Design Of Four-channel Synchronized Energy Spectrum Measurement System And Energy Spectrum Analysis

At present,CdZnTe detector are widely used in nuclear radiation detection,cosmic ray detection,medical PET and SPECT imaging.In the field of imaging,on the one hand,CdZnTe detector is evolving towards multi-pixel array structure,on the other hand,its readout system is evolving towards integration,multi-channel and digital direction.Charge sharing is an important factor which restrict the imaging resolution of multi-pixel array CdZnTe detector.In this paper,a novel readout circuit system of CdZnTe detector based on pixel array is designed by using the existing ²⁴¹Am radiation source and four-pixel CdZnTe detector in the laboratory.It improves the resolution of single-pixel spectrum.Meanwhile,it has smaller system volume,more channels and higher digitization degree.In addition,the readout system is used to test and analyze the charge sharing problem of the pixel array detector creatively from the perspective of synchronous measurement and photon position density map.It provides experience and guidance for the readout system optimization and charge sharing test of CdZnTe detector with large array pixels.The four-channel energy spectrum measurement system includes the amplitude spectrum measurement system and the width spectrum measurement system.The near Gauss pulse signal which is from the front-end processing circuit board are measured by the amplitude spectrum measurement system and square wave signal are measured by the width spectrum measurement system.Then frame discrimination,statistics and processing are carried out.Finally,the energy spectrum and photon position density are displayed.Amplitude spectrum measurement system is composed of ADC processing circuit,FPGA circuit,USB and PC software.Width spectrum measurement system is composed of FPGA digital circuit,USB and software.ADC processing circuit includes power supply circuit,reference circuit,ADC circuit,single-ended differential circuit and signal conditioning circuit.It amplify nuclear Gauss pulse to meet the input range of ADC and convert to digital signal.It only used in amplitude spectrum measurement system.There are many similar modules in amplitude spectrum measurement system and width spectrum measurement system,including frame recognition module,data aggregation module,USB control module.Meanwhile,two systems are similar in frame synchronization recognition method.In addition,a new single threshold real-time peak detection method is proposed in FPGA,which detect the pulse amplitude of nuclear signal in real time.The threshold is adjustable and the peak detection efficiency is high.The communication between FPGA and PC is USB,and the two systems use the same USB firmware.The two systems upload the same frame data format in FPGA,so it share the PC software.The software includes data processing,array spectrum and summary spectrum display,photon position density map display,file management,filtering and other functions.It can display the real-time energy spectrum and photon position density map of the radiation source.At the same time,it control system by sending instructions and threshold data to the FPGA.The following aspects are tested experimentally:?1?The energy spectra of the two systems under 20s are measured by using sinusoidal and square wave signals with a frequency of 50kHz.The total counts,spectral resolution,phase drift and measurement error rate are analyzed.?2?The single channel of the amplitude spectrum system is tested by ²⁴¹Am.The reasonable threshold is about 330?the amplitude is 161.0mV?,and the location of the noise peak in the amplitude spectrum is determined.?3?The same channel of the amplitude spectrum system and the width spectrum system is tested by ²⁴¹Am,detector bias 650V,measuring time 600s.Combining with the test results of analog signals,it is proved that the width spectrum system is superior to the amplitude spectrum system in terms of volume,total counts,phase drift,detection efficiency and measurement error,although it is slightly lower in spectral resolution than the amplitude spectrum system.?4?Using the optimized front-end processing circuit board,the four-channel signal is tested synchronously through the width spectrum system.Detector bias 700V and measurement time 600s.Four channels of energy spectrum are obtained with resolution of 5.6%,6.0%,5.9%and 5.7%,respectively.?5?Set the frame recognition time to 0.1?s.The photon position density map is obtained by?4?.The effect of charge sharing on four-pixel CdZnTe detector is visualized.By dividing the photon position density map region,the charge sharing rates of the four pixels are calculated to be 0.84%,0.99%,0.93%and 1.05%,respectively.By comparing primary spectrum and energy spectrum after charge sharing elimination,it is confirmed that ²⁴¹Am has less influence on charge sharing of four-pixel CdZnTe detector.