Concept of formation length in radiation theory

The features of electromagnetic processes are considered which connected with finite size of space region in which final particles (photon, electron–positron pair) are formed. The longitudinal dimension of the region is known as the formation length. If some external agent is acting on an electron while traveling this distance the emission process can be disrupted. There are different agents: multiple scattering of projectile, polarization of a medium, action of external fields, etc. The theory of radiation under influence of the multiple scattering, the Landau–Pomeranchuk–Migdal (LPM) effect, is presented. The probability of radiation is calculated with an accuracy up to “next to leading logarithm” and with the Coulomb corrections taken into account. The integral characteristics of bremsstrahlung are given, it is shown that the effective radiation length increases due to the LPM effect at high energy. The LPM effect for pair creation is also presented. The multiple scattering influences also on radiative corrections in a medium (and an external field too) including the anomalous magnetic moment of an electron and the polarization tensor as well as coherent scattering of a photon in a Coulomb field. The polarization of a medium alters the radiation probability in soft part of spectrum. Specific features of radiation from a target of finite thickness include: the boundary photon emission, interference effects for thin target, multi-photon radiation. The theory predictions are compared with experimental data obtained at SLAC and CERN SPS. For electron–positron colliding beams following items are discussed: the separation of coherent and incoherent mechanisms of radiation, the beam-size effect in bremsstrahlung, coherent radiation and mechanisms of electron–positron creation.     

Quantum effects on polarization bremsstrahlung from electron-atom collisions in partially ionized dense hydrogen plasmas

The quantum effects on the polarization bremsstrahlung emission due to the low-energy electron-atom collisions are investigated in partially ionized dense hydrogen plasmas. The impact parameter analysis is employed to describe the motion of the projectile electron in order to investigate the variation of the bremsstrahlung emission spectrum as a function of the impact parameter, de Broglie wave length, Debye length, and radiation photon energy. The results show that the quantum effects strongly suppress the polarization bremsstarhlung emission. It is also found that the polarization bremsstarhlung emission cross section shows the maximum value at the position of the Bohr radius. It is interesting to note that the quantum effects are found to be more important than the screening effects in the polarization bremsstarhlung emission.     

Understanding the polarization signal of spherical particles for microwave limb radiances

This paper presents a simple conceptual model to explain that even spherical scatterers lead to a polarization difference signal for microwave limb radiances. The conceptual model relates the polarization difference measured by a limb-looking sensor situated inside a cloud with the anisotropy of the radiation. In the simulations, it was assumed that the cloud consists of spherical ice particles with a radius of which were situated between 10.6 and 12.3 km altitude. The frequencies 318 and 500 GHz were considered.The results of the conceptual model were compared to the results of the fully polarized scattering model ARTS-1-1. The comparison showed a good qualitative agreement. The polarization difference decreases inside the cloud with increasing height and changes sign. This behavior can be related to a different amount of radiation coming from the atmosphere above and below the cloud, compared to the amount of radiation coming from the sides. The sign of polarization difference of the scattered radiation is opposite for these two radiation sources.     

Magnetic circular dichroism in the luminescence of F-centres in KI

Magnetic field induced circular polarization of the F-centre emission has been measured in KI at 1.9 K in fields up to 80 kG. Both the diamagnetic (field dependent) and the paramagnetic contributions (dependent upon the spin polarization of the relaxed excited state) have been detected. The spin polarization is produced by a pump beam modulated between left and right circular polarization. The expected behaviour of the spin polarization as a function of the frequency of the modulation has been observed. Knowledge of these small dichroic effects gives further insights into the nature of the relaxed excited state.     

Cloud 3D effects on broadband heating rate profiles: I. Model simulation

Solar broadband heating directly drives the atmospheric and ocean circulations, and is largely determined by cloud spatial 3-diminesional (3D) structures. To study the cloud 3D effects on radiation, a 3D broadband Monte-Carlo radiative transfer model, along with an Independent Pixel/Column Approximation (IPA) method, is used to simulate radiation and heating rate of three typical cloud fields generated by cloud resolving models (CRM). A quantitative and statistical estimation of cloud 3D effects has been developed to investigate the impact of cloud 3D structures on both heating rate strength, STD_Bias, and vertical distribution, CorrCoef. The cloud 3D structures affect some clouds more in heating rate strength and others more in vertical distribution. It is crucial to use the combination of CorrCoef and STD_Bias for better quantitative evaluation of the 3D effects. Furthermore, there is no simple way to define a critical resolution (or average radius), within which the IPA heating rate profiles closely represent the true 3D heating rate profiles. The critical radius (or resolution) strongly depends on solar incident angle as well as cloud vertical distribution. Also, the critical radii for clear-sky columns are larger than for cloudy columns, although the corresponding STD_Bias for clear-sky columns are smaller than for cloudy columns. Analysis based on two different statistical average methods illustrates that the cloud 3D effects due to the dimensionality difference between the 3D clouds (circle average) and 2D clouds (line average) significantly impact on the heating rate profiles.     

Similarity effects in multiple scattering of coherent radiation: Phenomenology and experiments

Based on phenomenological concepts of statistics of effective optical paths for multiple scattering of coherent radiation, an analysis is carried out of similarity effects observed for the dependences of statistical moments of the scattered field on the relaxation parameters with a dimension of reciprocal length. Within the framework of the diffusion approximation, expressions are obtained that describe the autocorrelation function of fluctuations of the scattered-field amplitude, the degree of polarization, and the normalized intensity of scattered light for media with a finite absorption length in the case of forward scattering of coherent radiation in a plane layer of an isotropic scattering medium. The results of the analysis show the similarity of the dependences of these quantities on the corresponding spatial scales. Experiments with model scattering media (aqueous suspensions of polystyrene spherical particles) supported the existence of similarity effects in multiple scattering. An experimental study was made of the relation between the characteristic depolarization length and the transport length for multiple scattering of coherent radiation in a plane layer. The effective value of the radiation diffusion coefficient providing the best agreement between the experimental and the calculated values of parameters of the scattered field is shown to be independent of the absorption coefficient of a medium.     

Studying the Polarization Characteristics of Thermal Microwave Emission from a Cloudy Atmosphere

We present the results of spectral studies of thermal microwave emission from a cloudy atmosphere at 37 and 85 GHz. The experimental data are interpreted on the basis of modeling of microwave radiation transfer in the mixed-type clouds containing ice crystals of different shapes and supercooled water drops. All orders of scattering are taken into account. It is shown that polarization radiometric measurements allow one to separately determine ice-water and liquid-water contents of the clouds and also to diagnose the cloud microstructure (crystal shapes and characteristic sizes).     

Calculation of light fields of concentrated sources in turbid media with strongly anisotropic scattering

An efficient approach to solving the equation of radiation transfer in a turbid medium with strongly anisotropic scattering is proposed based on the small-angle approximation of the theory of radiation transfer with allowance for the dispersion of the radiation propagation path length, as well as with allowance for polarization. The approach is applied to the solution of the problem of the field of a pointlike isotropic radiation source in a two-dimensional medium.     

Time-domain, nuclear-resonant, forward scattering: theclassical approach

This paper deals with the interaction of electromagnetic radiation with matter assuming the matter to have nuclear transitions in resonance with incident electromagnetic radiation. The source of the radiation is taken to be of two types; natural radioactive gamma decay and synchrotron radiation. Numerical examples using ⁵⁷Fe are given for the two types of source radiation. Calculated results are contrasted for the two cases. Electromagnetic radiation produced by recoil-free gamma-ray emission has essentially the natural linewidth. Electromagnetic radiation from a synchrotron, even with the best monochromators available, has a relatively broad-band spectrum, essentially constant for these considerations. Polarization effects are considered. In general, the nuclear-resonant medium changes the polarization of the input radiation on traversing the medium. Calculations are presented to illustrate that synchrotron radiation studies using nuclear-resonant forward scattering have the potential for making high-precision measurements of hyperfine fields and recoilless fractions. An interesting aspect of nuclear-resonant forward scattering, relative to possible gamma-ray laser development, is the so-called “speed-up” effect. This revised version was published online in August 2006 with corrections to the Cover Date.     

三维燃烧介质和壁面温度的非接触联合重建研究

温度分布在线实时测量对于燃烧过程优化和污染物控制具有重要意义, 针对以往非接触三维温度分布重建过程的耗时性问题和忽略壁面辐射的不足, 本文提出了一种新的离散重建模型, 用于三维吸收、 发射和散射性高温燃烧介质以及壁面温度的快速联合非接触测量. 该模型以四个CCD(Charge Coupled Device) 为测量传感器, 通过构建辐射逆问题求解方程, 从CCD输出的辐射投影图像重建温度分布. 介质中不同投影方向内的辐射传递过程通过离散传递法来描述, 介质的散射和壁面反射则通过离散坐标法来近似. 离散后计算局部辐射强度的病态方程通过最小二乘余量法来求解, 论文对其计算速度进行了优化. 通过非对称温度分布测量算例分析了该模型的有效性, 讨论了测量噪音、 介质和壁面辐射特性对重建精度的影响, 并与其他方法对比分析了模型的重建速度. 计算结果表明本文提出的离散模型可以有效地用于大型高温燃烧介质和壁面温度分布的联合非接触测量. 即使在有噪声的情况下, 该模型也能获得准确的测量结果, 与其他计算方法相比, 采用改进的最小二乘余量法, 能有效地提高温度分布的重建计算速度.     

Visualization of scattering media upon backscattering of a linearly polarized nonmonochromatic light

The method of polarization visualization of multiply scattering macroinhomogeneous media, based on analysis of the spatial distributions of polarization characteristics of a linearly polarized radiation backscattered from a medium, is discussed. The effect of optical characteristics of the medium and the scattering geometry on the quality of the images obtained in the case of visualization of an absorbing heterogeneity immersed into a multiply scattering medium is considered. The comparative analysis of the quality of formed images was performed with the use of different polarization characteristics of the backscattered radiation as a visualization parameter. The theoretical interpretation of the obtained experimental results is given within the framework of the phenomenological approach based on the concept of the distribution of the effective optical paths of partial components of the scattered optical field. To calculate the probability density of the effective optical paths, the statistical simulation method was used.     

Three-dimensional visualization of objects in scattering medium using integral imaging and spectral analysis

This paper presents a three-dimensional visualization method of 3D objects in a scattering medium. The proposed method employs integral imaging and spectral analysis to improve the visual quality of 3D images. The images observed from 3D objects in the scattering medium such as turbid water suffer from image degradation due to scattering. The main reason is that the observed image signal is very weak compared with the scattering signal. Common image enhancement techniques including histogram equalization and contrast enhancement works improperly to overcome the problem. Thus, integral imaging that enables to integrate the weak signals from multiple images was discussed to improve image quality. In this paper, we apply spectral analysis to an integral imaging system such as the computational integral imaging reconstruction. Also, we introduce a signal model with a visibility parameter to analyze the scattering signal. The proposed method based on spectral analysis efficiently estimates the original signal and it is applied to elemental images. The visibility-enhanced elemental images are then used to reconstruct 3D images using a computational integral imaging reconstruction algorithm. To evaluate the proposed method, we perform the optical experiments for 3D objects in turbid water. The experimental results indicate that the proposed method outperforms the existing methods.     

Quasi-stellar objects as Čerenkov sources

If a beam of ultrarelativistic charges enters a gas cloud ?erenkov radiation may be produced. The condition for such radiation is given, and the ?erenkov power radiated per charge is compared to the synchrotron power. Intense ?erenkov pulses will excite emission lines via stimulated Raman scattering. Due to anomalous dispersion in the presence of ac Stark effect the ?erenkov field will be most intense at frequencies well red-shifted from atomic resonances, and this red-shift will be a feature of the stimulated Raman line emissions. The intensity ratios and anomalous red-shifts of the broad emission lines of QSOs can be explained. ?erenkov emission froma collimated beam will be highly directional. The emission evidently does not occur in BL Lacs, which are pure synchrotron emitters.     

Monte Carlo simulation of photon transport in a randomly oriented sphere-cylinder scattering medium

A Monte Carlo simulation tool for simulating photon transport in a randomly oriented sphere-cylinder medium has been developed. The simulated medium represents a paper pulp suspension where the constituents are assumed to be mono-disperse micro-spheres, representing dispersed fiber fragments, and infinitely long, straight, randomly oriented cylinders representing fibers. The diameter of the micro-spheres is considered to be about the order of the wavelength and is described by Mie scattering theory. The fiber diameter is considerably larger than the wavelength and the photon scattering is therefore determined by an analytical solution of Maxwell’s equation for scattering at an infinitely long cylinder. By employing a Stokes–Mueller formalism, the software tracks the polarization of the light while propagating through the medium. The effects of varying volume concentrations and sizes of the scattering components on reflection, transmission and polarization of the incident light are investigated. It is shown that not only the size but also the shape of the particles has a big impact on the depolarization.     

Effect of absorption of multiply scattering media on the degree of residual polarization of backscattered light

The effect of absorption in a scattering medium on the degree of residual polarization of backscattered radiation is studied in the case of probing of multiply scattering media by a linearly polarized light. An approximate expression describing the dependence of the degree of residual linear polarization of the backscattered radiation on the optical characteristics of a multiply scattering medium is derived within the framework of the phenomenological approach, based on the concept of the distribution of the optical paths of partial components of the scattered optical field under the conditions of multiple scattering, and with the use of the ideas about the similarity of statistical moments of the multiply scattered optical fields. The cut-off of the partial components, characterized by a large value of the optical path, because of their absorption, results in a substantial increase of the degree of residual polarization for the bands of the selective absorption caused by the presence of chromophores in the scattering medium. The results of experiments with model scattering media (whole milk) and biological tissues (human skin in vivo) are presented.     

Coherent backscattering under conditions of pulsed radiation trapping

Coherent backscattering of pulsed radiation emitted by optically dense atomic ensembles is considered. The diagrammatic technique is used for deriving analytic expressions for correlation functions of scattered light, which make it possible to take into account all main factors affecting the dynamics of the process, including the hyperfine and Zeeman structure of the ground and first excited states of atoms, polarization of probe radiation, the actual shape and size of an atomic cloud, its spatial inhomogeneity, motion of atoms, and angular-momentum polarization of atoms. On the basis of these relations, the time dependence of the total intensity and the dependence of enhancement factor of backscattered light on the pulse duration, type of polarization of the polarization system of observation, optical thickness of the scattering medium, and the carrier frequency of the pulse are investigated. The calculations are performed for an ensemble of rubidium-85 atoms cooled in magnetooptical traps.     

Two-Color Dual-Polarization Pulsed Bistatic Lidar for Measuring Water Cloud Droplet Size

The concept of a pulsed bistatic lidar for measuring water cloud particle size is presented. The method uses a two-color laser and a receiver with a polarization analyzer located at a suitable scattering angle. The dependence of Mie scattering on scattering angle, wavelength, and polarization is used to derive water cloud droplet size. The measurement was simulated for the C₁ and C₂ clouds, and the technique for determining mode radius was studied. The result shows the lidar system with a two-wavelength laser (1064 nm and 532 nm) and a dual-polarization receiver fixed at a scattering angle of around 178 deg can be used to measure a cloud particle size (mode radius) of 4 to 12 μm. Evaluation of the effect of multiple scattering showed that the method can be applied not only for the measurement at the cloud base but also in the cloud where multiple scattering is not negligible.     

Intraband coherence of Bloch oscillations after momentum scattering

Resonantly excited secondary emission of light (RSE) in GaAs/AlGaAs superlattices gives a direct proof for the stability of coherent Bloch dynamics beyond the optical coherent regime. The polarization analysis of the RSE signal confirms its nonlinear origin resulting from coherent intraband density dynamics. It is shown that the intraband coherence of Bloch oscillations (BO) persists after momentum scattering of the photoexcited states. A variation of emission angle and excitation density allows to distinguish two momentum scattering processes contributing to the RSE. A static disorder mediated contribution is restricted to directions near the reflection direction. A further contribution of carrier induced momentum scattering is resolved in directions well separated from diffraction directions. The dependence of the RSE as a function of an external electric field demonstrates an enhancement of momentum scattering with increasing field. PACS 78.47.+p; 63.20Kr; 71.35.-y.