Weak current measuring system

Abstract

The invention provides a weak current measurement system capable of measuring weak current with high precision in a wide measurement range. The weak current measurement system comprises a weak current source for generating the weak current to be measured; the weak current measuring circuit is used for measuring weak current generated by the weak current source; the control host is used for controlling the work of the weak current source and the weak current measuring circuit, and the weak current measuring circuit comprises a negative feedback amplifier with a reverse input end, an output end and a grounded positive input end; the electrometer is connected between the positive input end and the output end of the negative feedback amplifier; and the plurality of feedback capacitors are connected between the reverse input end and the output end, and one feedback capacitor is selected from the plurality of feedback capacitors to access the weak current measuring circuit for measurement based on the magnitude range of the weak current to be measured.

Description

Technical Field

[0001] The invention relates to the measurement of weak current, and in particular to the measurement of weak current in the technical field of radiation measurement. Background Art

[0002] The measurement of weak electrical signals is an important part of current measurement technology. Weak current generally refers to currents less than 10 -6 A current, weak current measurement technology is widely used in the fields of ionizing radiation detection, semiconductors, electrostatic detection, space radiation detection, biotechnology, etc., involving high impedance discharge circuit design, leakage current protection and many other technologies. At present, there are commercial products with excellent technical performance such as Keithley 6517 electrometer on the market for measuring weak current. The limit of measuring current directly with such measuring instruments is 10 -8 A, but for example, the ionization current generated by medium-energy X-rays irradiating the ionization chamber is 10 -12 A to 10 -10 A is much smaller than the limit of direct measurement by such measuring instruments, so such measuring instruments are not suitable for measuring weak currents such as the ionization current generated by the medium-energy X-ray irradiation ionization chamber.

[0003] At present, the methods for measuring weak current include charge integration method and high-gain negative feedback method. Charge integration method is to collect the charge generated by ionization within the set charge collection time, and compare the collected charge with time to obtain ionization current. High-gain negative feedback method, as disclosed in patent document 1, is to use the inherent characteristic that the output voltage of the high-gain negative feedback amplifier changes linearly with time. The ionized charge applies voltage to the connected standard capacitor via the high-gain negative feedback amplifier, charges the standard capacitor, and measures the current generated by the ionized charge, i.e., weak current, by measuring the voltage change within a specified time. Since the weak current is very small, its measurement result is easily affected by various environments and instruments. For example, when measuring the ionization current generated by the ionization chamber irradiated by X-rays, the temperature in the ionization chamber will affect the measurement result of the weak current, so it is necessary to measure the temperature in the ionization chamber and other influencing factors to correct the value of the measured weak current.

[0004] Patent document 1: CN204789709U Summary of the invention

[0005] The charge integration method is a method of directly measuring charge in principle. However, it cannot collect all the charges generated by ionization and cannot accurately measure weak currents. Therefore, it is generally preferred to use a high-gain negative feedback method to measure weak currents.

[0006] The high-gain negative feedback method can measure weak currents more accurately than the charge integration method, but the weak current to be measured generally covers a relatively large range, and it is difficult to accurately measure weak currents in such a large range at the same time using the measurement circuit of the prior art. In addition, the correction of the measured weak current in the prior art is performed using the parameters and formulas built into the measurement circuit that cannot be corrected, and the correction cannot be applied to the measurement results in all cases without error.

[0007] That is, the existing weak current measurement system may have errors both during measurement and correction, and needs to be improved.

[0008] Therefore, the purpose of the present invention is to provide a weak current measurement system that can measure weak current with high accuracy in a wide measurement range, and further provide a weak current measurement system that can perform appropriate correction on the measured weak current to obtain a more accurate final measurement result.

[0009] In order to achieve the above object, the present invention includes the following technical solutions.

[0010] A weak current measurement system includes: a weak current measurement circuit for measuring the weak current generated by the weak current source; and a control host for controlling the operation of the weak current source and the weak current measurement circuit, wherein the weak current measurement circuit includes: a negative feedback high-precision capacitor having a grounded input terminal, a ground terminal and an output terminal; an electrometer is connected to the output terminal of the negative feedback precision capacitor; the input terminal of the precision capacitor is the current to be measured (ionization chamber), and a feedback capacitor is selected from a plurality of the feedback capacitors based on the magnitude range of the weak current to be measured and connected to the weak current measurement circuit for measurement.

[0011] In a weak current measurement system using a high-gain negative feedback method, multiple feedback capacitors are set, and based on the magnitude range of the weak current to be measured, one feedback capacitor is selected from the multiple feedback capacitors to be connected to the weak current measurement circuit for measurement. Thus, the weak current can be measured with high accuracy within a wide measurement range.

[0012] Wherein, control software is installed in the control host, the feedback capacitor is manually switched and the switched feedback capacitor is selected in the control software for measurement.

[0013] The control host stores parameters for correcting the measured weak current, and the control software can be used to select or modify the parameters.

[0014] The control software can be used to conveniently select or modify the parameters used to modify the measured weak current, and the measurement results can be corrected with appropriate parameters at any time to obtain more accurate final measurement results. The control software also stores various parameters for correcting weak current measurement results, including temperature, pressure, and air density. Through the user interface, the operator can select correction formulas and parameters, such as selecting temperature parameters to correct air density. In free air ionization chamber measurements, the system will prompt to use 273.15K as the reference temperature, while in transfer ionization chambers, the reference temperature is 293.15K. The operator can manually modify these parameters to ensure that the corrected measurement results are more accurate.

[0015] The weak current source is an ionization chamber, and X-rays irradiate the ionization chamber, so that the air in the ionization chamber is ionized to generate the weak current.

[0016] The weak current generated by X-ray irradiation of the ionization chamber is relatively small, and the weak current measurement system of the present invention is particularly suitable for measuring such relatively small weak current.

[0017] The ionization chamber includes a temperature sensor for measuring the temperature inside the chamber and a temperature sensor for measuring the temperature on the optical path of the X-ray outside the chamber. The measured internal temperature is corrected with the measured external temperature to obtain a temperature measurement result.

[0018] Use dual-channel measurement to measure accurate temperature. The dual-channel results can supervise each other to avoid deviation in the measurement results.

[0019] The parameters include the temperature measurement result and the reference temperature for correcting the air density, and the reference temperature is selected or modified according to the type of the ionization chamber by using the control software.

[0020] As an application example of the weak current measurement system of the present invention, when the weak current measurement system of the present invention is used to measure the weak current generated by the X-ray irradiation ionization chamber, the control software can be used to conveniently select or modify the temperature parameters used to modify the measured weak current, and the measurement results can be corrected with appropriate temperature parameters at any time to obtain a more accurate final measurement result.

[0021] The weak current is used to calculate the air kerma, the parameter is a reference temperature for correcting the air density, and the control software is used to select or modify the reference temperature to correct the calculated air kerma.

[0022] The weak current measurement system of the present invention can not only measure the weak current generated by the X-ray irradiation ionization chamber, but also can modify the parameters used in the subsequent calculation of air kerma by using the control software, so as to obtain a more accurate air kerma calculation result.

[0023] Effects of the Invention

[0024] The weak current measurement system of the present invention can measure weak currents with high accuracy within a wide measurement range. On the other hand, the measured weak current can be properly corrected to obtain a more accurate final measurement result. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a schematic diagram of the structure of a weak current measurement system of the present invention.

[0026] Figure 2 It is a schematic diagram showing the structure of an ionization chamber.

[0027] Figure 3 An example of the control software interface of the present invention is shown.

[0028] Figure 4 It is a schematic diagram of the structure of a weak current measurement system in the prior art. DETAILED DESCRIPTION

[0029] The specific implementation modes of the present invention are described below to facilitate those skilled in the art to understand the present invention. However, it should be clear that the present invention is not limited to the scope of the specific implementation modes. For those of ordinary skill in the art, as long as various changes are within the scope of the claimed rights and the determined spirit and scope of the present invention, these changes are obvious, and all inventions and creations utilizing the concept of the present invention are protected.

[0030] In the fields of ionizing radiation detection, semiconductors, electrostatic detection, space radiation detection, biotechnology, etc., it is necessary to measure less than 10 -6 For example, in a state without light irradiation, the dark current flowing in a photodiode is as small as 10 -9 A, the weak current generated by the activity meter and air kerma meter used in the field of ionizing radiation detection is as small as 10 -12 A. The weak current measurement system of the present invention can accurately measure currents as small as 10 -12 A weak current.

[0031] The weak current measurement system of the present invention uses a high-gain negative feedback method to measure weak current. The following first describes the circuit structure and principle of measuring weak current using the high-gain negative feedback method in the prior art.

[0032] like Figure 4As shown, the weak current measurement circuit 1000 of the prior art includes: a high-gain negative feedback amplifier 1001, a feedback capacitor 1002 connected between the inverting input terminal and the output terminal of the high-gain negative feedback amplifier 1001, an electrometer 1003 connected between the grounded positive input terminal and the output terminal of the high-gain negative feedback amplifier 1001, a weak current source 1004 connected in series between the positive input terminal and the inverting input terminal of the high-gain negative feedback amplifier 1001, and a measurement result display unit 1005 not shown.

[0033] The high-gain negative feedback amplifier is a typical operational amplifier, such as an operational amplifier used in an electrometer, which has a high input impedance and the input terminal maintains the collector almost at ground potential. 14 Ω, gain up to 10 5 , so the input voltage Vi of the high-gain negative feedback amplifier is ≈ 0, and no current flows through the high-gain negative feedback amplifier. Therefore, Vu=Vc, and the current passing through the feedback capacitor is equal to the weak current to be measured, that is:

[0034] I=C f *ΔV u / Δt

[0035] Among them, C f is the capacitance value of the feedback capacitor 1002, ΔV u is the voltage change of the feedback capacitor 1002 within the Δt time.

[0036] The feedback amplifier in this case has an inherent characteristic: the output voltage changes linearly with time, that is, ΔV u / Δt is a constant, which basically transfers all the charge generated by the weak current source within the Δt time to the measurement circuit. Because the input terminal is grounded, the slope ΔV u / Δt is a constant and is basically isolated from the input circuit, that is, the voltage on the feedback capacitor does not affect the current generated by the weak current source. If this is not the case, the charge accumulated on the feedback capacitor will increase exponentially.

[0037] The input terminal is grounded so that the current signal of the weak current source will not be affected by the amplification circuit. The linearity of the feedback capacitor charging during the measurement process is checked regularly by linear regression. According to the feedback working principle, the stray capacitance has little effect on the small current measurement. The stray capacitance of the feedback circuit is composed of the signal cable, amplifier and weak current source, among which the cable contributes the most. However, due to the use of a high-gain negative feedback amplifier, it is possible to use a very long signal cable. For example, if the signal cable is 8 meters long, if the capacitance ratio is 0.10nF / m, the total capacitance of the signal cable is 0.8nF, and the gain of the amplifier is 105 Under the condition of , the capacitance of the signal cable increases by about 0.008pF, which is negligible compared to the feedback capacitance of the order of 100pF.

[0038] In the above-mentioned prior art circuit structure for measuring weak current, a feedback capacitor 1002 is directly integrated in the circuit and is a capacitor with a fixed capacitance value. The weak current to be measured by the prior art circuit for measuring weak current is, for example, 10 -8 A-10 -10 A. 10 -10 A-10 -12 A relatively large range, the difference between the upper and lower limits is as large as 100 times. Using a feedback capacitor can accurately measure weak currents in a small range, but it is not possible to accurately measure weak currents in such a large range. In this regard, the present invention improves the circuit and specially designs the control software to accurately measure 10 -10 A-10 -12 A is a weak current in a larger range.

[0039] First, the hardware structure of the weak current measurement system 100 of the present invention is described.

[0040] like Figure 1 As shown, the weak current measurement system 100 includes a weak current measurement circuit 200 for measuring weak current, a weak current source (an ionization chamber in the embodiment) 300 for generating weak current, and a control host 400 (not shown) for comprehensively controlling the weak current measurement circuit 200 and the weak current source.

[0041] The weak current measurement circuit 200 includes: a negative feedback amplifier 201, which has a positive input terminal 201A, a negative input terminal 201B and an output terminal 201C, wherein the positive input terminal 201A is grounded; a feedback capacitor 202 connected between the negative input terminal 201B and the output terminal 201C; an electrometer 203 connected between the positive input terminal 201A and the output terminal 201C of the negative feedback amplifier 201, which is used to measure the voltage of the feedback capacitor 202; and a switch S connected between the positive input terminal 201A and the negative input terminal 201B of the negative feedback amplifier 201.

[0042] A weak current source 300 that generates a weak current I to be measured is also connected between the positive input terminal 201A and the reverse input terminal 201B. When the switch S is closed, the feedback capacitor 202 is discharged in the negative feedback amplifier circuit. When the switch S is opened, the weak current source 300 is connected to the circuit, and its weak current I charges the feedback capacitor 202. At this time, the timing starts, and the voltage at this moment is measured. After a specified time has passed, the timing ends, and the voltage is measured again at this time, thereby obtaining the voltage change of one cycle within the specified time. After repeated cycles, the measurement cycle ends.

[0043] In terms of hardware, the difference between the weak current measurement system 100 of the present invention and the weak current measurement circuit 1000 of the prior art is that the weak current measurement system 100 of the present invention includes a plurality of feedback capacitors 202, and different numbers of feedback capacitors 202 are set for different ranges, and at least two feedback capacitors 202 are set. When measuring, the integral effect of the current on the capacitor is mainly used, so when measuring a large current such as several hundred pA, a capacitor with a large capacitance value is selected, and vice versa, a capacitor with a small capacitance value should be selected.

[0044] The feedback capacitor is the key to the feedback and compensation system. Therefore, the capacitor used in the weak current measurement system 100 of the present invention is a high-precision standard capacitor with a constant capacitance value and a capacitance value traceability. The capacitance temperature coefficient is only 2×10 -5 / ℃. In order to minimize the impact of temperature and humidity on capacitors, the capacitors are placed in electronic moisture-proof cabinets.

[0045] in addition, Figure 1 Three feedback capacitors 202 are shown in the figure, but the number of feedback capacitors 202 is not limited to three, and a suitable number can be set according to the length of the range, as long as there are two or more feedback capacitors 202.

[0046] In the software aspect of the weak current measurement system 100 of the present invention, a control software is installed in the control host, and the control software is used to select the feedback capacitor for measurement. The control software includes a user interface that allows an operator to select a feedback capacitor suitable for different ranges. Before the measurement begins, the control software will perform a pre-measurement and automatically select the appropriate electrostatic meter range based on the current range of the predicted result.

[0047] The control host 400 may be constituted by a personal computer, for example, including a CPU, memory, hard disk, input / output devices, etc. The software is installed on the hard disk, and various controls and settings can be performed on the weak current measurement circuit 200 and the weak current source by running the control software.

[0048] The input and output devices include, for example, a mouse, a keyboard, a control panel, a display, etc. By displaying various buttons and options on the display, the operator can perform various controls and settings, or the operator can automatically perform various controls and settings according to the processes and parameters set in the software and stored in the control host 400.

[0049] When the feedback capacitor 202 is automatically switched by the control software, the weak current is first measured by the weak current measurement system 100, and the range of the weak current measured is known. Then, according to the correspondence between the preset current range and the feedback capacitor 202, the feedback capacitor 202 with the capacitance value corresponding to the range of the weak current measured is selected and connected to the weak current measurement circuit 200. Finally, the weak current is formally measured using the weak current measurement circuit 200 connected to the selected feedback capacitor 202, thereby accurately measuring the weak current using a suitable high-precision standard capacitor.

[0050] When the feedback capacitor 202 is manually switched using the control software, the weak current may not be pre-measured, but the weak current measurement personnel may select the feedback capacitor 202 to be connected to the weak current measurement circuit 200 based on experience, parameters of the weak current source, and other information. Of course, as in the case of automatically switching the feedback capacitor 202, the weak current measurement system 100 may be used to pre-measure the weak current first, and the weak current measurement personnel may be prompted to switch the appropriate feedback capacitor 202 based on the range of the predicted weak current. After manually switching to a certain feedback capacitor 202, the feedback capacitor 202 may be selected in the interface of the control software.

[0051] By selecting a suitable feedback capacitor 202 for measurement based on the magnitude range of the weak current to be measured as described above, accurate measurement results can be obtained in each range.

[0052] The weak current I as the measurement result is a very small physical quantity. In actual measurement, various factors will affect the accuracy of the measurement result. Take the measurement of the weak current generated by the ionization of the air in the ionization chamber by medium-energy X-rays as an example. Due to the structure and physical mechanism of the ionization chamber, the slightest changes in various conditions such as temperature, humidity, air pressure, air attenuation, electric field distortion, scattered fluorescence, etc. may have a significant impact on the weak current I. Therefore, in order to measure the weak current I more accurately, the weak current I must be corrected.

[0053] In the weak current measurement circuit 1000 of the prior art, the weak current I is corrected by measuring temperature, air pressure, etc., and the correction is performed using a fixed formula built into the measurement device. For example, the difference in temperature and air pressure causes the difference in air density, and the air density will affect the accuracy of the measurement of the weak current I. Therefore, it is necessary to consider the difference between the air density during measurement and the air density under standard conditions to correct the measurement result of the weak current I. The correction of air density in the prior art is as follows:

[0054]

[0055] The standard condition refers to the condition where the air pressure is one standard atmosphere (101.325 kPa) and the temperature is 0 degrees Celsius (273.15 K). In formula 1, ρ is the corrected air density, and ρ0 is the air density under standard conditions, which is 1.293 kg / m 3 , P is the air pressure (in kPa) measured when measuring weak current, and T is the temperature (in K) measured when measuring weak current.

[0056] The corrected air density ρ calculated by formula 1 can be used to correct the measurement result of the weak current I. In the prior art, no matter what kind of ionization chamber is to be measured, the fixed formula 1 is used to correct the air density.

[0057] The air ionization chamber for measuring the radiation size of X-rays includes a free air ionization chamber for realizing absolute measurement of X-ray exposure according to the definition of exposure dose, and a transfer ionization chamber for transferring the value. The above formula 1 is well applicable to the case of measuring the weak current of the free air ionization chamber as a reference ionization chamber. However, when used to measure the transfer ionization chamber, the formula 1 will cause errors.

[0058] The inventors have found that when measuring the transfer ionization chamber, using formula 1 to correct the air density ρ will cause errors. The standard situation in formula 1 is the situation where the air pressure is one standard atmospheric pressure (101.325 kPa) and the temperature is 0 degrees Celsius (273.15 K). If the standard situation in formula 1 is replaced with the air pressure being one standard atmospheric pressure (101.325 kPa) and the temperature being 20 degrees Celsius (293.15 K), the weak current from the transfer ionization chamber can be corrected more accurately. The details are as follows:

[0059]

[0060] The standard condition refers to the condition where the air pressure is one standard atmosphere (101.325 kPa) and the temperature is 20 degrees Celsius (293.15 K). In the formula, ρ is the corrected air density, and ρ0 is the air density at one standard atmosphere and 20 degrees Celsius, which is 1.205 kg / m3 , P is the air pressure (in kPa) measured when measuring weak current, and T is the temperature (in K) measured when measuring weak current.

[0061] The prior art does not involve the use of different numerical values ​​to correct the measurement results for different situations. The various formulas and parameters used for correction in the prior art are fixed, and there may be errors when the numerical values ​​are directly substituted for the correction calculation. However, the present invention can select and modify the formulas and parameters through software to perform corrections suitable for various situations, and can better correct the measurement results to obtain more accurate final measurement results.

[0062] Example

[0063] The weak current measurement system 100 of the present invention can not only accurately measure 10 -6 A to 10 -8 A is a weak current that can be accurately measured by a commercially available electrometer. -12 A to 10 -10 A is a small weak current, which is commonly used in the field of medium-energy X-ray measurement for medical diagnosis and radiotherapy. As an embodiment, the weak current measurement system 100 of the present invention is used to measure the weak current generated by medium-energy X-rays ionizing the air in the ionization chamber.

[0064] Medium-energy X-rays are used, for example, in X-ray radiography to assist medical diagnosis, which is commonly referred to as taking X-rays. To accurately measure the radiation level of medium-energy X-rays, an ionization chamber is usually used for indirect measurement.

[0065] An ionization chamber is a gas detector for detecting ionizing radiation. Its working principle is that when the detector is irradiated by X-rays, gamma rays, etc., the rays interact with the molecules in the gas to produce ion pairs consisting of an electron and a positive ion. These ions diffuse freely to the surrounding area. During the diffusion process, electrons and positive ions can recombine to re-form neutral molecules. However, if a DC polarization voltage V is applied to the collecting electrode and the high-voltage electrode that constitute the gas detector to form an electric field, the electrons and positive ions will be pulled to the positive and negative poles, respectively, and collected. The collected electrons generate an ionization current, which is the weak current that can be measured by the weak current measurement system 100 of the present invention. A series of corrections and calculations are performed on the measured weak current to calculate the radiation dose.

[0066] Figure 2 2 shows an ionization chamber used in the present invention as an example, wherein the radiation source detected by the ionization chamber is an X-ray source. Figure 2The ionization chamber 300 in the embodiment includes: a high voltage electrode 301, a collecting electrode 302 and a guard electrode 303. The high voltage electrode 301 is applied with a high voltage of, for example, 4000V, the collecting electrode 302 is arranged opposite to the high voltage electrode 301, and is used to measure the amount of electricity or current generated by the sensitive volume of the ionization chamber 300, and the guard electrode 303 is used to cut off the leakage current and guide the leakage current to the ground by bypassing the collecting electrode 302. The guard electrode 303 can also ensure that the electric field in the sensitive volume of the ionization chamber 300 has good uniformity, thereby accurately collecting the ionization charge.

[0067] The weak current measurement system 100 of the present invention is used to measure the weak current generated by the ionization chamber 300 when it is irradiated with medium-energy X-rays. In other words, the ionization chamber 300 is Figure 1 In the weak current source 300 shown in FIG. 1 , the weak current I to be measured is the current generated by the medium-energy X-ray ionizing the air in the ionization chamber 300 .

[0068] The high voltage electrode 301 of the ionization chamber 300 is fully charged after voltage is applied. When the X-rays from the X-ray source enter the ionization chamber 300 through the aperture, the gas molecules in the ionization chamber 300 are ionized by the X-rays, generating positive ions and low-energy electrons. The low-energy electrons combine with oxygen molecules in the air to become negative ions. The positive and negative ions are collected, resulting in a decrease in the charge on the ionization chamber electrodes. The decrease in charge is proportional to the dose, and the strength of the weak current I generated is proportional to the dose rate. The weak current I is introduced by the collecting electrode into the weak current measurement circuit 200 connected to the collecting electrode 302.

[0069] In addition to the ionization chamber 300 in which the weak current source is ionized by medium-energy X-rays, the hardware of the weak current measurement system 100 of this embodiment mainly includes a Keithley 6517B electrometer, a high-precision standard capacitor, a Stanford PS350 high-voltage DC power supply, a digital temperature humidity barometer, an electronic moisture-proof cabinet, and a computer serving as a control host 400.

[0070] In the weak current measurement system 100, the negative feedback amplifier 201 constituting the weak current measurement circuit 200 uses an amplifier built into the Keithley 6517B electrometer, which has excellent performance of high gain. In addition, the Keithley 6517B electrometer is not only used as a voltmeter for measuring the voltage of the feedback capacitor 202, but also because it has a high-voltage power supply function and can provide a voltage of 0-1000V to the outside, the Keithley 6517B electrometer can also be used as a high-voltage power supply required for the ionization chamber, and bias is applied to the ionization chamber that requires a bias setting lower than 1000V.

[0071] The Stanford PS350 high voltage DC power supply applies bias to ionization chambers with bias setting requirements higher than 1000 V, and its limit is set to 5000 V. For example, the design working bias of a free air ionization chamber can reach 4000 V. In this case, the Stanford PS350 high voltage DC power supply provides voltage to it and can prevent the ionization chamber from breaking down due to excessive voltage caused by incorrect operation.

[0072] The digital temperature, humidity and barometer measure temperature in two ways, and can measure the temperature inside the ionization chamber and the temperature outside (optical path). If the measuring beam energy is low, it is affected by the temperature and pressure of the air path, and the air path has a greater impact on the measurement result. The temperature measurement result can be corrected by the temperature outside the ionization chamber to reduce the impact of air disturbance on the measurement result. That is, the two-way results can supervise each other to avoid deviations in the measurement results.

[0073] Since the X-rays used by the medical X-ray machine to be measured are medium-energy X-rays, the weak current generated by ionizing the air in the ionization chamber 300 is at 10 -10 A-10 -12 A. Therefore, three high-precision standard capacitors with capacitance values ​​of 50pF, 300pF, and 1000pF are used. The capacitance values ​​of these high-precision standard capacitors are constant, the capacitance value traceability has been completed, and the capacitance temperature coefficient is 2×10 -5 / ℃, in order to minimize the impact of temperature and humidity on capacitance, these high-precision standard capacitors are placed in an electronic moisture-proof cabinet.

[0074] When measuring the weak current I from the ionization chamber 300, the weak current is first measured in advance, and the approximate range of the weak current measured in advance can be known. Then, according to the correspondence between the preset current range and each feedback capacitor 202, the feedback capacitor 202 with the capacitance value corresponding to the range of the weak current measured in advance is selected and connected to the weak current measurement circuit 200. The measurement can be automatically performed by the control software installed in the computer serving as the control host 400, or can be manually performed by the operator through the control software.

[0075] After the value of the weak current I is measured, the measured value is corrected according to the type of the ionization chamber. For example, by selecting a correction formula or inputting correction parameters on the display by the control host 400, the above formula (1) is used for correction when the ionization chamber is a free air ionization chamber, and the above formula (2) is used for correction when the ionization chamber is a transfer ionization chamber, thereby obtaining an accurate value of the weak current.

[0076] Figure 3An example of a software interface displayed on a monitor is schematically shown. As shown in the figure, the left side of the software interface is a parameter display bar, which displays the measurement method, capacitance value of the feedback capacitor, correction temperature, etc. selected or input by the operator, and various parameters such as circuit parameters and high-voltage source parameters can be set by clicking the "Set" button. Figure 3 The upper dialog box in the dialog box is the dialog box displayed by clicking the "Settings" button. The middle part of the software interface is the process display bar that shows the measurement process, which can display the measurements in the form of curves or discrete points. On the right side of the software interface are the voltage source (high voltage source) setting bar and the weak current value display bar. In the voltage source setting bar, you can select the voltage source, set the voltage value of the voltage source, and switch the voltage source. In the weak current value display bar, the measured weak current value is displayed in digital form.

[0077] The operator can select and modify various items or modify parameters in the software interface, and can replace and select correction formulas and parameters suitable for different ionization chambers, thereby further improving measurement accuracy.

[0078] After the weak current measurement system 100 of the present invention is used to measure the weak current generated by the medium-energy X-rays ionizing the air in the ionization chamber 300, the weak current can be used to further calculate the air kerma that characterizes the radiation magnitude.

[0079] The size of air kerma is proportional to the weak current, and also needs to be corrected by parameters such as air density. By using the control software of the present invention, the size of various parameters can be easily adjusted. On the basis of measuring the accurate weak current, for example, a more appropriate air density can be used to correct the calculated air kerma, so that accurate air kerma can be obtained.

[0080] It should be understood that although this specification is described according to implementation methods, not every implementation method contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

[0081] The technical solutions disclosed in the present invention and the technical details disclosed in the embodiments are merely illustrative of the inventive concept of the present invention and do not constitute a limitation on the technical solutions of the present invention. Any conventional changes, substitutions or combinations of the technical details disclosed in the embodiments of the present invention have the same inventive concept as the present invention and are within the scope of protection requested by the present invention.

Claims

1. A weak current measurement system, characterized in that: include: A weak current source for generating the weak current to be measured; A weak current measuring circuit for measuring the weak current generated by the weak current source; and A control host that controls the operation of the weak current source and the weak current measurement circuit, Wherein, the weak current measurement circuit comprises: A negative feedback amplifier having an inverting input terminal, an output terminal and a grounded positive input terminal; an electrometer connected between the positive input terminal and the output terminal of the negative feedback amplifier; and a plurality of feedback capacitors connected between the inverting input terminal and the output terminal, Based on the magnitude range of the weak current to be measured, one feedback capacitor is selected from the plurality of feedback capacitors and connected to the weak current measurement circuit for measurement.

2. The weak current measurement system according to claim 1, characterized in that: Control software is installed in the control host, the feedback capacitor is manually switched and the switched feedback capacitor is selected in the control software for measurement.

3. The weak current measurement system according to claim 2, characterized in that: The control host stores parameters for correcting the measured weak current, and the control software can be used to select or modify the parameters.

4. The weak current measurement system according to claim 3, characterized in that: The weak current source is an ionization chamber. X-rays irradiate the ionization chamber, so that the air in the ionization chamber is ionized to generate the weak current.

5. The weak current measurement system according to claim 4, characterized in that: The ionization chamber includes a temperature sensor for measuring the temperature inside the ionization chamber and a temperature sensor for measuring the temperature on the optical path of the X-ray outside the ionization chamber. The measured internal temperature is corrected by the measured external temperature to obtain a temperature measurement result.

6. The weak current measurement system according to claim 5, characterized in that: The parameters include the temperature measurement result and a reference temperature for correcting the air density, The control software is used to select or modify the reference temperature according to the type of the ionization chamber.

7. The weak current measurement system according to claim 4, characterized in that: The weak current is used to calculate the air kerma, The parameter is the reference temperature used to correct the air density, The control software is used to select or modify the reference temperature for correcting the calculated air kerma.

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