Angular Distribution Of Radiation Research Articles

Mode composition analysis on experimental results of a Gigawatt-class Ka-band overmoded Cerenkov oscillator

In this paper, a modified numerical method is used to investigate the mode composition of a Gigawatt-class Ka-band overmoded Cerenkov oscillator, which has been proposed and studied in our previous experiment. In the experiment, the measured angular distribution of radiation did not fit a single TM01 mode. So the particle in cell code calculations and the antenna radiation calculations are carried out, which show a consistent picture: the dominant modes are the TM01 mode and the TM03 mode, and their phase relationship is constant with time; therefore, a steady radiation pattern is produced, which matches the experimental data. As a conclusion, the comprehensive analysis shows that the existing modes of the output microwave in our experiment are the first five TM0n modes (n = 1–5), with corresponding power ratios of 36.64%, 0.78%, 56.26%, 5.70%, and 0.52%, and relative phase differences of 0°, 146°, 54°, 169°, and 133°.

Combined Experimental-Numerical Approach to Determine Radiation Properties of Particle Suspensions

A combination of experimental measurements with a numerical model is used to find the volume-averaged radiation properties—extinction coefficient, scattering albedo and approximated scattering phase function—of SiC particle suspensions with varying particle loadings. The experimentally determined angular radiation distribution of irradiated SiC samples is applied to fit a collision-based Monte Carlo (MC) model with a continuous participating media defining the particle suspension. A validation case with glass microspheres and Mie theory is implemented to verify the modeling procedure. Two types of SiC particles with dissimilar optical characteristics are examined and the respective radiation properties are determined for particle loadings between 0.05 and 0.30. The extinction coefficients of both types of SiC particle are in good agreement with the dependent scattering correlation of Kaviany and Singh.

Analysis of transport of short-pulse radiation in a participating medium using lattice Boltzmann method

This article reports usage of the lattice Boltzmann method to analyze transport of short-pulse radiation in a participating medium. Radiatively, the planar medium is absorbing and anisotropically scattering. A new kind of lattice structure handles angular distribution of radiation in the 4π-spherical space. For the short-pulse laser with step temporal profile incident on the north boundary, effects of extinction coefficient, scattering albedo, anisotropy factor, angle of incidence and pulse width on temporal transmittance and reflectance signals are analyzed. To access the accuracy and computational efficiency of the lattice Boltzmann method, in all cases, results are also obtained by solving the same problem using the finite volume method. Lattice Boltzmann method provides accurate results, and computationally it has an advantage over the finite volume method.

Characterization of the solar light field within the ocean mesopelagic zone based on radiative transfer simulations

The solar light field within the ocean from the sea surface to the bottom of the mesopelagic zone was simulated with a radiative transfer model that accounts for the presence of inelastic radiative processes associated with Raman scattering by water molecules, fluorescence of colored dissolved organic matter (CDOM), and fluorescence of chlorophyll-a contained in phytoplankton. The simulation results provide a comprehensive characterization of the ambient light field and apparent optical properties (AOPs) across the entire visible spectral range within the depth range 200–1000m of the entire mesopelagic zone for varying chlorophyll-a concentration and seawater optical properties in the mixed surface layer of the ocean. With increasing depth in the mesopelagic zone, the solar irradiance is reduced by ~9–10 orders of magnitude and exhibits a major spectral maximum in the blue, typically centered around a light wavelength of 475nm. In the green and red spectral regions, the light levels are significantly lower but still important owing to local generation of photons via inelastic processes, mostly Raman scattering and to a lesser extent CDOM fluorescence. The Raman scattering produces a distinct secondary maximum in irradiance spectra centered around 565nm. Comparisons of our results with light produced by the radioactive decay of the unstable potassium isotope contained in sea salt (40K) indicates that the solar irradiance dominates over the 40K-produced irradiance within the majority of the mesopelagic zone for most scenarios considered in our simulations. The angular distribution of radiance indicates the dominance of downward propagation of light in the blue and approach to uniform distribution in the red throughout the mesopelagic zone. Below the approximate depth range 400–500m, the shape of the angular distribution is nearly invariant with increasing depth in the green and red and varies weakly in the blue. The AOPs at any light wavelength also assume nearly constant values within the deeper portion of the mesopelagic zone. These results indicate that the mesopelagic light field reaches a nearly-asymptotic regime at depths exceeding ~400–500m.

The anisotropic scattering coefficient of sea ice

AbstractRadiative transfer in sea ice is subject to anisotropic, multiple scattering. The impact of anisotropy on the light field under sea ice was found to be substantial and has been quantified. In this study, a large data set of irradiance and radiance measurements under sea ice has been acquired with a Remotely Operated Vehicle (ROV) in the central Arctic. Measurements are interpreted in the context of numerical radiative transfer calculations, laboratory experiments, and microstructure analysis. The ratio of synchronous measurements of transmitted irradiance to radiance shows a clear deviation from an isotropic under‐ice light field. We find that the angular radiance distribution under sea ice is more downward directed than expected for an isotropic light field. This effect can be attributed to the anisotropic scattering coefficient within sea ice. Assuming an isotropic radiance distribution under sea ice leads to significant errors in light‐field modeling and the interpretation of radiation measurements. Quantification of the light field geometry is crucial for correct conversion of radiance data acquired by Autonomous Underwater Vehicles (AUVs) and ROVs.

An empirical algorithm to estimate spectral average cosine of underwater light field from remote sensing data in coastal oceanic waters

The underwater average cosine is an apparent optical property of water that describes the angular distribution of radiance at a given point in water. Here, we present a simple empirical algorithm to estimate spectral underwater average cosine µ̅(λ) where the wavelength λ ranges from 400 nm to 700 nm, based only on the apparent optical property, remote sensing reflectance, Rrs(λ), and solar zenith angle. The algorithm has been developed using the measured optical parameters from the coastal waters off Goa, India, and eastern Arabian Sea and the optical parameters derived using the radiative transfer code using these measured data. The algorithm was compared with two earlier reported empirical algorithms of Haltrin (1998, 2000), and the performance of the algorithm was found to be better than these two empirical algorithms. The algorithm is based on single optical parameter; remote sensing reflectance, which can be easily measured in‐situ, and is available from the ocean color satellite sensors; hence this algorithm will find applications in the ocean color remote sensing.

Radiance distribution simulation in a transparent medium with fresnel boundaries containing aluminum nanoparticles

The radiative transfer equation in a slab with Fresnel boundaries was solved by the spherical harmonics method in the case of a transparent medium containing aluminum nanoparticles. A computer program was developed, the angular radiance distribution on the slab boundaries and in the slab center were calculated. It was concluded that Fresnel boundary conditions make the angular distribution on the slab boundaries asymmetric. The use of angular radiance distribution for solving the reverse problems of scattering spectroscopy was discussed.

Measured spectral bidirectional reflection properties of three mature hemiboreal forests

A combined spectrometer-BRF sensor was developed and built at Tartu Observatory, Estonia. The BRF sensor records angular distribution of target radiance with high angular resolution in red or near-infrared spectral band in a plain selected with sensor orientation up to 70° zenith angles. Bidirectional reflectance factor (BRF) measurements on board a low flying helicopter over three mature forest stands in South-East Estonia – a birch stand, a spruce stand, and a pine stand – are reported. Airborne measurements are supported by extensive ground truth data which have been reported in separate publications. The measurements revealed that both the BRF values and the BRF shape depend on the forest type and wavelength. The asymmetry of reflectance factor in the principal plane is strong and the hot-spot shape is very sharp in coniferous stands in the red spectral band. Multiple scattering of radiation in the NIR spectral band smooths the angular distribution of forest BRF. The collected data allow to validate forest radiative transfer models.

Experimental Determination of the Radiative Properties of Particle Suspensions for High-Temperature Solar Receiver Applications

This study combines experimental measurements with a numerical method to determine the effective radiative properties of particle suspensions. A spectroscopic goniometry system is applied to measure the attenuation and angular radiation distribution of irradiated samples produced by uniformly suspending particles within a transparent epoxy resin. Samples of varying porosity and thickness are examined. A collision-based Monte Carlo model is formulated for a continuous participating medium representing the particle suspension. The volume-averaged radiative properties, namely, the extinction coefficient, the scattering albedo, and the scattering phase function, are determined for porosities between 0.72 and 0.95 by fitting to the experimental data. Extended correlations of the independent scattering theory show good match with the experimental determined extinction coefficient and capture the dependent scattering effect. The modified double Henyey–Greenstein scattering phase function yields the best agreement between experimental and numerical results, and is characterized by a strong forward scattering preference.

Slowing-down of relativistic heavy ions and its influence on angular distributions of Vavilov–Cherenkov radiation

We performed calculations of Vavilov–Cherenkov radiation (VChR) angular distributions from relativistic heavy ions (RHI) taking account of their slowing-down in a radiator. The results of calculations show that the slowing-down of RHI in radiator leads to specific broadening of VChR ring thus forming specific VChR angular distribution which is different compared to well-known Tamm–Frank distribution. The key parameters which change drastically the VChR angular distribution from RHI are: energy, charge and mass of an ion; emission wave length and corresponding refractive index; stopping power and radiator thickness. The theory developed has been applied to the analysis of the earlier experiments (1996–2001) devoted to studies of the VChR angular distribution from Au RHI beam with energies ∼0.9GeV/u at SIS-18 heavy ion accelerator (GSI, Darmstadt, Germany).

Spatial and spectral characteristics of nonlinear Thomson scattering in the few-cycle regime

Based on classical radiation theory, the electron relativistic dynamics, full spatial and spectral characteristics of nonlinear Thomson scattering in a linearly polarized intense few-cycle laser pulse have been investigated theoretically and numerically. In the few-cycle laser pulse, the electron relativistic dynamics depends sensitively on the initial phase of the driving laser pulse. The Thomson scattering radiation also shows many remarkable initial phase-sensitive characteristics. For most of the initial phases, the x–y plane and y–z plane symmetry of radiation angular distributions is broken down, i.e., the well-known fourfold symmetry disappears. The radiation spectra also depend on the initial phase and show a y–z plane asymmetry for most initial phases. These phenomena can be attributed to the properties of the few-cycle pulse. Moreover, possible ways to observe these phenomena are suggested. In addition, we propose that the initial phase-sensitive characteristics of nonlinear Thomson scattering can be utilized to determine the initial phase of the intense few-cycle pulse.

Terahertz radiation from electrons moving through a waveguide with variable radius, based on Smith–Purcell and Cherenkov mechanisms

In this work we suggest as a source of terahertz radiation a waveguide – dielectric pipe with periodic internal radius and electrons moving through it. The resulting radiation is the sum of Cherenkov and Smith–Purcell ones. The intensities of radiation from the dielectric pipe with periodic internal radius and with the constant one are compared. It is shown that the radiation from periodic internal structure may be several times more intensive than that from usual pipe with the smooth internal surface. Also, the spectral–angular distribution of radiation is shown to oscillate with the radius of the waveguide, both for periodically changed and for straight internal channel.

Influence of slowing down in the radiator on the Cherenkov radiation angular distributions from relativistic heavy ions at FAIR, SPS and LHC energies

The calculations of Cherenkov radiation (ChR) angular distributions from relativistic heavy ions (RHI) at very high energies (from 30GeV/u up to 3000GeV/u) taking into account their slowing-down (ionization energy loss) in a radiator are performed for the first time. The slowing-down of RHI in an optically transparent solid radiator can drastically change the ChR angular distributions at RHI energies of order of 1GeV/u. The results of calculation show that at higher RHI energies (>30GeV/u) (FAIR, SPS and LHC), the ChR angular distribution (at the reasonable radiator thickness) is very close to the Tamm–Frank distribution and practically does not depend on the RHI slowing-down in a radiator, if only ionization energy loss is taken into account.

Resonant Modes of Single Silicon Nanocavities Excited by Electron Irradiation

High-index dielectric or semiconductor nanoparticles support strong Mie-like geometrical resonances in the visible spectral range. We use 30 keV angle-resolved cathodoluminescence imaging spectroscopy to excite and detect these resonant modes in single silicon nanocylinders with diameters ranging from 60 to 350 nm. Resonances are observed with wavelengths in the range of 400-700 nm, with quality factors in the range Q = 9-77, and show a strong red shift with increasing cylinder diameter. The photonic wave function of all modes is determined at deep-subwavelength resolution and shows good correspondence with numerical simulations. An analytical model is developed that describes the resonant Mie-like optical eigenmodes in the silicon cylinders using an effective index of a slab waveguide mode. It shows good overall agreement with the experimental results and enables qualification of all resonances with azimuthal (m = 0-4) and radial (q = 1-4) quantum numbers. The single resonant Si nanocylinders show characteristic angular radiation distributions in agreement with the modal symmetry.

Modeling the electromagnetic cavity mode contributions to the THz emission from triangular Bi2Sr2CaCu2O8+δ mesas

In order to understand the radiation observed from the intrinsic Josephson junctions in triangular Bi2Sr2CaCu2O8+δ mesas, we calculate the transverse magnetic (TM) electromagnetic modes for thin equilateral cavities. A new set of distinct but degenerate TM modes coexists with the known modes of Helszajn and James, but are expected to lead to distinct radiation angular distribution patterns. Although we have been unable to solve for the exact TM modes of a thin cavity of general acute isosceles triangular shape, we solved exactly the closely related problems of the TM cavity modes of two thin circumscribing “pie-shaped” wedges, which provide highly accurate approximations to very acute isosceles triangular cavities.

New peculiarities in angular distribution of Cherenkov radiation from relativistic heavy ions caused by their stopping in radiator: numerical and theoretical research

A new method and theoretical base for calculation of the Cherenkov Radiation (ChR) angular distribution from relativistic heavy ions (RHI) taking into account stopping in radiator is suggested. Our method is based on the thin radiator approximation and the Bethe-Bloch formula for ionization energy loss (stopping) and provides fast calculations without using any special software. The simple formula for estimation of the ChR angular distribution width in vicinity of the Cherenkov angle taking into account RHI stopping in radiator is obtained. New peculiarities of ChR - dependence on the RHI charge and mass (isotopic effect), emission wavelength (index of refraction) and radiator length - are studied. These new features of ChR from RHI allow for their possible applications to simultaneously reconstruction of both velocity and charge of RHI penetrating through radiator and even to measure the masses of isotopes or radiator stopping power.

Experimental study of coherent transition radiation from relativistic electron bunches entering a dihedral angle

The angular distributions of the intensity of transition radiation from a bunch of relativistic electrons entering a dihedral angle between two conducting planes have been measured in a millimeter wavelength range. A microtron with a particle energy of 7.4 MeV is used as a source of electrons. The effect of the particle injection direction and the magnitude of the dihedral angle on the angular distribution of the radiation intensity has been analyzed. The measurements show that the character of the distribution of radiation from a charge entering the dihedral angle significantly differs from that for a charge escaping the angle. A comparatively small change in the magnitude of the dihedral angle can lead to qualitative changes in the angular distribution of radiation from a charge entering the dihedral angle.

Taking the pulse of pyrocumulus clouds

Large forest fires are a known natural and dominant disturbance factor in high northern latitudes, and form pyrocumulus (pyroCu), and occasionally pyrocumulonimbus (pyroCb) clouds. These clouds can transport emissions into the upper troposphere/lower stratosphere (UT/LS) and produce significant regional and even global climate effects, as is the case with some volcanoes. However, the lack of observational data within pyroCu or pyroCb complicates our ability to investigate pyro-convection and to understand the vertical and cross-isentropic transport mechanisms responsible for UT/LS injection. Here, we report detailed airborne radiation measurements within strong pyroCu taken over boreal forest fires in Saskatchewan, Canada during the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) summer field campaign in 2008. We find a prominent smoke core within the pyroCu, which is defined by strong extinction in the UV, VIS and NIR, and high gas-particle concentrations. We also find that the angular distribution of radiance within the pyroCu is closely related to the diffusion domain in water clouds, which is dominated by multiple scattering processes. The radiation field of pyroCu can be described by diffusion approximations that are comprised of simple cosine functions, which can be used to calculate the spatial and temporal characteristics of the radiance field, and applied in cloud resolving models. We demonstrate with Monte Carlo simulations that radiation transport in pyroCu is inherently a 3D problem and must account for particle absorption.

Quantum mechanical calculation of electron spin flip in a helical undulator

We derive a four-component spinor wave function for an electron in a helical undulator which, in the relativistic limit, successfully reproduces all of the results of the classical calculation for the radiation angular distribution, polarization, and energy spectrum. This wave function also allows the nonclassical calculation of the spin flip. For electron energies below the several hundred GeV range, the spin-flip probability is negligible, but for higher energies and high undulator strengths it cannot be neglected if beam polarization is to be preserved, even though nonflip radiation still greatly dominates the radiation intensity. The anomalous magnetic moment ${a}_{e}$ is seen to play a dominant role in the helical undulator spin-flip process. The probability of spin flip is shown to have a ${\ensuremath{\gamma}}^{5}$ dependence on electron energy. For high energy electrons, the direction of spin flip is independent of the handedness of the undulator. As a result, at sufficiently high energy, a polarized electron or positron beam rapidly depolarizes by spontaneous radiation in the undulator. Because of the high correlation between the direction of spin flip and the handedness of the spin-flip radiation, we conjecture that it may be possible to polarize electrons by using the intense circularly polarized photons in the helical undulator to stimulate spin flip.

Effects of transverse electron beam size on transition radiation angular distribution

In this paper we consider the effect of the transverse electron beam size on the Optical Transition Radiation (OTR) angular distribution in case of both incoherent and coherent emission. Our results confute the theoretical argumentations presented first in Optics Communications 211, 109 (2002), which predicts a dependence of the incoherent OTR angular distribution on the beam size and emission wavelength. We present here theoretical and experimental data not only to validate the well-established Ginzburg–Frank theory, but also to show the impact of the transverse beam size in case of coherent emission.