Showing papers on "Scintillation published in 1984"

Flicker of Extragalactic Radio Sources at Two Frequencies

Abstract: Dual-frequency observations of flat and steep-spectrum extragalactic radio sources made at Arecibo Observatory over a 20-day period are analyzed. As first reported by Heeschen (1982, 1984), flat-spectrum sources generally have larger intensity variations than steep-spectrum ones. A structure function analysis demonstrates a qualitative difference in the time series of the sources. The case against interstellar scintillation is examined, including a review of applicable scintillation theory. Relativistic source motion is treated as a solution to the brightness-temperature problems which arise if the variations are assumed intrinsic to the sources. 16 references.

Measurement and modeling of scintillation intensity to estimate turbulence parameters in an Earth-space path

Abstract: The theoretical and experimental aspects of the analysis of amplitude scintillations as a possible practical remote-sensing tool of atmospheric turbulence parameters with special emphasis on earth-space paths are explored. Theoretical studies and experimentation reveal that the scintillation variance when complemented with spectral analysis and suitable modeling of the path leads to realistic values of the parameters. A layered turbulent structure has been studied in particular, the strategy of data analysis is given in detail, and the limitations due to the cross-path wind profile are discussed.

Large barium fluoride detectors

Abstract: Big BaF2 crystals of 1–2 1 volume and up to 15 cm thickness were investigated with respect to their application as gamma-ray detectors. In particular, we were interested in the light transmission in the UV region, and the energy and time resolution. We found that an energy resolution of ∼ 12% (662 keV) and a time resolution of ∼ 0.4 ns (60Co, 300 keV threshold) can be obtained simultaneously. For these features, BaF2 is superior to NaI or BGO in cases where good timing is essential. Gamma-rays and alpha particles can be clearly discriminated; as for the latter the fast component does not show up in the scintillation light.

Modeling and scientific application of scintillation results

Abstract: Efforts continue to update and improve empirical models of the mean morphological behavior of ionospherically produced scintillation. While often intended for engineering application, these models also can be scientifically useful to the extent that they describe not only the signal fluctuations but also the plasma density irregularities that cause them. Such a model is contained in the computer code called WBMOD, which employs the phase screen propagation theory to calculate the power law spectral index and strength (at a fluctuation frequency of 1 Hz) of phase scintillation and the standard deviation of phase, for a user-specified system. Explicit provision is made for an ionospheric outer scale, but the relevant phase spectrum is cut off by a system parameter in most applications. The intensity scintillation index S, is calculated from weak-scintillation theory and corrected for the saturating effect of multiple scatter on the basis of Rice statistics. The irregularity model in WBMOD describes threedimensionally anisotropic (sheetlike) irregularities on the nightside of the high-latitude scintillation zone and axially symmetric (rodlike) irregularities elsewhere. The current status of WBMOD, which is undergoing continued refinement, is described in this paper for all latitude regimes. Plans for future development of its irregularity model are stated, and it is suggested that such models should now move in the direction of incorporating irregularity dynamics.

Orbital inclination and mass of the binary pulsar PSR0655 + 64

Abstract: The full determination of the orbits of a binary star system requires measurements of both line-of-sight and transverse velocities. Except for orbits of large angular size, there is usually insufficient angular resolution for transverse velocities to be measured; the orbital parameters then contain an unknown orbital inclination. It is this that has so far limited the determination of the parameters of the binary system containing the pulsar PSR0655 + 64. Recent proper motion measurements have shown that the transverse velocities of pulsars can be determined from observations of the rate of scintillation of pulsar radio intensity due to irregularities in the interstellar medium1. We report here variations in the rate of scintillation that have been measured as the pulsar PSR0655 + 64 moves around its orbit, allowing a direct determination of the inclination of the orbit, the transverse velocity of the whole binary system through space and also of the linear scale of the interstellar scintillation pattern. The observations also provide an upper limit to the diameter of the companion star and the electron density within the orbit.

A Stationary Positron Emission Ring Tomograph Using Bgo Detector and Analog Readout

Abstract: A single ring PET camera has been constructed to demonstrate the use of one dimensional scintillation camera techniques in positron tomographic design. A pseudo-continuous BGO detector is used. The detector is 46 cm ID, 52 cm OD, 2 cm wide and is made using 360 elements. A narrow light guide and 90 PM Tubes are used for the readout. The position of coincident events are found using unique scintillation camera logic.

Scintillation Detection with Large-Area Reach-Through Avalanche Photodiodes

Abstract: Silicon avalanche photodiodes of the "reach-through" type have been used as scintillation detectors with device performance closely approaching that of PMT's. The device consists of a 6 × 6mm diode with a 5 × 5mm photosensitive area in a 6.7 × 6.7 × 2.0mm package which is suitable for mounting in an array. Typical characteristics at room temperature are: quantum efficiency ?50% at ?=415nm; surface dark current <200nA; bulk dark-current (before gain) <1nA; noise current (at optimum gain of 20 to 50) <1pA/Hz?; response time <5ns; capacitance ?21pF; biasing voltage ?200-400 Volts. When coupled to a NaI(Tl) scintillator (9 × 9 × 38mm) photo-peak resolutions (FWHM) for 662keV radiation of 10.4% at 21°C and 9.8% at 0°C were measured. The measured noise equivalent primary charge of one detector at a gain of 50 was 121 electrons (51 electrons rms) at room temperature. The significance of this new detector to high resolution imaging systems and detector arrays is discussed.

Numerical simulations of intensity scintillation using the power law phase screen model

Abstract: Simulations of radio wave scintillation in power law media were performed to study the structure of the intensity and phase spectra when the Fresnel radius is much smaller than the outer scale or the low-frequency limit of the power law regime in the in situ irregularity spectrum. The results verify the critical dependence of intensity scintillation on the power law index as predicted by both asymptotic and more refined theoretical computations. Because of more recent findings, the scintillation characteristics of propagation in media characterized by a two-component power law were also investigated. The results reconcile the interpretation of scintillation data from the Wideband satellite which showed a shallowly sloped power law phase spectrum with more recent intensity scintillation data.

A Modular Scintillation Camera for Use in Nuclear Medicine

Abstract: A "modular" scintillation camera is discussed as an alternative to using Anger cameras for gamma-ray imaging in nuclear medicine. Each module is an independent gamma camera and consists of a scintillation crystal, light pipe and mask plane, PMT's, and processing electronics. Groups of modules efficiently image radionuclide distributions by effectively utilizing crystal area. Performance of each module is maximized by using Monte-Carlo computer simulations to determine the optical design of the camera, optimizing the signal processing of the PMT signals using maximum-likelihood (ML) estimators, and incorporating digital lookup tables. Each event is completely processed in 2 ?sec, and FWHM of the PSF over the crystal area is expected to be 3 mm. Both one-dimensional and two-dimensional prototypes are tested for spatial and energy resolution.

Neutron and/or gamma radiation detecting system

Abstract: In a neutron and/or gamma radiation detection system, a large reception surface for the radiation to be detected is formed on a body of scintillation material which is adapted to convert neutron or gamma radiation into light energy. A large number of fiber light conductors is embedded in the body of scintillation material such that the fibers extend essentially parallel and fully across the reception surface of the body of scintillation material. The light energy, upon propagation along the fiber light conductors, is coupled into the conductors along the surface of the fibers, which fibers are anisotropic. This arrangement permits the use of anisotropic light conductor systems which, in contrast to optically isotropic systems, provide for a separation of light collecting and light transmitting functions which results in a substantial reduction of light absorption losses during light transmission so that most of the light energy coupled into the fiber light conductors reaches the optoelectronic amplifier coupled to the end of the light conductors.

Detector saturation effect on higher-order moments of intensity fluctuations in atmospheric laser propagation measurement

Abstract: We investigate the influence of the detection-system saturation on the intensity-fluctuation statistics in measurements of optical scintillation induced by atmospheric turbulence. The statistical distribution of the intensity is assumed to be log normal. The values of the nth moments detected by the apparatus are shown to depend on the ratio between the saturation intensity of the detector and the mean intensity of the distribution to be measured.

Multifrequency study of ionospheric scintillation at Ascension Island

Abstract: Multifrequency amplitude scintillation data at L and C bands from the equatorial region at Ascension Island are used to study the spectral index n for the scintillation index S4. The average frequency dependence of the scintillation index (S4) is found to be f−1.68. It is shown that the spectral index tends to be larger for events which occur after 2030 local time than for events which occur before 2030. This may indicate temporal evolution of the irregularity spectrum. Model studies based on a two component power law irregularity spectrum are applied to relate changes in n to spectral evolution and to show that the observations are consistent with recent in situ measurements. Possible geophysical implications are also discussed.

Photon interaction measurements of certain compounds in the energy range 30–660 keV

Abstract: The total photon interaction cross sections at six energies in the energy range 30–660 keV are determined in ten compounds, (NaNO3, KNO3, Sr(NO3)2, NaCl, SrCl2∙6H2O, BaCl2∙2H2O, NaClO3, (NH4)2SO4, MgSO4∙7H2O, and K2SO4) using a NaI(Tl) scintillation spectrometer in a good geometry setup. Analysis of the experimental data in terms of cross sections and effective atomic numbers is presented.

Light Emission from Tracking Discharges on Organic Insulation

Abstract: This paper describes measured spectra of light emitted from discharges generated on the surface of organic insulations. The corona discharge before carbon deposit showed many ultraviolet lines below 400 nm. These spectra are produced by the energy transition of molecular nitrogen in air. After carbon deposit, the scintillation discharge initiates. It contains many lines in the visible light above 400 nm. The spectra of the scintillation discharge in the visible light do not correspond with those of the dominant molecular gases in air. Therefore, the molecular structure of the materials may influence the line spectra for the scintillation discharge. The scintillation discharge approaches toward the sample's surface. The arc discharge showed many spectra in the ultraviolet and/or visible range. The arc discharge bridges between the electrodes, far away from the sample's surface as compared with other discharges.

Interpretation and modeling of quasiperiodic diffraction patterns observed in equatorial VHF scintillation due to plasma bubbles

Abstract: Quasiperiodic diffraction patterns have been observed in VHF scintillation recorded at Ascension Island in 1981. The patterns occur most often at the beginning or end of a scintillation patch and are shown to be consistent with those expected from irregularities having east-west scale sizes of a few hundred meters that are associated with the walls (edges) of equatorial plasma bubbles. By adjusting the geophysical parameters in a computer simulation of the amplitude fluctuations expected simultaneously at VHF, L-Band and C-Band, we model a specific event in detail. The model contains information on the size and strength of the structures that cause the regular fading patterns. In addition, multi-frequency diffraction patterns are computed for a 'phase screen' model based on high resolution measurements of a structured plasma bubble wall made using the AE-E satellite. The results are qualitatively very similar to the observations. Finally, implications of the model results for the extraction of information about the vertical structure of the irregularities are discussed.

Inorganic scintillator crystal having a highly reflective surface

Abstract: Scintillators, and more particularly inorganic scintillation crystals such as cadmium tungstate, bismuth germanate, barium fluoride and calcium fluoride are coated with successive coatings of particulate materials such as barium sulfate and titanium dioxide to provide a highly reflective surface layer capable of providing internal reflection of scintillation events in the region of 200-700 nm. The reflective surface enhances the pulse height and resolution of the scintillator when coupled to a suitable detector such as a photomultiplier tube or a silicon photodiode.

Germanium-Scintillation Camera Coincidence Detection Studies for Imaging Single Photon Emitters

Abstract: A prototype system has been used to measure the point spread function (PSF) and sensitivity characteristics of a germanium-scintillation camera (GSC) system for imaging single photon emitters. A detector consisting of two 5mm × 5mm × 6 mm elements, fabricated on a single piece of high purity germanium, and a Pho/gamma HP scintillation camera were used. Experiments were conducted with a point source of Tc-99m to specify the threshold and energy resolution required in the germanium detector and to study the timing characteristics of coincidence detection.

Scintillator crystal having a highly reflective surface

Abstract: Scintillators, and more particularly inorganic scintillation crystals such as cadmium tungstate, bismuth germanate, barium fluoride and calcium fluoride are coated with one or more coatings of a highly reflective material such as barium sulfate dispersed in a suitable binder such as sodium silicate to provide a highly reflective surface layer capable of providing internal reflection of scintillation events in the region of 200-700 nm. The reflective surface enhances the pulse height and resolution of the scintillator when coupled to a suitable detector such as a photomultiplier tube or a silicon photodiode.

Spatial registration correction for rotational gamma cameras

Abstract: A method for correcting predictable errors in the location of detected scintillation events acquired during emission computed tomography by a rotational scintillation gamma camera system. The method includes calibrating the rotational scintillation camera system and generating a look-up table comprising a set of pairs of x and y offset values, one pair for each viewing angle. The calibration reflects systemic deviations in the location of the rotating camera head detector measured in terms of x and y coordinates in the frame of reference of the detector for a plurality of view angles relative to the geometric ideal expected path. Once a look-up table is generated for a camera system, the detected location of each gamma event is altered in real time by adjusting in the camera head the x,y coordinate location of each event by the x,y offset values previously generated for the corresponding viewing angle associated with the detected event.

Radiation measuring element

Abstract: PURPOSE:To obtain a small light wt radiation measuring element having high resolving capacity, by forming a scintillation film having a large number of vertically extended fine needle like structures on an array type semiconductive light detecting element in a closely adhered state through an electromagnetic wave previous film CONSTITUTION:A scintillation film 5 comprising CsI(Tl) with a thickness of about 160mum is vapor deposited on an array type semiconductive light detecting element obtained by forming n type semiconductive regions 3 on the surface of a p type semiconductive substrate 2 in a heated state through a SiO2 vapor deposition film 4 with a thickness of about 01mum and, after cooling, cracks are generated in the vertical direction by the difference of coefficient of thermal expansion with the SiO2 film 4 The whole is coated with an Al vapor deposition protective film 6 Radioactive rays such as X-rays from the direction shown by the arrow are incident to the scintillator film 5 through the protective film 6 to generate fluorescence which is, in turn, subjected to multiple reflection in the needle like sections 51 to reach a light detecting element Therefore, a small light wt radiation measuring element having high resolving capacity is obtained

A comparison of radio wave and in situ observations of tropospheric turbulence and wind velocity

Abstract: The importance of radio wave scintillations introduced by tropospheric turbulence for both telecommunication and meteorological research is indicated. Amplitude, phase, and phase difference measurements of scintillation were performed at 30 GHz on an 8.2-km line-of-sight path. In situ measurements of temperature and humidity fluctuations and wind velocity and direction were done about midway on the propagation path. The measuring systems are described, and theory is presented to derive the refractive-index structure parameter C2n and wind velocity v from radio wave scintillations and the in situ measurements. Formulas are given to scale C2n and v with height. Finally, results of the experiments are presented and discussed.

Device for measuring nuclear radiation, and gamma or scintillation camera comprising such a device

Abstract: A device for measuring nuclear radiation which comprises a scintillator (10) for the detection of the radiation quanta which is optically coupled to an entrance window of a photodetector (20) for converting the scintillations produced by the radiation into current pulses. The device comprises a digital circuit which is connected to a sampling and analog-to-digital conversion circuit (40) and which is used to determine the energy of each nonsuperposed radiation quantum; when the radiation quanta produce partly superposed current pulses, this circuit determines the energy content of the individual radiation quanta by extrapolations and corrections, notably by means of a memory (170 and/or 180) in which the correction coefficients are stored for performing extrapolations and corrections under the control of a sequencer (200) as initiated by a pulse edge detector (80) which detects the incidence of a radiation quantum.

Statistical characterizations of equatorial scintillation in the Asian region

Abstract: The morphological aspect of gigahertz equatorial scintillations in the Asian region has been reported. As a follow-up, this paper studies the statistical aspect of equatorial scintillations in the same region. Power spectra of prominent scintillation events collected over a 16-month period during the solar maximum years in sunspot cycle 21 were analyzed. The slope ν of the spectrum at the high-frequency end was analyzed with respect to the strength of scintillation as indicated by scintillation index S4. The values of S4 were compared with theoretical calculations based on either phase screen formulation or Rytov approximation. These comparisons were used to assess ionospheric parameters such as the height and thickness of the irregularities layers, rms fluctuation of the total electron content, and the axial ratio of the irregularities. Autocorrelation calculation provided a characteristic coherent interval τ. The study suggests that equatorial ionospheric irregularities in the F region, with subkilometer sizes in the evening hours after local sunset, are the main cause of the gigahertz scintillations observed. The spectra of the irregularities appear to be the power law type, with spectral indices p that generally have values greater than 4. The characterizations of S4, ν, and τ for both the Pacific and Indian Ocean region links provide an important basis for improved modeling of ionospheric scintillation phenomena in the Asian region.

Fano factor in gaseous argon measured by the proportional scintillation method

Abstract: It is found that the apparent Fano factor in argon obtained by using 5.90 keV X-rays with the proportional scintillation method depends on the gas pressure and that the true Fano factor can be obtained by extrapolating the linear relation between the apparent Fano factor and the reciprocal of the gas pressure. The Fano factor thus obtained gives good agreement, within experimental errors, with the value recently obtained by using a gridded ionization chamber.