Point Source Of Radiation Research Articles

Hybrid characteristics: 3D radiative transfer for parallel adaptive mesh refinement hydrodynamics

Received * / Accepted * Abstract. We have developed a three-dimensional radiative transfer method designed specifically for use with parallel adaptive mesh refinement hydrodynamics codes. This new algorithm, wh ich we call hybrid characteristics, introduces a novel form of ray tracing that can neither be classified as long, nor as shor t characteristics, but which applies the underlying princi ples, i.e. efficient execution through interpolation and paralleliza bility, of both. Primary applications of the hybrid characteristics method are radiation hydrodynamics problems that take into account the effects of photoionization and heating due to point sources of radiation. The method is implemented in the hydrodynamics package FLASH. The ionization, heating, and cooling processes are modelled using the DORIC ionization package. Upon comparison with the long characteristics method, we find tha t our method calculates the column density with a similarly high accuracy and produces sharp and well defined shadows. We show the quality of the new algorithm in an application to the photoevaporation of multiple over-dense clumps. We present several test problems demonstrating the feasibility of our method for performing high resolution three-dimensional radiation hydrodynamics calculations that span a large range of scales. Initial performance tests show that the ray tra cing part of our method takes less time to execute than other parts of the calculation (e.g. hydrodynamics and adaptive mesh refinem ent), and that a high degree of efficiency is obtained in parallel ex ecution. Although the hybrid characteristics method is developed for problems involving photoionization due to point sources, the algorithm can be easily adapted to the case of more general radiation fields.

A simple mathematical method to determine the efficiencies of log-conical detectors

A new type of photon detector, log-conical, is proposed. The average path length traveled by an incident photon of arbitrary energy as well as the geometrical solid angle are calculated in a mathematical expression to determine the efficiencies of this detector for an arbitrarily positioned isotropic radiating point source. The off-axis effect of the source position was analyzed to demonstrate the powerful capability of the proposed method. The results are compared with those obtained using a standard 3″×3″ cylindrical detector of the same volume in order to show the enhanced efficiency of the log-conical detector.

Intensity fluctuations in a moving random medium

We consider the intensity fluctuations arising when a point source of radiation moves in a randomly inhomogeneous scattering medium. The medium itself can also move with a velocity whose component normal to the direction of propagation can have an arbitrary distribution. We derive an expression for the space–time autocorrelation function of the intensity fluctuations transverse to the direction of propagation. The result is analysed for some particular cases and it is shown how the resulting information can be useful in examining the behaviour of random media in situations of practical interest.

Intensity fluctuations in a moving random medium

We consider the intensity fluctuations arising when a point source of radiation moves in a randomly inhomogeneous scattering medium. The medium itself can also move with a velocity whose component normal to the direction of propagation can have an arbitrary distribution. We derive an expression for the space–time autocorrelation function of the intensity fluctuations transverse to the direction of propagation. The result is analysed for some particular cases and it is shown how the resulting information can be useful in examining the behaviour of random media in situations of practical interest.

Analysis of a setup for monitoring concave surfaces of revolution of nonspherical shape by a compensation method

This paper presents an aberrational analysis of a single-lens compensation setup for monitoring concave nonspherical surfaces of revolution with and without an intermediate image of a point source of radiation. Relationships are obtained that determine the functional possibilities of an additional lens in the Offner layout, located close to the center of curvature at the axial point of the test surface.

The first miniquasar

We investigate the environmental impact of the first active galactic nuclei that may have formed ∼150 Myr after the big bang in low-mass ∼106 M⊙ minihaloes. Using enzo, an adaptive-mesh refinement cosmological hydrodynamics code, we carry out three-dimensional simulations of the radiative feedback from ‘miniquasars’ powered by intermediate-mass black holes. We follow the non-equilibrium multispecies chemistry of primordial gas in the presence of a point source of X-ray radiation, which starts shining in a rare high-s peak at z= 21 and emits a power-law spectrum in the 0.2–10 keV range. We find that, after one Salpeter time-scale, the miniquasar has heated up the simulation box to a volume-averaged temperature of 2800 K. The mean electron and H2 fractions are now 0.03 and 4 × 10−5: the latter is 20 times larger than the primordial value, and will delay the build-up of a uniform ultraviolet (UV) photodissociating background. The net effect of the X-rays is to reduce gas clumping in the intergalactic medium (IGM) by as much as a factor of 3. While the suppression of baryonic infall and the photoevaporation of some halo gas lower the gas mass fraction at overdensities δ in the range 20–2000, enhanced molecular cooling increases the amount of dense material at δ > 2000. In many haloes within the proximity of our miniquasar, the H2-boosting effect of X-rays is too weak to overcome heating, and the cold and dense gas mass actually decreases. We find little evidence for an entropy floor in gas at intermediate densities preventing gas contraction and H2 formation: we show, instead, that molecular cooling can affect the dynamics of baryonic material before it has fallen into the potential well of dark matter haloes and virialized. Overall, the radiative feedback from X-rays enhances gas cooling in lower-s peaks that are far away from the initial site of star formation, thus decreasing the clustering bias of the early pregalactic population, but does not appear to dramatically reverse or promote the collapse of pregalactic clouds as a whole.

Least square error method to estimate individual power of noise sources under simultaneous operating conditions

People have been aware of the connection between noise and hearing loss for centuries. Hearing loss is not the only adverse effect of occupational noise, but also a number of non-auditory effects may endanger worker's safety. This paper proposes an approach enable engineers to point out quantitatively the noisiest source for modification, while multiple machines are operating simultaneously. The model with the point source and spherical radiation in a free field was adopted to formulate the problem. The proposed method requires input data that includes the coordinates ( x, y) of the noise sources and the locations where the sound pressure level is measured, and the measured sound pressure levels. Then, the method of least squares was applying to estimate system parameters. Finally, a set of solutions that represents the sound power of noise sources can be found and these solutions have minimum error in least squares sense. With help of sound powers obtained, engineers would be able to evaluate noise distribution whenever re-layout workplace in future. Relevance to industry Industry workers suffer psychological and physical stress as well as hearing loss due to industrial noise. Although noise source control can be profound process sometimes, it would be the most effective way to eliminate noise level on source. Therefore, identifying dominant source of noise shall be the first step to overcome the noise problem in industry.

Pixel-pyramid model for divergent projection geometry

Divergent projection refers to fan-beam or cone-beam projection originating from a point source, which is commonly seen in x-ray imaging. Conventionally, the divergent projection calculation is carried out using a ray model, in which each detector pixel (cell) receives radiations emanating from a point source, in a form of straight-line ray. The finite dimension of the detector pixel develops a zero-width ray into a nonzero-width strip, with its cross section assuming the effective aperture of a detector pixel facet. In the pursuit of accuracy, we portray the divergent projection from the point source radiation on a detector cell by a pyramidal element (pyrel), which is formed by a pyramidal base (detector pixel facet) and an apex (point source). A pyrel intersects the object and circumscribes a trapezoidal zone for fan-beam projection (a frustum volume for cone-beam projection), wherein the object voxels are accumulated (appropriately weighted) to produce the detector cell's signal. In addition to the forward divergent projection calculation, the pyrel model can be used for iterative object reconstruction by repeating the backprojection and reprojection. Since the pyrel model portrays more naturally the divergent projection geometry, it maximizes the calculation accuracy, though at the cost of more computations. This model may be rewarding in certain situations: (1) when the detector resolution is unacceptably low, (2) when the projection data set is severely incomplete (for example, the number of projections <100), and (3) when the projection-beam angle is too large (say, >40 deg). Simulations on an iterative fan-beam tomographic reconstruction, using the pyrel model, the ray model, and the voxel-splatting model, are demonstrated.

The Time Domain Green's Function and Propagator for Maxwell's Equations

The free space time domain propagator and corresponding dyadic Green's function for Maxwell's differential equations are derived in one-, two-, and three-dimensions using the propagator method. The propagator method reveals terms that contribute in the source region, which to our knowledge have not been previously reported in the literature. It is shown that these terms are necessary to satisfy the initial condition, that the convolution of the Green's function with the field must identically approach the initial field as the time interval approaches zero. It is also shown that without these terms, Huygen's principle cannot be satisfied. To illustrate the value of this Green's function two analytical examples are presented, that of a propagating plane wave and of a radiating point source. An accurate propagator is the key element in the time domain path integral formulation for the electromagnetic field.

Near-fault strong motion complexity of the 2000 Tottori earthquake (Japan) from a broadband source asperity model

The October 6/2000 Tottori earthquake that occurred in central Japan was an intermediate size strike-slip event that produced a very large number of near field strong motion recordings. The large amount of recorded data provides a unique opportunity for investigating a source asperity model of the Tottori earthquake that, combined with a hybrid strong motion simulation technique, is able to reproduce the observed broadband frequency near-fault ground motion. We investigated the optimum source asperity parameters of the Tottori earthquake, by applying a Genetic Algorithm (GA) inversion scheme to optimise the fitting between simulated and observed response spectra and Peak Ground Acceleration (PGA) values. We constrained the initial model of our inversion by using the heterogeneous slip distribution obtained from a kinematic inversion of the source of previous studies. We used all the observed near-fault ground motions (−100 m) from the borehole strong motion network of Japan (KiK-Net), which are little affected by surficial geology (site effects). The calculation of broadband frequency strong ground motion (0.1–10 Hz) is achieved by applying a hybrid technique that combines a deterministic simulation of the wave propagation for the low frequencies and a semi-stochastic modelling approach for the high frequencies. For the simulation of the high frequencies, we introduce a frequency-dependent radiation pattern model that efficiently removes the dependence of the pattern coefficient on the azimuth and take-off angle as the frequency increases. The good agreement between the observed and simulated broadband ground motions shows that our inversion procedure is successful in estimating the optimum asperity parameters of the Tottori earthquake and provides a good test for the strong ground motion simulation technique. The ratio of background stress drop to average asperity stress drop from our inversion is nearly 50%, in agreement with the theoretical asperity model of Das and Kostrov [Das, S., Kostrov, B.V., 1986. Fracture of a single asperity on a finite fault: a model for weak earthquakes? Earthquake Source Mechanics, AGU, pp. 91–96.], and an empirical ratio of asperities to rupture area [Seismol. Res. Lett. 70 (1999) 59–80.]. The simulated radiation pattern is very complex for epicentral distances within half the fault length, but it approaches the radiation of a double-couple point source for larger distances. The rupture velocity and rise time have a significant influence on the Peak Ground Velocity (PGV) distribution around the fault. An increase in rupture velocity produces a similar effect on the ground motion as a reduction in rise time.

Impulse Representation of Sound Field due to a Rigid Wedge

An impulse representation for calculating a diffraction wave due to a rigid wedge is described. The method is an approximation of the Biot-Tolstoy rigorous closed-form solution for the diffraction of point source radiation by an infinite rigid wedge. The band-limited time-domain function can be reconstructed to the original waveform if it satisfies the sampling theorem, which assumes that sampling takes place at the lowest permissible sampling rate. Therefore, if the energy is concentrated between the first sampling intervals immediately after the rise time of the time-domain function, the rigorous solution can be approximated as a delta function. This paper shows the description methods of the diffraction field near the ridge in three-dimensional space. Using the proposed impulse representation, numerical simulation was performed and the calculation accuracy was examined.

Geometrical Optics Approximation for Nonlinear Equations

A wide class of nonlinear equations is studied in the geometrical optics approximation. It is shown that a nonlinear equation with coefficients dependent on the amplitude of the function sought can be reduced to a system of quasi-linear equations of the gas-dynamics type. As an illustration, the Hamilton--Jacobi equation with a specific form of the nonlinear operator has been solved, and the propagation of monochromatic waves and of point source radiation in nonlinear media has been studied.

Micro electron bunch generation by intense short pulse laser

The generation of a high-energy electron bunch is studied by using an intense fs TEM(1, 0) mode laser in vacuum. We perform three-dimensional particle simulations in which the relativistic equation of motion is solved with an analytically expressed laser field. In this paper, we found that electrons are accelerated longitudinally and confined in the transverse direction by a ponderomotive force of an intense short pulse TEM(1, 0) mode laser. Using a TEM(1, 0) mode laser of 1017 W cm−2, the electrons are successfully trapped in the polarized direction and accelerated to the order of tens of megaelectronvolts with an acceleration gradient of about gigaelectronvolts per metre. The electron bunch generated has a small emittance in the transverse and longitudinal directions, and the spatial size of the electron bunch is about several hundred micrometres in each direction with a high electron number density. This electron acceleration mechanism may open a new engineering world towards a point radiation source, a precise measurement, applications to material sciences, and so on.

Formation of Optimal Dose Fields on a Segment Using Point and Linear Radiation Sources (Theoretical Aspects of the Problem)

Formation of Optimal Dose Fields on a Segment Using Point and Linear Radiation Sources (Theoretical Aspects of the Problem)

Source Effects on Regional Seismic Discriminant Measurements

Source effects on high-frequency regional seismic discriminant measurements involving P / S -type ratios ( Pn/Lg, Pg/Lg ) are isolated for earthquakes in southern California and southern Nevada. The scatter in earthquake population discriminant measurements for fixed propagation paths is statistically analyzed to assess the spatial and frequency-dependent contributions from source radiation pattern and source depth variations along with variable near-source scattering. This is useful because procedures for reducing the variance caused by propagation effects must be constrained by knowledge of the intrinsic scatter resulting from independent source effects, for which corrections are not available without a priori knowledge of the source type and depth. Broadband waveforms for earthquakes near the Nevada Test Site (NTS) and in southern California recorded at stations of the Livermore NTS Network (LNN), the Caltech TERRAscope Network, and the Berkeley Digital Seismic Network (BDSN) are used to obtain Pg / Lg and Pn / Lg measurements and to assess source depth and mechanism contributions to measurement variance. Pn/Lg and Pg/Lg spectral amplitude ratios for several frequency bands between 0.5 and 10 Hz from vertical velocity waveforms are computed for a total of 18 tightly located clusters of earthquakes. For each cluster, we examine the correlations between observed Pn/Lg and Pg/Lg ratios and estimated source depths or predictions based on previously determined source mechanisms. Focal mechanism predictions do not match the observations well, likely due to strong near-source scattering and directivity effects that overwhelm the point-source radiation pattern. However, the data do show significant scatter even for sources only a few kilometers apart. Source depth produces some coherent trends, which can be emphasized by averaging results from multiple stations. These trends vary from region to region, presumably due to specific local crustal layering, but most of the variance is not deterministically predictable and appears to arise from strong variations in near-source scattering. Variance estimates for logarithmic amplitude ratios normalized by mean amplitude ratios vary from 0.01 to 0.05 for different phase pairs. Frequency-dependent variations of the mean, variance, minimum, and maximum amplitude ratio parameters show strong distance dependence out to about 300 km and then appear to become independent of event-station distance. These variance parameters appear to be suitable for use in spatial interpolations of observed P/S -type ratios for a specific unidentified event path, as needed for nuclear test monitoring.

Collimated point-source x-ray nanolithography

The most efficient use of radiation point sources for x-ray lithography requires a collimator to capture a large fraction of spherically emitted radiation and deliver the rays with acceptable divergence properties. We describe an x-ray point source utilizing a polycapillary collimator. The optical performance of the collimator was measured and is described. Dense line-space patterns (∼100 nm lines) were printed in poly(methylmethacrylate) using a mask-wafer gap of 16 μm. Lithography results are consistent with modeling analysis.

Interference effects in high-order harmonic generation with molecules

We study high-order harmonic generation for ${\mathrm{H}}_{2}^{+}$ and ${\mathrm{H}}_{2}$ model molecules in linearly polarized laser pulses by numerical solution of the Schr\odinger equation. Maxima and minima due to intramolecular interference are found in the dependence of the harmonic intensities on the internuclear distance and on the orientation of the molecules. These extrema can be approximately predicted by regarding them as the result of interference between two radiating point sources located at the positions of the nuclei.

Search for point sources of gamma radiation above 15 TeV with the HEGRA AIROBICC array

A search for potential point sources of very high energy gamma rays has been carried out on the data taken simul- taneously by the HEGRA AIROBICC and Scintillator arrays from August 1994 to March 2000. The list of sought sources includes supernova remnants, pulsars, AGNs and binary systems. The energy threshold is around 15 TeV. For the Crab Nebula, a modest excess of 2.5 standard deviations above the cosmic ray background has been observed. Flux upper limits (at 90% c.l.) of around 1.3 times the flux of the Crab Nebula are obtained, in average, for the candidate sources. A dierent search procedure has been used for an all-sky search which yields absolute flux upper limits between 4 and 9crabs depending on declination, in the band from= 0t o= 60.

X pinch plasma development as a function of wire material and current pulse parameters

An X pinch plasma is produced using two or more fine metal wires arranged so that they cross and touch at a single point, in the form of an X, as the load of a pulsed power generator (pulser). X pinches produce very bright, small, short-lived sources of X-rays in the 1-10 keV range. The X-ray source size, pulse duration and photon energy spectra depend upon the wire material and diameter, and the current waveform from the pulser. Those with /spl sim/1 /spl mu/m source size can be used as point sources of X-ray radiation for point-projection radiography with high spatial resolution. We report experiments with a variety of different wire materials, including Al, Ti, Mo, W, among others, all of which produced high resolution images, and all of which have subnanosecond X-ray pulses for X-rays /spl ges/3 keV. X pinches produced using all of these materials had common development stages. However, the wire material and the rate of rise of the current pulse influenced the details of the dynamics, leading to variation in the timing of the X-ray burst(s) relative to the start of the current pulse. For a given wire material, wire diameter had only a small effect on X-ray emission timing. Final X pinch neck implosion speeds as high as 66-75 /spl mu/m/ns were estimated.

Calibration of a biconical antenna and its usage in EMI measurements

AbstractBiconical antennas, which are typical broadband antennas, are widely used to measure electromagnetic interference (EMI) in the band from 30 to 300 MHz. Thus, theoretical studies using the method of moments have investigated antenna arrangements suitable for biconical antenna calibration. The results showed that the proximity effect between the standard dipole antenna for calibration and the transmitting antenna in the low‐frequency band and the nonuniformity of the electric field strength near the biconical antenna being calibrated in the high‐frequency band greatly affected the calibration uncertainty. Based on these considerations, the antenna setup was determined to calibrate the biconical antenna with an uncertainty of 0.1 dB. Furthermore, the difference between the EMI measurements using a biconical antenna and the measurements using a dipole antenna was examined theoretically by means of a radiating point source. This result showed that the overall difference in the measurements is not very large at about 1 dB or less, but the difference increases in measurements for vertically polarized waves than for horizontally polarized waves, and this difference was conspicuous particularly at a measurement distance of 3 m. In addition, the difference in the measurements decreases when a free space antenna factor is used. © 2002 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 85(7): 23–31, 2002; Published online in Wiley Interscience (www.interscience. wiley.com). DOI 10.1002/ecja.1108