CN114578888A

CN114578888A - Photomultiplier gain stabilizing method

Info

Publication number: CN114578888A
Application number: CN202011415099.3A
Authority: CN
Inventors: 陈义新
Original assignee: Dalian Kexin Instrument Research Institute
Current assignee: Dalian Kexin Instrument Research Institute
Priority date: 2020-12-01
Filing date: 2020-12-01
Publication date: 2022-06-03

Abstract

A gain stabilizing method for photomultiplier tube features that at present, the number of used elements and devices is minimum, volume is minimum, cost is minimum, reliability is maximum, and it is simplified to an extreme PMT gain holding circuit.

Description

Photomultiplier gain stabilizing method

Technical Field

The invention relates to a method for stabilizing the gain of a photomultiplier, in particular to a method for stabilizing a visible light photoelectric detection system of an automatic component analyzer.

Background

A photomultiplier tube (PMT) is a highly sensitive detection device for converting weak light into an amplified electrical signal. A typical PMT includes an evacuated glass tube and a series of electrodes disposed within the tube. The series of electrodes includes a photocathode, a focusing electrode, dynodes (electron multipliers), and an anode where multiplied charge accumulates. In practice, a high voltage source is connected between the photocathode and the anode, which is at the highest potential, and the potential of the plurality of dynodes is successively increased. When multiple incident photons (incident light) strike the photocathode of the PMT, the photocathode emits multiple photoelectrons due to the photoelectric effect. The photoelectrons emitted from the photocathode are accelerated by an electric field and directed by the focusing electrode toward the series of dynodes. The electron multiplier multiplies the photoelectrons one at a time by a plurality of secondary emission processes. When these multiplied photoelectrons finally reach the anode, they are output as an electric signal. Due to the above multiple multiplication processes, even a very small photocurrent from the photocathode can provide a large output current at the anode of the PMT. This amplification, which may be referred to as "gain", depends on the number of dynodes, acceleration voltage, temperature, etc., and can typically be up to several million times, theoretically one photon can be detected. PMT is widely applied in the high-energy physical field due to its high sensitivity and quick response characteristics, and is also widely applied in the fields of weak chemiluminescence and fluorescence analysis.

However, the gain of the PMT is not stable and unchangeable, but changes along with the influence of a plurality of external factors and self factors, wherein the maximum influence is the influence of negative high voltage of power supply, the literature reports that the gain of the photomultiplier changes by 15% when the negative high voltage changes by 1%, so the stability requirement of the negative high voltage power supply matched with the PMT changes by not more than 0.02%, the negative high voltage power supply manufactured according to the requirement needs to use a constant-temperature high-precision reference voltage source, and a large pile of sampling resistors wound by manganese copper wires with the minimum temperature coefficient has large volume and high manufacturing cost, and is very inconvenient to use. How to compensate the temperature coefficient of the PMT itself is also quite complicated, and it is usually to measure the actual temperature coefficient of the PMT in advance, measure the actual temperature of the use environment where the PMT is located, and perform a temperature coefficient compensation operation.

Although this is done, there is a very important feature that is difficult to compensate by known methods, namely that the PMT, like all the photoreceiving elements, has a "light fatigue" phenomenon, which is characterized by: the PMT, when it is in the dark, has a very high photoelectric sensitivity, which drops sharply after being irradiated by light, with different results being certainly obtained at intervals between the two measurements. Since such measurement is open-loop, the gain is uncontrollable and becomes an important factor for instrument performance drift.

In the automatic testing and control technology, the use of a reference source is the most important and basic part, and the test reference necessary for analog comparison or A/D conversion is provided from the early standard battery to various high-precision reference voltage sources. A similar method can be provided to adjust the gain of the PMT to a relatively stable state, and many have long proposed closed-loop gain adjustment systems based on automatic control using a reference radiation source as a reference, and have applied numerous patents for similar principles, to list only a few more typical examples:

application number: 201010228915.X detects the gain change of the scintillation detector based on the reference radiation source, determines the gain change amount, and outputs a control signal to compensate the gain change; and stabilizing the gain relative to the reference radiation source based on the control signal.

Application No.: 200310117175.2 amplifying the energy spectrum pulse signals generated by the low-energy reference source and the measured source; and respectively detecting the two amplified energy spectrum pulse signals to adjust the gain of the system.

③ application no: 200610171576 when the LED inside the X scintillation detector emits light, the photon is conducted to the photomultiplier through the scintillator, the control calculating unit controls the luminous intensity and quantity of the LED according to the need, and the luminous intensity of the LED is automatically compensated according to the detection parameter of the temperature sensor; the digital preamplification circuit unit carries out A/D conversion according to the detected LED luminous parameters, then compares the A/D conversion with the quantity and the intensity position which should be obtained by a normal detector system, calculates the variation of each parameter of the detector system through the comparison obtained difference, and then automatically adjusts the digital gain and the digital threshold setting amount through the system so as to enable the system to recover to a normal state and realize the self-detection and the self-calibration of the system.

Application number: 200710138443.7 the scintillation crystal contains a calibration source that includes a small number of effective sources of known energy spectrum radiation. Such as cesium-137 because such source materials emit substantially monochromatic gamma photons of energy 662keV, control is applied to the high voltage source by a controller capable of executing an embedded computer program.

Application number: 201720906974.5 the device comprises a calibration source, at least two comparators, a counting module, a temperature sensor and a single-chip microcomputer, wherein each comparator is in communication connection with the photoelectric device to convert the analog voltage signals of different energy sections into digital pulse signals; the counting module is respectively in communication connection with each path of comparator and is used for measuring the counting rate of the digital pulse signals; actually measuring surface temperature data of the scintillation detector by a temperature sensor; the single-chip microcomputer is in communication connection with the counting module and calculates target gain and correction voltage according to the counting rate and the actually measured temperature data; the high voltage power supply is connected to the one-chip microcomputer to receive the correction voltage.

As represented by the above examples, various known closed-loop gain adjustment systems based on automatic control technology using a reference radiation source as a reference employ, in most cases and not for all, a processor-based controller capable of executing an embedded computer program, specially developed software, an essential signal pre-amplifying (conditioning) circuit, a high-resolution a/D converter, a high-precision reference voltage generator, and finally a high-resolution D/a converter, and the processor-based controller converts an operation result obtained by a complex operation into an analog signal to control and output an adjustable high-voltage power supply to perform gain adjustment.

Such a regulation system, the circuit and the specialized software thereof are very complex, and for some common instruments, the hardware cost is relatively high, the volume is too large, the software development cost is too high, and the power consumption of the circuit is too large. Since the input photoelectric signal is an analog quantity and the output is an analog quantity, the regulation system is simplified to the utmost extent because the analog operation cannot be directly carried out.

Disclosure of Invention

According to the thought, the invention compares the pure analog operation scheme or the full-digital full-intelligent operation scheme comprising the A/D and D/A conversion circuits in detail in all aspects, removes the factors which need to be considered in the application of the physical cost of parts, the development cost of special software, the volume, the power consumption and the like, and has the most important precision factor. The existing calculation and control technology is not a problem in designing and manufacturing a control part of a 16-bit (resolution 1/65536) or even 20-bit (resolution 1/1048576) A/D and D/A conversion circuit, and the problem is that it is very difficult to manufacture an analog resistor with one-ten-thousandth of precision, especially to manufacture a large stack of completely consistent, extremely high-precision and extremely-small temperature coefficient analog resistor networks on a small chip, which is almost impossible, and the high resolution is not equal to the high precision which can be achieved by practical application due to the practical influence of various performance factors such as the precision of a reference voltage source, temperature drift, noise and the like. This is why in practical applications 12 bit (resolution 1/4096) or even 10 bit (resolution 1/1024) a/D and D/a conversion circuits are more used. The most commonly used A/D and D/A conversion circuits adopt the input and output voltage standard of 0 to +5V, for 12 bit A/D and D/A conversion circuits, the minimum resolution is equivalent to 1.22mV, which is very coarse, for example, 0 to +5V output voltage is used to control the negative high voltage power supply of 0 to-1500V output, 1.22mV control voltage is corresponding to the variation of 0.366V negative high voltage output, when actually applied at-500V, is equivalent to 1/1366, in [0003], it is mentioned that the gain of PMT is changed by 15% for every 1% change in negative high voltage, so that the gain of PMT is changed by (1/1366) × 15 times, 1/91, corresponding to 1.1%, which has a great influence on the value of the light intensity to be measured, which, in other words, cannot be finely adjusted. Even for nominally 16 bit a/D conversion circuits, the minimum resolution is equivalent to 76.3 muv. The special software also needs to design a threshold value, and only if the optical signal voltage generated by the reference radiation source and detected by the A/D conversion circuit through multiple sampling is compared with the preset voltage to generate a certain deviation, and the set threshold value is confirmed to be broken through, the control part can take the action of correcting the deviation, and theoretically, the threshold voltage is undoubtedly greater than the minimum resolution of the A/D conversion circuit. It is difficult for the digitizing system to accurately adjust the gain of the PMT. For example, in the application effect provided by the background material iv, it is stated that when the typical voltage applied to the photomultiplier is in the range of 800 to 2200V, the voltage output of the high voltage power supply can be adjusted to the accuracy of 1-5V, which is a very good level in the applicant's opinion of the invention, but in the range of 800 to 1200V, the voltage output of the high voltage power supply can be adjusted to the minimum display bit number of 0.1V of the negative high voltage voltmeter, because the present invention is different from the above-mentioned digitizing system in that the resolution of the analog signal is infinite, the operational amplifier can correct for even a deviation voltage smaller than 1 μ V in time, and there is no threshold voltage to be set, so in the practical application, the effect is very desirable.

Drawings

Fig. 1 is a simplified block diagram of various known closed-loop gain adjustment systems based on automatic control techniques using a reference radiation source as a reference, as shown in fig. 1: the device is characterized in that 1 is a PMT, a controllable negative high-voltage power supply 2 with the output of 0 to-1500V is used for supplying power, a light input window of the PMT is provided with a reference radiation source 3 and a radiation source 4 with unknown light intensity to be measured, photocurrent output by the PMT is converted into digital quantity by an A/D converter 6 after being processed by a signal preamplification (conditioning) circuit 5, a processor-based controller 7 capable of executing an embedded computer program is input, a gain and threshold setting circuit 8 formed by specially developed hardware and software regulates and controls the controller 7, the digital output of the controller is converted into 0 to +5V analog voltage by a D/A converter 9, and then the output voltage of the negative high-voltage power supply 2 is controlled to complete a large closed-loop control system. Fig. 2 is a successful example of the invention developed in accordance with this concept and is also shown in block diagram form, 10 being a PMT, powered by a negative high voltage power supply 11 with a controlled output, and having, in the light input window of the PMT, a reference radiation source 12 and a radiation source 13 of unknown light intensity to be measured, all as in the part of fig. 1. Except that the regulating system 14 is a PI (proportional integral) regulator composed of an operational amplifier and a peripheral resistance-capacitance element, the output voltage of the PI regulator is transmitted to a voltage keeper 16 through a normally closed analog switch 15, and the output of the voltage keeper 16 is used for controlling the negative high-voltage power supply 11 to complete a large closed-loop feedback control system. Because the electric signal is straight-through, without any delay, without any A/D and D/A conversion circuit, and without the influence of the minimum resolution ratio on the adjustment precision, the method has no dead halt and no bug, and is simple and extremely simple.

Fig. 2 is extremely simplified compared to fig. 1. Only a double operational amplifier IC, half a PI regulator and half a voltage keeper, plus peripheral necessary resistance-capacitance elements and a miniature electromagnetic relay (if an analog switch IC is used, the volume is smaller), the total physical cost is less than 10 RMB, the volume is smaller than 1 RMB, the use effect is very ideal, and compared with a regulation system which is developed by the inventor before and is manufactured according to a known method, the technical indexes are not inferior, and even are much higher. This is also well understood: the performance of the prior high-precision operational amplifier is extremely close to that of an ideal amplifier, the gain is extremely high, the input impedance is extremely high, the temperature drift and offset voltage are extremely low, the noise voltage is extremely low, and the prior high-precision operational amplifier can be conveniently designed into circuits with various purposes. The resolution of the analog signal is infinite and the end result can be high precision. For many situations where there is no special program requirement and only the comparison and amplification of the output is performed, it is just about the handedness of the operational amplifier, and the application of the operational amplifier in the present invention is just a proper name. Fig. 3 is an electrical schematic of a preferred embodiment of the present invention. In the examples, the actual relationship between the photocurrent U1 to be tested and the standard voltage U2 formed by the reference radiation source (LED) is shown, and there are two schemes, scheme a is shown in fig. 4 and scheme B is shown in fig. 5.

Two design differences need to be explained: the design scheme is that the light intensity of a reference radiation source (LED) is adjusted to a certain common level and is stable and unchanged. The photocurrent (converted into voltage U1 in practical application) is compared with an adjustable standard voltage U2, and the obtained difference is amplified to become an output control voltage U3, which is applied to the control input terminal of a special high-voltage power supply for supplying power to the PMT to control the output voltage to increase or decrease, thereby achieving the purpose of adjusting the PMT. The design scheme is that the standard voltage U2 is stable and unchanged, and the light intensity of the reference radiation source (LED) is adjustable. The effects of the two designs are not significantly different in practical application.

Detailed Description

Fig. 3 is an electrical schematic of a preferred embodiment of the invention, which is simplified to the utmost and which is a most simplified embodiment of the invention, and additional improvements in accordance with this concept are also included in the scope of the invention. This preferred embodiment does not include positive and negative power supplies and a reference power supply, only 7 elements. In the

embodiments

17 and 18, the dual operational amplifier ICs with identical high input impedance are provided, wherein the left operational amplifier 17 and the peripheral resistor-capacitor element form a PI (proportional-integral) regulator, and amplify the very small difference obtained by comparing the photocurrent U1 with the standard voltage U2, which is the set PMT gain value, the standard voltage U2 increases the U2, the output voltage of the negative high voltage power supply increases accordingly, and although the light intensity of the reference radiation source (LED) does not change, the photocurrent U1 also increases due to the increase of the output voltage of the negative high voltage power supply, reaches equilibrium again, which means that the gain of the PMT increases, and vice versa. The positive input end of the operational amplifier 17 is grounded, the negative input end is connected with one end of the

resistors

19, 20 and 21 and one end of the capacitor 22, the other ends of the

resistors

19 and 20 are respectively connected with the input optical current U1 and the standard voltage U2, and the negative input end of the operational amplifier 17 forms a summing point. For simplicity of design, it is recommended that the

resistors

19, 20 use the same resistance values. The other ends of the resistor 21 and the capacitor 22 are connected to the output end of the operational amplifier 17, and form a feedback resistor and an integrator which are necessary for the PI operation. The operational relationship between U1 and U2,

resistors

19, 20, and 21, and output voltage belongs to the known technical field, and the description is not repeated here, and only two points are described here: in order to have strong deviation rectifying capability, the proportionality coefficient K (gain) of the circuit is designed to be large, preferably K is more than 1000, so that the resistance value of the resistor 21 is very large. Since the drift of the PMT system gain varies very slowly and the time constant of the integration operation is very large, the capacitance of the capacitor 22 matched to the resistor 21 is also large. The result of this design is that the output of the operational amplifier 17 has no voltage jump, but the very small difference (in microvolts) obtained by comparing the photocurrent U1 with the standard voltage U2 is also amplified sufficiently by integration over a period of time, and is timely regulated and controlled by the operational amplifier 17. Under the condition of proper parameter selection, even if the input U1 or U2 has mutation, the whole regulating system can achieve the complete balanced result within 2 to 3 oscillation periods, and completely meets the design and use requirements. The design is also set without adjusting the threshold value, the output control voltage tracks the voltage change of the addition point all the time and place, even the deviation less than 1 muV can be corrected in time, and the design is also superior to a digital adjusting mode.

The operational amplifier 18 is connected into an emitter follower output device with extremely high input impedance and extremely low output impedance, a holding capacitor 24 is connected at the input end to form a voltage retainer, the output voltage U4 is used for controlling the output negative voltage of the negative high-voltage power supply, the analog switch 23 is of a normally closed type and is controlled by a switching signal U3 of an instrument system, the output voltage of the operational amplifier 17 is cut off and isolated from the input end of the operational amplifier 18 at the moment before the system measures the light intensity to be measured, because the holding capacitor 24 is an organic film capacitor, the leakage of the organic film capacitor is extremely small and can be completely ignored, the input impedance of the operational amplifier 18 is extremely high, the organic film capacitor is connected into an emitter output device with higher input impedance, the electric quantity of the discharge capacitor 24 cannot be discharged, the voltage of the capacitor 24 cannot be detected to drop any more during the whole measuring period, and the output voltage of the negative high-voltage power supply is kept unchanged during the whole measuring period t1, the gain of the PMT also remains constant throughout the measurement period in a relatively short time. After the measurement period is finished, the analog switch 23 is switched on again, and the whole system adjusts the output voltage of the negative high-voltage power supply again according to the light intensity of the reference radiation source, so that the output voltage is balanced on the original level again.

The practical relationship between the tested photocurrent U1 and the standard voltage U2 formed by the reference radiation source (LED) is shown in the embodiment, there are two schemes, fig. 4 shows scheme a, the light intensity of the reference radiation source (LED) is unchanged, the formed standard voltage U2 is maintained at a lower level, usually not exceeding 10-20% of the tested maximum photocurrent (full scale value) U1, before the measurement, the analog switch 23 is turned off, so that the negative high voltage, i.e. the gain of the PMT, is kept unchanged, and after the measurement period is finished, the analog switch 23 is turned on again, so that the off time t2 is formed. During measurement, within a measurement window t1, the tested photocurrent U1 is superposed on the standard voltage U2, and the influence of the standard voltage U2 is processed by measurement-specific software.

Fig. 5 shows a scheme B, which differs from the scheme a in that before measurement, the analog switch 23 is turned off to keep the negative high voltage, i.e. the gain of the PMT, constant, and then the reference radiation source is shielded or the current (for the LED) is switched off, at which time the photocurrent U1 of the PMT immediately drops to very close to 0V, and only its weak pA-level dark current is negligible or processed by the measurement-specific software, before the measurement of U1 is performed. After the measurement period is over, the reference radiation source is restored (the LED is re-illuminated), the analog switch 2 is turned back on, and the whole system is restored. Therefore, in fig. 5, there are 3 time windows, t1 is the measurement window, the output is the value of the photocurrent to be tested, t2 is the time when the analog switch 23 is turned off, and t3 is the window time when the reference radiation source is shielded or turned off, when U1 drops to 0V, according to the operation procedure described above, t1 < t3 < t2, and the normal operation can be performed. Modern analytical instruments operate according to a fixed program, and it is only a matter of manual labor to introduce 2 control terminals (since the measurement window t1 is originally available) to control t2 and t 3.

In practical application, a dedicated voltage keeper IC with an internal analog switch, for example, model LF398, may also be selected to replace the voltage keeper formed by the operational amplifier 18 and the analog switch 23 in the embodiment, which not only further reduces the number and volume of components, but also is suitable for some special applications because the sampling time of LF398 is only 6 microseconds at the shortest. The luminous intensity of the LED as the reference radiation source is affected by the temperature, and how to control the luminous intensity and the quantity of the LEDs and automatically compensate the luminous intensity of the LEDs according to the detection parameters of the temperature sensor has been described in many documents, and the invention is not described in detail.

In summary, the present invention provides a PMT gain holding circuit with minimum number of components, minimum volume, minimum cost, maximum reliability, simplified to extreme, and ideal practical application in analytical instrument products due to its almost infinite resolution and extremely strong rectification capability, replacing the well-known complex gain holding circuit based on computer technology.

Claims

1. A PMT gain holding circuit based on computer technology is characterized in that a precise operational amplifier is used to form an analog regulating circuit, the deviation of a photocurrent value formed by PMT absorption of a reference radiation source and a set current value is subjected to proportional integral operation, the positive input end of an operational amplifier (17) is grounded, the negative input end of the operational amplifier is connected with one end of a resistor (19, 20, 21) and a capacitor (22), the other end of the resistor (19, 20) is respectively connected with an input optical current U1 and a standard voltage U2, the other end of the resistor (21) and the other end of the capacitor (22) are connected with the output end of the operational amplifier (17), the output end of the operational amplifier (17) is connected with the input end of a voltage holder (18) through a normally-on analog switch (23), the input end is also connected with a holding capacitor (24), the control end of the analog switch (23) is connected to and under the control of a central control system of a measuring instrument, the analog switch is turned off in the process of measuring the photocurrent by the PMT, the voltage retainer provides stable and non-fluctuating negative high-voltage output for the PMT, and the analog switch is turned off and returns to the on state after the process of measuring the photocurrent is finished.

2. A gain-hold circuit as claimed in claim, characterised in that the voltage keeper (18) is a general purpose high input impedance operational amplifier or a dedicated voltage keeper IC, such as model LF 398.

3. A gain-hold circuit as claimed in claim, characterized in that the holding capacitor (24) is an organic thin-film capacitor with a minimum leakage current.

4. A gain-maintaining circuit as claimed in claim, characterized in that the normally-on analog switch (23) can be implemented using a dedicated IC chip, or using a miniature electromagnetic relay, the former being smaller and faster but the latter being switched off with negligible leakage current.

5. The gain maintenance circuit of claim, wherein the scaling factor K (gain) of the operational circuit is designed to be > 1000 and the integration time is > 10S.