Positron transmission and effective mass absorption coefficient in nickel

The penetration of positrons from a²²Na source into Ni foils sandwiched between different backscattering materials (Kapton, NaCl, Ni, and Mo) has been studied for the geometry commonly used in PAT experiments. Transmitted intensities and effective mass absorption coefficients have been found and the dependence on the backscattering material is discussed. General formulas for the effective mass absorption coefficients are derived. The large geometrical effects are stressed. Suggestions for source corrections are supplied.     

The diffusion coefficient depends on absorption

The diffusion approximation is widely invoked to model the propagation of light in turbid media. When absorption is not weak in comparison with scattering, there is currently a controversy as to if, and how, the diffusion coefficient depends on absorption. Here it is shown that better agreement with random walk simulation is obtained if the photon-diffusion coefficient is taken as D=c/(3micro(s)(')+micro(a)) . One can reconcile this result with recent work advocating D=c/3micro(s)(') by noting that the diffusion equation must be correspondingly changed to a telegrapher's equation.     

Role of the mass attenuation coefficient database in standardization of a silicon drift X‐ray detector for PIXE analysis

A simple, inexpensive procedure is described for calibrating a silicon drift detector‐based PIXE system for the analysis of geological and other “thick‐target” materials. It rests on the use of single element, chemical compound, and National Institute for Standards and Technology multielement standards. Much less effort has been focussed on the impact of mass attenuation coefficients on PIXE's analytical accuracy than on ionization cross sections and fluorescence yields. The calibrated system enables us to investigate the effects of inserting different mass attenuation coefficient datasets into the GUPIX database. For the K X‐rays of light elements, accuracy is significantly improved by replacing the formerly used XCOM self‐attenuation coefficients by the corresponding FFAST values; this effect decreases rapidly with increasing atomic number. Similar improvement was found for L X‐rays of medium‐Z, but not for high‐Z atoms.     

Fluctuations in the absorption coefficient of a single molecule at different signal acquisition times

The relation between the absorption coefficient of a single molecule calculated within the framework of a dynamical theory and the fluctuations in the absorption coefficient measured in experiments on scanning the laser frequency is analyzed in relation to the acquisition time of fluorescence photons. The analysis is performed for the case where the optical band of an impurity center consists of two well-resolved lines. A method of processing the fluctuating fluorescence is proposed that makes it possible to obtain the averaged absorption coefficients, which can be compared with the coefficients calculated theoretically.     

Mg_2Ge吸收系数和能量损失函数的第一性原理计算

应用密度泛函理论框架下的第一性原理超软赝势平面波方法系统地计算了Mg_2Ge基态的电子结构、电子态密度、弹性常数以及主要光电性质.计算结果表示Mg_2Ge是一种间接带隙半导体,禁带宽度为0.2136 eV;其价带主要由Ge的4s,4p态电子组成,导带则主要由Mg的3s,3p以及Ge的4p态电子组成;静态介电常数ε_1(0)=25.294;折射率n_0=4.5043;吸收系数最大峰值为396560.9 cm~(-1);通过计算弹性常数解释了Mg_2Ge的脆性;并分析了所计算的Mg_2Ge光电性质和其能带结构,为Mg_2Ge提供了在光电应用领域的理论依据和实验指导.     

Diffusion coefficient depends on time, not on absorption

The recent controversy over whether the photon diffusion coefficient depends on absorption is addressed by use of the analytical solution of the photon transport equation in an infinite homogeneous scattering medium. The diffusion coefficient is found to be independent of absorption but temporally dependent. After a long period of time, the photon diffusion coefficient approaches D=1/3mu(s)(?) , which supports a claim made by Furutsu and Yamada [Phys. Rev. E 50, 3634 (1994)]. At early times, the diffusion coefficient is smaller than D=1/3mu(s)(?) , but this reduction cannot be expressed as D=1/3(mu(s)(?)+mu(a)) , since the time-dependent diffusion coefficient is found to be unrelated to absorption.     

High-temperature absorption coefficient of methane at 3.392μ

The spectral absorption coefficient of methane at 3.392μ has been measured in the temperature range 965 ?T, °K≤2710 behind incident and reflected shock waves. It is given by the relation P'=P'₀(T₀/T)ⁿ where P'₀=(1.34±0.58)x10²cm^-1atm^-1 at T₀=300°K and n=2.88±0.21. The empirically determined temperature exponent n may be approximately accounted for by a simplified theoretical analysis.     

Theoretical lower bound on the absorption coefficient of infrared transmitting materials

A calculation of the absorption due to anharmonicity-induced multi-phonon processes is employed to predict a lower bound for the absorption coefficient of infrared transmitting materials.     

应用PIXE技术对书写字迹的分析

在国内首次应用质子激发X射线荧光法对常见的书写字迹进行刑事检验分析,确认PIXE技术可以作为文件检验当中字迹同一认定的有效方法。同时,质子激发X射线荧光法也是一种有效的无损检验的方法。提出了不同于现在的解决途径的质子激发X射线荧光技术解决法庭科学当中的两大疑难问题——书写时间的鉴定、毒品源头的认定。     

Measurement of acoustic absorption coefficient with phase-conjugate ultrasonic waves

Experimental results on measurements of the acoustic absorption coefficient in test objects that were obtained with two methods, i.e., a standard insert-substitution method and a modification thereof using phase-conjugate waves, are given. Samples of gelatin and biological tissue in vitro (porcine muscle fibers) were used as test objects. Gelatin objects were manufactured that were both homogeneous and with inhomogeneities in the form of a rough surface or inclusions (air bubbles) distributed over the volume. A rough surface leads mainly to phase distortions of a probe beam, while bubble inclusions cause additional field scattering. For all homogeneous samples, both compared methods produce identical results. In the case of inhomogeneous samples including biological tissues, absorption measurement by a standard method may lead to significant errors. It is demonstrated that the use of properties of phase-conjugate waves provides an opportunity to eliminate almost completely the measurement error connected with phase distortions and reduce the error in the case of a medium with scatterers.     

硅的间接跃迁双光子吸收系数谱

利用皮秒Nd:YAG脉冲激光器作为激发光源,测量出光子能量介于1.36 μm (0.912 eV)—1.80 μm (0.689 eV)之间的硅间接跃迁双光子吸收系数谱.尽管此波段范围内的激光光子能量小于硅间接带隙,但当激光辐照在硅基光电二极管受光面时,在二极管两电极端仍然探测到了显著的脉冲光伏信号.光伏信号峰值强度与入射光强呈二次幂函数关系,表明其是双光子吸收过程.采用pn结等效结电容充放电模型,将光伏响应信号峰值与入射光强相关联,从中提取出硅的间接跃迁双光子吸收系数,改变入射波长得到系数谱.研究表明:     

银吸收边附近X射线衰减系数测量

在北京同步辐射装置的4B7A中能X光束线上,光源能区为2.1~6.0 keV,能量分辨大于5000,高次谐波小于0.1%,光源强度大于109光子/s。通过全能区多能点的透过率精确测量Ag样品质量厚度,然后采用Ag薄膜对单能X光子的透过率进行测量,给出了Ag薄膜在吸收边（3.4~3.9 keV）的衰减系数 。建立了Ag样品吸收边附近衰减系数同步辐射测量方法。通过不确定度分析给出衰减系数测量不确定度小于1％,填补了在该区间衰减系数的空白。     

GaAs半导体中三光子吸收系数的光学测量

本文首次报道了采用非线性光透射技术(NLT)测量λ=2.06μm激光激发下GaAs本征半导体中三光子吸收的实验研究.观察到了三光子吸收及所伴随的自由载流子的激发态吸收,并测得了三光子吸收系数.实验测量结果与理论计算结果比较,符合较好.