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关键词： germanium detectors   Monte Carlo simulation   gamma-ray interaction   multiple scattering coefficient   full-energy-peak efficiency  

摘要： Monte Carlo simulation results that were known to agree with efficiencies measured by means of radioactivity standards for two germanium detectors of very different sizes were used to examine efficiency responses to gamma rays of detectors that could not be measured directly or that would require use of numerous detectors. Contributions to the full-energy peak by multiple scattering of gamma rays preceding the photoelectric interaction were tabulated. The applicability of the commonly used straight-line relationship in logarithmic coordinates to plot intrinsic efficiency versus energy was tested for 14 detectors with widely different dimensions. Published empirical equations for predicting efficiency versus energy on the basis of detector volume were compared with simulation results. An equation that appears to be applicable to a wider range of volumes was obtained. The development of this equation makes it possible to evaluate the efficiencies of detectors with a wide range of volumes without necessitating direct measurement. (C) 2002 Elsevier Science Ltd. All rights reserved.

关键词： option valuation   currency options   GARCH   regime-switching   jump-diffusion  

摘要： We compare option valuation models based on regime-switching, GARCH, and jump-diffusion processes to a standard "smile" model, in which Black and Scholes (1973) implied volatilities are allowed to vary across strike prices. The regime-switching, GARCH, and jump-diffusion models provide significant improvement over a fixed smile model in fitting GBP and JPY option prices both in-sample and out-of-sample. The jump-diffusion model achieves the tightest fit. A time-varying smile model, however, provides hedging performance that is comparable to the other models for the GBP options. This result suggests that standard option valuation techniques may provide a reasonable basis for trading and hedging strategies. (C) 2003 Elsevier Science Ltd. All rights reserved.

关键词： Microenvironments   Neutrophils   Complex systems   Integrate   Neutrophil Infiltration   Infectious Agent   cell type   Dendritic Cells   Human   Epithelial Cells   Cell Locomotion   Host-Pathogen Interactions   basolateral membrane  

摘要： Many pathogens must overcome an epithelial barrier in order to establish an infection. Unsurprisingly, such pathogens have evolved different mechanisms to overcome this obstacle, targeting specific epithelial structures or functions. These include disruption of epithelial barrier function, transcytosing from the apical to the basolateral membrane domain or inducing cell movement such as neutrophil recruitment. When studying these processes in vivo, animal models often fail to mimic the disease observed in humans and present a complex system in which many variables cannot be controlled. Therefore, in vitro transepithelial models that permit the study of a relevant biological surface have been developed, to integrate not only interactions between bacteria and epithelial cells but also, under certain conditions, to integrate a third cell type, such as neutrophils or dendritic cells. Such models are particularly useful for studying the bacteria-host relationship as it would occur in the microenvironment of the human epithelium and have enhanced our understanding of the unique strategies by which pathogenic bacteria exploit host cells to overcome the initial epithelial hurdle.

关键词： 教学方法   CAI课件   寄生虫检验   实验教学  

摘要： 多媒体计算机辅助课堂教学(简称CAI)给教育带来了新的革命.随着现代科技的发展,CAI在课堂教学中的运用会越来越广泛.掌握现代教育技术,并能正确、合理地运用到教学工作中去,是信息时代的要求,也是素质教育的要求.在有效时间内使学生既掌握理论,又能准确辨认各种寄生虫的形态结构,是寄生虫检验实验课堂教学所追求的目标.CAI课件是现代教育技术的应用软件,是以文字、图像、图形、声音、影像、动画等多种合成媒体为辅助工具的教学方式.在寄生虫检验实验教学中使用CAI课件,能创造人或事的导入,激发学生的学习兴趣,促进课堂教学内容、教学方法和教学过程的优化,教学效果明显提高.

关键词： 财政政策   阶段性   非对称性   经济周期   中国   财政收入   经济波动  

摘要： 财政政策的非对称性主要是指它在经济周期的不同阶段对GDP的变化具有不同的响应。通过对经济周期状态的划分和度量 ,我们发现在我国财政收入和财政支出对不同程度的经济扩张和经济收缩具有非对称反应 ,而且预算盈余也表现了非对称迹象。因此 ,在我国经济的收缩期 ,应该加大财政政策的实施力度来刺激需求和促进经济增长 ,尤其要注意发挥财政收入对经济波动的调节作用。

关键词： 降水方案   降水深度   管道施工   检验   涌水量   地下水位   工程   地区   工时   成败  

摘要： 在地下水位较高地区进行管道施工时,经常会遇到施工排水问题,选用的降水方案是否合理,直接关系到工程的成败。本文作者通过分析群井的互阻作用后,提出了群井互阻涌水量的计算公式,并用其求出群井互阻后的降水深度,检验了降水方案的可行性。

关键词： 质量技术监督部门   标准化   计量工作   质量监督检验检疫工作会议  

关键词： Academic Achievement   Cognitive Development   Elementary Education   Learning Processes   Middle Schools   Science Activities   Science Instruction  

摘要： The current study examines the cognitive supports that underlie achievement in science by using a cultural historical framework (L. S. Vygotsky (1934/1986), Thought and Language, MIT Press, Cambridge, MA.) and the activity setting (AS) construct (R. G. Tharp & R. Gallimore (1988), Rousing minds to life: Teaching, learning and schooling in social context, Cambridge University Press, Cambridge, MA.) with its five features: personnel, motivations, scripts, task demands, and beliefs. Observations were made of the classrooms of seventh-grade science students, 32 of whom had participated in a prior achievement-related parent-child interaction or home study (P. R. Portes, M. F. Zady, & R. M. Dunham (1998), Journal of Genetic Psychology, 159, 163-178). The results of a quantitative analysis of classroom interaction showed two features of the AS: personnel and scripts. The qualitative field analysis generated four emergent phenomena related to the features of the AS that appeared to influence student opportunity for conceptual development. The emergent phenomenon were science activities, the building of learning, meaning in lessons, and the conflict over control. Lastly, the results of the two-part classroom study were compared to those of the home science AS of high and low achievers. Mismatches in the AS features in the science classroom may constrain the opportunity to learn. Educational implications are discussed. (C) 2002 Wiley Periodicals, Inc.

关键词： 卫生监督   执法   机构   非传染病   疾病控制   健康相关产品   卫生检验技术   计量认证   消毒管理   卫生行政   资质认证