CERN@school: demonstrating physics with the Timepix detector

This article shows how the Timepix hybrid silicon pixel detector, developed by the Medipix2 Collaboration, can be used by students and teachers alike to demonstrate some key aspects of any well-rounded physics curriculum with CERN@school. After an overview of the programme, the detector’s capabilities for measuring and visualising ionising radiation are examined. The classification of clusters – groups of adjacent pixels – is discussed with respect to identifying the different types of particles. Three demonstration experiments – background radiation measurements, radiation profiles and the attenuation of radiation – are described; these can used as part of lessons or as inspiration for independent research projects. Results for exemplar data-sets are presented for reference, as well as details of ongoing research projects inspired by these experiments. Interested readers are encouraged to join the CERN@school Collaboration and so contribute to achieving the programme’s aim of inspiring the next generation of scientists and engineers.     

Strong and Weak Disorder for Lévy Directed Polymers in Random Environment

Abstract

We consider Lévy directed polymers in the Poisson random environment. We give conditions for strong or weak disorder in terms of the Lévy exponent of symmetric Lévy process.     

PFPE脂润滑2Cr13钢摩擦副的真空摩擦磨损行为研究

利用销-盘式摩擦磨损试验机对PFPE脂润滑2Cr13钢摩擦副进行了不同滑动时间的真空滑动摩擦磨损试验.采用扫描电子显微镜(SEM)、X射线光电子能谱仪(XPS)和傅里叶变换红外光谱(FTIR)分别对磨损表面的形貌、化学状态及润滑脂的结构和成分进行了系统分析.结果表明:随着滑动时间的增加,由微切削磨损、轻微腐蚀磨损向严重腐蚀磨损、轻度局部剥落继续向严重黏着磨损、严重局部剥落的严重复合磨损转变.在真空摩擦过程中PFPE润滑脂主要以物理退化为主,即润滑脂中基础油和增稠剂的相对比例发生变化.同时,PFPE润滑脂与2Cr13钢在摩擦过程中发生化学反应,生成具有催化作用的Fe F3.但由于所生成的Fe F3量较少,并未使PFPE润滑脂发生化学降解.     

Mode-stirred reverberation chambers: A paradigm for spatio-temporal complexity in dynamic electromagnetic environments

We investigate probability distributions in dynamic multi-mode electromagnetic cavities, commonly referred to as mode-stirred reverberation chambers. We show that Bessel K$K$ and Bessel I$I$ distributions play a prominent role when a large but finite number of excited modes, loss of energy (through aperture leakage or dissipation), or nonstationary transient fields are involved. With the aim at reducing the number of simultaneously excited cavity modes as much as possible while maintaining a well-characterizable quasi-random field, measurement results indicate that single-mode stirring is feasible at certain frequencies well below the usual ‘lowest usable frequency’ of the cavity. Distributions for nonstationary fields are shown to allow for improved estimation of the maximum-to-mean ratio of the received power during stepwise rotation of the mode stirrer.     

基于综合故障的无线传感器网络无标度容错拓扑模型研究

针对无线传感器网络实际应用中遇到的环境损毁和能量耗尽的问题,本文首先对网络综合故障进行建模,获取满足综合故障容忍能力和网络生命期双重需求的网络节点度和节点度上限值的取值规律,并结合由无标度特征导出的两者关系,从而求得最优节点度上限值,最终引入关于节点度上限值的适应度函数,提出了容忍环境损毁和能量耗尽综合故障的无标度容错拓扑演化模型.仿真实验结果表明,该模型演化生成的无标度拓扑对环境损毁和能量耗尽具有较好的容错性,并能够有效地延长网络生命期.     

基于数字地图的二维电波传播问题仿真

为快速预测二维地理环境下的电磁环境特性,应用二维抛物方程模型对电磁环境进行仿真。为了得到真实有效的地形数据,研究了从GeoTiff中抽取网格点上地理信息的方法,并利用双线性插值法计算了任意位置处的高程值。同时研究了地球表面两点之间计算距离的方法,将该方法的计算结果与GIS软件结果进行对比,验证了该方法的可靠性。在标准大气环境下,利用二维抛物方程模型仿真分析了不同距离处不同高度的电波传播传播因子的变化情况,为预测真实地理环境中的电波传播特性提供了一种有效的方法。     

特征提取方法在多源混合信号分选中的应用

在复杂电磁环境中,脉冲的大量丢失(数据缺失)及信号参数空间的严重交叠,破坏了传统分选方法所利用的信号规律性,最终导致现有信号分选算法很难获得令人满意的分选效果甚至完全失效。在同时考虑多个关键特征指标(脉冲宽度、载波频率、到达时间)的基础上,设计多参数五层互耦的分选算法;特别是提出新的关键特征指标提取方法,研究数据内部蕴含的特征向量,建立相应的数学模型,最终应用于对各种通信信号的分选。通过数值结果可以看出,引入的五步分选法可以实现对严重交叠和部分数据缺失的信号的分选。     

基于非参数方法的频率密度分布计算及其并行优化

跳频序列预测是信息对抗的关键问题之一。基于跳频序列具有的伪随机特性和数据之间连续分布的相似性,采用非参数密度估计方法计算并预测出频率的大概率分布区间,进而用于引导通信的梳状灵巧干扰。针对实际应用对实时性的要求,在多结点多核平台上实现了基于消息传递机制的可扩展计算。测试表明,并行程序计算结果正确,并且具有较好的可扩展性。     

包含非线性组件的系统级电磁效应分析方法

提出一种包含非线性组件的场线耦合分析模型,在此基础上建立了包含非线性组件的系统级电磁效应分析方法。采用基于黑箱模型的非线性散射参数对非线性组件进行建模,给出了非线性散射参数的实验提取方法。利用焊接在微带传输线上的并联反向HSMS-282C型肖特基二极管构成非线性器件,并搭建原理性实验系统测量了电磁波辐射下连接在微带线上的该非线性器件输出端口的频谱和功率。实验结果与该分析方法的计算结果相吻合,证明了该方法的有效性和可行性,为系统级电磁效应分析提供了一种有效途径。