Hexagonal boron nitride neutron scintillator with high n / gamma discrimination ratio

Abstract

The application discloses a high n / gamma inhibition ratio hexagonal boron nitride neutron scintillator, and a preparation method thereof. The method comprises the following steps: placing a SiC single crystal substrate into a CVD furnace, vacuumizing the furnace, heating the substrate to a reaction temperature, feeding raw gas BCl3 and NH3, or BF3 and NH3 and carrier gas N2 into the furnace, adding a small amount of carbon-containing gas for doping to make the boron nitride film have high efficient light emitting performance, growing after heat preservation, cooling after power-off, and stripping from the substrate to obtain the hexagonal boron nitride neutron scintillator. The scintillator is composed of small atomic mass elements, and has high n / gamma resolution capability. The application provides a neutron scintillator, carbon-doped hexagonal boron nitride (h-BN:C), which has high n / gamma inhibition ratio. The application introduces a carbon source to dope h-BN during h-BN film growth, so that the h-BN film shows wide spectrum emission of 300-800 nm.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of neutron scintillation screens, and particularly relates to a hexagonal boron nitride neutron scintillation screen with high n / γ suppression ratio. BACKGROUND

[0002] In recent years, neutron imaging technology has developed rapidly, which allows penetration of thick metal layers while having high sensitivity to light elements (hydrogen, lithium, etc.). High-resolution neutron imaging technology can non-destructively reveal the internal structure and composition of materials, providing key information for nuclear reaction monitoring, material design and geological exploration, and has become a core technology to promote the continuous development of nuclear science, material science and geoscience. The mainstream technology of neutron imaging is to use a neutron scintillation screen to convert neutron information with spatial signals into visible light information, which is then collected by a rear-end camera. Therefore, the neutron scintillation screen is one of the core components of neutron imaging technology.

[0003] The neutron source is usually accompanied by a certain flux of gamma rays, also known as a mixed neutron and gamma radiation field. If the neutron scintillation screen is also sensitive to gamma rays, it will cause the neutron imaging pattern to be disturbed by the gamma ray background, affecting the imaging accuracy. Therefore, the scintillation screen for neutron imaging needs to have strong neutron / gamma (n / γ) discrimination ability, i.e. high n / γ suppression ratio. The commonly used neutron scintillation screens at present are 6 LiF / ZnS and Gd2O2S:Tb (GOS) scintillation screens. ZnS and GOS materials with heavy atomic mass have a certain sensitivity to gamma rays, so the n / γ suppression ratio of the neutron scintillation screen is not high (<10). Especially for GOS materials, their relatively high gamma ray sensitivity may introduce additional interference in practical applications, which limits their utility in situations where strict differentiation between neutrons and gamma rays is required. SUMMARY

[0004] In order to overcome the deficiencies of the above prior art, the present application provides a new neutron scintillation screen with high n / γ suppression ratio - carbon-doped h-BN (h-BN:C). 10B has a thermal neutron capture cross section of about 3840 barns, and is also one of the commonly used elements for neutron scintillation and imaging. The neutron scintillation screen materials are all composed of light elements, and the absorption cross section of gamma rays is small, and the sensitivity is low. Using the chemical vapor deposition (CVD) method, a 6-inch micron-level thickness h-BN:C film is grown on a silicon carbide (SiC) single crystal substrate, and a neutron scintillation screen is made by peeling it off from the substrate. During the growth process, the carbon defects introduced by using methane (CH4) as the carbon source for doping make the h-BN show strong blue light emission. In the radiation scintillation test experiment, the h-BN film shows a high n / γ suppression ratio of about 45000. Finally, the neutron imaging test of the h-BN scintillation screen proves the great potential of the h-BN film as a neutron scintillation screen.

[0005] In order to achieve the above-mentioned purpose, the technical scheme adopted by the present application is:

[0006] In the first aspect of the present application, a preparation method of a high n / γ suppression ratio hexagonal boron nitride neutron scintillation screen is provided, comprising the following steps:

[0007] Step S1, place the SiC single crystal substrate into a chemical vapor deposition furnace, and vacuumize the furnace body to 10 -3 Pa or below;

[0008] Step S2, heat the substrate to a reaction temperature, and pass two kinds of raw gas BCl3 and NH3, or BF3 and NH3, and carrier gas N2 into the furnace, while adding a small amount of carbon-containing gas for doping to make the boron nitride film have high-efficiency light-emitting performance; after the gas is passed, the temperature is maintained at the reaction temperature for growth;

[0009] Step S3, then power off, and after natural cooling to room temperature, a micron-level carbon-doped h-BN film is obtained, and after peeling off from the substrate, a hexagonal boron nitride neutron scintillation screen is obtained.

[0010] Further, the carbon-containing gas in step S2 includes at least one of CH4, C2H2, and CCl4.

[0011] Further, the reaction formula in step S2 is BCl3+NH3→BN+3HCl, or BF3+NH3→BN+3HF.

[0012] Further, the substrate is heated to a reaction temperature of 1800-1900℃ in step S2.

[0013] Further, the gas volume ratio of the two kinds of raw gas BCl3 and NH3, or BF3 and NH3 in step S2 is 1:2.

[0014] Further, the gas volume ratio of the carbon-containing gas to the raw gas NH3 in step S2 is 0.0001-0.1:1.

[0015] Furthermore, in step S2, the gas volume ratio of carbon-containing gas to raw material gas NH3 is 0.001:1.

[0016] Furthermore, the heat preservation growth time in step S2 is 2 to 5 hours.

[0017] Furthermore, in step S2, after the gas is introduced, the furnace temperature is maintained at the reaction temperature of 1800-1900℃ for 4-5 hours.

[0018] In a second aspect, the present invention provides a hexagonal boron nitride neutron scintillation screen with a high n / γ suppression ratio, which is prepared by the above-described method for preparing a hexagonal boron nitride neutron scintillation screen with a high n / γ suppression ratio.

[0019] Furthermore, the n / γ suppression ratio of the hexagonal boron nitride neutron scintillation screen is ≥40000.

[0020] Compared with the prior art, the beneficial effects of the present invention are:

[0021] Neutron imaging technology can non-destructively reveal the internal structure and composition of materials, and has wide applications in materials design, medical diagnostics, and geological exploration. Since neutron sources are usually accompanied by a certain flux of gamma rays, the scintillation screen used in neutron imaging technology needs to have strong neutron / gamma (n / γ) resolution. This invention provides a neutron scintillation screen with a high n / γ suppression ratio—carbon-doped hexagonal boron nitride (h-BN:C). A 6-inch thick h-BN film was grown on a silicon carbide substrate using chemical vapor deposition, and then peeled off from the substrate to form the neutron scintillation screen. The introduction of a carbon source during growth to dope h-BN resulted in the h-BN film exhibiting a broad spectrum of emission from 300 to 800 nm. Radiation scintillation tests showed that the h-BN film has an n / γ suppression ratio of approximately 45,000, which is higher than commonly used... 6 LiF / ZnS and GOS scintillation screens can perform neutron imaging with extremely low gamma background in mixed neutron and gamma-ray radiation fields. Finally, the neutron imaging demonstration also shows the great potential of h-BN as a neutron scintillation screen. Attached Figure Description

[0022] Figure 1 This is a schematic diagram and a physical image of the growth of the 6-inch micron-thick h-BN film in Embodiment 1 of the present invention;

[0023] Figure 2 This is a microscopic image of the h-BN film in Example 2 of the present invention.

[0024] Figure 3 This is a graph showing the results of elemental analysis of the h-BN membrane in Example 2 of the present invention;

[0025] Figure 4 This is a graph showing the results of elemental analysis of the extremely bright blue light and transient fluorescence spectra of the h-BN film in Example 3 of the present invention;

[0026] Figure 5 This is a graph showing the results of gamma-ray and neutron irradiation scintillation response tests of the h-BN film in Example 4 of the present invention;

[0027] Figure 6 This is a schematic diagram of the neutron imaging test device and scintillation screen in Embodiment 5 of the present invention, as well as the neutron and gamma imaging results of the copper-cased kerosene lighter by the h-BN scintillation screen. Detailed Implementation

[0028] The specific embodiments of the present invention will be further described below. It should be noted that these descriptions are for the purpose of aiding understanding the present invention and do not constitute a limitation thereof. Furthermore, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other. Unless otherwise specified, the experimental methods in the following embodiments are conventional methods, and the experimental materials used in the following embodiments are all available through conventional commercial channels.

[0029] In the following embodiments, SEM images of the h-BN film were acquired using an SU5000 scanning electron microscope system; XRD spectra were obtained using a Panalytical X'Pert Pro X-ray diffractometer (with Cu-Kα radiation); XPS measurements were performed using a Thermo ESCALAB 250Xi multi-functional imaging electron spectrometer; cross-sectional TEM samples of the h-BN thick film were prepared using the ion gun of a FEI Helios 5UX focusing dual-scanning electron microscope. TEM tests were performed using a FEITalos F200X transmission electron microscope.

[0030] h-BN film luminescence performance testing: Photoluminescence spectra were obtained by excitation with an EX5 / 250ArF excimer laser (193nm), and spectral acquisition was performed using a QE65PRO scientific-grade spectrometer (200-1000nm). Transient fluorescence spectra were acquired using an Ediburgh FLS1000 transient fluorescence spectrometer, with the excitation source being a 213nm pico second laser from CNI, model DPS-213-Pico, with a frequency of 5 MHz.

[0031] h-BN film radiation scintillation test: Radiation scintillation performance test adopts... 137 Cs Gamma Source and 252A Cf neutron-gamma source. The pulsed radiation source, sample, and photomultiplier tube (PMT) are arranged in a straight line within a dark chamber. The pulsed radiation source emits radioactive particles that strike the sample, which emits a visible light signal that is collected by the high-voltage PMT and converted into an electrical signal. This electrical signal is converted into a multichannel signal and transmitted to both a Tektronix TDS3052B oscilloscope for display as a pulse waveform and a digital multichannel analyzer and computer for acquisition, obtaining the sample's pulse height spectrum, such as... Figure 5 As shown in b. The PMT's high voltage was set to 1200V, and the radiative scintillation test time for both sources was the same, approximately 13.4h.

[0032] h-BN film neutron scintillation screen imaging test: The neutron source was the BL20 beamline of the China Spallation Neutron Source. The neutron beam spot diameter at the exit of this beamline was 20 mm. A 6-inch h-BN film was cut into 10×10 cm pieces and placed in a self-made Al metal fixture. The fixture was then installed in the neutron imaging system dark box. The neutron imaging system dark box mainly consists of a scintillation screen, a reflector, and a camera, such as... Figure 6 As shown in Figure a, the scintillation screen is located approximately 3.7 m from the BL20 neutron beam exit. Within the neutron detection system, the neutron beam passes through the object being imaged (a kerosene lighter with a copper metal casing) and is collected by the system. Inside the neutron detection system, the neutron beam strikes the scintillation screen, and the light emitted from the screen passes through a mirror and is collected by the camera, thus imaged the object. A direct-through beam with the same exposure time and number of frames is normalized to obtain the neutron imaging result. The exposure time is set to 30 minutes. The gamma-ray imaging optical path is similar to that of the neutron imaging system, and the exposure time is set to 2 hours.

[0033] Example 1: Preparation method of hexagonal boron nitride neutron scintillation screen with high n / γ suppression ratio

[0034] This embodiment provides a method for fabricating a hexagonal boron nitride neutron scintillation screen with a high n / γ suppression ratio according to the present invention. The method uses a 6-inch silicon carbide (SiC) single crystal as the substrate material and includes the following steps:

[0035] Step S1: Place the 6-inch SiC single crystal substrate into a chemical vapor deposition (CVD) furnace, and evacuate the furnace to 10°C. -3 Pa.

[0036] Step S2: Then, the substrate is heated to the reaction temperature of 1800-1900℃ using resistance heating and held at that temperature. Then, N2 is used as the carrier gas and protective gas, and two raw material gases, BCl3 and NH3 (or BF3 and NH3), are introduced at a gas volume ratio of 1:2. The two raw material gases react to generate BN, which is deposited on the substrate (reaction formula: BCl3+NH3→BN+3HCl, or BF3+NH3→BN+3HF). During this process, CH4 is used as the carbon doping source, and the gas volume ratio of CH4:NH3 is 0.001:1. After the gas is introduced, the temperature is maintained at 1800-1900℃ for 4 hours.

[0037] Step S3: Then, power is turned off, and the h-BN / SiC wafer is cooled to room temperature and removed from the furnace. The h-BN film is peeled off from the SiC substrate to obtain the hexagonal boron nitride film with high n / γ suppression ratio of the present invention.

[0038] In one embodiment, in step S2, the carbon-containing gas includes at least one of CH4, C2H2, and CCl4.

[0039] Figure 1 This image shows a schematic diagram and a physical photograph of a 6-inch, micrometer-thick h-BN film prepared using the method of this invention. Figure 1 a is a schematic diagram of the growth of h-BN:C film by chemical vapor deposition on SiC substrate; Figure 1 b is a physical image of the grown h-BN / SiC wafer; Figure 1 c is a photograph of the self-supporting h-BN membrane after peeling.

[0040] Figure 1 In step b, since the film thickness reaches the micrometer level and a slight decomposition occurs on the SiC substrate surface before h-BN growth to form a graphite interlayer, the h-BN film can be completely peeled off from the SiC substrate. Figure 1 c shows the peeled-off self-supporting h-BN membrane, which exhibits a certain degree of flexibility.

[0041] Example 2: Microscopic Images and Structural Characterization Analysis of h-BN Films

[0042] The growth surface of the h-BN film was observed using optical microscopy and scanning electron microscopy (SEM), and the results are as follows: Figure 2 a and Figure 2 As shown in b. Figure 2 a and Figure 2 In diagram b, the growth surface of the h-BN film can be seen to be rough, composed of stacked h-BN particles with a diameter of approximately 500 nm. The cross-section of the h-BN film was observed using a scanning electron microscope (SEM), and the results are as follows... Figure 2As shown in Figure c, the film thickness is approximately 36 μm. This micrometer-level thickness facilitates film peeling and enables self-support. X-ray diffraction (XRD) analysis of the h-BN film yielded the following results: Figure 2 As shown in Figure d, the main crystal plane of the h-BN film is the (002) plane, indicating that the h-BN film of the present invention mainly grows along the c-axis.

[0043] Furthermore, elemental analysis of the h-BN film was performed using X-ray electron spectroscopy (XPS) and energy-dispersive surface scanning (EDSMapping) of transmission electron microscopy (TEM), and the results are as follows: Figure 3 As shown. Among them, Figure 3 a represents the XPS plot of B1s. Figure 3 b is the XPS spectrum of N1s. Figure 3 c represents the TEM image and the corresponding EDS mapping spectra of B, N, C, and Si elements.

[0044] Figure 3 In the XPS spectrum of B1s, a BN bond is shown at 190.6 eV. Figure 3 a), while the XPS spectrum of N1s is linearly asymmetric ( Figure 3 (b) Peak fitting revealed a double peak at 398.2 eV and 398.8 eV, with the 398.2 eV peak originating from BN bonds and the 398.8 eV peak possibly representing CN bonds. TEM EDS mapping also detected the presence of carbon elements in the film. Figure 3 c) indicates that carbon has been incorporated into the h-BN film.

[0045] Example 3: Elemental analysis of h-BN film using extremely bright blue light and transient fluorescence spectra.

[0046] This embodiment analyzes the extremely bright blue light and transient fluorescence (TRPL) spectra of the h-BN film, and the results are as follows: Figure 4 As shown. Among them, Figure 4 a shows the photoluminescence (PL) spectrum of the h-BN film measured under 193 nm excitation. Figure 4 The illustration in the upper right corner is a photograph of the h-BN film emitting light under 213nm laser illumination; Figure 4 b shows the transient fluorescence (TRPL) spectra at wavelengths of 340, 370, 400, 430, and 470 nm at room temperature.

[0047] Figure 4In Figure a, the inset shows the extremely bright blue light emission of the h-BN film under 213 nm light excitation. The h-BN film exhibits a broad emission spectrum of 300–800 nm, with a peak emission at 387 nm. Despite the relatively thick sample, the emitted light can still pass through and be collected, indicating that the h-BN film has a high blue light emission intensity. Gaussian peak fitting was used to fit the line shape of the emission spectrum, and this broad spectrum can be mainly divided into four emission peaks: 350, 387, 451, and 580 nm. Figure 4 In b, due to its broadband emission, the TRPL spectra of the h-BN film at 340, 370, 400, 430, and 460 nm were measured. It can be seen that the emission decay time of the h-BN film is similar at these emission wavelengths, approximately 4.2 ns. This emission lifetime is significantly shorter than that of ZnS:Cu and Gd₂O₂S:Tb fluorescent materials commonly used in neutron scintillation screens.

[0048] Example 4: Analysis of Gamma-ray and Neutron Irradiation Scintillation Performance of h-BN Films

[0049] This embodiment investigates the gamma-ray and neutron irradiation scintillation properties of the h-BN film, and the results are as follows: Figure 5 As shown. Among them, Figure 5 a represents the theoretical gamma-ray absorption cross-sections of BN, LiF / ZnS, and GOS. Figure 5 b is a schematic diagram of the radiation scintillation luminescence testing system. Figure 5 c is 137 Pulse height spectrum of Cs gamma source excitation; Figure 5 d is 252 Pulse height spectrum of Cf neutron-gamma source excitation.

[0050] Figure 5 In Figure a, it can be seen that the absorption cross section of BN for gamma rays is smaller than that of LiF / ZnS and GOS, indicating that BN has extremely low sensitivity to gamma rays. Figure 5 b shows a schematic diagram of the radiation scintillation luminescence testing system. And by Figure 5 As seen in c and 5d, the h-BN film shows almost no response to gamma rays, but a significant response to neutrons. In fact, the collected gamma-ray response signal is almost identical to noise, making it impossible to determine the true source of the signal. Considering the source activity and the number of high-energy particles released, the calculated h-BN n / γ suppression ratio (number of photons produced by a single neutron-excited scintillator / number of photons produced by a single gamma-photon-excited scintillator) is approximately 45000. Therefore, both theoretical and experimental studies confirm that h-BN exhibits a higher gamma-ray suppression ratio than... 6 LiF / ZnS and GOS have high n / γ suppression ratios. In neutron experiments, gamma photon fluxes in the megaelectronvolt range are unavoidable. Therefore, h-BN films have a significant advantage in identifying neutrons independently in such complex radiation mixing fields.

[0051] Example 5: Analysis of Neutron and Gamma Imaging Results of h-BN Scintillation Screen

[0052] In this example, an h-BN film was used as a neutron scintillation screen for preliminary neutron imaging testing. The film was cut to the required scintillation screen size of 10 × 10 cm, and a neutron imaging experiment was conducted using the neutron beam from the China Spallation Neutron Source. The results are as follows: Figure 6 As shown. A schematic diagram of the imaging test setup and a photograph of the h-BN scintillation screen are shown below. Figure 6 As shown in a. Figure 6 The results are neutron and gamma imaging of the h-BN scintillation screen; among them, Figure 6 a shows the neutron imaging test setup and its optical path diagram. Figure 6 b represents a schematic diagram of the h-BN flickering screen. Figure 6 c is the neutron and gamma imaging result of a copper-cased kerosene lighter obtained by an h-BN scintillation screen.

[0053] Figure 6 In c, the contrast between light and dark areas in the image image obtained by the h-BN scintillation screen demonstrates its excellent luminous uniformity and imaging capability. Since neutrons are insensitive to heavy metallic elements, they can penetrate thick metal layers to image the kerosene inside a lighter, which has significant application value. Furthermore, to demonstrate the high n / γ suppression ratio of the h-BN scintillation screen, gamma-ray imaging was performed using the h-BN scintillation screen. The optical path diagram of the gamma-ray imaging is compared with... Figure 6 The optical path diagram for neutron imaging of type a is similar. Under gamma-ray irradiation, despite integration for 2 hours, it was still impossible to image the object. Figure 6 c).

[0054] In summary, this invention utilizes chemical vapor deposition to grow a 6-inch h-BN film with a thickness of approximately 36 μm, and after simple exfoliation, obtains a self-supporting h-BN film for use as a neutron scintillation screen. The use of methane as a carbon source during h-BN growth introduces carbon defects, resulting in strong blue light emission from the h-BN film, with a peak emission wavelength of 387 nm. The h-BN neutron scintillation screen exhibits an n / γ suppression ratio of approximately 45,000, which is higher than commonly used... 6 LiF / ZnS and GOS scintillation screens can resolve neutron signals and perform imaging in complex radiation mixing fields. Neutron imaging demonstration results showcase the enormous potential of h-BN scintillation screens for neutron imaging applications.

[0055] The embodiments of the present invention have been described in detail above, but the present invention is not limited to the described embodiments. For those skilled in the art, various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and these variations still fall within the protection scope of the present invention.

Claims

1. A method of producing a high n / gamma rejection ratio hexagonal boron nitride neutron scintillator screen, characterized by, The method comprises the following steps: Step S1, the SiC single crystal substrate is put into a chemical vapor deposition furnace, the furnace is vacuumed to 10 -3 Pa or below; Step S2, heating the substrate to a reaction temperature, and introducing two raw material gases BCl3 and NH3, or BF3 and NH3, and a carrier gas N2 into the furnace, and adding a small amount of carbon-containing gas to dope the boron nitride film to have high luminescent performance; after the gas is introduced, the temperature is kept at the reaction temperature to perform heat preservation growth; Step S3, then power off, and after natural cooling to room temperature, a micron-level carbon-doped h-BN film is obtained, and after being peeled off from the substrate, a hexagonal boron nitride neutron scintillator is obtained; The carbon-containing gas in the step S2 comprises at least one of CH4, C2H2 and CCl4; In the step S2, the substrate is heated to a reaction temperature of 1800-1900 DEG C. In the step S2, the gas volume ratio of the two raw material gases BCl3 and NH3, or BF3 and NH3 is 1:

2. In the step S2, the gas volume ratio of the carbon-containing gas to the raw material gas NH3 is 0.0001-0.1:

1. In the step S2, after the gas is introduced, the furnace body temperature is kept at the reaction temperature of 1800-1900 DEG C, and heat preservation is performed for 4-5 h.

2. The method of claim 1, wherein the high n / gamma rejection hexagonal boron nitride neutron scintillator is characterized by, In the step S2, the reaction formula is BCl3+NH3→BN+3HCl, or BF3+NH3→BN+3HF.

3. The method of claim 1, wherein the high n / gamma rejection hexagonal boron nitride neutron scintillator is characterized by, In the step S2, the gas volume ratio of the carbon-containing gas to the raw material gas NH3 is 0.001:

1.

4. A high n / gamma rejection ratio hexagonal boron nitride neutron scintillator, characterized by, Prepared by the preparation method in any one of claims 1-3.

5. The high n / gamma rejection ratio hexagonal boron nitride neutron scintillator according to claim 4, characterized in that, The n / γ suppression ratio of the hexagonal boron nitride neutron scintillator is greater than or equal to 40000.

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