0 Introduction
Nuclear spectrum radiation measurement technology is a highly comprehensive technology. It is the product of the cross-penetration of multiple disciplines such as nuclear detection technology, electronic technology, and computer technology. It has the characteristics of on-site and multi-element rapid analysis. Nuclear spectrum radiation measurement technology has not only been used in nuclear research, but also plays an increasingly important role in geology, medicine, environmental science, biology, chemistry, archaeology and other disciplines. Since the pulse signal amplitude output by nuclear radiation detectors such as scintillation counters and semiconductor detectors is proportional to the energy of the incident particles, the energy of the radiation can be known by measuring the amplitude of these pulses. However, the measurement of pulse amplitude is an important issue in nuclear spectrum radiation detection.
The multi-channel pulse amplitude analyzer can not only automatically obtain energy spectrum data, but also obtain the entire energy spectrum in one measurement, thus greatly reducing the data acquisition time. At the same time, its measurement accuracy is also significantly improved. Since the 1950s, the multi-channel pulse amplitude analyzer has developed rapidly and has now become a universal instrument for obtaining nuclear energy spectrum data.
Traditional nuclear geophysical data acquisition systems are mostly designed with discrete components and 8-bit single-chip microcomputers in hardware, so they have high power consumption, complex design, small memory capacity for storing data, low data transmission rate and are difficult to debug; and in software design, they are mostly implemented in lengthy and cumbersome assembly language, with low design efficiency, poor portability and difficult performance guarantee. With the development of electronic technology, the continuous introduction of some new low-power integrated circuits, ASIC integrated circuits, microprocessor technology and computer technology has made the functions of nuclear geophysical data acquisition systems increasingly perfect and powerful, and also provided the necessary conditions for the development of nuclear geophysical data acquisition systems in the direction of lightness, intelligence, microcomputer integration and networking.
The task of multi-channel analysis is to divide the measured pulse amplitude range into 2n amplitude intervals, then measure the number of input pulses in each amplitude interval, and finally obtain the pulse amplitude distribution curve of the input signal. Its measurement adopts A/D analog-to-digital conversion and data storage technology in computer technology.
A data buffer is opened in the computer's memory. There are 2n counters in the data buffer. Each pulse amplitude interval has a corresponding counter in the data buffer. When analyzing the amplitude of multiple pulses, the pulse signal to be analyzed can be first sent to the analog-to-digital converter under the control of the microprocessor, and a digital quantity (channel address) representing the pulse amplitude is formed through A/D conversion. Then the microprocessor converts the digital quantity into the corresponding counter address. And the counter content corresponding to the address is increased by one (reflecting the increase of the channel count). In this way, after a period of measurement, the number of counts of each counter in the counter buffer in the memory can reflect the amplitude distribution of the input pulse.
1 Structure of multi-channel pulse amplitude analyzer
A complete nuclear geophysical instrument can usually be divided into two parts: nuclear radiation detector and embedded system. The multi-channel pulse amplitude analyzer is the core part of the embedded system. On the one hand, the multi-channel pulse amplitude analyzer collects the signal from the amplifier and performs analog-to-digital conversion, and stores the conversion results at the same time; on the other hand, it performs data analysis on the stored conversion results and directly displays the spectrum, or sends it to the computer through the computer interface for data processing and spectrum display.
The design structure block diagram of the multi-channel pulse amplitude analyzer introduced in this article is shown in Figure 1. When the pulse signal passes through the discrimination circuit and the control circuit, the discrimination circuit gives the pulse peak information and starts the A/D conversion. The A/D conversion circuit can perform analog-to-digital conversion on the peak amplitude of the pulse signal and store the conversion result in the on-chip Flash, and then the microcontroller performs corresponding data processing.
2. Hardware Design of Multi-channel Pulse Amplitude Analyzer
2.1 Pulse Linear Main Amplifier
The multi-channel pulse amplitude analyzer consists of a discrimination circuit, a control circuit, a sampling and holding circuit, an analog-to-digital conversion circuit, and an ARM embedded system. Its control core is an embedded system. Its basic function is to classify and count the input pulses according to their amplitude. The multi-channel pulse amplitude analyzer divides the pulse amplitude range that can be analyzed into multiple amplitude intervals. The number of amplitude intervals is the number of channels of the pulse amplitude analyzer, and the width of the amplitude interval is the channel width of the pulse amplitude analyzer. The more channels there are, the more precise the amplitude distribution analysis is, the fewer the counts of each channel are, the longer the measurement time is, and the more complex the hardware circuit is. Therefore, the number of channels should not be pursued blindly. It is usually required that there should be 5 to 10 channels within the half-width range of the amplitude peak. For a multi-channel spectrometer using a NaI detector, due to its poor energy resolution, 128 to 256 channels can often meet the measurement requirements. For a semiconductor detector, 1024 to 8196 channels are required. This paper uses a semiconductor detector and a 12-bit AD converter, with a total of 4096 channels, but uses a parallel channel method to display 1024 channels.
The main amplifier should be placed between the preamplifier circuit and the discrimination circuit, but gain adjustment is required to compensate for changes in the pulse amplitude output by the nuclear radiation detector. Since the pulse signal amplitude output by the detector is relatively small (tens of millivolts to hundreds of millivolts) and the pulse width is relatively narrow, in order to perform signal amplitude analysis and achieve energy spectrum measurement, a pulse linear amplifier is usually required to linearly amplify the pulse signal amplitude and perform pulse shaping. In view of the pulse characteristics, the amplifier is required to have the following technical indicators:
First of all, the amplification factor should be determined according to the input pulse amplitude of the amplifier and the required output amplitude. Because the amplitude of the electrical pulse signal output by the preamplifier can generally be adjusted to about several hundred millivolts, and the amplifier output pulse amplitude is in the range of 1 to 5V, its amplification factor should be about 10 times. Considering the difference in the signal amplitude output by the preamplifier, its amplification factor should be adjustable.
The second is the bandwidth of the amplifier. Since the pulse width of the preamplifier output will be affected by the relevant circuits and is generally several μs, the bandwidth of the amplifier is required to be 1 to 2 MHz.
The third is the noise of the amplifier. Considering that the amplitude of the signal from the preamplifier is relatively small, the input noise of the selected amplifier should be as small as possible. Generally, the selection of low-noise operational amplifier components can effectively reduce the inherent noise inside the circuit.
In addition, factors such as the input impedance, anti-counting overload, amplifier stability, and power consumption of the amplifier should also be considered during circuit design and debugging. Since the α pulse signal has a pulse width of about 1 to 2 microseconds after shaping, and the γ pulse signal has a pulse width of about 3 to 5 microseconds after shaping, the conversion speed of the operational amplifier should be considered when selecting the operational amplifier. The operational amplifier of this system is CA3140, which has the characteristics of high input impedance, low noise, low power consumption, and small temperature drift.
2.2 Peak detection circuit
The peak detection circuit consists of two parts: the discrimination circuit and the control circuit. The discrimination circuit is used to detect the signal timing, and the control circuit controls the analog switch and ADC conversion according to the timing of the discrimination circuit. The control circuit must be strictly combined with the timing of the discrimination circuit to complete the peak detection task.
Since the pulse signal amplitude output by the nuclear radiation detector is proportional to the energy of the incident particles, the energy of the radiation can be known by measuring the amplitude of these pulses. It can be seen that pulse amplitude measurement technology is an important issue in nuclear energy spectrum measurement. The discrimination circuit needs to solve three signal-related information: one is the information of exceeding the threshold signal; the second is the peak time information, that is, the time information of starting ADC conversion; the third is the time information of ADC completing conversion. There are also three key issues in the discrimination circuit, which should be paid attention to in the research:
First, since the pulse width of the α and γ rays output by the amplifier is relatively narrow (about 1μs to 5μs), and the ADC conversion speed selected in this system is 10μs, the peak value of the pulse signal needs to be peak stretched. The sampling and holding circuit requires a fast sampling speed so that the holding time can reach the ADC sampling time indicator.
Secondly, due to the randomness of pulse signals, in order to prevent missed counting due to too dense signals, this system uses an ADC with a conversion speed of 10μs. Therefore, theoretically, if two signals are within 10μs of each other, missed counting will occur. However, due to the existence of issues such as CPU processing speed, in reality, this time interval may be 3 to 10 times longer, that is, between 30 and 100μs (depending on the CPU processing speed and the amount of code), or even more. In other words, the probability of this happening in actual signals is very small, so this problem can be ignored.
In addition, it is also necessary to solve the problem of erroneous recording of amplitude signals caused by overly dense signals, and signals in the high-energy area may also be mistakenly counted as signals in the low-energy area, which can easily cause the low-energy counts to be larger and the high-energy counts to be smaller.
Figure 2 shows the schematic diagram of the discrimination circuit and the control circuit. The main function of the discrimination circuit is to complete the peak detection and remove the signal noise. The noise in the signal with energy less than the threshold can be removed by setting the closed value. After the peak value passes, the information is provided to the control circuit; the main function of the control circuit is to complete the control of the A/D read/conversion state. The control circuit can be composed of a 74HC74 trigger.
The specific working process of the identification and control circuit is that the embedded microprocessor control center first sends a signal to the control circuit to put the control circuit into working state. When the pulse signal reaches the multi-channel pulse amplitude analyzer, it is identified by the identification circuit, and after passing the peak value, the time information of the peak value passing is provided to the control circuit; thereafter, the control circuit starts the analog-to-digital conversion, and after the digital-to-analog conversion is completed, the embedded microprocessor control center generates an interrupt, and at the same time stops the control circuit from working, and performs corresponding data processing; after the interrupt is completed, the single-chip microcomputer sends a signal to put the control circuit back into working state.
At the beginning of sampling, ARM starts A/D by controlling 74HC74, then makes RD of U2A and RD and SD of U2B output high level, and the control circuit is in the state of receiving signals. When the energy of the rising edge of the signal is lower than the set closing voltage, the CLK terminal of U2A is low voltage, at this time, the RD and SD terminals of U2A are both high level, and the output terminal 5 pin keeps the original low level unchanged. When the energy of the rising edge of the signal is higher than the set closing voltage value, the CLK terminal of U2A is high voltage, and the output terminal 5 pin outputs high level to start U2B. When the pulse does not reach the peak, the voltage of the in-phase input terminal of the comparator U1B is lower than the voltage of the inverting input terminal, and the 6 terminal outputs a low voltage. After the peak, the 6 terminal outputs a high level, and the R/C outputs a low level to start the A/D conversion. After the conversion is completed, the ARM re-controls the A/D to collect the next pulse signal. The working process of the discrimination circuit and the control circuit is shown in Figure 3.
2.3 Analog-to-digital conversion circuit
The function of the analog-to-digital conversion circuit is to convert analog quantity into digital quantity and feed back the conversion result to the microcontroller. The multi-channel pulse amplitude analyzer is mainly used to sample the input nuclear pulse signal quickly and accurately, and convert the pulse amplitude value into a digital quantity that can be processed by the microcontroller. As a key component of the multi-channel pulse amplitude analyzer, the performance of the analog-to-digital conversion circuit directly affects the parameters such as the energy resolution and conversion accuracy of the entire system. Considering the main performance of the ADC chip of the multi-channel pulse amplitude analyzer (such as conversion speed, power consumption, conversion accuracy), this system uses AD7994 from AD Company, and adopts the "parallel channel" method in actual work. Every 4 channels are paralleled into 1 channel, and the nonlinearity of the channel width can be reduced to 1/4 of the original. This method can reduce the nonlinear error caused by the ADC itself. The specific circuit design is shown in Figure 4.
2.4 ARM microcontroller peripheral circuit design
LPC2134 is a serial peripheral interface chip (SPI) with full-duplex communication capability. An SPI bus can connect multiple slave devices and multiple master devices, but at the same time, only one master is allowed to operate the bus. This system uses the SPI interface to expand the Flash memory. The Flash memory uses ATMEL's AT45DB041. The connection circuit between ARM and the serial Flash chip AT45DB041 is shown in Figure 5.
In this system, ARM works in host mode. When ARM works in host mode, if the SSEL pin is at a low level, the SPIO module will be prohibited from working. Therefore, in order to ensure reliable operation of the system, although this pin is not used here, it still needs to be connected to the power supply through a pull-up resistor. The CS chip select terminal of the serial Flash chip AT45DB041 is controlled by ARM. WP is the write protection terminal. If enabled, the first 256 pages of the memory cannot be erased or rewritten. Since this system does not require this function, this pin is directly connected to a high level. Since the storage capacity of the microprocessor is limited and the computing function is not strong, it is often necessary to use a computer system when performing more complex data processing. Because serial communication has the characteristics of using fewer transmission lines and is suitable for long-distance transmission, this system uses a serial port to connect the computer and the microcontroller. The hardware circuit of the serial communication is shown in Figure 6. The serial port signals TXD and RXD are directly connected to the serial port of LPC2134.
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3. Design of software related to multi-channel pulse amplitude analyzer
ARM microprocessor software can be designed using foreground/background or super-loop systems. The application is an infinite loop, and calling the corresponding function in the loop can complete the corresponding function. This part can be regarded as background behavior. The interrupt service program can handle asynchronous events, which can be regarded as foreground behavior. The background can also be called the task level, and the foreground is also called the interrupt level. Critical operations with strong time correlation must be guaranteed by interrupt services. Because the information provided by the interrupt service can only be processed when the background program runs to the point where it can process the information. This software system is less timely in processing information than it can actually do. The timeliness of processing information is called task-level response time. The worst-case task-level response time depends on the execution time of the entire loop. Because the execution time of the loop is not a constant, the exact time for the program to pass a specific part is determined. Furthermore, if the program is modified, the timing of the loop will also be affected. After the program is initialized, it will enter the super loop to wait for interrupts. When an interrupt arrives, it will first protect the scene and then go to the interrupt service program. After processing, it will restore the scene and then return to the super loop and continue to wait for interrupts. It can be seen that interrupt handling is a relatively important task in the program.
3.1 Main function programming process
The entire application system software program can adopt a modular design method, including two modules of C language and assembly language. The main program flow chart is shown in Figure 7.
3.2 A/D conversion interrupt service routine
After the A/D conversion is completed, the BUSY terminal of AD7994 can request an interrupt to ARM, and ARM starts to read and save the data from the A/D conversion. The program flow is shown in Figure 8.
AD7994 is a 4-channel 12-bit ADC. In order to quickly process the A/D interrupt service routine, this system sets the A/D interrupt as a fast interrupt.
3.3 Serial communication software design
The serial port interrupt program flow chart in this system is shown in Figure 9.
4 Simulation Debugging
By performing simulation on PROTEUS software and changing different circuit parameters, the changes in waveform can be observed.
A rectangular wave signal can be generated by a function generator, and then the simulation program can be run to directly read out the waveforms. By changing the value of capacitor Cll, the amplitude of the output waveform can be changed. Then adjust different input signals for testing, and record the amplitude and waveform of the output waveform. The waveform amplitude data is listed in Table 1, and its waveform simulation is shown in Figure 10.
It can be seen from the figure that when the value of C11 is less than 160 pF or greater than 1.5 nF, the waveform begins to be distorted.
5 Conclusion
The hardware circuit designed in this article has passed software debugging and simulation, and can achieve the expected effect. The capacitance value of the integral capacitor is not good if it is too large or too small. Choosing different capacitance values on PROTEUS has great advantages. Since the amplification factor of CA3140 is equal to the highest signal frequency at a certain time (4.5 MHz), when the input signal frequency is high, the amplification factor of CA3140 will not be close to 1, which will affect the final result. Therefore, in order to ensure that the system can work normally when the high-frequency input is applied, the CA3140 can be replaced with the LM6161 with better high-frequency characteristics, which can improve the high-frequency characteristics of the system.
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