Research On Automatic Baseline Recovery Technology Of Digital Multichannel Spectra Measurement

Nuclear spectrum measurement system is of great significance to the application of nuclear technology and nuclear physics.It is mainly composed of nuclear radiation detector,pulse signal conditioning circuit,multi-channel pulse height analyzer?MCA?,data display module and power supply module.With the development of digital signal processing technology,nuclear spectrum measurement system is gradually digitized.At the same time,MCA is gradually replaced by digital multi-channel pulse height analyzer.As the core of nuclear spectrum measurement system,because of high precision,high resolution,digital control and other advantages,DMCA is widely used in digital nuclear spectrum measurement system.Recently,the research on the key technologies of DMCA has become an important role in the field of nuclear technology application.Until now,in DMCA related technologies,the research of signal baseline recovery technology is mainly composed of two parts: the analog baseline recovery technology and the digital baseline recovery technology.Among them,the analog baseline recovery technology can not accurately estimate the baseline level because of the fixed parameters.Although there have been many improved baseline restorer,the complexity of the circuit increases the difficulty of design,more interference factors are also introduced.The digital baseline recovery technology uses the digital signal processor to estimate and deduct the digital signal which is quantized by ADC.Due to the ADC conversion of the signal baseline drift phenomenon has always existed,and even more serious,it increases the complexity of the baseline restoration algorithm and consumes the design resources,which affects the accuracy of the baseline restoration.Aiming at the above problems,based on the research of analog and digital baseline recovery technology,after analyzing and comparing the advantages and disadvantages of various methods,this paper proposed a new method based on DMCA.The method consists of two parts: digital baseline estimation algorithm and DAC feedback baseline recovery circuit.FPGA is used to implement digital baseline estimation algorithm,In order to simplify the algorithm complexity and improve the real-time performance of online analysis,multiple continuous sampling is used to obtain the minimum value,and then the average value is obtained,which is sent to MCU as the current baseline level.Based on the MCU,the baseline drift threshold setting and the baseline drift estimation algorithm over threshold range are realized.The baseline drift is given by comparing the baseline estimate and the setting threshold.By controlling the precision DAC output value,the nuclear pulse signal before the ADC is restored.The problem of baseline drift is solved from the analog pulse signal processing circuit,and the baseline error in the digital signal converted by the ADC is reduced.The DMCA test platform is used to test the¹³⁷Cs and ⁶⁰Co radioactive source samples.The baseline automatic recovery technique in this paper shows that the energy spectrum resolution of ¹³⁷Cs is about 6.55%,and the 0.661 MeV characteristic peak position is at 456-channel.The spectral resolution of the ¹³⁷Cs and ⁶⁰Co mixed nuclideenergy spectra was about 6.69%,and the 0.661 MeV characteristic peak position is at 456-channel.The method of digital baseline subtraction based on FPGA shows that the energy spectrum resolution of ¹³⁷Cs is about 6.62%,and the 0.661 MeV characteristic peak position is at 457-channel.The spectral resolution of the ¹³⁷Cs and ⁶⁰Co mixed nuclideenergy spectra was about 6.97%,and the 0.661 MeV characteristic peak position is at 445-channel.The method of baseline automatic recovery technique inthe paper indicate that the energy resolution of the system is better than 6.7%,and the peak drift error of ¹³⁷Cs 0.661 MeV peak is 2 channels.The method of digital baseline subtraction based on FPGA shows that the energy resolution of the system is better than 7.0%,and the peak drift error of ¹³⁷Cs 0.661 MeV peak is 12 channels.The design of automatic recovery technology based on DMCA is realized.Compared with the conventional digital baseline recovery method,it can effectively improve the peak position shift caused by the baseline drift in the high count measurement.It can be applied to the on-line energy spectrum measurement under unknown intensity radiation.