Study On Real-time Nuclear Pulse Readout System And Pulse Forming Method

For a long time,due to the limitation of communication technology,nuclear signals cannot be write into PC in real time,and the digital algorithm of digital spectrometer can only be implemented on digital chips such as FPGA.However,the deployment of digital algorithms on digital chips is difficult,the technical threshold is high,the development cycle is long,and the labor cost is high;digital chip resources are limited,and the implementation of algorithms is severely limited.When extracting multiple features,each feature can only be estimated roughly so the error is relatively large;As the signal data can only be kept for a short time,the long-term related information features cannot be extracted;the floating-point arithmetic capability is poor,then complex algorithms cannot be deployed.To solve this question,a real-time nuclear signal readout system is introduced to collect nuclear signal data in real time,and then implement nuclear signal digital processing methods and energy spectrum acquisition in MTALAB.This paper mainly contains three parts: hardware system introduction and signal characteristic analysis,nuclear signal real-time reading system program development and key technology of digital pulse program.Nuclear signal hardware system mainly includes SDD reset sensitive preamplifier,nuclear signal acquisition system and Zynq digital transmission system.The SDD detector mainly converts the radiation signal into an electrical signal and then the signal will be collected by the preamplifier.The signal output by the preamplifier is a step signal close to the step change of which each step indicates a detected nuclear signal.The nuclear signal acquisition system mainly separates the staircase signal and amplifies it to an amplitude which is suitable for ADC measurement.It mainly includes C-R differential circuit,cascade amplifier circuit and high-speed ADC.The CR differential circuit converts the staircase signal into a continuous exponential attenuation signal.The cascade amplifier circuit boosts the nuclear signal amplitude into the range which is suitable for the ADC.The ADC digitizes the nuclear signal,and then filtering,shaping,making a static of counting rates and amplitude with digital algorithms.Because the actual nuclear signal will have a certain charge collection time,all output signal shaped with C-R differentialcircuits exists a certain ballistic loss which could be suppress to a certain extent with the application of the later trapezoidal algorithm.Considering that each time the preamplifier is reset,the output voltage of the C-R differential circuit will have a relatively long negative voltage,making the entire measurement system overload.Therefore,in the later digital algorithm processing,it is necessary to repair the nuclear signal and correct the count rate in the energy spectrum.The Zynq system is a realtime data transmission hardware system.The system selects the model MZ7XCORE400 commercial grade system version produced by Millink.The digital transmission system is based on the ZYNQ high-performance digital chip.The main function of the design is transmitting digital nuclear signals into the PC in real time.The starting point of the digital nuclear signal is the ADC,then passes through the PL and PS unit of the Zynq chip,and finally it is transmitting to the PC via Gigabit Ethernet and is saving into the permanent storage device in the hard disk.The sampling clock of the ADC is 20 HMz,and the data transmission between PL and PS uses AXI bus communication.The bus supports DMA transmission,so it doesn't need to consume the PS system clock.AXI bus communication is frame transmission,with intermittent,and the communication clock is 100 MHz.Therefore,between the ADC and the AXI bus,an asynchronous ping-pong buffer module is designed to buffer the ADC data,and data leakage occurs when simulating the frame transfer of the AXI bus.The size of the ping-pong buffer is the same as the frame size of the AXI bus.After the AXI bus transmits the nuclear signal to the DDR3 of the PS,the digital nuclear signal can be transmitted to the PC using LWIP / TCP communication in the PS.Because there will be network delay in TCP communication,it may be tens to hundreds of milliseconds under bad network conditions.A multi-level buffer pool is designed to buffer data between AXI bus communication and Gigabit Ethernet communication to avoid data transmission confusion.The network conditions in the laboratory are relatively good.The buffer pool size in the multi-level cache mechanism is designed to be 1000 frames,and the network delay time is less than51.2ms,which can avoid data transmission confusion.Finally,a data receiving system is designed in the PC,and a data receiving sub-thread and a data storage sub-thread are designed based on the multi-threading mechanism,which improves the CPU utilization and reduces the risk of data blocking.The real-time nuclear signal data can be obtained by using the designed nuclear signal real-time acquisition system,and the key technology of digital pulse program can be realized in MATLAB to obtain the energy spectrum with high energyresolution under the circumstance of high-count rate.Based on the characteristics of the actual nuclear signal,a double exponential nuclear signal model is established,and then a double exponential trapezoidal algorithm and double exponential impulse shaping algorithm are constructed to extract the amplitude and count of the nuclear signal respectively.In the hardware circuit,when the preamplifier is reset,the nuclear signal will be damaged,so the exponential model is introduced to repair the nuclear signal,eliminating the false peaks that appear as a result of reset.In the nuclear signal recognition,statistical noise distribution is used to obtain the optimal threshold to identify the nuclear signal,and the problem of signal accumulation exists in the impulse shaping algorithm is analyzed.The trapezoidal signals are easy to accumulate,according to the different effects the accumulate of trapezoidal signals have in the amplitude,the accumulation characteristics peaks are divided into sum peak accumulation,platform accumulation,delayed accumulation and signal separation.The introduction of the accumulation rejection module is used to discard the accumulated nuclear signals,eliminate the accumulation platform in the acquired energy spectrum and reducing accumulation and peaks greatly.The last signal measurement dead time,reset dead time,is used to correct the count rate obtained in the fast channel,so that the measured count rate error is small.In combination with the fast and slow channel spectrum method,the energy spectrum in the slow channel is corrected to the actual energy spectrum,and then analyze the corrected total count rate of the energy spectrum and the total count rate in the full-energy peak.Regardless of the low or high count rate,the relative error between the measured total count rate and the true total count rate is relatively small;At the condition of low count rate,the relative error between the measured full-energy peak area and the true full-energy peak area is relatively small,but at the condition of high count rates,this error will increase greatly of which should be sum peak effect will reduces the full-energy peak area.