数字化多道中双指数脉冲梯形成形算法研究

本文在双指数脉冲梯形滤波成形算法原理基础上,从理论上分析了算法的滤波特性和弹道亏损特性。通过MATLAB仿真和基于FPGA的实验验证,证明了该算法的梯形上升沿时间越长滤波效果越好,修复弹道亏损所需梯形平顶长度小于探测器电荷收集时间。在设计数字化多道时可根据上述特性对梯形参数进行优化调整,从而最大限度地减少系统脉冲成形时间,降低脉冲堆积概率,提高多道脉冲幅度分析系统的脉冲通过率。     

三梯形成形法处理脉冲堆积仿真研究

在梯形成形算法基础上,提出了处理脉冲堆积的快中慢三梯形成形算法。该算法设置了三个成形时间不同的梯形通道,利用快梯形通道进行脉冲计数定位,慢梯形通道提取脉冲间隔较大的梯形幅度,中梯形通道提取脉冲堆积程度较轻的梯形幅度,降低丢弃脉冲的数量,减少原始核信息的丢失。实验测试结果表明,在高计数率和脉冲堆积严重时,该方法较快慢双通道梯形成形方法具有更高的能谱分辨率。     

基于MATLAB的核脉冲信号数字成形实现与性能分析

数字核谱仪系统中数字成形方法与成形参数的选取影响着脉冲信号的噪声抑制、堆积识别、幅度提取,进而直接影响系统的能量分辨率与最大脉冲计数率等主要性能指标。研发一套基于MATLAB的数字核信号处理平台,可分别实现仿真核信号,采样核信号在不同数字成形方法、不同成形参数下的高斯成形、梯形成形与三角成形输出的同时,可构建具有多套数字成形算法验证平台。通过对137Cs(NaI探测器)的γ能谱与Fe源(Si-PIN探测器)的X荧光能谱测试可得:数字高斯成形表现出更好的能量分辨率特性;三角成形表现出更好的最大脉冲计数率特性;梯形成形表现出更好的综合特性。采用梯形成形对137Cs能谱和高斯成形对Fe的特征X射线能谱测试可知:随着成形参数的增加,能量分辨率越好,最大脉冲计数率越低。     

基于小波变换的双指数信号高斯脉冲成形算法研究

高斯脉冲具有信噪比高、弹道亏损小的优点，因此，在核辐射测量系统中常将核辐射探测器输出信号成形为高斯波形。核辐射探测器实际输出的核脉冲信号更接近于双指数信号，因此在小波变换的基础上，利用卷积运算的微分特性，提出了双指数信号高斯脉冲成形算法，并建立了成形系统的冲激响应。采用模拟核脉冲信号，从时域和频域两方面研究了成形参数对成形脉冲形状、滤波特性的影响规律；采用FAST硅漂移探测器（Silicon Drift Detector,SDD）测量标准Mn样品获得实测核脉冲信号，分别进行高斯脉冲成形算法和梯形脉冲成形算法处理，并生成能谱；通过对比5.89 keV特征峰峰面积、能量分辨率，研究两种成形算法在能量分辨率和堆积脉冲分离方面的性能。结果表明：当达峰时间为3.2～6.4μs时，两种成形算法所得能谱的能量分辨率最佳，此时，两者之差小于5 eV；在相同达峰时间条件下，高斯脉冲成形算法的堆积脉冲分离能力优于梯形脉冲成形算法。     

实时核信号数字化脉冲成形关键技术研究

本文对理想核脉冲信号和实际探测器输出信号分别进行了计算机模拟仿真与分析,总结了不同成形时间的核脉冲信号的数字梯形成形参数的确定方法。在高计数率场合时提高了有效测量计数率,消除了部分脉冲的堆积并减少了系统死时间。同时,采用256点和512点数字三角成形方法测试了Si-PIN半导体探测器的性能,并与模拟电路成形方法进行了对比测试。测试结果表明,脉冲数字成形处理方法提高了探测器计数率和分辨率。     

基于脉冲宽度分析的实时数字堆积排除方法研究

高计数率下的核能谱获取中,随机脉冲的堆积效应造成获取到的核能谱分辨率变差。为了有效抑制脉冲堆积对γ谱仪分辨率的不利影响,基于高速全波形数字化采样技术,文中提出了脉冲宽度分析方法应用于高计数率下的核脉冲实时堆积判别和拒绝。采用仿核脉冲信号源论证了该堆积拒绝方法的可行性,也使用了Na I闪烁体探测器对137Cs和60Co两种放射源进行实验测试。结果表明:该方法可以有效实时识别出脉冲后沿和前沿堆积情况,在高计数率下可以有效改善数字γ谱仪分辨率。     

数字核谱仪系统中脉冲幅度提取方法研究

数字核谱仪系统中脉冲幅度提取的准确性与实时性直接影响到系统的能量分辨率与脉冲通过率。本文对高速ADC采样后的数字核信号脉冲幅度提取方法进行研究,将幅度提取方法按直接比较法、曲线拟合法与滤波成形法的工作原理展开讨论。采用上述三种方法对实际采样的数字核信号进行脉冲幅度提取,在40 MHz的采样频率下,分别采用曲线拟合法与滤波成形法对脉冲幅度进行提取,得到各自的能谱。通过理论分析,实验比较与测试,综合能量分辨率与脉冲计数率的性能,得到最佳的脉冲幅度提取方法为滤波成形法。     

基于余弦函数的核脉冲信号数字成形方法研究

核脉冲信号的数字余弦函数(cos)成形，因成形简单、可操作性强和灵活性高等优势被用于核脉冲信号的数字成形处理中。本文从单指数衰减脉冲信号和cos脉冲信号的数学模型出发，推导了对称cos成形、零面积cos成形与对称零面积cos成形三种成形方法在Z域中的传递函数和级联公式，分析了成形参数对数字cos成形结果的影响。针对仿真核信号和实际采样核信号，分别实现了三种数字cos成形；基于现场可编程逻辑门阵列(Field Programmable Gate Array,FPGA)系统，实现了核信号的数字cos成形、幅度提取、能谱构建等功能。通过对¹³⁷Cs(NaI(Tl)探测器)γ能谱的测试，结果表明三种数字cos成形方法在能量分辨率和计数方面均表现良好，且对称零面积cos成形性能指标更为优异，具有较大的应用前景。     

梯形成形算法中成形参数与成形脉冲波形关系研究

在梯形成形算法理论基础上,利用模拟和实测核脉冲信号研究了梯形成形算法中衰减时间常数(τ_(trap))、达峰时间(n_a)与成形脉冲波形、滤波效果的关系,以及nb在分离堆积脉冲时的取值选择。研究表明,当且仅当τ_(trap)等于输入信号衰减时间常数时,成形脉冲对称;n_a值越大,滤波效果越好,脉冲宽度增加;n_b小于两个核脉冲信号发生的时间间隔时,采用梯形成形算法可分离堆积脉冲。     

环境辐射监测数字伽玛谱仪设计

本文介绍了一种环境辐射监测数字伽玛谱仪设计方案。本设计采用Na I(Tl)探测器对伽玛射线进行探测,由FPGA控制可编程增益放大器对核脉冲信号进行调节,以适合后续模数转换输入要求。由FPGA控制高速模数转换器AD9235对核脉冲信号进行模数转换,然后采用海明窗低通FIR数字滤波器对数字核脉冲信号进行滤波,滤除高频噪声。本设计采用TMS320C6713对数字核脉冲信号进行梯形成形,改善弹道亏损和脉冲堆积,最后对其进行幅值判断形成能谱。谱仪经刻度后,不仅可以获得现场辐射计数总量还可以获取现场能谱对核素进行初步判别,这对于工业环境辐射监测非常有意义。本设计具有体积小、设计灵活、可扩展性好等特点,非常适合现场环境辐射监测。     

Implementation of a cusp-like for real-time digital pulse shaper in nuclear spectrometry

Pulse shaping,which improves signal-to-noise ratio excellently,has been extensively used in nuclear signal processing.This paper presents a cusp-like pulse-shaping technique developed through the recursive difference equation in time domain.It can be implemented in field programmable gate array hardware system.Another fiat-topped cusp-like shaper is developed to optimize the time constant of pulse shaping and reduce the influence of ballistic deficit.The methods of both baseline restoration and pile-up rejection are described.The 137Cs energy spectra measured with the digital cusp-like shaper are 6.6％ energy resolution,while those by traditional analog pulse shaper are 7.2％ energy resolution,under the same conditions.This technique offers flexibility,too,in adjusting the pulse shaper parameters.     

Ballistic deficit compensation method for a large-volume HPGe detector at high count rates

A large-volume HPGe detector normally has a severe ballistic deficit due to its long rise time of the output signals.Despite the trapezoidal shaping algorithm adopted as a remedy to deal with the signals,the algorithm cannot fully eliminate the ballistic deficit in the case of a high counting rate.To resolve the problem,we propose a ballistic deficit compensation method that is based on the measurement of rise time of the signals before shaping.We find that the ballistic deficit after trapezoidal shaping has little relation to the time constant,but shows a quadratic relationship with the rise time and has a negative correlation with the shaping time.In the case of high count rates,the high resolution is handled by fitting the curve to the rise time and the amplitude deficit of the signal after shaping and by compensating for the signal amplitude after trapezoidal shaping.Tests indicate that when the count rate is about 100 kcps,the resolution of Co-60 improves from traditional 2.32 up to 1.91 keV,thus reaching a higher level.     

New methods to remove baseline drift in trapezoidal pulse shaping

Trapezoidal pulse shaping algorithm is widely applied to improve signal-to-noise ratio(SNR), throughput and energy resolution with the properties of noise suppression, pile-up pulse separation and ballistic deficit correction. The algorithm can be acquired by z transform method which is easier for derivation. However, the baseline drift of trapezoidal pulse appears because the noise superimposes on the input signal. In this paper,two new methods based on convergence analysis and noise suppression are proposed to remove the baseline drift resulting from trapezoidal pulse shaping. Simulations and experimental tests are carried out to verify the methods. The results demonstrate that the proposed methods can remove baseline drift in trapezoidal pulse shaping.     

基于实时数字脉冲处理技术的核谱仪研究

本文提出一种基于80 MHz ADC的数字化γ能谱系统。系统由探测器、前端电路、ADC和数字处理单元组成。数字处理在FPGA中完成,主要包括FIR数字滤波、脉冲梯形成形、幅度甄别、数据通讯。为减小高速ADC在采集过程中引入的噪声信号,在数字处理单元实现FIR数字滤波,对数字脉冲信号先进行滤波处理,再进行脉冲梯形成形,得到高分辨率的能谱数据。测量系统中模拟信号全部采用直流耦合,数字脉冲宽度为1.6μs,对137 Cs的能量分辨率达6.88%。     

Optimal Optical Conditions and Positioning Scheme for an Ultrahigh-Resolution Silicon Drift Detector-Based Gamma Camera

In this study, we optimized the optical conditions and associated positioning scheme for an ultrahighspatial-resolution, solid-state gamma detector. The detector module consisted of an array of seven hexagonal silicon drift detectors (SDDs) packed hexagonally and coupled to a single slab of crystal via a light guide glass. Because the optical behavior and requirements of the detector module and noise characteristics of the SDD sensor are different from those of conventional photomultiplier tube (PMT)-based detectors, the following parameters were studied to determine the optimum condition: scintillator selection, the effect of cooling on signal-to-noise ratio (SNR), the depth dependence of the scintillation light distribution, and optimum shaping time. To that end, a modified, Anger-style positioning algorithm with a denoise scheme was also developed to address the estimation bias (pincushion distortion) caused by the excessively confined light distribution and the leakage current induced by the SDD sensor. The results of this study proved that the positioning algorithm, together with the optimized optical configuration of the detector module, improves the positioning accuracy of the prototype detector. Our results confirmed the ability of the prototype to achieve a spatial resolution of about 0.7mm in full width at half maximum (FWHM) for 122 keV gamma rays under the equivalent noise count (ENC) of 100 (e- rms) per SDD channel. The results also confirmed NaI(Tl) to be a more desirable scintillator for our prototype with an energy resolution performance of about 8%.     

一种新型溴化铈闪烁体探测器性能研究

闪烁探测器在射线能量与时间的探测中发挥着重要作用。针对国产新型溴化铈（CeBr₃）闪烁晶体，本文建立实验系统和方法对其关键性能指标进行了测试，具体包括：建立了快符合时间分辨系统测量装置，测试得到新型闪烁探测器对于⁶⁰Co源的时间分辨率为142 ps；建立了单光子计数法测量装置，测试得到溴化铈晶体的衰减时间为16.8 ns，上升时间为0.18 ns；建立了相对光输出测量装置，测试得到其相对于同规格NaI(Tl)闪烁体探测器对于661.66 keV射线的光输出为161%；同时针对其能量响应性能进行了测试，测得对于661.66 keV射线其能量分辨率为4.7%。将国产新型溴化铈闪烁晶体的性能测试结果与国际上同类晶体以及氟化钡、溴化镧、碘化钠等常用无机闪烁体的性能进行了比较，对其应用前景进行了探讨。     

ASIC With Multiple Energy Discrimination for High-Rate Photon Counting Applications

An ASIC for high rate photon counting measurements with multiple energy discrimination is presented. Optimized for pixelated CdZnTe sensors, the ASIC is composed of 64 channels with low-noise charge preamplification, high-order shaping (40 ns minimum peaking time), and five window-discriminators with associated 16-bit counters. An efficient readout scheme allows simultaneous measurement and readout through a 60 MHz 16-bit output bus. The ASIC architecture and experimental results are reported, and the impact of the shaper order on the resolution, ballistic deficit, and pile-up is discussed     

大体积NaI(Tl)数字式车载γ能谱仪的研制

车载γ能谱仪主要应用于区域放射性核素活度测量、放射源搜索及核污染检测,而数字化技术是提高车载γ能谱仪灵敏度与快速响应的有效方法。本文研制了可支持1～5个4 L大体积NaI(Tl)的数字式车载γ能谱仪系统。该系统采用数字核脉冲处理技术独立获取多个NaI（Tl）晶体的能谱,并通过无线WiFi、快速以太网、RS232等3种方式发送谱线数据给军用计算机。本系统在计算机上实现了实时核素识别、数字成图、色差能谱显示、ROI监控、数据回放等功能。结果表明,本系统采用数字技术后提升了车载γ能谱仪的能量分辨率、低能射线识别度和能量线性度,且具备更加稳定精确的数字稳谱效果。该系统不仅可用于放射性资源勘查,也可用于快速核应急、核污染检测、放射源搜索等。     

惰性气体β-γ符合测量系统探测器能量及分辨率刻度

β-γ符合法是全面禁止核试验条约(CTBT)放射性核素核查中惰性气体氙测量的一种重要方法,探测器能量及分辨率刻度是其首要解决的关键技术。本工作详细介绍了β-γ符合测量系统NaI(Tl)闪烁体和塑料闪烁体探测器能量及分辨率刻度的方法和结果,采用γ放射性核素点源刻度NaI(Tl)γ射线能量及分辨率,利用137Cs661.66keVγ射线康普顿散射电子刻度塑料闪烁体β射线能量及分辨率,并与131Xem内转换电子刻度的β射线能量分辨率结果进行了比较。结果表明:用137Cs康普顿散射电子刻度塑料闪烁体β射线能量是一种简便可行的方法,但用其刻度的β射线分辨率比实际的大。