Development Of Multi-Channel Pulse Amplitude Analyzer Of Airborne Gamma-Ray Spectrum Measurement

Airborne gamma spectrum detection technology is to measure the gamma-ray emitted by radioactive substances on the surface and in the atmosphere during flight,which obtains the distribution of radionuclides by analyzing the gamma spectrum data.It has the advantages of high efficiency,remarkable exploration effect and low cost.In addition to the application in searching for radioactive occurrence,the technology routinely used in resource exploration and geological mapping.In recent years,it's also commonly used in the fields of monitoring environmental radioactive pollution and nuclear power facilities.The performance index and detection accuracy of airborne gamma spectrum measurement are directly related to the effect of airborne measurement.And the multi-channel pulse amplitude analyzer is the core module of airborne gamma spectrum measurement system,which straightforwardly determines the accuracy of the whole measurement system.In order to guarantee pulse pass rate in a very short measurement period,the airborne gamma spectrum measurement requires multiple detectors to work synchronously,a digital multi-channel pulse amplitude analyzer with synchronous acquisition function is designed.According to the characteristics of the output signal of the detector,the hardware circuit of the digital multi-channel pulse amplitude analyzer is designed.The main functions include: programmable gain adjustment circuit,high-speed analog-to-digital conversion circuit,main control chip circuit,communication circuit and power supply circuit.After the analog nuclear pulse signal output by the detector passes through the programmable gain adjustment circuit,it enters the analog-to-digital conversion circuit to realize the digitization of the analog signal.In the FPGA chip,the nuclear pulse energy spectrum acquisition function is realized through digital signal processing technology.The main functional modules include: synchronous acquisition control module,trapezoidal forming module,pulse accumulation processing module,peak extraction module,peak finding and spectrum stabilization module,and communication module.The digital trapezoidal algorithm is realized by cascading modules to accurately extract the pulse amplitude,improve the system signal-to-noise ratio,and have better energy resolution and good ballistic loss performance.In order to solve the problem of spectrum drift caused by environmental changes in the aerial measurement process,a third-order polynomial 5-point smoothing first-order derivative algorithm is used to smoothly denoise the energy spectrum data and perform peak-finding analysis,and then use a combination of software and hardware according to the degree of spectrum drift,Adjust the pulse amplitude through the programmable gain circuit,realize automatic spectrum stabilization,and ensure the long-term stable operation of the system.The dual-port RAM is used to seamlessly send and receive data,and the data is sent to the upper computer in real time through the serial port or serial bus for subsequent data processing and energy spectrum analysis.The digital multi-channel pulse amplitude analyzer can still maintain a high pulse pass rate when the signal generator generates 50 KHz exponentially decayed pulses.Use single-box Na I(T1)scintillator detectors indoors to measure the environmental background,¹³⁷Cs,²³⁸U,and²³²Th test sources respectively,to test energy linearity,window count rate,energy resolution,and system stability.According to the result test,the energy linearity is good.The window count rate and energy resolution are compared with the GR-820 produced by AGRS,and the test result is slightly better than the GR-820.During the 7-hour measurement,the gamma spectrum line drift is within ±1,which can work stably for a long time.