Point source

About: Point source is a research topic. Over the lifetime, 5077 publications have been published within this topic receiving 94091 citations.

Radiative transport theory for optical molecular imaging

Abstract: We study the inverse fluorescent source problem for optical molecular imaging. In particular, we recover key properties of a fluorescent source inside a halfspace composed of a uniform absorbing and scattering medium from angularly resolved measurements at the boundary plane. We use the radiative transport equation to model the multiple scattering of light in tissues. Using Green's function, given as an analytical expansion in plane wave solutions, we subtract contributions from the measured angular data due to surface sources yielding a quantity that depends only on the interior fluorescent source. We analyse this reduced problem and obtain explicit solutions for a point source and a voxel source. Using the point source and voxel source solutions, we estimate the location, size and total strength of a general source. We perform numerical studies to validate this theory as well as investigate modelling errors due to incorrectly assumed optical properties of the medium.

Highly directional acoustic wave radiation based on asymmetrical two-dimensional phononic crystal resonant cavity

Abstract: The radiation properties of an asymmetrical two-dimensional phononic crystal resonant cavity with a point source inside are investigated experimentally. The resonant cavity is formed by two separated phononic crystals of different thickness, both of which consist of the same square array of steel rods in water. We observe highly directional acoustic wave radiation when a point acoustic source is put inside the cavity. The radiation field has a half-power beam width less than 6°. This design may serve as a highly directional acoustic source in applications.

Large-eddy simulation of passive scalar dispersion in an urban-like canopy

Abstract: Results from large-eddy simulations of short-range dispersion of a passive scalar from a point source release in an urban-like canopy are presented. The computational domain is that of a variable height array of buildings immersed in a pressure-driven, turbulent flow with a roughness Reynolds number . A comparative study of several cases shows the changes in plume behaviour for different mean flow directions and source locations. The analysis of the results focuses on utilizing the high-fidelity datasets to examine the three-dimensional flow field and scalar plume structure. The detailed solution of the flow and scalar fields within the canopy allows for a direct assessment of the impact of local features of the building array geometry. The staggered, skewed and aligned arrangements of the buildings with respect to the oncoming flow were shown to affect plume development. Additional post-processing quantified this development through parameters fundamental to reduced-order Gaussian dispersion models. The parameters include measures of concentration decay with distance from the source as well as plume trajectory and spread. The horizontal plume trajectory and width were found to be more sensitive to source location variations, and hence local geometric features, than vertical plume parameters.

Variable X-ray source near Cen X-3

Abstract: THE satellite Ariel V, launched on October 15, 1974, as part of the US-UK collaborative space programme, contains a rotation modulation collimator experiment designed to measure the positions of X-ray sources, which was built by the Mullard Space Science Laboratory. The experiment responds to X rays in the quantum energy range 3.1–9.3 keV, the area of the proportional counters being 256 cm2 which, allowing for grid transmission, counter efficiency and so on, leads to an effective area of 102 cm2 at 6.0 keV. The pitch-to-separation ratio of the collimator grids is 112, corresponding to a full width at half maximum (FWHM) of the image of a point source of 15′ (ref. 1). The spin axis was manoeuvred to point towards a number of sources in succession in the Centaurus region, including the important binary source Cen X-3, from December 17, 1974 to January 31, 1975, to carry out position and spectrum determinations. Soon after arriving at the first pointing direction in the sequence, a correlation map produced by overlaying data from orbits obtained on December 17–19 showed the presence of a new source not listed in the Uhuru (3U) catalogue, which we have designated A1118-61 from the position given below. The field of view of the experiment is 17° FWHM, so that the light curve could be followed over an interval of 46 d. The light curve is given in Fig. 1 in units of counting rate in our detector, which can be converted approximately to Uhuru counts by multiplication by 8. In the initial and final periods of observation, the source was rather weak, and could be detected at a satisfactory significance only by overlaying orbits, with a consequent reduction in time resolution. The latter improved to one orbit (101 min) when the source was near its maximum brightness. We have, for clarity, averaged the intensity over 12 h in this period, but significant intensity variations, exceeding those attributable to counting statistics, were observed in the one-orbit averages. Where an upper limit only is given, it corresponds to three standard deviations of the background level.

Travel Time Sensitivity Kernels

Abstract: We derive, following the standard first Born approximation approach used in the geophysics literature, an expression for the travel time perturbation caused by a perturbation to sound speed. In our simple model we employ a point source at one point and calculate the time taken for a wave packet created at the source to move to a second point. In the first Born approximation the travel time delay caused by a perturbation to the background model can be expressed as the integral over the whole sun of some function, called the travel time sensitivity kernel, multiplied by the perturbation. The sensitivity kernels are zero along the geometrical ray connecting the two points and have maximum weight in a tube around the ray; they are the solar equivalent of ‘the banana-doughnut’ kernels discussed in the geophysics literature. Calculating sensitivity kernels that are more accurate than those derived from ray theory is important for the future of inversions done with time-distance helioseismology data as they will allow greater confidence in the results as well as increased resolution.