A method for improving the visible-band radiation damage resistance of calcium fluoride crystals through trace element doping

Abstract

This invention belongs to the field of calcium fluoride crystal technology, specifically relating to a method for improving the radiation damage resistance of calcium fluoride crystals in the visible light band through trace element doping. To address the radiation damage resistance characteristics of calcium fluoride crystals in specific application scenarios, this invention controls the content of high-valence variable-valence impurity ions capable of competing for F-vacancy formation in the calcium fluoride crystal. This ensures that, upon exposure to high-energy radiation, these high-valence variable-valence impurity ions preferentially capture electrons compared to F-vacancy sites, thereby reducing or suppressing the formation of F-centers, ultimately improving the radiation damage resistance of calcium fluoride crystals in the visible light band. This invention utilizes the introduction of trace amounts of variable-valence ions (Yb). 3+ The radiation damage resistance of calcium fluoride crystals was significantly improved.

Description

Technical Field

[0001] This invention belongs to the field of calcium fluoride crystal technology, specifically relating to a method for improving the visible-band radiation damage resistance of calcium fluoride crystals through trace element doping. Background Technology

[0002] Space cameras are key components of deep space probes, playing a crucial role in major aerospace scientific and technological activities such as lunar exploration, deep space exploration, and high-orbit satellite navigation and positioning. Space optical cameras typically employ transmission or folding optical systems, which consist of a combination of positive and negative lenses made from optical materials with different refractive indices and dispersions. High-quality, high-performance optical systems are fundamental to high-precision deep space probes. Calcium fluoride single crystal is an excellent optical window material, exhibiting very high transmittance over a wide range. Furthermore, it possesses good thermomechanical properties, stable physicochemical properties, low refractive index, and low dispersion characteristics, making it an irreplaceable achromatic and apochromatic optical component. Due to these advantages, calcium fluoride crystals play a vital role in space camera optical systems.

[0003] When space cameras operate in the space environment, their lenses are inevitably exposed to electromagnetic radiation from ultraviolet rays, X-rays, gamma rays, and high-energy particles such as electrons and protons. Under the influence of high-energy electromagnetic radiation, calcium fluoride single crystals may form color center defects, resulting in absorption peaks in the visible light spectrum and causing lens tinting and reduced transmittance. This, in turn, reduces the imaging quality of the optical system, and excessive cumulative space radiation dose can ultimately render the optical system unusable. This will significantly affect the imaging accuracy, data reliability, and lifespan of the detector's optical system. Since gamma rays have strong penetrating power, gamma-ray irradiation-induced damage to CaF2 crystals is a key factor affecting their operational reliability.

[0004] Due to factors such as growth process, impurities in raw materials, and surface finishing, various defects inevitably occur on the surface, subsurface, and interior of CaF2 crystals. Studies generally agree that these defects are the primary cause of radiation damage.

[0005] Defects in CaF2 crystals can be categorized by size, from smallest to largest, into point defects, line defects, surface defects, and volume defects. The presence of defects in CaF2 crystals inevitably introduces absorption or scattering centers, severely affecting optical properties such as transmittance, stress birefringence, and optical homogeneity. Light absorption centers easily form at crystal defects, thus reducing the crystal's damage resistance. Therefore, the main way to improve the radiation damage resistance of CaF2 crystals is to reduce crystal defects. Specific approaches include: reducing impurity content through raw material purification, optimizing crystal growth and annealing processes to reduce inclusions and dislocation density, and optimizing processing techniques to reduce surface and subsurface defects. Summary of the Invention

[0006] To address the radiation damage resistance of calcium fluoride crystals in specific application scenarios, the inventors, starting from the damage mechanism and based on the operating wavelength, proposed doping with specific ions to introduce point defects to improve the radiation damage resistance of calcium fluoride crystals in the visible wavelength range.

[0007] Specifically, the present invention provides a method for changing the valence of Yb through trace doping. 3+ A method for improving the radiation damage resistance of calcium fluoride crystals in the visible light band involves controlling the content of high-valence variable impurity ions that can compete for F vacancies in the calcium fluoride crystal. This allows the high-valence variable impurity ions to preferentially capture electrons compared to F vacancies when exposed to high-energy radiation, thereby reducing or inhibiting the formation of F centers and ultimately improving the radiation damage resistance of calcium fluoride crystals in the visible light band.

[0008] Preferably, the high-valence variable impurity ion capable of competing for F-vacancy formation is Yb. 3+ ion;

[0009] Preferably, the content of the high-valence variable impurity ion capable of forming a competition mechanism for F vacancies in the calcium fluoride crystal is no more than 10 ppm, more preferably 2 to 7 ppm.

[0010] Damage mechanism analysis reveals that under high-energy radiation irradiation (such as gamma-ray irradiation), electrons in the valence band of CaF2 crystals are ionized due to their higher energy than the band width, forming electron-hole pairs. The diffusion of these electron-hole pairs causes lattice distortion, further relaxing to form self-trapped excitons (STEs). If the crystal is perfect, there are no defects to trap electrons, and they will rapidly recombine with holes. This recombination fades as STE luminescence, and no absorption band forms in the crystal. From a damage mechanism perspective, since fluorides often require long-term crystal growth in a vacuum environment, F vacancies inevitably exist. These F vacancies trap electrons to form F centers, which is the origin of damage. In particular, certain impurity ions further reduce the color center formation energy, significantly reducing the material's resistance to radiation damage. Therefore, to improve the damage characteristics of CaF2 crystals, considering technological capabilities and manufacturing costs, the impurity content is generally controlled to be as low as possible.

[0011] The inventors started from the origin of damage, namely the formation of F-centers by capturing electrons in F vacancies. By introducing high-valence variable impurity ions, the variable valence ions and F vacancies form a competitive mechanism. When exposed to high-energy radiation, the variable valence ions capture electrons first, thereby reducing or inhibiting the formation of F-centers, and ultimately improving the radiation damage resistance of CaF2 crystals.

[0012] In selecting variable-valence ions, functional considerations mainly focus on the application wavelength of the crystal, such as the position of the characteristic absorption peaks of the variable-valence ions (before and after electron binding) in the matrix. In terms of crystal growth technology, the feasibility of doping is mainly considered from the perspective of lattice distortion caused by the dopant ions.

[0013] This invention primarily focuses on the application of CaF2 crystals in the visible light band. Application scenarios include components for optical systems in aerospace cameras, and as protective materials for optical windows used in nuclear radiation environments, particularly in the atomic energy industry, high-energy physics, and nuclear radioactivity fields. 3+ Ce 4+ Cr 3+ Tm 3+ Ho 3+ Eu 3+ Dy 3+ It is a relatively common variable-valence ion that easily enters the CaF2 lattice, but Sm 3+ / Sm 2+ Cr 3+ / Cr 2+ Tm 3+ / Tm 3+ Ho 3+ / Ho 2+ Eu 3+ / Eu 2+Dy 3+ / Dy 2+ Plasma ions exhibit absorption in the visible light band, and their coloration affects the transmittance of the visible light spectrum; while Ce ions themselves do not have an absorption peak in the visible light band, Ce... 4+ Ce formed by trapped electrons 3+ The F-center in the crystal forms a stable color center through orbital hybridization. Therefore, these ions are all harmful impurities after entering the CaF2 crystal lattice, which will degrade its optical performance in the visible band.

[0014] The inventors have creatively proposed selecting Yb 3+ As a dopant ion, on the one hand, based on Yb 3+ It easily enters the CaF2 crystal lattice site; more importantly: Yb 3+ and Yb 2+ The spectral absorption bands of the ions in CaF2 crystal are located at 230 nm, 260 nm, 274 nm, and 365 nm, respectively, i.e., Yb 3+ Yb 2+ The ions have no absorption peaks in the visible light region, therefore they do not affect the transmittance of CaF2 in the visible band. (Containing Yb) 3+ When CaF2 crystals are subjected to high-energy radiation, high-speed electrons readily react with Yb. 3+ Yb is formed by ionic interactions 2+ Ions, namely Yb 3+ +e→Yb 2+ This reduces the probability of F vacancies trapping electrons, thereby improving the material's resistance to radiation damage in the visible light band.

[0015] As a high-precision optical component for space cameras, CaF2 crystals have extremely stringent requirements for optical properties such as transmittance and optical uniformity. The introduction of excessive impurities increases lattice distortion and affects the material's optical performance. Furthermore, Yb 2+ The ion has an absorption peak at 365 nm; excessive doping will reduce transmittance by around 400 nm, thus affecting its application in the visible wavelength range. Meanwhile, Yb... 3+ The doping amount must be sufficient to improve radiation resistance. This invention uses Yb... 3+ The doping level is extremely low, within 10 ppm, preferably 3 to 8 ppm.

[0016] The better one is Yb 3+ The methods of introducing ions include: solid-phase mixing during ball milling or liquid-phase introduction during powder synthesis;

[0017] The solid phase incorporated during ball milling includes: according to the chemical formula CaF2:xYb 3+CaF2 powder and YbF3 powder were weighed as raw materials and ball-milled and mixed. Crystal growth was carried out using the crucible descent method or the temperature gradient method to finally obtain calcium fluoride crystals with improved radiation resistance in the visible light band; wherein 0 < x ≤ 10 ppm.

[0018] Preferably, the purity of the CaF2 powder is ≥99.99%; the purity of the YF3 powder is ≥99.99%; and the purity of the PbF2 powder is ≥99.99%.

[0019] Preferably, PbF2 powder is added to the raw material powder as an oxygen scavenger, and the amount of PbF2 powder added is 0 to 5 wt.% of CaF2 powder, preferably 0.5 to 3.0 wt.%.

[0020] Preferably, the mixing is ball milling; the ball milling parameters include 80-160 rpm and a time of 12-24 hours.

[0021] Preferably, the parameters for the crucible lowering method include: after loading the furnace, evacuating to a vacuum level better than 5 × 10⁻⁶. -3 Pa;

[0022] Then, the temperature is raised to 200℃~300℃ and held for 5~10 hours to remove moisture from the raw materials; the temperature is then raised to 1400~1450℃ at a rate of 20~50℃ / h and held for 10 hours.

[0023] After the material is prepared, the crucible is slowly lowered to grow crystals, with the lowering speed controlled at 0.5–1.0 mm / h.

[0024] After crystal growth is completed, the temperature is lowered to room temperature at a rate of 20–50 °C / h; the crucible material used in the crucible lowering method is high-purity graphite with a purity of at least 99.5%; the crystal growth is carried out in a high vacuum atmosphere.

[0025] Alternatively, the parameters for the temperature gradient method may include: loading the mixed raw materials into a crucible and placing it into a temperature gradient crystal growth apparatus equipped with a high-vacuum sealed furnace cavity; the specific location varies depending on the temperature distribution design of the crystal growth apparatus. During the growth process, a high vacuum is maintained within the cavity, preferably better than 5 × 10⁻⁶. -3 Pa; The crucible is heated to a melting temperature 10-50°C higher than the melting point of the raw material at a heating rate of 20-80°C / h. During this process, the temperature is kept constant at 150-300°C for 5-30 hours to dry the moisture in the raw material. After keeping the melting temperature constant for another 5-30 hours, the temperature is lowered by 100-200°C at a cooling rate of 0.1-2°C / h to complete the crystallization process. The temperature is then increased to 900-1100°C at a heating rate of 20-50°C / h, and then kept constant for 20-60 hours. Finally, the temperature is lowered to room temperature at a cooling rate of less than 20°C / h.

[0026] Secondly, the present invention provides a Yb 3+ Doped calcium fluoride crystals, the Yb 3+ The chemical formula of the doped calcium fluoride crystal is CaF2:xYb 3+ Where 0 < x ≤ 10 ppm; preferably 2 ppm ≤ x ≤ 7 ppm.

[0027] Preferably, the Yb 3+ The doped calcium fluoride crystals did not exhibit absorption peaks in the 400–800 nm wavelength range under a 1000 Gy irradiation dose; the irradiation source for the 1000 Gy irradiation dose was X-rays or gamma rays. This means that the incorporation of trace Yb ​​ions significantly improved the radiation resistance of CaF2 crystals in the visible light band.

[0028] Thirdly, the present invention provides a Yb 3+ A method for preparing doped calcium fluoride crystals includes: according to the chemical formula CaF2:xYb 3+ CaF2 powder and YbF3 powder were weighed and mixed as raw materials. Yb was grown using either the crucible lowering method or the temperature gradient method. 3+ Doped calcium fluoride crystals; wherein 0 < x ≤ 10 ppm; preferably 2 ppm ≤ x ≤ 7 ppm.

[0029] Preferably, the mixing is ball milling; the ball milling parameters include 80-160 rpm and a time of 12-24 hours.

[0030] Preferably, the parameters for the crucible lowering method include: after loading the furnace, evacuating to a vacuum level better than 5 × 10⁻⁶. -3 Pa;

[0031] Then, the temperature is raised to 200℃~300℃ and held for 5~10 hours to remove moisture from the raw materials; the temperature is then raised to 1400~1450℃ at a rate of 20~50℃ / h and held for 10 hours.

[0032] After the material is prepared, the crucible is slowly lowered to grow crystals, with the lowering speed controlled at 0.5–1.0 mm / h.

[0033] After crystal growth is completed, the temperature is lowered to room temperature at a rate of 20–50 °C / h; the crucible material used in the crucible lowering method is high-purity graphite with a purity of at least 99.5%; the crystal growth is carried out in a high vacuum atmosphere.

[0034] Alternatively, the parameters for the temperature gradient method may include: loading the mixed raw materials into a crucible and placing it into a temperature gradient crystal growth apparatus equipped with a high-vacuum sealed furnace cavity; the specific location varies depending on the temperature distribution design of the crystal growth apparatus. During the growth process, a high vacuum is maintained within the cavity, preferably better than 5 × 10⁻⁶.-3 Pa; The crucible is heated to a melting temperature 10-50°C higher than the melting point of the raw material at a heating rate of 20-80°C / h. During this process, the temperature is kept constant at 150-300°C for 5-30 hours to dry the moisture in the raw material. After keeping the melting temperature constant for another 5-30 hours, the temperature is lowered by 100-200°C at a cooling rate of 0.1-2°C / h to complete the crystallization process. The temperature is then increased to 900-1100°C at a heating rate of 20-50°C / h, and then kept constant for 20-60 hours. Finally, the temperature is lowered to room temperature at a cooling rate of less than 20°C / h.

[0035] Fourthly, the present invention provides a Yb 3+ Applications of doped calcium fluoride crystals in the aerospace field: used as components in optical systems for space cameras.

[0036] Fifthly, the present invention provides a Yb 3+ Doped calcium fluoride crystals have applications in the atomic energy industry, high-energy physics, and nuclear radioactivity fields, serving as protective materials for observation optical windows in nuclear radiation environments or as key components in imaging optical systems.

[0037] Beneficial effects

[0038] The method described in this invention utilizes the artificial introduction of trace amounts of variable-valence ions Yb 3+ The radiation damage resistance of calcium fluoride crystals has been significantly improved. The radiation source is high-energy rays or particles such as gamma rays. The main application of calcium fluoride crystals is as lens elements in the optical system of space cameras. They can also be used in the nuclear energy industry, high-energy physics and nuclear radioactivity as protective materials for observation optical windows or imaging optical elements in nuclear radiation environments. Attached Figure Description

[0039] Figure 1 The images show the pure CaF2 crystals prepared in Example 1 before and after γ-ray irradiation. The irradiation dose was 300 Gy. It can be seen that the crystals were colorless and transparent before irradiation, but became colored after irradiation.

[0040] Figure 2 The absorption spectra of Sample 1 (pure CaF2 crystal) and Sample 2 (Yb doping concentration of 3 ppm) before and after irradiation with different doses of gamma rays are shown (the horizontal axis represents wavelength, and the vertical axis represents absorption coefficient). Before irradiation, neither crystal showed a significant absorption peak. After irradiation, the pure CaF2 crystal exhibited color center absorption peaks. These absorption peaks are mainly due to the formation of F centers and aggregates by electrons trapped by F vacancies. In contrast, the Yb-doped CaF2 crystal showed no significant absorption peaks after irradiation, indicating that doping the crystal with Yb ions can significantly improve its irradiation hardness in the visible light band.

[0041] Figure 3 The EPR spectra of the irradiated samples from Examples 1-2 are shown (the horizontal axis represents the magnetic field strength (Gauss), and the vertical axis represents the relative loudness (signal amplitude)). The EPR spectrum of the crystal after irradiation in Example 1 shows an intensity proportional to (1+x). 6 The seven spectral lines of the second expansion coefficient are mainly caused by the hyperfine interaction between F atoms and F centers in the calcium fluoride crystal, indicating that the color centers in the crystal are mainly caused by the presence of vacant electron trapping. However, the EPR spectrum of the crystal in Example 2 after irradiation showed no obvious signal peaks, indicating that the irradiated crystal did not have color centers.

[0042] Figure 4 The absorption spectra of the samples prepared in Examples 3-5 before and after irradiation with 100 Gy γ rays are shown (the horizontal axis is wavelength and the vertical axis is absorption coefficient). The Yb doping amounts are 2 ppm, 5 ppm and 7 ppm, respectively. No absorption peaks were observed in the visible band after irradiation.

[0043] Figure 5 The transmittance spectrum of the sample prepared for Comparative Example 1 before irradiation (the horizontal axis is wavelength and the vertical axis is in-line transmittance). Detailed Implementation

[0044] The present invention will be further illustrated by the following embodiments. It should be understood that the following embodiments are for illustrative purposes only and are not intended to limit the present invention.

[0045] The following exemplifies the method for improving the hardness of calcium fluoride crystals against gamma-ray irradiation according to the present invention. It can also be understood as a method for improving the resistance to antibody damage of calcium fluoride crystals under gamma-ray irradiation, using the visible wavelength range of 400–800 nm.

[0046] This invention compares the absorption spectra of damaged calcium fluoride crystals by examining the changes in the absorption spectra of calcium fluoride crystals containing Yb ions and those without. Specifically, the CaF2 crystal doped with trace amounts of Yb ions showed no significant absorption peak in the visible wavelength range after irradiation with 1000 Gy.

[0047] Taking the crucible lowering method as an example, the steps for growing calcium fluoride crystals are as follows:

[0048] Ingredients: Raw materials after impurity ions have been removed using 5N grade chemicals, with 2wt% PbF2 added as an oxygen scavenger.

[0049] Mixing: Place the weighed raw materials into the mixing bucket and stir on the mixer for 24 hours to make them evenly mixed.

[0050] Crystal growth: After loading the furnace, evacuate to a vacuum level greater than 5 × 10⁻⁶. -3 Pa, then the temperature is raised to 200℃ and held for 10 hours to remove moisture and air from the raw materials; the temperature is then raised to 800℃ at a rate of 20-50℃ / h and held for 15 hours; then the temperature is further raised to 1400-1450℃ and held for 10 hours. After the melting process is complete, the crucible is slowly lowered to grow crystals at a rate of 0.1-0.5 mm / h. After crystal growth is complete, the temperature is lowered to room temperature at a rate of 20-50℃ / h.

[0051] The steps for growing calcium fluoride crystals by doping with trace amounts of Yb are as follows:

[0052] Ingredients: The raw materials are 5N grade pharmaceutical products after removing cationic impurities. 2-10 ppm YbF3 and 2 wt% PbF2 are added as oxygen scavengers according to the composition design.

[0053] Mixing: Place the weighed raw materials into the mixing bucket and stir on the mixer for 24 hours to make them evenly mixed.

[0054] Crystal growth: After loading the furnace, evacuate to a vacuum level greater than 5 × 10⁻⁶. -3 Pa, then the temperature is raised to 200℃ and held for 10 hours to remove moisture from the raw materials; the temperature is then raised to 800℃ at a rate of 20–50℃ / h and held for 15 hours; then the temperature is further raised to 1400–1450℃ and held for 10 hours. After the melting process is complete, the crucible is slowly lowered to grow crystals at a rate of 0.1–0.5 mm / h. After crystal growth is complete, the temperature is lowered to room temperature at a rate of 20–50℃ / h.

[0055] In summary, introducing variable-valence ions (Yb) into the crystal can significantly improve its radiation resistance. After irradiation with 1000 Gy of gamma rays under vacuum, no significant damage was observed.

[0056] The grown crystal was processed to obtain a size of 15×15×10mm. 3 Calcium fluoride crystals. The crystals undergo ultra-precision polishing using an energy of 1.24 MeV. 60 Co-rays were used to conduct irradiation damage tests on crystals in a sealed vacuum tube.

[0057] The following examples further illustrate the present invention in detail. It should also be understood that the following examples are only for further explanation of the present invention and should not be construed as limiting the scope of protection of the present invention. Non-essential improvements and adjustments made by those skilled in the art based on the above description of the present invention are all within the scope of protection of the present invention. The specific process parameters, etc., in the following examples are merely examples within a suitable range; that is, those skilled in the art can make appropriate selections within the range based on the description herein, and are not intended to be limited to the specific values ​​in the examples below. In the following examples, calcium fluoride crystals containing Yb and those without Yb are prepared respectively. The crystals are grown using the crucible lowering method.

[0058] Example 1

[0059] The calcium fluoride raw material was purchased commercially;

[0060] CaF2 single crystals were grown using the crucible lowering method. A graphite crucible was selected, and a CaF2 single crystal rod with its end face normal aligned to

[111] was placed at the bottom of the crucible using an X-ray diffractometer. Crystal growth was carried out in a high vacuum atmosphere. Specifically:

[0061] Ingredients: The pharmaceutical raw material is CaF2 with a purity of 5N, and 2wt% PbF2 is added as an oxygen scavenger.

[0062] Mixing: Place the weighed raw materials into the mixing bucket and stir on the mixer for 24 hours to make them evenly mixed;

[0063] Crystal growth: After loading the furnace, evacuate to a vacuum level greater than 5 × 10⁻⁶. -3 Pa, then the temperature is raised to 200℃ and held for 10 hours to remove moisture and air from the raw materials; the temperature is then raised to 800℃ at a rate of 20–50℃ / h and held for 15 hours; then the temperature is further raised to 1400–1450℃ and held for 10 hours. After the melting process is complete, the crucible is slowly lowered to grow crystals at a rate of 0.1–0.5 mm / h. After crystal growth is complete, the temperature is lowered to room temperature at a rate of 20–50℃ / h.

[0064] Example 2

[0065] The crystal growth process is basically the same as in Example 1, except that 3 ppm of YbF3 is added to the crystal growth raw material. CaF2 single crystals are grown using the crucible lowering method. A graphite crucible is selected, and a CaF2 single crystal rod with the normal direction of its end face aligned to

[111] by an X-ray diffractometer is placed at the bottom of the crucible. Crystal growth is carried out in a high vacuum atmosphere. Specifically:

[0066] Ingredients: Pharmaceutical raw materials with a purity of 5N CaF2 and YbF3 are used, with 2wt.% PbF2 added as an oxygen scavenger;

[0067] Mixing: Place the weighed raw materials into the mixing bucket and stir on the mixer for 24 hours to make them evenly mixed;

[0068] Crystal growth: After loading the furnace, evacuate to a vacuum level greater than 5 × 10⁻⁶. -3 Pa, then the temperature is raised to 200℃ and held for 10 hours to remove moisture and air from the raw materials; the temperature is then raised to 800℃ at a rate of 20–50℃ / h and held for 15 hours; then the temperature is further raised to 1400–1450℃ and held for 10 hours. After the melting process is complete, the crucible is slowly lowered to grow crystals at a rate of 0.1–0.5 mm / h. After crystal growth is complete, the temperature is lowered to room temperature at a rate of 20–50℃ / h.

[0069] Example 3

[0070] The Yb doping concentration was 2 ppm, and other parameters were as described in Example 2.

[0071] Example 4

[0072] The Yb doping concentration was 5 ppm, and other parameters were as described in Example 2.

[0073] Example 5

[0074] The Yb doping concentration was 7 ppm, and other parameters were as described in Example 2.

[0075] Comparative Example 1

[0076] The Yb doping concentration was 15 ppm, and other parameters were as described in Example 2.

[0077] The calcium fluoride crystals prepared in Examples 1 and 2 are labeled as samples 1# and 2#, respectively. The samples prepared in Examples 3, 4, and 5 are labeled as W1, W2, and W3, respectively.

[0078] Figure 1 The images show the pure calcium fluoride crystals prepared in Example 1 before and after γ-ray irradiation. The irradiation dose was 300 Gy. It can be seen that the crystals were colorless and transparent before irradiation (left image), and colored after irradiation (right image).

[0079] Figure 2 The absorption spectra of Sample 1 (pure CaF2 crystal) and Sample 2 (Yb doping concentration of 3 ppm) before and after irradiation with different doses of gamma rays are shown. Before irradiation, neither crystal showed a significant absorption peak, while after irradiation, the pure CaF2 crystal exhibited color center absorption peaks. These absorption peaks are mainly due to the formation of F centers and aggregates by electron capture at F vacancies. In contrast, the Yb-doped CaF2 crystal showed no significant absorption peaks after irradiation, indicating that doping the crystal with Yb ions can significantly improve its irradiation hardness in the visible light band.

[0080] Figure 3The images show the EPR spectra of the irradiated samples from Examples 1 and 2. The EPR spectrum of the crystal after irradiation in Example 1 shows an intensity proportional to (1+x). 6 The seven spectral lines of the second expansion coefficient are mainly caused by the hyperfine interaction between F atoms and F centers in the calcium fluoride crystal, indicating that the color centers in the crystal are mainly caused by the presence of vacant electron trapping. In contrast, the EPR spectrum of the crystal in Example 2 after irradiation showed no obvious signal peaks, indicating that the irradiated crystal did not contain color centers.

[0081] Figure 4 The absorption spectra of the samples prepared in Examples 3-5 (labeled as W1, W2 and W3 respectively) before and after irradiation with 100 Gy γ-rays are shown. The Yb doping concentrations are 2 ppm, 5 ppm and 7 ppm respectively. It can be seen that when the doping concentration is 5 ppm, the absorption peak is the lowest after irradiation. When the concentration is increased to 7 ppm, the transmittance at 400 nm will be slightly affected because the strongest absorption peak of Yb ions is located near 364 nm.

[0082] Figure 5 The sample prepared for Comparative Example 1 had a Yb doping concentration of 15 ppm. The transmittance spectrum before irradiation shows that when the doping concentration was increased to 15 ppm, the Yb ions exhibited relatively strong absorption, particularly the strong absorption at 364 nm, which would negatively impact applications near 400 nm. Therefore, the preferred doping concentration in this invention is below 10 ppm.

Claims

1. A calcium fluoride crystal having a trace amount of a dopant of a variable valence Yb 3+ The method for improving the anti-radiation damage performance of calcium fluoride crystal in the visible light band by trace doping variable valence Yb ions, characterized in that, By controlling the content of the high-valence variable-valence impurity ions capable of competing with F-vacancy formation in the calcium fluoride crystal, when subjected to high-energy radiation, the high-valence variable-valence impurity ions capable of competing with F-vacancy formation in the calcium fluoride crystal preferentially capture electrons compared to F-vacancies, thereby reducing or inhibiting the formation of F-centers, and ultimately achieving the improvement of the anti-radiation damage performance of the calcium fluoride crystal in the visible light band. The high-valence variable valence impurity ion capable of F-center formation competition mechanism is Yb 3+ Ion; The content of the high-valence variable-valence impurity ions capable of competing with F-vacancy formation in the calcium fluoride crystal is not more than 10 ppm.

2. The method of claim 1, wherein, The content of the high-valence variable-valence impurity ions capable of competing with F-vacancy formation in the calcium fluoride crystal is 2-7 ppm.

3. The method of claim 1, wherein, Yb 3+ The introduction method of ions includes: solid phase mixing during ball milling mixing; The solid phase incorporated during ball milling includes: according to the chemical formula CaF2:xYb 3+ CaF2 powder and YbF3 powder were weighed as raw materials and ball-milled together. Crystal growth was carried out using the crucible descent method or the temperature gradient method to finally obtain calcium fluoride crystals with improved radiation resistance in the visible light band; wherein 0 < x ≤ 10 ppm.

4. The method of claim 3, wherein, The purity of the CaF2 powder is ≥99.99%. PbF2 powder is added as an oxygen scavenger in the raw material powder, and the addition amount of the PbF2 powder is 0-5 wt.% of the CaF2 powder.

5. The method of claim 4, wherein, The addition amount of the PbF2 powder is 0.5-3.0 wt.% of the CaF2 powder.

6. The method of claim 3, wherein, The ball milling mixing parameters include 80-160 revolutions per minute for 12-24 hours.

7. The method of claim 3, wherein, The parameters of the crucible lowering method include: after the completion of charging, vacuumizing to better than 5 x 10 -3 Pa; Then, the material is heated and melted, first heated to 200-300°C, and kept for 5-10 hours to remove the moisture in the raw material; then heated to 1400-1450°C at a rate of 20-50°C / h, and kept for 10 hours to melt the material; After the melting is completed, the crucible is slowly lowered to grow the crystal, and the lowering speed is controlled to be 0.5-1.0 mm / h; After the crystal growth is completed, the temperature is decreased to room temperature at a rate of 20-50°C / h; The crucible material used in the crucible lowering method is high-purity graphite with a purity of at least 99.5%; and the crystal growth is carried out in a high-vacuum atmosphere; The parameters of the temperature gradient method include: the mixed raw materials are loaded into a crucible and placed in a temperature gradient method crystal growth device provided with a high vacuum sealed furnace chamber, and a high vacuum is maintained in the chamber during the growth process, with a vacuum degree better than 5×10 -3 Pa; The crucible is heated to a melting temperature of the raw material that is 10-50°C higher than the melting point of the raw material at a heating rate of 20-80°C / h, the moisture in the raw material is dried at a constant temperature of 150-300°C for 5-30 hours, and then the temperature is decreased by 100-200°C at a rate of 0.1-2°C / h to complete the crystal growth process; Then, the temperature is increased to 900-1100°C at a rate of 20-50°C / h, kept for 20-60 hours, and finally decreased to room temperature at a rate of less than 20°C / h.

8. A Yb 3+ A doped calcium fluoride crystal characterized in that, The Yb 3+ The chemical formula of the doped calcium fluoride crystal is CaF2: xYb 3+ ; wherein 0 < x ≤ 10 ppm.

9. The Yb of claim 8 3+ A doped calcium fluoride crystal characterized in that, 2 ppm≤x≤7 ppm.

10. The Yb of claim 8 3+ A doped calcium fluoride crystal characterized in that, The Yb 3+ The doped calcium fluoride crystal does not appear absorption peak in 400-800nm wave band under 1000Gy irradiation dose; the irradiation source for the 1000Gy irradiation dose is X ray or γ ray.

11. A Yb as claimed in claim 8 3+ Process for the preparation of doped calcium fluoride crystals, characterized in that, It comprises: CaF2: xYb 3+ CaF2 powder and YbF3 powder as raw material powder and mixed, and grown Yb 3+ doped calcium fluoride crystal; wherein 0 < x ≤ 10 ppm.

12. The production method according to claim 11, characterized by, 2 ppm≤x≤7 ppm.

13. The preparation method according to claim 11, characterized in that, The mixing is ball milling mixing; and the ball milling mixing parameters include 80-160 revolutions per minute for 12-24 hours.

14. The preparation method of claim 11, wherein The parameters of the crucible lowering method include: after the completion of charging, vacuumizing to better than 5 x 10 -3 Pa; Then, the material is heated and melted, first heated to 200-300°C, and kept for 5-10 hours to remove the moisture in the raw material; then heated to 1400-1450°C at a rate of 20-50°C / h, and kept for 10 hours to melt the material; After the melting is completed, the crucible is slowly lowered to grow the crystal, and the lowering speed is controlled to be 0.5-1.0 mm / h; After the crystal growth is completed, the temperature is decreased to room temperature at a rate of 20-50°C / h; The crucible material used in the crucible lowering method is high-purity graphite with a purity of at least 99.5%; and the crystal growth is carried out in a high-vacuum atmosphere; The parameters of the temperature gradient method include: the mixed raw materials are loaded into a crucible and placed into a temperature gradient method crystal growth device provided with a high vacuum sealed furnace chamber, high vacuum is maintained in the chamber during the growth process, and the vacuum degree is better than 5x10 -3 Pa; The crucible is heated to a melting temperature 10-50 DEG C higher than the melting point of the raw material at a heating rate of 20-80 DEG C / h, the water in the raw material is dried at a constant temperature of 150-300 DEG C for 5-30 h, and then the raw material is kept at the melting temperature for 5-30 h, and then the temperature is decreased at a rate of 0.1-2 DEG C / h by 100-200 DEG C to complete the crystal crystallization process; Then the temperature is increased to 900-1100 DEG C at a rate of 20-50 DEG C / h, and then kept at the temperature for 20-60 h, and finally decreased to room temperature at a rate less than 20 DEG C / h.

15. A Yb of claim 8 3+ Use of doped calcium fluoride crystals in the aerospace field, characterized by the fact that, Optical system element for space camera.

16. A Yb of claim 8 3+ Use of doped calcium fluoride crystals in the field of atomic energy industry, high-energy physics and nuclear radioactivity, characterized in that, Used as observation optical window protection material in nuclear radiation environment or as system key component to constitute imaging optical system.

Images