正电子湮没寿命谱的傅里叶变换分析法

本文提出了一种通过傅里叶变换在频域内进行正电子湮没寿命谱分析的方法。文中对正电子湮没寿命谱的傅里叶变换谱的基本特性和该方法的潜在优点进行了若干讨论。 关键词：     

The influence of several parameters on the lifetimes and intensities of positron lifetime spectra of metals

The influence of the instrumental time-resolution function and the source-surface spectrum on the lifetimes and intensities of positron lifetime spectra is studied by a computer simulation of such spectra. The prompt curve is described not by a sum of Gaussian curves but by the convolution of a Gaussian curve with a double-sided exponential. It is shown that the fitted lifetimes and intensities considerably depend on these parameters.     

Defect characterization of slow-cooled and quenched samples of calcium-copper-titanate through positron annihilation spectroscopy

Abstract

Positron annihilation lifetime and Doppler-broadened gamma-ray spectra have been analyzed for slow-cooled and thermally quenched polycrystalline samples of calcium-copper-titanate. Two positron lifetimes revealing the characteristic defects in the respective samples were carefully analyzed to compare and contrast the significance of their origin and implication. A third component arising from positronium formation at the powdered particle surfaces has been considered in the analysis although its significance is lost in its very small intensity (～ 1.1–1.2%). In the quenched sample, the defect-specific long positron lifetime (τ₂) is found to larger and the mean lifetime smaller while its intensity I₂ is found drastically smaller and the concentration of defects less by an order of magnitude as compared to the slow-cooled sample. The observed changes in electrical parameters of slow-cooled and quenched samples were found to have correlations with the positron annihilation lifetime and Doppler-broadened lineshape parameters.     

Positron annihilation studies of mesoporous silica films using a slow positron beam

Positron annihilation lifetime spectra were measured for mesoporous silica films, which were synthesized using triblock copolymer (EO₁₀₆PO₇₀EO₁₀₆) as a structure-directing agent. Different positron lifetime spectra for the deposited and calcined films indicated the formation of meso-structure after calcination, which was confirmed by Fourier transform infrared (FTIR) spectra and field emission-scanning electron microscopy (FE-SEM) observation. Open porosity or pore interconnectivity of a silica film might be evaluated by a two-dimensional positron annihilation lifetime spectrum of an uncapped film. Pore sizes and their distributions in the silica films were found to be affected by thermal treatments.     

利用正电子湮没技术研究钾掺杂钨合金中的缺陷

钨合金中钾的掺杂会引入大量的缺陷,如尺寸几十纳米的钾泡、高密度的位错以及微米量级的晶粒带来的晶界等,这些缺陷的浓度和分布直接影响合金的服役性能.本文运用正电子湮没谱学方法研究钾掺杂钨合金中的缺陷信息,首先模拟计算了合金中各种缺陷的正电子湮没寿命,发现钾的嵌入对空位团、位错、晶界等缺陷的寿命影响很小;然后测量了不同钾含量掺杂钨合金样品的正电子湮没寿命谱,建立三态捕获模型,发现样品中有高的位错密度和低的空位团簇浓度,验证了钾对位错的钉扎作用,阐述了在钾泡形成初期是钾元素与空位团簇结合并逐渐长大的过程;最后使用慢正电子多普勒展宽谱技术表征了样品中缺陷随深度的均匀分布和大量存在,通过扩散长度的比较肯定了钾泡、晶界等缺陷的存在.     

Investigation of nanocrystalline CoFe 2 O 4 by positron annihilation lifetime spectroscopy

Nanoparticles of cobalt ferrite prepared by the co-precipitation method with crystallite size varying from 4.7 to 41 nm have been characterized by positron annihilation lifetime spectroscopy. Three lifetime components are fitted to the lifetime data. The shortest lifetime component is attributed to the delocalized positron lifetime shortened by defect trapping. The intermediate lifetime is assigned to the positron annihilation in diffuse vacancy clusters or microvoids at the grain boundaries and at the grain-boundary triple points. The longest component corresponds to the pick-off annihilation of ortho-positronium formed at the larger voids. The variations in these lifetimes and their relative intensities with annealing temperature and crystallite size have been studied in detail.     

用正电子湮没技术研究USY沸石的“二次孔”结构

测量了NaY及USY沸石的正电子寿命谱．得到NaY沸石有四个寿命分量：两个短寿命分量和两个长寿命分量．两个长寿命分量中，τ₃，τ₄分别是o-Ps在β笼和超笼中的湮没．USY沸石有五个寿命分量：两个短寿命分量和三个长寿命分量，三个长寿命分量中，τ₃为o-Ps在笼中的湮没，τ₄，τ₅分别为o-Ps在“二次微孔”和“二次中孔”中的湮没．与NaY沸石相比，在真空中，USY的τ₄增加到 关键词：     

PdAgHx金属氢化物正电子湮没研究

在77—295 K温区和氢浓度0—0.35范围采用正电子湮没寿命测量方法研究了Pd0.75Ag0.25H_x氢化物合金. 充氢后正电子湮没寿命谱可以用两个寿命成分表征. 短寿命成分τ1不随温度和氢浓度变化, 是自由正电子湮没寿命; 长寿命成分τ2及其相对强度I2不随温度变化, 但随氢浓度的增加分别增大和减小, τ2是氢气泡捕获的正电子湮没寿命, τ2增大和I2减小说明随氢浓度增大氢聚集成的气泡的尺度增大, 而浓度减小. 实验结果表明, 氢脆的微观机理是氢气泡致脆. The metal hydride PdAgHx with a hydrogen concentration x ranging from 0 to 0.35 has been investigated by positron annihilation lifetime method in the temperature region between 77 K and 295 K. The measured lifetime spectra in metal hydride PdAgHx are characterized by two lifetimes τ1 and τ2. The short lifetime τ1 is independent of both hydrogen concentration and temperature, which is ascribed to the annihilation lifetime of free positrons. The long lifetime τ2 and its intensity I2 do not change with temperature, while τ2 increases and I2 decreases with increasing of hydrogen concentration. τ2 is attributed to the lifetime of positrons trapped at the hydrogen bubble. The increase of τ2 indicates the growth of the hydrogen bubble, and the decrease of I2 shows the reduction of the hydrogen bubble concentration. The experimental result shows a microscopic mechanism that the hydrogen bubble produced causes hydrogen embrittlement.     

基于北京慢正电子强束流的微脉冲化系统设计

慢正电子湮没寿命测量是研究材料表面微观缺陷的重要分析方法．束团化系统是实现慢正电子湮没寿命测量的核心部件, 主要由斩波、聚束两部分组成, 它可以将随时间连续分布的束流束团化, 从而获得正电子湮没寿命测量的时间起点及满足时间分辨率要求的束团．本文以粒子动力学计算为基础, 完成了束团化系统的物理设计, 其时间分辨率设计值为150ps(FWHM)．     

Design of a Pulsed Low Energy Positron System Based on Beijing Intense Slow Positron Beam

The design of a pulsing system for an intense slow positron beam is described in this paper. Slow positron annihilation lifetime measurement is an important method to study the depth-dependent characteristics of the surface and near surface. The start signal for slow positron lifetime measurement can be obtained from the pulsing system, which consists of a reflection type chopper, a prebuncher and a buncher. On the basis of the simulation of dynamics process by Parmela, the frequency of the buncher and the positron energy have been chosen to be 150MHz and 330eV respectively. The designed time resolution of this system is about 150ps (FWHM).     

Nb2O5掺杂对提高钨酸铅晶体发光性能的微观研究

通过透射谱、X射线激发发射谱(XSL)的测试，研究了布里奇曼(Bridgman)法生长的掺铌钨酸铅晶体的发光性能，并利用正电子湮没寿命谱(PAT)和X射线光电子能谱(XPS)的实验手段，对其微观缺陷进行了深入研究。结果表明，铌掺杂能够有效地改善钨酸铅晶体的350nm吸收带，提高钨酸铅晶体的发光快成分比例，并使得晶体中的正电子捕获中心浓度上升，低价氧浓度上升。提出掺铌钨酸铅晶体中Nb^5 将占据W^6 格位并使得晶体内部分(WO4)^2-根团成为(NbO3 Vo)^-，由此可改善钨酸铅的发光性能。     

氘化对KH2PO4晶体微观缺陷影响的正电子湮没研究

采用传统降温法从不同程度氘化(x=0, 0.51, 0.85)的生长溶液中生长氘化KH₂PO₄(KDP) 晶体, 利用正电子湮没技术(正电子寿命谱和多普勒展宽谱)、结合X射线衍射谱(XRD) 结构分析, 对KDP晶体氘化生长的微观缺陷进行了研究, 讨论了氘化程度对晶体内部微观结构特性、缺陷类型和浓度的影响. XRD结果显示晶胞参数a, b值随氘含量的增加而增加, c值无明显变化; 正电子寿命谱结果发现随着氘化浓度的提高, KDP晶体内部中性填隙缺陷以及氧缺陷不断增加, 引起晶体晶格畸变; 氢空位、K空位、杂质替位缺陷不断发生缔合反应形成复合缺陷, 缺陷浓度不断减少; 团簇、微空洞等大尺寸缺陷也在不断发生聚合反应, 缺陷浓度表现为不断减少. 多普勒实验结果表明随着氘化程度的提升, 晶体内部各类缺陷表现为同步变化. 实验结果表明, KDP晶体在低浓度氘化生长(50%以内)下缺陷反应较弱, 而在高浓度氘化(50%以上)下的缺陷反应显著增强.     

元素周期表中元素单晶基于局域密度理论的正电子寿命计算

在局域密度近似理论（LDA）的基础上用中性原子叠加模型和有限插分方法（SNA-FD）计算了元素周期表中各种元素单晶的正电子体寿命和单空位寿命.分析了不同结构的单晶中自由正电子的分布信息和湮没信息.元素单晶的正电子寿命计算值与文献中的实验测量值相符合,表明LDA基础上的SNA-FD方法可以作为单晶中正电子湮没理论计算的有效研究手段. 关键词： 局域密度近似理论 正电子寿命     

基于主成分分析和人工神经网络的激光诱导击穿光谱塑料分类识别方法研究

研究了人工神经网络在激光诱导击穿光谱(LIBS)塑料分类识别方面的应用。选用七种常见的塑料作为实验样品,获得每种样品的170组LIBS光谱数据,利用主成分分析获得前五个主成分的得分矩阵。用每种塑料样品的130组光谱数据的主成分得分矩阵作为训练集,建立反向传播(BP)人工神经网络模型。将其余40组主成分得分作为测试数据输入训练好的模型进行分类识别,其识别准确度达到97.5%。实验结果表明,通过采用主成分分析与BP人工神经网络相结合的方法,可以很好地进行塑料激光诱导击穿光谱的分类识别,对塑料的回收利用有重要价值。     

The criteria for selecting a method for unfolding neutron spectra based on the information entropy theory

To further expand the application of an artificial neural network in the field of neutron spectrometry, the criteria for choosing between an artificial neural network and the maximum entropy method for the purpose of unfolding neutron spectra was presented. The counts of the Bonner spheres for IAEA neutron spectra were used as a database, and the artificial neural network and the maximum entropy method were used to unfold neutron spectra; the mean squares of the spectra were defined as the differences between the desired and unfolded spectra. After the information entropy of each spectrum was calculated using information entropy theory, the relationship between the mean squares of the spectra and the information entropy was acquired. Useful information from the information entropy guided the selection of unfolding methods. Due to the importance of the information entropy, the method for predicting the information entropy using the Bonner spheres' counts was established. The criteria based on the information entropy theory can be used to choose between the artificial neural network and the maximum entropy method unfolding methods. The application of an artificial neural network to unfold neutron spectra was expanded.     

Doppler程序对金红石型TiO2的空位缺陷研究

金红石型TiO2是一种非常好的稀磁半导体材料,其自身的本征缺陷与室温铁磁性起源密切相关。本文利用Doppler程序在广义梯度理论（GGA）的基础上,计算了正电子在金红石型TiO2块材中不同缺陷处的湮没寿命。主要包括自由态正电子的湮没寿命,单空位和双空位处束缚态正电子的湮没寿命。并从理论上给出了含有空位缺陷时金红石型TiO2的符合多普勒展宽能谱。     

A simple approach to the analysis of positron lifetime spectra

A new method is proposed to deal with analysis of positron annihilation lifetime spectra which defy ordinary computer fitting. The method is particularly suited for investigations of defects in metals.     

Doppler程序对金红石型TiO_2的空位缺陷研究

金红石型Ti O2是一种非常好的稀磁半导体材料,其自身的本征缺陷与室温铁磁性起源密切相关.本文利用Doppler程序在广义梯度理论(GGA)的基础上,计算了正电子在金红石型Ti O2块材中不同缺陷处的湮没寿命.主要包括自由态正电子的湮没寿命,单空位和双空位处束缚态正电子的湮没寿命.并从理论上给出了含有空位缺陷时金红石型Ti O2的符合多普勒展宽能谱.     

Preparation of48V source for measurements of lineshapes of annihilation radiations and positron lifetimes

The preparation of the⁴⁸V positron source induced in a 1 μm thick Ti foil by (³He, pxn) reaction with a cyclotron is described. This source is very convenient for measurements of lineshapes of annihilation radiations and positron lifetimes at low or high temperatures, in a vacuum or for liquid metals. The absence of the mixing of long lifetime components is also convenient for the study of defects.     

碳纳米管束中的正电子理论

采用中性原子叠加模型和有限差分方法(SNA-FD)计算了大范围内不同管径的单壁碳纳米管束中的正电子情况，发现对于单壁碳纳米管束，正电子的主要湮没区域，湮没对象和正电子寿命随碳纳米管管径的不同而发生规律性变化.计算得到管径范围在0.8—1.6nm的碳纳米管束的正电子寿命范围为332—470ps，与实验测得的394ps符合较好.