| The nuclear spectrum measurement system acquires information such as element content,position and time in the nuclear radiation field by energy spectrum analysis,and this information needs to be captured by a nuclear radiation detector.The highpurity germanium detector is a new type of semiconductor detector developed around the 1970 s.It has excellent energy resolution,high detection efficiency,wide energy measurement range and extremely low self-detection.The radioactive level and stable performance are widely used in high-precision measurement occasions such as nuclide identification,environmental monitoring and trace element analysis.Therefore,it is necessary to equip the multi-channel pulse amplitude analyzer to reflect its advantages.In the traditional analog multi-channel measurement system,due to the use of a large number of analog devices,the simulation bandwidth is narrow,the ballistic loss is serious,the processing speed is slow,the rate is low,and the dead time is large,which can not meet some high-precision measurement occasions..The digital multi-channel performance is superior and has become a development trend.However,due to the high cost and high power consumption of digital multi-channel,the spectral line noise is not as good as analog multi-channel.For those requiring low power consumption,low spectral line noise and high spectral line resolution.In the case of high counting rate,it is obvious that the traditional analog multi-channel and digital multi-channel cannot be satisfied at the same time.Therefore,this paper combines the advantages of analog multi-channel and digital multi-channel as much as possible to propose an analogdigital hybrid multi-channel design.The topic is derived from the National Key R&D Program(2017YFC0602100)“High-Resolution Aviation Gamma Spectrometry and Airborne Imaging Spectral Measurement Technology”.Based on the research results and theoretical basis of highpurity germanium detectors at home and abroad,combined with digital multi-channel Combining with the advantages of analog multi-channel technology,an analog-digital hybrid multi-channel with practical application value has been developed.The main results of the paper:1.Design of low-noise analog front-end circuit,including range switching and protection circuit,pole-zero cancellation circuit,polarity conversion circuit and hardware gain adjustment circuit with flexible adjustment signal,complete nuclear pulse output to high-purity germanium detector The signal performs the function of preliminary shaping filtering.2.Low-noise narrow-pulse fast-channel design eliminates the dead time of traditional analog spectrometers.The traditional analog spectrometer does not count the pulse signal output by the detector during the peak hold sampling of the nuclear pulse signal,that is,the dead time of the spectrometer,and the fast channel is designed so that the whole process is in the particle information output to the detector.Respond to eliminate dead time issues.3.Slow channel design with high SNR,reducing ballistic losses and increasing the energy resolution of the line.According to the optimal filter principle,the Gaussian shaping circuit is designed to ensure the signal-to-noise ratio of the output signal after the filter is formed,and the pulse signal amplitude is accurately extracted by the fast peak holder,so that the mixed multi-channel has higher precision and Resolution.4.Hybrid architecture design with high-speed and high-precision SAR ADC as the core.SAR ADCs have extremely low noise performance,especially for integral nonlinearity,differential nonlinearity,and signal-to-noise ratio over other types of ADCs.Through the timing control of the fast and slow channel hardware and software circuits,the SAR ADC is time-multiplexed,and the baseline estimation and peak sampling are realized in the analog circuit design to accurately extract the pulse signal amplitude..5.This paper adopts the FPGA solution based on SOC on-chip system,which replaces the implementation scheme of traditional single-chip microcomputer,has the advantages of fast and accurate timing control,and eliminates the slow processing speed of the traditional controller for the timing control of the peak sample-and-holder.Defects with increased dead time.6.System low noise design.From the aspects of power system topology,power chip selection,PCB circuit layout and circuit device parameter selection,etc.,the power supply is low noise,the noise level of the whole system is controlled within 1.5mVpp,and the power supply voltage input range is wide.It can support an input voltage range of 7V to 20 V.7.Perform the performance test on the hybrid multi-channel designed in this paper.The linearity is up to 0.999998 and the integral nonlinearity is as low as 0.065%.It has excellent energy response linearity.The P-type coaxial GEM40P4-76 high-purity germanium detector connected to ORTEC of the United States was used for energy spectrum measurement.The artificial radioactive source 137Cs(662 keV)was used for gamma spectroscopy,and the energy resolution was 0.21%.The test was performed using an artificial radioactive source 60Co(1332 keV),and the obtained energy resolution was 0.16%.. |
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