A nuclear radiation measurement system and a method for calibrating a mean square voltage mode thereof

Abstract

The application relates to the technical field of nuclear radiation measurement, and discloses a nuclear radiation measurement system and a calibration method of a mean square voltage mode of the nuclear radiation measurement system. The method comprises the following steps: reading a shaped pulse width output by a main amplifier, and assigning the pulse width as an initial dead time; adjusting a neutron yield, so that a first observed count rate output by a counter satisfies a first product constraint; triggering a variance analysis module to collect corresponding signal variances; readjusting the neutron yield, so that a second observed count rate output by the counter satisfies a second product constraint; triggering the variance analysis module to collect corresponding signal variances, and calculating corrected count rates respectively; and performing a linear fitting on two groups of data pairs by using the mean square voltage mode calibration module, and outputting a slope and an intercept corresponding to a current dead time. The application simplifies a calibration operation process, and greatly improves calibration efficiency.

Description

Technical Field

[0001] This application relates to the field of nuclear radiation measurement technology, and in particular to a nuclear radiation measurement system and its mean square voltage mode calibration method. Background Technology

[0002] In nuclear radiation measurement systems, after an incident particle is detected by the detector, the system needs a certain amount of time to process the signal (such as amplification, shaping, and discrimination). During this processing period, the system cannot respond to newly arriving signals, which may cause subsequent signals to be missed or discarded. This unresponsive time window is called "dead time." Under high count rate conditions, signal loss caused by dead time is particularly severe, with the observed count rate being far lower than the true count rate, making it difficult for pulse counting mode to work accurately.

[0003] To address the measurement challenges at high count rates, the mean square voltage mode (MSV) is an effective alternative. In this mode, the variance of the detector output signal is proportional to the incident particle count rate. By measuring the signal variance, count rate information can be indirectly obtained, thus avoiding signal loss caused by dead time. Therefore, the MSV mode is particularly suitable for neutron measurements in high count rate and high radiation environments, such as those found in fusion reactors.

[0004] However, the application of mean square voltage mode (MSV) relies on accurately knowing the proportional relationship between signal variance and count rate, which requires system calibration. How to conveniently and accurately obtain this calibration relationship is a key issue in the practical application of MSV.

[0005] Currently, the closest existing technical solution to this application is the reference detector comparison method. The principle of this method is as follows: the detector to be calibrated and a low-sensitivity detector (or secondary standard detector) with known efficiency are simultaneously placed in the same radiation field. Under the same radiation field conditions, the dead time effect of the low-sensitivity detector can be ignored due to its low sensitivity, and the measured count rate can be approximated as the true count rate. By changing the radiation field intensity, multiple sets of data corresponding to the signal variance of the detector to be calibrated and the count rate of the low-sensitivity detector are obtained. Then, linear fitting is performed to establish the proportional relationship between the variance and the count rate, thus completing the calibration of the mean square voltage mode. The specific implementation steps of this method are as follows: the ionization chamber detector to be calibrated and a low-sensitivity reference detector with known efficiency (such as a secondary standard ionization chamber) are simultaneously placed in a neutron radiation field. By changing the distance to the radiation source or adjusting the reactor power, the radiation field intensity is changed, and multiple sets of data on the signal variance of the detector to be calibrated and the count rate of the reference detector under different count rate conditions are recorded. The collected data are linearly fitted to (variance, reference count rate), and the resulting slope and intercept are the calibration coefficients for the mean square voltage mode. While this method can calibrate the mean square voltage mode, it has the following inherent drawbacks: First, it requires an additional reference detector, increasing system complexity and cost; second, in high-radiation environments such as fusion devices, the placement and maintenance of the reference detector are extremely difficult; and third, the calibration process is separated from the measurement process, failing to reflect the detector's true response characteristics under actual operating conditions. This scheme has the following disadvantages: 1. Requires an external reference detector, resulting in high system complexity and cost: The reference detector comparison method requires an additional low-sensitivity detector (or secondary standard detector) with known efficiency as a reference. This not only increases the hardware cost of the system but also increases its complexity and maintenance workload.

[0006] 2. The calibration process is cumbersome and time-consuming: This method requires changing the radiation field intensity to obtain multiple sets of data points. After each change, the system must be allowed to stabilize before recording, resulting in numerous steps and a long processing time. Especially on large experimental devices such as fusion reactors, changing operating parameters requires complex coordination and control.

[0007] 3. Inability to self-calibrate: This method relies on an external reference detector to provide a reference value for the true count rate and cannot be calibrated using the measurement system's own output signal, making it an offline calibration method. When system parameters change, online recalibration is not possible. Summary of the Invention

[0008] In view of this, this application provides a nuclear radiation measurement system and a calibration method for its mean square voltage mode.

[0009] This application discloses a calibration method for the mean square voltage mode of a nuclear radiation measurement system, comprising: Step 1: Read the width of the shaping pulse output by the main amplifier, and assign this pulse width as the initial dead time t; Step 2: Adjust the neutron yield so that the first observed count rate m1 output by the counter satisfies the first product constraint m1×t=k1, where k1 is a preset constant; trigger the variance analysis module to collect the corresponding signal variance var1; then adjust the neutron yield so that the second observed count rate m2 output by the counter satisfies the second product constraint m2×t=k2, where k2 is a preset constant; trigger the variance analysis module to collect the corresponding signal variance var2, and calculate the corrected count rates respectively , ; Step 3: The mean square voltage mode calibration module performs unary linear fitting on the two data pairs (var1, n1) and (var2, n2), and outputs the slope a and intercept b corresponding to the current dead time t.

[0010] Further, after Step 3, the method further comprises: Step 4: Adjust the neutron yield so that the third observed count rate m3 output by the counter satisfies the third product constraint m3×t=k3, where k3 is a preset constant and k3≥k1+k2; trigger the variance analysis module to collect the corresponding signal variance var3, and calculate the corrected count rate ; Step 5: If n3>npred, it is determined that the current dead time t is too large, the current dead time t is reduced, and the dead time parameter is updated; if n3<npred, it is determined that the current dead time t is too small, the current dead time t is increased; npred is the predicted count rate; Step 6: Replace the current dead time t with the adjusted dead time, repeat Step 2 to Step 5 until the difference between n3 and npred is less than a preset threshold, and at this time, the slope and intercept obtained from the last fitting in Step 3 are output as the final calibration result of the mean square voltage mode.

[0011] Further, said Step 3 comprises: For a non-paralyzable counting system, the dead time correction formula is satisfied between the observed count rate m and the true count rate n: n=m / (1-m×t), where t is the dead time; In the mean square voltage mode, there is a linear relationship between the signal variance var and the true count rate n: n=a×var+b, where a is the slope and b is the intercept, and this linear relationship is the result to be calibrated in the mean square voltage mode; According to the dead time correction formula and the linear relationship, m / (1-m×t)=a×var+b is obtained, where m and var are measurable quantities, and t, a, and b are unknown parameters; The optimal dead time t, a and b are solved through actual measurement under a plurality of product conditions and iterative fitting.

[0012] Furthermore, the expression for the predicted count rate is: .

[0013] Furthermore, adjusting the first observation count rate m1, the second observation count rate m2, and the third observation count rate m3 includes: The first mode adjusts the radiation intensity of the radioactive source; the second mode adjusts the physical measurement distance between the ionization chamber detector and the radioactive source. Executing either of the two modes will match the corresponding first product condition, second product condition, and third product condition.

[0014] Furthermore, the multiple product conditions include a first product condition, a second product condition, and a third product condition; The first product condition is m1×t=k1, the second product condition is m2×t=k2, and the third product condition is m3×t=k3, where m1 is the first observation count rate, m2 is the second observation count rate, m3 is the third observation count rate, t is the currently set dead time, and k1, k2, and k3 are three different preset constants, with k3≥k1+k2.

[0015] Furthermore, the preset threshold is determined by the calibration accuracy requirements.

[0016] This application also discloses a nuclear radiation measurement system that implements the calibration method of the mean square voltage mode of the nuclear radiation measurement system described above. The system includes an ionization chamber detector, a preamplifier, a main amplifier, a pulse amplitude discriminator, a counter, a variance analysis module, and a mean square voltage mode calibration module. The output terminal of the ionization chamber detector is connected to the input terminal of the preamplifier. The ionization chamber detector is used to receive external neutron radiation and output pulse signals. The output of the preamplifier is connected to the input of the main amplifier. The preamplifier is used to perform a first-stage linear amplification of the pulse signal. The main amplifier has two parallel signal output channels: the first output channel is connected to the input of the pulse amplitude discriminator, and the second output channel is connected to the input of the variance analysis module. The main amplifier is used to amplify the pulse signal after the first-stage amplification a second time and complete pulse shaping, and simultaneously output the shaped complete pulse signal to both channels. The output of the pulse amplitude discriminator is connected to the input of the counter. The pulse amplitude discriminator is used to filter out noise and interference pulses of the complete pulse signal according to a preset amplitude threshold, and only outputs the valid neutron pulse to the counter. The counter is used to acquire valid pulses and output the observation count rate m. The output interface of the counter is connected to the first input port of the mean square voltage mode calibration module. The variance analysis module is used to statistically analyze and calculate the signal variance var of the shaped pulses output by the main amplifier. The output interface of the variance analysis module is connected to the second input port of the mean square voltage mode calibration module. The mean square voltage mode calibration module is used to receive the observation count rate m and the signal variance var, and iteratively solve for the optimal dead time t and calibration coefficients, including the slope a and the intercept b.

[0017] Furthermore, the nuclear radiation measurement system is a fission ionization chamber used for neutron measurement.

[0018] Furthermore, the nuclear radiation measurement system is a nuclear radiation pulse counting system, and the mean square voltage mode is a working mode in which the signal variance characterizes the count rate under the first product condition, the second product condition, and the third product condition.

[0019] Due to the adoption of the above technical solution, this application has the following advantages: 1. Reduced system complexity and cost: This application does not require an external reference detector. It only uses the observation count rate and signal variance information of the measurement system itself to obtain the calibration coefficient of the mean square voltage mode through self-calibration, thereby reducing the system hardware cost and complexity.

[0020] 2. Simplified operation process and improved efficiency: This application only requires data collection under three product conditions, and calibration can be completed through closed-loop feedback iteration. There is no need to repeatedly place and remove the reference detector, which is simple to operate and takes less time.

[0021] 3. Self-calibration: This application uses the system's own signal variance as a reference benchmark. Through the mutual constraint of the dead time correction formula and the linear relationship between variance and count rate, the mean square voltage mode is self-calibrated without the need for an external reference and can be repeated when needed. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments recorded in the embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings.

[0023] Figure 1 This is a block diagram of a nuclear radiation measurement system according to an embodiment of this application; Figure 2 This is a schematic flowchart illustrating a calibration method for the mean square voltage mode of a nuclear radiation measurement system according to an embodiment of this application. Detailed Implementation

[0024] The present application will be further described in conjunction with the accompanying drawings and embodiments. The described embodiments are only a part of the embodiments of the present application, and not all of the embodiments. All other embodiments obtained by those skilled in the art should fall within the protection scope of the embodiments of the present application.

[0025] The technical problems to be solved by this application include: (1) To solve the problem that the existing mean square voltage mode calibration method requires external reference detectors (such as low sensitivity detectors or secondary standard detectors) which increases the system complexity and cost, a method is provided that can obtain calibration coefficients by self-calibration without external reference detectors.

[0026] (2) To solve the problem that the calibration process is separated from the measurement process in the existing calibration method and cannot reflect the true response characteristics of the detector under actual working conditions, a method that can be completed under actual measurement conditions is provided.

[0027] This application provides an embodiment of a calibration method for the mean square voltage mode of a nuclear radiation measurement system, comprising: Step 1: Read the shaping pulse width output by the main amplifier and assign this pulse width as the initial dead time t; Step 2: Adjust the neutron yield so that the counter outputs the first observation count rate m1 satisfying the first product constraint m1×t=k1, where k1 is a preset constant; trigger the variance analysis module to collect the corresponding signal variance var1; then adjust the neutron yield so that the counter outputs the second observation count rate m2 satisfying the second product constraint m2×t=k2, where k2 is a preset constant; trigger the variance analysis module to collect the corresponding signal variance var2, and calculate the corrected count rates respectively. , ; Step 3: The mean square voltage mode calibration module performs univariate linear fitting on the two sets of data pairs (var1,n1) and (var2,n2), and outputs the slope a and intercept b corresponding to the current dead time t.

[0028] Optionally, after step 3, the method further includes: Step 4: Adjust the neutron yield so that the counter output third observation count rate m3 satisfies the third product constraint m3×t=k3, where k3 is a preset constant, k3≥k1+k2; trigger the variance analysis module to collect the corresponding signal variance var3 and calculate the corrected count rate. ; Step 5: If n3>npred, it is determined that the current dead time t is too large, the current dead time t is reduced, and the dead time parameter is updated; if n3<npred, it is determined that the current dead time t is too small, and the current dead time t is increased; wherein npred is the predicted count rate; Step 6: Replace the current dead time t with the adjusted dead time, repeat steps 2 to 5 until the difference between n3 and npred is less than a preset threshold, and at this time, the slope and intercept obtained from the last fitting in step 3 are output as the final calibration result of the mean square voltage mode.

[0029] Optionally, said step 3 comprises: For a non-paralyzable counting system, the dead time correction formula is satisfied between the observed count rate m and the true count rate n: n=m / (1-m×t), where t is the dead time; In the mean square voltage mode, there is a linear relationship between the signal variance var and the true count rate n: n=a×var+b, where a is the slope and b is the intercept, and this linear relationship is the result to be calibrated in the mean square voltage mode; According to the dead time correction formula and the linear relationship, m / (1-m×t)=a×var+b is obtained, where m and var are measurable quantities, and t, a, and b are unknown parameters; The optimal dead time t, a and b are solved through actual measurement and iterative fitting under a plurality of product conditions.

[0030] Optionally, the expression of the predicted count rate is: .

[0031] Optionally, adjusting the first observed count rate m1, the second observed count rate m2, and the third observed count rate m3 comprises: The first mode is adjusting the radiation intensity of the radioactive source; the second mode is adjusting the physical measurement distance between the ionization chamber detector and the radioactive source; Either of the two modes can be implemented to match the corresponding first product condition, second product condition, and third product condition.

[0032] Optionally, the plurality of product conditions include a first product condition, a second product condition, and a third product condition; The first product condition is m1×t=k1, the second product condition is m2×t=k2, and the third product condition is m3×t=k3, where m1 is the first observed count rate, m2 is the second observed count rate, m3 is the third observed count rate, t is the currently set dead time, k1, k2, and k3 are three different preset constants, and k3≥k1+k2.

[0033] Optionally, the preset threshold is determined by calibration accuracy requirements.

[0034] See Figure 1This application also provides an embodiment of a nuclear radiation measurement system that implements the calibration method of the mean square voltage mode of the nuclear radiation measurement system described in the above embodiment. The system includes an ionization chamber detector, a preamplifier, a main amplifier, a pulse amplitude discriminator, a counter, a variance analysis module, and a mean square voltage mode calibration module. The output terminal of the ionization chamber detector is connected to the input terminal of the preamplifier. The ionization chamber detector is used to receive external neutron radiation and output pulse signals. The output of the preamplifier is connected to the input of the main amplifier. The preamplifier is used to perform a first-stage linear amplification of the pulse signal. The main amplifier has two parallel signal output channels: the first output channel is connected to the input of the pulse amplitude discriminator, and the second output channel is connected to the input of the variance analysis module. The main amplifier is used to amplify the pulse signal after the first-stage amplification a second time and complete pulse shaping, and simultaneously output the shaped complete pulse signal to both channels. The output of the pulse amplitude discriminator is connected to the input of the counter. The pulse amplitude discriminator is used to filter out noise and interference pulses of the complete pulse signal according to a preset amplitude threshold, and only outputs the valid neutron pulse to the counter. The counter is used to acquire valid pulses and output the observation count rate m. The output interface of the counter is connected to the first input port of the mean square voltage mode calibration module. The variance analysis module is used to statistically analyze and calculate the signal variance var of the shaped pulses output by the main amplifier. The output interface of the variance analysis module is connected to the second input port of the mean square voltage mode calibration module. The mean square voltage mode calibration module is used to receive the observation count rate m and the signal variance var, and iteratively solve for the optimal dead time t and calibration coefficients, including the slope a and the intercept b.

[0035] Optionally, the nuclear radiation measurement system is a fission ionization chamber used for neutron measurement.

[0036] Optionally, the nuclear radiation measurement system is a nuclear radiation pulse counting system, and the mean square voltage mode is a working mode in which the signal variance characterizes the count rate under the first product condition, the second product condition, and the third product condition.

[0037] For ease of understanding, this application provides a more specific embodiment: like Figure 1 As shown, the measurement system of this application includes: an ionization chamber detector, a preamplifier, a main amplifier (including shaping circuit), a pulse amplitude discriminator, a counter, a variance analysis module, and a mean square voltage mode calibration module.

[0038] The connection relationships and functions of each module are as follows: Ionization chamber detector: Receives neutron radiation and generates pulse signals.

[0039] Preamplifier: Connected to the ionization chamber detector, it amplifies the weak pulse signal output by the detector.

[0040] Main amplifier (including shaping circuit): connected to the preamplifier, it further amplifies the signal and performs pulse shaping processing.

[0041] Pulse amplitude discriminator: Connected to the main amplifier, it is used to distinguish the amplitude of pulse signals and eliminate noise and interference signals.

[0042] Counter: Connected to the pulse amplitude discriminator, it counts valid pulses and outputs the observation count rate m.

[0043] Variance analysis module: Connected to the main amplifier, it calculates the signal variance var.

[0044] Mean square voltage mode calibration module: connected to the counter and variance analysis module, receives the observation count rate m and signal variance var, executes the calibration method of this application, and outputs calibration coefficients (slope a and intercept b).

[0045] The system employs mean square voltage mode for neutron measurements under high count rate conditions.

[0046] Method Principles This application is based on the following principles: For a non-paralyzed counting system, the observed count rate m and the true count rate n satisfy the dead time correction formula: n=m / (1-m×t), where t is the dead time.

[0047] In mean square voltage mode, there is a linear relationship between the signal variance var and the true count rate n: n = a × var + b, where a is the slope and b is the intercept. This linear relationship is the result that needs to be calibrated in mean square voltage mode.

[0048] Combining the two relationships above, we get: m / (1-m×t)=a×var+b. In this formula, m and var are measurable parameters, while t, a, and b are unknown parameters. This application solves for these unknown parameters simultaneously through measurement and iterative fitting under multiple product conditions, where a and b are the required calibration coefficients.

[0049] Methods and Steps like Figure 2 As shown, the method of this application includes the following steps: Step (1): Set the initial dead time t.

[0050] The initial dead time is estimated based on the forming width of the signal in the measurement system. For example, if the pulse signal width after forming is approximately 1 microsecond, then the initial dead time is set to t = 1 μs.

[0051] Step (2): Two-condition data acquisition and correction.

[0052] The measurement system is set to operate under the first product condition m1×t=k1 and the second product condition m2×t=k2. Here, m is the observation count rate, t is the currently set dead time, and k1 and k2 are two different preset constants (preferably k1=0.1 and k2=0.25).

[0053] The signal variances var1 and var2 under various conditions are collected, and the dead time correction formula n=m / (1-m×t) is applied to the observed count rate to obtain the corrected count rates n1 and n2.

[0054] Step (3): Linear fitting.

[0055] The two data pairs (var1,n1) and (var2,n2) obtained in step (2) are linearly fitted to obtain the slope and intercept. These slope and intercept are the candidate values ​​of the calibration coefficients under the current dead time setting.

[0056] Step (4): Third condition verification.

[0057] The measurement system is operated under the third product condition m3×t=k3, where k3 is a preset constant different from k1 and k2 (preferably k3=0.4), and k3≥k1+k2 is satisfied. The signal variance var3 under this condition is collected, and the corrected count rate n3 is obtained by applying the dead time correction formula to the observed count rate.

[0058] The constraint k3≥k1+k2 ensures that the verification point is located outside the two fitted points, which can effectively verify the accuracy of the fitting extrapolation.

[0059] Step (5): Calculate the predicted count rate.

[0060] Substitute the variance var3 of the signal collected in step (4) into the slope and intercept obtained in step (3) to calculate the predicted count rate npred=a×var3+b.

[0061] Step (6): Compare and judge.

[0062] Compare the sizes of n3 and npred: If n3 > npred, it means that the current dead time t is too large, resulting in overcorrection. The current dead time t should be reduced. If n3 < npred, it indicates that the currently set dead time t is too small, resulting in insufficient correction, and the current dead time t should be increased.

[0063] Step (7): Iteration and output.

[0064] The adjusted dead time is used to replace the current dead time t, and steps (2) to (6) are repeated until the difference between n3 and npred is less than a preset threshold (this threshold is determined by the calibration accuracy requirement, for example, 0.5%). At this point, the slope and intercept obtained from the last fitting in step (3) are output as the final calibration result of the mean-square voltage mode.

[0065] Application Scenarios Taking a typical fission ionization chamber neutron measurement system as an example, the implementation process of the mean-square voltage mode calibration method of the present application is described in detail.

[0066] I. System Composition and Initial Parameter Setting The measurement system in this embodiment is as shown in Figure 1 including: an ionization chamber detector, a preamplifier, a main amplifier (including a shaping circuit), a pulse amplitude discriminator, a counter, a variance analysis module, and a mean-square voltage mode calibration module. The system adopts the mean-square voltage mode for neutron measurement under high count rate conditions.

[0067] The calibration accuracy requirement of the measurement system is 0.5%.

[0068] After measurement, the width of the shaped pulse signal is about 1 μs. According to step (1), the initial dead time t is set to 1 μs.

[0069] II. Measurement Method for System Dead Time In this embodiment, the values of the preset constants are: k1≈0.1, k2≈0.25, k3≈0.4. The verification condition satisfies k3≥k1+k2.

[0070] By adjusting the operating parameters of the fusion device, the system is made to work under three product conditions respectively, and the observed count rates m1, m2, m3 and the corresponding signal variances var1, var2, var3 are collected, with the results shown in Table 1.

[0071] III. Iterative Correction Process In each iteration, the value of the dead time t and the corresponding correction results n1, n2, n3, the predicted count rate npred, and the ratio n3 / npred are shown in Table 2.

[0072] 1st iteration (t=1 μs): The calculated n3 / npred=1.035, that is, n3>npred, so the dead time should be reduced according to step (6).

[0073] Second iteration (t=0.8 μs): The calculated n3 / npred=0.983, that is, n3<npred, so the dead time should be increased.

[0074] Third iteration (t=0.9 μs): The calculated n3 / npred=1.007, that is, n3>npred, so the dead time should be decreased.

[0075] Fourth iteration (t=0.85 μs): The calculated n3 / npred=0.995, that is, n3<npred, so the dead time should be increased.

[0076] Fifth iteration (t=0.875 μs): The calculated n3 / npred=1.001, with a difference from 1 of 0.1%, which is less than the preset threshold (0.5%), and the iteration converges.

[0077] IV. Output Result At this point, the fitting slope 9.210e+00 and intercept -2.702e+00 obtained from the fifth iteration are output as the final calibration result of the mean-square voltage mode. This calibration result can be used for subsequent calculation of the neutron count rate from the signal variance under high count rate conditions.

[0078] Table 1 Acquired data under the conditions of k1, k2 and k3

[0079] Table 2 Calibration coefficients for each iteration

[0080] It should be finally noted that the above embodiments are only used to illustrate the technical solutions of the present application rather than to limit them. Although the present application is described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: modifications or equivalent substitutions can still be made to the specific implementations of the present application, and any modifications or equivalent substitutions that do not depart from the spirit and scope of the present application shall be covered within the protection scope of the claims of the present application.

Claims

1. A calibration method for the mean square voltage mode of a nuclear radiation measurement system, characterized in that, comprising: Step 1: Read the width of the shaped pulse output by the main amplifier, and assign the pulse width as the initial dead time t; Step 2: Adjust the neutron yield so that the counter outputs the first observation count rate m1 satisfying the first product constraint m1×t=k1, where k1 is a preset constant; trigger the variance analysis module to collect the corresponding signal variance var1; then adjust the neutron yield so that the counter outputs the second observation count rate m2 satisfying the second product constraint m2×t=k2, where k2 is a preset constant; trigger the variance analysis module to collect the corresponding signal variance var2, and calculate the corrected count rates respectively. , ; Step 3: The mean square voltage mode calibration module performs unary linear fitting on the two sets of data pairs (var1, n1) and (var2, n2), and outputs the slope a and intercept b corresponding to the current dead time t.

2. The calibration method for the mean square voltage mode of the nuclear radiation measurement system according to claim 1, characterized in that, After Step 3, the method further comprises: Step 4: Adjust the neutron yield so that the counter output third observation count rate m3 satisfies the third product constraint m3×t=k3, where k3 is a preset constant, k3≥k1+k2; trigger the variance analysis module to collect the corresponding signal variance var3 and calculate the corrected count rate. ; Step 5: If n3>npred, it is determined that the current dead time t is too large, the current dead time t is decreased, and the dead time parameter is updated; if n3<npred, it is determined that the current dead time t is too small, the current dead time t is increased; npred is the predicted count rate; Step 6: Replace the current dead time t with the adjusted dead time, repeat Steps 2 to 5 until the difference between n3 and npred is less than a preset threshold, and at this time, the slope and intercept obtained from the last fitting in Step 3 are output as the final calibration result of the mean square voltage mode.

3. The calibration method for the mean square voltage mode of the nuclear radiation measurement system according to claim 1, characterized in that, Said Step 3 comprises: For a non-paralyzable counting system, the following dead time correction formula is satisfied between the observed count rate m and the true count rate n: n=m / (1-m×t), where t is the dead time; In the mean square voltage mode, there is a linear relationship between the signal variance var and the true count rate n: n=a×var+b, where a is the slope and b is the intercept, and this linear relationship is the result to be calibrated in the mean square voltage mode; According to the dead time correction formula and the linear relationship, m / (1-m×t)=a×var+b is obtained, where m and var are measurable quantities, and t, a, and b are unknown parameters; The optimal dead time t, a and b are solved through actual measurement under a plurality of product conditions and iterative fitting.

4. The calibration method for the mean square voltage mode of the nuclear radiation measurement system according to claim 2, characterized in that, The expression for the predicted count rate is: .

5. The calibration method for the mean square voltage mode of the nuclear radiation measurement system according to claim 2, characterized in that, Adjusting the first observed count rate m1, the second observed count rate m2, and the third observed count rate m3 comprises: The first mode: adjusting the radiation intensity of the radioactive source; the second mode: adjusting the physical measurement distance between the ionization chamber detector and the radioactive source; Either of the two modes can be selected to execute to match the corresponding first product condition, second product condition and third product condition.

6. The calibration method for the mean square voltage mode of the nuclear radiation measurement system according to claim 1, characterized in that, The plurality of product conditions include a first product condition, a second product condition, and a third product condition; The first product condition is m1×t=k1, the second product condition is m2×t=k2, the third product condition is m3×t=k3, where m1 is the first observed count rate, m2 is the second observed count rate, m3 is the third observed count rate, t is the currently set dead time, k1, k2, and k3 are three different preset constants, and k3≥k1+k2.

7. The calibration method for the mean square voltage mode of the nuclear radiation measurement system according to claim 2, characterized in that, The preset threshold is determined by the calibration accuracy requirement.

8. A nuclear radiation measurement system, implementing the calibration method for the mean square voltage mode of the nuclear radiation measurement system according to any one of claims 1-7, characterized in that, comprising an ionization chamber detector, a preamplifier, a main amplifier, a pulse amplitude discriminator, a counter, a variance analysis module, and a mean square voltage mode calibration module; The output end of the ionization chamber detector is connected to the input end of the preamplifier, and the ionization chamber detector is configured to receive external neutron radiation and output a pulse signal; The output of the preamplifier is connected to the input of the main amplifier. The preamplifier is used to perform a first-stage linear amplification of the pulse signal. The main amplifier has two parallel signal output channels: the first output channel is connected to the input of the pulse amplitude discriminator, and the second output channel is connected to the input of the variance analysis module. The main amplifier is used to amplify the pulse signal after the first-stage amplification a second time and complete pulse shaping, and simultaneously output the shaped complete pulse signal to both channels. The output of the pulse amplitude discriminator is connected to the input of the counter. The pulse amplitude discriminator is used to filter out noise and interference pulses of the complete pulse signal according to a preset amplitude threshold, and only outputs the valid neutron pulse to the counter. The counter is used to acquire valid pulses and output the observation count rate m. The output interface of the counter is connected to the first input port of the mean square voltage mode calibration module. The variance analysis module is used to statistically analyze and calculate the variance var of the shaped pulse output by the main amplifier. The output interface of the variance analysis module is connected to the second input port of the mean square voltage mode calibration module. The mean square voltage mode calibration module is used to receive the observation count rate m and the signal variance var, and iteratively solve for the optimal dead time t and calibration coefficients, including the slope a and the intercept b.

9. The nuclear radiation measurement system according to claim 8, characterized in that, The nuclear radiation measurement system is a fission ionization chamber used for neutron measurement.

10. The nuclear radiation measurement system according to claim 8, characterized in that, The nuclear radiation measurement system is a nuclear radiation pulse counting system, and the mean square voltage mode is a working mode in which the signal variance characterizes the count rate under the first product condition, the second product condition, and the third product condition.

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