X射线与影像增强器作用过程的蒙特卡罗研究

用蒙特卡罗方法研究了能量为（３０～１１０）ｋｅＶ的单能Ｘ射线在铁中的吸收及散射特性，给出了散射光子的角人布及背向向射光子的能量分布，结果表明，在入射Ｘ光子能量较低的情况下，背向散射光子主要是铁的特征Ｘ射线光子，随着入射能量的升高，背向散射光了中高能量的散射光子数增多，特征Ｘ光子数目迅速减少。     

Predetector Processing of Lidar Returns in Atmospheric Laser Sensing

A range of devices used in predetector processing of a backscattered radiation flux in the laser receiving system is considered. These include a variety of spatial filters improving the signal-to-noise ratio and compressing the dynamic range by a controlled vignetting of backscattered radiation and special masks and rasters, including those for separating multiply scattered signals. Different configurations of polarization analyzers are proposed to determine the Stokes parameters in lidar returns.     

Photon Diffusion in Random Media and Anisotropy of Scattering in the Henyey–Greenstein and Rayleigh–Gans Models

Journal of Experimental and Theoretical Physics - Monte Carlo (MC) numerical simulation is carried out for the intensity of multiply backscattered radiation as a function of the...     

Multiple scattering and intensity fluctuations in optical coherent tomography of randomly inhomogeneous media

An expression for signal intensity fluctuations in optical coherent tomography has been obtained for the first time in the framework of the theory of the multiple scattering of low-coherent optical radiation in a random medium. The contribution of the multiple scattering of low-coherent radiation backscattered from a randomly inhomogeneous layer, as well as the speckles of the interference component in optical coherent tomography, has been calculated.     

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.     

Verification of terahertz-wave spectrophotometry by Compton backscattering of coherent synchrotron radiation

We developed a continuous-spectrum light beam from Compton backscattering by using coherent synchrotron radiation in an L-band linac at the Kyoto University Research Reactor Institute. The ratio of Compton backscattered photons to background photons when coherent synchrotron radiation was used was three times larger than when coherent transition radiation was used. The transmission spectrum of a polystyrene film in the terahertz-wave region was evaluated by measuring the spectrum of the Compton backscattered photons and it roughly agreed with that measured by a Martin–Puplett-type interferometer. The spectrophotometry using Compton backscattering shows promise as a new tool for investigations in terahertz-wave science.     

Monte Carlo calculations of the energy deposited in biological samples and shielding materials

The energy deposited by gamma radiation from the Cs-137 isotope into body tissues (bone and muscle), tissue-like medium (water), and radiation shielding materials (concrete, lead, and water), which is of interest for radiation dosimetry, was obtained using a simple Monte Carlo algorithm. The algorithm also provides a realistic picture of the distribution of backscattered photons from the target and the distribution of photons scattered forward after several scatterings in the scatterer, which is useful in studying radiation shielding. The presented method in this work constitutes an attempt to evaluate the amount of energy absorbed by body tissues and shielding materials.     

Coherent resonant scattering of nuclear radiation in a paramagnetic absorber placed in a magnetic field

When a thick Mössbauer absorber of (paramagnetic) ferric alum enriched in⁵⁷Fe is placed in a longitudinal magnetic field of 270 G, a reduction of the intensity of backscattered resonant radiation from a⁵⁷CORh is observed. This is interpreted as caused by an increase of the degree of coherence of the resonant radiation re-emitted by the absorber. Further experiments are needed to corroborate this interpretation.     

Collimation of the initial wavefront of a partially coherent light beam by the backscattered signal

The possibility of compensating distortions of the initial wavefront of a partially coherent beam by the backscattered radiation power is analyzed. A new algorithm for controlling the flexible mirror by the signal of the volume atmospheric scattering is proposed and its efficiency is studied. It is shown that the developed algorithm makes it possible to considerably reduce the compensation time of distortions compared to the gradient method.     

Dependence of the circular polarization of backscattered light in random media on anisotropy of scatterers

The scattering of linearly or circularly polarized light from a semi-infinite randomly inhomogeneous medium is considered. Using the Monte Carlo method, it is shown that, in the case of a wide front of incident and scattered optical radiation and irrespective of the degree of scattering anisotropy, the copolarized component of backscattered light dominates the cross-polarized component for the linear polarization and the cross-polarized component dominates the copolarized component for the circular polarization. If the beams of incident and scattered radiation are spatially separated and the size of scatterers exceeds the wavelength, the circular copolarized component dominates the cross-polarized one. A similar effect of the change in direction of the rotation of the plane of polarization in relation to the size of scatterers is revealed for pulsed radiation.     

Doppler velocimetry and backscattering spectroscopy according to a homodyne scheme using a single-mode Er fiber laser

The prospects for the homodyne recording of backscattered radiation of an Er fiber laser in measuring velocity and in Doppler spectroscopy are investigated. The sensitivity and accuracy of measuring velocity are estimated. It is shown that this method can be used to control laser-induced hydrodynamic flows that form upon the laser evaporation of biotissues.     

Application of the Monte Carlo method for modeling passage of ultrashort laser pulses through an inhomogeneous medium with moving scatterers

We present the results of numerical modeling of passage of ultrashort laser pulses through an inhomogeneous layered medium with moving scatterers, based on solution of the nonsteady-state radiation transport equation by the Monte Carlo method. We consider the effects of Doppler broadening of the backscattered radiation spectrum in biological tissues. We have analyzed the dynamics of propagation of a short laser pulse within a multilayer model of human skin. We have studied the possibilities for tomography of different layers of biological tissue based on analysis of the spectrum of the scattered radiation pulse.     

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.     

Determination of optical parameters of a medium by the profile of backscattered radiation

The possibilities of determining the scattering coefficient, the absorption coefficient, and the scattering anisotropy parameter of strongly scattering media are analyzed on the basis of comparison of experimentally obtained profiles of backscattered radiation with the profiles calculated with help of the numerical Monte Carlo method.     

Backscattering of an intense laser beam by an electron

We present a novel, simple asymptotic expansion for the spectrum of radiation that is backscattered from a laser by a counterpropagating (or copropagating) electron. The solutions are presented in such a way that they explicitly show the relative merit of using an intense laser and of an energetic electron beam in x-ray production in the single particle regime. Simple scaling laws are given.     

Modeling of optical radiation energy distribution in plant tissue

A three-dimensional mathematical model of interactions of optical radiation with plant tissue taking into account its structural inhomogeneity, spectral properties, and the effects of fluorescence is constructed. The developed model is implemented using the statistical Monte Carlo method for the Henyey-Greenstein phase function. The dependence of differential backscattering and fluorescence coefficients on the concentration of photosynthetic pigments (chlorophylls) is numerically studied. It is demonstrated that numerical characteristics agree with results of physical experiment. The approximate solution based on the expansion of the diffusion and fluorescence radiation fluxes into a series in terms of a small parameter is found. This expansion makes it possible to calculate the field of backscattered radiation with satisfactory accuracy and to qualitatively correctly describe the experimentally observed dependences of the fluorescence coefficient in the region of high chlorophyll concentration.     

Investigation of primary cosmic rays at the Moon’s surface

The possibility of experimentally studying primary cosmic rays at the Moon’s surface is considered. A mathematical simulations of showers initiated in the lunar regolith by high-energy particles of primary cosmic rays is performed. It is shown that such particles can in principle be recorded by simultaneously detecting three components of backscattered radiation (secondary neutrons, gamma rays, and radio emission).     

Three-dimensional vector radiative transfer in a semi-infinite, Rayleigh scattering medium exposed to spatially varying polarized radiation: generalized reflection matrix

Three-dimensional vector radiative transfer in a semi-infinite medium exposed to spatially varying, polarized radiation is studied. The problem is to determine the generalized reflection matrix for a multiple scattering medium characterized by a 4×4 scattering matrix. A double integral transform is used to convert the three-dimensional vector radiative transfer equation to a one-dimensional form, and a modified Ambarzumian's method is then applied to derive a nonlinear integral equation for the generalized reflection matrix. The spatially varying backscattered radiation for an arbitrarily polarized incident beam can be found from the generalized reflection matrix. For Rayleigh scattering and normal incidence and emergence, the generalized reflection matrix is shown to have five non-zero elements. Benchmark results for these five elements are presented and compared to asymptotic results. When the incident radiation is polarized, the vector approach used in this study correctly predicts three-dimensional behavior, while the scalar approach does not. When the incident radiation is unpolarized, both the vector and scalar approaches predict a two-dimensional distribution of the intensity, but the error in the scalar prediction can be as high as 20%.     

日盲紫外域拉曼激光雷达探测大气水汽技术研究

拉曼激光雷达通过探测与水汽浓度相关的大气水汽振动拉曼散射回波信号,可实现大气水汽混合比廓线的探测。然而由于振动拉曼信号非常微弱,在白天测量时振动拉曼散射光谱会淹没在太阳背景光中,多在夜间测量。为实现大气水汽的全天时测量,设计开发一套日盲紫外波段拉曼激光雷达系统。该系统选择Nd∶YAG脉冲激光器的四倍频输出-266.0 nm日盲紫外波段作为拉曼激光雷达系统的激励波长,采用镀高增益介质膜的牛顿式望远镜作为接收器,同时利用二向色镜和超窄带干涉滤光片设计高效率的高光谱分光系统,实现了大气氧气、氮气和水汽振动拉曼散射回波信号277.5,283.6和294.6 nm的精细提取。计算仿真结果表明,臭氧吸收对日盲紫外域拉曼激光雷达探测存在一定的影响,主要是探测距离的影响;氮气通道不受白天太阳背景光噪声的影响;水汽通道存在少量太阳背景光噪声,对系统探测距离略有影响。而系统信噪比计算结果表明,设计的日盲紫外域拉曼激光雷达系统可实现白天3.5 km大气水汽的探测。实际进行水汽探测时,可利用氮气和氧气通道反演出臭氧浓度廓线,修正臭氧对发射波长、各通道拉曼散射波长的吸收,进一步提升系统的探测能力和探测精度。     

A rectangular proportional detector for backscattered57Fe gamma- and X-rays

We constructed a counter for backscattered gamma radiation and/or characteristic X-ray radiation and tested it using the 6.4 keV X-rays reemitted from⁵⁷Fe nuclei exited by Mössbauer effect resonance absorption. Our counter has a rectangular shape due to an arrangement of four proportional counter tubes with a common wire. We present details of the construction. Additionally we present energy spectra taken with different areas of the counter and show the operating features of our rectangular proportional detector.