CZ37255U1 - A multicomponent monocrystalline scintillator - Google Patents

Description

Multicomponent monocrystalline scintillator

Field of technology

The technical solution falls into the field of technology dealing with the scintillation detection of ionizing radiation and accelerated charged particles, specifically it concerns a multi-component monocrystalline scintillator.

Current state of the art

A scintillation detector is a device that allows detecting photons of ionizing radiation or charged accelerated particles. It consists of a scintillation material and a photodetector, which are optically connected. Scintillating material captures photons of ionizing radiation or charged accelerated particles and converts their energy into a flash of light, when the greater number of photons in it are in the ultraviolet or UV region, visible or VIS region or near infrared or NIR spectral region. This light flash is then converted into an electrical signal by a photodetector, which is further processed by subsequent electronic circuits. In many applications, a fast response of the scintillation material is required in the range of tens or hundreds of nanoseconds, or even shorter, since the scintillation detector works in the mode of counting individual incoming photons or particles.

Single crystals of dielectric materials with a wide band gap (above approx. 3 eV) are often used as scintillation material, which are further doped with fast luminescence centers that efficiently convert the energy stored in the base material into the mentioned UV/VIS/NIR photons, which are emitted from the scintillator . The most common such dopant is Ce ³⁺ , whose luminescence transition 5di-4f has typical lifetimes in the range of tens of nanoseconds and thus meets the above-mentioned requirements for the speed of the scintillation response at the same time as the high quantum efficiency of its luminescence at the application temperature, most often around room temperature. In addition to the speed of the scintillation response, an important parameter of the scintillator is the so-called light yield, or LY, which is defined as the number of emitted luminescent photons per unit of absorbed energy (typ. 1 Me V) from the incoming ionizing radiation and characterizes the efficiency of the scintillator. LY is measured in a time window, typically 1 to 2 microseconds, defined by the detection electronics.

One of the most frequently used material groups for this purpose are cerium-doped A3B5O12 garnet structures, whose basic representative is Y3AI5O12, referred to as YAG:Ce. This material has been used as a scintillator since the 1980s and is also produced by the Czech company CRYTUR spol. s ro With the requirement for a better attenuation or weakening ability for penetrating ionizing gamma radiation, its structural analog, cerium-doped LU3AI5O12 (LuAG:Ce) with significantly higher density and effective atomic number and thus with an order of magnitude higher attenuation ability, was developed. However, its scintillation response contains intense slower components in the time range above about 10 ps, which are no longer registered during LY measurements, and its value is therefore lower in the case of LuAG:Ce (20000 to 22000 ph/MeV) than in the case of YAG:Ce (25000 to 30000 ph/MeV). In 2011, a so-called multicomponent garnet with the general formula (Gd,Lu,Y)3(Ga,Al)50i2 was invented, which due to the suppression of electron capture in traps in the optimized composition Gd3Ga _x A15- _x Oi2:Ce (GGAG:Ce) , where 2.5 < x < 3, reaches LY values of 50,000 to 60,000 ph/MeV and is the most effective scintillation material in the class of single-crystal oxygen compounds, see review article [1]. e.g. in medical imaging, high-tech industry or monitoring of radioactive materials in the environment, including safety techniques.

The growth of bulk single crystals of GGAG:Ce in industrial conditions is carried out from a high-temperature melt using the Czochralski method, and the largest crystals reported in the literature reach a diameter of about 10 cm with a length of the cylindrical part over 10 cm. Due to the presence of Ga in the composition of the material, it is necessary to use an iridium crucible for cultivation, which significantly affects the production

- 1 CZ 37255 UI increases the price due to the rapidly increasing price of iridium. Therefore, the effort is to modify the composition of this multicomponent garnet so that it is possible to use molybdenum or tungsten crucibles for growing from the melt, the price of which is many times lower. Melt growth in a molybdenum crucible was recently published for the composition Gd3Sc2AhOi2:Ce (GSAG:Ce) [2], but the achieved LY value of about 10000 ph/MeV is several times lower than that of GGAG:Ce.

The task of the technical solution is therefore to create such a multi-component monocrystalline scintillator with a garnet structure based on the GSAG:Ce composition, which will achieve higher values of light yield or LY than published single crystals [2], and it will be possible to grow it as a bulk single crystal from a melt in molybdenum or tungsten crucible.

[1] M. Nikl, A. Yoshikawa, Recent R&D trends in inorganic single crystal scintillator materials for radiation detection. Adv. Opt. Mater. 3, 463 to 481 (2015). doi: 10.1002/adom.201400571

[2] O. Zapadlík, J. Pejchal, R. Kučerková, A. Beitlerová, M. Niki, Composition-Engineered GSAG Garnet: Single-Crystal Host for Fast Scintillators. Crystal Growth & Design 21 (2021) 7139-7149. DOI: 10.1021/acs.cgd.lc01007

The essence of the technical solution

The set task is solved using a multi-component monocrystalline scintillator based on cerium-doped garnet, or on the basis of a garnet structure (Gd,Y)3(Sc,Al)50i2 doped with cerium according to this technical solution. The essence of the technical solution is that it corresponds to the general chemical formula Gd3-xyYxCe _y Sc2A130i2, where the stoichiometric coefficient "x" lies in the interval 0.01 < x < 1 and the stoichiometric coefficient "y" is 0.01.

The multi-component single crystal scintillator enables the scintillation detection of X-ray or gamma radiation or accelerated particles with efficiency for optimized composition significantly exceeding the standard scintillator BGO or bismuth-germanium-oxide Bi4Ge30i2 or for the known single crystals GSAG:Ce. The advantage of the multicomponent monocrystalline scintillator compared to the standard BGO scintillator is a higher light yield and also a faster scintillation response than in the case of BGO, which is about 300 ns = 1/e lifetime.

A multicomponent monocrystalline scintillator based on a multicomponent Gd-Y-Sc-Al garnet is grown with the help of cost-effective technology by growing the crystal in a reducing atmosphere using a molybdenum or tungsten crucible. Such single crystals were grown by the micro-pulling-down method from a molybdenum crucible in a reducing atmosphere and subsequently annealed at a high temperature in air.

The advantages of the multi-component monocrystalline scintillator according to this technical solution are mainly that it achieves higher values of light yield or LY than known single crystals, and it can also be grown as a bulk single crystal from a melt in a molybdenum or tungsten crucible.

Clarification of drawings

The mentioned technical solution will be explained in more detail in the following illustrations, where:

Fig. 1a shows a photograph of a grown and annealed single crystal

Gd2,79Yo,2Ce ₀ , ₀ iSc ₂ A130i2,;

-2CZ 37255 UI Fig. 1b shows a photograph of the grown and annealed Gd2,49Yo,5Ce ₀ , ₀ iSc2Al ₃ Oi2 single crystal;

Fig. 1c shows a photograph of a grown and annealed Gd2,i9Yo,8Ce ₀ , ₀ iSc2Al ₃ Oi2 single crystal;

Fig. 2 shows absorption spectra of single crystals with marked Ce ³⁺ and Gd ³⁺ transitions;

Fig. 3 shows the radioluminescence spectra of single crystals under continuous X-ray excitation; and Fig. 4 shows the _spectrally undifferentiated scintillation afterglow _of the single crystal Gd2,79Yo, _{2Ce0,0iSc2Al3Oi2} upon excitation by gamma photons of ₆₆₂ keV from the radioisotope ¹³⁷ Cs.

An example of the implementation of a technical solution

Fig. 1 shows monocrystalline rods of a multicomponent monocrystalline scintillator according to this technical solution grown by the micro-pulling-down method of Czochralski technology in a reducing atmosphere using a molybdenum crucible.

Example 1

A single crystal sample of a multicomponent monocrystalline scintillator with the composition Gd ₃ -x-yYxCe _y Sc2Al ₃ 0i2 was prepared, where x=0.2 and ay=0.01, so it is Gd2.79Yo.2Ceo.oiSc2Al ₃ Oi2, which would be prepared by the following by procedure:

The binary oxides _Gd2O3 , _Y3O3 , _Sc2O3 , _ABO3 and CeO2 with _a purity of 5N were mixed in a ratio corresponding to the above chemical formula in a total weight of about 1 g, placed in a molybdenum crucible and melted at a temperature of about 1820 °C. The melt was homogenized at this temperature for about 1 hour, then crystal growth was initiated by touching the YAG:Ce crystal seed with the melt. Growth took place in a reducing atmosphere of Ar+5%H2, which prevents oxidation of the crucible, at a rate of 0.03 mm/min. All the melt in the crucible has been used up. Cooling the crystal to room temperature took about 2 hours. The grown Gd2,79Yo,2Ceo,oiSc2Al ₃ Oi2 crystal was then annealed for 12 hours in air at 1200 °C and its final appearance is shown in Fig. 1a. A circle was cut about 0.5 cm from the end of the rod and polished on both sides to a final thickness of 1 mm, which was used for further characterization.

Example 2

A single crystal sample of a multicomponent monocrystalline scintillator with the composition Gd ₃ -x-yYxCe _y Sc2Al ₃ 0i2 was prepared, where x=0.5 and ay=0.01, so it is Gd2.49Yo.5Ceo.oiSc2Al ₃ Oi2, which would be prepared as follows by procedure:

The binary oxides _Gd2O3 , _Y3O3 , _Sc2O3 , _ABO3 and CeO2 with _a purity of 5N were mixed in a ratio corresponding to the above chemical formula in a total weight of about 1 g, placed in a molybdenum crucible and melted at a temperature of about 1820 °C. The melt was homogenized at this temperature for about 1 hour, then crystal growth was initiated by touching the YAG:Ce crystal seed with the melt. Growth took place in a reducing atmosphere of Ar+5%H2, which prevents oxidation of the crucible, at a rate of 0.03 mm/min. All the melt in the crucible has been used up. Cooling the crystal to room temperature took about 2 hours. The grown Gd2,49Yo,5Ceo,oiSc2Al ₃ Oi2 crystal was then annealed for 12 hours in air at 1200 °C and its final appearance is shown in Fig. 1b. A circle was cut about 0.5 cm from the end of the rod and polished on both sides to a final thickness of 1 mm, which was used for further characterization.

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Example 3

A single crystal sample of a multicomponent monocrystalline scintillator with the composition Gd ₃ - _x -yY _x CeySc ₂ A130i2 was prepared, where x=0.8 and ay=0.01, so it is Gd2,i9Yo.sCeo,oiSc2Al ₃ Oi2, which would be prepared by the following by procedure:

The binary oxides Gd2O ₃ , Y2O ₃ , Sc2O ₃ , ABO ₃ and CeO2 with a purity of 5N were mixed in a ratio corresponding to the above chemical formula in a total weight of approx. 1 g, placed in a molybdenum crucible and melted at a temperature of approx. 1820 °C. The melt was homogenized at this temperature for about 1 hour, then crystal growth was initiated by touching the YAG:Ce crystal seed with the melt. Growth took place in a reducing atmosphere of Ar+5%H2, which prevents oxidation of the crucible, at a rate of 0.03 mm/min. All the melt in the crucible has been used up. Cooling the crystal to room temperature took about 2 hours. The grown Gd2,i9Yo,sCeo,oiSc2Al ₃ Oi2 crystal was then annealed for 12 hours in air at 1200 °C and its final appearance is shown in Fig. 1c. A circle was cut about 0.5 cm from the end of the rod and polished on both sides to a final thickness of 1 mm, which was used for further characterization.

Monocrystalline rods have a matte surface in places due to the reducing atmosphere during crystal growth, but are transparent in the bulk, which is documented by the absorption spectra in Fig. 2, measured on 1 mm polished wheels. Absorption transitions of Ce ³⁺ and Gd ³⁺ ions are marked in the spectra. The scintillation spectra are in Fig. 3 and are dominated by the luminescence of the Ce ³⁺ center (5di-4f transition) with a maximum at 550 nm. Their intensity increases with decreasing yttrium content and reaches approx. 2.4 to 2.5 times higher amplitude compared to the standard BGO scintillator. An example of the scintillation afterglow when excited by the radioisotope 137Cs (662 Ke V) is shown in Fig. 4 for x=0.2. The practically important so-called 1/e life time is estimated from the afterglow to be about 220 ns, so more than 90% of the intensity of the scintillation output can be captured when measuring the light yield LY with a standard electronics time constant (shaping time) of 1 microsecond.

The light yield of the multicomponent single crystal scintillator samples is shown in Table 1 and reaches a value of 13970 ph/MeV for x = 0.2, which is 37% higher than for GSAG:Ce in Ref. [2] mentioned above and 1.85x higher than for the standard BGO scintillator.

Table 1 which lists the LY light yield values measured with a detection electronics time constant of 1 microsecond.

V zorek multicomponent single crystal scintillator LY (ph/MeV) x = 0.8, y = 0.01 Gd2.7 _? Y ₀ .2Ceo, _e iS€2AhOn 8450 x = 0.5, y = 0.01 Gd ₂ ^Yo, ₅ Ceo, _OÍ Sc ₂ AhOi2 11990 x - 0.2, y - 0.01 Gdi.^Yo^Ceo^rSciALOu 13970 BGO standard 7570

As part of the routine performance of the assigned task, the expert will be able to design materials for a multicomponent monocrystalline scintillator with a different composition in a defined interval of compositional values while maintaining the technical idea of this technical solution.

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Industrial applicability

The multi-component monocrystalline scintillator according to this technical solution can be used especially in optical or optoelectronic applications. In general, however, a multicomponent 5 monocrystalline scintillator can be used where a standard BGO scintillator is used.

Claims (1)

1. A multicomponent monocrystalline scintillator based on cerium-doped garnet, characterized in that 5 corresponds to the general chemical formula Gd3-xyYxCe _y Sc2A130i2, where the stoichiometric coefficient "x" lies in the interval 0.01 < x < 1, and where the stoichiometric coefficient "y" is 0.01.