Showing papers on "Point source published in 1986"

Large intensity fluctuations for wave propagation in random media.

Abstract: The intensity pattern generated by a monochromatic point source in a random medium is studied. The intensity-intensity correlation function is calculated and it is shown that the intensity, as a function of coordinate, exhibits large fluctuations (the speckle pattern). The sensitivity of this speckle pattern to small changes in the source frequency is also studied.

Complex point-source representation of real point sources and the Gaussian beam summation method

Abstract: A new exact representation for point sources is given in terms of complex point sources. In the simplest configuration, a point source is equivalent to a distribution of sources on the surface of a sphere in complex space. The representation can be used to consider the propagation of point disturbances through inhomogeneous media and across interfaces. In the high-frequency limit, these solutions may be obtained by the use of complex ray-tracing methods, which are just the analytic extension of ordinary ray methods. It is shown that the additional use of the paraxial approximation yields a procedure that is similar to the Gaussian beam summation method. The latter technique is normally based on a matched asymptotics argument. However, the complex point-source representation now offers an exact basis for this widely used method.

Low-energy scanning transmission electron microscope

Abstract: Low-energy scanning transmission electron microscopy is achieved by using a sharply pointed electrode as a source of electrons having energies less than 10 eV and scanning the electron emitting pointed source across the surface of a self-supported thin film of material to be investigated at an essentially constant distance on the order of nanometers. The electrons transmitted through the specimen are sensed by a suitable detector and the output signal of the detector is used to control a display unit, such as a CRT display or a plotter. A scanning signal generating means simultaneously controls both the scanning of the electron emitting point source and the display unit while a separation control unit holds the distance between the point source and surface at a constant value. The electron emitting point source and associated mechanical drives as well as the specimen film and electron detector are all positioned in a vacuum chamber and isolated from vibration by a damped suspension apparatus.

A construction of the fields radiated by z‐directed point sources of current in the presence of a cylindrically layered obstacle

Abstract: Expressions for the electric and magnetic vector potentials for z-directed magnetic and electric current point sources are developed as spectral integrals over longitudinal and azimuthal wave numbers. The representations are valid outside the source region. The construction is systemized, with the aid of Mason's signal-flow graphs, in order to facilitate numerical implementation. The azimuthal spectral integral form is chosen rather than the azimuthally periodic form so that computations may be carried out in the complex plane associated with angular wave number without recourse to the Watson transform. Some example calculations are provided to demonstrate the practicability of the method.

Gaussian beams in anisotropic elastic media

Abstract: Summary We derive the parabolic equation for high-frequency solutions of anisotropic elasticity propagating in the neighbourhood of a ray. The dynamic ray tracing system governing the evolution of a Gaussian beam along its central ray is derived. It is shown that a Gaussian beam is non-singular at the caustics. The decomposition of a point source in an anisotropic medium into Gaussian beams is derived and its application to the complex DRT of seismic waves in anisotropic media is discussed.

Concentration fluctuations according to fluctuating plume models in one and two dimensions

Abstract: Concentration probability density functions (pdfs) calculated according to fluctuating plume models in one- and two-dimensions, representing the limiting cases of one-dimensional dispersion from a line source or a point source in strongly anisotropic turbulence and of axisymmetric dispersion from a point source in isotropic turbulence, are discussed and analyzed in terms of the location of the sampling point within the mean plume and of the ratio, s/m, of the standard deviations for relative dispersion and meandering.

Raster scan anode X-ray tube

Abstract: An apparatus and method are provided for X-ray imagery of an object comprising an X-ray source adapted to variably dispose of a point source of X-rays about a first surface of the X-ray source and a detector adapted to receive the X-rays and translate the X-rays into electrical signals. The detector is adapted to selectively respond to incident X-rays, e.g., in accordance with the angle of incidence of the X-rays upon the surface of the detector. The detector may be formed of an array of individual detector elements, or may comprise a light transducer disposed adjacent the input to a television camera. The interrogation of the detector elements, or scan of the television camera image, may be synchronized with the movement of the X-ray source such that the spacial relationship between the movement of the point source and the sampled detector element, or scan point of the television camera, remain substantially constant as the point source moves.

Constraints on neutron star models of gamma-burst sources from the Einstein Observatory

Abstract: Six Einstein observations of five gamma-ray burst sources are presented and discussed. With one possible exception, no point source was detected in any of the observations. The data are interpreted in the framework of neutron star models for gamma bursters. Upper limits are derived for the surface temperatures of the neutron stars assumed to be responsible for the bursts. It is shown that the lack of soft X-ray emission may impose stringent constraints on accretion rates onto neutron stars.

Measuring interstellar dust grains from the haloes of binary X-ray sources

Abstract: A method for determining the location, size distribution, and composition of interstellar dust grains from the haloes of binary X-ray sources is described. The method is based on the time dependence of the halo X-ray emission during an eclipse of the point source. The distance of the dust cloud is inferred directly from observations of the growth of the dark circle around the eclipsed point source. Grain size distribution in the interstellar dust clouds is derived from the halo brightness profile at a given photon energy. The composition of the grains is obtained by observing the spectral features in the total optical depth due to X-ray scattering near the photoionization thresholds of the elements in the grains. The haloes become dim just below the ionization threshold, and sharply increase in function brightness as the photon energy increases beyond the absorption edge of the relevant atom. The method makes it possible to observe and analyze the halo emission during eclipse even without imaging the source. It is suggested that haloes probably account for the soft X-rays observed during eclipses and dips observed in sources including 4U1915-05 and X1755-338.

QUANTITATIVE IN SITU HYBRIDIZATION AND DETERMINATION OF mRNA CONTENT

Abstract: The determination of mRNA levels by in situ hybridization in brain tissue is a prominent topic of concern of workers in this field Two degrees of quantitation are theoretically possible The ability to provide a minimum estimate of mRNA amounts for comparing conditions is the easier of the two The ability to measure absolute amounts of mRNA by in situ hybridization is a much less certain and more contentious proposition, as discussed below In both cases, it is necessary to convert grain densities to tissue radioactivity to, finally, mRNA levels The first conversion is discussed in the second portion of this chapter Briefly, for isotopes with higher energy than tritium, emulsions of uniform thickness are required to eliminate variability in grain density due to variations in emulsion thickness For this purpose, one can use x-ray film or emulsion-coated coverslips (1) In addition, one must simultaneously expose standards containing known amounts of the isotope used in the experimental sections (2) Once the autoradiograms are developed, one must decide on the appropriate approach to gathering the raw data The choices among grain densities, grain reflectances, or optical densi ties are a critical part of the analysis as discussed in detail in Rogers' excellent text on autoradiography (3) A radioactive isotope has a spectrum of energetic emissions A point source produces a bell-shaped distribution of grain densities An extended uniform source produces a plateau with a decline in grain density beginning within the perimeter of emulsion over the source The latter situation more closely approximates the case in tissue where a cell or cluster of cells may represent the uniform source There are a number of factors which may influence the resolution and the spread of grains beyond the edge of the source Isotopic emission energy, source and emulsion thickness, and distance between source and emulsion are the most important factors in light microscopic autoradiography To insure an accurate measurement over a uniform source, the diameter of the source should exceed that of the central measuring area by twice the track length of the highest energy particle emitted However, since most particles do not follow a straight path, such a stringent requirement may not be followed in practice By measuring grain densities over a source in this way, one can compare radioactivity concentrations

Variable Background Born Inversion by Wavefield Backpropagation

Abstract: The inverse scattering problem for an acoustic medium is formulated by using the variable background Born approximation. A constant density acoustic medium is probed by a wide-band point source, and the scattered field is observed along a curved receiver array located outside the region where the medium velocity is different from the assumed background velocity function. The solution proposed relies on the introduction of a backpropagated field. This field is obtained by using a finite-difference scheme backward in time to backpropagate into the medium the scattered field observed along the receiver array. The backpropagated field is imaged at the source travel times, giving an image of the same type as obtained by reverse-time finite-difference migration techniques. The gradient of this image is then taken along rays linking the source to points in the medium and, after scaling, this gives the reconstructed potential. To relate the reconstructed potential to the true scattering potential, high frequency ...

Diffusion Model for a Convective Layer. Part II: Plume Released from a Continuous Point Source

Abstract: A two-dimensional diffusion model for a convective planetary boundary layer is presented. Three prognostic equations are employed for the concentration, the concentration flux, and the covariance of temperature and concentration. The environmental background, which includes the mean wind, the mean virtual potential temperature, the turbulent kinetic energy and other second moments, is generated from Part I of this study. For an elevated point source, the plume concentration centerline descends until it intercepts the ground, then it rebounds and returns to the mixed layer, while the material released from a surface point source gradually ascends into the mixed layer, carried aloft by the updrafts of the convective motion. Compared to the laboratory and field experiments, this model simulates well the diffusion of passive pollutants released from a continuous point source into the convective layer.

Is there a common origin for the cosmic γ-ray lines at 0.51 and 1.81 MeV near the galactic centre?

Abstract: The discovery of the 1.81-MeV 26Al decay line in the present interstellar medium1 provided direct experimental proof that intermediate mass nuclei are being synthesized continuously in the Galaxy. The instruments making this and a subsequent observation2 had broad angular responses, thus, although both observations were consistent with a source near the centre of the Galaxy, interpretations for the origin of this radiation have included objects distributed throughout the Galaxy capable of synthesizing 26Al, such as type II supernova, Wolf Rayet stars, novae and red giants. New observations3 have localized this source to the galactic centre region consistent with a point source at the centre within the resolution function of the telescope (∼10° FWHM, full width at half maximum). Observations of the galactic 0.51-MeV positron annihilation line have also localized this line to the galactic centre with an angular resolution ∼15° FWHM4. Time variability requires that it come from a region ∼1 pc in size, hence it is probably produced near a compact object at or near the galactic centre5. Here we discuss the possibility that the 0.51- and 1.81-MeV lines have a common origin: the initial decay of 26Al to 26Mg with the most probable emission of a positron which eventually annihilates producing the 0.51-MeV line, followed by the decay of 26Mg to the ground state with the emission of the 1.81-MeV line. For this process to be solely responsible for both lines the time-integrated intensities of the lines must be compatible, and we show that this is not contradicted by the existing data. The measurement of the properties of the two lines individually and taken together place strong constraints on the source region of this radiation.

Calibration of a portable intrinsic Ge gamma-ray detector using point sources and testing for field applications.

Abstract: A simple procedure using point sources is described for the calibration of portable high-resolution gamma-ray detectors used in field surveys. The point source data are fitted by empirical equations that are used to calculate efficiency for field applications. Two models for the distribution of radionuclides are treated: a thin surface-film model and a vertically homogeneous model. The success of the procedure has been demonstrated by field applications in plots where the levels of contamination are known.

Elastic strain energy and forces on point defects in a two-phase medium

Abstract: Elastic strain energy and forces on point defects in a two-phase medium with a planar interface are analyzed employing the surface dislocation analysis developed earlier for three-dimensional distortions. The important field components, namely, the tractions and the displacements arising due to the point source at the interface, are determined. Furthermore, the field components at the interface are used to determine the elastic strain energy associated with the point source in the two-phase medium and the elastic force exerted by the second phase on the point defect. The significance of these results to the force acting on a vacancy or an interstitial at the interface is emphasized.

The electromagnetic edge wave due to a point source of current radiating in the presence of a conducting wedge

Abstract: Cylindrical wave expansions for the dyadic Green's functions for electric and magnetic fields of a point source of electric current radiating in the presence of a perfectly conducting wedge are derived using a scalarization procedure developed by Levine and Schwinger. The forms derived from this procedure involve a sum over angular wavenumbers and a continuous spectral integral which may be expressed either as an integration over a longitudinal or a radial spectral variable. Some relationships between these two representations are discussed. The longitudinal spectrum integral has a pair of branch points as its only singularities and may be evaluated asymptotically along a steepest descent path away from one of the branch points. The resulting asymptotic representation is found to agree with an earlier result obtained by Kouyoumjian and Buyukdura. The edge-guided wave interpretation of the asymptotic field is discussed, both in light of the longitudinal spectral representation and of the physical content of the asymptotic representation.

A Statistical Analysis and Overview of the IRAS Point Source Catalog

Abstract: The IRAS Point Source Catalog can be segregated into stars, “IR galactic components”, galaxies, cirrus, and “disk and bulge stars” categories. For each source type, the types and distances of the sources are discussed, and the sky distribution, log N/log S, and identification rate are given.

Radiation transfer due to a point source in an isotropically scattering, inhomogeneous solid sphere

Abstract: A method of analysis is presented for solving radiation transfer due to a point source in an absorbing, isotropically scattering, inhomogeneous sphere, which is subjected to a diffusely reflecting boundary. The space-dependent single-scattering albedo is represented in the form ω(r) = ΣKk=0 DK rk, where r is the optical variable and Dk are the known expansion coefficients. The effects of various functional forms of albedo on the incident radiation, angular distribution of radiation intensity and emissivity are examined.

Instantaneous concentration fluctuations in point‐source plumes

Abstract: A model for the variance σ of instantaneous plume concentrations, at a level corresponding to that of the Gaussian formulas for the mean field, is developed and tested against available data. Starting from the Eulerian species transport equation, theoretical and empirical information is used to estimate parameters and simplify the form of the transport equation for σ in a meandering frame of reference. Then, introduction of a localized production of fluctuations (LPF) scheme allows construction of analytical expressions for σ that provide a direct means for calculating the concentration variance inside the instantaneous boundaries of plumes resulting from continuous point emissions in turbulent flows with uniform mean velocity.

The strong UV source in the active E Galaxy NGC 4552

Abstract: 1200-3200 A IUE spectra of the nucleus of NGC 4552 (M89) were obtained in order to investigate the nature of the strong 10 micron source in this Galaxy. There is a strong, extended UV source in NGC 4552 which has a spatial distribution nearly identical with that at optical wavelengths and is undoubedly stellar in origin. Its properties are consistent with the correlation between UV source strength and metallicity pointed out by Faber (1983). There is no evidence for a nonthermal point source in the UV. It appears unlikely that the 10 micron emission is from heated dust grains. Instead, it is believed the 10 micron radiation is nonthermal in origin, implying a remarkably small size of only 0.1 AU for this source.

Diffusion of Matter from a Continuous Point Source in Uniform Mean Shear Flows : 1st Report, Characteristics of the Mean Concentration Field

Abstract: The turbulent diffusion process has been examined experimentally in the case of a continuous point source in turbulent water flows with three different uniform mean shear rates Shear-flow generators used were non-uniformly spaced rods, and the diffusing matter was a dye solution The present study mainly deals with the characteristics of the mean concentration field It is revealed that the horizontal and vertical profiles of the mean concentration are nearly similar, and vertical profiles in particular are skewed to the side with a smaller flow velocity by the shear effect A diffusion model for a continuous point source by Okubo & Karweit (1969) is investigated and compared with the experimental results The effect of shear rate on the spread of matter is discussed on the basis of this model

Diffusion from a continuous source near a surface in steady reversing shear flow

Abstract: The dispersion of continuous emissions from a line-source in a reversed-flow layer is analysed by means of diffusion equations; a family of exact solutions is found in the form of infinite series and/or integrals. It is shown that the concentration within the layer decays exponentially with the streamwise distance in the direction of reversed flow. The ground-level concentration near the source is found to be governed largely by the local mean flow; the value of the diffusivity affects the position of the maximum of ground-level concentration, but has little influence upon its magnitude. A useful upper limit is deduced for the background concentration due to recirculation effects. Further, a simple formula is given for the maximum value of the ground-level concentration for cases where the source is not too near the ground. The predictions for ground-level concentration are validated against experimental data for the particular case of a line source in the recirculating wake behind a two-dimensional backward-facing step. The extension of the analysis to the case of a point source is also discussed.

Point Source Location Sensitivity Analysis

Abstract: This paper presents the results of an analysis of point source location accuracy and sensitivity as a function of focal plane geometry, optical blur spot, and location algorithm. Five specific blur spots are treated: gaussian, diffraction-limited circular aperture with and without central obscuration (obscured and clear bessinc, respectively), diffraction-limited rectangular aperture, and a pill box distribution. For each blur spot, location accuracies are calculated for square, rectangular, and hexagonal detector shapes of equal area. The rectangular detectors are arranged on a hexagonal lattice. The two location algorithms consist of standard and generalized centroid techniques. Hexagonal detector arrays are shown to give the best performance under a wide range of conditions.

A study of bipolar and compact nebulae at radio, submillimetre and infrared wavelengths

Abstract: Radio observations have been made of a sample of optically selected bipolar nebulae, to examine their spatial structures and to determine the nature of the sources which excite them. IRAS point source survey data, sub-millimeter wavelength molecular line and continuum measurements have been combined with the radio data to constrain the estimates of the nature of the excitation sources. Two thirds of the sample are found to be associated with luminous (L > 1000 L⊙) objects which are characterized as ZAMS B0-B2 stars. Several cases show characteristics of ionized outflowing gas, and may be pre-main sequence objects. The remaining third of the sample have ZAMS exciting stars between B3 and A5.

Apparent axial properties of transversely isotropic media

Abstract: Byun (1984) described some important surface seismic characteristics (such as “apparent” velocities) for transversely isotropic media. His theoretical development was based on the elliptical surface fit to the actual wave surface of energy propagation from a point source to describe local properties of the ray. In this note we give more theoretical insight into the apparent axial properties of the surface observations made in Byun’s (1984) paper. More comprehensive studies were reported in Helbig (1983) and Hake et al. (1984) for other surface seismic properties of transversely isotropic media.

Absorption effects on plane waves in layered media

Abstract: We present equations which can be used in conjunction with estimates of material properties to determine whether exact linear viscoelastic modeling will give significantly different results from elastic ray calculations. We apply the viscoelastic version of Snell9s law to assess the effects of anelasticity on phase velocities, attenuation coefficients, transmission angles, and travel times for a plane wave propagating through a stack of plane viscoelastic layers described by the velocity and Q for a homogeneous wave in each layer. We include a survey of the viscoelastic theory of plane waves to provide a context for the results. Our analysis shows that the most general version of Snell9s law leads to a complex two-dimensional vector ray parameter in which case plane P -wave motion does not decouple from horizontal transverse S -wave motion. We show that in a layered medium the fields generated by a symmetric point source, a vector point force, a double-couple point force, or a plane of tractions corresponding to a complex vector parameter, as well as other “reasonable” sources, can be calculated by integrating over real ray parameters, but in all of these cases, one must consider the coupling in laterally heterogeneous media. Differences from elastic ray behavior are found to be well correlated with the behavior of the parameter χ ≡ 1 + Q - 2 cos 2 γ where γ is the angle of inhomogeneity of the wave. We present equations for χ in terms of layer parameters and both the vector ray parameter and incident wave parameters. Examples presented to illustrate the method show significant (5 to 10 per cent) deviations from elastic travel times, and offsets in a given layer may be possible in very lossy layered materials.

The generalized equation of pseudo‐Kossel lines

Abstract: The generalized equation of pseudo-Kossel lines obtained in a back-reflection divergent-beam pattern is derived for the general case where the X-ray point source and the plane of the film are not co-planar This treatment of the pseudo-Kossel lines permits lattice-parameter determinations even from incomplete diffraction conics