Gev gamma ray energy disperse spectroscopy and use method thereof

Abstract

The invention discloses a Gev gamma ray energy dispersive spectrometer and a use method, and relates to the technical field of spectral measurement of gamma ray sources, the Gev gamma ray energy dispersive spectrometer comprises a conversion body, a shielding part, a magnetic field area part, a detection area and a detector, the conversion body, the shielding part and the magnetic field area part are sequentially arranged in the advancing direction of a main gamma ray beam, and the detection area is arranged in the detection area. The detector is inserted into a detection area at the rear end of the magnetic field area; the conversion body is made of a high-Z target material and is used for converting incident gamma rays into electron and positron pairs; under the action of the magnetic field area, incident electrons and positron pairs deflect and enter a detector behind; the detector is inserted into the detection area along different energy electron deflection tracks; the device can be used in experimental environments such as vacuum and atmosphere, and has the characteristics of online detection, high signal-to-noise ratio, high resolution (about several percent), large energy range (20Mev-1 Gev) and large flux.

Description

Technical Field

[0001] The present invention relates to the technical field of spectrum measurement of gamma ray sources, and more specifically to the technical field of a GEV gamma ray spectrometer and a use method thereof. Background Art

[0002] In the research of high energy density physics, high energy physics and laboratory astrophysics, gamma ray beams with energies ranging from multi-MeV to GeV are a very important research object and also have a wide range of potential practical applications. High-flux high-energy gamma ray beams can usually be generated in the laboratory in two ways: one is the bremsstrahlung radiation generated when the electron beam passes through a solid target. The other is the inverse Compton scattering of the electron beam in a strong laser field. In addition, with the emergence of multi-PW laser devices, other mechanisms such as direct laser irradiation of solids or electromagnetic cascade effects can also be used to generate high-energy gamma ray beams.

[0003] High-energy and precisely characterized gamma-ray sources are not only crucial for nuclear physics research, but also key to studying strong-field quantum electrodynamics (SFQED) processes. For example, the Compton scattering of ultra-relativistic electrons in a strong laser field is significantly affected by quantum effects such as quantum radiation damping and electron-positron pair production, which in turn changes the spectrum and angular distribution of the emitted photons. Although traditional experiments have mainly focused on studying the electrons and positrons produced in these processes, recent studies have found that key features of strong-field quantum electrodynamics phenomena, such as multiphoton Compton scattering, are also reflected in the spectrum and spatial distribution of Compton scattered gamma-ray beams. In order to better study these complex physical phenomena, accurate spectral measurements of gamma-ray sources are indispensable. Existing patents disclose the following technologies:

[0004] The patent with the publication number CN113484900A and the patent name "Electron and Gamma-ray Spectrometer Based on Gradual Magnetic Field" discloses the following content: An electron and gamma-ray spectrometer based on a gradual magnetic field, including a conversion target, a shielding part, a spectrometer main part, and a detector. Along the direction of the gamma ray, the conversion target, the shielding part, and the spectrometer main part are arranged in sequence, and the detector is inserted into the interior of the spectrometer main part. The shielding part is composed of a plurality of lead bricks with central holes of the same diameter and a tungsten tube inserted into the central hole of the lead brick. The spectrometer main part is composed of an injection hole, a magnetic steel group, an assembly shell, and a rear collimator. The magnetic steel group generates a linearly enhanced gradual magnetic field. Under the action of the gradual magnetic field, the incident electrons are deflected, and the deflection trajectories of electrons with different energies converge on a nearly straight line. The assembly shell is used to encapsulate the magnetic steel group, fix the injection hole and the rear collimator, and has a certain magnetic field shielding and noise shielding effect. The injection hole and the rear collimator are used for spectrometer alignment. The detector is inserted into the main part of the spectrometer along the straight line where the convergence points of the deflection trajectories of electrons with different energies are located. The detector is composed of a detector plug-in plate, an imaging plate or a fluorescent screen, and the imaging plate or the fluorescent screen is mounted on the detector plug-in plate. The conversion target is a low-Z target material, such as LiF, C, etc., which is used to convert incident gamma rays into electrons. The inner diameter of the tungsten tube diameter has a variety of sizes to choose from, which is used to adjust the size of the limiting hole, and the number of lead bricks can be adjusted according to demand. The imaging plate is mainly used for electron and gamma ray energy spectrum detection under non-repetitive laser conditions; the fluorescent screen is used together with a camera to achieve real-time detection under repetitive laser conditions.

[0005] The above patent has the following defects:

[0006] 1. Using low-z targets allows gamma-ray spectrometers to operate at energies of multiple MeV, but the flux of gamma photons is very low.

[0007] 2. The magnetic field layout used can only deflect electrons of MeV at most, but cannot deflect electrons of 100 MeV, resulting in the inability to measure gamma rays close to the Gev energy range.

[0008] 3. The multiple scattering caused by the secondary particles generated by the conversion target of high-energy gamma rays and the noise such as off-axis photons are not completely shielded, which will cause serious distortion in measurement and difficulty in spectrum interpretation. Summary of the invention

[0009] The purpose of the present invention is to solve the above technical problems and provide a Gev gamma ray spectrometer and a method of using the same.

[0010] In order to achieve the above-mentioned purpose, the present invention specifically adopts the following technical solutions:

[0011] One aspect of the present invention provides a Gev gamma-ray spectrometer, comprising a converter, a shielding part, a magnetic field region part, a detection region and a detector, wherein the converter, the shielding part and the magnetic field region part are arranged in sequence along the advancing direction of the main gamma-ray beam, and the detector is inserted into the detection region at the rear end of the magnetic field region;

[0012] The material of the converter is a high-Z target material, which is used to convert incident gamma rays into electron and positron pairs; the incident electron and positron pairs are deflected under the action of the magnetic field area and enter the detector at the rear; the detector is inserted into the detection area along the deflection trajectory of electrons with different energies.

[0013] In one embodiment, the high-Z target is tungsten or tantalum, which is used to convert incident gamma rays into electron and positron pairs.

[0014] In one embodiment, the shielding part includes a first lead brick, a second lead brick and a collimation system, the first lead brick is provided with a cylindrical hole for shielding noise around the gamma ray, and the second lead brick is provided with a truncated cone hole for shielding noise around the gamma ray;

[0015] The collimation system includes a front collimation hole aperture and a rear collimation hole aperture. The front collimation hole is placed at the front end of the second lead block during collimation, and the rear collimation hole is placed at the rear end of the second lead block during collimation.

[0016] Specifically, the design of cylindrical holes and frustum-conical holes can maximize the shielding of noise around gamma rays.

[0017] In one embodiment, the magnetic field region is partially composed of permanent magnets.

[0018] In one embodiment, the detection area includes a detector board and an imaging board, and the imaging board is mounted on the detector board.

[0019] Specifically, the imaging plate is mainly used for the detection of positron-electron pairs and gamma-ray energy spectra under non-repetitive laser conditions to facilitate single-shot measurements.

[0020] Another aspect of the present invention provides a method for using a Gev gamma-ray spectrometer, using the above-mentioned Gev gamma-ray spectrometer.

[0021] In one embodiment, the steps include:

[0022] S1. Reconstruct the main gamma-ray beam using the measured electron pair spectrum through a deconvolution algorithm. When the main gamma-ray beam passes through the converter of the high-Z target, the incident main gamma-ray beam will be converted into positive and negative electron pairs due to its high energy.

[0023] S2. The shielding part consists of two lead bricks with holes and a collimation system. The aperture of the lead bricks is specially designed. The first lead brick uses a cylindrical hole. The second lead brick is placed at a certain position behind the first lead brick and uses a truncated cone hole that is small in the front and large in the back. Since the first lead block itself acts as a noise source, the second lead block can filter out the noise generated by the first lead block.

[0024] The collimation system is composed of a front collimation aperture and a rear collimation aperture. The front collimation aperture is placed at the front end of the second lead block during collimation; the rear collimation aperture is placed at the rear end of the second lead block during collimation; an adjustment-free collimation seat is added and placed on the center line of the rear side of the magnetic field during collimation;

[0025] S3, the laser passes through the center of the second lead block from the rear collimator tube, passes through the center of the limiting hole of the first lead brick, and hits the target point, thereby adjusting the position and direction of the spectrometer;

[0026] S4. After alignment is completed, remove and store.

[0027] In one embodiment, in step S1, the specific method of using the measured electron pair spectrum through a deconvolution algorithm is as follows:

[0028] If we consider an energy spectrum of The photon beam collides with the conversion target with a thickness of Δz, and the energy spectrum of the generated electrons and positrons is Then we get formula (1):

[0029]

[0030] In the formula, is the atomic mass of the conversion target, is the energy differential cross section of the positron-electron pair generation process, ρ is the material density, ∈ - ,∈ + are the energies of the electron and positron, respectively, so that ∈ - +∈ + =ω; the GEANT4 Monte Carlo particle transport code is used to simulate the interaction between gamma rays of various energy bands and the conversion target to produce positron-electron pairs, and the energy spectrum of the positron-electron pairs is statistically compared with that of gamma rays to obtain That is, the energy differential cross section of the positron-electron pair generation process; by inverting equation (1), we can define an inversion operator: The inversion equation is written as formula (2):

[0031]

[0032] Then the deconvolution problem can be expressed as the inversion of the linear operator problem. In principle, this problem can be solved by direct back substitution. However, this method will lead to a large uncertainty in the inverted photon spectrum. The following method is developed to achieve deconvolution:

[0033] The data is deconvolved using the expectation maximization (EM) algorithm as follows:

[0034] S11. Define the observation data g as formula (3):

[0035] g=K·f true +factor·noise (3);

[0036] Where K is the data matrix loaded from the file, f true is the desired true function, which is of the form f true (i) = e -λi (i=1,2,…,n), λ is the rate of exponential decrease, noise is the Gaussian noise generated by noise=|randn(m,1)|, and factor is the noise amplification factor;

[0037] S12. In the EM algorithm, an iterative solution is performed; the key formula of the expected step (E step) is as follows:

[0038] Calculate the covariance matrix∑ f =(K T Kβ+θI) -1 ; Where I is the unit matrix, β is the noise accuracy, and θ is the inverse of the prior variance:

[0039] Calculate the expected value μ of x f =∑ f (K T gβ+θf0), where f0 is the prior mean;

[0040] In the maximization step (M step), update the estimated value of f new f new =μ f And f new (i) = max(0, f new (i)) To ensure that it is non-negative, update the noise variance Use the formula At the same time, update the prior mean f0 to f new , update the prior variance θ to var(f new ), and update the noise accuracy β to

[0041] In each iteration, by calculating shift = max | f new-f est |To determine whether it has converged, the iteration is stopped when shift is less than the set convergence tolerance tol or the maximum number of iterations max_iter is reached;

[0042] S13, the estimated f est With the original f true Compare and stop the iteration when shift is less than the set convergence tolerance tol or reaches the maximum number of iterations max_iter.

[0043] In one embodiment, a Bayesian statistical deconvolution method is used, and the specific steps are as follows:

[0044] A11. In order to pose the deconvolution problem in a statistical way, a noise term is introduced into the inversion operator equation, namely

[0045] A12. Then apply a statistical distribution to the noise. A common choice is Gaussian noise that is independent of the signal, i.e. in is the noise variance, β is the equivalent accuracy, and the noise mean is zero;

[0046] Available And then launched

[0047]

[0048] In the formula, is the likelihood function of the problem, is the normalization factor, n = dimg(∈), I is the identity matrix;

[0049] A13. To apply the Bayesian method, a prior distribution needs to be given. Only a general assumption is made, that is, the spectral values ​​cannot be negative, that is, f(ω)∈[0,∞). The maximum entropy prior that satisfies this condition is selected: a normal distribution with an unknown variance to be inferred and a mean determined by the back-substitution solution of equation (2);

[0050] A14, the error associated with the spectral reconstruction is represented by the highest posterior density (HPD) interval of the solution;

[0051] The 100(1-α)% HPD interval for x is a specific 100(1-α)% level confidence interval, defined as: Let is a subset of X that satisfies

[0052] Where k is a constant such that ∫ C The maximum value of dxπ(x)=1-α.

[0053] The beneficial effects of the present invention are as follows:

[0054] 1 The present invention can be used in experimental environments such as vacuum and atmosphere, and has the characteristics of online detection, high signal-to-noise ratio, high resolution (about a few percent), large energy range (20Mev-1Gev), and large flux.

[0055] 2. The present invention can provide real-time information of the accurately measured spectrum of high-energy (expandable from hundreds of Mev to Gev) and high-flux gamma-ray beams. The shielding design used cleverly maximizes the elimination of various noises. The designed magnetic field layout can deflect electrons of hundreds of Mev or even Gev. The energy resolution can reach a level of several percent, meeting the needs of the next generation of strong-field quantum electrodynamics experiments. In short, photons are converted into electron-positron pairs when passing through a thin high-Z solid target. The measured electron pair spectrum is then used to reconstruct the main gamma-ray beam through a deconvolution algorithm, and the characteristics of spectra with complex structures can be accurately identified.

[0056] 3. Through optimized design, such as the use of high-Z targets and high-Z material double collimators, the background noise is effectively reduced and the signal-to-noise ratio is improved. This enables the target particle signal to be detected more clearly during the measurement process, reduces interference, and improves the accuracy and reliability of the measurement.

[0057] 4. Through the optimization of the collimator arrangement, aperture size, and magnet position by Geant4 Monte Carlo simulation, particles or photons in the energy range from MeV to GeV or even higher can be detected, which broadens the energy scale of the research, reduces the secondary radiation generated by the magnet frame, and improves the accuracy and stability of the experiment. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.

[0059] Figure 1 It is a structural schematic diagram of a Gev gamma-ray spectrometer of the present invention.

[0060] Figure 2 It is a schematic diagram of the trajectories of positive and negative electrons with different energies after being incident on the magnet of Example 1.

[0061] Figure 3 This is a diagram showing the effect of deconvolution of data using the expectation maximization (EM) algorithm in Example 2.

[0062] Figure 4 This is a diagram showing the effect of using the Bayesian statistical deconvolution method in Example 3. DETAILED DESCRIPTION

[0063] In order to make the technical problems, technical solutions and technical effects of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings here can be arranged and designed in various different configurations.

[0064] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the invention claimed for protection, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0065] It should be noted that similar reference numerals and letters represent similar items in the following drawings, so once an item is defined in one drawing, it does not need to be further defined and explained in the subsequent drawings. In addition, the terms "first", "second", etc. are only used to distinguish the description and cannot be understood as indicating or implying relative importance.

[0066] In the description of the embodiments of the present invention, it should be noted that the terms "inside", "outside", "upper", etc. indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, or are the orientations or positional relationships in which the inventive product is usually placed when used. They are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention.

[0067] like Figure 1 As shown, the present invention provides a Gev gamma-ray spectrometer, comprising a converter, a shielding part, a magnetic field region part, a detection region and a detector, wherein the converter, the shielding part and the magnetic field region part are sequentially arranged along the advancing direction of the main gamma-ray beam, and the detector is inserted into the detection region at the rear end of the magnetic field region;

[0068] The material of the converter is a high-Z target material, which is used to convert incident gamma rays into electron and positron pairs; the incident electron and positron pairs are deflected under the action of the magnetic field area and enter the detector at the rear; the detector is inserted into the detection area along the deflection trajectory of electrons with different energies.

[0069] In one embodiment, the high-Z target is tungsten or tantalum, which is used to convert incident gamma rays into electron and positron pairs.

[0070] In one embodiment, the shielding part includes a first lead brick, a second lead brick and a collimation system, the first lead brick is provided with a cylindrical hole for shielding noise around the gamma ray, and the second lead brick is provided with a truncated cone hole for shielding noise around the gamma ray;

[0071] The collimation system includes a front collimation hole aperture and a rear collimation hole aperture. The front collimation hole is placed at the front end of the second lead block during collimation, and the rear collimation hole is placed at the rear end of the second lead block during collimation.

[0072] Specifically, the design of cylindrical holes and frustum-conical holes can maximize the shielding of noise around gamma rays.

[0073] In one embodiment, the magnetic field region is partially composed of permanent magnets.

[0074] In one embodiment, the detection area includes a detector board and an imaging board, and the imaging board is mounted on the detector board.

[0075] Specifically, the imaging plate is mainly used for the detection of positron-electron pairs and gamma-ray energy spectra under non-repetitive laser conditions to facilitate single-shot measurements.

[0076] Example 1

[0077] This embodiment provides a gamma ray spectrometer with an energy range of 20 MeV-1 GeV. Figure 1 The gamma-ray spectrometer shown in the figure is drawn to scale. The converter of the spectrometer is a very thin tungsten target with a thickness of 0.03 mm. The shielding part consists of two lead blocks and a collimation system. The length of the first lead block is 500 mm. The radius of the cylindrical hole in the center of the first lead block is 4 mm. A second lead block with a length of 200 mm is placed 300 mm behind the first lead block. The second lead block needs to be drilled with a truncated cone hole with a transition radius from 6.5 mm to 8 mm. The diameter of the front collimator is 13 mm, and the diameter of the rear collimator is 16 mm. Insert the laser pen into the rear collimator and adjust the position of the spectrometer so that the beam passes through the front collimator of the spectrometer and is aimed at the target. The length*width*air gap of the permanent magnet magnetic field area is 160*80*20 mm, and the magnetic field strength is 1 T.

[0078] The detector is inserted into the detection area through the plug-in board. The distance between the tilting plug-in board seat joint and the end of the magnetic field is 540mm. The plug-in board seat is 350mm long. The joint can rotate 45° to 90° and can indicate the angle. The detection area is made into a magnetic spectrometer closed box with a plug-in board fixing slot. The positions of the detectors on both sides (IP1, IP2) are: y = -2.57x + 1.87, and the positive and negative electrons with different energies ( Figure 2The trajectory of the electron after entering the magnet of this embodiment can be intercepted by the straight line, ensuring that the spectrometer of this embodiment has high resolution in the energy range of 20MeV-1Gev. The 28mm aperture between the two IP plates is convenient for the unreacted gamma rays to pass through for subsequent research.

[0079] Example 2

[0080] This embodiment provides a method for using a Gev gamma-ray spectrometer, using the above-mentioned Gev gamma-ray spectrometer, and comprising the following steps:

[0081] S1. Reconstruct the main gamma-ray beam using the measured electron pair spectrum through a deconvolution algorithm. When the main gamma-ray beam passes through the converter of the high-Z target, the incident main gamma-ray beam will be converted into positive and negative electron pairs due to its high energy.

[0082] S2. The shielding part consists of two lead bricks with holes and a collimation system. The aperture of the lead bricks is specially designed. The first lead brick uses a cylindrical hole. The second lead brick is placed at a certain position behind the first lead brick and uses a truncated cone hole that is small in the front and large in the back. Since the first lead block itself acts as a noise source, the second lead block can filter out the noise generated by the first lead block.

[0083] The collimation system is composed of a front collimation aperture and a rear collimation aperture. The front collimation aperture is placed at the front end of the second lead block during collimation; the rear collimation aperture is placed at the rear end of the second lead block during collimation; an adjustment-free collimation seat is added and placed on the center line of the rear side of the magnetic field during collimation;

[0084] S3, the laser passes through the center of the second lead block from the rear collimator tube, passes through the center of the limiting hole of the first lead brick, and hits the target point, thereby adjusting the position and direction of the spectrometer;

[0085] S4. After alignment is completed, remove and store.

[0086] In step S1, the specific method of using the measured electron pair spectrum through a deconvolution algorithm is as follows:

[0087] If we consider an energy spectrum of The photon beam collides with the conversion target with a thickness of Δz, and the energy spectrum of the generated electrons and positrons is Then we get formula (1):

[0088]

[0089] In the formula, is the atomic mass of the conversion target, is the energy differential cross section of the positron-electron pair generation process, ρ is the material density, ∈-, ∈ +are the energies of the electron and positron, respectively, so that ∈-+∈ + =ω;

[0090] The GEANT4 Monte Carlo particle transport code is used to simulate the interaction between gamma rays and conversion targets in various energy ranges to produce positron-electron pairs. The energy spectrum of positron-electron pairs is statistically compared with that of gamma rays to obtain That is, the energy differential cross section of the positron-electron pair generation process; by inverting equation (1), we can define an inversion operator: The inversion equation is written as formula (2):

[0091]

[0092] Then the deconvolution problem can be expressed as the inversion of the linear operator problem. In principle, this problem can be solved by direct back substitution. However, this method will lead to a large uncertainty in the inverted photon spectrum. The following method is developed to achieve deconvolution:

[0093] The data is deconvolved using the expectation maximization (EM) algorithm as follows:

[0094] S11. Define the observation data g as formula (3):

[0095] g=K·f true +factor·noise (3);

[0096] Where K is the data matrix loaded from the file, f true is the desired true function, which is of the form f true (i) = e -λi (i=1,2,…,n), λ is the rate of exponential decrease, noise is the Gaussian noise generated by noise=|randn(m,1)|, and factor is the noise amplification factor;

[0097] S12. In the EM algorithm, an iterative solution is performed; the key formula of the expected step (E step) is as follows:

[0098] Calculate the covariance matrix∑ f =(K T Kβ+θI) -1 ; Where I is the unit matrix, β is the noise accuracy, and θ is the inverse of the prior variance:

[0099] Calculate the expected value μ of x f =∑ f (K T gβ+θf0), where f0 is the prior mean;

[0100] In the maximization step (M step), update the estimated value of f new f new =μ f And f new (i) = max(0, f new (i)) To ensure that it is non-negative, update the noise variance Use the formula At the same time, update the prior mean f0 to f n ew, update the prior variance θ to var(f n ew), and update the noise accuracy β to

[0101] In each iteration, by calculating shift = max | f new -f est |To determine whether it has converged, the iteration is stopped when shift is less than the set convergence tolerance tol or the maximum number of iterations max_iter is reached;

[0102] S13, the estimated f est With the original f true Compare and stop the iteration when shift is less than the set convergence tolerance tol or reaches the maximum number of iterations max_iter.

[0103] Specifically, the estimated f est With the original f true By comparison, it is found that the maximum error is about 10%, and in most cases the error is less than 5%. The denoising effect is very good, but it is not stable enough for multi-Gev gamma rays. The deconvolution effect is better for the case of more than ten Mev to 1-2 Gev. The effect of evaluation estimation is as follows Figure 3 .exist Figure 3 The uncertainty in edge identification at 1 GeV is about 4%, while the uncertainty in edge identification at 1.8 GeV is about 10%.

[0104] Example 3

[0105] This embodiment provides a method for using a Gev gamma-ray spectrometer, using the above-mentioned Gev gamma-ray spectrometer, and comprising the following steps:

[0106] S1. Reconstruct the main gamma-ray beam using the measured electron pair spectrum through a deconvolution algorithm. When the main gamma-ray beam passes through the converter of the high-Z target, the incident main gamma-ray beam will be converted into positive and negative electron pairs due to its high energy.

[0107] S2. The shielding part consists of two lead bricks with holes and a collimation system. The aperture of the lead bricks is specially designed. The first lead brick uses a cylindrical hole. The second lead brick is placed at a certain position behind the first lead brick and uses a truncated cone hole that is small in the front and large in the back. Since the first lead block itself acts as a noise source, the second lead block can filter out the noise generated by the first lead block.

[0108] The collimation system is composed of a front collimation aperture and a rear collimation aperture. The front collimation aperture is placed at the front end of the second lead block during collimation; the rear collimation aperture is placed at the rear end of the second lead block during collimation; an adjustment-free collimation seat is added and placed on the center line of the rear side of the magnetic field during collimation;

[0109] S3, the laser passes through the center of the second lead block from the rear collimator tube, passes through the center of the limiting hole of the first lead brick, and hits the target point, thereby adjusting the position and direction of the spectrometer;

[0110] S4. After alignment is completed, remove and store.

[0111] In step S1, the specific method of using the measured electron pair spectrum through a deconvolution algorithm is as follows:

[0112] If we consider an energy spectrum of The photon beam collides with the conversion target with a thickness of Δz, and the energy spectrum of the generated electrons and positrons is Then we get formula (1):

[0113]

[0114] In the formula, is the atomic mass of the conversion target, is the energy differential cross section of the positron-electron pair generation process, ρ is the material density, ∈-, ∈ + are the energies of the electron and positron, respectively, so that ∈-+∈ + =ω;

[0115] The GEANT4 Monte Carlo particle transport code is used to simulate the interaction between gamma rays and conversion targets in various energy ranges to produce positron-electron pairs. The energy spectrum of positron-electron pairs is statistically compared with that of gamma rays to obtain That is, the energy differential cross section of the positron-electron pair generation process; by inverting equation (1), we can define an inversion operator: The inversion equation is written as formula (2):

[0116]

[0117] Then the deconvolution problem can be expressed as the inversion of the linear operator problem, which can be solved in principle by direct back substitution. However, this approach will lead to a large uncertainty in the inverted photon spectrum: the following method is developed for deconvolution:

[0118] The Bayesian statistical deconvolution method is used, and the specific steps are as follows:

[0119] A11. In order to pose the deconvolution problem in a statistical way, a noise term is introduced into the inversion operator equation, namely

[0120] A12. Then apply a statistical distribution to the noise. A common choice is Gaussian noise that is independent of the signal, i.e. in is the noise variance, β is the equivalent accuracy, and the noise mean is zero;

[0121] Available And then launched

[0122]

[0123] In the formula, is the likelihood function of the problem, is the normalization factor, n = dimg(∈), I is the identity matrix;

[0124] A13. To apply the Bayesian method, a prior distribution needs to be given. Only a general assumption is made, that is, the spectral values ​​cannot be negative, that is, f(ω)∈[0,∞). The maximum entropy prior that satisfies this condition is selected: a normal distribution with an unknown variance to be inferred and a mean determined by the back-substitution solution of equation (2);

[0125] A14, the error associated with the spectral reconstruction is represented by the highest posterior density (HPD) interval of the solution;

[0126] The 100(1-α)% HPD interval for x is a specific 100(1-α)% level confidence interval, defined as: Let is a subset of X that satisfies

[0127] Where k is a constant such that ∫ C The maximum value of dxπ(x)=1-α.

[0128] Assume that the spectrum of the gamma-ray beam is a Gaussian function or an inverse proportional function, and then let it pass through the spectrometer. Next, use the above Bayesian method to deconvolve the resulting electron and positron spectra to obtain the inverted gamma-ray spectrum and its associated uncertainty. From these two examples, it can be seen that the spectrometer can reconstruct the gamma-ray spectrum with high accuracy and can accurately identify the edges in the spectrum. The effect of using Bayesian statistical deconvolution is shown below. Figure 4 .exist Figure 4 The uncertainty in edge identification at 2 GeV is about 8%, while the uncertainty in edge identification at 6 GeV is about 3%. It must be pointed out that these uncertainties, associated with the confidence level of the inverted gamma-ray spectrum, are much larger than the intrinsic resolution of the spectrometer and are therefore the dominant factor in the estimated uncertainty when reconstructing the gamma-ray spectrum from the detector. Note that while in principle the choice of deconvolving the electron or positron spectrum is arbitrary due to the symmetry of electron pair production in the conversion target, the electron spectrum is susceptible to higher noise due to additive noise (e.g. Compton scattering in the conversion target). Therefore, it is experimentally necessary to deconvolute the positron spectrum.

Claims

1. A Gev gamma-ray spectrometer, characterized in that: It includes a converter, a shielding part, a magnetic field region part, a detection region and a detector, wherein the converter, the shielding part and the magnetic field region part are arranged in sequence along the advancing direction of the main gamma ray beam, and the detector is inserted into the detection region at the rear end of the magnetic field region; The material of the converter is a high-Z target material, which is used to convert incident gamma rays into electron and positron pairs; the incident electron and positron pairs are deflected under the action of the magnetic field area and enter the detector at the rear; the detector is inserted into the detection area along the deflection trajectory of electrons with different energies.

2. A Gev gamma-ray spectrometer according to claim 1, characterized in that: The high-Z target is tungsten or tantalum, and is used to convert incident gamma rays into electron and positron pairs.

3. A Gev gamma-ray spectrometer according to claim 1, characterized in that: The shielding part includes a first lead brick, a second lead brick and a collimation system, the first lead brick is provided with a cylindrical hole for shielding noise around the gamma ray, and the second lead brick is provided with a truncated cone hole for shielding noise around the gamma ray; The collimation system includes a front collimation hole aperture and a rear collimation hole aperture. The front collimation hole is placed at the front end of the second lead block during collimation, and the rear collimation hole is placed at the rear end of the second lead block during collimation.

4. A Gev gamma-ray spectrometer according to claim 1, characterized in that: The magnetic field region is partially composed of permanent magnets.

5. A Gev gamma-ray spectrometer according to claim 1, characterized in that: The detection area includes a detector plug-in board and an imaging board, and the imaging board is mounted on the detector plug-in board.

6. A method for using a Gev gamma-ray spectrometer, characterized in that: A Gev gamma-ray spectrometer according to any one of claims 1 to 5 is used.

7. The method for using a Gev gamma-ray spectrometer according to claim 6, characterized in that: The steps include: S1. Reconstruct the main gamma-ray beam using the measured electron pair spectrum through a deconvolution algorithm. When the main gamma-ray beam passes through the converter of the high-Z target, the incident main gamma-ray beam will be converted into positive and negative electron pairs due to its high energy. S2. The shielding part consists of two lead bricks with holes and a collimation system. The aperture of the lead bricks is specially designed. The first lead brick uses a cylindrical hole. The second lead brick is placed at a certain position behind the first lead brick and uses a truncated cone hole that is small in the front and large in the back. Since the first lead block itself acts as a noise source, the second lead block can filter out the noise generated by the first lead block. The collimation system is composed of a front collimation aperture and a rear collimation aperture. The front collimation aperture is placed at the front end of the second lead block during collimation; the rear collimation aperture is placed at the rear end of the second lead block during collimation; an adjustment-free collimation seat is added and placed on the center line of the rear side of the magnetic field during collimation; S3, the laser passes through the center of the second lead block from the rear collimator tube, passes through the center of the limiting hole of the first lead brick, and hits the target point, thereby adjusting the position and direction of the spectrometer; S4. After alignment is completed, remove and store.

8. The method for using a Gev gamma-ray spectrometer according to claim 7, characterized in that: In step S1, the specific method of using the measured electron pair spectrum through a deconvolution algorithm is as follows: If we consider an energy spectrum of The photon beam collides with the conversion target with a thickness of Δz, and the energy spectrum of the generated electrons and positrons is Then we get formula (1): In the formula, is the atomic mass of the conversion target, is the energy differential cross section of the positron-electron pair generation process, ρ is the material density, ∈ - ,∈ + are the energies of the electron and positron, respectively, so that ∈ - +∈ + =ω; The GEANT4 Monte Carlo particle transport code is used to simulate the interaction between gamma rays and conversion targets in various energy ranges to produce positron-electron pairs. The energy spectrum of positron-electron pairs is statistically compared with that of gamma rays to obtain That is, the energy differential cross section of the process of electron-positron pair production; By inverting equation (1), we can define an inversion operator: The inversion equation is written as formula (2): Then the deconvolution problem can be expressed as the inversion of the linear operator problem. In principle, this problem can be solved by direct back substitution. However, this method will lead to a large uncertainty in the inverted photon spectrum. The following method is developed to achieve deconvolution: The data is deconvolved using the expectation maximization (EM) algorithm as follows: S11. Define the observation data g as formula (3): g=K·f true +factor·noise (3); Where K is the data matrix loaded from the file, f true is the desired true function, which is of the form f true (i) = e -λi (i=1,2,…,n), λ is the rate of exponential decrease, noise is the Gaussian noise generated by noise=|randn(m,1)|, and factor is the noise amplification factor; S12. In the EM algorithm, an iterative solution is performed; the key formula of the expected step (E step) is as follows: Calculate the covariance matrix∑ f =(K T Kβ+θI) -1 ; Where I is the unit matrix, β is the noise accuracy, and θ is the inverse of the prior variance; Calculate the expected value μ of x f =∑ f (K T gβ+θf0), where f0 is the prior mean; In the maximization step (M step), update the estimated value of f new f new =μ f And f new (i)=max(0,f new (i)) To ensure that it is non-negative, update the noise variance Use the formula At the same time, update the prior mean f0 to f n ew, update the prior variance θ to var(f n ew), update the noise accuracy β to In each iteration, by calculating shift = max | f new -f est |To determine whether it has converged, the iteration is stopped when shift is less than the set convergence tolerance tol or the maximum number of iterations max_iter is reached; S13, the estimated f est With the original f true Compare and stop the iteration when shift is less than the set convergence tolerance tol or reaches the maximum number of iterations max_iter.

9. The method for using a Gev gamma-ray spectrometer according to claim 8, characterized in that: The Bayesian statistical deconvolution method is used, and the specific steps are as follows: A11. In order to pose the deconvolution problem in a statistical way, a noise term is introduced into the inversion operator equation, namely A12. Then apply a statistical distribution to the noise. A common choice is Gaussian noise that is independent of the signal, i.e. in is the noise variance, β is the equivalent accuracy, and the noise mean is zero; Available And then launched In the formula, is the likelihood function of the problem, is the normalization factor, n = dimg(∈), I is the identity matrix; A13. To apply the Bayesian method, a prior distribution needs to be given. Only a general assumption is made, that is, the spectral values ​​cannot be negative, that is, f(ω)∈[0,∞). The maximum entropy prior that satisfies this condition is selected: a normal distribution with an unknown variance to be inferred and a mean determined by the back-substitution solution of equation (2); A14, the error associated with the spectral reconstruction is represented by the highest posterior density (HPD) interval of the solution; The 100(1-α)% HPD interval for x is a specific 100(1-α)% level confidence interval, defined as: Let is a subset of X that satisfies Where k is a constant such that ∫ c The maximum value of dxπ(x)=1-α.

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