Micron-scale ultra-high-resolution flight time neutron imaging detector based on boron nitride film scintillation screen and manufacturing method of micron-scale ultra-high-resolution flight time neutron imaging detector

Abstract

The invention discloses a micron-sized ultrahigh-resolution flight time neutron imaging detector based on a boron nitride film scintillation screen, and the detector comprises the boron nitride film scintillation screen which is used for absorbing neutron beams and generating scintillation light; the light path system is used for collecting the scintillation light, amplifying the scintillation light and projecting the amplified scintillation light to a photosensitive chip of a time-resolved high-speed camera; and the time-resolved high-speed camera is used for receiving the scintillation light processed by the light path system and recording photon information. According to the invention, micron-level spatial resolution and sub-microsecond time resolution can be realized. The method can be widely applied to the field of neutron imaging.

Description

Technical Field

[0001] The present invention relates to the field of neutron imaging, and in particular to a micron-level ultra-high resolution time-of-flight neutron imaging detector based on a boron nitride film scintillating screen and a manufacturing method thereof. Background Art

[0002] Neutron imaging plays an irreplaceable role in X-ray nondestructive testing and nondestructive evaluation due to its unique advantages. The spatial resolution of traditional neutron imaging can only reach tens of microns, and it cannot distinguish neutrons of different energies, which limits the application of neutron imaging technology in many fine structure studies. Based on the development of spallation neutron sources internationally, energy-resolved neutron imaging technology combined with the time-of-flight method has been realized. For pulsed neutron sources, since each beam of pulsed neutrons has an accurate starting time, the energy and wavelength of the neutrons can be obtained by combining the time-of-flight method (TOF), and the imaging results of neutrons of different wavelengths on the sample can be obtained simultaneously in one experiment.

[0003] Traditional neutron imaging detectors generally use a 6LiF / ZnS scintillator screen with CCD readout, which does not have time resolution capability. Due to the range limitation of secondary particles (~30μm), the spot size generated in the scintillator screen determines that the spatial resolution of the detector is difficult to exceed 30μm. Therefore, the development of a new scintillator screen with a smaller scintillation spot is a key issue to further improve the spatial resolution. Summary of the invention

[0004] In view of this, in order to solve the technical problem that the existing neutron imaging detector cannot achieve microsecond-level high time resolution due to the limitation of the scintillation screen, the present invention proposes a micron-level ultra-high resolution time-of-flight neutron imaging detector based on a boron nitride thin film scintillation screen, the detector comprising:

[0005] Boron nitride film scintillator screen, used to absorb neutron beam and produce scintillation light;

[0006] An optical path system, used for transmitting and projecting the scintillation light to a time-resolved high-speed camera;

[0007] The time-resolved high-speed camera is used to receive the scintillation light processed by the optical path system and record the photon information.

[0008] The computer unit realizes micron-level spatial resolution through an event reconstruction algorithm based on the data recorded by the time-resolved high-speed camera.

[0009] The working process of the neutron imaging detector is as follows: after the neutron is transmitted through the sample, it enters the boron nitride film scintillator screen and is absorbed to generate scintillation light, which is deflected 90 degrees by the reflector and optically amplified by the lens, and then multiplied and amplified by the image intensifier and enters the time-resolved high-speed camera. The camera records the time and position information of the incident photons, and finally the computer completes the event reconstruction and data analysis and processing.

[0010] In some embodiments, the steps of preparing the boron nitride thin film scintillator are as follows:

[0011] Micron-sized boron nitride films were heteroepitaxially grown on sapphire single crystal substrates using chemical vapor deposition. In this process, boron chloride was used as the boron source, ammonia as the nitrogen source, and nitrogen as the carrier gas. Furthermore, the boron nitride film was cut into a preset size according to the field of view of the imaging detector and fixed on a double-sided polished mirror aluminum plate.

[0012] Through this setting, the mirrored aluminum plate can reflect the scintillator's scintillation light, thereby increasing the scintillator's light output and improving experimental efficiency.

[0013] Based on the above scheme, the present invention provides a micron-level ultra-high resolution time-of-flight neutron imaging detector based on a boron nitride thin film scintillator screen. Since the secondary particle range of the boron nitride thin film scintillator screen is at the micron level and has a certain degree of transparency, it can effectively reduce the spot size and improve the light output efficiency, thereby achieving micron-level spatial resolution. The light decay time of the scintillator is tens of nanoseconds, and combined with a time-resolving high-speed camera, it can simultaneously achieve sub-microsecond time resolution. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a basic structural diagram of the neutron imaging detector of the boron nitride thin film scintillator screen of the present invention;

[0015] Figure 2 It is a basic diagram of energy-resolved neutron imaging combined with time of flight;

[0016] Figure 3 This is a typical neutron signal waveform of a boron nitride film scintillator screen;

[0017] Figure 4 is a schematic diagram of the case reconstruction algorithm;

[0018] Figure 5 It is a structural block diagram of a sub-imaging detector according to a specific embodiment of the present invention. DETAILED DESCRIPTION

[0019] For pulsed neutron sources, since each pulsed neutron has an accurate starting time, the energy and wavelength of the neutron can be obtained by combining the time-of-flight method (TOF). The basic principle is as follows: Figure 2 shown.

[0020] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0021] It should be noted that, for the convenience of description, only the parts related to the relevant invention are shown in the drawings. In the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other.

[0022] It should be understood that the "system", "device", "unit" and / or "module" used in this application is a method for distinguishing different components, elements, parts, portions or assemblies at different levels. However, if other words can achieve the same purpose, the word can be replaced by other expressions.

[0023] As shown in this application and claims, unless the context clearly indicates an exception, the words "a", "an", "a kind" and / or "the" do not refer to the singular, but also include the plural. Generally speaking, the terms "include" and "comprise" only indicate the inclusion of clearly identified steps and elements, and these steps and elements do not constitute an exclusive list. The method or device may also include other steps or elements. The elements defined by the sentence "includes a..." do not exclude the existence of other identical elements in the process, method, commodity or device that includes the elements.

[0024] In the description of the embodiments of the present application, "plurality" means two or more than two. The following terms "first" and "second" are used for descriptive purposes only and are not to be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Thus, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features.

[0025] In addition, flow charts are used in the present application to illustrate the operations performed by the system according to the embodiments of the present application. It should be understood that the preceding or following operations are not necessarily performed accurately in order. On the contrary, the various steps may be processed in reverse order or simultaneously. At the same time, other operations may also be added to these processes, or a certain step or several steps of operations may be removed from these processes.

[0026] Reference Figure 1 , is a schematic structural diagram of an optional example of a micron-level ultra-high resolution time-of-flight neutron imaging detector based on a boron nitride thin film scintillator screen proposed in the present invention. The neutron imaging detector proposed in this embodiment may include but is not limited to:

[0027] Boron nitride film scintillator screen, used to absorb neutron beam and produce scintillation light;

[0028] An optical path system, used for transmitting and projecting the scintillation light to a time-resolved high-speed camera;

[0029] The optical path system includes: a reflector for adjusting the direction of the scintillation light of the boron nitride film scintillation screen; a filter for filtering light outside the main emission wavelength range of the scintillator to reduce dispersion, and the light transmission range is determined according to the scintillator; a lens for optically amplifying the scintillation light; and an image intensifier for multiplying and amplifying the scintillation light.

[0030] Among them, filters and lenses are both optical lenses.

[0031] The time-resolved high-speed camera is used to receive the scintillation light processed by the optical path system and record the photon information.

[0032] Since the boron nitride film scintillator screen is transparent and has a thickness of micrometer level, combined with the redesigned high-precision optical system, the neutron imaging detector can effectively reduce the spot size and increase the light yield, thereby achieving micrometer-level spatial resolution. A high-speed camera with ns-level time resolution is used to record the light signal of the scintillator screen, and combined with the light decay time of the scintillator of tens of nanoseconds, sub-microsecond time resolution is achieved. The time-of-flight method can obtain an energy resolution of ~1‰ on the spallation neutron source, realizing high-energy resolution neutron imaging. The addition of an image intensifier to the optical system to amplify the scintillation light can make up for the lack of camera sensitivity and realize the detection of extremely weak light. According to the physical characteristics of the detector, the developed event reconstruction algorithm can be used to reconstruct the neutron event information from the event information of the photon signal, break through the spatial resolution limit of the camera pixel size, reduce the temporal dispersion of the neutron event, improve the spatial resolution and temporal resolution of neutron imaging, and remove noise signals at the same time to improve the signal-to-noise ratio of the image. In this embodiment, the high-resolution neutron imaging detector specifically refers to a neutron imaging detector with micrometer-level spatial resolution and microsecond-level time resolution. The time-of-flight neutron imaging detector can also be called an energy-resolution neutron imaging detector.

[0033] Among them, boron nitride is a neutron scintillator material. 10 After B is captured, the secondary particles produced are α ions and 7 Li particles, with a total energy of about 2.3 MeV, have a range of several microns in the scintillator, and the corresponding spot size is also in the micron level.

[0034] The present invention adopts a high-resolution boron nitride thin film scintillator combined with a ns-level time-resolution high-speed camera to build a micron-level ultra-high-resolution time-of-flight neutron imaging detector, which can simultaneously achieve micron-level spatial resolution and microsecond-level time resolution. The structural design of the detector is used to deviate the camera from the beam, which can reduce the radiation damage of the beam and increase the working life of the detector.

[0035] In some feasible embodiments, the steps of preparing the boron nitride thin film scintillator screen are as follows:

[0036] Micron-sized boron nitride thin films are grown heteroepitaxially on sapphire single crystal substrates using chemical vapor deposition. In order to meet the requirements of neutron imaging technology for the size of the scintillator screen, the size of the sapphire single crystal substrate is 8 inches. In this process, boron chloride (BCl 3 ) as a boron source, ammonia (NH 3 ) as a nitrogen source, nitrogen (N 2 ) is used as carrier gas. First, the substrate is placed in the deposition chamber and evacuated to 10 -3 Pa, then heated to 1500°C and kept at a constant temperature. Then, BCl 3 and NH 3 The raw gas is introduced into the deposition chamber respectively. At a high temperature of 1500℃, boron nitride is gradually deposited on the substrate. After 3 hours of deposition, the power is cut off and the film is taken out after natural cooling to room temperature. Due to the low lattice matching between sapphire and boron nitride films, the micron-sized boron nitride epitaxial layer automatically falls off from the sapphire substrate. Furthermore, the boron nitride film is cut into 3cm×3cm according to the field of view of the imaging detector, and then fixed on a double-sided polished mirror aluminum plate. The mirror aluminum plate can reflect the scintillation light of the scintillator, thereby increasing the scintillator light output and improving the experimental efficiency.

[0037] Time resolution is the key to achieving energy-resolved neutron imaging on a pulsed neutron source. Traditional imaging detectors use integral CCD or sCMOS cameras for optical imaging and do not have time resolution capabilities. The time-resolved high-speed camera TPX3Cam, the camera's photosensitive element uses the Timepix3 chip developed by CERN (pixel size is 55μm, pixel 256*256), which can read out the position (X, Y), arrival time (ToA) and signal amplitude (ToT, Time OverThreshold) of the photosensitive pixel. The signal amplitude is obtained through ToT technology and can be used to reconstruct the position using the center of gravity method to improve resolution. The time resolution of the detector for each neutron is mainly determined by the scintillator emission time and the time resolution of the camera itself. For the boron nitride thin film scintillator screen, the typical neutron signal waveform we tested on the neutron source through a photomultiplier tube (PMT) has a widening of about 50ns. Figure 3, showing a very short light decay time. The image intensifier is a vacuum electron multiplier device that amplifies weak light signals. It is mainly composed of a photocathode, a microchannel plate, a fluorescent screen and a relay lens. The fluorescent screen uses a P46 or P47 fluorescent screen with a decay time of hundreds of nanoseconds, which can achieve a fast response to the light signal. The time resolution of the Timepix3 chip is 1.6ns. Therefore, considering the time resolution capabilities of each component, the detector's final time resolution for neutron measurement is better than 1μs, which fully meets the requirements of Bragg edge neutron imaging for the detector's time resolution.

[0038] In some feasible embodiments, it also includes:

[0039] Computer unit, achieving micron-level spatial resolution through case reconstruction algorithm.

[0040] When the pixel size of the camera in the imaging system is large, the pixel size will become the main factor limiting the spatial resolution. The pixel size of the Timepix3 chip is 55μm, which is much larger than the micron-level spatial resolution. In order to achieve high spatial resolution imaging, a high-magnification, high-precision optical path system is required. The high-precision optical path system of the present application is composed of a high-magnification, high-precision optical lens, a high-flatness, high-reflection efficiency reflector, and a high-gain, fast-decay time image intensifier. The optical lens uses an optical magnification of more than 5 times to reduce the effective pixel size of the camera chip to less than 10 microns. It is still difficult to achieve ultra-high spatial resolution at the micron level using only lenses for optical magnification. After multiplication and optical dispersion by the image intensifier, each neutron event will have multiple pixel responses. The event reconstruction algorithm can be used to obtain a spatial resolution smaller than the effective pixel size, and ultimately achieve micron-level spatial resolution. Figure 4 As shown in the figure, when reconstructing an event, it is necessary to cluster the light spots recorded by the chip, and the pixels that are sensitive and adjacent at the same time (marked by the same circle) are regarded as a valid neutron event. The TPX3Cam high-speed camera can not only output the two-dimensional position coordinates of each event, but also output the signal arrival time (TOA) and signal threshold width (TOT) at the same time. The signal threshold width is linearly related to the signal charge size within a certain range. Therefore, the charge centroid method can be used to reconstruct the event after clustering processing to improve the spatial resolution. The signal that only responds to a single pixel can be considered as the afterglow of the scintillator or the gamma signal. These signals can be removed to improve the signal-to-noise ratio.

[0041] In some feasible embodiments, the overall structure is as follows Figure 5 As shown, it also includes:

[0042] The scintillator screen and the reflector are installed in the same darkroom, and the lens is installed in a retractable light-proof telescopic sleeve. The darkroom and the light-proof telescopic sleeve together form a complete light-proof environment, which reduces the interference of ambient light and protects the image intensifier from overexposure and damage. The two ends of the image intensifier are connected to the camera and the lens respectively through threads, and the connection is stable and reliable while having a good light-proof effect. The reflector deflects the light path by 90°, and the camera can be away from the center of the neutron beam, reducing radiation damage, extending the service life, and reducing the impact of the strong gamma background on the beam on the imaging results. The mobile platform can meet the focus of the detector at different optical magnifications, and obtain clear imaging results by moving the camera and the lens. In order to reduce the irradiation of neutrons to the detector components and reduce the stray neutron background, the detector is shielded by a boron aluminum alloy plate. An incident window matching the size of the scintillator is opened at the neutron incident position. In order to reduce the distance between the sample and the scintillator and improve the spatial resolution, the scintillator is directly installed on the incident window.

[0043] The above is a specific description of the preferred implementation of the present invention, but the invention is not limited to the embodiments. Those skilled in the art may make various equivalent modifications or substitutions without violating the spirit of the present invention. These equivalent modifications or substitutions are all included in the scope defined by the claims of this application.

Claims

1. A micron-scale ultra-high resolution time-of-flight neutron imaging detector based on a boron nitride thin film scintillator screen, characterized in that: include: Boron nitride film scintillator screen, used to absorb neutron beam and produce scintillation light; An optical path system, used for transmitting and projecting the scintillation light to a time-resolved high-speed camera; The time-resolved high-speed camera is used to receive the scintillation light processed by the optical path system and record the photon information.

2. The micron-level ultra-high resolution time-of-flight neutron imaging detector based on a boron nitride thin film scintillator screen according to claim 1, characterized in that: The preparation steps of the boron nitride thin film scintillator screen are as follows: Sapphire is used as substrate, boron chloride is used as boron source, ammonia is used as nitrogen source, and nitrogen is used as carrier gas; The substrate is placed in a deposition chamber and evacuated to 10 -3 Pa, then heated to the preset temperature and kept constant; Boron chloride and ammonia in a preset ratio are introduced into the deposition chamber as raw material gases respectively; At a preset temperature, boron nitride is gradually deposited onto the substrate; After the preset deposition time, the power is cut off and the sample is taken out after naturally cooling to room temperature.

3. The micron-level ultra-high resolution time-of-flight neutron imaging detector based on a boron nitride thin film scintillator screen according to claim 1, characterized in that: The optical path system comprises: A reflector, used for adjusting the direction of the scintillation light of the boron nitride thin film scintillation screen; A filter, used to filter light outside the main emission wavelength range of the scintillator; A lens, used for optically amplifying the scintillation light; An image intensifier is used to multiply and amplify the scintillation light.

4. The micron-level ultra-high resolution time-of-flight neutron imaging detector based on a boron nitride thin film scintillator screen according to claim 1, characterized in that: Also includes: Computer unit, achieving micron-level spatial resolution through case reconstruction algorithm.

5. The micron-level ultra-high resolution time-of-flight neutron imaging detector based on a boron nitride thin film scintillator screen according to claim 1, characterized in that: The process of achieving micron-level spatial resolution through the case reconstruction algorithm specifically includes: The recorded light spots are clustered, and the pixels that are simultaneously photosensitive and adjacent are considered as a valid neutron event. After clustering, case reconstruction is performed.

6. The micron-level ultra-high resolution time-of-flight neutron imaging detector based on a boron nitride thin film scintillator screen according to claim 5, characterized in that: Also includes: Signals that only respond to a single pixel are removed.

7. The micron-level ultra-high resolution time-of-flight neutron imaging detector based on a boron nitride thin film scintillator screen according to claim 1, characterized in that: Also includes: The boron nitride film scintillator screen and the reflector are installed in the same dark room; Installing the filter and the lens in a light-proof telescopic sleeve; Two ends of the image intensifier are connected to the time-resolved high-speed camera and the lens respectively through threads; The entire neutron imaging detector is shielded by a boron-aluminum alloy plate.

8. The micron-level ultra-high resolution time-of-flight neutron imaging detector based on a boron nitride thin film scintillator screen according to claim 7, characterized in that: Also includes: The boron nitride thin film scintillator screen is mounted on the incident window.

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