Angular Distribution Of Radiation Research Articles

The generalized Milne problem in gas-dusty atmosphere

We consider the generalized Milne problem in non-conservative plane-parallel optically thick atmosphere consisting of two components - the free electrons and small dust particles. Recall, that the traditional Milne problem describes the propagation of radiation through the conservative (without absorption) optically thick atmosphere when the source of thermal radiation located far below the surface. In such case, the flux of propagating light is the same at every distance in an atmosphere. In the generalized Milne problem, the flux changes inside the atmosphere. The solutions of the both Milne problems give the angular distribution and polarization degree of emerging radiation. The considered problem depends on two dimensionless parameters W and (a+b), which depend on three parameters: $\eta$ - the ratio of optical depth due to free electrons to optical depth due to small dust grains; the absorption factor $\varepsilon$ of dust grains and two coefficients - $\bar b_1$ and $\bar b_2$, describing the averaged anisotropic dust grains. These coefficients obey the relation $\bar b_1+3\bar b_2=1$. The goal of the paper is to study the dependence of the radiation angular distribution and degree of polarization of emerging light on these parameters. Here we consider only continuum radiation.

Linear microfocus bremsstrahlung generated in light and heavy narrow targets in B-18 betatron

The first results of studying the properties of X- and G-ray beams generated at the grazing incidence of 18-MeV electrons with the 50 and 8-μm-thick Si crystals and a 13-μm-thick Ta foil of 4 mm in length along the electron beam are presented. The target has been placed in a goniometer inside the chamber of a B-18 betatron. The results exhibit strong changes in the angular distribution of radiation at the variation of the orientation of the target. This effect is not observed in the case of the normal incidence of electrons on the surface of a thin target. Images of a reference microstructure have been obtained with a high resolution of details of the microstructure owing to the smallness of the source of radiation. The dependence of the contrast of an image on the position of the microstructure in the radiation cone has been demonstrated, which is determined by the change in the effective size of the radiation source when the emission angle of the source is changed.

Scattering of Electromagnetic Radiation by Dimers of Two Finite Dielectric Cylinders

We performed a numerical simulation of the scattering phase function for scattering of light by dimers of dielectric cylinders of finite length, using the volume integral equation formalism. We have studied the dependence of the scattering phase function on the optical and geometric parameters of the individual cylinders and their relative positions. We show that electrodynamic interaction between the cylinders leads to a substantial difference between the angular distribution of radiation scattered by this system and the angular distribution of radiation scattered by two cylinders with no electrodynamic coupling.

Ratio of the contributions real and virtual photons diffraction in thin perfect crystals. Comparison of calculation and experiment

To evaluate and improve the previously proposed method of calculating diffracted photon yields in thin perfect crystals, a comparison between calculated and experimental results in wide range of photons and electrons energy was carried out. It is shown that the proposed method describes all investigated experimental results for bremsstrahlung diffraction and transition radiation one with an error less than ten-fifteen percent. Consequently, the method may be used for calculation of the electron beam divergence influence on the diffracted transition radiation angular distribution.

Sub-THz radiation from dielectric capillaries with reflectors

In this report we present experimental investigations of the THz radiation generated from a corrugated and a non-corrugated capillary with reflectors, using a femtosecond electron beam of LUCX accelerator at KEK, Japan. We discuss measurements of the radiation angular distributions and their comparison with Particle In Cell simulations, and also investigate an off-central propagation of the beam in the capillaries based on experimental measurements and simulations.

Nonlinear Thomson scattering of a relativistically strong tightly focused ultrashort laser pulse

The problem of nonlinear Thomson scattering of a relativistically strong linearly polarized ultrashort laser pulse tightly focused into a spot with a diameter of DF ≳ λ (where λ is the laser wavelength) is solved. The energy, spectral, and angular distributions of radiation generated due to Thomson scattering from test electrons located in the focal region are found. The characteristics of scattered radiation are studied as functions of the tightness of laser focusing and the initial position of test particles relative to the center of the focal region for a given laser pulse energy. It is demonstrated that the ultratight focusing is not optimal for obtaining the brightest and hardest source of secondary electromagnetic radiation. The hardest and shortest radiation pulse is generated when the beam waist diameter is ≃10λ.

Cr/Sc multilayer radiator for parametric EUV radiation in “water-window” spectral range

The results of experimental investigation of parametric radiation generated by 5.7 MeV electrons in a multilayer structure consisting of 100 Cr/Sc bi-layers deposited on a Si3N4 membrane are presented. The multilayer structure was specially created for generation of parametric radiation with photon energy in “water-window” spectral range. First test measurements of angular distributions of radiation have been done and discussed.

Mixed optical Cherenkov–Bremsstrahlung radiation in vicinity of the Cherenkov cone from relativistic heavy ions: Unusual dependence of the angular distribution width on the radiator thickness

The Cherenkov radiation (ChR) angular distribution is usually described by the Tamm–Frank (TF) theory, which assumes that relativistic charged particle moves uniformly and rectilinearly in the optically transparent radiator. According to the TF theory, the full width at half maximum (FWHM) of the ChR angular distribution inversely depends on the radiator thickness. In the case of relativistic heavy ions (RHI) a slowing-down in the radiator may sufficiently change the angular distribution of optical radiation in vicinity of the Cherenkov cone, since there appears a mixed ChR–Bremsstrahlung radiation. As a result, there occurs a drastic transformation of the FWHM of optical radiation angular distribution in dependence on the radiator thickness: from inversely proportional (TF theory) to the linearly proportional one. In our paper we present the first analysis of this transformation taking account of the gradual velocity decrease of RHI penetrating through a radiator.

Asymmetry of the angular distribution of radiation of channeled relativistic electrons in optically transparent crystals

It has been shown that optical and ultraviolet radiation from relativistic electrons at planar channeling in optically transparent crystals is characterized by an unusual dependence on the polar and azimuth angles. A fraction of radiation with the frequency ω near which the derivative of the refractive index is nonzero, n'(ω) = dn(ω)/dω ≠ 0, should be observed at an angle close to π/2 with respect to the electron beam. For normal dispersion (n'(ω) > 0), this angle is smaller than π/2, whereas for anomalous dispersion (n'(ω) < 0), it is larger than π/2 (“backward” radiation). A pronounced dependence of the radiation intensity on the azimuth angle φ, i.e., azimuthal asymmetry, appears beyond the region of normal and anomalous dispersion at a fixed polar angle θ. In particular, the ratio of radiation intensities at angles φ = 0 and π/2 at θ = π/2 reaches a maximum value of about the square of the refractive index.

Infrared and terahertz radiation of a crystal at axial channeling

Basic properties of radiation of a crystal lattice excited by an axial channeling particle are considered. It is shown that a coherent radiation of atoms occurs if the frequency of oscillations of the channeled particle comes to a resonance with the vibrational mode of the crystal. Spectral and angular distribution of radiation and its polarization are calculated. In case of a relativistic channeled particle, the radiation of atoms is generated into a narrow cone in the direction of a crystallographic axis along which the particle is channeling. The radiation of atoms exited at axial channelling has significant degree of circular polarization.

Influence of radiative cascades on the polarization and angular distribution of radiation following electron-impact excitation of He-like Fe24+ ions

Detailed calculations are carried out for the electron-impact excitation cross sections from the ground state 1s2(J = 0) to the individual magnetic sublevels of the excited state 1s2s1/2 (J = 1) of highly charged He-like Fe24+ ions using a fully relativistic distorted-wave method. The cascades contributions from high-lying levels 1s2p, 1snl (n = 3, 4, 5, 6; l = s, p, d) to the cross sections and the polarization properties of the 1s2s(J = 0) line are investigated systematically. It is found that the 1s2p, 1s3l, and 1s4l (l = s, p, d) excited states have significant effects and make the cross sections increase evidently. These dramatic influences also lead to a remarkable decrease in the linear polarization and angular distribution of subsequent x-ray radiation. The present results are in good agreement with the previous theoretical results and the polarization measurements performed at the Livermore electron beam ion trap.

Influence of real photon diffraction on parametric X-ray radiation angular distribution in thin perfect crystals

Using the previously proposed method of calculating diffracted photon yields in thin perfect crystals, analyzed a relative contribution of parametric X-ray radiation and diffracted photons in thin crystals. It is shown that for average energy of electrons and the center of the PXR spot diffracted real photon contribution is comparable to the yield of parametric X-ray radiation and determines the shape of the angular distribution of the total emission in this range of observation angles. The possibility of estimating electron beams parameters based on the results of PXR angular distribution measurements is discussed. It is shown that for energy of electrons by far larger than 1GeV yield of diffracted transition radiation in narrow angular cone becomes predominating and determines the shape of the angular distribution of total emission in the center of radiation spot.

A Novel Microsensor for Measuring Angular Distribution of Radiative Intensity.

This article presents the design, construction and characterization of a novel type of light probe for measuring the angular radiance distribution of light fields. The differential acceptance angle (DAA) probe can resolve the directionality of a light field in environments with steep light gradients, such as microbial mats, without the need to remove, reorient, and reinsert the probe, a clear advantage over prior techniques. The probe consists of an inner irradiance sensor inside a concentric, moveable light-absorbing sheath. The radiative intensity in a specific zenith direction can be calculated by comparing the irradiance onto the sensor at different acceptance angles. We used this probe to measure the angular radiance distribution of two sample light fields, and observed good agreement with a conventional radiance probe. The DAA probe will aid researchers in understanding light transfer physics in dense microbial communities and expedite validation of numerical radiative transfer models for these environments.

Gamma-ray emission in near critical density plasmas at laser intensities of 1021 W/cm2

We study synchrotron radiation emission from laser interaction with near critical density (NCD) plasmas at intensities of 1021 W∕cm2 using three-dimensional particle-in-cell simulations. It is found that the electron dynamics depend on the laser shaping process in NCD plasmas, and thus the angular distribution of the emitted photons changes as the laser pulse evolves in space and time. The final properties of the resulting synchrotron radiation, such as its overall energy, the critical photon energy, and the radiation angular distribution, are strongly affected by the laser polarization and plasma density. By using a 420 TW∕50 fs laser pulse at the optimal plasma density (∼1nc), about 108 photons/0.1% bandwidth are produced at multi-MeV photon energies, providing a route to ultraintense, femtosecond gamma ray pulses.

Numerical Optimization of THz Source Based on Interaction between Electrons Flow and Dielectric Media

THz radiation source from electrons moving inside a corrugated channel in dielectric media is considered. Spectral angular distribution of radiation is shown to depend strongly on target and bunch parameters. Numerical optimization of THz source is performed for the parameters of LUCX facility in KEK, Japan.

Observation of quasimonochromatic radiation in the vacuum ultraviolet region generated by 5.7-MeV electrons in a multilayer mirror

Angular distributions of radiation in the vacuum ultraviolet region with a photon energy of about 70 eV, which have been measured for several angles of incidence of 5.7-MeV electrons on the surface of a periodic Mo/Si nanostructure of a multilayer mirror, have been analyzed. It has been shown experimentally that vacuum ultraviolet radiation in the direction of backward transition radiation has a quasimonochromatic component associated with coherent radiation of the periodic structure of the target.

Effect of directional dependency of wall reflectivity and incident concentrated solar flux on the efficiency of a cavity solar receiver

Managing the optical properties of a cavity solar receiver to create spectral and directional selectivities is a solution to improve receiver efficiencies. A reduction in the incident solar power lost by reflection and by emission in a solar receiver allows the absorption of the solar flux to be maximized. This report investigates the influence of the cavity walls directional reflectivity on the thermal radiative efficiency of a cubic cavity solar receiver. A Monte Carlo ray-tracing method is used to calculate the power lost by reflections and by emission with respect to the incident radiation angular distribution and the bidirectional reflectance distribution function of the cavity walls. To study the influence of the directional dependency of the incident flux on the radiative efficiency, four patterns are considered: collimated, diffuse, focused, and Themis incidences. The directional-hemispherical reflectivity for the bottom wall (face to aperture) and lateral walls are distinguished. For diffuse walls, the absorption efficiency is primarily affected by the lateral walls reflectivity because of the back reflection losses. For specular walls, the driving parameter is the bottom wall reflectivity. In addition, the radiative efficiency with thermal emission was studied for the Themis configuration and a slightly weakest dependency of the efficiency on the lateral walls reflectivity was found.

Development of synthetic hemispheric projections suitable for assessing the sky view factor on vertical planes

The solar radiation balance in buildings has a significant impact on their energy needs, as well as on their potential BIPV energy production. It also influences the potentials of daylight, its healthiness and sustainability. Solar radiation models for urban environments require the characterization of the obstruction degree to which each point is subjected due to other buildings, topography, vegetation, etc. This characterization is carried out with the parameter known as sky view factor (SVF). In this paper, we deepen and extend the study of SVF on vertical surfaces that have a high level of obstruction. This paper aims to present a general mathematical method to obtain projection equations in vertical planes, which allow the study of SVF as a surface ratio. It is also proposed an adequate projection for vertical planes under the hypothesis of angular distribution of diffuse radiance based on Moon-Spencer’s model.

A fast open source code for determining the intensity and angular distribution of radiation transmitted through homogenous cloud cover

Predicting the transmission of solar radiation through clouds is critical to forecasting the performance of solar power systems. The interaction of solar radiation with cloud particles is described by Mie scattering and the subsequent radiation transport equations must be solved numerically. A standard implementation is typically done using a discrete ordinates or Monte Carlo approach, both of which can be computationally expensive. An alternate approach, called the doubling-method, approximates a cloud as being comprised of optically thin layers for which transmission and reflection functions can be formulated. A thick cloud is then modeled by stacking the layers in such a way that the reflection and transmission from one layer act as inputs to the layers above and below it. The approach allows for rapid computation of the direct and diffuse components of surface solar radiation as a function of solar zenith angle, wavelength, and cloud optical thickness. Here we present a simple, platform independent, fully benchmarked, algorithm for implementing the doubling-method. The code runs orders of magnitude faster than does DISORT for an equivalent angular resolution and is shown to produce equivalent results.

Development of synthetic hemispheric projections suitable for assessing the sky view factor on horizontal planes

The solar radiation balance in buildings has a significant impact on their energy needs, as well as on their potential for buildings-integrated photovoltaics (BIPV) energy production. It also influences the potentials of daylight, its healthiness and sustainability. Solar radiation models for urban environments require the characterization of the obstruction degree to which each point is subjected due to other buildings, topography, vegetation, etc. This characterization is carried out with the parameter known as sky view factor (SVF). In this paper, we check that significant disagreements exist in the literature with respect to the definition of SVF. Most published methods show that SVF admits a geometric interpretation as a ratio of projected sky surface versus the global surface of the sky vault projected on the same system. Nevertheless, the type of projection depends only on the authors’ considerations. The geometric comparison of the methods opens a new way to explain their differences. This paper presents a general mathematical method to obtain projection equations, which allows the interpretation of SVF as a surface ratio. It is also proposed the adequate projection for horizontal surfaces under the hypothesis of angular distribution of diffuse radiance based on Moon–Spencer's model.