CN116413762A - Gas ionization chamber detection system and method for pulse beam current of laser proton accelerator - Google Patents

Abstract

The invention discloses a gas ionization chamber detection system and method of laser proton accelerator pulse beam, which are optimized and improved aiming at the physical design and measurement method of a gas ionization chamber so as to avoid signal saturation of the gas ionization chamber, thereby completing measurement of laser proton beam signals. The gating integration circuit is used on the gas ionization chamber detection system to realize accurate measurement for the wide range (40-100 MeV) of laser proton pulse beam current. In the measurement method, simulation analysis is mainly carried out through three aspects of working gas medium optimization of a gas ionization chamber, gas pressure value setting optimization of the gas ionization chamber and voltage value setting optimization of the gas ionization chamber, an optimal measurement scheme is provided, and the signal saturation problem caused by ultrahigh peak flow intensity of laser accelerated protons is effectively solved.

Description

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

technical field

The invention relates to the technical field of laser proton accelerator treatment of tumors, in particular to a gas detection method and device based on beam uniformity monitoring at the horizontal end, vertical end and common end of a laser proton accelerator test.

Background technique

According to the survey of the World Health Organization, cancer is the second leading cause of death in the world. About one sixth of the deaths worldwide are caused by cancer. The clinical manifestations appear late and it is difficult to obtain diagnosis and treatment. Nearly 70% of cancer deaths occur in low income and middle-income countries. Currently, there are three common treatment modalities for cancer treatment: surgery, radiation therapy, and chemotherapy. Compared with the other two methods, radiotherapy can greatly increase the cure rate and improve the quality of life of patients.

Compared with photon, electron and other radiation therapy methods, the unique Bragg peak of proton rays deposits most of the beam energy in the designated area, achieving more precise treatment of diseased tissues while avoiding damage to healthy tissues. At present, the radio frequency medical accelerators used in most hospitals are expensive, occupy a large area, and have high maintenance and operation costs, resulting in high treatment costs. Due to its acceleration principle, laser accelerators can more easily obtain higher acceleration gradients than traditional accelerators, thereby reducing the size of the equipment and reducing the cost. The emerging science pioneered by ultra-intense and ultra-short laser technology also promotes the research and development of new accelerators.

The main function of the accelerator beam diagnosis system is to monitor the beam information during the process from the accelerator beam to the treatment beam terminal. How to accurately measure the dose and position of the laser pulse proton beam is the key to the entire beam diagnosis system. The essence of accelerator beam diagnosis is to study the electromagnetic field generated by charged particles, that is, to obtain the photoelectric signal carrying beam information through the beam detector. The non-intercepting beam diagnostic components in the beam distribution system include particle detectors such as pixel detectors, multi-filament proportional chambers, and strip-type gas ionization chambers. Pixel detectors are used to measure the spatial distribution information and electrical characteristics of traditional accelerator beams. The parameter effect is excellent, but the application is easy to damage the device in the laser pulse radiation. The working principle of the multi-filament proportional chamber is particle-by-particle measurement, which is not suitable for laser pulse beams. The strip-type gas ionization chamber can be used to measure the beam spot shape and spatial distribution information of the beam, but the current design of the strip-type gas ionization chamber is based on the slow extraction of the beam, which is mainly used to measure the gyration and synchronously generated long pulse low If you want to measure the beam current generated by laser acceleration, the beam current with peak current intensity will lead to measurement saturation and the problem that the electronic system cannot work normally. Therefore, it is necessary to improve the physical design of the gas ionization chamber and the supporting electronic system, and develop a dedicated Gas ionization chamber detection technology for ion beams generated by laser acceleration.

Contents of the invention

The invention optimizes and improves the physical design and measurement of the gas ionization chamber to avoid the signal saturation of the gas ionization chamber, thereby completing the measurement of the laser proton beam current signal. The measurement method is mainly designed through the optimization of the working gas medium of the gas ionization chamber, the optimization of the pressure value setting of the gas ionization chamber, and the optimization of the voltage value setting of the gas ionization chamber. caused by signal saturation.

Basic design principle of the present invention is based on:

The working principle of the gas ionization chamber is that when the charged particles pass through the sensitive volume in the gas flat ionization chamber, the charged particles will interact with the gas, and the gas molecules will be excited and ionized in the path of the charged particles to generate a positive ion and an electron. That is ion pairs. The ion pairs produced by ionization have a certain initial kinetic energy and continuously collide with the gas molecules in the ionization chamber. Three physical processes will occur in the ion pairs. One is that the positive ions and electrons will drift from the area with high density to the area with low density, which is called It is a diffusion movement; electrons will be captured by neutral gas molecules in the ionization chamber during the movement to form negative ions, which is called electron capture; there is also the recombination effect, that is, the positive ions and electrons recombine to form neutral during the movement. molecular. Therefore, a voltage needs to be applied to the two plates of the gas ionization chamber, and an electric field is formed between the two plates so that the positive ions and electrons overcome the above three movements and drift along the direction of the electric field until the two plates are collected. Multiple plates are accumulated, and the electronic system linked in the dose ionization chamber area reads out the charge information, and in the position ionization chamber area, the position of the beam is jointly determined according to the X and Y directions. Beam distribution information is obtained from the signal collected from the signal anode (collector) of the detector.

In terms of circuit composition, an operational amplifier, an integrating capacitor, and two high-speed switches can be used to implement a gated integrator. For the proton beam pulse signal with a large span (40-100MeV), the controllable gated integrator (as shown in Figure 10) can just match it very well to achieve a proton beam with a large beam energy range Signals are accurately measured. When the input signal strength is high, the integration time of the gated integrator can be shortened, and when the input signal strength is small, the integration time of the gated integrator can be increased. In short, in a gated integrator, the conversion amplification gain of the circuit is determined by the integration time length and the integration capacitance. The output voltage is proportional to the integration time length and inversely proportional to the integration capacitance. Small integral capacitor for higher gain. It can be seen that the gated integrator is flexible and changeable. It can not only realize the conversion and amplification of weak current to voltage, but also effectively reduce noise and interference. This is because it integrates the weak current over a period of time, based on the baseline The noise and interference signals fluctuating up and down can cancel part of each other after integration, reducing the impact on useful signals.

The working principle of the electronics module is to set the trigger signal that the beam is about to act on the ionization chamber, and the electronics module starts working immediately. After waiting for the beam to act on the ionization chamber, the signal during the period when the beam is acting Integrating, converts all charge generated by the collection into a voltage. The workflow of the electronics module is that the data acquisition card sends a trigger signal to synchronize the back-end and front-end.

In order to achieve the above object, the present invention takes the following technical solutions: a laser proton accelerator pulse beam cross-section measurement device based on a flat plate ionization chamber—a gas ionization chamber detection system (referring to Fig. 1), consisting of a gas ionization chamber detector part and The electronic part of the gas ionization chamber consists of two parts. Among them, the detector part of the gas ionization chamber is the main body of the system. The core unit in the ionization chamber is the place where the radiation ionization signal is generated after the proton beam is injected. It is installed in the gas box assembly of the ionization chamber. The high vacuum sealed shell and the precession system are used for Controls the indentation of the gas ionization chamber. The ionization inner core unit includes a dose ionization chamber and a position ionization chamber. The electronics part of the gas ionization chamber detection system includes the front-end electronics module (including the front-end electronics of the position ionization chamber and the front-end electronics of the dose ionization chamber) and the back-end chip data acquisition system. The front-end electronics module is composed of the hardware circuit of the front-end integration board, which mainly uses the gated integration circuit to convert the measured beam current signal into a voltage signal through the gated integrator composed of the integration capacitor and the operational amplifier, and then converts the measured beam current signal into a voltage signal through the analog-to-digital conversion. The voltage is converted into a digital signal. This method is a time-segment processing method developed from the integrator technology. The continuous signal is cut off and processed, and the accumulated charge information in a certain time period is obtained. The signal processed by the front-end electronic module is combined with the DAQ system built by commercial CRIO components to realize digital readout. Finally, the data is transmitted to the FPGA (Field Programmable Gate Array) through the serial peripheral interface, and the FPGA uploads the data to the host computer through the Gigabit Ethernet interface or the USB interface for display and storage.

The dose ionization chamber is composed of 3 layers of polar plates, of which the outermost two layers of membranes are charged with negative high voltage, the middle layer is the signal lead-out layer at ground potential, and isolation plates are installed between the layers. Usually, the central signal readout electrode and the high-voltage electrodes on both sides are made of 13μm double-sided aluminized Kapton film, the electrode spacing is 4mm, and the sensitive area is 60mm×60mm. When the proton beam passes through the ionization chamber, the signal of the anode of the ionization chamber passes through the charge frequency converter (QFC) to obtain the beam current information, which is converted into dose information, and the dose information is provided to the control system for real-time comparison with the given dose.

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 have high precision, and the position information can be sent to the control system in real time. Generally, the central electrode of the position ionization chamber adopts 13 μm double-sided aluminum-plated Kapton film, and the readout electrode adopts the process of coating copper bars on 25 μm Kapton film. The thickness of copper skin is 18 μm, the width of copper bars is 0.8 mm, and the distance between copper bars is 1 mm periodic arrangement. The sensitive area is 60mm×60mm, and the total number of readout channels is 120. The copper strips of the readout electrodes on both sides are perpendicular to each other to achieve X-Y position resolution.

The front-end electronics module adopts the design of the gated integration circuit (see Figure 10), in which 2n gated integrators convert the current signal into a voltage signal, and then pass through two multiplexers, each multiplexer will The parallel signals in the n gated integrator channels are converted into serial signal outputs, where n represents an integer greater than 1, and n=32 in an embodiment of the present invention. When outputting the signal, a differential output drive circuit is used to enhance the anti-interference ability, and the output differential signal (voltage range ±4V) is sent to the data acquisition card at the back end. The DAQ data acquisition card sends an external trigger signal to control the switch of the front-end integrator. After the external trigger signal arrives, the signal processing starts, and when the falling edge of the signal arrives, the processing stops. Keep. Before the output, the serial signal sends out the acquisition signal through the acquisition clock, so that the back-end and the front-end signal acquisition are synchronized.

In order to increase the measurement signal range and avoid the signal overflow of ultra-high peak current intensity, in the electronic design of the gated integration circuit, a parallel capacitor is added to the low-noise precision gated integrator, thereby increasing the input current peak range. In order to meet the measurement requirements of the laser proton pulse signal. The gated integrator + ADC digital readout method, its adjustable integral capacitance can meet the measurement range requirements of different energy protons and different gas medium ionization chambers, and the parallel integral capacitance design can greatly upgrade the electronic performance of the gated integral circuit. Solve the overflow situation of ultra-high peak flux laser proton pulse signal. The back-end high sampling rate ADC chip can meet the sampling accuracy of narrow pulse width pulsed proton beam. This solution can effectively meet the data acquisition of nanosecond-scale laser proton pulse beam and the measurement of wide energy beam signal range, solve the problem of gas ionization chamber detection of proton beam generated by laser acceleration, and improve the quality of laser proton pulse beam. Flow gas ionization chamber detection technology.

In order to implement the above-mentioned physical optimization design of the gas ionization chamber detection system and meet and match the electronic measurement range of the gas ionization chamber, the measurement method of the pulsed beam current of the laser proton accelerator provided by the present invention is based on the selection of the working gas medium of the gas ionization chamber and the voltage value of the ionization chamber. The simulation analysis is carried out in three aspects: range setting and air pressure value range setting, and the working gas that can effectively reduce the saturation problem of the measurement signal caused by the high peak value of the transient flow intensity is selected, and according to the gas ionization chamber pressure, voltage and the measurement signal Set the appropriate air pressure value and voltage value according to the relationship and law. Specifically, the present invention carries out simulation analysis based on the Garfield++ platform, comprising the following steps:

1) Ansys modeling of the gas ionization chamber and finite element analysis of the electrostatic field were performed. After the analysis results converged, the model was imported to the Garfield++ platform for subsequent particle transport simulation analysis;

2) The unit node and material attribute information ELIST.lis, node position coordinate information NLIST.lis, material resistivity and relative permittivity MPLIST.lis, and node voltage boundary generated by the finite element electric field analysis of the ansys model of the gas ionization chamber Import the constraint result PRNSOL.lis into Garfield++, and start the code setting and simulation;

3) Generate the working medium files of various classic gas ionization chambers including nitrogen, air, helium, p10, etc., and study the signals detected by the gas ionization chambers under these different gas conditions when the pressure value is 740 Torr based on the Garfield++ platform value, select the gas that can effectively reduce the peak value of the transient current as the working gas;

4) Carry out Garfield++ simulation with the working gas selected in step 3), and study the relationship and law between the gas ionization chamber pressure, voltage and measurement signal under different pressure value ranges and different voltage value ranges;

5) Two-way verification of theoretical calculations and Garfield++ simulation results, and finally form a relationship diagram of voltage, air pressure and ionization chamber measurement signals, providing a reference for actual detection operations.

In the specific implementation, the present invention selects helium as the working gas, and the pressure value range of step 3) Garfield++ simulation research is 480-740 Torr, and the voltage value range is 100-1000V. Due to the non-proportional relationship between the electron drift velocity and the reduced field strength, when the pressure value is in the range of 540-600 Torr, the gas ionization chamber measurement signal will appear arched and protruding signal oscillations, which suggests that the setting of the pressure value should avoid this range.

This measurement method can effectively improve the signal saturation phenomenon of the gas ionization chamber during beam diagnosis caused by the transient ultra-high current intensity of laser protons, and can effectively avoid the signal oscillation interval and ensure the high precision and stability of the measured signal. Compared with the traditional measurement method, the working medium of the measurement gas in the traditional measurement method is mostly air or nitrogen, etc., and there is no effective guidance range for the setting range of air pressure and voltage, and it is easy to reach signal saturation and generate signal oscillation and instability , the measurement accuracy is poor.

Advantages of the present invention:

The acceleration gradient of the laser proton accelerator is more than three orders of magnitude higher than that of the traditional radio frequency accelerator. The proton beam generated by laser acceleration has the characteristics of short pulse (ps-ns) and high peak current intensity (up to kA) that traditional accelerators do not have. . Therefore, when a traditional ionization chamber is used to measure the laser-accelerated proton beam, it is easy to exceed the electronic measurement range of the gas ionization chamber, resulting in signal saturation. Now optimize and improve the physical design of the ionization chamber, and conduct simulation analysis from three aspects: the selection of the working gas medium of the gas ionization chamber, the setting of the ionization chamber voltage, and the range of pressure values. The problem of the ultra-high transient peak flow intensity of the flow is given, and the best physical design scheme of the ionization chamber is given.

1. Due to the ultra-high peak current intensity characteristics of the laser proton beam, it is the primary key to choose which gas to use as the working gas in the gas ionization chamber to effectively reduce the measurement signal saturation problem caused by the high peak value of the transient current intensity. Based on the Garfield++ platform, the signal results measured under the ionization chamber filled with several different gas conditions were studied (as shown in Figure 3), and helium was finally selected as the working gas of the gas ionization chamber.

2. Due to the different integration time settings of the gas ionization chamber, the measured ranges are also different. Taking the electronics of this gas ionization chamber as an example, when the integration time is set to 500ns, the current measurement range is ±16μA. In order to match the measurement range of the ionization chamber electronics and avoid excessive transient current signals, which may cause signal saturation, when the gas When the measurement result of the ionization chamber is the current value, the optimal gas pressure value and the selection range of the ionization chamber voltage value should be: the pressure value is set to be lower than 540Torr, and the voltage value is set to be lower than 300V.

3. When the measurement type is accumulative charge, due to the saturation characteristics of helium, the ionization chamber voltage reaches 100V to reach a saturated state, and the change of the accumulative charge has nothing to do with the voltage, and can be adjusted by changing the gas pressure value of the ionization chamber. Measure the size range of the charge.

4. Due to the non-proportional relationship between the electron drift velocity and the reduced field strength, there will be a raised fluctuation area when the pressure value is in the range of 540-600Torr. The fluctuation error of the measurement signal, when the measurement signal is a transient current signal, can reduce the current peak fluctuation up to 0.55μA. When the measurement signal is a cumulative charge signal, the fluctuation of the cumulative charge amount can be reduced or up to 0.7pC.

5. Verify the fluctuating convex area of the measurement signal through the theoretical formula (as shown in the black box area in Figure 9), and provide a dual basis for the selection of the subsequent air pressure range, both theoretical and actual simulation data. Avoiding the pressure range value in the convex area can obtain a stable and high-precision measurement signal.

6. Use the gated integration circuit to realize accurate measurement for the wide range (40-100MeV) of laser proton pulse beam current.

In summary, the gas ionization chamber detection system of the present invention can directly and effectively obtain the measured laser pulse beam current information, and by improving and optimizing the physical design of the gas ionization chamber, the signal saturation caused by the ultra-high peak current intensity of laser-accelerated protons can be effectively solved question. At present, the laser-driven proton therapy equipment under construction by Peking University will use a combination of flat-plate and strip-type ionization chambers to measure the uniformity and distribution of the laser pulse proton beam section, and promote the laser-driven proton therapy device project.

Description of drawings

Fig. 1 is a measurement flowchart of a gas ionization chamber detection system according to an embodiment of the present invention.

Fig. 2 is a structural principle diagram of a gas ionization inner core unit according to an embodiment of the present invention.

Fig. 3 is the current signal generated by charging four kinds of classical gases under the pressure of 740 Torr based on the simulation of the Garfield++ platform according to the embodiment of the present invention.

Fig. 4 is the relationship curve of the gas ionization chamber voltage, gas pressure and transient current peak value simulated based on the Garfield++ platform according to the embodiment of the present invention.

Fig. 5 is the relationship curve of the gas ionization chamber voltage, air pressure and accumulated charge based on the simulation of the Garfield++ platform according to the embodiment of the present invention.

Fig. 6 is a three-dimensional diagram of the relationship between the voltage, pressure and transient current peak value of the gas ionization chamber simulated based on the Garfield++ platform according to the embodiment of the present invention.

Fig. 7 is a three-dimensional diagram of the relationship between the voltage, air pressure and accumulated charge of the gas ionization chamber simulated based on the Garfield++ platform according to the embodiment of the present invention.

Fig. 8 is a curve diagram of electron drift velocity and reduced field strength in helium gas simulated based on the Garfield++ platform according to an embodiment of the present invention.

FIG. 9 is a graph showing the relationship between ionization chamber pressure and accumulated charge when the voltage is 500V based on Garfield++ platform simulation according to an embodiment of the present invention.

Fig. 10 shows the design of the gate integration circuit of the electronic part of the gas ionization chamber according to the embodiment of the present invention.

Detailed ways

The present invention will be further elaborated below through specific embodiments in conjunction with the accompanying drawings. The examples given are only for explaining the present invention, not for limiting the scope of the present invention.

Example:

The laser pulse proton beam energy to be measured is from 40MeV to 230MeV, the beam intensity is 10 ⁷ -10 ⁹ pps, and the beam spot area is about 3cm ² .

Referring to FIG. 1 , the main component of the gas ionization chamber detection system used in this embodiment is a gas ionization chamber detector, and a radiation ionization signal is generated after the proton beam is injected into the ionization chamber core unit. Figure 2 shows the core unit in the ionization chamber. The arrangement order of each unit plate from the beam incident direction is the first high voltage plate 1, the first position signal plate (X direction) 2, the second high voltage plate 3, the second Position signal plate (Y direction) 4 , third high voltage pole plate 5 , dose signal plate 6 , fourth high voltage pole plate 7 . The material used in the sensitive area of each unit plate of the ionization inner core unit is an organic aluminum film, which is convenient for conduction. Isolation plates are added between the unit plates of the ionization indoor core unit to ensure that the distance in the working area is the same and a uniform electric field can be formed. The first position signal board 2 and the second position signal board 4 measure the uniformity of the beam current in the horizontal and vertical directions respectively.

The unit plates that need to be connected to high voltage include first to fourth high voltage pole plates, and the high voltage electrode terminals are connected in series. The high-voltage electrode end of the unit plate is on the front of the unit plate, and the high-voltage connection method is that the first high-voltage pole plate 1 is connected to the electrode end of the second high-voltage pole plate 3; the electrode end of the second high-voltage pole plate 3 is connected to the third high-voltage pole plate 5 The electrode ends are connected; the electrode end of the third high-voltage plate 5 is connected with the electrode end of the fourth high-voltage plate 7; the high-voltage electrode end of the dose signal plate 6 is connected to an external high-voltage device.

The ionization indoor core unit is installed on the fixed support, and the fixed support is connected with the fixed base at the same time. The beam signal received by the ionization indoor core unit is transmitted to the fixed base through the signal transmission plate, and the signal socket of the fixed base is connected with the electronic system to transmit and process the signal. The external gas source exhausts the air in the ionization chamber through the gas pipe and creates helium as the working gas to realize the stable operation of the beam detection.

In order to implement the physical optimization design of the gas ionization chamber and meet and match the electronic measurement range of the gas ionization chamber, through theoretical calculations and research simulations based on the Garfield++ platform, effective solutions and theoretical basis are obtained.

The steps for the simulation study part are as follows:

Step 1: Ansys modeling of the gas ionization chamber and finite element analysis of the electrostatic field are performed. After the analysis results converge, the model is imported to the Garfield++ platform for subsequent particle transport simulation analysis.

The second step: the unit node and material property information ELIST.lis, node position coordinate information NLIST.lis, material resistivity and relative permittivity MPLIST.lis, node generated after the finite element electric field analysis of the ansys model of the gas ionization chamber Import the voltage boundary constraint result PRNSOL.lis into Garfield++, and start code setting and simulation.

Step 3: Generate several classic gas ionization chamber working media files, such as nitrogen, air, helium, and p10, and study the signals detected by the gas ionization chambers under these different gas conditions when the pressure value is 740 Torr based on the Garfield++ platform value, as shown in Figure 3. Selecting helium as the working gas of the gas ionization chamber is the first and key step in the optimization design work, which can effectively reduce the peak value of the transient current (the maximum transient current value can be reduced up to 1700pA). Subsequent Garfield++ simulations were performed under the premise of helium as the gas medium.

The fourth step: study the relationship and law between the gas ionization chamber pressure, voltage and measurement signal under the conditions of different pressure value ranges (480-740Torr) and different voltage value ranges (100-1000V). Reducing the non-proportional relationship of field strength, there will be arched and protruding signal oscillations in the measurement signal of the gas ionization chamber in the pressure range of 540-600 Torr. And through the reasoning and verification of theoretical formulas, the finally formed voltage, air pressure, and ionization chamber measurement signal relationship curves (as shown in Figures 4 and 5) and three-dimensional diagrams (as shown in Figures 6 and 7) provide a solid foundation for the actual detection operation. important reference.

Step 5: Two-way verification of theoretical calculations and Garfield++ simulation results.

Unlike ions, the drift velocity of electrons is not proportional to the reduced field strength (as shown in the simulation results in Figure 8), which can be expressed as:

Where v is the rate of directional movement of free charges, 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.

其中n表示单位长度内的自由电荷数；e为自由电荷的电量；v代表自由电荷定向移动的速率；I代表单位长度内的电流大小；Q代表单位积分时长下的累积电荷量；E/p为约化场强。The final measurement signal will have a large bulge in the pressure range of 540-600Torr. Taking the relationship between the pressure of the gas ionization chamber and the accumulated charge at 500V as an example (as shown in Figure 9), it is deduced through theoretical formulas To verify, convert the formula I=nev to

Where n represents the number of free charges per unit length; e is the quantity of free charges; v represents the rate of directional movement of free charges; I represents the magnitude of the current per unit length; Q represents the accumulated charge per unit integration time; E/p is the reduced field strength.

Because under the same conditions and within the same integration time, the collected accumulated charge Q basically remains unchanged. When the electron drift speed reaches the concave area, the size of f(E/p) also has a concave range value. At this time, due to the Q If kept constant, the electronic charge ne will have a corresponding arched protruding area (between 540-600 Torr) as shown in the black box in Figure 9 . This verifies the reliability of the simulation, and also verifies the scientificity of the optimal voltage and air pressure value setting interval.

When specifically using the present invention to measure the laser proton accelerator pulsed beam current, realize the online measurement section uniformity according to the following steps, and according to the simulation analysis diagram (Fig. 6, Fig. 7 and Table 1) as a reference standard, set the optimized Air pressure and voltage selection range:

(1) Fix the gas ionization chamber detector at the mouth of the beam flow pipe, the incident window is perpendicular to the beam flow direction, and adjust it to a suitable position through lifting control, so that the center of the gas ionization chamber detector is on the horizontal line of the beam flow center;

(2) Connect the signal interface of 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 data acquisition system;

(3) Open the gas valve of the gas cylinder, let the working gas helium carry out the ventilation cycle in the gas ionization chamber, and use the flowing gas to reduce the aging of the gas;

(4) Set the optimal air pressure and voltage values. If the measurement signal is a transient current, it can be selected according to Figure 6. When the voltage value is fixed, in order to avoid large fluctuations in the signal, try to avoid setting the air pressure between 540-600Torr. The setting of the rest of the air pressure range basically obeys that the larger the air pressure value, the larger the transient current, and vice versa. When the pressure value is fixed, according to the measurement range requirements of the gas ionization itself, if it is necessary to measure a small transient current, a lower voltage value can be selected. It can be seen from Figure 6 that as the voltage value becomes smaller, the overall curved surface slopes down , the minimum transient current can reach 13μA, otherwise increase the voltage value until it can match the measurement range of its own gas ionization chamber, and the signal will not overflow;

(5) Set the optimal air pressure and voltage values. If the measurement signal is the accumulated charge, it can be selected according to Figure 7. It can be seen that when the pressure value is constant, the change of the voltage has almost no influence on the measured accumulated charge signal. This is due to the saturation properties of helium. At this point, you can focus on optimizing the setting of the air pressure value. Also when the voltage value is set, in order to avoid large fluctuations in the signal, try to avoid setting the air pressure between 540-600Torr. The setting of the other air pressure intervals basically follows that the larger the air pressure value, the greater the measured accumulated charge, and vice versa. The cumulative charge range covered by this simulation result is 3-12pC, and the appropriate air pressure value can be selected according to the electronic measurement range of the gas ionization chamber itself, so that the signal does not overflow;

(6) Apply negative high voltage to the cathode to check whether the system is operating normally;

(7) According to the actual proton beam energy, working gas medium, etc., set the integral capacitance value of the gated integral circuit. After calculation in this example, set it to 10pF.

(8) The beam passes through the sensitive volume of the detection area, and the collector conducts the signal and transmits it to the data acquisition system. The voltage offset generated by the system error before is deducted, and the beam current information is obtained after processing.

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

Finally, it should be noted that the purpose of the disclosed embodiments is to help further understand the present invention, but those skilled in the art can understand that various replacements and modifications can be made without departing from the spirit and scope of the present invention and the appended claims. It is possible. Therefore, the present invention should not be limited to the content disclosed in the embodiments, and the protection scope of the present invention is subject to the scope defined in the claims.

Claims (10)

1. The gas ionization chamber detection system of the pulse beam of the laser proton accelerator comprises a gas ionization chamber detector and a gas ionization chamber electronic system, wherein the gas ionization chamber detector adopts a flat-plate ionization chamber, and an ionization chamber inner core unit comprises a position ionization chamber and a dose ionization chamber; the gas ionization chamber electronic system comprises a front-end electronic module and a rear-end chip data acquisition system and is characterized in that the front-end electronic module adopts a gate-controlled integrating circuit, wherein 2n gate-controlled integrators convert current signals into voltage signals, and then the voltage signals pass through two multiplexers, each multiplexer converts parallel signals in n gate-controlled integrator channels into serial signals for output, wherein n is an integer greater than 1; when the signals are output, two paths of differential output driving circuits are adopted, and the output differential signals are transmitted to a data acquisition card at the rear end; the data acquisition card sends an external trigger signal to control the switch of the front-end integrator, the external trigger signal starts to process after reaching, and the processing is stopped when the signal falling edge arrives, and at the moment, the peak of the voltage signal with a certain amplitude of the front-end integrator is kept; before outputting, the serial signal sends out an acquisition signal through an acquisition clock, so that the acquisition of the signal at the rear end and the front end is synchronous.

2. The gas ionization chamber detection system of claim 1 wherein said gated integrator comprises an op-amp and a plurality of integrating capacitors in parallel therewith.

3. The gas ionization chamber detection system of claim 1 wherein the back-end chip data acquisition system is a DAQ system built for a commercial CRIO module, which digitally reads out signals processed by the front-end electronics module, and finally transmits the data to the FPGA through the serial peripheral interface, and the FPGA uploads the data to the host computer for display and storage through the gigabit ethernet interface or the USB interface.

4. The gas ionization chamber detection system of claim 1 wherein the ionization chamber core unit is arranged in the order of the first high voltage plate, the first position signal plate, the second high voltage plate 3, the second position signal plate, the third high voltage plate, the dose signal plate, and the fourth high voltage plate from the beam incident direction, wherein the first position signal plate and the second position signal plate measure uniformity of the beam in the horizontal direction and the vertical direction, respectively; the sensitive area of each unit plate is made of organic aluminum film, and a separation plate is added between each unit plates.

5. A measuring method of pulse beam current of laser proton accelerator is characterized in that by adopting the gas ionization chamber detection system of claim 1, simulation analysis is carried out from three aspects of gas ionization chamber working gas medium selection, ionization chamber voltage value range setting and gas pressure value range setting, working gas capable of effectively reducing the problem of measuring signal saturation caused by transient current high peak value is selected, and proper gas pressure value and proper voltage value are set according to the relation and rule among gas ionization chamber gas pressure, voltage and measuring signal.

6. The measurement method according to claim 5, wherein the step of simulation analysis is as follows:

1) Carrying out ansys modeling on the gas ionization chamber and carrying out finite element analysis of an electrostatic field, and after the analysis result is converged, introducing the model into a Garfield++ platform for carrying out subsequent particle transport simulation analysis;

2) The method comprises the steps of importing unit node and material attribute information ELIST.lis, node position coordinate information NLIST.lis, material resistivity and relative dielectric constant MPLIST.lis, and node voltage boundary constraint result PRNSOL.lis generated after an ansys model finite element electric field of a gas ionization chamber is analyzed into Garfield++, and starting code setting and simulation;

3) Generating a plurality of classical gas ionization chamber working medium files comprising nitrogen, air, helium and p10, researching the signal value conditions respectively detected by the gas ionization chambers under different gas conditions when the gas pressure value is 740Torr based on a Garfield++ platform, and selecting gas capable of effectively reducing transient current peak values as working gas;

4) Performing Garfield++ simulation by using the working gas selected in the step 3), and researching the relationship and rules among the gas pressure, the voltage and the measurement signals of the gas ionization chamber under the conditions of different gas pressure value ranges and different voltage value ranges;

5) And (3) carrying out bidirectional verification on theoretical calculation and a Garfield++ simulation result to finally form a relation diagram of voltage, air pressure and ionization chamber measurement signals, and providing a reference basis for actual detection operation.

7. The method of measuring of claim 6, wherein step 2) selects helium as the working gas.

8. The method of claim 6, wherein the helium is used as the working gas in step 3), the gas pressure of the garfield++ simulation study is in the range of 480-740Torr, and the voltage is in the range of 100-1000V.

9. The method of claim 6, wherein step 3) simulates a signal oscillation in which the dome-shaped protrusions appear in the gas ionization chamber measurement signal when the gas pressure is in the range of 540to 600Torr due to the non-proportional relationship between the electron drift velocity and the reduced field strength.

10. The method of measuring of claim 5, wherein helium is selected as the working gas; when the measurement signal is a current value, the air pressure value is set to be lower than 540Torr, and the ionization chamber voltage value is set to be lower than 300V; when the measurement signal is the accumulated charge quantity, the size range of the measured charge quantity is adjusted by changing the gas pressure value of the ionization chamber; meanwhile, the air pressure value is set so as to avoid a section where a bump fluctuation region occurs in a measurement signal of a transient current or an accumulated charge amount.

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