JP4605588B2 - Fluoride single crystal for radiation detection, method for producing the same, and radiation detector - Google Patents

Description

  The present invention relates to a novel fluoride single crystal for radiation detection and a radiation detector. In particular, the present invention mainly relates to an X-ray computed tomography (X-ray CT), a positron emission computed tomography (Positron emission computed tomography). : PET), and a fluoride single crystal for radiation detection and a radiation detector used in medical diagnostic apparatuses such as Time-of-Flight Positron Emission Tomography (TOF-PET). Moreover, the fluoride single crystal for radiation detection of the present invention can also be used as a light emitting material such as PCE.

  Conventionally, radiation has been used for medical diagnosis, industrial nondestructive inspection, and the like. For example, X-ray CT and PET have been put to practical use as medical devices. In such an apparatus using radiation, a detector for detecting radiation such as γ-rays and X-rays, for example, a scintillator is used.

  Here, the scintillator is a substance that radiates visible light or electromagnetic waves having a wavelength close to visible light by stimulation of radiation such as γ-rays or X-rays, and has a high density, a short fluorescence decay time, and radiation resistance. It is required to have excellent properties.

As such a scintillator material for PET, bismuth germanate (Bi ₄ Ge ₃ O ₁₂ single crystal (BGO)) has been conventionally used. However, cerium-doped gadolinium silicate ( A Ce: Gd ₂ SiO ₅ (Ce: GSO)) single crystal was developed and put to practical use (see Patent Document 1).

After that, various studies were conducted to obtain higher performance characteristics, and cerium-doped lutetium oxyorthosilicate crystals (Ce: Lu ₂ SiO ₅ : (Ce: LSO)) were developed. (See Patent Documents 2, 3, 4, etc.).

  However, because oxide scintillators have a high melting point, growing using the Czochralski method causes significant damage to the hot zone and increases the energy cost due to the high melting point, and there are many types of elements included in the composition. There is a problem that it is necessary to severely control the charging ratio in order to prevent the composition deviation of the growth.

Therefore, other than oxides, scintillators using BaF ₂ and CsF, which are manufactured for optical lenses and the like, are currently being applied to TOF-PET, but CsF has a problem of deliquescence. Further, CeF ₃ which has been studied conventionally has problems such as excellent fluorescence lifetime but slightly low fluorescence intensity (Non-patent Document 1).

In addition to such single crystals, various ceramic materials have been studied as scintillators, and polycrystals (ceramics) such as BaFCl: Eu, LaOBr: Tb, CsI: Tl, CaWO ₄ , and CdWO ₄ (see Patent Document 5, etc.) , (Gd, Y) ₂ O ₃ : Polycrystalline rare earth oxide (ceramics) having a cubic structure such as Eu (see Patent Document 6), rare earth oxysulfide such as Gd ₂ O ₂ S: Pr Known are polycrystals (ceramics) of products (see Patent Document 7, etc.).

Since such a ceramic scintillator material is manufactured by sintering powder, various proposals have been made regarding improvement of transparency (translucency), improvement of sinterability, and the like. For example, there is a proposal to improve the light output of the scintillator by setting the amount of impurities in phosphor ceramics such as Gd ₂ O ₂ S: Pr, particularly the content of phosphate (PO ₄ ) to 100 ppm or less (patent) Reference 8). Also, fluorides such as LiF, Li ₂ GeF ₆ , and NaBF ₄ are added to the rare earth oxysulfide powder as a sintering aid, and these mixed powders are sintered by hot isostatic pressing (HIP). A highly densified phosphor ceramic has been proposed (see Patent Document 9).

In addition, since fluoride single crystals have a high transmittance over a wide range, a small crystal field, and a negative temperature coefficient of refractive index, they have attracted great expectations as laser crystals. However, CeF ₃ , CsF and BaF For single crystals other than ₂ , almost no application studies have been made as radiation detection materials such as scintillators. On the other hand, since there are many factors that make it difficult to produce a fluoride single crystal, such as the production atmosphere, production temperature, purity of raw materials, and control of composition, it is difficult to produce a large bulk crystal. Therefore, the present inventor has developed a technique for producing a high-quality single crystal safely and easily (see Patent Document 10), and as a single crystal for laser, lithium calcium aluminum fluoride (see Patent Document 11), barium lithium fluoride. A single crystal (see Patent Document 12) is manufactured. Furthermore, it has been reported that lithium calcium aluminum fluoride has characteristics that can be used as a scintillator (see Non-Patent Document 2 and Non-Patent Document 3), but the density is as small as 2.94 g / cm ^3, and γ rays There was a problem that the absorption coefficient of was small.

Japanese Examined Patent Publication No. 62-8472 (Claims) U.S. Pat. No. 4,958,080 (I CLAIM) US Pat. No. 5,025,151 (WHAT IS CLAIMED IS) JP-A-9-118593 (Claims) Japanese Patent Publication No.59-45022 (Claims) JP 59-27283 A (Claims) JP 58-204088 A (Claims) JP-A-7-188655 (Claims) Japanese Patent Publication No. 5-017566 (Claims) Japanese Patent No. 3062753 (Claims) Japanese Patent No. 3089418 (Claims) Japanese Patent No. 3168294 (Claims) "Cerium Fluoride, A New Fast, Heavy Scintillator", W. W. Moses et al., IEEE Transactions on Nuclear Science, vol. 36, No. 1, Feb. 1989, pp. 173-176. "Scintillation decay of LiCaAlF6: Ce3 + single crystals" M.Nikl, N.Solovieva, E.Mihokova, M.Dusek, A.Vedda, M.Martini, K.Shimamura and T.Fukuda, Phys.Stat.Sol. a) 187 (2001) R1-R3. "LiCaAlF6: Ce crystal: a new scintillator" A. Gektin, N. Shiran, S. Neicheva, V. Gavrilyuk, A. Bensalah, T. Fukuda and K. Shimamura, Nuclear Instruments and Methods in Physics Research A 486 (2002) 274-277.

  In view of the circumstances described above, the present invention has an object to provide a radiation detection fluoride single crystal that can grow crystals more easily than conventional materials and has a response to radiation, and a radiation detector using the same. To do.

A first aspect of the present invention to solve the above problems is a light rare earth fluoride single crystal, Pr, containing Lu及 beauty with _{F, Pr 1-x Lu x F 3} (x ≦ 0.5) It exists in the fluoride single crystal for radiation detection characterized by being represented.

According to a second aspect of the present invention, there is provided a radiation detector comprising the scintillator made of the fluoride single crystal for radiation detection according to the first aspect and a photodetector for detecting light emitted from the scintillator. is there.
In the third aspect of the present invention, while maintaining a high vacuum of 10 ⁻⁴ to 10 ⁻⁵ torr, the powder or bulk polycrystalline fluoride raw material is heated from room temperature to a temperature not higher than the melting point of the raw material, After introducing argon gas, the temperature is raised to the melting point or higher, impurities generated on the surface of the melt or solution and impurities present in the melt or solution react with the gas to remove the impurities, and the resulting melt is obtained. The method for producing a fluoride single crystal for radiation detection according to the first aspect, characterized by growing from a liquid or a solution.

  The fluoride single crystal for radiation detection according to the present invention has the effect of being able to grow crystals more easily than conventional materials and being responsive to radiation.

The fluoride single crystal for radiation detection of the present invention is a light rare earth fluoride single crystal, and is composed of REF ₃ (RE is at least one selected from Nd and Pr). Praseodymium Pr is particularly preferable because the fluorescence wavelength is in the visible region and the higher the fluorescence intensity, the more the resolution is improved.

  Such a fluoride single crystal is useful as a radiation detection material such as a scintillator because it has a high single crystal density even in a light rare earth, has a short fluorescence decay time, and has a high fluorescence intensity.

  The light rare earth fluoride single crystal, which is a fluoride single crystal for radiation detection of the present invention, has characteristics as a scintillator that emits visible light or electromagnetic waves having a wavelength close to visible light upon stimulation of radiation such as γ-rays or X-rays. If it has, it will not specifically limit as a manufacturing method.

  The light rare earth fluoride of the present invention may be a single crystal in which a plurality of rare earth fluorides are combined, or a part of the rare earth element may be substituted with another element. By replacing a part of the rare earth element with another element, the light emission amount can be adjusted and the light emission wavelength can be shifted.

  Such other elements include rare earth elements other than RE and La, that is, cerium Ce, praseodymium Pr, neodymium Nd, promethium Pm, samarium Sm, europium Eu, gadolinium Gd, terbium Tb, dysprosium Dy, holmium Ho, erbium. Although at least one element R selected from Er, thulium Tm, ytterbium Yb, and lutetium Lu can be mentioned, Nd, Pr, Lu, Gd, Sm, Eu, Tm, Ho, Er, Tb, Dy are preferable. Can be mentioned.

Such elements can be substituted up to about 50 mol%. That is, the rare earth fluoride single crystal is represented by RE _1-x R _x F ₃ , where x is 0.5 or less, preferably 0.2 or less, and more preferably 0.01 to 0.1. When x is larger than 0.5, the characteristics of the rare earth fluoride single crystal greatly depend on the substituted element R, so that the light emission amount may be reduced and the property may be different from that of the host material. Such a substitution element is also called a dopant up to about 1 mol%.

Further, the substitution element may be lanthanum La. Since La is the same light rare earth, it is possible to replace Nd and Pr with few cracks and inclusions, and the emission amount and emission wavelength can be changed widely depending on the amount used. La can be substituted up to about 95 mol%. Rare earth fluoride single crystals containing La _is represented by _{(RE 1-x R x)} 1-y La y F 3 (0 ≦ x ≦ 0.5), y is 0.95 or less, preferably 0.7 Hereinafter, it is more preferably 0.2 to 0.5. If y is less than 0.2, the characteristics do not change significantly, and if it is greater than 0.95, the characteristics of RE and R deteriorate because it depends greatly on the added lanthanum.

  On the other hand, in order to increase the intensity of fluorescence generated when such a fluoride single crystal absorbs radiation and improve the characteristics as a scintillator, to prevent the phase transition of the fluoride single crystal, or to increase the fluorescence peak For shifting, other dopants may be added as necessary. Such a dopant is preferably added as a fluoride, but it is preferable to use a dopant having no absorption at the fluorescence wavelength. Such a dopant can be added, for example, in a molar ratio of about 0.01% to 1%. Note that when such a dopant is added, the fluorescence intensity, emission wavelength, and fluorescence lifetime may change, so it is necessary to select them appropriately in accordance with desired characteristics.

  Since the fluoride single crystal for radiation detection of the present invention is used as a scintillator for a PET or TOF-PET detector, it is necessary to obtain a high-quality and homogeneous bulk crystal. In order to obtain such a bulk crystal, it is preferable to use a production method as described below.

That is, the fluoride single crystal for radiation detection of the present invention is preferably produced by a melt growth method or a solution growth method, but in order to produce the rare earth fluoride of the present invention, it is melted under the following conditions. It is preferable to manufacture by a liquid growth method or a solution growth method. This condition is that while maintaining a high vacuum of 10 ⁻⁴ to 10 ⁻⁵ torr, the powder or bulk polycrystalline fluoride raw material is heated from room temperature to a temperature below the melting point of the raw material, for example, a temperature of 500 to 800 ° C., After introducing a fluorocarbon gas such as CF ₄ and an argon gas into the furnace (mixing ratio, fluorocarbon gas: argon gas = 100: 0 to 0: 100, volume ratio), the temperature is raised above the melting point, The impurities generated on the solution surface and the melt or impurities present in the solution are reacted with the gas to remove the impurities and grow from the obtained melt or solution.

  According to the above manufacturing method, for example, even when a fluoride raw material having a purity of about 99.9% by weight is used, it is possible to manufacture a high-quality single crystal more easily than the conventional method. The fluoride single crystal of the present invention can be prepared from a melt or solution obtained by removing impurities by a melt growth method or a solution growth method in an inert gas atmosphere such as Ar. Is possible.

  Hereinafter, the fluoride single crystal for radiation detection of the present invention will be described in more detail.

The fluoride single crystal for radiation detection of the present invention is a powder or polycrystalline rare earth fluoride raw material, that is, a matrix raw material such as praseodymium fluoride (PrF ₃ ), and neodymium fluoride (NdF ₃ ) as required. The above-mentioned fluoride is maintained while maintaining a high vacuum of about 10 ⁻⁴ to 10 ⁻⁵ torr in order to remove moisture and oxygen contained in the furnace and the raw material. The raw material is heated from room temperature to about 500 to 800 ° C., that is, a predetermined temperature not exceeding the melting point. Next, after introducing a chlorofluorocarbon gas such as CF ₄ and an argon gas into the production furnace (mixing ratio, chlorofluorocarbon gas: argon gas = 100: 0 to 0: 100, volume ratio), the temperature is raised to the melting point or higher. Then, impurities generated on the surface of the melt or solution and impurities present in the melt or solution are reacted with the fluorocarbon gas to remove the impurities. Then, a rare earth fluoride single crystal is produced from the obtained melt or solution. In addition, when adding a dopant, what is necessary is just to add the fluoride of a dopant, for example.

  The method for producing a single crystal from the melt or solution thus obtained is not particularly limited, and a pulling method, a Bridgman method, or the like may be used. For example, according to the pulling method, the temperature of the melt is kept in the vicinity of the melting point of each compound, and the seed crystal is lifted at a speed of 0.1 to 10 mm / h while rotating at 1 to 50 rpm. A transparent high-quality single crystal without a ring center is obtained. In addition, the fluoride single crystal of the present invention can be obtained only by slow cooling after melting, and if the conditions are set appropriately, the single crystal can be obtained by slow cooling without using a seed crystal. Has features.

  The rare earth fluoride single crystal thus obtained is useful as a scintillator for PET or TOF-PET.

  A scintillator obtained by cutting such a rare earth fluoride single crystal into a predetermined size is a photodetector that matches the wavelength of fluorescence generated by absorbing radiation, for example, X-rays or γ-rays, such as visible light or ultraviolet light. By combining with a photodetector such as a photomultiplier tube, a radiation detector can be obtained.

Example 1
PrF ₃ , which is a commercially available bulk pulverized raw material with a purity of 99.99%, was prepared and filled in a crucible. It was placed in a single crystal production furnace as it was, evacuated to about 10 ⁻⁴ to 10 ⁻⁵ torr, and heated as it was in a vacuum state to about 700 ° C. to remove moisture and oxygen in the furnace and raw materials. Here, CF ₄ and argon gas were introduced into the single crystal production furnace, the raw material was heated and melted in a mixed gas atmosphere, and kept in a liquid state for 1 hour. At this time, all impurities appearing on the liquid surface disappeared by reacting with CF ₄ gas. Next, the seed crystal was brought into contact with the melt, and a single crystal was grown and produced in the c-axis direction at a pulling speed of 2 mm / h and a rotation speed of 10 rpm. The produced crystal was a green transparent, high-quality praseodymium fluoride (PrF ₃ ) single crystal having a diameter of about 15 mm and a length of about 70 mm, without bubbles, cracks, and a scattering center. The density of the obtained PrF ₃ single crystal was 6.29 g / cm ³ .

(Example 2)
After preparing NdF ₃ which is a commercially available bulk pulverized raw material having a purity of 99.99% and filling this into a crucible, crystals were grown in the same manner as in Example 1. The produced crystal was a green transparent and high-quality neodymium fluoride (NdF ₃ ) single crystal having a diameter of about 25 mm and a length of about 70 mm, without bubbles, cracks, and a scattering center. The density of the obtained NdF ₃ single crystal was 6.50 g / cm ³ .

(Example 3)
PrF ₃ , which is a commercially available bulk pulverized raw material with a purity of 99.99%, was prepared and filled in a crucible. It was placed in a single crystal production furnace as it was, evacuated to about 10 ⁻⁴ to 10 ⁻⁵ torr, and heated as it was in a vacuum state to about 700 ° C. to remove moisture and oxygen in the furnace and raw materials. Here, CF ₄ gas and argon gas were introduced into the single crystal production furnace, the raw material was heated and melted in a mixed gas atmosphere, and kept in a liquid state for 1 hour. At this time, all impurities appearing on the liquid surface disappeared by reacting with CF ₄ gas. Next, the melt was gradually cooled over 24 hours to produce a green transparent and high-quality praseodymium fluoride (PrF ₃ ) single crystal without bubbles and cracks. The produced crystal was a green transparent and high-quality praseodymium fluoride (PrF ₃ ) single crystal having a diameter of about 60 mm and a length of about 15 mm, without bubbles, cracks, and a scattering center.

Example 4
PrF ₃ which is a commercially available bulk pulverized raw material with a purity of 99.99% was prepared, and this was filled in a crucible, and then NdF ₃ was filled in a 1 mol% crucible as a dopant. Nurtured.

(Example 5)
A crystal was grown in the same manner as in Example 4 except that 1 mol% of TbF ₃ was used instead of NdF ₃ as a dopant.

(Example 6)
A crystal was grown in the same manner as in Example 4 except that 1 mol% of DyF ₃ was used instead of NdF ₃ as a dopant.

(Example 7)
A crystal was grown in the same manner as in Example 4 except that 1 mol% of EuF ₃ was used instead of NdF ₃ as a dopant.

(Example 8)
A crystal was grown in the same manner as in Example 4 except that 1 mol% of ErF ₃ was used instead of NdF ₃ as a dopant.

Example 9
A crystal was grown in the same manner as in Example 4 except that 1 mol% of TmF ₃ was used instead of NdF ₃ as a dopant.

(Example 10)
A crystal was grown in the same manner as in Example 4 except that 1 mol% of HoF ₃ was used instead of NdF ₃ as a dopant.

(Example 11)
A crystal was grown in the same manner as in Example 4 except that 1 mol% of CeF ₃ was used instead of NdF ₃ as a dopant.

(Example 12)
A crystal was grown in the same manner as in Example 4 except that 1 mol% of LuF ₃ was used instead of NdF ₃ as a dopant.

(Test example)
The X-ray diffraction profiles of the PrF ₃ single crystal and the NdF ₃ single crystal of Example 1 and Example 2 are shown in FIG. As a result, it was confirmed that the single crystals of Example 1 and Example 2 were a PrF ₃ single crystal and an NdF ₃ single crystal, respectively.

The result of measuring the X-ray fluorescence spectrum of the PrF ₃ single crystal obtained in Example 1 is shown in FIG. As a result, it was found that the emission wavelength had no peak in the green-red region and a sharp peak at 400 nm.

Further, the PrF ₃ single crystal obtained in Example 1 shows the results of measurement of the fluorescence lifetime at X-ray irradiation in FIG. As a result, afterglow was not observed at a fluorescence lifetime of 510 nsec.

Furthermore, the results of comparison of X-ray fluorescence intensity of PrF ₃ single crystal obtained in Example 1 and BGO in FIG. As a result, it was found that the fluorescence intensity was slightly lower than that of BGO, but showed almost the same value.

  The result of having measured the X-ray fluorescence spectrum of the single crystal of Examples 4-10 obtained by adding 1 mol% of various dopants is shown in FIG. As a result, it was confirmed that the single crystals of Examples 4 to 10 had a fluorescence intensity lower than that of Example 1, but a peak was obtained at about 400 nm. Therefore, it was found that the light emission amount can be adjusted by adding various dopants.

  The results of measuring the X-ray fluorescence spectra of the single crystals of Examples 1, 11 and 12 are shown in FIG. In Example 11, as in Examples 4 to 10, it was confirmed that a peak near 400 nm was smaller than that in Example 1, but a new emission peak was obtained at 300 nm. Therefore, the fluorescence intensity at 400 nm decreased in Examples 4 to 10 and no new large emission peak was observed in the vicinity of the visible region, but in Example 11 using Ce as a dopant, the peak appeared at 300 nm. It was found that the amount of light emitted in the vicinity of the visible region can be increased according to the amount of Ce added. In Example 12 using Lu as a dopant, a large peak was obtained at about 400 nm, and it was confirmed that the fluorescence intensity was higher than that in Example 1.

  The fluoride single crystal for radiation detection of the present invention is mainly composed of an X-ray computed tomography (X-ray CT), a positron emission computed tomography (PET), a time of flight. Although it is used for medical diagnostic apparatuses such as a positron emission tomography apparatus (TOF-PET), it is used for various other purposes for radiation detection.

Is a diagram showing an X-ray diffraction profile of PrF ₃ single crystals of Examples 1 and 2 and NdF ₃ single crystal. ₃ is a diagram showing an X-ray fluorescence spectrum of a PrF ₃ single crystal of Example 1. FIG. Is a diagram showing the results of measurement of the fluorescence lifetime for PrF ₃ single crystal of Example 1. The X-ray fluorescence intensity of PrF ₃ single crystal of Example 1 is a diagram showing the result of comparison with BGO. It is a figure which shows the X-ray fluorescence spectrum of the single crystal of Examples 4-10 obtained by adding 1 mol% of various dopants. It is a figure which shows the X-ray fluorescence spectrum of the single crystal of Example 1, 11 and 12.

Claims (3)

A light rare earth fluoride single crystal, Pr, Lu及 beauty contain _{F, Pr 1-x Lu x} F 3 (x ≦ 0.5) in the radiation detection fluoride single, characterized by being represented crystal. A radiation detector comprising: the scintillator comprising the radiation-detecting fluoride single crystal according to claim 1; and a photodetector for detecting light emitted from the scintillator. While maintaining a high vacuum of 10 ⁻⁴ to 10 ⁻⁵ torr, the powder or bulk polycrystalline fluoride raw material is heated from room temperature to a temperature not higher than the melting point of the raw material, and after introducing the chlorofluorocarbon gas and argon gas, the temperature is increased. The temperature is raised above the melting point, the impurities generated on the surface of the melt or solution and the impurities present in the melt or solution react with the gas to remove the impurities and grow from the obtained melt or solution. A method for producing a fluoride single crystal for radiation detection according to claim 1 .

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