【作者】 覃章健；

【导师】 葛良全； 赖万昌；

【作者基本信息】 成都理工大学 ， 地球探测与信息技术， 2008， 博士

【摘要】 核信息的获取与处理在许多基础科学研究和应用科学研究中都有重要的意义。多道脉冲幅度分析是核辐射能谱测量的一种基本方法,核谱仪是用于获取和处理多道脉冲幅度的基本设备。受ADC和数字信号处理器的发展制约,基于多道脉冲幅度分析的传统核谱仪大多是以对模拟脉冲信号峰值进行采样保持并作AD转换为特点的模拟核谱仪。随着高速高分辨率ADC和数字信号处理技术的迅猛发展,使以直接采集前置放大器输出脉冲波形、由数字信号处理进行多道脉冲幅度提取和分析为特点的全数字化核谱仪的实现成为可能。本文采用超高宽带集成运算放大器、125Msps的高速ADC和集逻辑控制与数据处理于一体的高速FPGA,完成了新一代全数字化的便携式核谱仪的研制。数字核谱仪对模拟放大电路的带宽和ADC的采样率要求较高,然而高带宽、高采样率要求往往以降低数据采集系统的信噪比为代价。因此,在噪声背景相对较高的情况下如何较精准地提取各数字脉冲幅度是本研究首先要解决的问题,否则,数字核谱仪因精确度和准确度还赶不上模拟核谱仪而失去研制的价值。本文采取曲线拟合算法来解决高噪声背景下的数字脉冲幅度提取问题。针对不同类型核辐射探测器,研究其相应的输出波形特征,采用快速曲线拟合算法提取其输出脉冲的幅度。实验表明,在信噪比为31.4dB左右的噪声背景下仿真半导体探测器的输出脉冲的后沿,采用曲线拟合算法提取其脉冲幅度,其结果与直接以该脉冲采样点中的最大值作为其幅度相比较,曲线拟合算法提取脉冲幅度的测量准确度和精确度分别改善了1个数量级。本文研制的数字核谱仪的信噪比达到了45.3dB左右,在该噪声背景下仿真计算的结果相较于信噪比为31.4dB左右的噪声背景下仿真计算的结果,其测量的准确度和精确度又改善了1个数量级。高信噪比数字化、实时甄别并存储核辐射探测器输出的随机脉冲信号是本文研究又一重点内容。通过附加相位补偿电路,本文设计出低噪声超宽带的信号放大电路,同时采用采样率高达125Msps的14位ADC实现了高信噪比的高速数据采集系统,为高保真数字化核辐射探测器输出的随机脉冲信号提供了保证。此外,本文通过巧妙地逻辑设计,实现了数字化的核辐射探测器输出的随机脉冲信号的实时甄别与存储。最后,论文通过实验详细测试了数字核谱仪的信噪比、精确度、能量分辨率等重要指标参数。实验结果表明,数字核谱仪测得的能谱与模拟核谱仪测得的能谱的能量刻度一致,数字核谱仪的能量分辨率要优于模拟核谱仪。论文的主要创新点主要表现在三个方面:1.基于FPGA集逻辑控制、数据处理、数据存储于一体的数字核谱仪设计方案,较传统的CPLD+RAM(或FIFO)+DSP实现逻辑控制和数据存储与处理功能的设计方案,减少了印制板走线数量,缩小了印制板面积,并且降低了功耗,更好地达到便携式要求。另外,通过巧妙地逻辑设计,在FPGA中实现了无处理器介入下的数字随机脉冲实时甄别与存储,突破了模拟核谱仪中因采样保持而产生脉冲处理死时间的局限。2.根据不同类型核辐射探测器输出的脉冲信号特征曲线,采用相应曲线拟合算法提取脉冲幅度,既能保证脉冲幅度提取的准确性和精确性,又简单易行且少占CPU处理时间以保证实时处理要求。3.本文研究了有关算法,对数字化后的核辐射探测器输出脉冲进行了弹道亏损补偿,突破了模拟核谱仪系统难以实现弹道亏损补偿的局限。这是数字核谱仪研制的又一创新点。更多还原

【Abstract】 It is very important to extract and process nuclear information for the purpose of basic and applied researches.In nuclear spectroscopy measurement,multi-channel pulse height analysis is a commonly-adopted method,and the nuclear spectroscopy instrument is the key equipment.On account of the restrictions of ADC and digital signal processor development,traditional nuclear spectroscopy instrument is an analog equipment,which samples and holds the peak of analog pulse signal,and then converts it to digital pulse signal.The rapid development of high-sample-rate and high-resolution ADC and digital processing technology has made it feasible to design a digital nuclear spectroscopy instrument,which can sample the pulse signal output by employing preamplifier of nuclear detector,and then extract the pulse height via using digital signal processing method.The author of this paper has accomplished the design of a portable digital nuclear spectroscopy instrument by utilizing ultra-wideband amplifier,125Msps high-speed ADC and high-speed FPGA with the function of logic control and data processes.A digital nuclear spectroscopy instrument sets a high demand for broad bandwidth in analog amplifier circuit and high sample rate in ADC,which are usually realized at the cost of reducing the signal-to-noise ratio of analog-to-digital conversion.Consequently,the correct and accurate extraction of the digital pulse height in the background of intense noise becomes the foremost problem to be solved. Otherwise,the digital nuclear spectroscopy instrument,not superior to an analog one in terms of correction and accuracy,is not worthy of more researches.This paper, applying curve-fitting method to the extraction of the digital pulse height,firstly analyzes the pulse’s curve feature output by various types of nuclear detectors,and then the high-speed curve-fitting algorithm is adopted to extract the pulse’s height. With a simulation of the ideal pulse curve output via semiconductor detector when the signal-to-noise ratio is 31.4dB,the curve-fitting algorithm is utilized,and the noise-curve’s height is extracted.The comparative study of the height to the maximum in the noise-curve’s sample-point data reveals that the height’s correction and accuracy extracted by curve fitting method is 1 magnitude better than that of the maximum.If the signal-to-noise ratio is improved—for example,it is up to 45.3dB, which is the instrument’s signal-to-noise ratio designed by the paper—the height’s correction and accuracy measured by the instrument via curve fitting method will be further improved by 1 magnitude.Another important task of this paper is to design a high signal-to-noise ratio data acquisition system so as to sample the random-pulse output by the nuclear detector and plan a logic unit to discriminate the pulse and noise and then only record the pulse. The author of this paper designs a low noise and ultra-wideband amplifier circuit with phase-compensated technology and uses a 14bit,125Msps ADC to accomplish a data acquisition system,which can convert the analog signal output by nuclear detector to digital signal with high fidelity.A logic unit is then ingeniously invented to capture the conditional pulse and record it at real time.In this paper,some parameters,e.g.signal-to-noise ratio,accuracy and energy resolution of the instrument,have been tested,which is better than that of analog nuclear spectroscopy instrument.To sum up,the following points are innovative:Firstly,the instrument is designed based on FPGA,which can simultaneously do logic control,data process and data recording.In contrast to CPLD+RAM(or FIFO)+DSP design,the new scheme can decrease power and reduce printed circuit area,which contributes to the realization of portability.Secondly,curve-fitting is employed to extract nuclear detector’s pulse height. The algorithm is not only of easy feasibility,but also help improve the correction and accuracy on the pulse height measurement.Thirdly.the ingenious algorithm is successful in compensating the ballistic deficit of digital pulse while the analog nuclear spectroscopy instrument fails to do so. In the sight of this,digital nuclear spectroscopy instrument is better than an analog one.更多还原