CN103757702A - Method for preparing high-temperature inorganic scintillation crystal - Google Patents

Abstract

A method for preparing high-temperature inorganic scintillation crystals, using a molybdenum metal, tungsten metal crucible or a tungsten-molybdenum alloy crucible to replace the conventionally used iridium metal crucible for crystal growth; and using a reducing atmosphere to replace the conventional single nitrogen or argon crystal growth atmosphere , while ensuring that the crucible material is not oxidized, the valence state change of doped cerium ions is avoided, thereby greatly reducing the preparation cost of crystal growth and obtaining high-quality crystals.

Description

A kind of preparation method of high temperature inorganic scintillation crystal

technical field

The invention relates to the field of radiation detection materials, in particular to a preparation method of high-temperature inorganic scintillation crystals.

Background technique

Inorganic scintillation crystals are crystalline energy converters that can convert the energy of high-energy photons (X/γ-rays) or particles (protons, electrons, etc.) into easily detectable ultraviolet/visible photons. Scintillation crystals can be made into detectors, and scintillation crystal detectors have great application prospects in the fields of high-energy physics, nuclear physics, imaging nuclear medicine diagnosis (XCT, PET), geological exploration, astronomy and space physics, and safety inspection. With the rapid development of nuclear science and technology and other related technologies, its application fields are constantly expanding. Different application fields also put forward more and higher requirements for inorganic scintillators. Traditional scintillation crystal detectors such as NaI(Tl) and BGO cannot meet the special requirements of new application fields.

At present, the development trend of scintillation crystals is to carry out exploration and research on new scintillation crystals centering on the performance of high output, fast response, and high density. Growth difficulty; optimize crystal growth process, conduct engineering growth research, reduce growth cost; study the relationship between crystal defects and their scintillation performance. By reducing various defects in the crystal and improving the optical uniformity of the crystal, its scintillation performance can be improved. Cerium-doped silicate and aluminate crystals are two important types of high-temperature inorganic scintillation crystals that have attracted the attention of the industry this year.

Rare earth orthosilicate crystal Ln ₂ SiO ₅ (Ln-lanthanide ions, such as: Y, Gd, Lu) is a kind of high-temperature scintillation crystal with relatively excellent performance, represented by LSO, YSO, GSO, LYSO, etc. According to the difference in the size of lanthanide ions, Ln ₂ SiO ₅ has two different spatial structures: "monoclinic P21/c or monoclinic C2/c". In monoclinic P21/c (represented by GSO), Ln ₂ SiO ₅ forms a two-dimensional network structure connected by vertices of (OLn ₄ ) tetrahedrons in the spatial structure, and the network structure layer and the interlayer gap are composed of (SiO ₄ ) tetrahedral filling, and the oxygen coordination numbers of rare earth ions _are 7 _{and 9} respectively; (OLn ₄ ) tetrahedra share edges to form a chain connected by separated (SiO ₄ ) tetrahedra, and the oxygen coordination numbers of the rare earth ions are 6 and 7, respectively. Ln ₂ SiO ₅ crystal uses Ce ³⁺ as the active ion, and the 5d-4f transition of Ce ³⁺ is very sensitive to its surrounding lattice environment. Different crystal matrixes have great differences in scintillation performance. YSO, LSO and LYSO crystals have high light output, but YSO has low density, and LSO and LYSO are slightly radioactive; GSO has strong radiation resistance, but the conversion efficiency is slightly low, and the crystal is easy to dissociate. LSO and LYSO are scintillation crystals with excellent comprehensive performance. Compared with other scintillation crystals, they have obvious advantages: 1) High light output, up to 25,000-295,000ph/Mev, which is equivalent to 76% of NaI(Tl) and 4-4% of BGO. 5 times; 2) The decay time is short, up to 40ns, far better than 300ns of BGO, 230ns of NaI(Tl), 700ns of CsI(Tl), even compared with 30ns of CeF ₃ ; 3) has High density and high atomic number, the radiation length is equivalent to BGO, good absorption of X-rays and γ-rays, high detection efficiency, far superior to NaI(Tl), CsI(Tl) and other crystals, and the crystal size is relatively small , which is conducive to the miniaturization of the device and ultimately reduces the cost of the PET machine; 4) The dominant wavelength of light is at 420nm and is located in the sensitive area of the photomultiplier tube, which can effectively detect light pulses; 5) The hardness against radiation is high, and when the radiation dose is 10 ⁶ No damage occurred, and slight damage was shown at doses up to 10 ⁸ .

Cerium-doped aluminates are an important class of high-temperature inorganic scintillation crystals, mainly including 2 types of 4 types of crystals, namely Ce:YAG (yttrium aluminum garnet), Ce:LuAG (yttrium lutetium garnet), Ce:YAP (aluminate Yttrium), Ce:LuAP (lutetium aluminate), among which Ce:LuAG and Ce:LuAP crystals are relatively more practical. The Ce:LuAG crystal luminescence center wavelength is 550nm, which can be effectively coupled with detection equipment such as silicon photodiodes. Compared with CsI scintillation crystal, Ce:LuAG scintillation crystal has a fast decay time (about 60ns, while CsI decay time is about 300ns). Ce:LuAG crystals also have a good ability to distinguish gamma rays and alpha particles by light pulses, and Ce:LuAG scintillation crystals are non-deliquescent, high temperature resistant, and thermodynamically stable, and can be used in extreme detection environments. Ce:LuAG high-temperature scintillation crystal is mainly used in light particle detection, alpha particle detection, gamma ray detection and other fields. In addition, it can also be used in electronic detection imaging (SEM), high-resolution microscopic imaging fluorescent screen and other fields. The decay time of Ce:LuAP crystal is 18ns, which is the fastest known oxide scintillator so far, and its decay time is approximately constant in the range of 100-600K, and the quantum efficiency Q of the fluorescent center is also basically constant , its light yield is 12000ph/MeV, it has high density and high atomic number, its radiation length is equivalent to that of BGO, it has good absorption of X-rays and γ-rays, and its detection efficiency is high, which is far superior to that of NaI (TI) and CsI (T1 ) and other crystals, which are conducive to the miniaturization of devices; the temperature effect of its light output is small in the range of 100-600K, and the thermal quenching effect begins to appear after the temperature is higher than 600K, while Ce:LSO is higher than 300K. The output starts to drop significantly.

When cerium-doped silicate and aluminate crystals enter the field of civilian use, there are still three important problems. One is the high cost of preparation: the current conventional crystal growth technology must use an iridium crucible as a container and an induction heating body for crystal growth, and the price of iridium is expensive (the price per kilogram of iridium exceeds 100,000 yuan, and each crucible depends on the size The weight is generally 3 to 10 kg. At the same time, the processing cost of the iridium crucible and the iridium processing loss are very large, resulting in a high cost of crystal growth. The second is that the internal packaging defects of the crystal are serious: the melting point of LYSO and LSO exceeds 2100 ° C, The melting point of YAG and LuAP exceeds 1900°C. The conventional crystal growth technology must use an iridium crucible as a container for induction crystal growth. However, such a high growth temperature has reached the working limit of the iridium crucible, and the iridium metal in the crucible is particularly easy to be oxidized. Become impurities into the melt and form inclusions, resulting in crystals that cannot be used normally. Third, crystal scintillation performance fluctuates greatly: the luminescent center of silicate and aluminate scintillation crystals is Ce ³⁺ , which replaces matrix cations (Lu ³⁺ , Y ^3+, etc.) into the crystal, because the radius difference between the two is large, the segregation coefficient of Ce ³⁺ in the silicate crystal is low (k=0.28), resulting in extremely uneven distribution of Ce ³⁺ , and at the same time, the cerium ion in the crystal is mainly in the form of Ce ³⁺ exists, partly Ce ⁴⁺ , the growth and annealing atmosphere will affect the distribution and ratio of the two. Ce ⁴⁺ (without weakly bound 4f electrons) does not emit light by itself, but also absorbs the scintillation photons from Ce ³⁺ ions, which The existence and proportion of the crystal have a greater negative impact on the luminous efficiency and response uniformity of the crystal. Among the two sites of Ce ³⁺ (Ce ₁ and Ce ₂ ), Ce ₂ itself is due to the strong fluorescence quenching , there is almost no photon emission, which will affect the transmission of scintillation photons of Ce _1. There is competition between the two Ce lattice points for light emission, which will also reduce the energy resolution of the crystal.

Contents of the invention

In order to overcome the disadvantages of the prior art, the present invention provides a method for preparing high-temperature inorganic scintillation crystals, which can greatly reduce the cost of crystal preparation under the same conditions and suppress fluctuations in crystal scintillation performance.

The present invention is realized through the following technical means: one is to use molybdenum metal, tungsten metal crucible or tungsten-molybdenum alloy crucible to replace the conventionally used iridium gold crucible for crystal growth, thereby greatly reducing the preparation cost of crystal growth; the other is to use nitrogen or argon The inert atmosphere of gas is mixed with a reducing atmosphere of hydrogen or carbon monoxide with a concentration of 0.1%-5%, replacing the traditional nitrogen or argon crystal growth atmosphere, while ensuring that the crucible material is not oxidized, avoid doping with cerium ions The valence state transformation can suppress the fluctuation of crystal scintillation performance.

In terms of crucible material and shape design: the most commonly used heating method for the pulling method is induction heating, and the crucible itself is often a heater. The crucible material must be able to withstand the required working temperature, not pollute the melt, nor react with the growth atmosphere and surrounding insulating materials, and have good thermal shock resistance and machining performance. By changing the geometric conditions of the crucible (such as the ratio of diameter to height) and changing the relative position of the crucible in the growth device, the liquid flow conditions and temperature distribution in the melt can be changed. Commonly used crucible materials are platinum, iridium, molybdenum, graphite, silicon dioxide or other high melting point oxides. The melting point of cerium-doped silicate and aluminate crystals is very high, exceeding the use temperature of platinum, so iridium crucibles are generally used as growth vessels. The size of the crucibles used is generally barrel-shaped, and the height and diameter of the crucibles are generally the same. , For example, one of the most commonly used specifications is an outer diameter of 126mm, a height of 123mm, and a wall thickness of 3mm. The inventor used the size specifications to process the molybdenum crucible for crystal growth. The test found that the heating performance of molybdenum metal under intermediate frequency induction is different from that of the iridium crucible. Using the molybdenum crucible with the above specifications, the convection of the melt is very disordered, and it is impossible to Crystal isodiametric growth is carried out. The present invention combines the characteristics of cerium-doped silicate and aluminate melts to design a wide-mouth special-shaped crucible, which can effectively stabilize the melt convection, form a stable growth temperature field, and meet the requirements of crystal growth with equal diameters.

In terms of crystal growth atmosphere: due to the high growth temperature, the crucible material is iridium, tungsten or molybdenum metal, which will be oxidized at high temperature and must be protected by an inert atmosphere (generally high-purity nitrogen). However, since the _CeO2 component in the original raw material cannot be completely decomposed during the sintering process of the polycrystalline raw material, during the crystal growth process, the remaining trace _CeO2 component of the melt group will still decompose oxygen, which will decompose iridium, tungsten or molybdenum The metal oxidizes into impurities and enters the melt, forming inclusions, which makes the crystal unable to be used normally; at the same time, a small amount of oxygen will also make the activated ion Ce ³⁺ change into Ce ⁴⁺ , and Ce ⁴⁺ not only does not emit light itself because it has no weakly bound 4f electrons , will also absorb scintillation photons from Ce ³⁺ ions, and its existence will have a great negative impact on the luminous efficiency and response uniformity of the crystal. The present invention adopts a reducing atmosphere instead of the conventional single nitrogen or argon inert atmosphere, and avoids the valence change of doping cerium ions while ensuring that the crucible material is not oxidized. The reducing atmosphere described in the present invention is a mixed gas, that is, the inert atmosphere of nitrogen or argon is mixed with 0.1%-5% hydrogen or carbon monoxide; at the same time, in order to avoid the problems that hydrogen tanks are easy to explode and carbon monoxide is easy to poison people, the present invention It is required to mix a specific proportion of inert gas nitrogen or argon with reducing gas hydrogen or carbon monoxide in the factory in advance and then pour it into the cylinder, which can effectively avoid the personal danger caused by the use of reducing gas.

Detailed ways

Example 1: Crystal growth of cerium-doped lutetium silicate

1. Synthesis of raw materials by solid-phase raw material synthesis method

Assuming that the Ce _2x :Lu _2(1-x) SiO ₅ polycrystalline raw material doped with cerium ion concentration is x to be synthesized, the chemical cooperation reaction formula for sintering the solid phase raw material is:

2x CeO ₂ ＋(1-x)Lu ₂ O ₃ ＋SiO ₂ ＝Ce _2x :Lu _2(1-x) SiO ₅ ＋x/2O ₂ ↑

If it is planned to prepare raw materials with an active ion concentration of 0.5 mol%, then x=0.5 mol%, according to the molar ratio of 0.01:0.995:1, respectively weigh CeO ₂ , Lu ₂ O3 and SiO ₂ powder raw materials with a purity of 99.95%.

Put the three raw materials into an agate tank, and mix them on the mixer for 12 hours to ensure that the three components are evenly mixed; then add a small amount of pure water, and use hydraulic equipment to press the mixed raw materials into a 80mm in diameter and 20mm in thickness Cylindrical block of raw material. Put the raw material block into the corundum crucible, first pre-sinter in the electric oven at 200°C to remove the H ₂ O in the raw material, and then sinter in the muffle furnace, the sintering temperature is 1300°C ~ 1400°C, the sintering time for 24h. The raw material after sintering has completed the solid phase reaction of the three components to form Ce _0.01 : Lu _1.99 SiO ₅ polycrystalline raw material.

2. Crystal Growth

Adopt domestic DJL-800 type lead-up method single crystal growth furnace, 50KW thyristor intermediate frequency induction power supply heating, double platinum rhodium (Pt/Rh30-Pt/Rh10) thermocouple, British Continental 818 temperature regulator, temperature control accuracy up to ±0.1 ℃.

Typical crystal growth parameters are:

Table 1 Technical parameters of growing cerium-doped lutetium silicate crystals by pulling method

Crystal growth steps: 1) Preparatory work before crystal growth: including cleaning of the furnace, chemical materials, centering of the seed crystal and filling the furnace with a protective atmosphere; 2) Seeding, necking and shouldering: introducing the seed crystal Near the liquid surface, after a period of stability (about 10min), seeding is performed. After pulling for an hour, adjust the temperature so that the crystals gradually grow radially (shouldering); 3) Diameter control: when the crystals are shouldered to a predetermined diameter (for example ), use an appropriate amount of temperature rise to control the continued increase of the crystal diameter. In this way, the growth interface of the crystal will gradually be 1-2 mm higher than the liquid level, and the diameter control is completed; 4) Equal-diameter growth of the crystal: During the growth process, the temperature must be adjusted according to the growth situation to control the equi-diameter growth of the crystal; 5) Diameter reduction Process: When the crystal grows to a predetermined length and needs to stop growing, keep the pulling speed unchanged, and use a higher heating rate (such as 15-40°C/h) to continue pulling. After the crystal tail becomes a flat interface or a slightly convex surface, it is very It will be out of the liquid level soon, then stop pulling, and turn to the annealing stage; 6) Furnace annealing: In the high temperature zone (>1200°C), use a cooling rate of 20-50K/h. Below 1200°C, generally adopt a cooling rate of 80-100°C/h; 6) Out-of-furnace annealing: put the crystal in an Al ₂ O ₃ corundum crucible, put it in a muffle furnace, and heat it at high temperature (1200°C) in the atmosphere , Long time (12h) annealing, heating and cooling were 50K/h.

Embodiment 2: Crystal growth of cerium-doped yttrium-lutetium silicate

Assuming the synthesis of Ce _2x : Y _2y Lu _2(1-xy) SiO ₅ polycrystalline raw materials doped with cerium ion concentration x and yttrium ion concentration y, the chemical cooperation reaction formula for sintering solid phase raw materials is:

2x CeO ₂ ＋y Y ₂ O ₃ ＋(1-xy)Lu ₂ O ₃ ＋SiO ₂ ＝Ce _2x :Y _2y Lu _2(1-xy) SiO ₅ ＋x/2O ₂ ↑

If the concentration of activated ions is planned to be 0.5mol%, and the doping ratio of yttrium ions is 10mol%, then x=0.5mol%, y=10mol%, according to the molar ratio of 0.01:0.1:0.895:1, the purity is 99.95 % CeO ₂ , Y ₂ O ₃ , Lu ₂ O ₃ and SiO ₂ powder raw materials.

Subsequent parameters and steps of polycrystalline raw material sintering and crystal growth are consistent with Embodiment 1.

Embodiment 3: Crystal growth of cerium-doped lutetium aluminum garnet

Assuming the synthesis of Ce _3x : Lu _3-3x Al ₅ O ₁₂ polycrystalline raw materials with a cerium ion concentration of x, the chemical cooperation reaction formula for sintering of solid-phase raw materials is:

6x CeO ₂ ＋3(1-x)Lu ₂ O ₃ ＋5Al ₂ O ₃ ＝2Ce _3x :Lu _3-3x Al ₅ O ₁₂ ＋4.5O ₂ ↑

If the active ion concentration is planned to be 0.5mol%, then x=0.5mol%, according to the molar ratio of 0.03:2.985:5, respectively weigh CeO ₂ , Lu ₂ O ₃ and Al ₂ O ₃ powder raw materials with a purity of 99.95% .

Put the three raw materials into an agate tank, and mix them on the mixer for 12 hours to ensure that the three components are evenly mixed; then add a small amount of pure water, and use hydraulic equipment to press the mixed raw materials into a 60mm in diameter and 10mm in thickness Cylindrical block of raw material. Put the raw material block into the corundum crucible, first pre-sinter in the electric oven at 200°C to remove _H2O in the raw material, and then sinter in the muffle furnace, the sintering temperature is 1100°C ~ 1200°C, the sintering time for 8h. The sintered raw material has completed the solid-state reaction of the three components to form a Ce _3x : Lu _3-3x Al ₅ O ₁₂ polycrystalline raw material.

2. Crystal Growth

Adopt domestic DJL-400 type lead-up method single crystal growth furnace, 25KW thyristor medium frequency induction power supply heating, double platinum rhodium (Pt/Rh30-Pt/Rh10) thermocouple, British Continental 818 temperature regulator, temperature control accuracy up to ±0.1 ℃.

Typical crystal growth parameters are:

Table 2 Technical parameters of growing cerium-doped lutetium aluminum garnet crystals by pulling method

The crystal growth steps include: preparatory work before crystal growth, seeding, necking and shouldering, equal diameter control, equal diameter growth, diameter narrowing process, furnace annealing, and crystal removal.

Example 4: Crystal growth of cerium-doped lutetium aluminate

Assuming that _Cex :Lu _1-x AlO ₃ polycrystalline raw materials doped with cerium ion concentration x is synthesized, the chemical cooperation reaction formula for sintering solid-phase raw materials is:

2x CeO ₂ ＋(1-x)Lu ₂ O ₃ ＋Al ₂ O ₃ ＝2Ce _x :Lu _1-x AlO ₃ ＋0.5O ₂ ↑

If the active ion concentration is planned to be 0.5mol%, then x=0.5mol%, according to the molar ratio of 0.01:0.995:1, respectively weigh CeO ₂ , Lu ₂ O ₃ and Al ₂ O ₃ powder raw materials with a purity of 99.95% .

The subsequent parameters and steps of polycrystalline raw material sintering and crystal growth are the same as those in Embodiment 3.

Claims (5)

1. A preparation method for high-temperature inorganic scintillation crystals, characterized in that: molybdenum metal, tungsten metal crucibles or tungsten-molybdenum alloy crucibles are used to grow high-temperature non-polar scintillation crystals under reducing atmosphere conditions. 2. The method for preparing a high-temperature inorganic scintillation crystal according to claim 1, wherein the high-temperature inorganic scintillation crystal can be a silicate crystal or an aluminate crystal. 3. The method for preparing a high-temperature inorganic scintillation crystal according to claim 1, wherein the crucible is a wide-mouth special-shaped crucible. 4. The method for preparing a high-temperature inorganic scintillation crystal according to claim 1, wherein the reducing atmosphere is inert atmosphere of nitrogen or argon mixed with hydrogen gas or 0.1%-5% hydrogen or carbon monoxide. 5. The preparation method of a kind of high-temperature inorganic scintillation crystal according to claim 1, characterized in that: the reducing atmosphere gas is the inert gas nitrogen or argon in a specific proportion, which is directly mixed with hydrogen or carbon monoxide Pour into cylinders.