Point source

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

Scanning X-ray source

Abstract: Radiographic images of high definition and clarity are produced quickly and with reduced radiation exposure of the subject by utilizing a scanning X-ray source in which a moving point source of X-rays is created by sweeping an electron beam in a raster pattern on a broad anode. A radiation detector having a very small radiation sensitive area is situated on the opposite side of the subject from the source. The output of the detector controls electron beam intensity within a cathode ray type display tube wherein the raster pattern is synchronized with that of the X-ray source to produce an image of internal structure of the subject. In some embodiments of the invention, the small radiation detector is mounted on a probe suitable for insertion into internal regions of a living body or into recesses in mechanical structure to be examined. Stereo images may be produced by employing two spaced apart detectors controlling two separate images which are directed to separate eyes of the observer or by using a single detector alternately controlling each of the two images while the raster pattern at the source is alternately shifted between two at least partially separate areas of the anode. As the detector output is an electronic signal, the image data may be stored on magnetic tape or the like and may also be readily processed by electronic techniques for such purposes as image enhancement, and addition, subtraction or superimposition of images. Automatic brightness control may also be provided to produce uniform contrast in different areas of the image where the corresponding different regions of the subject have different average densities.

Tycho-2 stars with infrared excess in the MSX Point Source Catalogue

Abstract: Stars of all evolutionary phases have been found to have excess infrared emission due to the presence of circumstellar material. To identify such stars, we have positionally correlated the infrared Mid-Course Space Experiment (MSX) Point Source Catalogue and the Tycho-2 optical catalogue. Near-mid-infrared colour criteria have been developed to select infrared excess stars. The search yielded 1938 excess stars; over half (979) have never previously been detected by IRAS. The excess stars were found to be young objects such as Herbig Ae/Be and Be stars, and evolved objects such as OH/IR (infrared) and carbon stars. A number of B-type excess stars were also discovered whose infrared colours could not be readily explained by known catalogued objects.

A search for point sources of EeV photons

Abstract: Measurements of air showers made using the hybrid technique developed with the fluorescence and surface detectors of the Pierre Auger Observatory allow a sensitive search for point sources of EeV photons anywhere in the exposed sky. A multivariate analysis reduces the background of hadronic cosmic rays. The search is sensitive to a declination band from -85{\deg} to +20{\deg}, in an energy range from 10^17.3 eV to 10^18.5 eV. No photon point source has been detected. An upper limit on the photon flux has been derived for every direction. The mean value of the energy flux limit that results from this, assuming a photon spectral index of -2, is 0.06 eV cm^-2 s^-1, and no celestial direction exceeds 0.25 eV cm^-2 s^-1. These upper limits constrain scenarios in which EeV cosmic ray protons are emitted by non-transient sources in the Galaxy.

Theoretical study of magnetic and ultrasonic sensors: dependence of magnetic potential and acoustic pressure on the sensor geometry

Abstract: In this paper, we describe a model based on a spatial distribution of point sources, called 'DPSM' (Distributed Point Sources Method), applied to magnetic and ultrasonic sensors modelling. Magnetic and acoustic fields are theoretically generated for two types of sensors. The sensor surface is discretized into a finite number of elemental surfaces. A point source is placed at the centroid position of every elemental surface. Point source strength is proportional to the elemental surface area for acoustic sensors and it is obtained by inverting a matrix to satisfy the equipotential boundary conditions for magnetic sensors. Total field is computed at a given point by adding fields generated by all sources. The main difference between the magnetic and acoustic field modelling is that for a magnetic sensor the magnetic potential remains constant on the sensor surface and the magnetic flux varies from point to point, while for the acoustic sensor the particle velocity remains constant on the sensor surface and the acoustic pressure varies. This difference causes an additional matrix inversion in the magnetic field modeling, which is not necessary for the acoustic field modeling. Like other numerical modeling schemes, accuracy of the computation depends on the sensor surface discretization or mesh generation. Effect of the spacing between two neighboring point sources on the accuracy of the field computation is studied and the optimum spacing for accurate numerical computation is given. For accurately modelling acoustic fields the spacing between two neighboring sources should be less than the acoustic wavelength. Flat sensors with circular and rectangular cross-sections as well as point focused concave sensors have been modelled by this technique.