Thulium-doped fluoride crystal and method of making same, and solid nuclear optical clock using the thulium-doped fluoride

Abstract

This invention provides a thorium-doped fluoride crystal, its preparation method, and a solid-state nuclear optical clock using the thorium-doped fluoride. The thorium-doped fluoride crystal uses SrF2, LiF, BaF2, and LiF+BaF2 as matrix materials, respectively, and ThF4 as a dopant material. Single crystals of Th:SrF2, Th:LiF, Th:BaF2, and Th:BaLiF3 are obtained through a melt growth process according to a pre-set molar percentage of the raw materials. The thorium-doped fluoride crystal of this invention has a low melting point and simple composition, which is beneficial for achieving high-quality crystal growth. It has an extremely wide band gap, high transmittance in the vacuum ultraviolet band, and a high Th doping concentration, which helps reduce absorption loss and background luminescence in the nuclear transition emission band, and is beneficial for the excitation and detection of nuclear transition signals. It can be used as a preferred material for applications based on… 229 A solid-state nuclear optical clock based on the transition of the Th atom nucleus.

Description

Technical Field

[0001] This invention relates to the fields of optical materials and precision measurement, specifically to thorium-doped lithium fluoride crystals and their preparation methods, as well as solid-state nuclear optical clocks using thorium-doped fluoride. Background Technology

[0002] The second, a standard unit of time and frequency, is one of the seven base units of the International System of Units (SI) and boasts the highest precision. Currently, the second is defined by the frequency determined by the transition between the two hyperfine levels of the electronic ground state of a cesium atom (cesium fountain microwave clock). On the other hand, the most accurate optical clocks today use the frequency determined by the ultra-narrow electronic transitions of atoms in the optical band, achieving an accuracy of 10⁻⁶. -18 This is two orders of magnitude more accurate than the cesium fountain microwave clock, and the frequency stability of the optical clock is also more than two orders of magnitude higher. Based on the superior performance of the optical clock in terms of accuracy and frequency stability, it has great application and development prospects in fields requiring higher time accuracy, such as the measurement of gravitational waves and dark matter, and global navigation and positioning systems.

[0003] Furthermore, since the atomic nucleus radius is five orders of magnitude smaller than the electron shell radius, it is less susceptible to interference from the surrounding environment, thus eliminating many side effects that interfere with measurements. Therefore, utilizing the atomic nucleus (e.g., 229 The frequency determined by the energy level transitions of the Th atom nucleus is used as the standard definition of "second" to further improve the accuracy of the time-frequency standard. This is disclosed in Non-Patent Document 1. 229 The first metastable excited state of Th (also known as the allotropic state) 229m The energy of Th is only less than that of its ground state. 229g The Th value is 8.28±0.17eV, and the corresponding transition excitation wavelength is around 150nm, which is in the vacuum ultraviolet band. Therefore, existing technology may enable the direct excitation of nuclear transitions using narrowband vacuum ultraviolet lasers to realize the operation of nuclear optical clocks. 229 The Th nuclear level transition has the advantages of relatively narrow linewidth and small systematic error. Nuclear-optical clocks fabricated using this transition will outperform the best optical clocks currently available, improving the accuracy of time and frequency standards to 10^6. -19 .

[0004] To date, experimental research on Th atomic nuclei optical clocks has mainly focused on two approaches: one employs the same ion trap scheme as atomic optical clocks, where... 229 Th ions are cooled in an ion trap; secondly, Th ions are cooled in an ion trap. 229 Th is incorporated into vacuum ultraviolet transparent crystals to develop solid-state nuclear optical clocks. The latter has the following advantages: (1) Using crystals as the matrix material allows for high concentrations 229 Th doping is more effective than ion trapping. 229 The density of Th is more than eight orders of magnitude higher. High229 Th density is beneficial to increasing the probability of atomic nuclei being excited and the detection of radiation decay signals, while improving the stability of nuclear optical clocks (proportional to the square root of the number of atomic nuclei); (2) Small size. Compared with nuclear optical clocks using ion trap schemes, solid nuclear optical clocks do not require laser cooling. Their core components can be Th-doped crystals of centimeter or even millimeter size, which have great potential in miniaturization, chip-based, spaceborne and commercial applications.

[0005] Currently reported experimental doping 229 The matrix crystals for Th include LiSrAlF6 (Non-Patent Document 2) and CaF2 (Non-Patent Documents 3 and 4). Among them, the LiSrAlF6 matrix crystal can achieve a size of 4.1 × 10⁻⁶. 17 cm -3 of 229 With a Th doping concentration of , a CaF2 matrix crystal can achieve 8.8 × 10⁻⁶ wt%. 15 cm -3 of 229 Th doping concentration. However, in neither of these crystals, direct vacuum ultraviolet excitation was experimentally achieved, nor was any observed... 229 The optical signal released by the isomer decay transition of the Th atom nucleus. Currently reported doping... 229 The main problems with Th crystals are as follows: (1) The transmittance of light in the vicinity of 150nm is low, not exceeding 50%. 229 The light emitted during the nuclear transition of Th atoms is mostly absorbed by the crystal itself; (2) 229 Th has a low doping concentration and a small number of nucleons in crystals. Therefore, it is necessary to find new vacuum ultraviolet (VUV) transparent crystal materials as... 229 Th matrix material, satisfying the requirements for incorporation 229 After Th, it exhibits high transmittance around 150 nm and can achieve high concentrations. 229 Th doping is very important for the research and development of solid-state nuclear optical clocks.

[0006] On the other hand, fluoride crystals (such as SrF2, LiF, BaF2, and BaLiF3) have extremely wide band gaps and good VUV transmittance, and can be used as Th... 4+ Candidate materials for host crystals of ions, but currently for Th 4+ Whether ions can easily be incorporated into these host crystals, and whether Th doping will negatively affect the VUV transmittance of the crystal itself, remains questionable. Due to Sr 2+ Li + Ba 2+ ionic radius and Th 4+ The ionic radii exhibit a significant mismatch, and the ionic valence states differ considerably. Therefore, it is generally believed that Th...4+ These cation sites are not easily replaced in the crystal, or even if they are incorporated, the resulting concentration will be very low, or additional defect compensation mechanisms may be introduced due to valence state issues, ultimately causing a significant reduction in the transmittance of the crystal near 150 nm due to defect absorption.

[0007] Existing technical documents

[0008] Non-patent literature 1: Seiferle, B. et al. Energy of the 229 Th nuclear clocktransition.Nature 573,243–246(2019)

[0009] Non-Patent Literature 2: Justin Jeet, et al. Results of a Direct Search Using Synchrotron Radiation for the Low-Energy 229 Th Nuclear Isometric Transition,Phys.Rev.Lett.2015,114,253001

[0010] Non-patent literature 3: Stellmer, et al. Attempt to optically excite the nuclearisomer in 229 Th,Phys.Rev.A,2018,97,062506;

[0011] Non-patent literature 4: Simon Stellmer, et al. Radioluminescence and photoluminescence of Th:CaF2 crystals, Sci. Rep, 2015, 5, 15580 Summary of the Invention

[0012] The technical problem to be solved by the present invention

[0013] The purpose of this invention is to overcome the defects of the prior art and provide a thorium-doped fluoride crystal and its preparation method. The thorium-doped fluoride crystal can achieve a high thorium (Th) doping concentration and has high transmittance in the vacuum ultraviolet band (e.g., 150 nm). As a material for solid-state nuclear optical clocks, it is beneficial to improve the probability of nuclear energy level transition processes and the detection probability of nuclear energy level decay processes, and also beneficial to improve the stability of nuclear optical clocks.

[0014] Technical means used to solve technical problems

[0015] To achieve the above objectives, the thorium-doped fluoride crystal according to the first aspect of the present invention is prepared from a matrix material and a dopant material, wherein the matrix material is any one of the fluorides selected from LiF, SrF2, BaF2, and mixtures of LiF and BaF2, and the dopant material is ThF4.

[0016] (1) When the matrix material is LiF, the raw materials for preparing the thorium-doped fluoride crystal, i.e., Th:LiF single crystal, satisfy the following:

[0017] M(ThF4):[M(ThF4)+M(LiF)]=x1:1,

[0018] Wherein, M(ThF4) and M(LiF) are the molar percentages of ThF4 and LiF raw materials, respectively.

[0019] The value of x1 ranges from 0.001 to 0.01;

[0020] (2) When the matrix raw material is a mixture of LiF and BaF2, the raw materials for preparing the thorium-doped fluoride crystal, i.e., Th:BaLiF3 single crystal, satisfy the following:

[0021] M(ThF4):[M(ThF4)+M(LiF)+M(BaF2)]=x2:1,

[0022] Wherein, M(ThF4), M(LiF), and M(BaF2) are the molar percentages of ThF4, LiF, and BaF2 raw materials, respectively, and M(BaF2) / [M(ThF4)+M(LiF)+M(BaF2)] = 0.36~0.47.

[0023] The value of x2 ranges from 0.001 to 0.03.

[0024] (3) When the matrix material is SrF2, the raw materials for preparing the thorium-doped fluoride crystal, i.e., Th:SrF2 single crystal, satisfy the following:

[0025] M(ThF4):[M(ThF4)+M(SrF2)]=x3:1,

[0026] Wherein, M(ThF4) and M(SrF2) are the molar percentages of ThF4 and SrF2 raw materials, respectively.

[0027] The value of x3 ranges from 0.001 to 0.05;

[0028] (4) When the matrix material is BaF2, the raw materials for preparing the thorium-doped fluoride crystal, i.e., Th:BaF2 single crystal, satisfy the following:

[0029] M(ThF4):[M(ThF4)+M(BaF2)]=x4:1,

[0030] Wherein, M(ThF4) and M(BaF2) are the molar percentages of ThF4 and BaF2 raw materials, respectively.

[0031] The value of x4 ranges from 0.001 to 0.01.

[0032] Furthermore, the second aspect of the present invention relates to a thorium-doped fluoride crystal, which is the same as that in the first aspect of the present invention, wherein the dopant material is selected from... 229 ThF4 232 ThF4 and 229 ThF4 and 232 Any of the raw materials in the ThF4 mixture, wherein 229 Th、 232 Th is two isotopes of Th.

[0033] A third aspect of the present invention provides a method for preparing thorium-doped fluoride crystals, using a matrix material and a dopant material as the fluoride, wherein the matrix material is any one of LiF, SrF2, BaF2, and a mixture of LiF and BaF2, and the dopant material is ThF4.

[0034] The mixture of the matrix material and the dopant material is melted under a protective atmosphere or vacuum atmosphere, and the thorium-doped fluoride crystal is grown by melt method.

[0035] (1) When the matrix material is LiF, the raw materials for preparing the thorium-doped fluoride crystal, i.e., Th:LiF single crystal, satisfy the following:

[0036] M(ThF4):[M(ThF4)+M(LiF)]=x1:1,

[0037] Wherein, M(ThF4) and M(LiF) are the molar percentages of ThF4 and LiF raw materials, respectively.

[0038] The value of x1 ranges from 0.001 to 0.01;

[0039] (2) When the matrix raw material is a mixture of LiF and BaF2, the raw materials for preparing the thorium-doped fluoride crystal, i.e., Th:BaLiF3 single crystal, satisfy the following:

[0040] M(ThF4):[M(ThF4)+M(LiF)+M(BaF2)]=x2:1,

[0041] Wherein, M(ThF4), M(LiF), and M(BaF2) are the molar percentages of ThF4, LiF, and BaF2 raw materials, respectively, and M(BaF2) / [M(ThF4)+M(LiF)+M(BaF2)] = 0.36~0.47.

[0042] The value of x2 ranges from 0.001 to 0.03;

[0043] (3) When the matrix material is SrF2, the raw materials for preparing the thorium-doped fluoride crystal, i.e., Th:SrF2 single crystal, satisfy the following:

[0044] M(ThF4):[M(ThF4)+M(SrF2)]=x3:1,

[0045] Wherein, M(ThF4) and M(SrF2) are the molar percentages of ThF4 and SrF2 raw materials, respectively.

[0046] The value of x3 ranges from 0.001 to 0.05;

[0047] (4) When the matrix material is BaF2, the raw materials for preparing the thorium-doped fluoride crystal, i.e., Th:BaF2 single crystal, satisfy the following:

[0048] M(ThF4):[M(ThF4)+M(BaF2)]=x4:1,

[0049] Wherein, M(ThF4) and M(BaF2) are the molar percentages of ThF4 and BaF2 raw materials, respectively.

[0050] The value of x4 ranges from 0.001 to 0.01.

[0051] Furthermore, the method for preparing thorium-doped fluoride crystals according to the fourth aspect of the present invention is based on the method described in the third aspect of the present invention.

[0052] The dopant material is selected from... 229 ThF4 232 ThF4 and 229 ThF4 and 232 Any of the raw materials in the ThF4 mixture, wherein 229 Th、 232 Th is two isotopes of Th.

[0053] Furthermore, the method for preparing thorium-doped fluoride crystals according to the fifth aspect of the present invention is based on the method described in the third or fourth aspect of the present invention.

[0054] The melt crystal growth process includes any one of the following methods: Czochralski method, crucible lowering method, heat exchange method, temperature gradient method, mold guiding method, and micro-pulling method.

[0055] Furthermore, the method for preparing thorium-doped fluoride crystals according to the sixth aspect of the present invention is based on the fifth aspect of the present invention.

[0056] When preparing the thorium-doped fluoride crystal using the Czochralski crystal growth process, the following steps are included:

[0057] (1) Weigh the matrix material and the dopant material according to the molar percentage of the raw materials required to prepare the thorium-doped fluoride crystal, and mix them evenly;

[0058] (2) Place the mixed raw materials into a crucible and place it in a Czochralski furnace. Heat the mixed raw materials under a vacuum atmosphere or a protective atmosphere to melt them and obtain the initial melt for crystal growth and mix them evenly.

[0059] (3) Using the corresponding matrix single crystal of the matrix raw material as a seed crystal and placing it into the initial melt, the thorium-doped fluoride crystal is grown in the Czochralski furnace;

[0060] (4) The thorium-doped fluoride crystal obtained by growth is pulled out and removed from the melt.

[0061] Furthermore, the method for preparing thorium-doped fluoride crystals according to the seventh aspect of the present invention is based on the method described in the sixth aspect of the present invention.

[0062] The parameters of the crystal growth process of the Czochralski method are as follows:

[0063] Shoulder angle 90-120°, lifting speed 0.5-2mm / h, rotation speed 2-10 rpm, cooling rate 5-15℃ / h.

[0064] Furthermore, the method for preparing thorium-doped fluoride crystals according to the eighth aspect of the present invention is described in the sixth or seventh aspect of the present invention.

[0065] The crucible is made of graphite or platinum.

[0066] Furthermore, the method for preparing thorium-doped fluoride crystals according to the ninth aspect of the present invention is described in the sixth or seventh aspect of the present invention.

[0067] The protective atmosphere consists of a mixture of CF4 and Ar gas.

[0068] The tenth aspect of the present invention relates to a solid-state nuclear optical clock, wherein the time frequency of the solid-state nuclear optical clock is determined by exciting thorium nuclei transitions in a thorium-doped fluoride crystal as described in the first or second aspect of the present invention.

[0069] Technical effect

[0070] The thorium-doped fluoride crystals (Th:SrF2, Th:LiF, Th:BaF2, and Th:BaLiF3 single crystals) according to the present invention are prepared using SrF2, LiF, BaF2, and LiF+BaF2 as matrix materials and ThF4 as dopant material, respectively, and can obtain high Th doping concentrations (>10). 18 cm -3 This invention relates to a thorium-doped fluoride crystal with high transmittance (>40% @ 150 nm) in the vacuum ultraviolet band. The crystal also exhibits a low refractive index, a large X-ray penetration depth, and, due to its simple composition and low melting point, facilitates the growth of high-quality, large-size crystals. 229 Solid-state nuclear optical clocks based on Th atomic nuclei transitions (transition energies in the vacuum ultraviolet band) have important application prospects in the fields of precision measurement and precision measurement. Attached Figure Description

[0071] Figure 1 This is a flowchart of the preparation process of the thorium-doped fluoride crystal of the present invention.

[0072] Figure 2 This is a thorium-doped lithium fluoride (Th:LiF) crystal sample and its VUV transmittance curve according to Embodiment 1 of the present invention.

[0073] Figure 3 The image shows the XRD pattern of the thorium-doped lithium fluoride (Th:LiF) crystal according to Embodiment 1 of the present invention.

[0074] Figure 4 This is a thorium-doped barium lithium fluoride (Th:BaLiF3) crystal sample and its VUV transmittance curve according to Embodiment 2 of the present invention.

[0075] Figure 5 The XRD pattern of the thorium-doped barium lithium fluoride (Th:BaLiF3) crystal according to Embodiment 2 of the present invention is shown.

[0076] Figure 6 This is a schematic diagram of a solid-state nuclear optical clock using the thorium-doped fluoride crystal of the present invention. Detailed Implementation

[0077] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. These embodiments are implemented based on the technical solution of the present invention, providing detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0078] For ease of description, spatial relative terms such as “below,” “under,” “down,” “above,” “up,” etc., may be used herein to describe the relationship of one element or feature relative to another element or feature as shown in the figure. It should be understood that spatial relative terms are intended to include different orientations of the device used or operated in addition to those shown in the figure. For example, if the device in the figure were flipped, an element described as “below” or “under” other elements or features would be oriented as “above” other elements or features.

[0079] Unless otherwise specified, the terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Terms should be understood to have the meaning consistent with their meaning in the context of the relevant art and should not be interpreted in an idealized or overly formalized manner, unless explicitly stated otherwise herein.

[0080] The thorium-doped fluoride crystals provided by this invention include four types: Th:SrF2 single crystal, Th:LiF single crystal, Th:BaF2 single crystal, and Th:BaLiF3 single crystal. These are prepared using strontium fluoride (SrF2), lithium fluoride (LiF), barium fluoride (BaF2), and a mixture of LiF and BaF2 as matrix materials, respectively, and thorium tetrafluoride (ThF4) as the dopant material. The following describes different embodiments of these four thorium-doped fluoride crystals and their preparation methods.

[0081] <Implementation Method 1>

[0082] In Embodiment 1, thorium-doped lithium fluoride (Th:LiF) single crystals and their preparation method are described. Th:SrF2 single crystals are prepared from the matrix material LiF and the dopant material ThF4, and the molar percentages M(LiF) and M(ThF4) of the two materials satisfy the following relationship (1):

[0083] M(ThF4):[M(ThF4)+M(LiF)]=x1:1 (1)

[0084] The value of x1 ranges from 0.001 to 0.01.

[0085] In the following Examples 1 to 6, refer to Figure 1 The flowcharts illustrate the Th:LiF crystals obtained under different preparation conditions and their preparation methods.

[0086] Example 1

[0087] The preparation method of Th:LiF crystal in this embodiment includes the following steps.

[0088] like Figure 1As shown, in the raw material preparation process of step S1, the raw materials ThF4 and LiF are weighed according to the relationship (1). In this example 1, x1 = 0.002. After the refining processes such as batching, mixing, pressing, and fluorination, the raw materials are mixed evenly for later use.

[0089] Then, in the furnace loading process of step S2, the uniformly mixed raw materials from step S1 are placed into a graphite crucible with a diameter of 60 mm and a height of 60 mm, and the graphite crucible is loaded into the Czochralski furnace.

[0090] In the gas filling process of step S3, after closing the furnace door and drawing the furnace chamber into a vacuum (below 10 Pa), a mixture of CF4 and Ar gas is filled in as a protective gas. In this embodiment 1, the ratio of the two gases is CF4:Ar = 50%:50%.

[0091] Next, in the material mixing process of step S4, the raw materials are melted by induction heating and kept at a constant temperature for 2 hours to ensure thorough mixing.

[0092] In the growth process of step S5, this embodiment 1 grows a single crystal using the Czochralski method. Specifically, a LiF single crystal in the

[110] direction is used as the growth seed crystal. After the growth process including seeding, shoulder formation, equal diameter formation, tailing, and cooling, a Th:LiF single crystal with a diameter of 50 mm and an equal diameter length of 20 mm is finally obtained. The crystal rotation speed during the growth process is 5-10 rpm, and the growth pulling speed is 0.8-2 mm / hour.

[0093] After the growth process is completed, the crystal is slowly cooled to room temperature over 18 hours, and then the Th:LiF single crystal is removed (step S6, the process of obtaining the single crystal).

[0094] Figure 2 The image shows a sample of the Th:LiF single crystal grown in Example 1 and its VUV (vacuum ultraviolet) transmittance curve. Figure 2 The central portion shows a schematic diagram of the Th:LiF crystal sample (size: 20x20x1mm) obtained in Example 1, as shown below. Figure 2 As shown in the VUV transmittance curves, the Th:LiF crystal achieves a transmittance of 75% at 152.7 nm. Furthermore, the effective Th doping concentration in the Th:LiF crystal is approximately 8.3 × 10⁻⁶. 18 cm -3 .

[0095] Figure 3 The image shows the X-ray diffraction (XRD) pattern of the Th:LiF crystal from Example 1. Figure 3 As can be seen from the characteristic peaks in the XRD pattern, the characteristic peaks of the LiF crystal before and after Th doping are very similar, indicating that Th doping did not change the crystal structure of LiF. In the LiF crystal, Li... +The radius of the ion is Th 4+ The radius of the ion is Although Th 4+ Ionic radius greater than Li + Despite the significant difference in ionic radius and charge between the two molecules, a certain level of doping can still be achieved. Based on analysis, the inventors believe that Th... 4+ Ion-substituted Li + After the ion's position, three Li vacancies were formed in its vicinity as a charge compensation mechanism, reducing the defect formation energy and creating conditions favorable for Th. 4+ Ion-doped lattice environment.

[0096] Example 2

[0097] The preparation method of Th:LiF crystal in this embodiment includes the following steps.

[0098] Reference Figure 1 In the raw material preparation process of step S1, the raw materials ThF4 and LiF are weighed according to the relationship (1). In this embodiment 2, x1 = 0.001. After the refining processes such as batching, mixing, pressing, and fluorination, the raw materials are mixed evenly for later use.

[0099] Then, in the furnace loading process of step S2, the uniformly mixed raw materials from step S1 are placed into a platinum crucible with a diameter of 60 mm and a height of 60 mm, and the platinum crucible is loaded into the Czochralski furnace.

[0100] In the gas filling process of step S3, after closing the furnace door and drawing the furnace chamber into a vacuum (below 10 Pa), a mixture of CF4 and Ar gas is filled in as a protective gas. In this embodiment 2, the ratio of the two gases is CF4:Ar = 20%:80%.

[0101] Next, in the material mixing process of step S4, the raw materials are melted by induction heating and kept at a constant temperature for 2 hours to ensure thorough mixing.

[0102] In the growth process of step S5, this embodiment 2 grows a single crystal using the Czochralski method. Specifically, a LiF single crystal in the

[110] direction is used as the growth seed crystal. After the growth process including seeding, shoulder formation, equal diameter formation, finishing, and cooling, a Th:LiF single crystal with a diameter of 50 mm and an equal diameter length of 20 mm is finally obtained. The crystal rotation speed during the growth process is 5-10 rpm, and the growth pulling speed is 0.8-2 mm / hour.

[0103] After the growth process is completed, the crystal is slowly cooled to room temperature over 18 hours, and then the Th:LiF single crystal is removed (step S6, the process of obtaining the single crystal).

[0104] The Th:LiF crystal grown by the method in Example 2 of this paper achieved a transmittance of 75% at 150 nm, and the effective Th doping concentration was approximately 4.1 × 10⁻⁶. 18 cm -3 The VUV transmittance curve and XRD pattern of the Th:LiF crystal in Example 2 are basically the same as those in Example 1, so the description is omitted.

[0105] Example 3

[0106] The preparation method of Th:LiF crystal in this embodiment includes the following steps.

[0107] Reference Figure 1 In the raw material preparation process of step S1, the raw materials ThF4 and LiF are weighed according to the relationship (1). In this embodiment 2, x1 = 0.01. After the refining processes such as batching, mixing, pressing, and fluorination, the raw materials are mixed evenly for later use.

[0108] Then, in the furnace loading process of step S2, the uniformly mixed raw materials from step S1 are placed into a platinum crucible with a diameter of 60 mm and a height of 60 mm, and the platinum crucible is loaded into the Czochralski furnace.

[0109] In the gas filling process of step S3, the furnace door is closed and the furnace chamber is evacuated to a vacuum (below 10 Pa) before CF4 is filled in as a protective gas.

[0110] Next, in the material mixing process of step S4, the raw materials are melted by induction heating and kept at a constant temperature for 2 hours to ensure thorough mixing.

[0111] In the growth process of step S5, this embodiment 3 grows a single crystal using the Czochralski method. Specifically, a LiF single crystal in the

[110] direction is used as the growth seed crystal. After the growth processes of seeding, shoulder formation, equal diameter formation, finishing, and cooling, a Th:LiF single crystal with a diameter of 50 mm and an equal diameter length of 20 mm is finally obtained. The crystal rotation speed during the growth process is 5-10 rpm, and the growth pulling speed is 0.8-2 mm / hour.

[0112] After the growth process is completed, the crystal is slowly cooled to room temperature over 18 hours, and then the Th:LiF single crystal is removed (step S6, the process of obtaining the single crystal).

[0113] The Th:LiF crystal grown by the method in Example 3 of this paper achieved a transmittance of 65% at 150 nm, and the effective Th doping concentration was approximately 1.02 × 10⁻⁶. 19 cm -3 The VUV transmittance curve and XRD pattern of the Th:LiF crystal in Example 3 are basically the same as those in Example 1, so the description is omitted.

[0114] It is worth noting that when the value of x1 in equation (1) is too high (x1 > 0.01), i.e., the Th doping concentration is too high, a second phase is generated in the crystal, which leads to a significant decrease in the transmittance of the crystal at 150 nm. Therefore, there is an upper limit to the effective doping concentration of Th:LiF crystal, which is approximately 10. 19 cm -3 The level.

[0115] Example 4

[0116] The preparation method of Th:LiF crystal in this embodiment includes the following steps.

[0117] Reference Figure 1 In the raw material preparation process of step S1, the raw materials ThF4 and LiF are weighed according to the relationship (1). In this embodiment 2, x1 = 0.005. After the refining processes such as batching, mixing, pressing, and fluorination, the raw materials are mixed evenly for later use.

[0118] Then, in the furnace loading process of step S2, the uniformly mixed raw materials from step S1 are placed into a graphite crucible with a diameter of 20 mm and a height of 50 mm, and the graphite crucible is placed into the high-temperature zone of the crucible lowering furnace.

[0119] In the gas filling process of step S3, the furnace door is closed and the furnace chamber is evacuated to a vacuum (below 10 Pa) but no protective gas is filled in.

[0120] Next, in the material melting process of step S4, the raw materials are melted by resistance heating to ensure thorough mixing.

[0121] In the growth process of step S5, this embodiment 4 grows a single crystal by crucible descent. Specifically, a LiF single crystal in the

[110] direction is used as the growth seed crystal. After the descent growth process, a Th:LiF single crystal with a diameter of 20 mm and a constant length of 40 mm is finally obtained.

[0122] After the growth process is complete, the crystal is slowly cooled to room temperature, and then the Th:LiF single crystal is taken out (the single crystal acquisition process in step S6).

[0123] The Th:LiF crystal grown by the method in Example 4 of this paper achieved a transmittance of 71% at 150 nm, and the effective Th doping concentration was approximately 1.0 × 10⁻⁶. 19 cm -3 The VUV transmittance curve and XRD pattern of the Th:LiF crystal in Example 4 are basically the same as those in Example 1, so the description is omitted.

[0124] Example 5

[0125] The preparation method of Th:LiF crystal in this embodiment includes the following steps.

[0126] Reference Figure 1 In the raw material preparation process of step S1, the raw materials ThF4 and LiF are weighed according to the relationship (1). In this embodiment 2, x1 = 0.003. After the refining processes such as batching, mixing, pressing, and fluorination, the raw materials are mixed evenly for later use.

[0127] Then, in the furnace loading process of step S2, the uniformly mixed raw materials from step S1 are placed into a graphite crucible with a diameter of 100 mm and a height of 50 mm, and the graphite crucible is loaded into the heat exchange furnace.

[0128] In the gas filling process of step S3, the furnace door is closed and the furnace chamber is evacuated to a vacuum (below 10 Pa) before Ar gas is filled in as a protective gas.

[0129] Next, in the material melting process of step S4, the raw materials are melted by resistance heating to ensure thorough mixing.

[0130] In the growth process of step S5, this embodiment 5 grows single crystals by heat exchange method. Specifically, a LiF single crystal in the

[110] direction is used as the growth seed crystal. After the heat exchange growth process, a Th:LiF single crystal with a diameter of 20 mm and a constant diameter length of 40 mm is finally obtained.

[0131] After the growth process is complete, the crystal is slowly cooled to room temperature, and then the Th:LiF single crystal is taken out (the single crystal acquisition process in step S6).

[0132] The Th:LiF crystal grown by the method in Example 5 of this paper achieved a transmittance of 62% at 150 nm, and the effective Th doping concentration was approximately 9.5 × 10⁻⁶. 18 cm -3 The VUV transmittance curve and XRD pattern of the Th:LiF crystal in Example 5 are basically the same as those in Example 1, so the description is omitted.

[0133] Example 6

[0134] The preparation method of Th:LiF crystal in this embodiment includes the following steps.

[0135] Reference Figure 1 In the raw material preparation process of step S1, the raw materials ThF4 and LiF are weighed according to the relationship (1). In this embodiment 2, x1 = 0.001. After the refining processes such as batching, mixing, pressing, and fluorination, the raw materials are mixed evenly for later use.

[0136] Then, in the furnace loading process of step S2, the uniformly mixed raw materials from step S1 are placed into a platinum crucible with a diameter of 20 mm and a height of 50 mm, and the platinum crucible is loaded into the micro pull-down furnace.

[0137] In the gas filling process of step S3, the furnace door is closed and the furnace chamber is evacuated to a vacuum (below 10 Pa) before Ar gas is filled in as a protective gas.

[0138] Next, in the material melting process of step S4, the raw materials are melted by resistance heating to ensure thorough mixing.

[0139] In the growth process of step S5, this embodiment 6 grows single crystals using a micro-pull-down method. Specifically, a LiF single crystal in the

[110] direction is used as the growth seed crystal, and after a micro-pull-down growth process, a Th:LiF single crystal with a diameter of 1 mm and a length of 50 mm is finally obtained.

[0140] After the growth process is complete, the crystal is slowly cooled to room temperature, and then the Th:LiF single crystal is taken out (the single crystal acquisition process in step S6).

[0141] The Th:LiF crystal grown by the method in Example 6 of this paper achieved a transmittance of 68% at 150 nm, and the effective Th doping concentration was approximately 5.0 × 10⁻⁶. 18 cm -3 The VUV transmittance curve and XRD pattern of the Th:LiF crystal in Example 6 are basically the same as those in Example 1, so the description is omitted.

[0142] <Implementation Method 2>

[0143] In Embodiment 2, a thorium-doped barium lithium fluoride (Th:BaLiF3) single crystal and its preparation method are described. The Th:BaLiF3 single crystal is prepared from a mixture of LiF and BaF2 as matrix materials and ThF4 as dopant material, and the molar percentages M(LiF), M(BaF2), and M(ThF4) of the three materials satisfy the following relationship (2):

[0144] M(ThF4):[M(ThF4)+M(LiF)+M(BaF2)]=x2:1 (2)

[0145] Wherein, M(BaF2) / [M(ThF4)+M(LiF)+M(BaF2)]=0.36~0.47,

[0146] The value of x2 ranges from 0.001 to 0.03.

[0147] In the following Examples 7-12, refer to Figure 1 The flowcharts illustrate the Th:BaLiF3 crystals obtained under different preparation conditions and their preparation methods.

[0148] Example 7

[0149] The preparation method of Th:BaLiF3 crystal in this embodiment includes the following steps.

[0150] like Figure 1 As shown, in the raw material preparation process of step S1, the raw materials ThF4, LiF, and BaF2 are weighed according to the relationship (2). In this embodiment 7, x2 = 0.01 and M(BaF2) = 0.43. After the refining processes such as batching, mixing, pressing, and fluorination, the raw materials are mixed evenly for later use.

[0151] Then, in the furnace loading process of step S2, the uniformly mixed raw materials from step S1 are placed into a graphite crucible with a diameter of 60 mm and a height of 60 mm, and the graphite crucible is loaded into the Czochralski furnace.

[0152] In the gas filling process of step S3, after closing the furnace door and drawing the furnace chamber into a vacuum (below 10 Pa), a mixture of CF4 and Ar gas is filled in as a protective gas. In this embodiment 7, the ratio of the two gases is CF4:Ar = 60%:40%.

[0153] Next, in the material mixing process of step S4, the raw materials are melted by induction heating and kept at a constant temperature for 2 hours to ensure thorough mixing.

[0154] In the growth process of step S5, this embodiment 7 grows a single crystal using the Czochralski method. Specifically, a BaLiF3 single crystal with the

[100] orientation is used as the growth seed crystal. After growth processes such as seeding, shoulder formation, equal diameter formation, finishing, and cooling, a Th:BaLiF3 single crystal with a diameter of 50 mm and an equal diameter length of 20 mm is finally obtained. During the growth process, the crystal rotation speed is 5-10 rpm, and the growth pulling speed is 0.5-2 mm / hour.

[0155] After the growth process is completed, the crystal is slowly cooled to room temperature over 18 hours, and then the Th:BaLiF3 single crystal is taken out (the single crystal acquisition process in step S6).

[0156] Figure 4 The image shows a sample of the Th:BaLiF3 single crystal grown in Example 7 and its VUV transmittance curve. Figure 4 The central portion shows a schematic diagram of the Th:BaLiF3 crystal sample obtained in Example 7, as shown below. Figure 4 As shown in the VUV transmittance curves, the Th:BaLiF3 crystal achieves a transmittance of 55% at 152 nm. Furthermore, the effective Th doping concentration in the Th:BaLiF3 crystal is approximately 3.3 × 10⁻⁶. 18 cm -3 .

[0157] Figure 5The image shows the XRD pattern of the Th:BaLiF3 crystal from Example 7. Figure 5 As can be seen from the characteristic peaks of the XRD pattern, the characteristic peaks of BaLiF3 crystal before and after Th doping are very similar, indicating that Th doping did not change the crystal form of BaLiF3.

[0158] Example 8

[0159] The preparation method of Th:BaLiF3 crystal in this embodiment includes the following steps.

[0160] like Figure 1 As shown, in the raw material preparation process of step S1, the raw materials ThF4, LiF, and BaF2 are weighed according to the relationship (2). In this embodiment 7, x2 = 0.02 and M(BaF2) = 0.36. After the refining processes such as batching, mixing, pressing, and fluorination, the raw materials are mixed evenly for later use.

[0161] Then, in the furnace loading process of step S2, the uniformly mixed raw materials from step S1 are placed into a platinum crucible with a diameter of 60 mm and a height of 60 mm, and the platinum crucible is loaded into the Czochralski furnace.

[0162] In the gas filling process of step S3, after closing the furnace door and drawing the furnace chamber into a vacuum (below 10 Pa), a mixture of CF4 and Ar gas is filled in as a protective gas. In this embodiment 8, the ratio of the two gases is CF4:Ar = 70%:30%.

[0163] Next, in the material mixing process of step S4, the raw materials are melted by induction heating and kept at a constant temperature for 2 hours to ensure thorough mixing.

[0164] In the growth process of step S5, this embodiment 8 grows a single crystal using the Czochralski method. Specifically, a BaLiF3 single crystal in the

[111] direction is used as the growth seed crystal. After the growth processes of seeding, shoulder formation, equal diameter formation, tailing, and cooling, a Th:BaLiF3 single crystal with a diameter of 40 mm and an equal diameter length of 20 mm is finally obtained. The crystal rotation speed during the growth process is 5-10 rpm, and the growth pulling speed is 0.8-2 mm / hour.

[0165] After the growth process is completed, the crystal is slowly cooled to room temperature over 18 hours, and then the Th:BaLiF3 single crystal is taken out (the single crystal acquisition process in step S6).

[0166] The Th:BaLiF3 crystal grown by the method in Example 8 of this paper achieved a transmittance of 45% at 150 nm, and the effective Th doping concentration was approximately 6.8 × 10⁻⁶. 18 cm -3The VUV transmittance curve and XRD pattern of the Th:BaLiF3 crystal in Example 8 are basically the same as those in Example 7, so they will not be described again.

[0167] Example 9

[0168] The preparation method of Th:BaLiF3 crystal in this embodiment includes the following steps.

[0169] like Figure 1 As shown, in the raw material preparation process of step S1, the raw materials ThF4, LiF, and BaF2 are weighed according to the relationship (2). In this embodiment 9, x2 = 0.03 and M(BaF2) = 0.40. After the refining processes such as batching, mixing, pressing, and fluorination, the raw materials are mixed evenly for later use.

[0170] Then, in the furnace loading process of step S2, the uniformly mixed raw materials from step S1 are placed into a platinum crucible with a diameter of 60 mm and a height of 60 mm, and the platinum crucible is loaded into the Czochralski furnace.

[0171] In the gas filling process of step S3, the furnace door is closed and the furnace chamber is evacuated to a vacuum (below 10 Pa) before CF4 is filled in as a protective gas.

[0172] Next, in the material mixing process of step S4, the raw materials are melted by induction heating and kept at a constant temperature for 2 hours to ensure thorough mixing.

[0173] In the growth process of step S5, this embodiment 9 grows a single crystal using the Czochralski method. Specifically, a BaLiF3 single crystal with the

[100] orientation is used as the growth seed crystal. After growth processes such as seeding, shoulder formation, equal diameter formation, finishing, and cooling, a Th:BaLiF3 single crystal with a diameter of 20 mm and an equal diameter length of 20 mm is finally obtained. During the growth process, the crystal rotation speed is 5-10 rpm, and the growth pulling speed is 0.8-2 mm / hour.

[0174] After the growth process is completed, the crystal is slowly cooled to room temperature over 18 hours, and then the Th:BaLiF3 single crystal is taken out (the single crystal acquisition process in step S6).

[0175] The Th:BaLiF3 crystal grown by the method in Example 8 of this paper achieved a transmittance of 40% at 150 nm, and the effective Th doping concentration was approximately 1.0 × 10⁻⁶. 20 cm -3 The VUV transmittance curve and XRD pattern of the Th:BaLiF3 crystal in Example 9 are basically the same as those in Example 7, so they will not be described again.

[0176] Example 10

[0177] The preparation method of Th:BaLiF3 crystal in this embodiment includes the following steps.

[0178] like Figure 1 As shown, in the raw material preparation process of step S1, the raw materials ThF4, LiF, and BaF2 are weighed according to the relationship (2). In this embodiment 10, x2 = 0.005 and M(BaF2) = 0.47. After the refining processes such as batching, mixing, pressing, and fluorination, the raw materials are mixed evenly for later use.

[0179] Then, in the furnace loading process of step S2, the uniformly mixed raw materials from step S1 are placed into a graphite crucible with a diameter of 20 mm and a height of 50 mm, and the graphite crucible is loaded into the high-temperature zone of the descending furnace.

[0180] In the gas filling process of step S3, the furnace door is closed and the furnace chamber is evacuated to a vacuum (below 10 Pa) but no protective gas is filled in.

[0181] Next, in the material melting process of step S4, the raw materials are melted and thoroughly mixed using resistance heating.

[0182] In the growth process of step S5, this embodiment 10 grows a single crystal by a descending method. Specifically, a BaLiF3 single crystal in the

[100] direction is used as a growth seed crystal. After a descending growth process, a Th:BaLiF3 single crystal with a diameter of 20 mm and a constant diameter length of 40 mm is finally obtained.

[0183] After the growth process is complete, the crystal is slowly cooled to room temperature, and then the Th:BaLiF3 single crystal is taken out (the single crystal acquisition process in step S6).

[0184] The Th:BaLiF3 crystal grown by the method in Example 10 achieved a transmittance of 52% at 150 nm, and the effective Th doping concentration was approximately 1.6 × 10⁻⁶. 18 cm -3 The VUV transmittance curve and XRD pattern of the Th:BaLiF3 crystal in Example 10 are basically the same as those in Example 7, so they will not be described again.

[0185] Example 11

[0186] The preparation method of Th:BaLiF3 crystal in this embodiment includes the following steps.

[0187] like Figure 1 As shown, in the raw material preparation process of step S1, the raw materials ThF4, LiF, and BaF2 are weighed according to the relationship (2). In this embodiment 11, x2 = 0.004 and M(BaF2) = 0.45. After the refining processes such as batching, mixing, pressing, and fluorination, the raw materials are mixed evenly for later use.

[0188] Then, in the furnace loading process of step S2, the uniformly mixed raw materials from step S1 are placed into a graphite crucible with a diameter of 100 mm and a height of 50 mm, and the graphite crucible is loaded into the heat exchange furnace.

[0189] In the gas filling process of step S3, the furnace door is closed and the furnace chamber is evacuated to a vacuum (below 10 Pa), and then Ar gas is filled in as a protective gas.

[0190] Next, in the material melting process of step S4, the raw materials are melted and thoroughly mixed using resistance heating.

[0191] In the growth process of step S5, this embodiment 11 grows a single crystal by heat exchange method. Specifically, a BaLiF3 single crystal in the

[111] direction is used as the growth seed crystal. After the heat exchange growth process, a Th:BaLiF3 single crystal with a diameter of 100 mm and a constant diameter length of 40 mm is finally obtained.

[0192] After the growth process is complete, the crystal is slowly cooled to room temperature, and then the Th:BaLiF3 single crystal is taken out (the single crystal acquisition process in step S6).

[0193] The Th:BaLiF3 crystal grown by the method in Example 11 of this paper achieved a transmittance of 48% at 150 nm, and the effective Th doping concentration was approximately 1.2 × 10⁻⁶. 18 cm -3 The VUV transmittance curve and XRD pattern of the Th:BaLiF3 crystal in Example 11 are basically the same as those in Example 7, so they will not be described again.

[0194] Example 12

[0195] The preparation method of Th:BaLiF3 crystal in this embodiment includes the following steps.

[0196] like Figure 1 As shown, in the raw material preparation process of step S1, the raw materials ThF4, LiF, and BaF2 are weighed according to the relationship (2). In this embodiment 12, x2 = 0.008 and M(BaF2) = 0.45. After the refining processes such as batching, mixing, pressing, and fluorination, the raw materials are mixed evenly for later use.

[0197] Then, in the furnace loading process of step S2, the uniformly mixed raw materials from step S1 are placed into a platinum crucible with a diameter of 20 mm and a height of 50 mm, and the platinum crucible is loaded into the micro pull-down furnace.

[0198] In the gas filling process of step S3, the furnace door is closed and the furnace chamber is evacuated to a vacuum (below 10 Pa), and then Ar gas is filled in as a protective gas.

[0199] Next, in the material melting process of step S4, the raw materials are melted and thoroughly mixed using resistance heating.

[0200] In the growth process of step S5, this embodiment 11 grows a single crystal by heat exchange method. Specifically, a BaLiF3 single crystal in the

[110] direction is used as the growth seed crystal, and after a micro-pull-down growth process, a Th:BaLiF3 single crystal with a diameter of 1 mm and a length of 50 mm is finally obtained.

[0201] After the growth process is complete, the crystal is slowly cooled to room temperature, and then the Th:BaLiF3 single crystal is taken out (the single crystal acquisition process in step S6).

[0202] The Th:BaLiF3 crystal grown by the method in Example 11 achieved a transmittance of 46% at 150 nm, and the effective Th doping concentration was approximately 2.1 × 10⁻⁶. 18 cm -3 The VUV transmittance curve and XRD pattern of the Th:BaLiF3 crystal in Example 12 are basically the same as those in Example 7, so they will not be described again.

[0203] <Implementation Method 3>

[0204] In Embodiment 3, thorium-doped strontium fluoride (Th:SrF2) crystal and its preparation method are described. Th:SrF2 single crystal is prepared from matrix material SrF2 and dopant material ThF4, and the molar percentages M(SrF2) and M(ThF4) of the two materials satisfy the following relationship (3):

[0205] M(ThF4):[M(ThF4)+M(SrF2)]=x3:1 (3)

[0206] The value of x3 ranges from 0.001 to 0.05.

[0207] In the following Examples 13-18, refer to Figure 1 The flowcharts illustrate the Th:SrF2 crystals obtained under different preparation conditions and their preparation methods.

[0208] Example 13

[0209] The preparation method of Th:SrF2 crystal in this embodiment includes the following steps.

[0210] like Figure 1 As shown, in the raw material preparation process of step S1, the raw materials ThF4 and SrF2 are weighed according to the relationship (3). In this embodiment 13, x3 = 0.02. After the refining processes such as batching, mixing, pressing, and fluorination, the raw materials are mixed evenly for later use.

[0211] Then, in the furnace loading process of step S2, the uniformly mixed raw materials from step S1 are placed into a graphite crucible with a diameter of 60 mm and a height of 60 mm, and the graphite crucible is loaded into the Czochralski furnace.

[0212] In the gas filling process of step S3, after closing the furnace door and drawing the furnace chamber into a vacuum (below 10 Pa), a mixture of CF4 and Ar gas is filled in as a protective gas. In this embodiment 13, the ratio of the two gases is CF4:Ar = 50%:50%.

[0213] Next, in the material mixing process of step S4, the raw materials are melted by induction heating and kept at a constant temperature for 2 hours to ensure thorough mixing.

[0214] In the growth process of step S5, this embodiment 13 grows a single crystal using the Czochralski method. Specifically, an SrF2 single crystal in the

[110] direction is used as the growth seed crystal. After growth processes such as seeding, shoulder formation, equal diameter formation, finishing, and cooling, a Th:SrF2 single crystal with a diameter of 40 mm and an equal diameter length of 20 mm is finally obtained. The crystal rotation speed during the growth process is 5-10 rpm, and the growth pulling speed is 0.8-2 mm / hour.

[0215] After the growth process is completed, the crystal is slowly cooled to room temperature over 20 hours, and then the Th:SrF2 single crystal is removed (step S6, the process of obtaining the single crystal).

[0216] The Th:SrF2 crystal grown by the method in Example 13 achieved a transmittance of 42% at 150 nm, and the effective Th doping concentration was approximately 1.2 × 10⁻⁶. 20 cm -3 .

[0217] Example 14

[0218] The preparation method of Th:SrF2 crystal in this embodiment includes the following steps.

[0219] like Figure 1 As shown, in the raw material preparation process of step S1, the raw materials ThF4 and SrF2 are weighed according to the relationship (3). In this embodiment 14, x3 = 0.01. After the refining processes such as batching, mixing, pressing, and fluorination, the raw materials are mixed evenly for later use.

[0220] Then, in the furnace loading process of step S2, the uniformly mixed raw materials from step S1 are placed into a platinum crucible with a diameter of 60 mm and a height of 60 mm, and the platinum crucible is loaded into the Czochralski furnace.

[0221] In the gas filling process of step S3, after closing the furnace door and drawing the furnace chamber into a vacuum (below 10 Pa), a mixture of CF4 and Ar gas is filled in as a protective gas. In this embodiment 14, the ratio of the two gases is CF4:Ar = 20%:80%.

[0222] Next, in the material mixing process of step S4, the raw materials are melted by induction heating and kept at a constant temperature for 2 hours to ensure thorough mixing.

[0223] In the growth process of step S5, this embodiment 14 grows a single crystal using the Czochralski method. Specifically, an SrF2 single crystal in the

[110] direction is used as the growth seed crystal. After growth processes such as seeding, shoulder formation, equal diameter formation, finishing, and cooling, a Th:SrF2 single crystal with a diameter of 50 mm and an equal diameter length of 20 mm is finally obtained. The crystal rotation speed during the growth process is 5-10 rpm, and the growth pulling speed is 0.8-2 mm / hour.

[0224] After the growth process is completed, the crystal is slowly cooled to room temperature over 18 hours, and then the Th:SrF2 single crystal is removed (step S6, the process of obtaining the single crystal).

[0225] The Th:SrF2 crystal grown by the method in Example 14 of this paper achieved a transmittance of 50% at 150 nm, and the effective Th doping concentration was approximately 6.5 × 10⁻⁶. 19 cm -3 .

[0226] Example 15

[0227] The preparation method of Th:SrF2 crystal in this embodiment includes the following steps.

[0228] like Figure 1 As shown, in the raw material preparation process of step S1, the raw materials ThF4 and SrF2 are weighed according to the relationship (3). In this embodiment 15, x3 = 0.03. After the refining processes such as batching, mixing, pressing, and fluorination, the raw materials are mixed evenly for later use.

[0229] Then, in the furnace loading process of step S2, the uniformly mixed raw materials from step S1 are placed into a graphite crucible with a diameter of 60 mm and a height of 60 mm, and the graphite crucible is loaded into the Czochralski furnace.

[0230] In the gas filling process of step S3, the furnace door is closed and the furnace chamber is evacuated to a vacuum (below 10 Pa) before CF4 is filled in as a protective gas.

[0231] Next, in the material mixing process of step S4, the raw materials are melted by induction heating and kept at a constant temperature for 2 hours to ensure thorough mixing.

[0232] In the growth process of step S5, this embodiment 15 grows a single crystal using the Czochralski method. Specifically, an SrF2 single crystal in the

[110] direction is used as the growth seed crystal. After growth processes such as seeding, shoulder formation, equal diameter formation, finishing, and cooling, a Th:SrF2 single crystal with a diameter of 40 mm and an equal diameter length of 20 mm is finally obtained. The crystal rotation speed during the growth process is 5-10 rpm, and the growth pulling speed is 0.8-2 mm / hour.

[0233] After the growth process is completed, the crystal is slowly cooled to room temperature over 24 hours, and then the Th:SrF2 single crystal is removed (step S6, the process of obtaining the single crystal).

[0234] The Th:SrF2 crystal grown by the method in Example 15 achieved a transmittance of 40% at 150 nm, and the effective Th doping concentration was approximately 2.0 × 10⁻⁶. 20 cm -3 .

[0235] Example 16

[0236] The preparation method of Th:SrF2 crystal in this embodiment includes the following steps.

[0237] like Figure 1 As shown, in the raw material preparation process of step S1, the raw materials ThF4 and SrF2 are weighed according to the relationship (3). In this embodiment 16, x3 = 0.05. After the refining processes such as batching, mixing, pressing, and fluorination, the raw materials are mixed evenly for later use.

[0238] Then, in the furnace loading process of step S2, the uniformly mixed raw materials from step S1 are placed into a graphite crucible with a diameter of 20 mm and a height of 50 mm, and the graphite crucible is placed into the high-temperature zone of the crucible lowering furnace.

[0239] In the gas filling process of step S3, the furnace door is closed and the furnace chamber is evacuated to a vacuum (below 10 Pa) but no protective gas is filled in.

[0240] Next, in the material melting process of step S4, the raw materials are melted by resistance heating to ensure thorough mixing.

[0241] In the growth process of step S5, this embodiment 16 grows a single crystal by crucible descent. Specifically, a SrF2 single crystal in the

[110] direction is used as a growth seed crystal. After a descent growth process, a Th:SrF2 single crystal with a diameter of 20 mm and a constant length of 40 mm is finally obtained.

[0242] After the growth process is complete, the crystal is slowly cooled to room temperature, and then the Th:SrF2 single crystal is removed (step S6, the process of obtaining the single crystal).

[0243] The Th:SrF2 crystal grown by the method in Example 16 of this paper achieved a transmittance of 35% at 150 nm, and the effective Th doping concentration was approximately 4.2 × 10⁻⁶. 20 cm -3 .

[0244] Example 17

[0245] The preparation method of Th:SrF2 crystal in this embodiment includes the following steps.

[0246] like Figure 1 As shown, in the raw material preparation process of step S1, the raw materials ThF4 and SrF2 are weighed according to the relationship (3). In this embodiment 17, x3 = 0.005. After the refining processes such as batching, mixing, pressing, and fluorination, the raw materials are mixed evenly for later use.

[0247] Then, in the furnace loading process of step S2, the uniformly mixed raw materials from step S1 are placed into a graphite crucible with a diameter of 100 mm and a height of 50 mm, and the graphite crucible is loaded into the heat exchange furnace.

[0248] In the gas filling process of step S3, the furnace door is closed and the furnace chamber is evacuated to a vacuum (below 10 Pa) before Ar gas is filled in as a protective gas.

[0249] Next, in the material melting process of step S4, the raw materials are melted by resistance heating to ensure thorough mixing.

[0250] In the growth process of step S5, this embodiment 17 grows a single crystal by heat exchange method. Specifically, a SrF2 single crystal in the

[110] direction is used as a growth seed crystal. After the heat exchange growth process, a Th:SrF2 single crystal with a diameter of 100 mm and a constant diameter length of 40 mm is finally obtained.

[0251] After the growth process is complete, the crystal is slowly cooled to room temperature, and then the Th:SrF2 single crystal is removed (step S6, the process of obtaining the single crystal).

[0252] The Th:SrF2 crystal grown by the method in Example 17 of this paper achieved a transmittance of 50% at 150 nm, and the effective Th doping concentration was approximately 1.8 × 10⁻⁶. 19 cm -3 .

[0253] Example 18

[0254] The preparation method of Th:SrF2 crystal in this embodiment includes the following steps.

[0255] like Figure 1As shown, in the raw material preparation process of step S1, the raw materials ThF4 and SrF2 are weighed according to the relationship (3). In this embodiment 18, x3 = 0.001. After the refining processes such as batching, mixing, pressing, and fluorination, the raw materials are mixed evenly for later use.

[0256] Then, in the furnace loading process of step S2, the uniformly mixed raw materials from step S1 are placed into a platinum crucible with a diameter of 20 mm and a height of 50 mm, and the platinum crucible is loaded into the micro pull-down furnace.

[0257] In the gas filling process of step S3, the furnace door is closed and the furnace chamber is evacuated to a vacuum (below 10 Pa) before Ar gas is filled in as a protective gas.

[0258] Next, in the material melting process of step S4, the raw materials are melted by resistance heating to ensure thorough mixing.

[0259] In the growth process of step S5, this embodiment 18 grows a single crystal by micro-pull-down method. Specifically, a SrF2 single crystal in the

[110] direction is used as a growth seed crystal. After the micro-pull-down growth process, a Th:SrF2 single crystal with a diameter of 1 mm and a length of 50 mm is finally obtained.

[0260] After the growth process is complete, the crystal is slowly cooled to room temperature, and then the Th:SrF2 single crystal is removed (step S6, the process of obtaining the single crystal).

[0261] The Th:SrF2 crystal grown by the method in Example 18 achieved a transmittance of 60% at 150 nm, and the effective Th doping concentration was approximately 6.3 × 10⁻⁶. 18 cm -3 .

[0262] <Implementation Method 4>

[0263] In Embodiment 4, thorium-doped barium fluoride (Th:BaF2) crystal and its preparation method are described. Th:BaF2 single crystals are prepared from the matrix material BaF2 and the dopant material ThF4, and the molar percentages M(BaF2) and M(ThF4) of the two materials satisfy the following relationship (4):

[0264] M(ThF4):[M(ThF4)+M(BaF2)]=x4:1 (4)

[0265] The value of x4 ranges from 0.001 to 0.01.

[0266] In the following Examples 19-24, refer to Figure 1 The flowcharts illustrate the Th:BaF2 crystals obtained under different preparation conditions and their preparation methods.

[0267] Example 19

[0268] The preparation method of Th:BaF2 crystal in this embodiment includes the following steps.

[0269] like Figure 1 As shown, in the raw material preparation process of step S1, the raw materials ThF4 and BaF2 are weighed according to the relationship (4). In this embodiment 19, x4 = 0.002. After the refining processes such as batching, mixing, pressing, and fluorination, the raw materials are mixed evenly for later use.

[0270] Then, in the furnace loading process of step S2, the uniformly mixed raw materials from step S1 are placed into a graphite crucible with a diameter of 60 mm and a height of 60 mm, and the graphite crucible is loaded into the Czochralski furnace.

[0271] In the gas filling process of step S3, after closing the furnace door and drawing the furnace chamber into a vacuum (below 10 Pa), a mixture of CF4 and Ar gas is filled in as a protective gas. In this embodiment 19, the ratio of the two gases is CF4:Ar = 50%:50%.

[0272] Next, in the material mixing process of step S4, the raw materials are melted by induction heating and kept at a constant temperature for 2 hours to ensure thorough mixing.

[0273] In the growth process of step S5, this embodiment 19 grows a single crystal using the Czochralski method. Specifically, a BaF2 single crystal in the

[110] direction is used as the growth seed crystal. After the growth processes of seeding, shoulder formation, equal diameter formation, finishing, and cooling, a Th:BaF2 single crystal with a diameter of 40 mm and an equal diameter length of 20 mm is finally obtained. The crystal rotation speed during the growth process is 5-10 rpm, and the growth pulling speed is 0.5-2 mm / hour.

[0274] After the growth process is completed, the crystal is slowly cooled to room temperature over 20 hours, and then the Th:BaF2 single crystal is removed (step S6, the process of obtaining the single crystal).

[0275] The Th:BaF2 crystal grown by the method in Example 19 of this paper achieved a transmittance of 45% at 150 nm, and the effective Th doping concentration was approximately 8.6 × 10⁻⁶. 18 cm -3 .

[0276] Example 20

[0277] The preparation method of Th:BaF2 crystal in this embodiment includes the following steps.

[0278] like Figure 1As shown, in the raw material preparation process of step S1, the raw materials ThF4 and BaF2 are weighed according to the relationship (4). In this embodiment 20, x4 = 0.001. After the refining processes such as batching, mixing, pressing, and fluorination, the raw materials are mixed evenly for later use.

[0279] Then, in the furnace loading process of step S2, the uniformly mixed raw materials from step S1 are placed into a platinum crucible with a diameter of 60 mm and a height of 60 mm, and the platinum crucible is loaded into the Czochralski furnace.

[0280] In the gas filling process of step S3, after closing the furnace door and drawing the furnace chamber into a vacuum (below 10 Pa), a mixture of CF4 and Ar gas is filled in as a protective gas. In this embodiment 20, the ratio of the two gases is CF4:Ar = 50%:50%.

[0281] Next, in the material mixing process of step S4, the raw materials are melted by induction heating and kept at a constant temperature for 2 hours to ensure thorough mixing.

[0282] In the growth process of step S5, this embodiment 19 grows a single crystal using the Czochralski method. Specifically, a BaF2 single crystal in the

[110] direction is used as the growth seed crystal. After the growth processes of seeding, shoulder formation, equal diameter formation, finishing, and cooling, a Th:BaF2 single crystal with a diameter of 30 mm and an equal diameter length of 20 mm is finally obtained. The crystal rotation speed during the growth process is 5-10 rpm, and the growth pulling speed is 0.8-2 mm / hour.

[0283] After the growth process is completed, the crystal is slowly cooled to room temperature over 24 hours, and then the Th:BaF2 single crystal is removed (the single crystal acquisition process in step S6).

[0284] The Th:BaF2 crystal grown by the method in Example 20 achieved a transmittance of 48% at 150 nm, and the effective Th doping concentration was approximately 4.3 × 10⁻⁶. 18 cm -3 .

[0285] Example 21

[0286] The preparation method of Th:BaF2 crystal in this embodiment includes the following steps.

[0287] like Figure 1 As shown, in the raw material preparation process of step S1, the raw materials ThF4 and BaF2 are weighed according to the relationship (4). In this embodiment 21, x4 = 0.005. After the refining processes such as batching, mixing, pressing, and fluorination, the raw materials are mixed evenly for later use.

[0288] Then, in the furnace loading process of step S2, the uniformly mixed raw materials from step S1 are placed into a platinum crucible with a diameter of 60 mm and a height of 60 mm, and the platinum crucible is loaded into the Czochralski furnace.

[0289] In the gas filling process of step S3, the furnace door is closed and the furnace chamber is evacuated to a vacuum (below 10 Pa) before Ar gas is filled in as a protective gas.

[0290] Next, in the material mixing process of step S4, the raw materials are melted by induction heating and kept at a constant temperature for 2 hours to ensure thorough mixing.

[0291] In the growth process of step S5, this embodiment 21 grows a single crystal using the Czochralski method. Specifically, a BaF2 single crystal in the

[110] direction is used as the growth seed crystal. After growth processes such as seeding, shoulder formation, equal diameter formation, finishing, and cooling, a Th:BaF2 single crystal with a diameter of 50 mm and an equal diameter length of 20 mm is finally obtained. The crystal rotation speed during the growth process is 5-10 rpm, and the growth pulling speed is 0.8-2 mm / hour.

[0292] After the growth process is completed, the crystal is slowly cooled to room temperature over 18 hours, and then the Th:BaF2 single crystal is removed (step S6, the process of obtaining the single crystal).

[0293] The Th:BaF2 crystal grown by the method in Example 21 achieved a transmittance of 41% at 150 nm, and the effective Th doping concentration was approximately 1.6 × 10⁻⁶. 19 cm -3 .

[0294] Example 22

[0295] The preparation method of Th:BaF2 crystal in this embodiment includes the following steps.

[0296] like Figure 1 As shown, in the raw material preparation process of step S1, the raw materials ThF4 and BaF2 are weighed according to the relationship (4). In this embodiment 22, x4 = 0.01. After the refining processes such as batching, mixing, pressing, and fluorination, the raw materials are mixed evenly for later use.

[0297] Then, in the furnace loading process of step S2, the uniformly mixed raw materials from step S1 are placed into a graphite crucible with a diameter of 20 mm and a height of 50 mm, and the graphite crucible is placed into the high-temperature zone of the crucible lowering furnace.

[0298] In the gas filling process of step S3, the furnace door is closed and the furnace chamber is evacuated to a vacuum (below 10 Pa) but no protective gas is filled in.

[0299] Next, in the material melting process of step S4, the raw materials are melted by resistance heating to ensure thorough mixing.

[0300] In the growth process of step S5, this embodiment 19 grows a single crystal by crucible descent. Specifically, a BaF2 single crystal in the

[110] direction is used as the growth seed crystal. After the descent growth process, a Th:BaF2 single crystal with a diameter of 20 mm and a constant length of 40 mm is finally obtained.

[0301] After the growth process is complete, the crystal is slowly cooled to room temperature, and then the Th:BaF2 single crystal is taken out (the single crystal acquisition process in step S6).

[0302] The Th:BaF2 crystal grown by the method in Example 22 achieved a transmittance of 40% at 150 nm, and the effective Th doping concentration was approximately 3.2 × 10⁻⁶. 19 cm -3 .

[0303] Example 23

[0304] The preparation method of Th:BaF2 crystal in this embodiment includes the following steps.

[0305] like Figure 1 As shown, in the raw material preparation process of step S1, the raw materials ThF4 and BaF2 are weighed according to the relationship (4). In this embodiment 23, x4 = 0.005. After the refining processes such as batching, mixing, pressing, and fluorination, the raw materials are mixed evenly for later use.

[0306] Then, in the furnace loading process of step S2, the uniformly mixed raw materials from step S1 are placed into a graphite crucible with a diameter of 100 mm and a height of 50 mm, and the graphite crucible is loaded into the heat exchange furnace.

[0307] In the gas filling process of step S3, the furnace door is closed and the furnace chamber is evacuated to a vacuum (below 10 Pa) before Ar gas is filled in as a protective gas.

[0308] Next, in the material melting process of step S4, the raw materials are melted by resistance heating to ensure thorough mixing.

[0309] In the growth process of step S5, this embodiment 23 grows a single crystal by heat exchange method. Specifically, a BaF2 single crystal in the

[110] direction is used as the growth seed crystal. After the heat exchange growth process, a Th:BaF2 single crystal with a diameter of 100 mm and a constant diameter length of 40 mm is finally obtained.

[0310] After the growth process is complete, the crystal is slowly cooled to room temperature, and then the Th:BaF2 single crystal is taken out (the single crystal acquisition process in step S6).

[0311] The Th:BaF2 crystal grown by the method in Example 23 achieved a transmittance of 42% at 150 nm, and the effective Th doping concentration was approximately 1.8 × 10⁻⁶. 19 cm -3 .

[0312] Example 24

[0313] The preparation method of Th:BaF2 crystal in this embodiment includes the following steps.

[0314] like Figure 1 As shown, in the raw material preparation process of step S1, the raw materials ThF4 and BaF2 are weighed according to the relationship (4). In this embodiment 24, x4 = 0.02. After the refining processes such as batching, mixing, pressing, and fluorination, the raw materials are mixed evenly for later use.

[0315] Then, in the furnace loading process of step S2, the uniformly mixed raw materials from step S1 are placed into a platinum crucible with a diameter of 20 mm and a height of 50 mm, and the platinum crucible is loaded into the micro pull-down furnace.

[0316] In the gas filling process of step S3, the furnace door is closed and the furnace chamber is evacuated to a vacuum (below 10 Pa) before Ar gas is filled in as a protective gas.

[0317] Next, in the material melting process of step S4, the raw materials are melted by resistance heating to ensure thorough mixing.

[0318] In the growth process of step S5, this embodiment 24 grows a single crystal by micro-pull-down method. Specifically, a BaF2 single crystal in the

[110] direction is used as the growth seed crystal. After the micro-pull-down growth process, a Th:BaF2 single crystal with a diameter of 1 mm and a length of 50 mm is finally obtained.

[0319] After the growth process is complete, the crystal is slowly cooled to room temperature, and then the Th:BaF2 single crystal is taken out (the single crystal acquisition process in step S6).

[0320] The Th:BaF2 crystal grown by the method in Example 24 achieved a transmittance of 38% at 150 nm, and the effective Th doping concentration was approximately 4.3 × 10⁻⁶. 19 cm -3 .

[0321] <Other Implementation Methods>

[0322] The thorium-doped fluoride crystals described in Embodiments 1 to 4 include Th:LiF, Th:SrF2, Th:BaF2, and Th:BaLiF3, respectively, using LiF, SrF2, BaF2, and LiF+BaF2 as matrix materials and ThF4 as dopant material. Here, ThF4 as the dopant material can be... 229 ThF4 232 ThF4 and 229 ThF4 and 232 Any of the mixed feedstocks of ThF4, wherein 229 Th、 232 Th is two isotopes of Th.

[0323] The raw materials for preparing Th:LiF crystals satisfy the following:

[0324] M(ThF4):[M(ThF4)+M(LiF)]=x1:1,

[0325] Wherein, M(ThF4) and M(LiF) are the molar percentages of ThF4 and LiF raw materials, respectively.

[0326] The value of x1 ranges from 0.001 to 0.01.

[0327] The raw materials for preparing Th:BaLiF3 crystals satisfy the following:

[0328] M(ThF4):[M(ThF4)+M(LiF)+M(BaF2)]=x2:1,

[0329] Wherein, M(ThF4), M(LiF), and M(BaF2) are the molar percentages of ThF4, LiF, and BaF2 raw materials, respectively, and M(BaF2) / [M(ThF4)+M(LiF)+M(BaF2)] = 0.36~0.47.

[0330] The value of x2 ranges from 0.001 to 0.03.

[0331] The raw materials for preparing Th:SrF2 crystals satisfy the following:

[0332] M(ThF4):[M(ThF4)+M(SrF2)]=x3:1,

[0333] Wherein, M(ThF4) and M(SrF2) are the molar percentages of ThF4 and SrF2 raw materials, respectively.

[0334] The value of x3 ranges from 0.001 to 0.05;

[0335] The raw materials for preparing Th:BaF2 crystals satisfy the following:

[0336] M(ThF4):[M(ThF4)+M(BaF2)]=x4:1,

[0337] Wherein, M(ThF4) and M(BaF2) are the molar percentages of ThF4 and BaF2 raw materials, respectively.

[0338] The value of x4 ranges from 0.001 to 0.01.

[0339] The thorium-doped fluoride crystals of the present invention can be produced at higher doping concentrations (>10). 18 cm -3 In particular, the Th doping concentration of Th:SrF2 crystals can reach 10. 20 cm -3 The thorium-doped fluoride crystal meets the requirement of high light transmittance (>40%@150nm) in the vacuum ultraviolet band, which is beneficial for increasing the probability of nuclear level transitions and the detection probability of nuclear level decay processes, and also helps to improve the stability of the nuclear optical clock, making it a preferred material for solid-state nuclear optical clocks. Furthermore, the thorium-doped fluoride crystal has a low refractive index and a large X-ray penetration depth, which helps to suppress harmful Cherenkov radiation and reduce background interference. At the same time, due to its simple composition and low melting point, it is easier to achieve the growth of high-quality, large-size crystals. Especially thorium-doped (… 229 Th) fluoride crystals based on 229 Solid-state nuclear optical clocks based on Th atomic nuclei transitions (transition energies in the vacuum ultraviolet band) have important application prospects in the fields of precision measurement and precision measurement. Figure 6 This is a schematic diagram of a solid-state nuclear optical clock using the thorium-doped fluoride crystal of the present invention. Figure 6 As shown, utilizing the high doping concentration and high VUV transmittance of the thorium-doped fluoride crystal of the present invention, nuclear level transitions ( 229m Th→ 229g The probability of Th) increases, therefore, when the laser emitted by the ultra-narrow linewidth frequency-stabilized laser generator excites the thorium-doped fluoride crystal to generate 229m Th→ 229g When the Th transition is detected by the subsequent detector, a solid-state nuclear optical clock can be realized through the control of the servo control system, or it can be applied to the field of precision measurement.

[0340] Furthermore, compared to existing thorium-doped LiSrAlF6 and thorium-doped CaF2 crystals, thorium-doped lithium fluoride (Th:LiF) crystals can achieve a transmittance of over 70% near 150 nm, and the doping concentration can reach ~1×10⁻⁶. 19 cm -3Therefore, it can greatly reduce the absorption loss and background interference of the crystal to the excitation light and nuclear decay signal, increase the probability of nuclear transition process, and facilitate the detection of nuclear transition signal.

[0341] Moreover, compared with CaF2 (melting point: 1420℃), LiF and BaLiF3 crystals have lower melting points, at 870℃ and 820℃ respectively, making it easier to achieve high-quality, large-size crystal growth, reduce crystal defect concentration, and thus reduce the harmful effects of defect absorption, scattering, and background luminescence.

[0342] Furthermore, for X-rays that can induce nuclear transitions, their penetration depths in LiSrAlF6 crystal and CaF2 crystal are 0.325 mm and 1.32 mm, respectively. However, the constituent elements of LiF crystal have a lighter mass, resulting in a deeper penetration depth and a greater probability of interaction with Th atoms.

[0343] In addition, in the methods for preparing thorium-doped fluoride crystals described in Embodiments 1 to 4, the raw materials are prepared and mixed according to the molar percentage of each crystal, and high-quality thorium-doped fluoride crystals are obtained by using a melt crystal growth process.

[0344] Examples of methods described in the various embodiments include the Czochralski method, crucible lowering method, heat exchange method, and micro-pulling method. However, temperature gradient method, guided mold method, etc., can also be used. As long as it is a melt crystal growth process, it can be used to prepare the thorium-doped fluoride crystal of the present invention. The preferred Czochralski method is beneficial for removing impurities from the crystal and for obtaining thorium-doped fluoride crystals with low defects, high quality, and high light transmittance.

[0345] In each of the above embodiments, the mixed raw materials prepared according to the molar percentage of the raw materials are placed in a crucible for processing. The material of the crucible can be graphite or platinum, and there is no particular limitation.

[0346] In the embodiments described above, the crucible containing the raw materials is placed into a single crystal growth furnace such as a Czochralski furnace or a heat exchange furnace, and the furnace cavity is evacuated or filled with a protective gas, i.e., the raw materials are melted under a vacuum atmosphere or a protective atmosphere. Preferably, a mixture of CF4 and Ar gas is used as the protective atmosphere for crystal growth. This helps ensure the purity of the raw materials during the growth process, improves crystal quality, and enhances crystal transmittance. Furthermore, increasing the proportion of CF4 in the mixed atmosphere can promote the formation of a charge compensation mechanism, which is beneficial for Th ion doping.

[0347] In each of the above embodiments, induction or resistance heating is used to melt the raw material to obtain the initial melt for crystal growth. However, there is no particular limitation on the method of melting the raw material, and other methods may also be used.

[0348] Furthermore, any combinations, substitutions, and improvements made by those skilled in the art without departing from the essential content, concept, and spirit of this invention should also be within the scope of protection of this invention.

[0349] Industrial practicality

[0350] The thorium doping of the present invention includes 229 Th、 232 Th and 229 Th and 232 Th mixed raw material) fluoride crystals (Th:SrF2, Th:LiF, Th:BaF2 and Th:BaLiF3 single crystals) can be used in solid-state nuclear optical clocks and precision measurement fields.

Claims

1. A thorium (Th)-doped fluoride crystal, characterized in that, It is prepared from a matrix material and a dopant material, wherein the matrix material is any one of the fluorides selected from LiF, SrF2, BaF2, and mixtures of LiF and BaF2, and the dopant material is ThF4. (1) When the matrix material is LiF, the raw materials for preparing the thorium-doped fluoride crystal, i.e., Th:LiF single crystal, satisfy the following: M(ThF4):[M(ThF4)+M(LiF)]=x1:1, Wherein, M(ThF4) and M(LiF) are the molar percentages of ThF4 and LiF raw materials, respectively. The value of x1 ranges from 0.001 to 0.01; (2) When the matrix raw material is a mixture of LiF and BaF2, the raw materials for preparing the thorium-doped fluoride crystal, i.e., Th:BaLiF3 single crystal, satisfy the following: M(ThF4):[M(ThF4)+M(LiF)+M(BaF2)]=x2:1, Wherein, M(ThF4), M(LiF), and M(BaF2) are the molar percentages of ThF4, LiF, and BaF2 raw materials, respectively, and M(BaF2) / [M(ThF4)+M(LiF)+M(BaF2)] = 0.36~0.

47. The value of x2 ranges from 0.001 to 0.03; (3) When the matrix material is SrF2, the raw materials for preparing the thorium-doped fluoride crystal, i.e., Th:SrF2 single crystal, satisfy the following: M(ThF4):[M(ThF4)+M(SrF2)]=x3:1, Wherein, M(ThF4) and M(SrF2) are the molar percentages of ThF4 and SrF2 raw materials, respectively. The value of x3 ranges from 0.001 to 0.05; (4) When the matrix material is BaF2, the raw materials for preparing the thorium-doped fluoride crystal, i.e., Th:BaF2 single crystal, satisfy the following: M(ThF4):[M(ThF4)+M(BaF2)]=x4:1, Wherein, M(ThF4) and M(BaF2) are the molar percentages of ThF4 and BaF2 raw materials, respectively. The value of x4 ranges from 0.001 to 0.

01.

2. The thorium-doped fluoride crystal as described in claim 1, characterized in that, The dopant material is selected from... 229 ThF4 232 ThF4 and 229 ThF4 and 232 Any of the raw materials in the ThF4 mixture, wherein 229 Th、 232 Th is two isotopes of Th.

3. A method for preparing thorium-doped fluoride crystals, characterized in that, The preparation is carried out using a matrix material and a dopant material as fluorides, wherein the matrix material is any one of the fluorides selected from LiF, SrF2, BaF2, and mixtures of LiF and BaF2, and the dopant material is ThF4. The mixture of the matrix material and the dopant material is melted under a protective atmosphere or vacuum atmosphere, and the thorium-doped fluoride crystal is grown by melt method. (1) When the matrix material is LiF, the raw materials for preparing the thorium-doped fluoride crystal, i.e., Th:LiF single crystal, satisfy the following: M(ThF4):[M(ThF4)+M(LiF)]=x1:1, Wherein, M(ThF4) and M(LiF) are the molar percentages of ThF4 and LiF raw materials, respectively. The value of x1 ranges from 0.001 to 0.01; (2) When the matrix raw material is a mixture of LiF and BaF2, the raw materials for preparing the thorium-doped fluoride crystal, i.e., Th:BaLiF3 single crystal, satisfy the following: M(ThF4):[M(ThF4)+M(LiF)+M(BaF2)]=x2:1, Wherein, M(ThF4), M(LiF), and M(BaF2) are the molar percentages of ThF4, LiF, and BaF2 raw materials, respectively, and M(BaF2) / [M(ThF4)+M(LiF)+M(BaF2)] = 0.36~0.

47. The value of x2 ranges from 0.001 to 0.03; (3) When the matrix material is SrF2, the raw materials for preparing the thorium-doped fluoride crystal, i.e., Th:SrF2 single crystal, satisfy the following: M(ThF4):[M(ThF4)+M(SrF2)]=x3:1, Wherein, M(ThF4) and M(SrF2) are the molar percentages of ThF4 and SrF2 raw materials, respectively. The value of x3 ranges from 0.001 to 0.05; (4) When the matrix material is BaF2, the raw materials for preparing the thorium-doped fluoride crystal, i.e., Th:BaF2 single crystal, satisfy the following: M(ThF4):[M(ThF4)+M(BaF2)]=x4:1, Wherein, M(ThF4) and M(BaF2) are the molar percentages of ThF4 and BaF2 raw materials, respectively. The value of x4 ranges from 0.001 to 0.

01.

4. The method for preparing thorium-doped fluoride crystals according to claim 3, characterized in that, The dopant material is selected from... 229 ThF4 232 ThF4 and 229 ThF4 and 232 Any of the raw materials in the ThF4 mixture, wherein 229 Th、 232 Th is two isotopes of Th.

5. The method for preparing thorium-doped fluoride crystals according to claim 3 or 4, characterized in that, The melt crystal growth process includes any one of the following methods: Czochralski method, crucible lowering method, heat exchange method, temperature gradient method, mold guiding method, and micro-pulling method.

6. The method for preparing thorium-doped fluoride crystals according to claim 5, characterized in that, When preparing the thorium-doped fluoride crystal using the Czochralski crystal growth process, the following steps are included: (1) Weigh the matrix material and the dopant material according to the molar percentage of the raw materials required to prepare the thorium-doped fluoride crystal, and mix them evenly; (2) Place the mixed raw materials into a crucible and place it in a Czochralski furnace. Heat the mixed raw materials under a vacuum atmosphere or a protective atmosphere to melt them and obtain the initial melt for crystal growth and mix them evenly. (3) Using the corresponding matrix single crystal of the matrix raw material as a seed crystal and placing it into the initial melt, the thorium-doped fluoride crystal is grown in the Czochralski furnace; (4) The thorium-doped fluoride crystal obtained by growth is pulled out and removed from the melt.

7. The method for preparing thorium-doped fluoride crystals according to claim 6, characterized in that, The parameters of the crystal growth process of the Czochralski method are as follows: Shoulder angle 90-120°, lifting speed 0.5-2mm / h, rotation speed 2-10 rpm, cooling rate 5-15℃ / h.

8. The method for preparing thorium-doped fluoride crystals according to claim 6 or 7, characterized in that, The crucible is made of graphite or platinum.

9. The method for preparing thorium-doped fluoride crystals according to claim 6 or 7, characterized in that, The protective atmosphere consists of a mixture of CF4 and Ar gas.

10. A solid-state nuclear optical clock, characterized in that, The time frequency of the solid-state nuclear optical clock is determined by exciting nuclear transitions of thorium atoms in the thorium-doped fluoride crystal according to claim 1 or 2.

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