Micron-resolved boron nitride neutron scintillator and method of making same

Abstract

The application discloses a micron-resolved boron nitride neutron scintillator and a preparation method thereof. The preparation method comprises the following steps: taking an inch-level sapphire single crystal as a substrate, and selecting an aluminum polar surface as a growth surface; using a graphite clamp to fix the substrate, and putting the substrate into a chemical vapor deposition furnace; after vacuumizing, using a graphite heater to heat the furnace body to a reaction temperature; then, feeding raw gas and carrier gas N2 into the furnace, and decomposing the graphite clamp and the graphite heater at high temperature to provide a proper amount of carbon source, so that the gas mixture is reacted to deposit a crystalline boron nitride film on the substrate; using alcohol immersion to separate the boron nitride film from the substrate, fixing the boron nitride film on an aluminum plate after cutting, and manufacturing a neutron scintillator. The boron nitride film is dense and transparent, and exhibits strong blue light emission characteristics due to carbon defects, and can be used as a micron-resolved neutron scintillator. The micron-resolved boron nitride neutron scintillator is successfully realized, and the potential of the boron nitride neutron scintillator in high-resolution neutron imaging is verified.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of neutron scintillation screen, in particular to a micron resolution boron nitride neutron scintillation screen and a preparation method thereof. BACKGROUND

[0002] The neutron imaging community strives to achieve high-resolution neutron imaging, which can capture smaller features and provide finer images, helping to better understand the microstructure of objects. This has important application value in many disciplines such as archaeology, geology, and biomedical science. However, although X-ray imaging can usually achieve sub-micron spatial resolution, neutron imaging has long been limited to a resolution of greater than 10 μm.

[0003] Tb-doped gadolinium oxysulfide (Gd2O2S:Tb, GOS) scintillation screens are widely used in high-resolution neutron imaging research due to their high spatial resolution. This is due to 157 The electrons generated by the reaction of Gd with neutrons have a relatively low energy (~ keV), which limits the range of the electrons in the scintillation screen material to ~ 5 μm. The Paul Scherrer Institute in Switzerland successfully achieved neutron imaging with a resolution as low as 4.5 μm using a 3.5 μm thick GOS film enriched in Gd. 157 The conventional method of making a GOS scintillation screen is to mix GOS powder with a binder and then coat it on a substrate. However, the scattering of photons on the surface of the powder particles leads to a decrease in light output, a larger spot size, and the resulting screen is opaque. The minimum size of the particles determines the lower limit of the thickness of the scintillation screen, and thus the spatial resolution. The particle size of GOS is still in the micron range, and the uniformity during the preparation of the scintillation screen cannot be guaranteed.

[0004] In theory, boron-based scintillation screens also have high spatial resolution. Theoretical simulation results show that 10 The range of secondary particles formed by the reaction of B with neutrons is similar to that of Gd. The boron-based scintillation screens (B2O3 / ZnS, NaB5O8 / ZnS) prepared by B. Schillinger et al. have a spatial resolution comparable to that of a 10 μm GOS scintillation screen. These boron-based scintillation screens are still in the form of powder plus binder, and have low transparency, facing the same difficulties as GOS scintillation screens. In comparison with powder scintillation screens, crystalline film scintillation screens are dense and transparent, which helps to reduce the size of the scintillation spot and provide high imaging resolution. The present application first reports a boron nitride (BN) film-based neutron scintillation screen, which has good crystalline properties, high transparency, and the potential to achieve high spatial resolution neutron imaging. SUMMARY

[0005] The application first reports a boron-based single film type neutron scintillation screen, which directly uses a chemical vapor deposition method to grow an 8-inch self-supporting micron boron nitride film, which can effectively absorb neutrons and can scintillate and emit light. The boron nitride film has good crystallization performance and high density, which can effectively reduce the scattering of scintillation light and reduce the light spot area, thereby improving the spatial resolution. In addition, the film thickness reaches 71 microns, but still has high transparency, which can effectively improve the light output efficiency. The boron nitride scintillation screen is applied to a high-resolution neutron imaging system, and the test obtains that the spatial resolution of the scintillation screen can be as low as 10 microns. The application proves the potential of the boron nitride scintillation screen in high-resolution neutron imaging, which will provide a new choice for future high-resolution neutron imaging technology, and is expected to promote the development of related fields.

[0006] In order to achieve the above-mentioned purpose, the technical scheme adopted by the application is as follows.

[0007] In a first aspect, the application provides a preparation method of a micron-resolution boron nitride neutron scintillation screen, comprising the following steps:

[0008] Step S1, taking an inch-level sapphire single crystal as a substrate, and selecting an aluminum polar surface as a growth surface; the substrate is fixed by using a graphite clamp and is placed in a chemical vapor deposition furnace, the furnace body is vacuumed to 10 -3 Pa or below;

[0009] Step S2, using a graphite heater to heat the furnace body to a reaction temperature laterally, then two kinds of raw gas BCl3 and NH3, or BF3 and NH3, and carrier gas N2 are introduced into the furnace, at the same time, the graphite clamp and the graphite heater are decomposed to provide a carbon source, and the gas mixture is reacted to deposit a crystalline boron nitride film on the substrate; after the gas is introduced, the temperature is kept at the reaction temperature for heat preservation growth;

[0010] Step S3, power off, boron nitride / sapphire is cooled to room temperature and taken out of the furnace, the boron nitride film is separated from the substrate, and after cutting, a boron nitride neutron scintillation screen is prepared.

[0011] In the preparation method of the application, the graphite clamp and the graphite heater provide a small amount of carbon source at high temperature, so that the boron nitride film has high light emission characteristics. After heat preservation growth and power-off cooling, a micron-thick transparent boron nitride film is obtained. The boron nitride film has good crystallization performance and high transparency, and the formed scintillation light spot is small, which can be applied to high spatial resolution neutron imaging.

[0012] Further, in step S2, the graphite heater heats the furnace body to a reaction temperature of 1400-1600℃ laterally.

[0013] Further, in step S2, the two kinds of raw gas BCl3 and NH3, or BF3 and NH3 enter the deposition cavity from the bottom, so that the two kinds of raw gas are mixed and reacted in the middle part of the deposition cavity.

[0014] Further, the gas volume ratio of the two raw gases BCl3 and NH3, or BF3 and NH3 in the step S2 is 1:1.4-1.5.

[0015] Further, the furnace temperature after aeration in the step S2 is kept at the reaction temperature of 1400-1600 DEG C for 3-5 h.

[0016] Further, the step S3 uses alcohol soaking to separate the boron nitride film from the substrate, and after cutting, the boron nitride film is fixed on an aluminum plate to form a boron nitride neutron scintillation screen.

[0017] In the second aspect of the present application, a micron resolution boron nitride neutron scintillation screen is provided, which is prepared by the preparation method of the micron resolution boron nitride neutron scintillation screen.

[0018] Further, the spatial resolution of the micron resolution boron nitride neutron scintillation screen is ≤10 μm.

[0019] Compared with the prior art, the present application has the following beneficial effects:

[0020] In the method of the present application, the graphite clamp and the graphite heater provide a small amount of carbon source at high temperature, so that the boron nitride film has high efficient light emitting characteristics. After heat preservation growth and power-off cooling, a micron level thickness transparent boron nitride film is obtained, which has good crystallization performance, high transparency, and small scintillation light spot, and can be applied to high spatial resolution neutron imaging.

[0021] The present application successfully realizes the micron resolution boron nitride neutron scintillation screen, and proves its potential in high resolution neutron imaging, which will provide a new choice for future high resolution neutron imaging technology. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 It is a schematic diagram of BN film growth and alcohol soaking peeling in the embodiment 1 of the present application, and an excitation spectrum diagram of ultraviolet lamp irradiation;

[0023] Figure 2 It is an XRD spectrum, a Raman spectrum and a cross section SEM diagram of the micron level BN film in the embodiment 2 of the present application;

[0024] Figure 3 It is a schematic diagram of scintillation light transmission and emission process after neutron incidence in the powder scintillation screen and the crystalline film scintillation screen in the embodiment 2 of the present application;

[0025] Figure 4 It is a structure schematic diagram of the BN scintillation screen based neutron imaging system in the embodiment 3 of the present application, and a neutron imaging result diagram of the resolution test target. DETAILED DESCRIPTION

[0026] The specific embodiments of the present application are further described below. It is to be understood that the description of these embodiments is intended to help understand the present application and is not intended to limit the present application. Furthermore, the technical features involved in the various embodiments of the present application described below can be combined with each other as long as there is no conflict.

[0027] The experimental methods in the following examples are all conventional methods unless otherwise specified. The experimental materials used in the following examples are all commercially available unless otherwise specified.

[0028] Example 1, micron-resolved boron nitride neutron scintillator and a method for preparing the same

[0029] This embodiment is used to provide a method for preparing a micron-resolved boron nitride neutron scintillator of the present application. The method uses a vertical chemical vapor deposition (CVD) furnace to grow a micron-level BN film. Considering the specific requirements of neutron imaging technology for the size of the scintillator, this embodiment selects an 8-inch sapphire single crystal wafer as the substrate and selects the aluminum polarity surface of the substrate as the growth surface because the aluminum surface has a higher surface energy, which is beneficial to adsorption and nucleation. At the same time, NH3 and BCl3 (or NH3 and BF3) are selected as the reaction raw gas, which includes the following steps:

[0030] Step S1, use a graphite clamp to fix an 8-inch sapphire substrate, and put it into a chemical vapor deposition furnace. The furnace body is vacuumed to 10 -3 Pa.

[0031] Step S2, then use a graphite heater to heat laterally to 1500℃ and keep warm. N2 is used as the carrier gas and protective gas. BCl3 and NH3 (or NH3 and BF3) are introduced at a volume ratio of 1:1.45. The two raw gas BCl3 and NH3 (or NH3 and BF3) enter the deposition cavity from the bottom, so that the two gases are mixed and reacted in the middle part of the deposition cavity. After the gas is introduced, the temperature is kept at 1500℃, and the temperature is kept for 3h. In this process, NH3 reacts with BCl3 (or NH3 and BF3) to form solid BN deposited on the substrate, and the by-product HCl (or HF) is discharged from the chamber in the form of gas.

[0032] Step S3, then turn off the power, and the BN / sapphire is cooled to room temperature with the furnace, and the h-BN film is peeled off from the sapphire substrate using the alcohol immersion method, thereby obtaining the micron-resolved boron nitride film of the present application.

[0033] Figure 1 a shows a schematic diagram of the BN film grown by the chemical vapor deposition method of the present application and the alcohol immersion peeling, Figure 1 b shows a real object diagram of an 8-inch micron-level BN film. Figure 1In a, the BN film can be separated from the sapphire substrate by simple alcohol immersion treatment due to the low lattice matching degree between the sapphire and the BN film and the micron level of the BN film. The BN film after peeling can be self-supporting, smooth in surface, and high in transparency Figure 1 b). Under ultraviolet light irradiation, the BN film shows high-efficiency blue light emission.

[0034] Further, the micron-level BN film of the present application is subjected to ultraviolet lamp irradiation excitation spectrum analysis, and the results are shown in Figure 1 c and Figure 1 d, wherein Figure 1 c is a fluorescence map of the BN film under 254 nm ultraviolet lamp irradiation excitation, Figure 1 d is a photoluminescence spectrum of the BN film under 257 nm laser excitation. Figure 1 In c, the fluorescence map shows that the luminescence has good uniformity. Figure 1 In the photoluminescence spectrum of d, the wavelength of the luminescence peak is 387 nm, and the luminescence range is 300-600 nm. The luminescence is derived from BN film defects related to carbon. The results show that during the growth of the BN film, high temperature causes slight decomposition of the graphite clamp and the graphite heater, thereby introducing a small amount of carbon source.

[0035] Example 2, crystal structure and cross-sectional morphology analysis of micron-level BN film

[0036] In order to explore the crystallization performance of the BN film, the X-ray diffraction (XRD) spectrum and Raman spectrum and cross-sectional scanning electron microscope (SEM) image of the BN film are measured, and the results are shown in Figure 2 , wherein Figure 2 a is the XRD spectrum of the BN film, Figure 2 b is the Raman spectrum of the BN film, Figure 2 c is the cross-sectional SEM image of the BN film.

[0037] Figure 2 In a, the characteristic diffraction peak of the XRD spectrum shows that the BN film is a layered structure and mainly grows along the c-axis. Figure 2 In b, the Raman spectrum shows that the BN film has an E -1 characteristic vibration mode at 1369 cm 2g , and the half-height width is about 35 cm -1 . These all show that the BN film has good crystallization performance. Further, the cross-sectional morphology of the BN film is observed by SEM, and the cross-sectional SEM image also shows that the BN film has a layered structure, and the thickness is about 71 μm Figure 2 c).

[0038] Figure 3Schematic diagram of the transmission and emission process of the scintillation light after the neutron incident in the powder scintillator and the crystalline film scintillator. As shown in the figure, the BN film still has a high transmittance at a thickness of 71 μm. Compared with the powder scintillator, the crystalline film scintillator has small grain scattering and high optical transmittance, which not only makes the scintillation light spot size small, but also ensures that most of the scintillation light can be effectively transmitted from the inside of the scintillator, thereby improving the light output efficiency of the scintillator.

[0039] Example 3, high-resolution neutron imaging analysis of micron-level BN scintillator

[0040] The BN scintillator is used for high-resolution neutron imaging demonstration, and the structure schematic diagram of the BN scintillator-based neutron imaging system is shown as Figure 4 Figure 4 In a, first, it is cut into 3cm×3cm, and then it is fixed on a 1mm-thick double-polished mirror aluminum plate. Thanks to the self-supporting characteristics of the BN film, it only needs to be fixed at the edge without using any adhesive. This not only simplifies the installation process, but also avoids the adhesive material that may affect the optical performance. In addition, the use of double- polished mirror aluminum plate can effectively reflect the scintillation light, thereby increasing the light output and improving the detection efficiency. The neutron imaging system mainly consists of a scintillator, a mirror, a lens, an image intensifier and a high-sensitivity camera. The neutron beam passes through the imaging object to reach the scintillator, the light emitted by the scintillator is deflected by 90° by the mirror and then passes through the lens, is enhanced by the image intensifier and is finally collected by the camera.

[0041] The above BN scintillator-based neutron imaging system is used to image the resolution test target, and the result is shown as Figure 4 Figure 4 The imaging result of b shows that the BN scintillator of the application can achieve a spatial resolution as low as 10 μm, which fully proves the application potential of the BN scintillator in the field of high-resolution neutron imaging.

[0042] Finally, it is worth mentioning that the GOS scintillator is extremely sensitive to gamma rays during imaging, which makes it difficult to avoid the interference of gamma rays. In contrast, the BN film composed of light atoms shows low gamma ray sensitivity, which helps to ensure the accuracy of neutron imaging. At the same time, BN has excellent radiation resistance and thermal stability, and the scintillator does not need to use adhesive, which can effectively prolong the service life of the neutron imaging system.

[0043] The embodiments of the application are described in detail above, but the application is not limited to the described embodiments. For those skilled in the art, various changes, modifications, replacements and variations of these embodiments can be made without departing from the principles and spirits of the application, and still fall within the protection scope of the application.​​

Claims

1. A method of making a micron-resolved boron nitride neutron scintillator screen, characterized by, The method comprises the following steps: Step S1, taking an inch-level sapphire single crystal as a substrate, and selecting an aluminum polar surface as a growth surface; The substrate is fixed with a graphite jig and put into a chemical vapor deposition furnace. The furnace is evacuated to 10 -3 Pa or less. Step S2, using a graphite heater to heat the furnace body to a reaction temperature of 1400-1600 ℃, then passing two kinds of raw material gases BCl3 and NH3, or BF3 and NH3 according to a volume ratio of 1:1.4-1.5, and a carrier gas N2 into the furnace, and simultaneously decomposing the graphite clamp and the graphite heater to provide a carbon source, so that the gas mixture reaction deposits a crystalline boron nitride film on the substrate; after the gas is passed, the temperature is kept at the reaction temperature of 1400-1600 ℃ for 3-5 h for heat preservation growth; Step S3, power off, boron nitride / sapphire is cooled to room temperature in the furnace, the boron nitride film is separated from the substrate, and after cutting, a boron nitride neutron scintillation screen is prepared; In the boron nitride growth process, high temperature causes slight decomposition of the graphite clamp and the graphite heater, thereby introducing a small amount of carbon source.

2. The method of claim 1, wherein the boron nitride micrometer resolved neutron scintillator is prepared by the steps of: In the step S2, the two kinds of raw material gases BCl3 and NH3, or BF3 and NH3 enter the deposition cavity from the bottom, so that the two kinds of raw material gases are mixed and reacted in the middle part of the deposition cavity.

3. The method of claim 1, wherein the boron nitride micrometer resolved neutron scintillator is prepared by the steps of: In the step S3, the boron nitride film is separated from the substrate by using alcohol soaking, and after cutting, the boron nitride neutron scintillation screen is fixed on an aluminum plate.

4. A micron-resolved boron nitride neutron scintillator, characterized in that, The micron-resolved boron nitride neutron scintillation screen is prepared by the method of any one of claims 1-3.

5. The micron-resolved boron nitride neutron scintillator of claim 4, wherein, The spatial resolution of the micron-resolved boron nitride neutron scintillation screen is ≤10 μm.

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