Instantaneous high-brightness gamma ray detection method based on copper activation

Abstract

A method for detecting instant high-brightness gamma rays based on copper activation comprises the steps that a copper sheet is perpendicularly placed in the propagation direction of a gamma ray source to be detected, the copper sheet is activated in the process of being irradiated by the gamma ray source, the copper sheet is attached to an imaging plate, and the gamma ray source to be detected is detected in the mode that the activated copper sheet and the imaging plate are combined. An imaging plate responding to positron is used for measuring the positron generated by Cu-62 decay, the space divergence angle of a high-energy gamma radiation source and the flux of Giant Dipole Resonance (Giant Dipole Resonance, GDR) energy region (17 + / -5) MeV of a 63Cu (gamma, n) 62Cu reaction channel can be rapidly measured within the time period of 15-20 minutes, and the measurement accuracy is high. The problem that in the prior art, a high-brightness and ultra-short-pulse high-energy X / gamma radiation source cannot be rapidly measured is solved.

Description

Technical Field

[0001] The present invention relates to the field of high energy density physics and nuclear science technology, and in particular to a prompt high brightness gamma ray detection method based on copper activation, specifically a prompt high brightness gamma ray detection method combining Cu-62 positron decay and imaging plate response. Background Art

[0002] With the rapid development of femtosecond / picosecond high-power laser technology, it has become possible to generate dense matter under laboratory conditions, study particle generation and acceleration in near-critical density plasma environments, and generate secondary high-brightness radiation sources. By interacting high-power picosecond ultrashort laser pulses with near-critical density plasmas, high-energy electron beams with temperatures reaching MeV and charges of approximately 100 nC can be generated in the laboratory. By bombarding high-atomic-number metal conversion targets with the electron beam, bremsstrahlung can generate high-brightness X-ray / gamma radiation sources. These laser-driven high-energy and high-brightness radiation sources have attracted extensive attention and research in fields such as fundamental physics research, nuclear science and technology, and medical imaging.

[0003] High-energy X / gamma radiation sources driven by high-power lasers have the characteristics of ultra-high brightness and ultra-short time resolution, and therefore have important applications in industrial material flaw detection and photography, nuclear energy level property research, isotope production, and other fields. The high-brightness radiation source generated by laser drive has a cutoff energy of up to tens of MeV and a flux of about 10 9 -10 10 The continuous energy spectrum of photons / MeV / sr and the complex environment in which lasers interact with plasma make it difficult for existing detection technologies to non-destructively, accurately, and rapidly measure the spatial divergence angle and energy spectrum. Therefore, a rapid diagnostic technique is needed to assess the experimental completion within a short period of time after laser targeting and simultaneously obtain information on the spatial divergence angle and energy spectrum of the gamma radiation source.

[0004] Currently, the measurement technologies for high-energy X / gamma radiation sources in laboratories mainly include the following categories:

[0005] Passive imaging plate: Photoluminescent phosphor screens (such as BaFBr:Eu) are widely used to record integrated X / gamma ray images in laser-plasma and X-ray experiments. Incident photons generate trapped charges in the phosphor layer, which excite electrons to high energy levels. The scanner de-excites them to emit characteristic X-rays, enabling the measurement of the radiation source. The imaging plate is sensitive to the flux range of 10 5It has a good response to photons of the order of magnitude and energy of about tens to hundreds of keV. For example, the "Intense Pulsed Gamma Radiation Dose Field Distribution Measurement and Diagnosis Device and Method" published in the patent application with application number 201811168588.6 is based on this principle. The spatial resolution of this method is determined by the pixel size of the imaging plate and the resolution of the scanner, which is about tens of microns. The imaging plate does not have time resolution and performs integral statistics on the entire pulse width of the radiation source. However, compared with the high-brightness secondary radiation source generated by the interaction between the laser and the near-critical density plasma, the imaging plate method cannot measure the flux of about 10 10 / MeV / sr high-luminosity gamma-ray source for direct measurement.

[0006] Activated stacks: High-energy gamma photons induce photonuclear reactions with activated stack elements, such as the (γ, n) and (γ, p) reaction channels, producing radioactive isotopes. By counting the characteristic photons of isotopic decay for a long time and combining this with deconvolution calculations from multiple sample data, the energy spectrum of the gamma-ray source can be inferred. For example, in the article "Ultra-high-efficiency bremsstrahlung production in the interaction of direct laser-accelerated electrons with high-Z material," author Tavana used a high-purity germanium detector to count the characteristic photons emitted by the gamma decay of activated Ta-178m, Ta-176, and Ta-175, and derived the energy spectrum of the bremsstrahlung source. Photonuclear reactions have energy thresholds, typically as low as a few MeV, and this method involves time-delayed integration measurements. The experiment, detection completion, and spectrum analysis typically take dozens of hours, making rapid evaluation of experimental results impossible and the spatial divergence of the gamma-ray source inaccessible.

[0007] Scintillator detector: Inorganic scintillators can convert high-energy photons into visible light pulse signals, which are read out by photomultiplier tubes and diodes. For example, in the "Liquid Scintillator Detector" published in the patent application with application number 202121654993.6, liquid scintillators are used to convert gamma signals into light signals and amplify them, thereby achieving high-precision detection of gamma signals. The detection capabilities of various scintillators range from hundreds of keV to several MeV, such as NaI (Tl), CsI (Tl), LaBr3, organic plastic scintillators, and liquid scintillators (EJ-301). This method uses a radiation source for detection with a time resolution of about nanoseconds, and an efficiency of about 3% to 12% in the 0.5-1MeV energy range. It is impossible to measure high-brightness radiation sources with pulse widths on the order of picoseconds, and the signal will be oversaturated and unable to be read.

[0008] Semiconductor detectors: When radiation source photons enter the detector's semiconductor material, electron-hole pairs are generated through the photoelectric effect, Compton scattering, and the electron pair effect. An external bias forms a depletion layer, causing the electron-hole pairs to drift in the electric field, generating an electrical signal output. The signal amplitude of the semiconductor detector is proportional to the energy of the radiation source photons. Commonly used semiconductor detectors in laboratories include high-purity germanium (HPGe) and silicon (PIN) detectors. However, the pulse width of high-energy laser-generated radiation sources is in the picosecond range, and semiconductor detectors have a time resolution of approximately nanoseconds to microseconds, making them unable to function properly in the environment of ultrashort laser pulses.

[0009] In summary, there is currently no good technical solution to ensure data evaluation and azimuth and energy spectrum diagnosis of high-brightness, short-pulse high-energy X / gamma radiation sources in a short period of time. Summary of the Invention

[0010] In order to overcome the defects of the above-mentioned prior art, the purpose of the present invention is to provide a method for measuring prompt high-brightness gamma rays by combining Cu-62 positron decay with imaging plate response. By using an imaging plate that responds to positrons to record the number and spatial distribution of positrons generated by Cu-62 decay, it is possible to achieve the spatial divergence angle and spatial distribution of high-energy gamma radiation sources within a period of 15 to 20 minutes. 63 Cu(γ,n) 62 The flux in the giant dipole resonance (GDR) energy region of Cu reaction channel (17±5) MeV is measured rapidly, which solves the problem in existing technology that it is impossible to quickly measure high-brightness, ultrashort pulse high-energy X / gamma radiation sources.

[0011] In order to achieve the above-mentioned object of the invention, the technical solution adopted by the present invention is as follows:

[0012] A method for detecting prompt high-brightness gamma rays based on copper activation comprises the following steps:

[0013] Step 1: Conduct a laser target shooting experiment in the target chamber. Before the laser shooting, place a copper sheet perpendicular to the propagation direction of the gamma ray source to be measured. In a vacuum environment, the high-brightness gamma rays generated by the laser drive irradiate the copper sheet. During the irradiation process by the gamma ray source, the copper sheet undergoes a photoneutron reaction and is activated, generating the radioactive isotope Cu-62. The laser shooting time is recorded as t0.

[0014] Step 2: After the target shooting is completed, wait for the target room to be ventilated, take out the irradiated activated copper sheet, and place the activated copper sheet close to the imaging plate in a dark room environment. The time the copper sheet is placed on the imaging plate is recorded as t1;

[0015] Step 3: Set the exposure time of the imaging plate to Cu-62 irradiation to 15-20 minutes. After the irradiation is completed, record the time when the activated copper sheet is removed as t2;

[0016] Step 4: Place the imaging plate in the scanner and set the scanning parameters: scanning voltage is 500-2000V; gear L is 1-5; the high-voltage sensitivity curve needs to be calibrated, and the calibration result is h(V). After scanning, the grayscale value G of the imaging plate is obtained. The scanned grayscale value G is converted into a photoluminescence signal PSL according to formula (1):

[0017]

[0018] Where R is the image pixel size set when reading the imaging plate;

[0019] Combined with the response coefficient of the imaging plate to the incident positron parameters, the number N of Cu-62 decay positrons and their spatial distribution on the imaging plate during the measurement period are calculated according to formula (2):

[0020]

[0021] Where g(E) is the response efficiency of the imaging plate to positrons with energy E, f(t) is the signal attenuation coefficient, and k(θ) is the correction factor for the response efficiency of the imaging plate to positrons at different incident angles.

[0022] Step 5: Use ImageJ software to perform integral statistics on the spatial distribution of positrons obtained on the imaging plate. Obtain the intensity curve of the positron number through Gaussian fitting. Calculate the full width at half maximum of the curve, that is, the circular spot diameter of the irradiated area of ​​the imaging plate. Calculate the azimuth angle and substitute the distance between the copper sheet and the gamma source to calculate the spatial divergence angle of the ray source.

[0023] Step 6: Calculate the ratio ρ of the number of Cu-62 decayed during the measurement to the total generated Cu-62 using the decay formula (3):

[0024]

[0025] Where λ is the decay constant of Cu-62. The total number of Cu-62 is obtained by dividing the number of Cu-62 decayed during the measurement by ρ, that is, the yield Y of Cu-62 generated by the photonuclear reaction. i , then the relationship between the photonuclear reaction yield and the incident gamma photon flux, the number of target atoms and the reaction cross section is (4):

[0026]

[0027] Where N T is the number of target nuclei, N γ(E) is the photon flux of the gamma radiation source when the energy is E, σ i (E) 63 Cu(γ,n) 62 Cu is the cross-section value of the reaction channel when the incident photon energy is E; the cross-section curve of the reaction channel is integrated in the giant dipole resonance energy region to obtain the integrated cross-section value σ; N T The circular spot area obtained by scanning the imaging plate is calculated by multiplying the copper thickness and copper density. T Substituting into formula (4), we can calculate that the flux of the gamma-ray source in the giant dipole resonance (17±5) MeV energy region is N γ .

[0028] In the step 1, the thickness of the copper sheet is less than 2 mm, and the area of ​​the copper sheet is larger than the area of ​​the gamma radiation source.

[0029] The imaging plate scanner used was a GE Typhoon 7000 fast laser imager.

[0030] The imaging plate used is of model Fuji BAS-SR, with a size of 10 cm×15 cm.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] (1) The present invention uses Cu-62 decay positrons and an imaging plate to detect the spatial intensity distribution of the decayed positrons. The spatial angular distribution of a high-brightness, ultrashort pulse gamma-ray source is measured within a 15-20 minute period.

[0033] (2) The present invention is based on photonuclear reactions. By statistically measuring the number of decaying Cu-62 in a time period, the flux of a high-brightness, ultrashort pulse gamma-ray source in the (17±5) MeV energy region is quickly inferred.

[0034] In summary, the present invention utilizes a combination of activated copper sheets and imaging plates to quickly measure the spatial divergence angle and partial energy spectrum of high-brightness, ultrashort-pulse gamma-ray sources. The copper sheet, used as an activated target, is inexpensive, simple in structure, and can withstand extremely high radiation intensities. It can be configured in a laboratory environment based on the actual gamma-ray source divergence angle and flux. The method is clear, uses readily available parameter data, and facilitates calculations, making it suitable for rapidly evaluating the quality of high-energy gamma-ray sources. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is a schematic diagram of the experimental arrangement of the present invention.

[0036] Figure 2 It is a scanned image of the imaging plate of the present invention.

[0037] Figure 3 This is a schematic diagram of the spatial angular distribution of gamma-ray sources.

[0038] Figure 4 This is a comparison chart of the ray source flux obtained using the present invention and the simulation results. DETAILED DESCRIPTION

[0039] The present invention will be described in detail below with reference to the accompanying drawings.

[0040] A method for detecting prompt high-brightness gamma rays based on copper activation comprises the following steps:

[0041] Reference Figure 1 , step 1: carry out laser target shooting experiment in the target chamber. Before laser target shooting, place a copper sheet with a thickness of less than 2mm vertically in the propagation direction of the gamma ray source to be measured. The copper sheet with this thickness is selected because the energy of the positrons from the decay of Cu-62 is 1.3MeV. The penetration depth of positrons under this energy in the copper sheet is 2mm. In order to avoid self-absorption of positrons during transmission in the copper sheet, a copper sheet with a thickness of less than 2mm is selected; the area of ​​the copper sheet is larger than the area of ​​the gamma radiation source; in a vacuum environment, a high-power picosecond laser first interacts with the near-critical density plasma to generate a supermassive power electron beam, which then bombards the tantalum conversion target to generate bremsstrahlung and generate high-brightness gamma rays, which is the gamma ray source to be measured. The gamma rays irradiate the copper sheet in the target chamber. During the irradiation process by the gamma ray source, the copper sheet undergoes a photonuclear neutron reaction and is activated to generate a radioactive isotope Cu-62. In the experiment, the time of laser target shooting is recorded as t0;

[0042] Step 2: After the target shooting is completed, wait for the target room to be ventilated, take out the irradiated activated copper sheet after taking out the target room with proper protection. Since the positrons emitted by Cu-62 decay are emitted at 4π and the imaging plate is sensitive to visible light signals and easily exposed, the activated copper sheet needs to be placed close to the imaging plate in a darkroom environment. The time the copper sheet is placed on the imaging plate is recorded as t1.

[0043] Step 3: According to the National Nuclear Data Center (NNDC) database, the half-life of Cu-62 is T 1 / 2 The time of exposure of the imaging plate to Cu-62 irradiation is set to 15-20 minutes, which is about two half-lives of Cu-62. After the exposure, the time of removing the activated copper sheet is recorded as t2.

[0044] Step 4: Place the imaging plate with the information of Cu-62 decay positrons on the scanning stage of the imaging instrument GE Typhoon 7000. According to the specific experimental conditions, set the scanning parameters: scanning voltage 500-2000V; gear L is 1-5; the high-voltage sensitivity curve needs to be calibrated, and the calibration result is h (V). After scanning, the grayscale value G of the imaging plate is obtained. According to formula (1)

[0045]

[0046] Where R is the image pixel size set when reading the imaging plate;

[0047] The scanning grayscale value G is converted into a photoluminescence signal PSL. The number N of Cu-62 decay positrons and its spatial distribution on the imaging plate during the measurement period are calculated by formula (2) and the response coefficient of the imaging plate to the incident positron parameters. Since the secondary gamma ray source generated by the interaction between the laser and the near-critical density plasma is a point source, the activated area of ​​the copper sheet appears as a circular spot, and the irradiated area of ​​the imaging plate is also a circular spot. Figure 2 .

[0048]

[0049] Where g(E) is the response efficiency of the imaging plate to positrons with energy E, f(t) is the signal attenuation coefficient, and k(θ) is the correction factor for different positron incident angles.

[0050] Step 5: Use ImageJ software to perform integral statistics on the spatial intensity distribution of positrons obtained on the imaging plate, and obtain the intensity curve of the positron number by Gaussian fitting. Calculate the full width half maximum (FWHM) of the curve, that is, the diameter of the circular spot in the irradiated area of ​​the imaging plate. Calculate the azimuth angle, substitute the distance between the copper sheet and the gamma source, and refer to Figure 3 , the spatial divergence angle of the ray source is calculated. This method realizes the measurement of the spatial angular distribution of prompt high-energy gamma-ray sources.

[0051] Step 6: Calculate the ratio ρ of the number of Cu-62 decayed during the measurement to the total generated Cu-62 using the decay formula (3):

[0052]

[0053] Where λ is the decay constant of Cu-62. The total number of Cu-62 is obtained by dividing the number of Cu-62 decayed during the measurement by ρ, that is, the yield Y of Cu-62 generated by the photonuclear reaction. i , the relationship between the photonuclear reaction yield and the incident gamma photon flux, the number of target atoms and the reaction cross section (4) is:

[0054]

[0055] Here, N T is the number of nuclei in the irradiated area, N γ (E) is the incident bremsstrahlung photon flux at energy E, σ i(E) 63 Cu(γ,n) 62 The cross-section value of the Cu reaction channel when the incident photon energy is E. Since the gamma-ray source energy spectrum is continuous, the cross-section integral value of the reaction channel in the giant dipole resonance energy region is used as the parameter σ. N T is the number of atomic nuclei in the area irradiated by the copper sheet, which can be obtained by multiplying the circular spot area obtained by scanning the imaging plate by the thickness of the copper sheet and the copper density. T Substituting into formula (4), the flux of the gamma-ray source in the giant dipole resonance (17±5) MeV energy region is obtained, which enables the measurement of part of the energy spectrum of the gamma-ray source.

[0056] Figure 4 This is a comparison chart of the ray source flux obtained using the method of the present invention and the Monte Carlo simulation results. It can be seen from the figure that the ray source flux at (17±5) MeV obtained using this method is in good agreement with the ray source energy spectrum results obtained by simulation, proving the reliability of the method of the present invention.

Claims

1. A method for detecting prompt high-brightness gamma rays based on copper activation, characterized in that: The following steps are involved: Step 1: Conduct a laser target shooting experiment in a target chamber. Before the laser shooting, place a copper sheet perpendicular to the propagation direction of the gamma ray source to be measured. Under a vacuum environment, the laser is fired at the copper sheet in the target chamber. During the irradiation process by the gamma ray source, the copper sheet is activated and undergoes a photoneutron reaction, generating the radioactive isotope Cu-62. The time of laser shooting is recorded as t0. Step 2: After the target shooting is completed, wait for the target room to be ventilated, take out the irradiated activated copper sheet, and place the activated copper sheet close to the imaging plate in a dark room environment. The time the copper sheet is placed on the imaging plate is recorded as t1; Step 3: Set the exposure time of the imaging plate to Cu-62 irradiation to 15-20 minutes. After the exposure, record the time of removing the activated copper sheet as t2; Step 4: Place the imaging plate in the scanner and set the scanning parameters: scanning voltage is 500-2000V; gear L is 1-5; the high-voltage sensitivity curve needs to be calibrated, and the calibration result is h(V). After scanning, the grayscale value G of the imaging plate is obtained. The scanned grayscale value G is converted into a photoluminescence signal PSL according to formula (1): Where R is the image pixel size set when reading the imaging plate; Combined with the response coefficient of the imaging plate to the incident positron parameters, the number N of Cu-62 decay positrons and their spatial distribution on the imaging plate during the measurement period are calculated according to formula (2): Where g(E) is the response efficiency of the imaging plate to positrons with energy E, f(t) is the signal attenuation coefficient, and k(θ) is the correction factor for the response efficiency of the imaging plate to positrons at different incident angles. Step 5: Use ImageJ software to perform statistics on the spatial distribution of positrons obtained on the imaging plate. Obtain the intensity curve of the positron number through Gaussian fitting. Calculate the full width at half maximum of the curve, that is, the circular spot diameter of the irradiated area of ​​the imaging plate. Calculate the azimuth angle and substitute the distance between the copper sheet and the gamma source to calculate the spatial divergence angle of the ray source. Step 6: Calculate the ratio ρ of the number of Cu-62 decayed during the measurement to the total generated Cu-62 using the decay formula (3): Here, λ is the decay constant of Cu-62. The total number of Cu-62, i.e. the yield Y of Cu-62 generated by the photonuclear reaction, is obtained by dividing the number of Cu-62 decayed during the measurement by ρ. i , the relationship between the photonuclear reaction yield and the incident gamma photon flux, the number of target atoms and the reaction cross section (4) is: Where N T is the number of target nuclei, N γ (E) is the photon flux of the gamma radiation source when the energy is E, σ i (E) 63 Cu(γ,n) 62 Cu is the cross-section value of the reaction channel when the incident photon energy is E; the cross-section curve of the reaction channel is integrated in the giant dipole resonance energy region to obtain the integrated cross-section value σ; N T The circular spot area obtained by scanning the imaging plate is calculated by multiplying the copper thickness and copper density. T Substituting into formula (4), we can calculate the gamma-ray source flux N in the giant dipole resonance (17±5) MeV energy region. γ .

2. The method for detecting prompt high-brightness gamma rays based on copper activation according to claim 1, characterized in that: In the step 1, the thickness of the copper sheet is less than 2 mm, and the area of ​​the copper sheet is larger than the area of ​​the gamma radiation source.

3. The method for detecting prompt high-brightness gamma rays based on copper activation according to claim 1, characterized in that: The laser imager used was a GE Typhoon 7000 fast laser imager.

4. The method for detecting prompt high-brightness gamma rays based on copper activation according to claim 1, wherein: The imaging plate used is of model Fuji BAS-SR, with a size of 10 cm×15 cm.

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