光子传输过程中g因子的计算

在电离辐射空气比释动能基准量值复现过程中,g因子作为关键组成部分之一参与量值复现和不确定度计算,其量值的确定需要通过光子的质能传输系数和质能吸收系数计算得到.采用不同的软件,在不同参数设置条件下模拟计算并对结果进行比较.结果 显示采用EGSnrc计算结果更为合理,而且对于低能光子,尽管g因子相对较小,但在不同的参数设置...     

低能X射线基准辐射装置的建立

建立了低能X射线基准辐射装置.在离辐射源1 m处得到非均匀性＜1％的辐射野直径＞40 mm,建立了5个CCRI推荐的低能X射线标准辐射质,研究低能X射线衰减规律,测定了半值层和同质系数.该装置的建立为低能X射线空气比释动能量值复现、完成2010年亚太地区APMP.RI(Ⅰ)-K2国际关键比对、建立低能X射线空气比释动能...     

250~600 kV X射线石墨空腔电离室壁修正因子及物理常数的蒙卡模拟北大核心CSCD

石墨空腔电离室由于对较高能量段有较好的能量响应,广泛应用于137Cs(有效能量为662keV)、60Co(有效能量为1250keV)以及平均能量为350keV的192Irγ射线空气比释动能的测量。运用EGSnrc程序模拟自制石墨空腔电离室在250~600kV窄谱辐射质下量值复现过程中的壁修正因子及物理常数。结果显示,石墨与空气的阻止本领比、空气与石墨的质能吸收系数之比、韧致辐射份额常数均与平均能量间有比较明显的关系可循,壁修正因子随着X射线能量的增加,则会有先变大后减小的趋势。蒙特卡洛模拟的结果为建立250~600kVX射线空气比释动能国家基准、实现量值复现提供了技术数据,达到了复现结果测量不确定度的预期要求。     

防护水平X射线电离室的校准及不确定度分析

防护水平电离室剂量计是辐射防护的主要计量器具,需要在窄谱参考辐射质下进行检定和校准。利用EGSnrc软件模拟了参考辐射质X射线能谱,分析得到的能谱分辨率和平均能量与ISO 4037-1推荐值的最大偏差分别为7.1％和1.04％,均满足规范要求。依托60~250kV X射线空气比释动能国家基准装置,在窄谱系列参考辐射质下完成了距离X光机1m处参考点的空气比释动能量值复现;然后通过替代法对两个传递电离室进行校准并完成量值传递;最后利用传递电离室复现的2.25m处的空气比释动能率对PTW-32002球形电离室进行校准,获得相应的校准因子,校准因子相对扩展不确定度为2.2%(k=2)。     

60Coγ射线空气比释动能绝对测量

为解决⁶⁰Coγ射线空气比释动能量值溯源与传递的问题以及进一步提升量值溯源与传递能力,中国测试技术研究院采用石墨空腔电离室以布拉格-格雷理论为基础测量光子辐射空气比释动能的方法建立⁶⁰Coγ射线空气比释动能绝对测量装置,复现⁶⁰Coγ射线空气比释动能,其量值复现的相对标准不确定度为0.2%。文中阐述和研究石墨空腔电离室结构和⁶⁰Coγ射线空气比释动能量值复现的影响因子。为进一步验证⁶⁰Coγ射线空气比释动能量值复现结果的准确性与一致性,中国测试技术研究院与加拿大国家实验室NRC (National Recerche Coucil),就⁶⁰Coγ射线空气比释动能量值复现进行测量比对,比对结果在相对标准不确定度0.52%以内一致,E_n值为–0.07。     

计量比对组织与实施中需关注的问题

正0引言计量比对是指在规定条件下,在相同量的计量基准、计量标准所复现或保持的量值之间进行比较、分析和评价的过程,简称比对。也就是指两个或两个以上实验室,在一定时间范围内按照预先规定的条件,测量同一性能稳定的传递标准,通过分析测量结果的量值确定量值的一致程度,确定实验室的测量结果是否在规定的范围内,从而判断实验室量值传递的准确性活动。比对能考察实验室测量量值和出具测量结果的准确一致程度;考核实验室在计     

低能原电子下形状因子对金属曲面二次发射系数的影响

提出金属曲面二次电子发射的形状因子概念,并从理论上论述了形状因子对曲面二次电子发射系数的影响。从半经验理论出发,推导出低能原电子入射下,金属曲面二次发射系数与入射角的近似关系,并结合曲面形状,得到形状因子一般表达式,进而求出金属曲面的二次发射系数。以圆柱曲面为例,计算其形状因子和低能入射下其二次发射系数,并用实验数据证明了该理论的正确性,最后对结果进行了讨论,得到如下结论:在低能原电子入射情况下,金属曲面二次发射系数为金属材料二次发射系数与形状因子相乘所得。     

可动屏蔽型计算电容复现电容单位的方法研究

中国计量科学研究院建立了标准不确定度为2×10-8的可动屏蔽型计算电容装置。该装置复现电容单位量值只与屏蔽电极所移动的轴向长度有关。采用激光干涉仪测量这个长度时,一般无法提供干涉条纹数。经典方法采用替换方法,逐步从已知的小整数扩大到大整数。过程复杂,所需条件多。提出了一种新的复现电容单位的方法。对应复现电容量值的干涉条纹整数由一个准确度等级相对较低的仪器直接确定。其干涉条纹小数则通过电容电桥的两次平衡确定。所需结构及过程简单,操作方便。给出了该方法的详细内容及其优点,报告了检查装置线性的实验及其结果。     

低能X射线空气比释动能国际关键比对

为促进我国在低能X射线空气比释动能计量领域内的量值统一并达到国际等效,中国计量科学研究院(NIM)于2018年采用平板自由空气基准电离室复现低能X射线空气比释动能,并通过传递电离室Radcal-RC6M在国际计量局(BIPM)低能X射线辐射场中开展了间接比对,其比对结果在合成标准不确定度为0.37%范围内与国际计量局等效一致,该比对结果已纳入国际计量局的关键比对数据库(KCDB)。     

用于放疗量传的照射量、空气比释动能和水吸收剂量量值体系比较

用照射量和空气比释动能校准的电离室进行剂量测量时,依据IAEA TRS 277报告,需要经历四级量值转换过程,不确定度也较大,但目前仍是国内使用的量值体系.我国正在建立60Co γ射线以及高能光子下的水吸收剂量基准装置并进行国际比对,之后将拥有水吸收剂量的量值复现的能力.在60Co γ射线参考辐射场和加速器高能X射线辐射场下,使用NE2571和NE2570/1A、PTW TW30013和PTW UNIDOS两套电离室剂量仪,分别按照277和398报告的要求计算并比较2种方法计算出的水吸收剂量值,从而验证了277报告和398报告的一致性.     

Imaging of highly turbid media by the absorption method

The results of a study on imaging that is based on the absorption method are presented. This method is based on attenuation measurements carried out in the presence of a sufficiently high absorption coefficient by the use of a continuous-wave source. The benefit of absorption on image quality comes from the strong attenuation of photons traveling along long trajectories. When the absorption coefficient is increased, the received energy decreases, but the mean path length of received photons decreases. The effect of increasing the absorption coefficient is similar to that of decreasing the gating time when the time-gating technique is used. Experimental results showed that the spatial resolution obtained with the absorption technique is similar to that obtained with the time-gating technique.     

Electrical conductivity in thin and very thin platinum films

The two-photon transition probability between electron energy bands in a semiconductor film is discussed, taking quantum size effects into account. An expression for the absorption rate of one of the two photons is given as a function of the frequency corresponding to all the types of critical points. The anisotropy of non-cubic materials is taken into account. The numerical calculation of the absorption coefficient is performed for the case of ZnS near the extremal point.     

Computational Analysis of the Effect of Nano Particle Material Motion on Mixed Convection Flow in the Presence of Heat Generation and Absorption

The present study is concerned with the physical behavior of the combined effect of nano particle material motion and heat generation/absorption due to the effect of different parameters involved in prescribed flow model. The formulation of the flow model is based on basic universal equations of conservation of momentum, energy and mass. The prescribed flow model is converted to non-dimensional form by using suitable scaling. The obtained transformed equations are solved numerically by using finite difference scheme. For the analysis of above said behavior the computed numerical data for fluid velocity, temperature profile, and mass concentration for several constraints that is mixed convection parameter λ_t, modified mixed convection parameter λ_c, Prandtl number Pr, heat generation/absorption parameter δ, Schmidt number Sc, thermophoresis parameter N_t, and thermophoretic coefficient k are sketched in graphical form. Numerical results for skin friction, heat transfer rate and the mass transfer rate are tabulated for various emerging physical parameters. It is reported that in enhancement in heat, generation boosts up the fluid temperature at some positions of the surface of the sphere. As heat absorption parameter is decreased temperature field increases at position X = π/4 on the other hand, no alteration at other considered circumferential positions is noticed.     

Efficiency factors and radiation characteristics of spherical scatterers in an absorbing medium

The radiative properties of bubbles or particles embedded in an absorbing medium are investigated. We aim first to determine the conditions under which absorption by the surrounding medium must be accounted for in the calculation of the efficiency factors by comparing results from Mie theory and the far-field and near-field approximations. Then, we relate these approximations for a single particle to the effective radiation characteristics required for solving the radiative transfer in an ensemble of scatterers embedded in an absorbing medium. The results indicate that the efficiency factors for a spherical particle can differ significantly from one model to another, in particular for large particle size parameter and matrix absorption index. Moreover, the effective scattering coefficient should be expressed based on the far-field approximation. Also, the choice of the absorption efficiency factor depends on the model used for estimating the effective absorption coefficient. However, for small void fractions, absorption by the matrix dominates, and models for the absorption coefficient and efficiency factor are unimportant. Finally, for bubbles in water, the conventional Mie theory can be used between 0.2 and 200 mum except at some wavelengths at which absorption by water must be accounted for.     

剧院舞台台口等效吸声系数研究

混响时间是剧院建筑声学设计的核心指标.在进行混响时间仿真计算时,由于耦合空间的缘故,通常会将观众厅独立出来,并在舞台台口处设置一个等效吸声面,以反映舞台空间和观众厅的相互作用.然而,对于不同体型、不同吸声条件的观众厅与舞台空间,舞台台口应有不同的等效吸声参数.通过对观众厅与舞台空间混响混响时间的分别计算,推导出一组舞台...     

Feasibility Investigation of NbN Nanowires as Detector in Time-of-Flight Mass Spectrometers for Macromolecules of Interest in Biology (Proteins)

We are investigating the possibility of using NbN nanowires as detectors in time-of-flight mass spectrometers for investigation of macromolecules of interest in biology (proteins). NbN nanowires could overcome the two major drawbacks encountered so far by cryogenic detectors, namely the low working temperature in the mK region and the slow temporal response. In fact, NbN nanowires can work at 5 K and the response time is at least a factor 10–100 better than that of other cryogenic detectors. We present a feasibility study based on a numerical code to calculate the response of a NbN nanowire. The parameter space is investigated at different energies from IR to macromolecules (i.e. from eV to keV) in order to understand if larger value of film thickness and width can be used for the keV energy region. We also present preliminary experimental results of irradiation with X-ray photons of NbN to simulate the effect of macromolecules of the same energy.      

Hydromagnetic stagnation point flow by a perturbation technique

An analysis is performed to study the momentum, heat and mass transfer characteristics of MHD natural convection flow and heat generating/absorbing fluid at the stagnation point of an isothermal two-dimensional porous body immersed in a fluid saturated porous medium. The results are obtained by solving the coupled non-linear partial differential equations describing the conservation of mass, momentum and energy by a perturbation technique [A. Aziz, T.Y. Na, Perturbation Methods in Heat Transfer, Springer-Verlag, Berlin, 1984 (pp. 1–184), R. Kenneth Cramer, Shih-I Pai, Magnetofluid Dynamics for Engineers and Applied Physicists, McGraw-Hill Book Company, New York, 1973 (pp. 164–171).]. These results are presented to illustrate the influence of the Hartmann number, Prandtl number, and dimensionless heat generation/absorption coefficient and suction injection parameter. Numerical results for the dimensionless velocity profiles, the temperature profiles, the local friction coefficient and the local Nusselt number are presented for various parameters. These effects of the different parameters on the velocity and temperature as well as the skin friction and wall heat transfer are presented graphically.     

结霜翅片管换热器热质传递分析

低温工况下翅片管换热器的结霜是一个复杂的热质传递过程.从能量的角度出发,由换热器的对数平均温差(LMTD)引出对数平均焓差(LMED),改进了传统的基于对数平均温差(LMTD)的结霜翅片管换热器传热、传质模型,并通过实验研究了影响翅片管换热器结霜及能量传递系数(E0)的主要因素.结果表明,能量传递系数(E0)更准确地反映了结霜工况下换热器的热质交换情况,可以作为系统在霜运行及实时化霜的重要性能参数.     

湿工况下翅片管式换热器中刘易斯因子的数值研究

刘易斯因子是反映湿工况下空气侧热质传递特性的重要参数。在此提出了可计算湿工况下翅片侧刘易斯因子的数值模型和适用于工程应用的关联式,通过建立相间传质数值模型、潜热传递数值模型,利用CFD分别计算出空气侧的热质传递系数,从而求得刘易斯因子的数值解。利用数值仿真方法分析了翅片管换热器结构和运行工况对刘易斯因子的影响和已有热质传递类比关系的适用性;同时采用刘易斯因子的数值计算结果拟合得到关联式。传热和传质数值计算结果和实验的相对误差分别为6.93%和12.1%;关联式与数值模型之间的相对误差为5.52%。该数值模型有足够的精度与适用面,所得关联式可代替数值模型。     

纤维截面形状对聚酯纤维材料吸声性能的影响研究

为探讨聚酯纤维截面形状对吸声性能的影响,通过热压法制备了4种不同纤维截面形状的聚酯纤维板,利用阻抗管对4种聚酯纤维板在80~6 300 Hz频率范围内的吸声性能进行测试。根据流阻率模型和声阻抗模型对材料吸声性能进行预测,并将该模型的计算结果与测试结果进行比较。结果表明：扁平截面的聚酯纤维材料的吸声性能最佳,平均吸声系数达到0.404;对于聚酯纤维材料的吸声系数与模型的计算值,圆形截面聚酯纤维材料吻合较好（全频段误差率为2.036%）,异形截面聚酯纤维材料的误差较大。