Hexagonal boron nitride neutron scintillation screen with high n / gamma inhibition ratio

Abstract

The invention discloses a hexagonal boron nitride neutron scintillation screen with a high n / gamma inhibition ratio, and a preparation method of the hexagonal boron nitride neutron scintillation screen comprises the following steps: putting a SiC single crystal substrate into a CVD furnace, vacuumizing the furnace body, heating the substrate to a reaction temperature, introducing raw material gases BCl3 and NH3, or BF3 and NH3 and a carrier gas N2 into the furnace, heating the substrate to a reaction temperature, and carrying out a reaction for 2-3 hours; meanwhile, a small amount of carbon-containing gas is added for doping, so that the boron nitride film has efficient light-emitting performance, and the hexagonal boron nitride neutron scintillation screen is obtained after heat-preservation growth, power-off cooling and stripping from the substrate. The scintillation screen is composed of elements with small atomic mass and has high n / gamma resolution capability. The invention provides a neutron scintillation screen-carbon doped hexagonal boron nitride (h-BN: C) with a high n / gamma inhibition ratio. A carbon source is introduced in the growth process of an h-BN film to dope the h-BN, so that the h-BN film displays 300-800nm wide spectrum emission.

Description

Technical Field

[0001] The invention belongs to the field of neutron scintillator screens, and in particular relates to a hexagonal boron nitride neutron scintillator screen with a high n / γ suppression ratio. Background Art

[0002] In recent years, neutron imaging technology has developed rapidly, which allows penetration of thick metal layers and has high sensitivity to light elements (hydrogen, lithium, etc.). High-resolution neutron imaging technology can non-destructively reveal the internal structure and composition of materials, provide key information for nuclear reaction monitoring, material design and geological surveys, and become a core technology for promoting the continuous development of nuclear science, material science and geological science. 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 the back-end camera. Therefore, the neutron scintillation screen is one of the core components of neutron imaging technology.

[0003] Neutron sources are usually accompanied by a certain flux of gamma rays, also known as a mixed radiation field of neutrons and gamma rays. If the neutron scintillator is also sensitive to gamma rays, the neutron imaging pattern will be interfered by the gamma ray background, affecting the imaging accuracy. Therefore, the neutron imaging scintillator needs to have a strong neutron / gamma (n / γ) resolution capability, that is, a high n / γ suppression ratio. The commonly used neutron scintillator screens are: 6 LiF / ZnS and Gd2O2S:Tb(GOS) scintillator screens. ZnS and GOS materials with heavy atomic mass are sensitive to gamma rays, so the n / γ suppression ratio of their neutron scintillator screens is not high (<10). Especially for GOS materials, their relatively high gamma ray sensitivity may introduce additional interference in practical applications, which limits their usefulness in situations where strict distinction between neutrons and gamma rays is required. Summary of the invention

[0004] In order to overcome the above-mentioned deficiencies of the prior art, the present invention proposes to provide a new neutron scintillator screen having a 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 scintillator screen material is composed of light elements, with a small absorption cross section for gamma rays and low sensitivity. A 6-inch micron-thick h-BN:C film was grown on a silicon carbide (SiC) single crystal substrate using chemical vapor deposition (CVD) and peeled off from the substrate to make a neutron scintillator screen. The carbon defects introduced by doping with methane (CH4) as a carbon source during the growth process make h-BN show strong blue light emission. In the radiation scintillation test experiment, the h-BN film showed a high n / γ suppression ratio of about 45,000. Finally, the h-BN scintillator screen was tested for neutron imaging, and the results proved the great potential of the h-BN film as a neutron scintillator screen.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is:

[0006] In a first aspect, the present invention provides a method for preparing a hexagonal boron nitride neutron scintillator screen with a high n / γ suppression ratio, comprising the following steps:

[0007] Step S1: Place the SiC single crystal substrate into a chemical vapor deposition furnace, and evacuate the furnace to 10 -3 Below Pa;

[0008] Step S2, heating the substrate to the reaction temperature, introducing two raw material gases BCl3 and NH3, or BF3 and NH3, and carrier gas N2 into the furnace, and adding a small amount of carbon-containing gas for doping so that the boron nitride film has high-efficiency luminescent properties; after ventilation, the temperature is maintained at the reaction temperature for heat preservation growth;

[0009] Step S3, then turn off the power, cool naturally to room temperature to obtain a micron-scale carbon-doped h-BN film, and peel it off from the substrate to obtain a hexagonal boron nitride neutron scintillator screen.

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

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

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

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

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

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

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

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

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

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

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] Neutron imaging technology can reveal the internal structure and composition of materials in a non-destructive manner, and has a wide range of applications in the fields of material design, medical diagnosis, and geological survey. Since neutron sources are usually accompanied by a certain flux of gamma rays, the scintillator screen used in neutron imaging technology needs to have a strong neutron / gamma (n / γ) resolution capability. The present invention provides a neutron scintillator screen with a high n / γ suppression ratio - carbon-doped hexagonal boron nitride (h-BN:C). A 6-inch h-BN thick film was grown on a silicon carbide substrate by chemical vapor deposition, and then peeled off the substrate to make a neutron scintillator screen. During the growth process, a carbon source was introduced to dope the h-BN so that the h-BN film displayed a wide spectrum emission of 300 to 800 nm. The radiation scintillation test results show that the h-BN film has an n / γ suppression ratio of about 45,000, which is higher than the commonly used 6 LiF / ZnS and GOS scintillator screens can perform neutron imaging with extremely low gamma background in a mixed neutron and gamma ray radiation field. Finally, the neutron imaging demonstration also shows the great potential of h-BN as a neutron scintillator screen. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 The growth diagram and physical diagram of a 6-inch micrometer-thick h-BN film in Example 1 of the present invention are shown;

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

[0024] Figure 3 This is a diagram showing the result of elemental analysis of the h-BN film in Example 2 of the present invention;

[0025] Figure 4 This is a result diagram 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 result diagram of the scintillation response test of the h-BN film to gamma rays and neutron irradiation in Example 4 of the present invention;

[0027] Figure 6 It is a schematic diagram of the neutron imaging test device and the scintillation screen in Example 5 of the present invention, and a diagram showing the results of neutron and gamma imaging of a copper shell kerosene lighter by the h-BN scintillation screen. DETAILED DESCRIPTION

[0028] The specific embodiments of the present invention are further described below. It should be noted that the description of these embodiments is used to help understand the present invention, but does not constitute a limitation of the present invention. In addition, the technical features involved in each embodiment of the present invention described below can be combined with each other as long as they do not conflict with each other. The experimental methods in the following examples are conventional methods unless otherwise specified, and the test materials used in the following examples are all available through conventional commercial channels unless otherwise specified.

[0029] In the following examples, the SEM photos of h-BN film characterization were collected using a SU5000 scanning electron microscope system; the XRD spectrum was measured using a Panalytical X'Pert Pro X-ray diffractometer (with Cu-Kαradiation); the XPS measurement was performed using a Thermo ESCALAB 250Xi multifunctional imaging electron spectrometer; the cross-sectional TEM sample preparation of the h-BN thick film was performed using the ion gun of a FEI Helios 5UX focusing dual-number scanning electron microscope. The TEM test was performed using a FEITalos F200X transmission electron microscope.

[0030] h-BN film luminescence performance test: The photoluminescence spectrum was obtained by laser excitation of EX5 / 250ArF excimer laser (193nm), and the spectrum was collected using QE65PRO scientific-grade spectrometer (200-1000nm). The transient fluorescence spectrum was collected using Ediburgh FLS1000 transient fluorescence spectrometer, where the excitation light source was a 213nm picosecond laser of DPS-213-Pico model from CNI, with a frequency of 5 MHz.

[0031] h-BN film radiation scintillation test: The radiation scintillation performance test adopts 137 Cs gamma source and 252Cf neutron-gamma source. The pulse radiation source, sample and photomultiplier tube (PMT) are placed in a dark box in the same straight line. The pulse radiation source emits radioactive particles that hit the sample, and the sample emits visible light signals that are collected by the high-voltage PMT and converted into electrical signals. The electrical signal is transmitted to the Tektronix TDS3052B oscilloscope through a one-to-many converter to be displayed in the form of a pulse waveform, and to a digital multi-channel analyzer and computer for acquisition to obtain the pulse height spectrum of the sample, such as Figure 5 As shown in Figure b, the high voltage of the PMT was set to 1200V, and the radiation flicker test time of the two sources was the same, about 13.4h.

[0032] h-BN film neutron scintillation screen imaging test: The neutron source comes from the BL20 beam line of the China Spallation Neutron Source. The neutron beam spot diameter at the exit of the beam line is 20mm. Cut the 6-inch h-BN film into 10×10cm and put it into a homemade Al metal fixture. Install the fixture in the dark box of the neutron imaging system. The dark box of the neutron imaging system mainly consists of a scintillation screen, a reflector and a camera, such as Figure 6 As shown in a. The scintillation screen is about 3.7m away from the neutron BL20 beam outlet. In the neutron detection system, the neutron beam passes through the imaged object (copper metal shell kerosene lighter) and is collected by the neutron detection system. In the neutron detection system, the neutron beam hits the scintillation screen, and the light emitted by the scintillation screen passes through the reflector and is collected by the camera to image the imaged object. The straight-through beam with the same exposure time and frame number is normalized to obtain the neutron imaging result. The exposure time is set to 30min. The γ-ray imaging optical path diagram 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 scintillator screen with high n / γ suppression ratio

[0034] This embodiment is used to provide a method for preparing a hexagonal boron nitride neutron scintillator screen with a high n / γ suppression ratio of the present invention. The preparation method selects a 6-inch silicon carbide (SiC) single crystal as a substrate material and comprises the following steps:

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

[0036] Step S2, then use resistance heating to heat the substrate to a reaction temperature of 1800-1900°C and then keep it warm; then use N2 as a carrier gas and protective gas, and introduce two raw material gases BCl3 and NH3 (or BF3 and NH3) at a gas volume ratio of 1:2; the two raw material gases react to generate BN deposited on the substrate (reaction formula: BCl3+NH3→BN+3HCl, or BF3+NH3→BN+3HF); in this process, CH4 is used as a doping carbon source at the same time, and the gas volume ratio of CH4:NH3 is 0.001:1; after ventilation, the temperature is maintained at 1800-1900°C and kept warm for 4 hours.

[0037] Step S3, then turn off the power, and the h-BN / SiC wafer is taken out of the furnace as it cools down to room temperature, and the h-BN film is peeled off from the SiC substrate to obtain the hexagonal boron nitride film with a high n / γ suppression ratio of the present invention.

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

[0039] Figure 1 The growth diagram and the actual image of the 6-inch micron-thick h-BN film prepared by the method of the present invention are shown in FIG. Figure 1 a is a schematic diagram of the growth of chemical vapor deposited h-BN:C film on SiC substrate; Figure 1 b is a physical picture of the grown h-BN / SiC wafer; Figure 1 c is the actual picture of the self-supporting h-BN film after peeling.

[0040] Figure 1 In b, since the thickness of the film growth reaches the micron level and the surface of the SiC substrate is slightly decomposed to form a graphite intermediate layer before the growth of h-BN, the h-BN film can be completely peeled off from the SiC substrate. Figure 1 c shows the peeled-off free-standing h-BN film, which exhibits certain flexibility.

[0041] Example 2: Microscopic image and structural characterization analysis of h-BN film

[0042] The growth surface of the h-BN film was observed using an optical microscope and a scanning electron microscope (SEM). Figure 2 a and Figure 2 As shown in b. Figure 2 a and Figure 2 In b, it can be seen that the growth surface of the h-BN film is rough and is composed of h-BN particles with a diameter of about 500nm. The cross section of the h-BN film was observed using a scanning electron microscope (SEM). Figure 2As shown in Figure c, the film thickness is about 36 μm. This micron-level thickness also makes it easy to peel off the film and make it self-supporting. The h-BN film was analyzed by X-ray diffraction (XRD) and the results are shown in Figure 5. Figure 2 As shown in d, the figure shows that 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 by X-ray electron spectroscopy (XPS) and transmission electron microscopy (TEM) energy spectrum scanning (EDSMapping). Figure 3 As shown. Among them, Figure 3 a is the XPS spectrum of B1s, Figure 3 b is the XPS spectrum of N1s, Figure 3 c is the TEM image and the corresponding EDS Mapping spectra of B, N, C and Si elements.

[0044] Figure 3 The XPS spectrum of B1s shows the BN bond at 190.6 eV ( Figure 3 a), while the XPS spectrum of N1s is linearly asymmetric ( Figure 3 b), the double peaks of 398.2eV and 398.8eV were obtained by peak fitting, of which the peak of 398.2eV comes from the BN bond, and the peak of 398.8eV may be the CN bond. TEM EDS Mapping also detected the presence of carbon in the film ( Figure 3 c), indicating that carbon has been incorporated into the h-BN film.

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

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

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

[0048] Example 4: Gamma-ray and neutron irradiation scintillation performance analysis of h-BN films

[0049] In this example, the gamma ray and neutron irradiation scintillation performance of h-BN film was investigated. Figure 5 As shown. Among them, Figure 5 a is the theoretical gamma-ray absorption cross section of BN, LiF / ZnS and GOS, Figure 5 b is a schematic diagram of the radiation scintillation luminescence test 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 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 the schematic diagram of the radiation scintillation luminescence test system. Figure 5 c and 5d, it can be seen that the h-BN film hardly responds to gamma rays, but has a significant response to neutrons. In fact, the collected gamma-ray response signal is almost the same as the noise, and it is impossible to determine the true source of the signal. Considering the activity of the source and the number of high-energy particles released, the calculated h-BN n / γ suppression ratio (the number of photons generated by a single neutron-excited scintillator / the number of photons generated by a single gamma photon-excited scintillator) is about 45,000. Therefore, both theory and experiment have confirmed that h-BN has a higher suppression ratio than 6 High n / γ suppression ratio of LiF / ZnS and GOS. In neutron experiments, gamma photon flux with energy in the mega-electron-volt range is inevitable. Therefore, h-BN film has a huge advantage in identifying neutrons alone in this complex mixed field of radiation.

[0051] Example 5: Analysis of neutron and gamma imaging results of h-BN scintillation screen

[0052] In this example, h-BN film was used as a neutron scintillator screen to conduct preliminary neutron imaging tests. The film was cut into the required scintillation screen size of 10×10 cm for neutron imaging tests, and the neutron imaging experiment was carried out using the neutron beam of the National Spallation Neutron Source. The results are shown in the figure. Figure 6 The schematic diagram of the imaging test device and the photo of the h-BN scintillation screen are shown in Figure 6 As shown in a. Figure 6 are the h-BN scintillation screen neutron and gamma imaging results; among them, Figure 6 a is the neutron imaging test device and optical path diagram, Figure 6 b are schematic diagrams of the h-BN scintillation screen, Figure 6 c is the neutron and gamma imaging results of the copper shell kerosene lighter using the h-BN scintillation screen.

[0053] Figure 6 In c, the image contrast of the h-BN scintillator screen shows that the scintillator screen has good luminescence uniformity and imaging ability. Since neutrons are insensitive to heavy metal elements, they can image the kerosene inside the lighter through thick metal layers, which has important application value. Furthermore, in order to prove the high n / γ suppression ratio of the h-BN scintillator screen, the h-BN scintillator screen was used for gamma-ray imaging. The optical path diagram of gamma-ray imaging is similar to Figure 6 The optical path diagram of neutron imaging of a is similar. Under gamma ray irradiation, despite integration for 2 hours, it is still impossible to image the object ( Figure 6 c).

[0054] In summary, the present invention uses chemical vapor deposition to grow a 6-inch h-BN film with a thickness of about 36μm, and after simple peeling, a self-supporting h-BN film is obtained for use as a neutron scintillator. In the h-BN growth process, methane is used as a carbon source to introduce carbon defects, which makes the h-BN film exhibit strong blue light emission with a peak emission of 387nm. The h-BN neutron scintillator exhibits an n / γ suppression ratio of about 45,000, which is higher than the commonly used 6 LiF / ZnS and GOS scintillator screens can distinguish neutron signals and perform imaging in complex radiation mixed fields. The neutron imaging demonstration results show the great potential of h-BN scintillator screens in neutron imaging applications.

[0055] The embodiments of the present invention are 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 of these embodiments are made without departing from the principles and spirit of the present invention, and still fall within the protection scope of the present invention.

Claims

1. A method for preparing a hexagonal boron nitride neutron scintillator screen with a high n / γ suppression ratio, characterized in that: The following steps are involved: Step S1: Place the SiC single crystal substrate into a chemical vapor deposition furnace, and evacuate the furnace to 10 -3 Below Pa; Step S2, heating the substrate to the reaction temperature, introducing two raw material gases BCl3 and NH3, or BF3 and NH3, and carrier gas N2 into the furnace, and adding a small amount of carbon-containing gas for doping so that the boron nitride film has high-efficiency luminescent properties; after ventilation, the temperature is maintained at the reaction temperature for heat preservation growth; Step S3, then turn off the power, cool naturally to room temperature to obtain a micron-scale carbon-doped h-BN film, and peel it off from the substrate to obtain a hexagonal boron nitride neutron scintillator screen.

2. The method for preparing a hexagonal boron nitride neutron scintillator screen with a high n / γ suppression ratio according to claim 1, characterized in that: The carbon-containing gas in step S2 includes at least one of CH4, C2H2, and CCl4.

3. The method for preparing a hexagonal boron nitride neutron scintillator screen with a high n / γ suppression ratio according to claim 1, characterized in that: The reaction formula in step S2 is BCl3+NH3→BN+3HCl, or BF3+NH3→BN+3HF.

4. The method for preparing a hexagonal boron nitride neutron scintillator screen with a high n / γ suppression ratio according to claim 1, characterized in that: In the step S2, the substrate is heated to a reaction temperature of 1800-1900°C.

5. The method for preparing a hexagonal boron nitride neutron scintillator screen with a high n / γ suppression ratio according to claim 1, characterized in that: In step S2, the gas volume ratio of the two raw materials BCl3 and NH3, or BF3 and NH3, is 1:

2.

6. The method for preparing a hexagonal boron nitride neutron scintillator screen with a high n / γ suppression ratio according to claim 5, characterized in that: 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.

7. The method for preparing a hexagonal boron nitride neutron scintillator screen with a high n / γ suppression ratio according to claim 5, characterized in that: In step S2, the gas volume ratio of the carbon-containing gas to the raw material gas NH3 is 0.001:

1.

8. The method for preparing a hexagonal boron nitride neutron scintillator screen with a high n / γ suppression ratio according to claim 1, characterized in that: In the step S2, after ventilation, the temperature of the furnace body is maintained at the reaction temperature of 1800-1900° C. for 4-5 hours.

9. A hexagonal boron nitride neutron scintillator screen with a high n / γ suppression ratio, characterized in that: The compound is prepared by the preparation method according to any one of claims 1 to 8.

10. The hexagonal boron nitride neutron scintillator with high n / γ suppression ratio according to claim 9, characterized in that: The n / γ suppression ratio of the hexagonal boron nitride neutron scintillator screen is ≥40000.

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