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

Abstract

A micron-scale ultra-high-resolution time-of-flight neutron imaging detector based on a boron nitride thin-film scintillation screen, comprising: a boron nitride thin-film scintillation screen, used for absorbing a neutron beam and generating scintillation light; an optical path system, used for collecting and amplifying the scintillation light and then projecting the amplified scintillation light onto a photosensitive chip of a time-resolved high-speed camera; and the time-resolved high-speed camera, used for receiving the scintillation light processed by the optical path system and recording photon information. The present invention can be widely used in the field of neutron imaging, and achieves micron-scale spatial resolution and sub-microsecond temporal resolution.

Description

A micrometer-scale ultra-high resolution time-of-flight neutron imaging detector based on a boron nitride thin-film scintillation screen and its fabrication method Technical Field

[0001] This invention relates to the field of neutron imaging, and more particularly to a micron-scale ultra-high resolution time-of-flight neutron imaging detector based on a boron nitride thin-film scintillator and its manufacturing method. Background Technology

[0002] Neutron imaging plays an irreplaceable role in X-ray nondestructive testing and evaluation due to its unique advantages. Traditional neutron imaging has a spatial resolution limited to tens of micrometers and cannot distinguish between neutrons of different energies, restricting its application 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 pulse has a precise start time, the energy and wavelength of the neutrons can be obtained using the time-of-flight method (TOF), allowing for simultaneous imaging of samples by neutrons of different wavelengths in a single experiment.

[0003] Traditional neutron imaging detectors typically employ a 6LiF / ZnS scintillation screen with CCD readout, lacking time resolution capability. Due to the range limitation of secondary particles (~30μm), the size of the light spot generated within the scintillation screen determines that the spatial resolution of the detector is difficult to exceed 30μm. Therefore, developing a new type of scintillation screen with a smaller scintillation spot is a key issue for further improving spatial resolution. Summary of the Invention

[0004] In view of this, in order to solve the technical problem that existing neutron imaging detectors cannot achieve microsecond-level high temporal resolution due to the limitations of the scintillation screen, this invention proposes a micrometer-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 thin-film scintillator screens are used to absorb neutron beams and generate scintillating light.

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

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

[0008] The computer unit achieves micrometer-level spatial resolution based on the data recorded by the time-resolved high-speed camera using a case reconstruction algorithm.

[0009] The neutron imaging detector operates as follows: after being transmitted through the sample, neutrons are absorbed by a boron nitride thin-film scintillation screen, generating scintillation light. This light is then deflected by a mirror by 90 degrees, optically magnified by a lens, and further amplified by an image intensifier before entering a time-resolved high-speed camera. The camera records the time and position information of the incident photons, and finally, a computer performs event reconstruction and data analysis.

[0010] In some embodiments, the preparation steps of the boron nitride thin-film scintillator screen 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 films were cut to a preset size according to the field of view of the imaging detector and fixed on a double-sided polished mirror aluminum plate.

[0012] This setup allows the mirrored aluminum plate to reflect the scintillating light of the scintillator, thereby increasing the light output of the scintillator and improving experimental efficiency.

[0013] Based on the above scheme, this invention provides a micrometer-scale ultra-high resolution time-of-flight neutron imaging detector based on a boron nitride thin-film scintillator. Since the secondary particle range of the boron nitride thin-film scintillator is in the micrometer range and has a certain degree of transparency, the spot size can be effectively reduced and the light output efficiency improved, thereby achieving micrometer-scale spatial resolution. The light decay time of this scintillator is in the tens of nanoseconds, and combined with a time-resolved high-speed camera, sub-microsecond time resolution can be achieved simultaneously. Attached Figure Description

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

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

[0016] Figure 3 shows a typical neutron signal waveform of a boron nitride thin-film scintillator screen.

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

[0018] Figure 5 is a structural block diagram of the sub-imaging detector of a specific embodiment of the present invention. Detailed Implementation

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

[0020] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0021] It should be noted that, for ease of description, only the parts relevant to the invention are shown in the accompanying drawings. Unless otherwise specified, the embodiments and features described in this application can be combined with each other.

[0022] It should be understood that the terms "system," "apparatus," "unit," and / or "module" used in this application are a method of distinguishing different components, elements, parts, sections, or assemblies at different levels. However, if other terms can achieve the same purpose, they may be replaced by other expressions.

[0023] As indicated in this application and claims, unless the context clearly indicates otherwise, the words "a," "an," "a," and / or "the" are not specifically singular and may include the plural. Generally, the terms "comprising" and "including" only indicate the inclusion of expressly identified steps and elements, which do not constitute an exclusive list, and the method or apparatus may also include other steps or elements. An element defined by the phrase "comprising an..." does not exclude the presence of other identical elements in the process, method, product, or apparatus that includes the element.

[0024] In the description of the embodiments of this application, "a plurality of" refers to two or more. The terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature.

[0025] Furthermore, flowcharts are used in this application to illustrate the operations performed by the system according to embodiments of this application. It should be understood that the preceding or following operations are not necessarily performed precisely in sequence. Instead, the steps can be processed in reverse order or simultaneously. Additionally, other operations can be added to these processes, or one or more steps can be removed from them.

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

[0027] Boron nitride thin-film scintillator screens are used to absorb neutron beams and generate scintillating light.

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

[0029] The optical path system includes: a reflector for adjusting the direction of the scintillating light from the boron nitride thin-film scintillator; a filter for filtering light outside the main emission wavelength range of the scintillator to reduce dispersion, with the light transmission range depending on the scintillator; a lens for optically amplifying the scintillating light; and an image intensifier for multiplying and amplifying the scintillating light.

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

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

[0032] Due to the transparency and micrometer-level thickness of the boron nitride thin-film scintillator, combined with a redesigned high-precision optical system, this 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 nanosecond-level time resolution is used to record the optical signal from the scintillator, and combined with the tens of nanosecond light decay time of the scintillator, sub-microsecond time resolution is achieved. Using the time-of-flight method at a spallation neutron source, an energy resolution of ~1‰ can be obtained, achieving high-energy-resolution neutron imaging. An image intensifier is added to the optical system to amplify the scintillating light, compensating for insufficient camera sensitivity and enabling the detection of extremely weak light. Based on the physical characteristics of the detector, a developed event reconstruction algorithm can reconstruct neutron event information from the photon signal event information, overcoming the spatial resolution limitations of camera pixel size, reducing the temporal dispersion of neutron events, improving the spatial and temporal resolution of neutron imaging, and simultaneously removing noise signals to improve the image signal-to-noise ratio. In this embodiment, a high-resolution neutron imaging detector specifically refers to a neutron imaging detector with micrometer-level spatial resolution and micrometer-level time resolution. A time-of-flight neutron imaging detector can also be called an energy-resolution neutron imaging detector.

[0033] Among them, boron nitride, as a neutron scintillator material, thermal neutrons are... 10 After B is captured, the secondary particles produced are α ions and... 7 Li particles, with a total energy of approximately 2.3 MeV, form a beam with a range of several micrometers within the scintillator, corresponding to a spot size in the micrometer range.

[0034] This invention employs a high-resolution boron nitride thin-film scintillator combined with a nanosecond-level time-resolved high-speed camera to construct a micrometer-level ultra-high-resolution time-of-flight neutron imaging detector. It can simultaneously achieve micrometer-level spatial resolution and microsecond-level time resolution. Furthermore, by designing the detector structure to deflect the camera off the beam, it can reduce beam radiation damage and improve the detector's operational lifespan.

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

[0036] Micrometer-scale boron nitride thin films were heteroepitaxially grown on sapphire single-crystal substrates using chemical vapor deposition. To meet the size requirements of the scintillator screen for neutron imaging technology, the sapphire single-crystal substrate was 8 inches in size. In this process, boron chloride (BCl3) was used as the boron source, ammonia (NH3) as the nitrogen source, and nitrogen (N2) as the carrier gas. First, the substrate was placed in the deposition chamber, and a vacuum of 10... -3 The temperature was initially set below Pa, then heated to 1500℃ and maintained at a constant temperature. BCl3 and NH3 feed gases, in a volume ratio of 1:1.45, were then introduced into the deposition chamber. At 1500℃, boron nitride gradually deposited onto the substrate. After 3 hours of deposition, the power was cut off and the substrate was allowed to cool naturally to room temperature before removal. Due to the low lattice matching between the sapphire and boron nitride films, the micron-sized boron nitride epitaxial layer automatically detached from the sapphire substrate. Further, the boron nitride film was cut to 3cm × 3cm according to the field of view required by the imaging detector, and then fixed onto a double-sided polished mirror aluminum plate. The mirror aluminum plate reflects the scintillator light, thereby increasing the scintillator light output and improving experimental efficiency.

[0037] Temporal resolution is crucial for achieving energy-resolved neutron imaging at pulsed neutron sources. Traditional imaging detectors use integrating CCD or sCMOS cameras for optical imaging, lacking temporal resolution capabilities. The TPX3Cam, a high-speed time-resolved camera, utilizes the Timepix3 chip (55μm pixel size, 256*256 pixels) developed by CERN. It can read out the position (X, Y), arrival time (ToA), and signal amplitude (ToT, Time Over Threshold) of the photosensitive pixels. The signal amplitude is obtained through ToT technology and can be used for centroid reconstruction to improve resolution. The temporal resolution of each neutron is primarily determined by the scintillator emission time and the camera's own temporal resolution. For a boron nitride thin-film scintillator, the typical neutron signal waveform broadened by a photomultiplier tube (PMT) at the neutron source is approximately 50ns, as shown in Figure 3, indicating a very short light decay time. An image intensifier is a vacuum electron multiplier device that amplifies weak light signals. It mainly consists of a photocathode, a microchannel plate, a fluorescent screen, and a relay lens. The fluorescent screen uses a P46 or P47 screen with a decay time of hundreds of nanoseconds, enabling a rapid response to the light signal. The Timepix3 chip has a time resolution of 1.6 ns. Therefore, considering the time resolution capabilities of each component, the detector's final time resolution for neutron measurement is better than 1 μs, fully meeting the time resolution requirements for Bragg edge neutron imaging.

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

[0039] The computer unit achieves micrometer-level spatial resolution through a case reconstruction algorithm.

[0040] When the pixel size of a camera in an imaging system is large, pixel size becomes the main factor limiting spatial resolution. The Timepix3 chip has a pixel size of 55μm, which is much larger than the micrometer-level spatial resolution. To achieve high spatial resolution imaging, a high-precision optical path system with high magnification is required. The high-precision optical path system of this application consists of a high-magnification, high-precision optical lens, a high-flatness, high-reflectivity mirror, and a high-gain, fast-fading-time image intensifier. The optical lens uses an optical magnification of more than 5 times, reducing the effective pixel size of the camera chip to less than 10 micrometers. However, using only lenses for optical magnification is still insufficient to achieve ultra-high spatial resolution at the micrometer level. After multiplication and optical diffusion by the image intensifier, each neutron event will have multiple pixel responses. Using the event reconstruction algorithm, a spatial resolution smaller than the effective pixel size can be obtained, ultimately achieving micrometer-level spatial resolution. As shown in Figure 4, during event reconstruction, the light spots recorded by the chip need to be clustered. Simultaneously, adjacent pixels that are photosensitive (marked by the same circle) are considered as a single effective neutron event. The TPX3Cam high-speed camera can output not only the two-dimensional position coordinates of each event, but also the time of arrival (TOA) and the overthreshold width (TOT) of the signal. The overthreshold width is linearly related to the signal charge magnitude within a certain range. Therefore, the charge centroid method can be used for event reconstruction after clustering, thereby improving spatial resolution. Signals with only a single pixel response can be considered as scintillator afterglow or gamma signals; removing these signals improves the signal-to-noise ratio.

[0041] In some feasible embodiments, the overall structure is shown in Figure 5, and also includes:

[0042] The scintillator screen and reflector are installed in the same dark chamber, while the lens is mounted in a retractable light-shielding sleeve. Together, the dark chamber and the sleeve create a complete light-shielding environment, reducing ambient light interference and protecting the image intensifier from overexposure damage. The image intensifier is threaded to the camera and lens at both ends, ensuring stable and reliable connections while providing excellent light shielding. The reflector deflects the optical path by 90°, allowing the camera to be positioned away from the neutron beam center, reducing radiation damage, extending its lifespan, and minimizing the impact of the strong gamma background on the imaging results. The moving platform allows the detector to focus at different optical magnifications, enabling clear imaging results by moving the camera and lens. To reduce neutron radiation to detector components and lower the stray neutron background, the entire detector is shielded using boron-aluminum alloy plates. An entrance window, matching the size of the scintillator, is located at the neutron incident position. To reduce the distance between the sample and the scintillator and improve spatial resolution, the scintillator is directly mounted on the entrance window.

[0043] The above is a detailed description of the preferred embodiments of the present invention. However, the present invention is not limited to the embodiments described. Those skilled in the art can make various equivalent modifications or substitutions without departing from the spirit of the present invention. All such equivalent modifications or substitutions are included within the scope defined by the claims of this application.

Claims

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

2. The micron-scale ultra-high resolution time-of-flight neutron imaging detector based on a boron nitride thin-film scintillation screen according to claim 1, characterized in that, The preparation steps of the boron nitride thin film scintillator screen are as follows: Using sapphire as a substrate, boron chloride as a boron source, ammonia as a nitrogen source, and nitrogen as a carrier gas; The substrate was placed in the deposition chamber and the vacuum was evacuated to 10°C. -3 Below Pa, then heat to the preset temperature and maintain a constant temperature; Boron chloride and ammonia gas in a preset ratio are respectively introduced into the deposition chamber; At a preset temperature, boron nitride is gradually deposited onto the substrate; After a preset deposition time, the power is turned off and the material is allowed to cool naturally to room temperature before being removed.

3. The micron-scale ultra-high resolution time-of-flight neutron imaging detector based on a boron nitride thin-film scintillation screen according to claim 1, characterized in that, The optical path system includes: A reflector is used to adjust the direction of the scintillation light from the boron nitride thin-film scintillation screen. Filters are used to filter light outside the main emission wavelength range of a scintillator; A lens is used to optically amplify the scintillation light; An intensifier is used to amplify the flickering light.

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

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

6. The micron-scale ultra-high resolution time-of-flight neutron imaging detector based on a boron nitride thin-film scintillation screen according to claim 5, characterized in that, Also includes: Remove signals that only have a single pixel response.

7. The micron-scale ultra-high resolution time-of-flight neutron imaging detector based on a boron nitride thin-film scintillation screen according to claim 1, characterized in that, Also includes: The boron nitride thin-film scintillator screen and the reflector are installed in the same dark chamber; The filter and the lens are installed inside the light-shielding telescopic sleeve; The two ends of the image intensifier are respectively connected to the time-resolved high-speed camera and the lens via threads; The neutron imaging detector is shielded using boron-aluminum alloy plates.

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

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