Gas ionization chamber detection system and method for laser proton accelerator pulsed beam

Abstract

The application discloses a kind of gas ionization chamber detection system and method of laser proton accelerator pulse beam, the physical design and measurement method of gas ionization chamber are optimized and improved, to avoid gas ionization chamber signal saturation, to complete the measurement of laser proton beam signal. The gate integration circuit is used on the gas ionization chamber detection system to realize accurate measurement for a wide range (40-100 MeV) of laser proton pulse beam. In the measurement method, simulation analysis is mainly carried out from three aspects of gas ionization chamber medium optimization, gas ionization chamber pressure value optimization and gas ionization chamber voltage value optimization, and the best measurement scheme is given, which effectively solves the signal saturation problem caused by the ultra-high peak value of laser accelerated protons.

Description

Technical Field

[0001] This invention relates to the field of laser proton accelerator therapy for tumors, and specifically to a gas detection method and device based on monitoring the beam uniformity at the horizontal, vertical and common ends of a laser proton accelerator test. Background Technology

[0002] According to a World Health Organization survey, cancer is the second leading cause of death globally, accounting for approximately one-sixth of all deaths worldwide. Due to its late clinical presentation and difficulty in diagnosis and treatment, nearly 70% of cancer deaths occur in low- and middle-income countries. Currently, the three main treatment methods for cancer are surgery, radiation therapy, and chemotherapy. Radiation therapy, compared to the other two, significantly improves the cure rate and enhances patients' quality of life.

[0003] Compared to photon and electron radiotherapy, proton beams, with their unique Bragg peak, deposit most of the beam energy in a designated area, achieving more precise treatment of diseased tissue while avoiding damage to healthy tissue. Currently, most hospitals use expensive radiofrequency medical accelerators, which are large in size and have high maintenance and operating costs, resulting in high treatment costs. Laser accelerators, due to their acceleration principle, can more easily achieve higher acceleration gradients than traditional accelerators, thus enabling a reduction in equipment size and a significant drop in cost. The emerging science pioneered by ultra-intense, ultra-short laser technology is also driving the research and development of new accelerators.

[0004] The main function of an accelerator beam diagnostic system is to monitor beam information during the process from the accelerator-derived beam to the treatment beam terminal. Accurate measurement of the dose and position of the laser pulse proton beam is crucial to the entire beam diagnostic system. Accelerator beam diagnostics essentially studies the electromagnetic field generated by charged particles, that is, obtaining photoelectric signals carrying beam information through beam detectors. Non-interceptor beam diagnostic elements in the beam delivery system include pixel detectors, multi-wire proportional chambers, and strip-type gas ionization chambers. Pixel detectors are excellent at measuring the spatial distribution and electrical characteristics of traditional accelerator beams, but they are prone to damage when used in laser pulse radiation. Multi-wire proportional chambers operate on a particle-by-particle measurement principle and are not suitable for laser pulse beams. Strip-type gas ionization chambers can be used to measure the beam spot shape and spatial distribution information of a beam. However, current strip-type gas ionization chamber designs are based on slow-extraction beams and are mainly used to measure long-pulse, low-peak-intensity beams generated by cyclotron and synchronous ionization. If the beam generated by laser acceleration is to be measured, it will lead to measurement saturation and the inability of the electronic system to function properly. Therefore, it is necessary to improve the physical design of the gas ionization chamber and the supporting electronic system, and develop gas ionization chamber detection technology specifically for ion beams generated by laser acceleration. Summary of the Invention

[0005] This invention optimizes and improves the physical design and measurement of the gas ionization chamber to avoid signal saturation and thus enable the measurement of the laser proton beam signal. The measurement method is primarily designed through three aspects: optimization of the working gas medium, optimization of the gas pressure setting, and optimization of the gas voltage setting. These improvements effectively solve the signal saturation problem caused by the ultra-high peak current of laser-accelerated protons.

[0006] The basic design principle of this invention is based on:

[0007] The working principle of a gas ionization chamber is as follows: When charged particles pass through the sensitive volume of the gas plate ionization chamber, they interact with the gas, exciting gas molecules and ionizing them to produce a positive ion and an electron, i.e., an ion pair. The ion pair produced by ionization has a certain initial kinetic energy and continuously collides with gas molecules in the ionization chamber. Three physical processes occur in the ion pair: first, the positive ion and electron drift from a region of high density to a region of low density, called diffusion; second, the electron is captured by neutral gas molecules in the ionization chamber during its motion, forming a negative ion, called electron capture; and third, the recombination effect, where the positive ion and electron recombine to reform a neutral molecule. Therefore, a voltage needs to be applied to the two plates of the gas ionization chamber to create an electric field between them, causing the positive ion and electron to overcome the above three motions and drift along the electric field direction to the two plates for collection. The accumulated charge from multiple plates is read out by an electronic system linked in the dose ionization chamber region, and the beam position is determined jointly by the X and Y directions in the position ionization chamber region. The beam distribution information is obtained by collecting the signal from the detector signal anode (collector electrode).

[0008] In terms of circuit composition, a gated integrator can be implemented using one operational amplifier, one integrating capacitor, and two high-speed switches. For proton beam pulse signals with a large range (40-100 MeV), a controllable gated integrator (such as...) is required. Figure 10The diagram (shown) perfectly matches this, enabling accurate measurement of proton beam signals over a wide beam energy range. When the input signal strength is high, the integration time of the gated integrator can be shortened; when the input signal strength is low, the integration time can be increased. In short, in a gated integrator, the conversion amplification gain is determined by the integration time and the integrating capacitor. The output voltage is directly proportional to the integration time and inversely proportional to the integrating capacitor; increasing the integration time or decreasing the integrating capacitor can achieve higher gain. Therefore, the gated integrator is flexible and versatile, not only converting and amplifying weak current to voltage but also effectively reducing noise and interference. This is because it integrates a weak current over a period of time; noise and interference signals fluctuating around the baseline can be partially canceled out after integration, reducing their impact on the useful signal.

[0009] The electronics module works by triggering a signal indicating that the beam is about to act on the ionization chamber. The module then begins operation, waiting for the beam to reach the chamber. Afterward, it integrates the signal generated during the period of beam action, converting all the collected charge into voltage. The electronics module's workflow involves the data acquisition card sending a trigger signal to synchronize the backend and frontend.

[0010] To achieve the above objectives, the present invention adopts the following technical solution: a laser proton accelerator pulse beam cross-section measurement device based on a planar ionization chamber—a gas ionization chamber detection system (see...). Figure 1The gas ionization chamber detector consists of two main parts: a gas ionization chamber detector section and a gas ionization chamber electronics section. The gas ionization chamber detector section is the main body of the system. The inner core unit of the ionization chamber is where the radiation ionization signal is generated after the proton beam is injected. It is installed inside the ionization chamber gas box assembly. A high-vacuum sealing shell and a precession system are used to control the retraction of the gas ionization chamber. The inner core unit of the ionization chamber includes a dose ionization chamber and a position ionization chamber. The electronics section of the gas ionization chamber detection system includes a front-end electronics module (including the position ionization chamber front-end electronics and the dose ionization chamber front-end electronics) and a back-end chip data acquisition system. The front-end electronics module consists of a front-end integrating board hardware circuit. It mainly uses a gated integrating circuit to convert the measured beam current signal into a voltage signal via a gated integrator composed of an integrating capacitor and an operational amplifier, and then converts the voltage into a digital signal through analog-to-digital conversion. This method is a time-segmentation processing method developed from integrator technology, which processes the continuous signal to obtain the accumulated charge information within a certain time period. The signal, after being processed by the front-end electronics module, is then digitally read out using a DAQ system built with commercial CRIO components. Finally, the data is transmitted to the FPGA (Field Programmable Gate Array) via a serial peripheral interface. The FPGA then uploads the data to a host computer for display and storage via a Gigabit Ethernet interface or a USB interface.

[0011] The dose ionization chamber consists of three electrode layers. The outermost two layers are subjected to a negative high voltage, while the middle layer is the signal extraction layer at ground potential. Isolation plates are installed between the layers. Typically, the central signal readout electrode and the two high-voltage electrodes both use 13μm double-sided aluminum-coated Kapton film, with an electrode spacing of 4mm and a sensitive area of ​​60mm × 60mm. When the proton beam passes through the ionization chamber, the signal at the ionization chamber anode is converted into beam current information by a charge-frequency converter (QFC), which is then converted into dose information and provided to the control system for real-time comparison with a given dose.

[0012] The position ionization chamber has the function of real-time monitoring of beam position, beam spot size, and beam profile uniformity. The monitored beam position information, beam spot shape, and beam distribution data are highly accurate and can be transmitted to the control system in real time. Typically, the center electrode of the position ionization chamber uses a 13μm double-sided aluminum-coated Kapton film, and the readout electrodes use a 25μm Kapton film with copper strips on top. The copper layer is 18μm thick, the copper strips are 0.8mm wide, and the strips are arranged periodically with a 1mm spacing. The sensitive area is 60mm × 60mm, and the total number of readout channels is 120. The copper strips on both sides of the readout electrodes are perpendicular to each other to achieve XY position resolution.

[0013] The front-end electronics module adopts a gated integrator circuit design (see...). Figure 10The system consists of 2n gated integrators that convert current signals into voltage signals. These signals are then passed through two multiplexers, each of which converts the parallel signals from the n gated integrator channels into serial output signals. Here, n represents an integer greater than 1; in one embodiment, n = 32. To enhance anti-interference capabilities, a differential output drive circuit is used during signal output. The output differential signal (voltage range ±4V) is sent to the back-end data acquisition card. The DAQ data acquisition card sends an external trigger signal to control the switching of the front-end integrators. Signal processing begins upon the arrival of the external trigger signal and stops when the falling edge of the signal arrives. At this point, a peak hold is applied to the voltage signal of the front-end integrator. Before output, the serial signal is then transmitted via an acquisition clock to synchronize the back-end signal acquisition with the front-end signal acquisition.

[0014] To increase the measurement signal range and avoid signal overflow at extremely high peak currents, a parallel capacitor is added to the low-noise precision gated integrator in the electronic design of the gated integrator circuit. This increases the peak input current range to meet the measurement requirements of laser proton pulse signals. The gated integrator + ADC digital readout method, with its adjustable integrating capacitor, can meet the measurement range requirements under different proton energies and different gas medium ionization chambers. The parallel integrating capacitor design significantly upgrades the electronic performance of the gated integrator circuit, resolving the overflow issue of extremely high peak current laser proton pulse signals. The high sampling rate ADC chip at the back end can meet the sampling accuracy requirements of narrow pulse width proton beams. This solution effectively meets the overall data acquisition requirements of nanosecond-level laser proton pulse beams and the measurement of wide-energy beam signal ranges, solving the problem of gas ionization chamber detection for laser-accelerated proton beams and improving the gas ionization chamber detection technology for laser proton pulse beams.

[0015] To implement the aforementioned physical optimization design of the gas ionization chamber detection system and meet and match the electronic measurement range of the gas ionization chamber, the laser proton accelerator pulse beam measurement method provided by this invention conducts simulation analysis from three aspects: selection of the working gas medium in the gas ionization chamber, setting of the voltage range of the ionization chamber, and setting of the gas pressure range. It selects a working gas that can effectively reduce the measurement signal saturation problem caused by the peak value of transient flow intensity, and sets appropriate gas pressure and voltage values ​​based on the relationship and law between the gas pressure, voltage, and measurement signal of the gas ionization chamber. Specifically, this invention performs simulation analysis based on the Garfield++ platform, including the following steps:

[0016] 1) Model the gas ionization chamber using ANSYS and perform electrostatic field finite element analysis. After the analysis results converge, import the model into the Garfield++ platform for subsequent particle transport simulation analysis.

[0017] 2) Import the element node and material property information ELIST.lis, node position coordinate information NLIST.lis, material resistivity and relative permittivity MPLIST.lis, and node voltage boundary constraint results PRNSOL.lis generated after the finite element electric field analysis of the gas ionization chamber model into Garfield++, and start code setup and simulation.

[0018] 3) Generate working medium files for various classic gas ionization chambers, including nitrogen, air, helium, and p10. Based on the Garfield++ platform, study the signal values ​​detected by the gas ionization chambers under these different gas conditions when the gas pressure is 740 Torr, and select the gas that can effectively reduce the peak value of transient current as the working gas.

[0019] 4) Using the working gas selected in step 3), Garfield++ simulation was performed to study the relationship and law between the gas pressure, voltage and measurement signal of the gas ionization chamber under different pressure ranges and voltage ranges.

[0020] 5) The theoretical calculations and Garfield++ simulation results are verified in both directions, and finally a relationship diagram of voltage, air pressure and ionization chamber measurement signals is generated to provide a reference for actual detection operations.

[0021] In this invention, helium is selected as the working gas. In step 3), the Garfield++ simulation studies cover a pressure range of 480-740 Torr and a voltage range of 100-1000 V. Due to the non-proportional relationship between electron drift velocity and reduced field strength, when the pressure is in the 540-600 Torr range, the gas ionization chamber measurement signal will exhibit an arched spike in signal oscillation. This suggests that the pressure setting should avoid this range.

[0022] This measurement method effectively mitigates the signal saturation phenomenon in the gas ionization chamber during beam diagnosis caused by the transient ultra-high current intensity of laser protons, and effectively avoids signal oscillation ranges, ensuring high accuracy and stability of the measured signal. In contrast, traditional measurement methods often use air or nitrogen as the working gas, lack effective pressure and voltage setting ranges, easily leading to signal saturation, signal oscillation, and instability, resulting in poor measurement accuracy.

[0023] Advantages of this invention:

[0024] The acceleration gradient of laser proton accelerators is more than three orders of magnitude higher than that of traditional radio frequency accelerators. The proton beams generated by laser acceleration possess characteristics not found in traditional accelerators, such as short pulse length (ps-ns) and high peak current (up to kA). Therefore, when using a traditional ionization chamber to measure laser-accelerated proton beams, it is easy to exceed the electronic measurement range of the gas ionization chamber, resulting in signal saturation. This paper optimizes and improves the physical design of the ionization chamber, conducting simulation analysis on three aspects: the selection of the working gas medium, and the setting of the ionization chamber voltage and pressure range. Addressing the problem that traditional ionization chamber electronics cannot measure the ultra-high transient peak current of laser-accelerated proton beams, the paper presents the optimal physical design scheme for the ionization chamber.

[0025] 1. Due to the ultra-high peak flux characteristics of laser proton beams, selecting the appropriate gas for the working gas in the gas ionization chamber to effectively reduce the saturation of the measurement signal caused by the high transient flux peak is of paramount importance. The signal results measured under several different gas filling conditions in the ionization chamber were studied using simulations on the Garfield++ platform (e.g., Figure 3 As shown in the figure, helium was ultimately chosen as the working gas for the gas ionization chamber.

[0026] 2. The measured range varies depending on the integration time setting of the gas ionization chamber. Taking the electronics of this gas ionization chamber as an example, when the integration time is set to 500 ns, the current measurement range is ±16 μA. To match the measurement range of the ionization chamber electronics and avoid excessive transient current signals that could lead to signal saturation, when the gas ionization chamber measurement result is a current value, the optimal selection range for the gas pressure and ionization chamber voltage values ​​should be: gas pressure set below 540 Torr, and voltage set below 300 V.

[0027] 3. When the measurement type is cumulative charge, due to the saturation characteristics of helium, the ionization chamber voltage reaches saturation at 100V. The change in cumulative charge is independent of the voltage magnitude. The range of the measured charge can be adjusted by changing the gas pressure in the ionization chamber.

[0028] 4. Due to the non-proportional relationship between electron drift velocity and reduced field strength, transient current or accumulated charge measurements within the 540-600 Torr pressure range will exhibit a bulging fluctuation region. Setting the pressure value outside this region can effectively reduce measurement signal fluctuation errors. When the measured signal is a transient current signal, it can reduce current peak fluctuations by up to 0.55 μA. When the measured signal is an accumulated charge signal, it can reduce accumulated charge fluctuations by up to 0.7 pC.

[0029] 5. Verify the fluctuation bulge region of the measured signal using theoretical formulas (e.g., Figure 9The area shown in the black box provides both theoretical and practical simulation data as a basis for selecting the subsequent pressure range. Avoiding the pressure range value of the convex area can obtain a stable and high-precision measurement signal.

[0030] 6. A gated integration circuit is used to achieve accurate measurement of laser proton pulse beams over a wide range (40-100MeV).

[0031] In summary, the gas ionization chamber detection system of this invention can directly and effectively obtain information on the measured laser pulse beam. By improving and optimizing the physical design of the gas ionization chamber, the signal saturation problem caused by the ultra-high peak current of laser-accelerated protons can be effectively solved. Currently, the laser-driven proton therapy equipment being constructed by Peking University will use an ionization chamber based on a combination of planar and strip types to measure and locate the uniformity of the laser pulse proton beam cross-section, thus advancing the laser-driven proton therapy device project. Attached Figure Description

[0032] Figure 1 This is a flowchart of the gas ionization chamber detection system according to an embodiment of the present invention.

[0033] Figure 2 This is a schematic diagram of the structural principle of the gas ionization chamber core unit according to an embodiment of the present invention.

[0034] Figure 3 The current signals generated by charging four classical gases at a pressure of 740 Torr, based on the Garfield++ platform, are shown in this embodiment of the invention.

[0035] Figure 4 The curves showing the peak values ​​of voltage, gas pressure, and transient current in a gas ionization chamber, simulated using the Garfield++ platform, are provided in this embodiment of the invention.

[0036] Figure 5 The curves showing the relationship between voltage, gas pressure, and accumulated charge in a gas ionization chamber, simulated using the Garfield++ platform in an embodiment of the present invention.

[0037] Figure 6 This is a three-dimensional graph showing the relationship between voltage, gas pressure, and peak transient current in a gas ionization chamber, simulated using the Garfield++ platform according to an embodiment of the present invention.

[0038] Figure 7 This is a three-dimensional graph showing the relationship between voltage, gas pressure, and accumulated charge in a gas ionization chamber, simulated using the Garfield++ platform according to an embodiment of the present invention.

[0039] Figure 8 This is a graph showing the helium electron drift velocity versus reduced field strength simulated using the Garfield++ platform in an embodiment of the present invention.

[0040] Figure 9 The curve showing the relationship between the ionization chamber pressure and the accumulated charge at a voltage of 500V is simulated using the Garfield++ platform in an embodiment of the present invention.

[0041] Figure 10 This is the design of the gated integrator circuit for the electronic part of the gas ionization chamber in an embodiment of the present invention. Detailed Implementation

[0042] The present invention will be further illustrated below with reference to the accompanying drawings and specific embodiments. The examples given are for illustrative purposes only and are not intended to limit the scope of the invention.

[0043] Example:

[0044] The laser pulse proton beam energies to be measured range from 40 MeV to 230 MeV, with a beam intensity of 10. 7 -10 9 pps, beam spot area is 3cm 2 about.

[0045] See Figure 1 In this embodiment, the main component of the gas ionization chamber detection system is a gas ionization chamber detector. After the proton beam is injected into the core unit of the ionization chamber, it generates a radiation ionization signal. Figure 2 The diagram shows the core unit within the ionization chamber. From the beam injection direction, the arrangement of the unit plates is as follows: first high-voltage plate 1, first position signal plate (X direction) 2, second high-voltage plate 3, second position signal plate (Y direction) 4, third high-voltage plate 5, dose signal plate 6, and fourth high-voltage plate 7. The sensitive areas of each unit plate within the ionization chamber core unit use an organic aluminum film material for easy conductivity. Isolation plates are added between the unit plates to ensure equal distances within the working area, thus forming a uniform electric field. The first position signal plate 2 and the second position signal plate 4 measure the beam uniformity in the horizontal and vertical directions, respectively.

[0046] The unit board requiring high-voltage connection includes first to fourth high-voltage electrodes, with the high-voltage electrodes connected in series. The high-voltage electrodes of the unit board are located on the front side of the unit board. The high-voltage connection method is as follows: the first high-voltage electrode 1 is connected to the electrode of the second high-voltage electrode 3; the electrode of the second high-voltage electrode 3 is connected to the electrode of the third high-voltage electrode 5; the electrode of the third high-voltage electrode 5 is connected to the electrode of the fourth high-voltage electrode 7; and the high-voltage electrode of the dose signal board 6 is connected to an external high-voltage device.

[0047] The core unit inside the ionization chamber is mounted on a fixed bracket, which is connected to a fixed base. The beam signal received by the core unit inside the ionization chamber is transmitted to the fixed base through a signal transmission electrode. The signal port on the fixed base is connected to the electronics system for signal transmission and processing. An external gas source expels air from the ionization chamber through a gas pipe and creates helium as the working gas, enabling stable beam detection operation.

[0048] To implement the physical optimization design of the gas ionization chamber and meet and match the electronic measurement range of the gas ionization chamber, effective solutions and theoretical basis were derived through theoretical calculations and research simulations based on the Garfield++ platform.

[0049] The steps for the simulation study are as follows:

[0050] Step 1: Model the gas ionization chamber using ANSYS and perform electrostatic field finite element analysis. After the analysis results converge, import the model into the Garfield++ platform for subsequent particle transport simulation analysis.

[0051] Step 2: Import the element node and material property information ELIST.lis, node position coordinate information NLIST.lis, material resistivity and relative permittivity MPLIST.lis, and node voltage boundary constraint results PRNSOL.lis generated after the finite element electric field analysis of the gas ionization chamber model into Garfield++, and start code setup and simulation.

[0052] Step 3: Generate working medium files for several classic gas ionization chambers, including nitrogen, air, helium, and p10. Based on the Garfield++ platform, study the signal values ​​detected by the gas ionization chambers under these different gas conditions at a pressure of 740 Torr. Figure 3 As shown. Choosing helium as the working gas for the gas ionization chamber is the first and crucial step in the optimization design, as it can effectively reduce the peak value of transient current (up to 1700 pA). Subsequent Garfield++ simulations were conducted using helium as the gas medium.

[0053] Step 4: Investigate the relationship and patterns between gas pressure, voltage, and measurement signals in the gas ionization chamber under different pressure ranges (480-740 Torr) and voltage ranges (100-1000 V). It is found that due to the non-proportional relationship between electron drift velocity and reduced field strength, the gas ionization chamber measurement signal exhibits an arched oscillation in the pressure range of 540-600 Torr. This is verified through theoretical formula deduction, and the final curves showing the relationship between voltage, pressure, and ionization chamber measurement signals (e.g., [image of curves]). Figure 4 and Figure 5As shown), 3D diagram (such as) Figure 6 and Figure 7 As shown, this provides an important reference for actual detection operations.

[0054] Step 5: Two-way verification between theoretical calculations and Garfield++ simulation results.

[0055] Unlike ions, the drift velocity of electrons is not directly proportional to the strength of the reduced field (e.g., ...). Figure 8 As shown in the simulation results, it can be expressed by the formula:

[0056]

[0057] Where v is the velocity of the free charge moving in a directional manner, E / p is the reduced field strength, and f(E / p) represents the functional relationship between the electron drift velocity v and the reduced field strength E / p.

[0058] The final measurement signal will exhibit a significant oscillation within the pressure range of 540-600 Torr. For example, consider the relationship between gas pressure and accumulated charge in the gas ionization chamber at 500V (e.g.,...). Figure 9 As shown), through theoretical formula derivation and verification, the formula I = nev is transformed into... Where n represents the number of free charges per unit length; e is the amount of free charge; v represents the rate of directional movement of free charges; I represents the magnitude of the current per unit length; Q represents the cumulative charge per unit integration time; and E / p is the reduced field strength.

[0059] Under the same conditions, within the same integration time, the accumulated charge Q remains essentially constant. When the electron drift velocity reaches the concave region, the magnitude of f(E / p) also exhibits a concave range. At this point, since Q remains constant, the electron charge ne will... Figure 9 The black box indicates the corresponding arched protrusion area (between 540-600 Torr). This verifies the reliability of the simulation and the scientific validity of the optimal voltage and pressure setting range.

[0060] When using this invention to measure the pulsed beam of a laser proton accelerator, the following steps are followed to achieve online measurement of the profile uniformity, and the results are analyzed based on the simulation diagram ( Figure 6 , Figure 7 Using Table 1 as a reference standard, the optimized air pressure and voltage selection ranges are set:

[0061] (1) Fix the gas ionization chamber detector at the beam pipe opening, with the entrance window perpendicular to the beam direction. Adjust it to a suitable position by lifting and lowering the detector so that the center of the gas ionization chamber detector is on the horizontal line of the beam center.

[0062] (2) Connect the signal interface at the gas ionization chamber end to the electronic data acquisition system, preheat the front-end electronic system for a certain period of time, and record the voltage bias introduced by the front-end electronic system and the data acquisition system.

[0063] (3) Open the gas cylinder valve to allow the working gas helium to circulate in the gas ionization chamber. Using a flowing gas can reduce gas aging.

[0064] (4) Set optimized air pressure and voltage values. If the measured signal is a transient current, it can be based on... Figure 6 For reference selection. When the voltage value is fixed, to avoid large signal fluctuations and oscillations, try to avoid setting the air pressure between 540-600 Torr. For other air pressure ranges, the general rule is that the higher the air pressure, the larger the transient current, and vice versa. When the air pressure value is fixed, based on the measurement range requirements of the gas ionization's own electrons, if a smaller transient current needs to be measured, a lower voltage value can be selected. Figure 6 It can be seen that as the voltage value decreases, the overall surface tilt decreases, and the lowest transient current can reach 13μA. Conversely, the voltage value is increased until it can match the measurement range of its own gas ionization chamber and the signal will not overflow.

[0065] (5) Set optimized air pressure and voltage values. If the measured signal is the accumulated charge, it can be based on... Figure 7 For reference selection. It can be seen that when the gas pressure is constant, voltage changes have almost no effect on the measured cumulative charge signal. This is due to the saturation characteristics of helium. Therefore, optimizing the gas pressure setting is crucial. Similarly, once the voltage value is set, to avoid significant signal fluctuations, avoid setting the gas pressure between 540-600 Torr. For other pressure ranges, the principle is that the higher the pressure, the larger the measured cumulative charge, and vice versa. The cumulative charge range covered by this simulation is 3-12 pC. A suitable gas pressure value can be selected based on the electronic measurement range of your gas ionization chamber to ensure the signal does not overflow.

[0066] (6) Apply a negative high voltage to the cathode and check whether the system is operating normally;

[0067] (7) The integration capacitor value of the gated integrator circuit is set according to the actual proton beam energy, working gas medium, etc. In this example, it is set to 10pF after calculation.

[0068] (8) The beam passes through the sensitive volume of the detection area, and the collected electrode conducts the signal to the data acquisition system. After deducting the voltage bias caused by the system error, the beam information is obtained.

[0069] Table 1. Simulation data of gas ionization chamber based on Garfield++ platform under different voltages and pressures

[0070]

[0071] Finally, it should be noted that the purpose of disclosing the embodiments is to help further understand the present invention. However, those skilled in the art will understand that various substitutions and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the present invention should not be limited to the content disclosed in the embodiments, and the scope of protection of the present invention is defined by the claims.

Claims

1. A gas ionization chamber detection system for a laser proton accelerator pulsed beam, comprising two parts: a gas ionization chamber detector and a gas ionization chamber electronics system, wherein, The gas ionization chamber detector employs a planar ionization chamber, whose inner core unit includes a position ionization chamber and a dose ionization chamber. The gas ionization chamber electronics system includes a front-end electronics module and a back-end chip data acquisition system. The front-end electronics module uses a gated integrator circuit, where 2n gated integrators convert current signals into voltage signals. These signals are then passed through two multiplexers, each multiplexer converting the parallel signals from the n gated integrator channels into serial signals for output, where n is an integer greater than 1. A two-way differential output drive circuit is used for outputting the differential signals to the back-end data acquisition card. The data acquisition card sends an external trigger. The signal controls the switching of the front-end integrator. Signal processing begins upon the arrival of an external trigger signal and stops when the falling edge of the signal arrives. At this time, a voltage signal of a certain amplitude is peak-held at the front-end integrator. Before output, the serial signal is transmitted to the acquisition clock to synchronize the back-end signal acquisition with the front-end signal acquisition. The arrangement order of the unit plates of the core unit in the ionization chamber from the beam injection direction is as follows: first high-voltage plate, first position signal plate, second high-voltage plate, second position signal plate, third high-voltage plate, dose signal plate, and fourth high-voltage plate. The first and second position signal plates measure the beam uniformity in the horizontal and vertical directions, respectively.

2. The gas ionization chamber detection system as described in claim 1, characterized in that, The gated integrator includes an operational amplifier and multiple integrating capacitors connected in parallel with it.

3. The gas ionization chamber detection system as described in claim 1, characterized in that, The back-end chip data acquisition system is a DAQ system built on commercial CRIO components. It digitizes the signals processed by the front-end electronics module and transmits the data to the FPGA via a serial peripheral interface. The FPGA then uploads the data to the host computer for display and storage via a gigabit Ethernet interface or a USB interface.

4. The gas ionization chamber detection system as described in claim 1, characterized in that, The sensitive areas of each unit plate in the ionization chamber core unit are made of organic aluminum film, and an isolation plate is added between each unit plate.

5. A method for measuring the pulsed beam of a laser proton accelerator, characterized in that, Using the gas ionization chamber detection system as described in claim 1, simulation analysis is conducted from three aspects: selection of working gas medium for the gas ionization chamber, setting of voltage range for the ionization chamber, and setting of pressure range. A working gas that can effectively reduce the saturation problem of the measurement signal caused by the peak value of transient flow intensity is selected, and appropriate pressure and voltage values ​​are set according to the relationship and law between gas pressure, voltage and measurement signal in the gas ionization chamber.

6. The measurement method as described in claim 5, characterized in that, The steps of simulation analysis are as follows: 1) Model the gas ionization chamber using ANSYS and perform electrostatic field finite element analysis. After the analysis results converge, import the model into the Garfield++ platform for subsequent particle transport simulation analysis. 2) Import the element node and material property information ELIST.lis, node position coordinate information NLIST.lis, material resistivity and relative permittivity MPLIST.lis, and node voltage boundary constraint results PRNSOL.lis generated after the finite element electric field analysis of the gas ionization chamber model into Garfield++, and start code setup and simulation. 3) Generate working medium files for various classic gas ionization chambers, including nitrogen, air, helium, and p10. Based on the Garfield++ platform, study the signal values ​​detected by the gas ionization chambers under these different gas conditions when the gas pressure is 740 Torr, and select the gas that can effectively reduce the peak value of transient current as the working gas. 4) Using the working gas selected in step 3), Garfield++ simulation was performed to study the relationship and law between the gas pressure, voltage and measurement signal of the gas ionization chamber under different pressure ranges and voltage ranges. 5) The theoretical calculations and Garfield++ simulation results are verified in both directions, and finally a relationship diagram of voltage, air pressure and ionization chamber measurement signals is generated, providing a reference for actual detection operations.

7. The measurement method as described in claim 6, characterized in that, Step 2) Select helium as the working gas.

8. The measurement method as described in claim 6, characterized in that, Step 3) Using helium as the working gas, the Garfield++ simulation studies the pressure range of 480-740 Torr and the voltage range of 100-1000 V.

9. The measurement method as described in claim 6, characterized in that, Due to the non-proportional relationship between electron drift velocity and reduced field strength, step 3) simulates the signal oscillation of the gas ionization chamber measurement signal when the gas pressure value is in the range of 540-600 Torr.

10. The measurement method as described in claim 5, characterized in that, Select helium as the working gas; when the measurement signal is a current value, set the gas pressure value below 540 Torr and the ionization chamber voltage value below 300 V; when the measurement signal is the accumulated charge, adjust the range of the measured charge by changing the gas pressure value of the ionization chamber; at the same time, the gas pressure value setting should avoid the range where the measurement signal of transient current or accumulated charge shows a bulging fluctuation.

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