CN114966813A - Array type structure detector for fast neutron beam large-area imaging - Google Patents
Array type structure detector for fast neutron beam large-area imaging Download PDFInfo
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- CN114966813A CN114966813A CN202210582084.9A CN202210582084A CN114966813A CN 114966813 A CN114966813 A CN 114966813A CN 202210582084 A CN202210582084 A CN 202210582084A CN 114966813 A CN114966813 A CN 114966813A
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Abstract
The invention discloses an array type structure detector for fast neutron beam large-area imaging, and belongs to the technical field of fast neutron detection. The arrayed fast neutron converter overcomes the contradiction between the detection efficiency and the space resolution capability in the current fast neutron imaging technology, and the silicon photomultiplier realizes the detection of scintillation photons. The detector has low working voltage, high detection efficiency and good spatial resolution. The design principle of the reading circuit is simple, and the reading circuit can be conveniently adjusted according to needs. Silicon photomultipliers are easy to integrate, have excellent detection performance on photons and low cost, and have the potential for imaging objects with larger areas.
Description
Technical Field
The invention belongs to the technical field of fast neutron detection, and particularly relates to an array type structure detector for fast neutron beam large-area imaging.
Background
In the nondestructive detection of internal defects of large thick sealing objects, especially the composition of low atomic number materials wrapped by the large thick sealing objects, the X-ray imaging and the thermal neutron imaging cannot achieve satisfactory results, so that the development of fast neutron imaging with higher penetration capability is necessary as a supplement of nondestructive detection technology. In previous reports, detector systems used in fast neutron imaging are mainly classified into the following categories: (1) a photosensitive element (including a PMT (photomultiplier tube), a CCD camera and a COMS camera) and a neutron-photon converter, mainly a hydrogen-rich plastic converter; (2) MCP (microchannel plate) and amorphous silicon array; (3) position sensitive GEM (gas electron multiplier) and hydrogen rich converter; (4) hydrogen rich converters and imaging plates. The systems show the contradiction between detection efficiency and spatial resolution, and overcoming the contradiction problem is also the key direction of research in the field of fast neutron imaging at present.
In order to realize large-area fast neutron imaging with high detection efficiency and good spatial resolution, the key point and difficulty lies in the selection and forming of the neutron conversion body, the forming of the detection unit and the design of the readout electronics thereof. At present, the measurement of fast neutrons is mainly based on a nuclear recoil method with hydrogen nuclei, the range of recoil protons generated by reaction is short, the direct measurement of the recoil protons requires that a neutron conversion body is as thin as possible, and as for the current materials, the extremely low fast neutron conversion efficiency is caused, so indirect measurement is mostly adopted. Commonly used neutron converters are: liquid scintillators and solid scintillators. The reflecting layer is wrapped on each scintillator unit to limit crosstalk of optical photons between different units, so that the scintillator thickness is increased, the efficiency is improved, and meanwhile good spatial resolution can be guaranteed as far as possible. Commonly used photon detectors are: CCD cameras, PMTs, silicon photomultipliers (sipms), etc., which are expensive and not suitable for large area applications, and the latter, which are inexpensive and easy to integrate, have a high demand on read-out electronics.
Disclosure of Invention
In order to overcome the defects of the prior art, the invention aims to provide an array type structure detector for fast neutron beam large-area imaging, which can solve the technical problems that the prior art is not suitable for large-area imaging, and has low spatial resolution and low detection efficiency.
In order to achieve the purpose, the invention adopts the following technical scheme to realize the purpose:
the invention discloses an array type structure detector for fast neutron beam large-area imaging, which comprises a two-dimensional scintillator array, wherein a light guide is arranged at one end of the two-dimensional scintillator array, and a silicon photomultiplier array is arranged at the other end of the light guide;
scintillation photons produced by the two-dimensional scintillator array are diffused through the light guide and transmitted to a silicon photomultiplier array at the end of the light guide where they are converted into electrical signals for radiometry.
Preferably, the two-dimensional scintillator array is formed by stacking a plurality of scintillator units, the radial outer wall and one axial outer wall of each scintillator unit are coated with a mixture of glue and titanium dioxide, as shown in fig. 1, the glue is used for fixing the positions of the scintillator units, and the titanium dioxide is used as a reflecting layer; the scintillator mainly comprises two elements of carbon and hydrogen, and each scintillator unit is internally enriched with hydrogen atoms.
Further preferably, the scintillator cell has a length of 30mm and a cross-sectional dimension of 1 × 1mm 2 。
Further preferably, the scintillator unit selects a scintillator having a high light yield and a short decay time.
Still further preferably, a high light yield is generally required to be equal to or greater than 10000/MeV, and a short decay time is generally less than 8 ns.
Preferably, the light guide is made of K9 optical glass.
Preferably, the silicon photomultiplier array is formed by integrating a plurality of silicon photomultipliers on the same PCB.
Further preferably, the silicon photomultiplier is formed by integrating hundreds of single photon avalanche diode unit arrays with the size of tens of micrometers on the same monocrystalline silicon piece.
Still further preferably, each single photon avalanche diode is connected in series with a resistance of several hundred kiloohms for quenching an avalanche after a photon avalanche occurs in the photodiode, restoring the voltage across the single photon avalanche diode to the initial operating bias in preparation for detecting the next incident photon.
Preferably, the silicon photomultiplier array employs a multiplexed readout method of capacitive charge division.
It is further preferred that the anodes of the silicon photomultipliers in the silicon photomultiplier array are connected to one or more weighting capacitors depending on the location, i.e., the anode signal is divided into as many sub-signals as the number of connected capacitors, and then the sub-signals are connected to one of the four location signal output channels P, Q, R or S for output.
Compared with the prior art, the invention has the following beneficial effects:
the invention discloses an array type structure detector for fast neutron beam large-area imaging, which is characterized in that a scintillator array and a silicon photomultiplier array are connected by a light guide, the arrayed fast neutron scintillator overcomes the contradiction between the detection efficiency and the space resolution capability in the conventional fast neutron imaging technology, and the silicon photomultiplier realizes the detection of scintillation photons. The scintillator can convert uncharged particles into charged ions or detectable photons, and further titanium dioxide is coated between scintillator units to reflect photons, so that the reconstruction of incident fast neutron positions due to crosstalk influence of scintillation light is prevented, and the imaging space resolution capability of the system is reduced. The detector has the advantages of low working voltage, high detection efficiency, good spatial resolution and short test time, in addition, the price of the silicon photomultiplier is lower, the integration process is not complicated, the size of the whole column of the silicon photomultiplier can be defined by user according to the requirement, and the potential is used in large-area imaging.
Drawings
FIG. 1 is a schematic view of a scintillator construction; wherein, (a) is a cross-sectional view of a plastic scintillator unit with a reflective layer; (b) is a right side view of the structure shown in (a);
FIG. 2 is a schematic diagram of a detector system of the present invention that may be used for fast neutron imaging; wherein 1 is a two-dimensional scintillator array; 2 is a light guide; 3 is a silicon photomultiplier array;
FIG. 3 is a schematic diagram of a scintillator array used in the present invention;
FIG. 4 is a schematic diagram of a silicon photomultiplier array of the present invention;
FIG. 5 is a schematic diagram of a data read circuit according to the present invention;
FIG. 6 is a physical diagram of a prototype of the detector;
FIG. 7 is a split view of the detector and electronics; wherein, 4 is a packaged 16 × 16SiPM array and a compression circuit thereof, 5 is a data acquisition module, and 6 is a pre-amplification module;
FIG. 8 is a graph of Modulation Transfer Function (MTF) curves of a prototype detector tested on a 14MeV fast neutron source under different threshold conditions.
Detailed Description
In order to make those skilled in the art better understand the technical solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
It should be noted that the terms "first," "second," and the like in the description and claims of the present invention and in the drawings described above are used for distinguishing between similar elements and not necessarily for describing a particular sequential or chronological order. It is to be understood that the data so used is interchangeable under appropriate circumstances such that the embodiments of the invention described herein are capable of operation in sequences other than those illustrated or described herein. Furthermore, the terms "comprises," "comprising," and "having," and any variations thereof, are intended to cover a non-exclusive inclusion, such that a process, method, system, article, or apparatus that comprises a list of steps or elements is not necessarily limited to those steps or elements expressly listed, but may include other steps or elements not expressly listed or inherent to such process, method, article, or apparatus.
The invention is described in further detail below with reference to the accompanying drawings:
as shown in FIG. 2, the array-type structure detector for fast neutron imaging disclosed by the invention comprises a two-dimensional scintillator array 1, a light guide 2 and a silicon photomultiplier array 3. A light guide 2 is arranged at one end of the two-dimensional scintillator array 1, and a silicon photomultiplier array 3 is arranged at the other end of the light guide 2; scintillation photons generated by the two-dimensional scintillator array 1 are diffused by the light guide 2 and transmitted to the silicon photomultiplier array 3 at the end of the light guide 2 to be converted into electrical signals for radiometric measurement.
The scintillator is rich in hydrogen elements, has high light yield and short decay time.
The two-dimensional scintillator array is formed by stacking numerous small-sized scintillator cells. Referring to fig. 1, the radial outer wall and one of the axial outer walls of each scintillator unit are coated with a mixture of glue and titanium dioxide, the glue fixes the positions of the scintillator units, the titanium dioxide serves as a reflecting layer, so that generated photons are constrained in the current scintillator unit and only transmitted to two ends of the current scintillator unit, hydrogen atoms are enriched in each scintillator unit, fast neutrons and the hydrogen atoms are elastically scattered to generate recoil protons, and the recoil protons deposit energy along the motion tracks of the recoil protons and generate scintillation photons.
In the process of detecting the fast neutrons, the fast neutrons cannot be directly measured because the fast neutrons are neutral. Therefore, when fast neutrons are detected, a neutron converter is required to be introduced to convert the fast neutrons into particles which can be directly detected, such as charged particles and photons. The fast neutrons have high self energy and extremely small action section with most substances, so that the fast neutrons are detected by an n-p reaction based on a large action section, namely, the fast neutrons collide with hydrogen nuclei in a fast neutron converter rich in hydrogen elements to generate recoil protons. If fast neutrons need to be indirectly measured by measuring the recoil protons, the thickness of the conversion body needs to be ensured to be smaller than the range of the recoil protons, and the thickness leads to extremely low detection efficiency. Therefore, the measurement of the fast neutrons is often indirectly measured on the basis of the measurement of scintillation photons generated by the energy of the recoil proton deposition, and therefore, the fast neutron converter selected here is a plastic scintillator rich in hydrogen elements.
Since the current main factor limiting the application of fast neutron imaging technology is the contradiction between the fast neutron detection efficiency and the system resolution, the contradiction is also the main research content in the current fast neutron imaging field. The scheme of the invention is as follows: the light yield of the scintillator determines how many scintillating photons are produced when the same energy is deposited, and under the same condition, the higher light yield is more beneficial to the measurement of the particles. In addition, the short decay time of the scintillator effectively ensures that the scintillator can work under the condition of high-frequency beam current environment.
In combination with the above discussion, plastic scintillators with high light yield and short decay time are particularly preferred for improving the measurement efficiency and extending the application range of the detector. High light yields generally require 10000/MeV or more and short decay times generally mean less than 8 ns.
The following specific embodiments are taken as examples, and the parameters of the scintillator used in the detector, such as the luminescence decay time, the light yield and the like, are shown in table 1:
TABLE 1 luminescence decay time of selected scintillator materials
| Properties | EJ200 |
| Optical yield (photon/MeV) | 10000 |
| Light attenuation length (cm) | 380 |
| Rise time (ns) | 0.9 |
| Decay time (ns) | 2.1 |
| Maximum emission wavelength (nm) | 425 |
As shown in fig. 2, when the incident ray is a fast neutron, the incident ray collides with a hydrogen atom in the scintillator with a certain probability to generate a recoil proton with positive charge, and through simulation research, the range of the recoil proton in the scintillator is about 0.5mm, and the recoil proton deposits energy along the motion path thereof and generates a scintillation photon. The larger the thickness of the scintillator is, the higher the probability that the fast neutron and the hydrogen atom in the scintillator react to generate scintillation photons is, namely, the conversion efficiency of the fast neutron is improved. Further, scintillation photons generated inside the scintillator are emitted isotropically, and the size of the light spot finally observed increases with the increase in the transmission distance, based on the size of the light spot generated by the fast neutron at the action point inside the scintillator, and the spatial characteristics of the fast neutron deteriorate. To alleviate the decrease of the space characteristics, a cross-sectional area of 1X 1mm is particularly adopted 2 The titanium dioxide is used as a reflecting layer between the scintillator units, so that the diffusion of light spots in the radial direction of the neutron incidence direction is limited, the spatial characteristic of fast neutrons is ensured, and the spatial resolution capability of the detector is improved.
The scintillator array structure with a sufficiently large thickness theoretically ensures the spatial resolution and fast neutron conversion efficiency of the system, but in practice, as the length increases, the crosstalk signal increases, and therefore, comprehensive consideration is required. It was found by theoretical studies that for a cross-sectional area of 1X 1mm 2 When the thickness of the scintillator unit is not more than 30mm, the detection efficiency of the detector on incident fast neutrons can reach more than 8%, and meanwhile, the crosstalk signal is about 1/7 of all signals, so that the influence is small. Comprehensively considering the influence of fast neutron conversion efficiency and crosstalk signals, and finally determining the size of the scintillator units forming the scintillator array to be 1 multiplied by 30mm 3 The final scintillator array is shown in fig. 3.
In the detector, a light guide is used for connection between the scintillator array and the silicon photomultiplier, and the principle is as follows:
the EJ200 scintillator has the refractive index of 1.58, the refractive index of air is about 1, the reflection of scintillation photons on the end face of the scintillator is intensified, and therefore K9 glass with the refractive index of about 1.52 is introduced, the penetration probability of photons on the end face of the scintillator is improved, and meanwhile, the diffusion of scintillation photons is enhanced when the photons are transmitted to a silicon photomultiplier at the other end.
Among the detectors, the reason for using a silicon photomultiplier as a photon detector is as follows:
as shown in fig. 4, the silicon photomultiplier is a solid-state detector developed based on P-N diodes to directly detect light from near ultraviolet to near infrared. Has the following main advantages: high quantum efficiency, low working voltage, no magnetic field interference, low cost and easy integration. The method is suitable for large-area imaging application, and can replace the traditional photomultiplier to measure scintillation photons.
As shown in fig. 5, the readout electronics of a silicon photomultiplier array employs a multiplexing method of charge division by capacitors. The anode of the silicon photomultiplier is connected to one or more weighting capacitors depending on the position, i.e., the anode signal is divided into as many partial signals as the number of connected capacitors, and these partial signals are then connected to one of four position signal output channels (P, Q, R, S) for output. Taking the circuit of fig. 5 for compressing 16 outputs into 4 outputs as an example, the relationship between the anode signal and the weighted value (i.e. capacitance) can be expressed as:
the amplitude E two-dimensional position (X, Y) of the trigger signal has the following relationship with the four position signals:
E＝P+Q+R+S
based on the simulation results and using the above capacitance multiplexing method, the prototype of the detector shown in fig. 6 is finally produced, and the silicon photomultiplier and its electronics are shown in fig. 7, wherein 1 is a 16 × 16 silicon photomultiplier integrated on a PCB board, 2 is a data output module, and 3 is a preamplifier.
The performance test of the detector prototype is carried out on a 14MeV D-T neutron source of China institute of engineering and physics, and the average fast neutron flux is 1 multiplied by 10 5 Neutron cm -2 ·s -1 In the case of (2), the net counting rate of the detector is 100230Hz, namely the detection efficiency of the sample machine on fast neutrons is about 4.0%. After the irradiation is carried out in the fast neutron beam for 5 minutes, the radiation imaging of the sample can be obtained, and the result is processed off line to obtain the result shown in fig. 8. In radiation imaging, it is generally accepted that the spatial frequency corresponding to a Modulation Transfer Function (MTF) value of 0.1 is the ultimate spatial resolution of the system, i.e., the spatial resolution of the prototype is 1.86mm and 1.26mm at thresholds of 25mV and 50mV, respectively. At present, the international fast neutron imaging space resolution is about 1mm generally, and the detection efficiency is basically less than 1%, which shows that the developed prototype can relieve the contradiction between the space resolution and the detection efficiency.
Based on the detector and the design of reading electronics, the invention can be used for imaging in fast neutron beam, and can obtain the results of higher detection efficiency and smaller spatial resolution only by needing shorter irradiation time. In addition, since silicon photomultipliers are easy to integrate, the silicon photomultipliers have the potential to play a role in large-area imaging.
The above-mentioned contents are only for illustrating the technical idea of the present invention, and the protection scope of the present invention is not limited thereby, and any modification made on the basis of the technical idea of the present invention falls within the protection scope of the claims of the present invention.
Claims (10)
1. The array type structure detector capable of being used for fast neutron beam large-area imaging is characterized by comprising a two-dimensional scintillator array (1), wherein one end of the two-dimensional scintillator array (1) is provided with a light guide (2), and the other end of the light guide (2) is provided with a silicon photomultiplier array (3); scintillation photons generated by the two-dimensional scintillator array (1) are diffused by the light guide (2) and transmitted to the silicon photomultiplier array (3) at the end of the light guide (2) to be converted into electric signals for ray measurement.
2. The array type structure detector for large-area imaging of fast neutron beam current according to claim 1, wherein the two-dimensional scintillator array (1) is formed by stacking a plurality of scintillator units, and a radial outer wall and an axial outer wall of each scintillator unit are coated with a mixture of glue and titanium dioxide, as shown in fig. 1, the glue is used for fixing the positions of the scintillator units, and the titanium dioxide is used as a reflecting layer; each scintillator cell is internally enriched in hydrogen atoms.
3. The array type structure detector for large-area imaging of fast neutron beam current according to claim 2, wherein the length of the scintillator unit is 30mm, and the cross-sectional dimension is 1 x 1mm 2 。
4. The array-type structure detector applicable to fast neutron beam large-area imaging according to claim 2, wherein the scintillator unit is selected from scintillators with high light yield and short decay time.
5. The array type structure detector for large-area imaging of fast neutron beam current according to claim 1, characterized in that the light guide (2) is made of K9 optical glass.
6. The array type structure detector for fast neutron beam large-area imaging according to claim 1, wherein the silicon photomultiplier array (3) is formed by integrating a plurality of silicon photomultipliers on the same PCB.
7. The array type structure detector for fast neutron beam large area imaging according to claim 6, wherein the silicon photomultiplier is formed by integrating hundreds of single photon avalanche diode unit arrays with the size of tens of microns on the same single crystal silicon wafer.
8. The array-type structure detector for fast neutron beam large-area imaging according to claim 7, wherein each single photon avalanche diode is connected in series with a resistor of several hundred kilo ohms, and the resistor is used for quenching avalanche after a photon is avalanche in the photodiode, so that the voltage across the single photon avalanche diode is restored to the initial working bias voltage to prepare for detecting the next incident photon.
9. The array type structure detector for fast neutron beam large area imaging according to claim 1, wherein the silicon photomultiplier array (3) adopts a multiplexing readout method of capacitive charge division.
10. The array type structure detector for fast neutron beam current large area imaging according to claim 9, characterized in that the anode of the silicon photomultiplier in the silicon photomultiplier array (3) is connected with one or more weighting capacitors according to the position, i.e. the anode signal is divided into the same number of sub-signals as the connected capacitors, and then the sub-signals are connected to one of four position signal output channels P, Q, R or S for output.
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