Study On Key Technologies Of Digital Nuclear Spectrometer System

Nuclear spectrum is the important information in domain of nuclear physics research, radiation detection and applications of nuclear technology, nuclear spectrum measurement technique has been a hot issue. Multi-channel pulse height analysis in nuclear spectrum measurement as a fundamental method, With the rapid development of ADC and digital signal processing technology, the realization of digital nuclear spectrometer has become a reality, which used the high-speed, high-resolution ADC to acquire nuclear signal and used the DSPs to accomplish the effective extraction and analysis of pulse amplitude, the acquisition and processing of nuclear signal is the main direction of development and application of nuclear spectrometer.Domestic and foreign research institutions have studied the key technologies of nuclear spectrometer system, and overseas digital commercial nuclear spectrometer has taken, but the domestic research and application are still in the preliminary stages, in which have not been in-depth study on the key technologies, such as data acquisition, filter shaping, baseline restoration, pile-up recognition, count rate correction and amplitude extraction, and not set up a effective platform for realization of key technologies.In response to these problems, the thesis focuses on the following key issues:(1) This thesis introduces the system components of digital nuclear spectrometer in domestic and international domain, and discusses the main factors on impacting on the performance of digital nuclear spectrometer; it describes the digital process of nuclear pulse waveform, and analyzes the main factors of the selection of ADC chip in nuclear spectrometer system.(2) This thesis analyzes and compares three amplitude extraction methods which include direct comparison method, curve fitting method and filter former method, it does the in-depth study on amplitude extraction method, and it uses the synthetic method to deal with the digital nuclear signal.(3) This paper analyzes the reasons of baseline drift and discusses the effects of baseline drift for amplitude extraction. It analyzes and compares three baseline restoration methods which include zero-area filtering method, segmentation filtering method and interpolation method, it studies a simple, fast and effective method, the interpolation method can realize the optimal baseline estimation.(4) This paper analyzes the reasons of pulse pile-up and discusses the method of pile-up identification and the effects of pile-up for amplitude extraction, it in-depth study on the pile-up identification which comes true via comparing the length between the pulse interval time and the time of shaping parameters.(5) This paper studies on the shaper optimization and the best shaping parameters, which must consider the effect on noise suppression, ballistic deficit, pile-up identification, amplitude extraction among the trapezoidal shape, triangular shape and Gaussian shape.(6) This thesis builds up a test platform that realizes these key technologies, which include filter shape, baseline restoration, pile-up identification and amplitude extraction.After studying the key technologies of nuclear spectrometer system, the innovative research results are as follows:(1) After comparing and analyzing several amplitude extraction methods, it introduces the synthetic method to deal with the digital nuclear signal, the synthetic method can realize the digital filter shape at any noise, and it achieves the different output shaper only to change the shaping parameters.(2) After comparing and analyzing several baseline restoration methods, it introduces the interpolation method which can realize the optimization of the baseline estimation.(3) It builds up an a test platform which has a USB communication interface to achieve nuclear signal transmission of actual acquisition, and the platform has the functions that include filter shape, baseline restoration, pile-up identification, count rate correction and amplitude extraction, it is used to validates the feasibility and rationality of optimal digital nuclear signal processing method and parameters which comes from the academic research. After testing, for the same number of data, when setting the different shaping parameters of trapezoidal shape (18μs⁵0μs), the energy resolution increases from 210eV to 165eV (⁵⁵Fe), but the count rate decreases from 8102 to 6823, at the same time, the test platform can use different processing methods to build up a digital nuclear signal processing system which has a multi-algorithms configuration.(4) This thesis independently designs a set of digital nuclear spectrometer system based on Si-PIN detector, the nuclear spectrometer uses a combination of FPGA and DSP to realize digital filter shape, baseline restoration, pile-up identification, amplitude extraction and count rate correction, it achieves good results in the actual experiment.