塑料闪烁光纤阵列成像探测器在高能X射线辐照下的串扰分析

采用蒙特卡罗方法对闪烁光纤阵列探测器在高能X射线入射下的串扰进行了模拟研究,并且分析比较了加铅层对串扰的影响.研究中采用对表征成像系统空间分辨率参量--调制传递函数进行模拟分析和比较,得到在光纤阵列之间加入不同铅层厚度后对系统调制传递函数参量曲线的影响.研究结果表明:在高能射线下,采用闪烁光纤阵列作为成像探测器存在严重...     

塑料闪烁光纤阵列成像探测器在高能X射线辐照下的串扰分析

采用蒙特卡罗方法对闪烁光纤阵列探测器在高能X射线入射下的串扰进行了模拟研究,并且分析比较了加铅层对串扰的影响.研究中采用对表征成像系统空间分辨率参量——调制传递函数进行模拟分析和比较,得到在光纤阵列之间加入不同铅层厚度后对系统调制传递函数参量曲线的影响.研究结果表明:在高能射线下,采用闪烁光纤阵列作为成像探测器存在严重的次级粒子相互串扰的现象,而在阵列之间加入铅介质能够减少这种效应;但另一方面,若所加铅层太厚又会导致成像探测器像素过大而使得空间分辨率下降.通过模拟计算得出:只要在阵列之间加入适当厚度的铅介质,既可以有效抑止阵列之间次级粒子的串扰,同时又能提高闪烁光纤阵列探测器系统的空间分辨率.     

基于目标运动的复眼式双目视觉测距法

昆虫（螳螂）复眼利用目标的动态差异实现立体视觉，具有视场大、计算简单、实时性高等特点，是立体视觉研究的新方向。为实现复眼立体视觉在机器人视觉导航中的应用，根据复眼结构以及信息处理机制，提出并研究了环形光电传感器的目标快速检测与定位方法。首先，搭建了基于60个光电二极管的等6°夹角分布式环形传感器，形成具有360°视场的环形仿生复眼；其次，建立了基于光流原理的运动目标方位角检测模型，采用傅里叶拟合法实现了方位角检测模型的优化，实现了运动目标方位角与目标距离大视野范围内的简单、快速检测。实验结果表明:1）可实现距离375 mm范围内运动速度为30 mm/s的目标方位角实时检测，测量误差在2°范围内；2）基于目标方位角检测模型，可实现双目阵列传感器在300 mm×375 mm视野重叠区域范围内、平均测量误差在10 mm范围内的立体视觉测距。基于光流的运动目标方位角检测模型可用于实现对运动目标空间位置的动态实时检测，在运动检测、视觉导航等领域具有广泛的应用前景。     

基于Si-PM阵列的辐射作用位置定位方法研究

在辐射成像中准确估计射线与探测器相互作用的空间位置是保障成像质量的关键步骤。为进一步提升辐射作用事件的定位效果,有效抑制重心法在辐射事件作用位置靠近探测器边缘时造成的定位偏移,针对Si-PM阵列构成的位置灵敏探测系统建立了基于响应函数的定位算法。搭建了基于CsI阵列与Si-PM阵列构成的位置灵敏探测器和基于ASIC的电子学读出系统,使用等效电阻网络简化了Si-PM阵列的输出信号数量, 通过实验获得了辐射事件在Si-PM阵列上的响应函数。实验结果表明,在同一条件下使用响应函数法在最边缘像素散点的FWHM仅为使用重心法获得FWHM的46.2%,与中央像素散点的FWHM相当,基于响应函数的辐射事件方法定位效果明显优于传统重心法的定位效果,可有效克服重心法的边缘效应。     

利用照度测距圆重叠区重心估计的可见光室内定位方法研究

基于点光源照度函数测距原理,提出了一种利用相对照度测距圆重叠区重心估计的定位方法。通过探测器与光源投影点的照度比来测量探测器与光源投影点之间的距离,并建立以投影点为圆心的测距圆方程;将三测距圆的公共重叠区域的重心作为探测器位置的估计值,求出该公共重叠区域重心位置即可实现定位;实验中以LED模拟点光源,采用所提出的方法,定位误差约为3.51cm。实验验证了本方法可实现快速、精确地室内定位。     

应用半正定规划的目标方位超分辨方法

针对水下目标方位超分辨估计问题,提出了一种基于半正定规划(Sdp)的常规波束(CBF)方位超分辨算法(SdpCBF).Sdp-CBF算法基于常规波束形成获得多目标方位谱数据,利用阵列响应矩阵和半正定规划技术,精确估计目标数量和波达角方向.该算法的本质是利用阵列特性和信号能量信息获得超分辨方位估计,不用进行子空间分解,通过卷积反演的方式将阵列孔径的有限效应消除,在L₂范数约束条件下重构空间谱.仿真表明,Sdp-CBF算法具有较强的噪声抑制能力,对非相干和相干信号均具有目标方位超分辨能力,在低信噪比环境下的方位分辨性能超过多重信号分类(MUSIC)等经典高分辨算法。对消声水池以及湖上实验数据的处理结果显示,Sdp-CBF算法在复杂环境中对相干信号及微弱信号具有较强的分辨能力。      

基于大视场人工复眼定位技术

针对大视场目标探测提出了一种基于人工复眼大视场定位方法.通过分析子眼视场角与总视场角之间的关系,并结合多目视觉定位对子眼排布方式的要求,研究了包含多个子眼的人工复眼结构设计方法.通过分析子眼图像与三维空间映射关系,对二维图像进行裁剪并映射于三维立体空间,实现了二维子眼图像在三维空间的大视场拼接.利用子眼图像坐标、空间三维坐标及系统参数间的关系,建立了空间点多目定位数学模型,并编制目标定位算法.制备了包含19个子眼可实现120°大视场角的样机,通过张正友标定法获得系统参数,并进行目标定位实验.实验结果表明,使用设计的人工复眼大视场成像系统对5.35m处目标进行探测,定位误差为0.19%.     

高分辨率快速数字化光声CT乳腺肿瘤成像

提出了一种基于聚焦线性阵列探测器的快速光声计算机断层成像技术（光声CT）.在光声二维图像重建中,根据阵列探测器机械扫描和电子扫描相结合的组合扫描模式,提出了改进的有限场滤波反投影重建算法.一方面该算法适合多元探测器旋转扫描模式,另一方面探测器的指向性函数作为反投影的权重因子提高了系统的横向分辨率.同时,该成像系统还利用柱面声透镜实现Z轴方向上的聚焦扫描以实现三维层析成像.实验中,这套成像系统空间分辨率达到0.2mm,Z轴方向分辨率为1.5mm,扫描一幅二维图像仅需150s,得到 关键词： 光声CT 有限场滤波反投影算法 声透镜聚焦 乳腺肿瘤检测     

基于倾斜阵的距离与方位联合估计*

针对阵列倾斜引起的阵不变量方法定位精度下降的问题,该文结合倾斜阵声源定位的三维模型提出一种基于倾斜阵的距离与方位联合估计算法。该算法通过联合自校正方位距离联合估计方法对阵列倾角在声源-接收平面的投影量进行估计和补偿,改善目标距离估计误差的同时,利用匹配出的阵列倾角投影量反向估计目标方位,获得对声源二维位置的估计。通过仿真验证了该算法能够在阵列倾角较小时实现目标方位与距离的联合估计。     

无人机载型曲面仿生复眼成像测速系统

基于仿生复眼的视觉优势,将曲面仿生复眼结构应用于无人机载光电探测系统,实现机载宽视场高分辨运动目标探测。根据生物复眼的结构形态,设计了六边形排列的曲面透镜阵列作为曲面仿生复眼,辅以光学中继转像子系统和CMOS图像传感器,构成曲面仿生复眼成像测速系统。该系统的成像视场可达98°×98°,系统焦距为5 mm,角分辨率为1.8 mrad,F数为3.5,系统体积为Ф123 mm×195 mm,重量为1.35 kg。根据仿生复眼的成像原理,利用仿生复眼成像系统中相邻小眼存在的视场重叠优势,提出了曲面仿生复眼的测速原理,使得多个小眼能够同时探测场景中的同一个目标。运动汽车的测速实验表明该测速方法能够有效提高运动目标测试的可靠性和准确性。     

HOW TO EXTEND ANY DYNAMICAL SYSTEM SO THAT IT BECOMES ISOCHRONOUS,ASYMPTOTICALLY ISOCHRONOUS OR MULTI-PERIODIC

We indicate how one can extend any dynamical system (namely, any system of nonlinearly coupled autonomous ordinary differential equations) so that the extended dynamical system thereby obtained is either isochronous or asymptotically isochronous or multi-periodic, namely its generic solutions are either completely periodic with a fixed period or tend asymptotically, in the remote future, to such completely periodic functions or are multi-periodic (or become multi-periodic only asymptotically, in the remote future). In all cases the scale of the periodicity can be arbitrarily assigned. Moreover, the solutions of the extended systems are generally well approximated by those of the original, unmodified, systems, up to a constant rescaling of the independent variable (time), as long as their evolution is considered over time intervals short with respect to the (arbitrarily assigned) periodicities characterizing the extended systems. Several examples are displayed. In some cases the general solution of these dynamical systems is also exhibited; in others, this is impossible inasmuch as the models being manufactured are extensions of dynamical systems displaying chaotic evolutions, such as, for instance, the well-known Lorenz model of 3 nonlinearly coupled ODEs.     

Metriplectic framework for dissipative magneto-hydrodynamics

The metriplectic framework, which allows for the formulation of an algebraic structure for dissipative systems, is applied to visco-resistive Magneto-Hydrodynamics (MHD), adapting what had already been done for non-ideal Hydrodynamics (HD). The result is obtained by extending the HD symmetric bracket and free energy to include magnetic field dynamics and resistive dissipation. The correct equations of motion are obtained once one of the Casimirs of the Poisson bracket for ideal MHD is identified with the total thermodynamic entropy of the plasma. The metriplectic framework of MHD is shown to be invariant under the Galileo Group. The metriplectic structure also permits us to obtain the asymptotic equilibria toward which the dynamics of the system evolves. This scheme is finally adapted to the two-dimensional incompressible resistive MHD, that is of major use in many applications.     

Mayer coefficients in two-dimensional coulomb systems

We show that, for neutral systems of particles of arbitrary charges in two dimensions, with hard cores, coefficients of the Mayer series for the pressure exist in the thermodynamic limit below certain thresholds in the temperature. Our methods apply also to correlation functions and yield bounds on the asymptotic behavior of their Mayer coefficients.     

Visibility graph similarity: A new measure of generalized synchronization in coupled dynamic systems

Synchronization is defined as interdependencies among coupled dynamic systems. In most coupled systems the intrinsic and internal variants, and the interdependencies among their subsystems are not accessible. Therefore, in order to quantify the interdependencies among the coupled systems, attempts have been made through measuring the synchronization between their outputs represented mostly as time series. In this paper a new method, called Visibility Graph Similarity (VGS), is presented as a method of measuring Generalized Synchronization. First, each time series is reconstructed as a trajectory in a state space. Next, a Distance Time Series (DTS) is created from a sequence of relative distances of the states to a reference state. Subsequently, a visibility graph (VG) is constructed using DTS. Then, a sequence of degrees of the VG, called Degree Sequence (DS), is obtained. Correlation of the DSs of two coupled systems is called VGS and is presented as a measurement of similarity of dynamics of the coupled systems. The synchronization measurement performance of the VGS is compared with synchronization likelihood (SL) and the classical cross correlation method using two identical and non-identical models of two coupled Henon map over the entire time domain. Also, it is compared with SL for tracing temporal synchronization using both models. It is shown that VGS provides a more accurate measure of the overall synchronization compared with SL. It is more reliable for measuring weak couplings compared with the cross correlation method. Moreover, VGS uses fewer parameters and detects the temporal synchronization sooner than the SL.     

Heisenberg-Integrable Spin Systems

We investigate certain classes of integrable classical or quantum spin systems. The first class is characterized by the recursively defined property P saying that the spin system consists of a single spin or can be decomposed into two uniformly coupled or disjoint subsystems with property P. For these systems the time evolution can be explicitly calculated. The second class consists of spin systems where all non-zero coupling constants have the same strength (spin graphs) possessing N − 1 independent, commuting constants of motion of Heisenberg type. These systems are shown to have the above property P and can be characterized as spin graphs not containing chains of length four as vertex-induced sub-graphs. We completely enumerate and characterize all spin graphs up to N = 5 spins. Applications to the construction of symplectic numerical integrators for non-integrable spin systems are briefly discussed.      

Complex Classical Mechanics of a QES Potential

We study a combined parity (P) and time reversal (T) invariant non-Hermitian quasi-exactly solvable (QES) potential, which exhibits PT phase transition, in the complex plane classically to demonstrate different quantum effects. The particle with real energy makes closed orbits around one of the periodic wells of the complex potential depending on the initial condition. However interestingly the particle escapes to an open orbits even with real energy if it is placed beyond a certain distance from the center of the well. On the other hand when the particle energy is complex the trajectory is open and the particle tunnels back and forth between two wells which are separated by a classically forbidden path. The tunneling time is calculated for different pair of wells and is shown to vary inversely with the imaginary component of energy. Our study reveals that spontaneous PT symmetry breaking does not affect the qualitative features of the particle trajectories in the analogous complex classical model.     

Persistent current in an almost staggered Harper model

In this paper we study the persistent current (PC) of a staggered Harper model, close to the half-filling. The Harper model is different than other one dimensional disordered systems which are always localized, since it is a quasi-periodic system with correlated disorder resulting in the fact that it can be in the metallic regime. Nevertheless, the PC for a wide range of parameters of the Harper model does not show typical metallic behavior, although the system is in the metallic regime. This is a result of the nature of the central band states, which are a hybridization of Gaussian states localized in superlattice points. When the superlattice is not commensurate with the system length, the PC behaves as an insulator. Thus even in the metallic regime a typical finite Harper model may exhibit a PC expected from an insulator.     

Fluctuations of expectation values in chaotic systems and broken time-reversal symmetry

Fluctuations of expectation values of observables are calculated in complex quantum systems, such as disordered metallic grains or quantum systems with classical chaotic motion. We derive an exact expression for these fluctuations valid for systems with and without time-reversal symmetry, as well as in the transition region between these two cases. We compare our results with those of a semiclassical theory and with simulations of random matrices.     

Comments on the use of loop reactors in sonochemical processes

For sonochemical processing on an industrial scale the traditional choice is either a batch or flow system. The former is straightforward in concept but it requires large scale powerful ultrasonic transducers capable of delivering high intensity ultrasound to large volumes of liquid. Unfortunately at the moment the cost and problems involved in building very large sonication devices for batch processes cannot justify the replacement of existing industrial processes. For this reason most sonochemists prefer some form of flow system where small quantities of reagents can be treated as they are pumped from a large vat of reagents through a smaller sonochemical reactor where high intensity ultrasound can be applied. In this short paper we draw attention to a problem which seems common in a number of papers dealing with such flow systems – a confusion between the terms continuous reactor and loop reactor. Further we emphasise the importance of calculating the actual amount of ultrasonic processing experienced by the reaction mixture within the sonication zone of a loop reactor during its operation. The parameters required for such a calculation are: ultrasonic processor volume (R_v in L), pump flow rate (F_r in L/min), stock solution volume in the reservoir (S_v in L) and the overall system operating time (S_o in min).     

Consensus of second-order multi-agent dynamic systems with quantized data

The consensus problem of second-order multi-agent systems with quantized link is investigated in this Letter. Some conditions are derived for the quantized consensus of the second-order multi-agent systems by the stability theory. Moreover, a result characterizing the relationship between the eigenvalues of the Laplacians matrix and the quantized consensus is obtained. Examples are given to illustrate the theoretical analysis.