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Showing papers on "Scintillation published in 1977"


Journal ArticleDOI
TL;DR: In this article, a review is presented of interstellar scattering and scintillation observations in the light of modern theory for extended, spatially homogeneous, power law inhomogenieties.
Abstract: A review is presented of interstellar scattering and scintillation observations in the light of modern theory for extended, spatially homogeneous, power law inhomogenieties. Formulas are presented for the experimentally observable quantities and are compared with data. A general discussion of the model is given. (GHT)

277 citations


01 Feb 1977
TL;DR: In this article, available observations of ionospheric scintillation are analyzed to evaluate the adequacy of existing models used for the interpretation of scintillant data. And the authors used the models were used to construct Scintillation occurrence distribution functions.
Abstract: Available observations of ionospheric scintillation are analyzed to evaluate the adequacy of existing models used for the interpretation of scintillation data. The theoretical models are reviewed and the frequency and propagation geometry dependences predicted by the models are compared with the observations. The models were used to construct scintillation occurrence distribution functions which show that scintillation phenomena significantly affect the design of transionospheric radar or communication systems operating at frequencies below 1 GHz. Diversity schemes useful for mitigation of scintillation effects are considered. Mention is made of the geophysical processes thought to be responsible for scintillation.

148 citations


Journal Article
TL;DR: The unit allows accurate positioning of a scintillation camera's detector at any angle around a patient in order to obtain the multiple projection images needed for transaxial tomography, and it is capable of imaging any area of the body.
Abstract: An emission transaxial tomographic system using a scintillation camera as the detector is described. The unit allows accurate positioning of a scintillation camera's detector at any angle around a patient in order to obtain the multiple projection images needed for transaxial tomography, and it is capable of imaging any area of the body. The camera can also be used for all types of conventional imaging procedures. Image processing is performed by a small on-line computer. A convolution algorithm and a mathematical technique for approximate absorption correction are used to obtain high-resolution and high-contrast images with good quantitative accuracy. The operation of the system is described and representative phantom and patient studies are presented to illustrate the capabilities of the system.

112 citations


Journal ArticleDOI
TL;DR: In this article, the latitudinal extent of the scintillation irregularity region has been established with a half occurrence width during years of moderate solar flux of plus and minus 12 degrees.
Abstract: With the advent of satellite communications systems at frequencies varying from \sim140 MHz to 1600 MHz as well as navigation and ranging systems in the 1200-1600 MHz portions of the spectrum, the effect of equatorial irregularities on fading signals has become of importance. Recent observations of the signal statistics of scintillations at frequencies ranging from 136 MHz to 6 GHz reveals a power law fall off of irregularity sizes. Power spectra are now available for a variety of conditions and for frequencies from VHF to microwaves. During periods of intense equatorial activity, at frequencies to 360 MHz, Rayleigh scattering is frequently experienced. The latitudinal extent of the scintillation irregularity region has been established with a half occurrence width during years of moderate solar flux of plus and minus 12\deg . A correlation of in-situ measurements of irregularities from satellites had revealed the great variations in longitudinal patterns during any season. It has also allowed the \Delta N obtained from in-situ measurements to be utilized to predict scintillation excursions. New facets of scintillation activity in the equatorial region recently reported include weak daytime scintillation and the patchy nature of irregularities (small irregularities embedded in large structures) particularly noted during periods of low solar flux. Future studies will assist in the delineation of the extent of the equatorial region at frequencies from 1.5 to 6 GHz, and in the UHF range. From the viewpoint of the communicator the morphology of scintillations at microwaves has still not been reported from any long term program of measurements.

111 citations


Journal Article
TL;DR: A computer-based scintillation camera has been designed for both dynamic and static radionuclide studies, and is being used for 133Xe dynamic studies of regional cerebral blood flow in man and for 99mTc and 197 Hg static imaging of the brain.
Abstract: A computer-based scintillation camera has been designed for both dynamic and static radionuclide studies. The detecting head has 254 independent sodium iodide crystals, each with a photomultiplier and amplifier. In dynamic measurements simultaneous events can be recorded, and 1 million total counts per second can be accommodated with less than 0.5% loss in any one channel. This corresponds to a calculated deadtime of 5 nsec. The multidetector camera is being used for 133Xe dynamic studies of regional cerebral blood flow in man and for 99mTc and 197 Hg static imaging of the brain.

92 citations


Journal ArticleDOI
TL;DR: The most persistent, identifiable feature on intensity and phase scintillation records of coherent beacon signals recorded near Fairbanks, Alaska during several months in 1976 is a narrow region of enhanced Scintillation to the south of the station, suggesting that it could be a geometric enhancement produced by fieldaligned irregularities as mentioned in this paper.
Abstract: The most persistent, identifiable feature on intensity and phase scintillation records of coherent beacon signals recorded near Fairbanks, Alaska during several months in 1976 is a narrow region of enhanced scintillation to the south of the station. On high-elevation passes the feature occurs near the magnetic zenith, suggesting that it could be a geometric enhancement produced by field-aligned irregularities. The feature also occurs, however, on medium and low-elevation passes when the minimum off-field angle is several tens of degrees. The observations, therefore, represent evidence for either (1) a hitherto unrecognized irregularity enhancement or layer thickening in the subauroral ionosphere, or (2) longitudinally extended irregularity sheets.

70 citations


Journal ArticleDOI
TL;DR: The role of the phase autocorrelation function in the generation of speckle, focusing and other scintillation effects produced when radiation is scattered by a deep random phase screen is investigated in this article.
Abstract: The role of the phase autocorrelation function in the generation of speckle, focusing and other scintillation effects produced when radiation is scattered by a deep random phase screen is investigated. New analytical formulae are obtained which reveal the mathematical origin of the observed physical phenomena, and these are supported by a range of numerically computed results.

65 citations


Journal ArticleDOI
TL;DR: In this article, the number of photons produced by a single electron travelling the unit of distance, divided by the gas pressure, depends only on the reduced electric field but not on the hydrogen pressure, and an empirical equation for this quantity is given.
Abstract: Data an the xenon uniform field gas proportional scintillation counter is presented. The energy resolution obtained for 8.1 MeV ?-particles was 1.2%. Measurements for pressures from 600 to 1500 Torr showed that the "reduced light output", i. e. the number of photons produced by a single electron travelling the unit of distance, divided by the gas pressure, depends only on the reduced electric field but not on the gas pressure. An empirical equation for this quantity is given. The role played by different mechanisms is discussed.

63 citations


Journal ArticleDOI
TL;DR: In this article, the effect of the equatorial irregularities on the propagation of radio waves is reviewed with emphasis on the effects of the irregularities upon the radio wave propagation, and a new composite model is presented which explains many properties of the scintillation phenomena associated with equatorial spread-F.

49 citations


Journal ArticleDOI
TL;DR: In this article, the power spectra of amplitude scintillation at a mid-latitude station of transmissions from the ATS 6 radio beacon experiments are used to find the power spectral shape.
Abstract: Simultaneous multifrequency scintillation data observed at a mid-latitude station of transmissions from the ATS 6 radio beacon experiments are used to find the power spectra of amplitude scintillation. With the frequency spacing of 40, 140, and 360 MHz, weak and strong scintillations are often observed. Single-scatter scintillation theory is applied to interpret the weak scintillation data, and the theory correctly predicts the spectral shape. For strong scintillations the spectra no longer show a distinct break frequency at which the spectra begin to roll off. Because of decorrelation by multiple scattering there is also broadening at the high-frequency portion of the spectrum, even though the slope of the high-frequency asymptote remains about the same as that in the weak scintillation case. Some of these features are compared with predictions based on the multiple-scatter scintillation theory.

48 citations


Journal ArticleDOI
TL;DR: In this paper, the authors used an optical detection system at Volcano Ranch Station near Albuquerque, New Mexico, to estimate the number of charged particles in air-shower trajectories and sizes.
Abstract: Extensive air-shower trajectories and sizes (numbers of charged particles) have been measured using an optical detection system at Volcano Ranch Station near Albuquerque, New Mexico. Light produced by atomospheric scintillation and Cherenkov emission by shower particles was measured at distances of 0.7 to approx. 10 km. The shower sizes determined by the optical measurements are in satisfactory agreement (an average of 10% higher) with measurements by the ground-level scintillation-counter array at Volcano Ranch.

Journal ArticleDOI
TL;DR: In this article, a Rayleigh distribution appears to describe the worst-case scintillations in the 225-400 MHz band for the auroral and equatorial regions of the Earth.
Abstract: Periods of intense ionospheric scintillations were analyzed in order to provide information for the design and evaluation of the performance of satellite communication links. Scintillation data were available for the auroral and equatorial regions. The amplitude fluctuations are described by cumulative amplitude distributions and show good agreement with the Nakagami m-distribution. A Rayleigh distribution appears to describe the ‘worst’ case scintillations in the 225–400 MHz band. The rate of scintillations is described by level crossing techniques, power spectra, and correlation functions. The effectiveness of diversity techniques is indicated. The spectral index was measured as approximately f−0.2 indicating that intense scintillations can become almost frequency-independent in the strong scattering limit.

Journal ArticleDOI
TL;DR: The emission spectrum of the ultra-violet light produced by a xenon gas scintillation counter has been experimentally determined in this paper, and the continuous spectrum peak intensity occurs around 1670 A. The production mechanism is discussed and probably molecular in origin.

Journal ArticleDOI
TL;DR: In this article, an accurate simulation of atmosphere induced scintillation was achieved by creating an unstable vertical temperature gradient in a 1.5m long, 40 cm wide, and 60 cm deep water tank.
Abstract: Accurate simulation, on a laboratory scale, of atmospherically induced scintillation phenomena is achieved by creating an unstable vertical temperature gradient in a 1.5-m long, 40-cm wide, and 60-cm deep water tank. The turbulent index of refraction has a statistical structure similar to that of the atmosphere, in particular, Kolmogorov’s inertial subrange model is verified. The measured saturation and supersaturation of the irradiance variance agree very well with theory and atmospheric data. The simulation results are properly scaled by the first-order expression for the log-amplitude variance, and the saturation distance is typically reduced by a factor of 300–400.

Journal ArticleDOI
TL;DR: In this paper, it was shown that under practical ionospheric conditions the thin screen method still gives results in very good agreement with those obtained from a single-scattering calculation, even at gigahertz frequencies.

Journal ArticleDOI
TL;DR: An experiment is described for the measurement of scintillation in a vertical path from an earth-based laser transmitter to the GEOS-III satellite and back to anearth-based receiver telescope.
Abstract: An experiment is described for the measurement of scintillation in a vertical path from an earth-based laser transmitter to the GEOS-III satellite and back to an earth-based receiver telescope. Measurements of the normalized variance, probability density function, and power spectral density of scintillation are presented. These results are discussed in terms of recent analytical results.

Journal ArticleDOI
TL;DR: In this article, measurements from several stations ranging from sub-auroral to auroral locations show that the irregularities producing the scintillation exist both equatorward and poleward of the optical aurora.
Abstract: Defense Meteorological Satellite Program photographs of the aurora are correlated with the scintillation of radio signals from both low-altitude and synchronous satellites. Measurements from several stations ranging from subauroral to auroral locations show that the irregularities producing the scintillation exist both equatorward and poleward of the optical aurora. The scintillations are seen to peak when the satellite path is through the aurora, the values falling off with distance from the aurora. Factors which allow for interpretation of the data under two different irregularity formations are applied to the median curves of the variation of scintillation with distance from the aurora for each station. The dominating irregularities for meter wavelength scintillation appear to be elongated and elliptical. To illustrate the extension of the F layer irregularities equatorward of the optical aurora, the magnetic storm of April 18, 1974, is examined. Different mechanisms for the irregularity production, such as heat flow and low-energy electron precipitation, are suggested.

Journal ArticleDOI
TL;DR: In this article, a noble gas scintillation counter for heavy ions is described, which was operated with pure argon and xenon at pressures ranging from 50 to 400 torr.

Journal ArticleDOI
TL;DR: Comparison of the normalized variance of scintillation calculated by using this function shows good agreement with results obtained by other methods.
Abstract: We study the statistics of scintillation at the ground-based receiver for the earth-space-earth retroreflector configuration of satellite laser ranging. These statistics are governed by the joint effects of atmospheric turbulence and speckle produced by the retroreflector array. An expression for the probability density function of scintillation is obtained and evaluated numerically. Comparison of the normalized variance of scintillation calculated by using this function shows good agreement with results obtained by other methods.

Journal ArticleDOI
TL;DR: In this article, the frequency dependence of the scintillation phenomenon is investigated using ATS-6 Radio Beacon Experiments for signals at 40, 140, and 360 MHz, and the spectral index, correlation coefficient, and correlation interval are investigated.
Abstract: Simultaneous scintillation data from the ATS-6 Radio Beacon Experiments for signals at 40, 140, and 360 MHz offer the opportunity to study the frequency dependence of the scintillation phenomenon. Using these data variations of the spectral index, correlation coefficient, and the correlation interval are investigated. It is found that for weak scintillations, the spectral indices lie between 1.5 and 1.7. When the scintillation is strong, the spectral indices vary from about 1.4 to 0.7. The correlation coefficients are found to decrease for increasing scintillation level. The interesting behavior of the correlation intervals as functions of signal frequency is also discussed. The observational results are compared with theoretical predictions.

Journal ArticleDOI
TL;DR: In this article, the authors presented a closed-form solution for S4 and the characteristic scale as a function of x = 2 k20/k2ƒ, where k0 is the outer scale wave number of the irregularity spectrum and kƒ the Fresnel wave number.
Abstract: Recent results from rocket, satellite, and radar experiments have greatly increased our understanding of equatorial spread-F and its effects upon communication systems. In situ measurements have shown that the typical irregularity spectrum is a power law with a one-dimensional index of −2. Previous applications of scintillation theory to such irregularities have utilized an anisotropic power law, in an effort to model elongation of irregularities along the magnetic field, and have found approximate solutions for the scintillation index S4, the rms phase deviation, and the characteristic scale size of the scintillation spectrum. We present here rigorous closed-form solutions for these quantities which are valid to the extent that weak scattering thin screen theory allows. In addition we introduce a second “hybrid” form for irregularity spectra which is gaussian along the magnetic field and a power law in the perpendicular plane. The index is chosen in such a way that a one-dimensional spectrum obtained on a spacecraft whose velocity V¯s makes a reasonable angle to the magnetic field and varies as k−2. Such a spectrum is introduced since it seems likely that at least at long wavelengths equatorial spread-F is an interchange instability and that the density variation along B¯ is that of the zero-order density variation. One-dimensional spectra for small angles between V¯s and B¯ would hence be steeper than k−2 at intermediate k values, a result consistent with some in situ measurements. If particle precipitation is responsible for high latitude irregularities at long wavelengths, the hybrid spectrum might also be more appropriate for their characterization. At longer k, waves with small but finite kz such as drift waves might be important and hence the anisotropic power law more appropriate. Using both spectral forms, curves are presented for S4 and the characteristic scale as a function of x = 2 k20/k2ƒ, where k0 is the outer scale wave number of the irregularity spectrum and kƒ the Fresnel wave number. The frequency dependence of S4 based on the power law spectrum is also plotted in the regime where the theory is applicable.

Journal ArticleDOI
TL;DR: Image quality, resolution and sensitivity of a scintillation camera equipped with various collimators have been investigated using high purity 123I and the modulation transfer function evaluated from the line spread functions shows a sudden drop at low frequencies for high resolution collimator due to their high degree of septa penetration.
Abstract: Image quality, resolution and sensitivity of a scintillation camera equipped with various collimators have been investigated using high purity 123I. Pulse height distributions of 123I from a thyroid phantom partly in air and partly immersed in water demonstrated the substantial septa penetration of the 440 and 529 keV gamma rays of 123I with high resolution collimators. Line spread functions were recorded and the modulation transfer function was evaluated from them. The two concepts of 'figure of merit' also used were QB=Sa mod MTF mod 2, where Sa is the plane sensitivity and QC=S2/(S+2B), where S is the true signal from the object and B is the total background or noise due to septa penetration and scatter.

Journal ArticleDOI
01 Jan 1977-Pramana
TL;DR: In this article, the amplitudes of radio beacon signals on 40, 140 and 360 MHz from ATS-6 (at 34° E longitude) recorded at Ootacamund, India (11.43° N, 76.70°E, dip 4°N, elevation angle 41°) have revealed the largest occurrence of scintillations for about 60% of cases around 2200 hr during the nighttime, and two secondary peaks (25%of cases) around 0900 hr and 1400 hours during the daytime.
Abstract: The recordings of the amplitudes of radio beacon signals on 40, 140 and 360 MHz from ATS-6 (at 34° E longitude) recorded at Ootacamund, India (11.43° N, 76.70°E, dip 4°N, elevation angle 41°) have revealed largest occurrence of scintillations for about 60% of cases around 2200 hr during the nighttime, and two secondary peaks (25% of cases) around 0900 hr and 1400 hr during the daytime. During the daytime, the scintillation decreases approximately as the inverse of the frequency for higher frequencies while for lower frequencies the law is valid till scintillation index at 40 MHz does not exceed 0.9. The temporal variation of daytime scintillation shows impulsive character, the duration of activity lasts for 1–2 hours at a time. During the nighttime, the scintillation decreases inversely with frequency for weak and moderate scintillation activity. The scintillation index at 360 MHz becomes independent of that at 140 MHz when the index at 140 MHz exceeds 0.85. For the set of frequencies 40–140 MHz, on some occasions scintillation index at 40 MHz is seen to be less than that at 140 MHz. The nighttime scintillations are in general stronger and remain so for extended length of time. The daytime scintillations are suggested to be due to blanketing or some other non-q type of sporadicE layer. The nighttime scintillations are most probably due to spreadF condition and the abnormal frequency variation of the scintillations may be due to multiple scattering layer during periods of intense spreadF.

Journal ArticleDOI
TL;DR: In this paper, the authors studied how the scintillation index is affected by the source bandwidth, the response time of the measuring instruments and the properties of the random medium and found that large-bandwidth sources lead to a scintillant index other than unity in very strong turbulence.
Abstract: We have studied how the scintillation index is affected by the source bandwidth, the response time of the measuring instruments and the properties of the random medium. It is found that large-bandwidth sources lead to a scintillation index other than unity in very strong turbulence.

Journal ArticleDOI
TL;DR: An improved large-area gas scintillation proportional counter has been built, suitable for use in X-ray astronomy as discussed by the authors, which employs electron focusing in a drift region to achieve uniform response.



Journal ArticleDOI
TL;DR: A technique for measuring path-averaged rain parameters by analyzing the rainfall-induced scintillations of a laser beam, which determines the average rainfall rate and drop-size distribution along the optical path from the time-lagged covariance function of two vertically spaced line detectors.
Abstract: We describe a technique for measuring path-averaged rain parameters by analyzing the rainfall-induced scintillations of a laser beam From the time-lagged covariance function of two vertically spaced line detectors, we determine the average rainfall rate and drop-size distribution along the optical path This technique requires no prior assumption of the form of the drop-size distribution Sample measurements on a 140-m path confirm that the path-averaged drop-size distribution of a steady rain follows a Marshall-Palmer distribution The optically measured path-averaged rain rate also shows good agreement with conventional tipping-bucket rain-gauge data

Journal ArticleDOI
TL;DR: In this article, a gas scintillation proportional counter (GSPC) was proposed for X-ray astronomy, which has an energy resolution of 8.35% at 6 keV and the ability to reject more than 90% of background events normally falling within a selected energy window.
Abstract: In X-ray astronomy there is a need for a large-area satellite-borne detector, which has a significantly better energy resolution than the classical proportional counter, in order to observe det-ailed X-ray spectral features in a variety of astrophysical objects. A gas scintillation proportional counter, (GSPC), seems to fulfill these requirements. Results are presented demonstrating: (a) an energy resolution of 8.35% (FWHM) at 6 keV, (b) the ability of the GSPC to reject more than 90% of the background events normally falling within a selected energy window. In addition results are presented on the optimisation of the working parameters of the GSPC, and on the development of a large area detector.

Journal ArticleDOI
B. W. Stuck1
TL;DR: In this article, the Radon transform was used to estimate the air pollutant density throughout the region that has been smoothed to minimize effects due to background sunlight fluctuations, laser fading and phase scintillation, and intrinsic sensor noise.
Abstract: Electro-optical sensors measure energy transmitted by lasers spaced about a region’s periphery. Both the lasers and sensors are tuned to infrared frequencies at which air pollutants strongly absorb electromagnetic energy. The sensor measurements are processed using an algorithm derived from the Radon transform. The result is an estimate of air pollutant density throughout the region that has been smoothed to minimize effects due to background sunlight fluctuations, laser fading and phase scintillation, and intrinsic sensor noise. Good agreement is found between a theoretical analysis of performance limitations and computer simulation results