模拟炸药及几种相关物质中氧元素探测技术研究

【作者】 胡永波；

【导师】 黄瑞良；

【作者基本信息】 中国工程物理研究院 ， 核能科学与工程， 2007， 硕士

【摘要】 准确、灵敏、快速地探测爆炸物的技术和方法是当今国内外很关心的问题。目前，国内外探测爆炸物的方法层出不穷，由于探测的要求和环境差异很大，各种方法都有使用的局限性。中子活化分析法是一种有效的无损探测爆炸物的方法，欲建立准确、灵敏、快速地用中子活化法探测炸药的装置仍需要深入研究解决一系列技术问题。各种爆炸物的主要化学成分是碳、氢、氧和氮，其中氮、氧元素的含量普遍高于日常用品，物质中氮、氧元素含量及其与氢元素的含量比是爆炸物属性的一个指标，因而探测爆炸物的重要手段是探测其中的碳、氢、氧、氮元素的含量。目前对爆炸物中氮元素探测技术研究较多，本文采用了中子活化法，开展了以模拟炸药及几种相关物质中氧、氢元素探测技术为主要方向的研究，用14MeV中子活化物质中的氧、氢元素，通过测量氧、氢元素活化产物衰变产生的特征γ射线强度，计算该物质中氧、氢元素含量及其比值，进而判断该物质的爆炸物属性。由于爆炸物品中含有多种元素，直接研究爆炸物品存在一定的难度，针对本文的研究目的，首先采用元素种类较少的水及酒精样品进行氧元素探测工作的研究。采用14MeV中子照射被探测物质，分析HPGe探测器探测到的氧元素6129keY特征γ射线及其单逃逸峰、双逃逸峰计数率和氢元素2223keV特征γ射线计数率。在对氧元素特征γ射线的归纳总结中发现了与中子活化法探测氮元素一样的规律：同一形状的物质，采用HPGe探测器探测到的特征γ射线强度随物质质量的增加呈现先增大后减小的趋势。根据这一规律，本文提出：采用固定形状的已知物质得到γ射线计数率随物质质量的变化关系曲线，对于同一形状未知物质，采用插值点的方法求出其氧元素含量。物质形状不一样时，氧氢元素比对应一条形状修正曲线，结合γ射线计数率随物质质量的变化关系曲线可以求出物质中氧氢元素含量。中子活化分析法的其中两个重要环节是对γ射线谱形的求解过程及对实验本底的扣除。本文针对这两方面展开工作，提出了对于存在部分重叠的γ射线能峰的解谱方法以及动态本底的分析方法。在分析过程中，被探测物质本身也是中子慢化剂，造成本底计数率的变化。因被探测物质的形状、质量不同，本底计数率也不同，在本文中，把本底看作被探测物质质量为零的情况，在作γ射线计数率与物质质量关系曲线时包含被探测物质对中子慢化效果的影响，由此得出了探测灵敏度与探测时间的关系，实际运用中可以根据不同需要选择合适的条件。由于模拟炸药中元素含量较多，探测得到的能峰存在部分重叠的现象。本文在研究了多种解谱方法的基础上，分析比较得出了采用单峰Gauss拟合法求解部分重叠能峰具有较好准确性的结论。本文数据均采用这一方法得到。本文的另一项工作是采用MCNP模拟计算了本次的实验过程，由于实验环境比较复杂，模拟程序进行了简化，只是进行了方法研究，旨在学习掌握使用MCNP模拟计算14MeV中子照射物质、HPGe探测器探测γ射线的计算方法、计算技巧，从模拟结果与实验结果的对比发现：模拟计算程序在设计过程中必须考虑环境中各种元素的影响，实验数据处理过程中应该考虑氧、氢元素反应截面随中子能量的变化关系以及中子源、被探测物质与探测器之间相对位置对数据处理结果造成的影响。通过本文的研究，初步掌握了中子活化法探测炸药的技术及其影响的因素，为中子活化法探测爆炸物提供一定的技术支撑。更多还原

【Abstract】 The problem how to detect explosives rapidly, accurately and sensitively is widelyconcerned in the world. Because detection precision and environment is different, everyexistent technology for detecting explosives has its own limitation.Neutron Activation Analysis (NAA) is an effective non-destructive examinationtechnology for detecting explosives. A series of technical problems should be seriouslyresearched if Neutron Activation Analysis is employed to build a rapid sensitive accuratemethod for detecting explosives. Most explosives are composed of the elements C, H, O andN. Further more, the concentration of O and N is usually higher than commodities. Theconcentration of O and N, as well as the ratio of the concentration of O to N are used toidentify different materials. So detecting the concentration of N, C, H, and O is an importantmethod to identify explosives. Now there are many studies in detecting the concentration of Nin explosives. Detecting the concentration of H and O in several relative materials andsimulative explosives using Neutron Activation Analysis is the main work in this paper. The Oor H atoms in materials are activated by 14MeV neutrons. These atoms subsequently decayand emit gamma rays, which are directly proportional to the concentration of the respectiveelement. Different materials can be identified by calculating the concentration of O and N andtheir ratio.As the explosives are usually composed of many elements, it is difficult to detect realexplosives directly. For the purpose of this paper, we research water and alcohol whichcontain a few elements in the beginning. 14 MeV neutrons are used to irritate the elements Oand H in samples. The excited oxygen atoms emit 6129 keV gamma rays and the excitedhydrogen atoms emit 2223 keV gamma rays, which can be detected by HPGe detector. Wecan get some information of samples by analyzing the count rate of characteristic peak, singleescape peak and double escape peak of the emitting gamma rays. If the shape of the samplesis definite, we discover that the intensity of gamma rays emitted by oxygen atoms becomeslarger when the mass of the sample increases at first, but when we continue to increase themass of the sample, the intensity of gamma rays emitted by oxygen atoms becomes smaller.This rule is similar to the result of the element N using NAA method. By researching theknown samples with a definite shape but different mass, we can get a curve that the intensityof gamma rays varies with the mass. The concentration of the element O in unknown samplewith the same shape can be calculated by interpolation method. The ratio of the concentration of O to H in samples of different shape correspond to a shape correctioncoefficient curve. Combining with the curve of gamma rays count rate varied with mass, wecan calculate the concentration of O or H in samples.Subtracting background and analyzing gamma rays spectroscopy are the two key pointsof NAA. In this paper, we draw a technique for unscrambling overlapped spectrum and adynamic background analysis method.Since the sample itself is the neutron moderator, the count rate of background varieswith the shape and mass of the samples in analysis process. In this paper, background is thecount rate when the mass of sample is zero. The moderative effect of samples is considered inthe curve that the count rate of gamma rays varies with mass. Thus we can analyze therelationship between detection sensitivity and detection time. The proper detectionconditions can be chosen when detecting.For simulative explosives are composed of many elements, it is possible that thedetection peaks of gamma rays overlap. On the basis of investigating many unscramblingspectrum methods, we find that the Gauss Fit of single peak is the fittest method. So thismethod is employed to analyze data of experiments in this paper.Another important work of this paper is simulating experiments with MCNP. But for theenvironments of our experiments are too complex, our code for simulating is simplified. Wejust study the technique of simulating detection process using MCNP. By comparing theresults of simulations and experiments, we find that the elements in environment must beconsidered when simulating, and the factor that the cross sections of the elements O and Hvary with the energy of neutrons also should be considered when analyzing data ofexperiments. Moreover, the positions of neutron source, samples and detector should beconsidered in data analysis.The technology of detecting explosives using NAA and the influence factors of thismethod are studied in this paper, which can be very useful for successive research in this area.更多还原