Ultrahigh-dose-rate proton beam monitoring system and method for dual-detector calibration

Abstract

The invention belongs to the technical field of radiation monitoring, and particularly relates to an ultrahigh dose rate proton beam monitoring method for double-detector calibration, which comprises the following steps of: 1, acquiring an efficiency ratio curve of a first detector and a second detector; 2, calculating a real-time dose rate according to the efficiency ratio curve; 3, adjusting the beam according to the real-time dose rate; and 4, irradiating the irradiation sample, and assisting in monitoring the beam current by the detector in real time. By applying the method, the problem that the detection efficiency of detectors such as an ionization chamber under the ultrahigh dose rate is reduced can be effectively corrected, the accuracy of ultrahigh dose rate proton dose measurement is greatly improved, and the method has an online real-time monitoring function to meet actual application.

Description

Technical Field

[0001] The invention belongs to the technical field of radiation monitoring, and in particular relates to an ultra-high dose rate proton beam monitoring system and method for dual-detector calibration. Background Art

[0002] Under the irradiation of ultra-high dose rate (>40Gy / s) particle beams, traditional ionization chamber detectors will suffer from reduced charge collection efficiency, signal attenuation or distortion due to effects such as charge coincidence and ion recombination, thereby reducing detection efficiency and accuracy and making it impossible to accurately monitor the beam dose rate in real time.

[0003] The highest energy proton accelerator currently in use in China, the 100MeV high-current proton cyclotron of the Institute of Atomic Energy, can achieve proton beam extraction with a current intensity of microamperes. Its established S3 beam line has already carried out relevant work on the biological effects of proton radiation.

[0004] The SEEM detector and Faraday cup benchmarking methods currently used can only obtain the proton fluence rate and fluence at conventional dose rates (<200Gy / min). Under ultra-high dose rate proton radiation, the capacitor will reach a saturated state in just a few seconds. Therefore, the proton dose calculated by the SEEM and Faraday cup methods can only reflect the beam state in a very short time, and the beam intensity of the accelerator will inevitably fluctuate under long-term operation. Traditional methods are difficult to monitor whether the proton beam is stable and whether the dose rate remains unchanged under ultra-high dose rates for a long time. Therefore, it is very necessary to carry out research on dose monitoring technology under ultra-high dose rates. Summary of the invention

[0005] In view of the problems existing in the prior art, the purpose of the present invention is to provide a method for ultra-high dose rate proton beam monitoring and data correction with dual detector calibration. A beam switch with adjustable duty cycle is added to the beam line to modulate the proton beam into a pulse-like beam; then the Faraday cup is still used as a reference detector; finally, a shield is built at the rear end of the sample by lead bricks, and another PTW ionization chamber detector is placed in the shield with a certain thickness as a calibration detector. Theoretical simulation and experimental data fitting are adopted, and the proton dose rate data at the irradiated sample can be obtained by fitting the three parameters of accelerator beam intensity, Faraday cup charge collection efficiency, and PTW ionization chamber parameters. The problem of short-time charge saturation of the traditional Faraday cup is solved, and real-time and accurate monitoring data of the proton dose rate in the process of ultra-high dose rate proton beam delivery is obtained.

[0006] In order to achieve the above-mentioned object of the invention, the technical solution of the present invention is as follows:

[0007] A dual-detector calibrated ultra-high dose rate proton beam monitoring method comprises the following steps:

[0008] Step 1, obtaining an efficiency ratio curve of a first detector and a second detector; the first detector is arranged in front of the irradiated sample, and the second detector is arranged in a shielded space behind the irradiated sample;

[0009] Step 2, calculating the real-time dose rate according to the efficiency ratio curve;

[0010] Step 3, adjusting the beam current according to the real-time dose rate;

[0011] Step 4, irradiate the irradiation sample, and the detector assists in monitoring the beam in real time.

[0012] Further, step 1 includes the following steps:

[0013] Step 11, aligning the beam center height by laser so that the beam center, the first detector center and the second detector center are at the same center height;

[0014] Step 12, starting a beam shutter disposed in front of the first detector, so that the beam shutter modulates the beam to form a quasi-pulse beam with a duty cycle of a set value according to a set working mode;

[0015] Step 13, adjusting the accelerator beam current to increase gradually from 0, collecting charge data of the first detector multiple times at each beam current value, then removing the first detector, and collecting dose rate data of the second detector disposed in the shielded space multiple times;

[0016] Step 14, using a Monte Carlo simulation program, respectively calculating theoretical values ​​of detection data collected by the first detector and the second detector disposed in the shielded space, and based on the measured values ​​of the detection data collected by the first detector and the second detector in step 13, respectively obtaining detection efficiencies of the first detector and the second detector corresponding to different beam current values ​​by dividing the measured values ​​by the corresponding theoretical values;

[0017] Step 15, using the detection efficiency data of the first detector and the second detector corresponding to different beam current values ​​obtained in step 14, respectively fitting the beam current and detection efficiency relationship curve equations of the first detector and the second detector;

[0018] Step 16, using the beam current and detection efficiency relationship curve equation of the first detector and the second detector in step 15, calculate an efficiency ratio curve describing the relationship between the detection efficiency ratio of the first detector and the second detector and the beam current.

[0019] Further, in step 2, the following steps are included:

[0020] Step 21, first preliminarily modulate the dose rate to the target dose rate, place a dose film at the irradiation sample, place a shielded space and a second detector therein behind it, and irradiate the dose film with an ultra-high dose rate proton beam;

[0021] Step 22, the dose rate reading of the second detector is divided by the result of the radiation dose at the irradiated sample obtained by scanning the dose film after calculating the shielding loss to obtain the detection efficiency E2 of the second detector;

[0022] Step 23, calculating the detection efficiency E1 of the first detector by using the efficiency ratio curve in step 1 and the detection efficiency E2 of the second detector in step 22;

[0023] Step 24 , moving the first detector to the beam online position, and calculating the real-time dose rate D according to the charge reading C of the first detector and the detection efficiency E1 of the first detector in step 23 .

[0024] Further, in step 3, the proton beam is adjusted according to the real-time dose rate in step 2 until the target dose rate is obtained at the irradiated sample, and the first detector is removed.

[0025] Furthermore, in step 4, the irradiation dose of the irradiated sample is controlled by controlling the time parameter or the charge parameter.

[0026] Further, in step 1, the first detector is a Faraday cup, and the second detector is a probe of a PTW ionization chamber.

[0027] Further, in step 12, the beam shutter operates in a mode of opening for 1 second and closing for 9 seconds, and the duty cycle of the pulse-like beam is 10%.

[0028] Further, in step 24, the calculation formula of the real-time dose rate D is as follows

[0029] D=C@E p / (C p ·E1), where C is the charge reading of the first detector, E p is the energy carried by a single proton, C p is the charge carried by a single proton, and E1 is the detection efficiency of the first detector.

[0030] Further, in step 13, the shielding space is a shielding space enclosed by lead bricks.

[0031] An ultra-high dose rate proton beam monitoring system that uses the above method to achieve dual-detector calibration includes a beam transport pipeline, a beam shutter, a first detector, an irradiated sample and a second detector located in a shielded space, which are sequentially arranged in the beam direction. The beam shutter is used to modulate the beam to form a pulse-like beam with a duty cycle of a set value, the first detector is used to detect charge data, and the second detector is used to detect dose rate data.

[0032] The beneficial effects of the present invention are as follows:

[0033] The application of this method can effectively correct the problem of decreased detection efficiency of detectors such as ionization chambers under ultra-high dose rates, greatly improve the accuracy of ultra-high dose rate proton dose measurement, and have an online real-time monitoring function to meet practical applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 Schematic diagram of the layout of the ultra-high dose rate proton beam monitoring system for dual detector calibration;

[0035] Figure 2 This is the dual detector detection efficiency calibration diagram.

[0036] In the above drawings, 1. beam transport pipeline; 2. beam shutter; 3. first detector; 4. irradiated sample; 5. second detector located in the shielded space. DETAILED DESCRIPTION

[0037] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0038] A dual-detector calibrated ultra-high dose rate proton beam monitoring method comprises the following steps:

[0039] Step 1, obtaining an efficiency ratio curve of a first detector and a second detector; the first detector is arranged in front of the irradiated sample, and the second detector is arranged in a shielded space behind the irradiated sample;

[0040] Step 2, calculating the real-time dose rate according to the efficiency ratio curve;

[0041] Step 3, adjusting the beam current according to the real-time dose rate;

[0042] Step 4, irradiate the irradiation sample, and the detector assists in monitoring the beam in real time.

[0043] An ultra-high dose rate proton beam monitoring system for realizing dual-detector calibration comprises a beam transport pipeline, a beam shutter, a first detector, an irradiated sample and a second detector located in a shielded space, which are sequentially arranged in the beam direction. The beam shutter is used to modulate the beam to form a quasi-pulse beam with a duty cycle of a set value. The first detector is used to detect charge data, and the second detector is used to detect dose rate data.

[0044] Aiming at the problem of online real-time dose monitoring in ultra-high dose rate proton irradiated samples, a dual-detector calibration monitoring method was designed. The specific technical solution is as follows:

[0045] 1. Record the beam intensity of the accelerator on the S3 beam line in real time, recorded as I.

[0046] 2. After the protons are led out of the atmosphere, a beam shutter is set at the isocenter height. Its thickness can completely block the proton beam. Its working mode is set to open for 1s and closed for 9s, thereby modulating the proton beam to form a pulse-like beam with a duty cycle of 10%.

[0047] 3. Set up an atmospheric Faraday cage behind the beam shutter, such as Figure 1 As shown. The outer diameter of the Faraday cup is 26mm, the length of the cup is 40mm, the inner diameter is 10mm, the depth of the cup is 25mm, and the vacuum chamber is built in. It is designed with a front panel, a rear panel and an exhaust nozzle. The front panel aligns the beam, the rear panel transfers the signal, and the exhaust nozzle is connected to the vacuum pump. The material is brass. The charge number is collected through the Faraday cup. The beam charge passing through within 1s each time the beam shutter is opened is collected and recorded as C. After the beam stops, the charge is automatically released and the next beam pulse is waited for to be counted again.

[0048] 4. Remove the Faraday cage and build a shielding space enclosed by lead bricks at the rear end of the irradiated sample. The thickness of the lead brick shielding is 2 cm. Place a probe of a PTW proton ionization chamber detector in the shielding space to record the real-time proton dose rate D.

[0049] 5. Start from 0 and continuously increase the accelerator beam current, obtain the changing relationship between I and C and between I and D respectively, and fit the response curve.

[0050] 6. Calculate the values ​​of C and D under ideal conditions in the Monte Carlo program and compare them with the measured values. The response curve corresponding to the ratio of the measured and theoretical values ​​is the E1 / E2-I curve (efficiency ratio curve).

[0051] 7. Remove the Faraday cage, preliminarily adjust the approximate beam current, obtain the measured dose through the dose film at the irradiated sample position, and simultaneously record the PTW value in the shielding body. By comparison, the detection efficiency E2 at this time can be obtained.

[0052] 8. According to the efficiency ratio curve, the detection efficiency E1 of the Faraday cup can be obtained at this time. After placing the irradiated sample at the irradiation position, first move the Faraday cup to the center of the beam. Since the Faraday cup has a beam blocking function, the irradiated sample is not irradiated at this time. However, the dose rate D at the irradiated sample after the Faraday cup is moved away can be calculated based on the Faraday cup reading and E1. r .

[0053] 9. After fine-tuning the beam to achieve the target dose rate at the irradiated sample, remove the Faraday cage and irradiate the sample.

[0054] 10. Record the PTW reading inside the shielding body at the same time as irradiation. Because the reading is recorded in real time during irradiation, it is possible to infer whether the irradiated dose of the irradiated sample is accurate and whether the beam is stable during irradiation based on E2, which serves as a dose verification method.

[0055] Example

[0056] The layout of a dual-detector calibrated ultra-high dose rate proton beam monitoring system of the present invention is as follows: Figure 1 As shown,

[0057] 1. First, use the laser to align the beam center height to ensure that the beam center, Faraday cup center and PTW probe center are at the same center height.

[0058] 2. Start the beam shutter and keep it open for 1 second and closed for 9 seconds in a reciprocating motion.

[0059] 3. The accelerator beam current starts to increase gradually from 0, and the charge data of the Faraday cage is collected ten times at each beam current value. Then the Faraday cage is removed and the dose rate values ​​collected by the probe of the PTW proton ionization chamber are recorded ten times.

[0060] 4. Using the Monte Carlo simulation program, the number of particles that should be deposited at the Faraday cup and in the shielded space can be calculated from the accelerator beam, and converted into corresponding charge and dose rate data as theoretical values. By dividing the measured values ​​by the theoretical values, the detection efficiency E1 of the Faraday cup and the detection efficiency E2 of the PTW proton ionization chamber probe under different beam conditions can be obtained.

[0061] The relationship curves between the accelerator beam current I and the detection efficiency of the Faraday cage and the PTW proton ionization chamber probe in the shielded space were fitted using the Matlab program, as shown in Figure 2 The relationship between the Faraday cup detection efficiency E1 and the accelerator beam current I is shown in the figure below: E1 = -1E-07I 2 -0.0002I+0.9982; the relationship between the detection efficiency E2 of the PTW proton ionization chamber probe in the shielded space and the accelerator beam current I is consistent with the curve: E2=-3E-08I 2-0.0002I+1.006; Under high dose rate, the detection efficiency E1 of the Faraday cup is a polynomial fit, while the detection efficiency E2 of the PTW proton ionization chamber probe in the shielded space is more similar to a straight line, thereby obtaining the efficiency ratio curve of E2 / E1 (or E1 / E2).

[0062] 5. Since the accelerator transmission efficiency is affected by many factors such as vacuum parameters and beam transmission line parameters, the curve relationship between the accelerator beam current I and the detection efficiency of the two detectors is not permanent. However, under the same beam conditions, that is, when the accelerator vacuum series parameters, beam line series parameters and magnetic field parameters are the same, the efficiency ratio curve of the two detectors is still stable and effective, and no recalibration is required.

[0063] 6. When irradiating the sample with ultra-high dose rate protons, first preliminarily modulate the dose rate to (near) the target dose rate, place a dose film at the irradiated sample, and then place a shield with the PTW proton ionization chamber probe in it. The reading of the PTW proton ionization chamber is divided by the irradiation dose at the irradiated sample obtained by scanning the dose film, and the result after calculating the shielding loss can be used to obtain the detection efficiency E2 of the PTW proton ionization chamber probe at this time. The detection efficiency E1 of the Faraday cup can be obtained by comparing it with the efficiency ratio curve. Move the Faraday cup to the beam online position, and calculate the accurate dose rate D based on the Faraday cup reading C: D = C·E p / (C p E1), E p is the energy carried by a single proton, C p The charge carried by a single proton.

[0064] 7. After calculating the precise dose rate based on the Faraday cup reading, fine-tune the beam until the target dose rate is obtained. Then remove the Faraday cup and irradiate the sample by controlling the dose through time parameters or charge parameters. The PTW proton ionization chamber probe in the shielding body after irradiating the sample can monitor the stability of the irradiation beam in real time.

[0065] 8. This method can also be extended to different types of detectors and different types of shielding layers. For example, using a flat-plate ionization chamber as a reference detector, using a water body as a shielding layer to shield and calibrate a detector, etc., can all be extended to use this method.

[0066] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims

1. A dual-detector calibrated ultra-high dose rate proton beam monitoring method, characterized in that: The following steps are involved: Step 1, obtaining an efficiency ratio curve of a first detector and a second detector; the first detector is arranged in front of the irradiated sample, and the second detector is arranged in a shielded space behind the irradiated sample; Step 2, calculating the real-time dose rate according to the efficiency ratio curve; Step 3, adjusting the beam current according to the real-time dose rate; Step 4, irradiate the irradiation sample, and the detector assists in monitoring the beam in real time.

2. The method for monitoring ultra-high dose rate proton beams with dual detector calibration according to claim 1, characterized in that: In step 1, the following steps are included: Step 11, aligning the beam center height by laser so that the beam center, the first detector center and the second detector center are at the same center height; Step 12, starting a beam shutter disposed in front of the first detector, so that the beam shutter modulates the beam to form a quasi-pulse beam with a duty cycle of a set value according to a set working mode; Step 13, adjusting the accelerator beam current to increase gradually from 0, collecting charge data of the first detector multiple times at each beam current value, then removing the first detector, and collecting dose rate data of the second detector disposed in the shielded space multiple times; Step 14, using a Monte Carlo simulation program, respectively calculating theoretical values ​​of detection data collected by the first detector and the second detector disposed in the shielded space, and based on the measured values ​​of the detection data collected by the first detector and the second detector in step 13, respectively obtaining detection efficiencies of the first detector and the second detector corresponding to different beam current values ​​by dividing the measured values ​​by the corresponding theoretical values; Step 15, using the detection efficiency data of the first detector and the second detector corresponding to different beam current values ​​obtained in step 14, respectively fitting the beam current and detection efficiency relationship curve equations of the first detector and the second detector; Step 16, using the beam current and detection efficiency relationship curve equation of the first detector and the second detector in step 15, calculate an efficiency ratio curve describing the relationship between the detection efficiency ratio of the first detector and the second detector and the beam current.

3. The dual-detector calibrated ultra-high dose rate proton beam monitoring method according to claim 1, characterized in that: In step 2, The steps include: Step 21, first preliminarily modulate the dose rate to the target dose rate, place a dose film at the irradiation sample, place a shielded space and a second detector therein behind it, and irradiate the dose film with an ultra-high dose rate proton beam; Step 22, the dose rate reading of the second detector is divided by the result of the radiation dose at the irradiated sample obtained by scanning the dose film after calculating the shielding loss to obtain the detection efficiency E2 of the second detector; Step 23, calculating the detection efficiency E1 of the first detector by using the efficiency ratio curve in step 1 and the detection efficiency E2 of the second detector in step 22; Step 24 , moving the first detector to the beam online position, and calculating the real-time dose rate D according to the charge reading C of the first detector and the detection efficiency E1 of the first detector in step 23 .

4. The method for monitoring ultra-high dose rate proton beams with dual detector calibration according to claim 1, characterized in that: In step 3, the proton beam is adjusted according to the real-time dose rate in step 2 until the target dose rate is obtained at the irradiated sample, and the first detector is removed.

5. The method for monitoring ultra-high dose rate proton beams with dual detector calibration according to claim 1, characterized in that: In step 4, the irradiation dose of the irradiated sample is controlled by controlling the time parameter or the charge parameter.

6. The method for monitoring ultra-high dose rate proton beams with dual detector calibration according to claim 1, characterized in that: In step 1, the first detector is a Faraday cup, and the second detector is a probe of a PTW ionization chamber.

7. The dual-detector calibrated ultra-high dose rate proton beam monitoring method according to claim 2, characterized in that: In step 12, the beam shutter operates in a mode of opening for 1 second and closing for 9 seconds, and the duty cycle of the pulse-like beam is 10%.

8. The dual-detector calibrated ultra-high dose rate proton beam monitoring method according to claim 3, characterized in that: In step 24, the calculation formula of the real-time dose rate D is as follows: D=C@E p / (C p ·E1), where C is the charge reading of the first detector, E p is the energy carried by a single proton, C p is the charge carried by a single proton, and E1 is the detection efficiency of the first detector.

9. The method for monitoring ultra-high dose rate proton beams with dual detector calibration according to claim 2, characterized in that: In step 13, the shielding space is a shielding space enclosed by lead bricks.

10. An ultra-high dose rate proton beam monitoring system that implements dual detector calibration using the method described in any one of claims 1 to 9, characterized in that: The invention comprises a beam transport pipeline (1), a beam shutter (2), a first detector (3), an irradiated sample (4) and a second detector (5) located in a shielded space, which are sequentially arranged in the beam direction; the beam shutter (2) is used to modulate the beam to form a quasi-pulse beam with a duty cycle of a set value; the first detector is used to detect charge data; and the second detector is used to detect dose rate data.

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