Modeling quantum noise of phosphors used in medical X-ray imaging detectors

The noise properties of the granular phosphor screens, which are utilized in X-ray imaging detectors, are studied in terms of the quantum noise transfer function (QNTF). An analytical model, taking into account the effect of K-characteristic X-rays reabsorption within the phosphor material and the optical properties of the phosphor, was developed. The optical properties of the phosphor material required by the model were obtained from literature, except for the optical diffusion length (σ) that was determined by data fitting and was found to be 26 cm²/g. The deviation between theoretical and experimental data is σ depended. Specifically for σ=26 cm²/g and σ=25 cm²/g the respective deviations between experimental and predicted results were 0.698% and −1.597%. However for relative differences in σ more than 15% from the value 26 cm²/g, the corresponding deviations exceed by 6 times the value of 0.698%. The model was tested via comparison to experimental results obtained by a set of Y₂O₃:Eu³⁺ phosphor screens prepared by sedimentation. The model may be used to evaluate the effect of screen thickness and irradiation geometry on quantum noise of phosphor materials for transmission and reflection mode.     

307 200 pixels/cm resolution phosphor screen in monochrome CRT

We have studied the screen luminance versus irradiation densities of electron beam on phosphor screens, by using the monochrome miniature CRTs. It is found that for irradiation of electron beam on phosphor screens, there is a critical condition which is determined by the electric field of the holes (∑E_h) in the surface volume of the phosphor particles. The clear, sharp, and flickerless images at a high luminance are obtained in the region of ∑E_h<E_a in which the screen luminance has a linear relation with the electron beam densities, where E_a is the electric field at top layer of screen by the anode. In the region of ∑E_h>E_a, the cathodoluminescence images smear, flicker, and fade due to the surface-bound-electrons on phosphor particles. A high resolution image (307 200 pixels/cm²) of VGA (640×480 pixels/cm²) at 2000 cd/m² is obtained with the screen of a practical 0.5 in. CRT(1 cm²), which is operated with the region of ∑E_h<E_a.     

X-Ray Efficiency and Modulation Transfer Function of Fluorescent Rare Earth Screens,Determined by the Monte Carlo Method

The overall, X-Rays to light conversion efliciency and the modulation transfer function of radiographic single front screens have been computed using the Monte Carlo method to simulate the generation and the multiple-scattering diffusion and absorption of light photons in the screen. The computation is based on a set of probability functions related to the absorption of X-Ray photons, free-path lengths and absorption of light photons and reflection conditions at the boundaries. A constant scattering angle has been used for light photons colliding with phosphor grains. For a given phosphor volume fraction, results are shown as a function of phosphor grain size, screen thickness and light wavelength. Some computed results have been checked experimentally.

A comparison is made between screens based on mainly green emitting terbium activated gadolinium oxysulphide, and conventional screens based on blue emitting lead activated calcium tungstate. Screens having equal packing volume, thickness and phosphor grain size show a much higher conversion efficiency and improvement in modulation transfer functions when made of Gd₂O₂S:Tb.     

YVO4:Eu3+@YPO4纳米核壳结构荧光粉的水热合成及表征

为了提高传统YVO₄:Eu³⁺荧光粉的发光效率及稳定性, 采用水热法制备合成了YVO₄:Eu³⁺@YPO₄纳米核壳结构荧光粉, 通过X射线衍射仪(XRD)、透射电镜(TEM)和荧光光谱仪(PL)等测试手段对所得样品进行表征. XRD和TEM测试结果表明: 由水热法合成的YVO₄:Eu³⁺@YPO₄荧光粉包含YVO₄:Eu³⁺核心和YPO₄壳层两种结构, 荧光粉粒径为10~30 nm, 壳厚为5~10 nm, 形态规则、粒径均匀、结晶度高; 荧光光谱测试结果表明: YVO₄:Eu³⁺@YPO₄荧光粉比单纯YVO₄:Eu³⁺荧光粉的发光效率高出66.75%, 且具有较高的色纯度. 结合第一性原理方法, 对YVO₄和YPO₄晶体的能带结构进行理论计算, 定性说明了电子跃迁和发光的关系.     

阻尼板材在高架轨道箱梁结构噪声控制中的应用研究

为探讨高架桥梁结构噪声的控制措施,以京沪高铁32 m无砟轨道箱梁结构为原型,设计制作1/10的模型试验系统。通过将TD09型高性能阻尼板材分别敷设于箱梁翼缘板、腹板等位置,进行多工况的桥梁结构噪声降噪的模型试验研究。结果表明:高架轨道箱梁结构噪声峰值频段为200~1000 Hz,敷设阻尼板材在峰值频段内具有一定的降噪效果。阻尼板材对桥梁结构降噪效果与阻尼板材的敷设位置有关,其在桥梁结构噪声控制中有一定的应用价值。在峰值频率500 Hz处,翼缘板敷设阻尼板材对翼缘板下侧降噪效果最好,降噪约为1.6 dB(A);腹板敷设阻尼板对底板处的降噪效果最好,降噪可达3.8 dB(A);腹板及翼缘板同时敷设阻尼板材也对底板处的降噪效果最好,降噪可达3 dB(A)。     

轨道交通噪声机理研究与控制

介绍了轨道噪声研究的现状和发展动态,以期为进一步研究提供参考.主要介绍了轨道交通噪声产生的机理,国外对轨道交通的研究过程中所涉及到的一些问题,诸如噪声源的定位,声屏障的计算,地面结构的振动,轮轨的优化设计和降噪手段等.     

Using thin film stress to produce precision, figured X-ray optics

We are studying the possibility of producing precision, aspherical mirrors for X-rays and visible light. Our study examines the use of ultrastructure processing to replace mechanical methods of material removal. The method starts with a chemically-mechanically polished, flat silicon wafer. The aim is to preserve atomic scale smoothness of the surface wafer while the wafer is bent to a desired figure. We report measurements of the mechanical properties of various stressing layers. This involves measuring the deformation of several thin silicon wafers coated with chemically vapor deposited nickel and boron films of known thickness. We have found that, under normal conditions, the film does not add to the microroughness of the substrate on either the front or the back surfaces. Film and substrate thicknesses, however, vary by as much as 10%. This is the present limit on figure accuracy. We have developed a model that describes bending of B/Si and Ni/Si structures. The model relates stress and Young's modulus to the measured thickness of the film, and the thickness and curvature of the substrate. This approach is used to measure the stress and Young's modulus for boron and nickel films. The Young's modulus E_f was 3.05 x 10¹² Pa for the boron films and 1.4 x 10¹⁰ Pa for the nickel films. From the relationship developed and verified for predicting the radii of curvature of the substrate, if may be possible to define a film thickness pattern which would provide a desired optical figure.     

两种非高斯海洋环境噪声统计模型分析

海洋环境噪声是影响声呐工作的主要因素之一,噪声模型的选取十分重要,普遍采用的高斯噪声模型在很多情况下存在局限性。引出两种非高斯噪声模型匹配实际非平稳的海洋环境噪声:广义自回归条件异方差模型(Generalized Autoregressive Conditional Heteroscedasticity,GARCH)和双模模型,通过分析两种噪声的概率密度分布函数,并与高斯模型噪声和实测海洋环境噪声比较得出两种噪声模型的适用性。GARCH(1,1)模型通过调节参数可以吻合大部分浅海和深海海洋环境噪声,双模模型则只对浅海某些情况存在适用性,而对深海吻合性较差。两种噪声模型的统计特性分析表明它们可以适用于高斯噪声模型存在局限的非平稳环境下。     

基于二次电子发射的FED中支撑结构的优化设计

基于二次电子发射的场致发射显示器(HOPFED)是一种新型的场致发射器件.其新颖之处在于其独特的支撑形状,这种结构的优点是充分提高了场致发射显示的性能,如对比度、色纯及像素内的发光均匀性,并减少了离子轰击发射源.在目前已有的结构中,环形的屏上光点不能使荧光粉得到充分的利用,而且光点尺寸较小,容易使荧光粉达到饱和.为了得到符合荧光粉要求的光点形状,本文提出了diabolo形状flu支撑、ZEUs形状hop支撑以及长flu支撑三种支撑形状,并采用数值计算的方法对这几种不同的支撑结构对屏上光点的影响进行了模拟计算.计算结果表明,对这三种不同的支撑形状进行优化以后都能得到符合荧光粉的要求的屏上光点.在现有的工艺条件下,长flu支撑是一种能改善光点和场强分布,而又易于制作的支撑结构.     

车轮参数对轮轨噪声的影响

铁路列车的运行噪声给沿线居住环境带来了严重影响,在已建立的轮轨滚动噪声预测模型的基础上,车轮参数的变化对轮轨噪声的控制进行分析,分析表明,适当减小车轮半径,增加车轮辐板厚度和车轮踏面质量有助于降低轮轨噪声。     

基于开槽管腔的高信噪比铷原子钟物理系统

物理系统提供的原子鉴频信号信噪比是决定铷原子钟频率稳定度的关键因素。借助高频结构仿真软件,设计了一款内径为20 mm的开槽管微波腔。分析和测试表明,该微波腔内微波场磁力线沿腔轴方向均匀密集分布,可激励高强度铷原子微波跃迁。基于这种微波腔,设计出分离滤光物理系统。借助F-P干涉仪光谱测量,优化了滤光效果。测试表明,这种物理系统具备高信噪比,可用于制造频率稳定度为5.0×10^-13t^-1/2的铷原子钟。     

高效管道有源降噪系统—理论与试验

本文建立了管道单极有源降噪系统的数学模型,在此基础上建立了两个管道单极有源降噪系统,分别称之为NCMS 系统和LFAM 系统。两个系统各具特点,分别适用于探测传声器的指向性因子α不同时的场合。NCMS 系统用补偿的方法对次级源的声反馈和系统组件的非理想相移特性进行了双重补偿。LFAM 系统通过改善探测传声器的工作指向性,使系统结构十分简单。两个系统都取得了良好的降噪效果,并实现了气流条件下的宽带降噪。     

船舶螺旋桨辐射噪声与尾流场的相关性及特征分析方法

研究螺旋桨几何特征和工况特征变化对螺旋桨尾流场和噪声特征的影响。采用黏性多相流理论对螺旋桨尾流场特征进行数值预报。利用N-S方程、混合多相流模型、湍流模型和空化模型数值求解尾流场压力脉动等物理量。将压力脉动数值解和测量噪声声压信号的功率谱低频线谱特征进行分析,表明两者特征具有一定的相关性。最后分析侧斜角和进速系数对压力信号频谱特征的影响,获得了低频线谱幅值分布特征与螺旋桨侧斜角、进速系数等参数的初步关系。     

机翼屏蔽对航空发动机噪声预测的影响分析

随着对飞机适航噪声的要求更加严格,利用机翼屏蔽效应对航空发动机进行噪声控制已成为一项有效策略与研究方向.基于惠更斯-菲涅尔原理,利用飞机噪声性能(Aircraft Noise and Performance,ANP)数据库计算飞机起飞航迹并在其上建立噪声源,声源位置作为噪声屏蔽计算坐标输入屏蔽效应算法,应用Matlab...     

Squeeze flow characterisation of thermoplastic polymer

New manufacturing concepts developed by the CRC-ACS involve co-curing a thermoplastic polymer layer onto the surface of a composite component. The layer can be reshaped to alter the dimensions of the laminate locally. A major physical phenomenon involved in these uses is squeeze flow of the thermoplastic polymer.

In the present work, the squeeze flow of a selected thermoplastic at high temperature and under given pressure was studied. A power-law model for viscosity variation with shear rate was assumed to describe the non-Newtonian behaviour of the thermoplastic at high temperatures. Based on the viscosity model, a fluid mechanics model was derived for the squeeze flow between approaching parallel plates of infinite length. The model relates the plate approaching speed to the applied pressure, thermoplastic geometry (both thickness and width) and power-law viscosity model parameters.

An experimental method and data analysis procedure were developed to determine the parameters of the power-law model that describes the viscosity of the thermoplastic material. Tests were conducted under isothermal conditions by squeezing composite laminates with integrated thermoplastic films. The transient thickness of the thermoplastic film was continuously measured and used to calculate the power-law model parameters for each test. A temperature range of 180–200 °C was investigated to establish the dependence of the power-law model parameters upon temperature.

Using the squeeze flow model, and experimentally determined power-law viscosity model parameters, the effect of various process conditions on the thermoplastic thickness was investigated. Predictions were found to be valid for situations where the shear rate range matched that achieved during the power-law viscosity model parameters tests.     

基于声品质的直升机舱内典型降噪措施分析

直升机舱内噪声严重降低了乘员的乘坐舒适性,进行直升机舱内降噪设计研究十分必要.本文以声品质最优为目标,以直升机舱内噪声烦恼度评价结果为依据,通过多元线性回归方法构建舱内声品质模型,利用计算声学软件模拟直升机舱内添加不同种类降噪措施后的双耳可听声信号,从不同角度对典型降噪措施的效果进行测评及对比分析.结果表明:在舱内添加...     

SMPDP放电单元中荧光粉层对放电特性的影响

本文采用流体模型 ,计算了新型荫罩式PDP(SMPDP)放电单元的放电特性。研究了放电单元中荧光粉层的厚度对放电过程和着火电压的影响。模拟结果表明 ,在SMPDP结构中 ,荧光粉层厚度的变化 ,对放电过程的影响小于通常的表面放电型PDP(ACCPDP)。最低着火电压随荧光粉层厚度的不同变化较小。因此 ,SMPDP结构中 ,荧光粉层涂敷的不均匀性对放电均匀性的影响小于ACCPDP结构     

Some High Intensity Flash Light Sources of the Luminescent Tube Type

The design and performance of vacuum flash tubes of the cathode ray tube type are described in which short-duration impulses of a high-energy electron beam are used to excite a phosphor screen and thus to produce luminescent radiation pulses of high intensity. Triode type tubes are shown which may be operated with 2kW pulses, and a diode type tube is described where the screen may be energized with I MW pulses. Either single-pulse radiation or trains of pulses with short pulse intervals can be produced, the duration of an individual radiation pulse ranging from about 0.1 microsec to the order of 100 microsec according to the rated value of the respective flash tube. The spectrum of the radiation is continuous with a form of the spectral curve depending on the type of phosphor used for the screen.     

高速列车阻尼喷涂式铝型材减振降噪特性试验

为探究某种阻尼材料对高速列车铝型材地板的减振降噪效果,以波纹状铝型材为基板,先后对其喷涂厚度为2 mm和4 mm的阻尼层,并在隔声室中进行空气声隔声及结构振动声辐射的测试及比对分析。结果显示,随阻尼层厚度的增加,铝型材的空气声隔声效果增加,尤其在500 Hz之后的中高频段;其中,2 mm阻尼层能在铝型材裸板的基础上使计权隔声量提高4.5 dB,阻尼层厚度增至4 mm,计权隔声量再提高2.4 dB。在100 Hz ～250 Hz,2 mm阻尼层对降低铝型材的振动声辐射水平起反作用,而4 mm阻尼层能够起到一定作用;在315 Hz ～400 Hz,阻尼层厚度对其振动声辐射几乎没有影响;500 Hz以上,随阻尼层厚度的增加,铝型材振动声辐射水平大大降低,其中,500 Hz、1 250 Hz和3 150 Hz 三个频段的降低量最为显著。