. Note that oxygen gas, inert gas, and a mixed gas of oxygen gas and inert gas can also be used in post-processes such as annealing after crystal growth.

  As a method for producing a single crystal of a crystal material according to the present embodiment, in addition to the micro pulling method, the Czochralski method (pulling method), the Bridgman method, the band melting method (zone melt method), and the edge limited thin film Examples thereof include a supply crystal growth (EFG) method and a floating zone method, but are not limited thereto, and various crystal growth methods can be used.

  In order to obtain a large single crystal, the Czochralski method or the Bridgman method is preferable. By using a large single crystal, it is possible to improve the yield of the single crystal and relatively reduce the processing loss. Therefore, compared with the method of taking out a single crystal from polycrystallized as described in Patent Document 2, a low-cost and high-quality crystal material can be obtained. However, the scintillator crystal material according to the present embodiment is not limited to a single crystal, and may be a polycrystalline sintered body such as ceramics.

  On the other hand, if only a small single crystal is used as the scintillator single crystal, there is no or little post-processing, so the floating zone method, the zone melt method, the EFG method, the micro pull-down method, or the Czochralski The method is preferred, and the micro-pulling method or the zone melt method is particularly preferred for reasons such as wettability with a crucible.

  Furthermore, from the viewpoint of industrial mass production of small single crystals as single crystals for scintillators, the micro pull-down method has the feature that it can grow single crystals at high speed and is easy to control the shape during growth. preferable.

  Examples of the crucible and afterheater material that can be used include Pt, Ir, Rh, Re, and alloys thereof.

  In manufacturing a scintillator single crystal, a high-frequency oscillator, a condenser heater, and a resistance heater may be further used.

  Hereinafter, a single crystal manufacturing method using the Czochralski method and the micro-pulling-down method will be described as an example of a method for manufacturing a single crystal of the crystal material according to the present embodiment. The method for producing the crystal is not limited to this.

  The Czochralski method (pulling method) can be performed by using a known single crystal pulling apparatus of an atmosphere control type by high frequency induction heating.

  The single crystal pulling apparatus includes a crucible filled with a raw material melt, induction heating means for remotely heating the crucible (for example, a high frequency induction heating coil), a seed crystal holder provided on the crucible, and a seed crystal holder upward. It consists of a moving mechanism that pulls up and a rotating mechanism that rotates around the axis of the seed crystal holder. The crucible is filled with a crystal material, and the crucible is heated by a high-frequency heating method or a resistance heating method to melt the crystal material. When the raw material is melted to become a raw material melt, the seed crystal cut in a predetermined crystal orientation is brought into contact with the surface of the raw material melt, and the seed crystal is rotated at a predetermined rotational speed. A single crystal is grown by pulling up.

  The micro pull-down method can be performed using a known atmosphere control type micro pull-down apparatus using high-frequency induction heating.

  The micro-pulling device is, for example, a crucible for containing a raw material melt, a seed crystal holder for holding a seed crystal to be brought into contact with the raw material melt flowing out from a fine hole provided at the bottom of the crucible, and a seed crystal holder downward A single crystal manufacturing apparatus including a moving mechanism for moving, a moving speed control device for controlling the speed of the moving mechanism, and induction heating means (for example, a high frequency induction heating coil) for remotely heating the crucible. According to such a single crystal production apparatus, a single crystal can be produced by forming a solid-liquid interface immediately below the crucible and moving the seed crystal downward.

  In the single crystal pulling apparatus and the micro pulling method apparatus described above, the crucible is made of C, Pt, Ir, Rh, Re, Mo, W or an alloy thereof. In the micro pull-down method, an after heater, which is a heating element made of C, Pt, Ir, Rh, Re, Mo, W, or an alloy thereof, is disposed on the outer periphery of the crucible bottom. Controlling the temperature and distribution of the solid-liquid boundary region of the raw material melt drawn from the fine holes provided at the bottom of the crucible by adjusting the heat output by adjusting the output of each induction heating means of the crucible and after-heater Can do.

The atmosphere control type single crystal pulling device and the atmosphere control type micro pulling device described above employ stainless steel (SUS) as the material of the chamber, quartz as the window material, and a rotary pump for enabling atmosphere control. And the internal vacuum degree can be reduced to 1 × 10 ⁻³ Torr or less before gas replacement. In addition, Ar, N ₂ , H ₂ , O ₂ gas, etc. can be introduced into the chamber at a flow rate precisely adjusted by an accompanying gas flow meter.

Using this apparatus, the crystal growth raw material prepared by the above method is put into a crucible, the inside of the furnace is evacuated to a high vacuum, and then N ₂ gas, Ar gas, or a mixed gas of Ar gas and O ₂ gas is used. Is introduced into the furnace to make the inside of the furnace an inert gas atmosphere or a low oxygen partial pressure atmosphere. Next, the crucible is slowly heated by gradually applying high-frequency power to the high-frequency induction heating coil to completely melt the raw material in the crucible.

  In the micro pull-down method, the seed crystal held by the seed crystal holder is gradually raised at a predetermined speed by a moving mechanism. Then, when the tip of the seed crystal is brought into contact with the fine hole at the lower end of the crucible and sufficiently blended, the crystal is grown by lowering the seed crystal while adjusting the melt temperature.

  As the seed crystal, it is preferable to use a seed crystal that is equivalent to or similar in structure and composition to the crystal growth object, but is not limited thereto. Moreover, it is preferable to use a crystal with a clear crystal orientation as a seed crystal.

  Crystal growth is completed when all of the prepared crystal growth raw materials are crystallized and the melt is exhausted. On the other hand, for the purpose of keeping the composition of the crystal to be grown uniform and for the purpose of lengthening it, a device for continuously charging the crystal growth raw material may be incorporated. Thereby, the crystal can be grown while charging the crystal growth raw material.

  Hereinafter, examples and comparative examples of the present invention will be described in detail with reference to the drawings, but the present invention is not limited thereto.

Example 1
Crystals represented by the composition of (Ce _0.01 La _0.29 Gd _0.70 ) ₂ Si ₂ O ₇ were grown by the micro-pulling down method. This crystal is a pyrochlore type oxide crystal represented by A ₂ B ₂ O ₇ .

(Example 2)
Crystals represented by the composition of (Ce _0.01 La _0.22 Gd _0.77 ) ₂ Si ₂ O ₇ were grown by the micro-pulling down method. This crystal is a pyrochlore type oxide crystal represented by A ₂ B ₂ O ₇ .

(Example 3)
Crystals represented by the composition of (Ce _0.01 La _0.215 Gd _0.775 ) ₂ Si ₂ O ₇ were grown by the micro-pulling down method. This crystal is a pyrochlore type oxide crystal represented by A ₂ B ₂ O ₇ .

Example 4
A crystal represented by a composition of (Ce _0.01 La _0.39 Gd _0.60 ) ₂ Si ₂ O ₇ was grown by the micro-pulling down method. This crystal is a pyrochlore type oxide crystal represented by A ₂ B ₂ O ₇ .

(Comparative Example 1)
A crystal represented by a composition of (Ce _0.01 La _0.44 Gd _0.55 ) ₂ Si ₂ O ₇ was grown by a micro-pulling down method. This crystal is a pyrochlore type oxide crystal represented by A ₂ B ₂ O ₇ .

(Comparative Example 2)
A crystal represented by a composition of (Ce _0.01 La _0.44 Gd _0.55 ) ₂ Si ₂ O ₇ was grown by a micro-pulling down method. This crystal is a pyrochlore type oxide crystal represented by A ₂ B ₂ O ₇ . The crystal of Comparative Example 2 is a brittle crystal and easily cracks, and it is difficult to evaluate optical characteristics.

(Comparative Example 3)
A crystal represented by a composition of (Ce _0.01 La _0.09 Gd _0.9 ) ₂ Si ₂ O ₇ was grown by a micro-pulling down method. This crystal is a pyrochlore type oxide crystal represented by A ₂ B ₂ O ₇ . The crystal of Comparative Example 3 was cloudy and was brittle and easily cracked, making it impossible to evaluate optical properties.

When crystal growth of Example 1 to Example 4 and Comparative Example 1 was carried out, a transparent bulk body could be stably grown, whereas Comparative Example 2 and Comparative Example 3 were stable. I could not train. The ionic radii of Gd and La (Shannon's ionic radius) are 0.94 and 1.03 respectively, and the ionic radii are different. For this reason, it is considered that when La is large, the strain of the grown crystal lattice becomes large and it is easy to break. Although Comparative Example 3 is a case where La is small, although it is close to the harmonic melt composition, this composition approaches the unstable peritectic composition of Gd ₂ Si ₂ O ₇ , so that crystal growth becomes unstable.

  Comparative Example 3 is a cloudy, cracked crystal with a crack, and the inside of the crystal is particularly cloudy, and optical characteristics such as an emission spectrum could not be obtained. Comparative Example 2 was a brittle crystal with a crack, but the structure inside the crystal was conspicuous but no cloudy portion was observed. Although the emission spectrum is not affected by cracks, the absolute value of the emission intensity and the amount of emitted scintillation light are affected by the composition and by the cracks inside the sample. Therefore, the evaluation was carried out with reference to optical characteristic evaluation and scintillator characteristics.

  In order to evaluate the optical characteristics of the crystals of Example 1 to Example 4 and Comparative Examples 1 and 2, light emission (photoluminescence) was measured by photoexcitation.

  For the measurement of photoluminescence, a spectroscope (model: Instrument FLS920) manufactured by Edinburgh Instruments was used, and 450 W xenon arc lamp (Xe900) of the same company was used as an excitation source.

  1 to 6 are diagrams showing emission spectra of photoluminescence of the obtained examples and comparative examples. 1 to 6, the horizontal axis represents the emission wavelength, and the vertical axis represents the count number (Arbitrary Unit) of each peak. 1 to 6, the solid line is the actual emission spectrum, the broken line is the Gaussian curve detected by the peak search, and the alternate long and short dash line is the result of performing the double Gaussian fitting on the actual emission spectrum.

  All of the crystals of the examples had a broad emission peak in the range of 300 nm to 500 nm.

  When a peak search by Gaussian was performed for each of the photoluminescences of FIGS. 1 to 6, peaks were detected at positions of 370 nm and 390 nm, respectively. Each peak at 370 nm and 390 nm is a light emission peak of a rare earth metal element that emits a 5d-4f transition, considering the band structure and the results of similar soot oxide materials. In addition, these peaks correspond to the energy of light emitted when transitioning from the 5d level, which is the excitation level of Ce added as an optical activator, to the 4f1 and 4f2 levels, respectively.

From the above, in the pyrochlore crystal containing La and Gd, the two wavelengths at the time of transition to two 4f orbitals ( ⁴ F _2/7 = 390 nm, ⁴ F _2/5 = 370 nm) having different spin states of the dopant Ce are obtained. A peak was confirmed.

Shorter (370 nm (short wavelength side)) emission wavelength intensity of the peak of the emission wavelength as _{P 1,} when a longer (390 nm (long-wavelength side)) emission wavelength intensity of the peak of the emission wavelength is _{P 2,} these Table 1 shows the intensity ratio (P ₁ / P ₂ ). In each of Examples 1 to 4, it was confirmed that there was a relationship of P ₁ / P ₂ ≧ 0.97 between P ₁ and P ₂ .

Further, the light emission amounts of the crystals obtained in Examples 1 to 4 and Comparative Examples 1 to 2 were estimated. Here, each crystal is optically mounted on a photomultiplier tube (R7600-200 manufactured by Hamamatsu Photonics), which is a photodetector, with optical grease (Applied Koken 6262A) and ¹³⁷ Cs having a radioactivity of 1 MBq. A sealed ray source (gamma ray source) or ²⁴¹ Am was used to excite and emit light by irradiating gamma rays.

  Note that 650 to 700 V was applied to the photomultiplier tube to convert the scintillation light into an electrical signal. Here, the electric signal output from the photomultiplier tube is a pulse-like signal reflecting the received scintillation light, and the pulse height represents the emission intensity of the scintillation light. The electric signal output from the photomultiplier tube was shaped and amplified by a shaping amplifier in this way, and then input to a multi-wave height analyzer (multi-channel analyzer: MCA) for analysis to create a wave height distribution spectrum.

The amount of luminescence was estimated from the wave height distribution spectrum obtained by irradiating the ¹³⁷ Cs with gamma rays (662 keV).

  The light emission amount of Example 1 exceeded 40,000 photons / MeV, and the light emission amount of Comparative Example 1 was less than 40,000 photons / MeV.

  In this embodiment, a light emission amount of 40,000 photons / MeV or more is regarded as a good characteristic. In Examples 1 to 4, the amount of luminescence exceeded 40,000 photons / MeV, which was good.

Table 1 shows the values of M _La / _MGd , short wavelength side intensity / long wavelength side intensity (P ₁ / P ₂ ) of the crystals of Example 1 to Example 4 and Comparative Example 1 to Comparative Example 3. 6 is a table summarizing values of relative light emission amounts based on Comparative Example 1. As shown in Table _1, P 1 / _{P 2} is 0.97 or more, and preferably one or more, more preferably in the case of 1.04 to 1.11, can be confirmed that favorable results.

Next, the scintillation light decay time of the crystals of Examples 1 to 4 was determined. Here, the crystal was optically bonded to the photomultiplier tube with the optical grease, and was excited and emitted by irradiating the gamma ray with the ¹³⁷ Cs gamma ray. Then, the time distribution of the signal from the photomultiplier tube was measured with an oscilloscope (Tektronix TDS 3034B) to determine the decay time. Here, 1000 ns or less is good, and 80 ns or less is particularly good.

  FIG. 9 is a graph showing a fluorescence decay curve profile of the crystal of Example 2. In FIG. 9, the horizontal axis represents time, and the vertical axis represents the voltage corresponding to the emission intensity. A line α is an actual measurement, and a line β is a result of fitting with the following function I (t) using the time t as a variable in order to obtain an attenuation constant (fluorescence lifetime).

I (t (ns)) = A 1 × exp (-t / τ 1 (ns)) + A 2 × exp (-t / τ 2 (ns)) + c

Here, A ₁ , τ ₁ , A ₂ , τ ₂ and c are fitting parameters, and in particular, τ ₁ and τ ₂ are called fluorescence lifetimes in the fluorescence decay curve. When the measurement result of Example 1 was fitted, the high-speed component fluorescence lifetime τ ₁ of the crystal was 75 ns , and a high-speed scintillator could be configured. Moreover, it was 80 ns or less in all Examples of Example 1 and Example 4, and was particularly favorable.

From the above, when the ratio M _La / M _Gd of the ratio of the molar fraction M _{La of La} to the molar fraction M _Gd of _Gd is 0.1, the melt is decomposed and transparent It becomes difficult to grow a crystal. Actually, the crystal of Comparative Example 3 was cloudy and fragile, and easily cracked, and could not be optically evaluated.

Within a range where the ratio ratio M _La / M _Gd is greater than 0.1 and less than or equal to 0.64, the scintillation light derived from the rare earth metal element emitting 5d-4f transition is within the wavelength range of 200 nm to 500 nm. Observed, the intensity distribution of the scintillation light for that wavelength showed two peaks. Among these peaks, by increasing the intensity at the short wavelength side peak compared to the intensity at the long wavelength side peak, it was possible to obtain a good light emission amount characteristic.

FIG. 7 is a graph plotting the relationship between the ratio M _La / M _Gd of the ratio of the molar fraction of La and Gd and the relative luminescence amount based on Comparative Example 1 for the results of Example 1 and Comparative Examples 1 and 2. It is. According to FIG. 7, the relative light emission amount decreases as the ratio ratio M _La / M _Gd increases. For the ratio _M La _{/ M Gd} ratio, the value (about 0.79) when more than relative light ratio _M La _{/ M Gd} ratio of Comparative Example 1 to be significantly reduced read from FIG.

FIG. 8 shows the ratio M _La / M _Gd of the ratio of the molar fraction of La and Gd, and the peak with the shortest emission wavelength for the peak with the highest emission wavelength intensity and the peak with the highest emission wavelength intensity next to the highest peak. relationship eXAMPLE ratio P _{1 /} P ₂ of the intensity P ₁ and the intensity P ₂ of the long peak emission wavelengths 1, example 4 is a graph plotting the results of Comparative example 1 and Comparative example 2. As shown in FIG. 8, when the ratio M _La / M _Gd is 0.64 or less, the ratio P ₁ / P ₂ between the peak intensity P ₁ of the short emission wavelength and the peak intensity P ₂ of the long emission wavelength is 0. It turns out that it becomes more than 97.

In addition, when the ratio ratio M _La / M _Gd is 0.27 or more and 0.64 or less, it is possible to grow a transparent crystal with stable and few distortion in the crystal lattice, and As a result, good light emission characteristics were obtained.

  As described above, the scintillator crystal material and the single crystal scintillator according to the present invention are useful because they have a high light emission amount, are easy to grow a single crystal, and are suitable for a production method for industrial use.

Claims (6)

A scintillator crystal material that emits scintillation light when irradiated with radiation,
The scintillator crystal material is a pyrochlore crystal containing La and Gd,
Including a rare earth metal element that emits 5d-4f transition light as an optical activator;
The relationship between the molar fraction M _{La of La} and the molar fraction M _Gd of _Gd satisfies 0.64 ≧ M _La / M _Gd ≧ 0.15,
The emission wavelength of the scintillation light is 200 nm or more and 500 nm or less,
The distribution of the emission wavelength intensity of the scintillation light with respect to the emission wavelength shows at least two peaks,
Among the highest peak of the emission wavelength intensity and the second highest peak of the emission wavelength intensity, the emission wavelength intensity of the shorter peak of the emission wavelength is P ₁ and the emission wavelength of the longer peak of the emission wavelength is P _1. A scintillator crystal material characterized by satisfying the relationship of P ₁ / P ₂ ≧ 0.97 when the strength is P ₂ . The scintillator crystal material according to claim 1, wherein an optical activator of the scintillator crystal material is Ce, and satisfies 0.64 ≧ M _La / M _Gd ≧ 0.27.   A single crystal scintillator comprising the scintillator crystal material according to claim 1. A scintillator composed of the scintillator crystal material according to claim 1 or 2 or the single crystal scintillator according to claim 3;
A radiation detector comprising: a photodetector that receives scintillation light from the scintillator.   An imaging apparatus comprising the radiation detector according to claim 4.   A nondestructive inspection apparatus comprising the radiation detector according to claim 4.

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