Showing papers on "Scintillation published in 1976"

Model of equatorial scintillations from in-situ measurements

Abstract: In-situ measurements of F-region irregularity amplitude and ambient electron density made by the retarding potential analyzer on OGO-6 near the perigee altitude of 400 km have been utilized to derive the variation of electron density deviation over the equatorial region. Based on these measured electron density deviations and other assumed model parameters including a three-dimensional power law form of irregularity spectrum with index 4, a model of equatorial scintillations is developed in the framework of diffraction theory. The percentage occurrence contours of estimated equatorial scintillations ≥ 4.5 dB at 140 MHz during 1900–2300 LMT for the period November–December 1969 and 1970 have been derived. The model is found to depict a pronounced longitude variation with the scintillation belt width and percentage occurrence being maximum over the African sector. The latitude extent of the spatial scintillation belt narrows over the American sector without much decrease in the scintillation occurrence whereas over the Indian and Far Eastern sectors both the extent and the occurrence are found to decrease. The percentage occurrence of scintillations estimated from this model is found to be consistent with VHF scintillation measurements at Ghana, Huancayo, and Calcutta. In addition, the model was found to be in qualitative agreement with GHz observations at various longitudes made by the COMSAT group. The effects of varying model parameters on scintillation estimates at VHF, UHF, and GHz are discussed. Implications of the observed longitudinal variation of scintillations on current theories of equatorial irregularity formation are indicated.

Maximum a Posteriori Estimation of Position in Scintillation Cameras

Abstract: We derive the form of the maximum likelihood (ML) estimate of the location of the scintillation point given the photomultiplier counts in an Anger Scintillation camera. This estimate is also Maximum a Posteriori (MAP) if the prior probability density on the scintillation point is uniform in the object plane. The form of the estimate suggests the possibility of an optical filtering implementation. We note that the ML estimate implies a solution that is remarkably similar to the "optimum position arithmetic" derived by Tanaka, et al.

Spectra of ionospheric scintillation

Abstract: Observations of amplitude and phase scintillation were made at the Millstone Hill radar facility by using phase coherent sources at 150 and 400 MHz on the U.S. Navy navigation system satellites. These observations have been processed to yield power spectra of the fluctuations in the logarithm of the received power (log power) at each frequency and the fluctuations in phase path length difference (differential phase) between the two frequencies. A theoretical analysis was performed to predict the log power and differential phase power spectra. The theoretical model was based upon weak scatter theory for a thick screen with a three-dimensional power law power spectrum representation for the electron density fluctuations. A −4 exponent for the power law best fit the observed spectra. For weak scintillation (rms fluctuations in log power less than 5 dB or S4 < 0.9) the theoretical calculations were in excellent agreement with the observations. For strong scintillation, saturation was observed, and the measured spectra were broader than those predicted on the basis of weak scatter theory. An increase in electron density fluctuations relative to the value predicted by the power law model was observed at scale sizes smaller than 0.8 km. This increase may be responsible for the anomalous scintillation observations at gigahertz frequencies.

Transaxial radionuclide emission camera apparatus and method

Abstract: A transaxial radionuclide emission camera apparatus and method for image reconstruction using data collected by the apparatus are disclosed. A significant feature of the camera apparatus is a detector means which is arcuately shaped. For example, a suitable detector means can be formed from an optically continuous, arcuately shaped strip of scintillation material and an array of photodetectors optically coupled to said scintillation material. These photodetectors sense light emitted by the scintillation material in response to absorbed radiation. Collimating means which converge to the center of an arc defined by the detector means are another significant feature. The transaxial camera apparatus and the method disclosed are useful in reconstructing images of the distribution of a source of penetrating radiation contained within a body. A particular example is the three-dimensional reconstruction of the distribution of a radiopharmaceutical in a human brain which is helpful in diagnosing and locating lesions or tumors of the brain. The apparatus can also be used in two-dimensional imaging.

Photon correlation study of stellar scintillation

Abstract: DURING the past decade photomultiplier-tube technology and signal processing using integrated circuits have progressed to the point at which the analysis of fluctuating light signals can now be carried out digitally, in real time, with accuracy close to the theoretical limit. These ‘photon correlation’ techniques have so far found their main application in laser light-scattering spectroscopy1 and laser doppler velocimetry2. The techniques need not, however, be limited to applications involving lasers and can be used for the analysis of fluctuating optical signals of any origin (provided the fluctuation time is ≥ 10−8 s). Here we report a preliminary study of the statistical and temporal correlation properties of stellar scintillation, taking advantage of the fast response time and high sensitivity of equipment now standard for laser light-scattering applications.

Liquid xenon scintillators for imaging of positron emitters

Abstract: The current understanding of xenon scintillation physics is summarized and keyed to the use of xenon as a ..gamma.. ray detector in medical radioisotope imaging systems. Liquid xenon has a short scintillation pulse (approx.10/sup -8/ sec) and high ..gamma..-ray absorption and scintillation efficiencies. The fast pulse may facilitate imaging in vivo distributions of hot positron sources and allow recovery of additional spatial information by time-of-flight techniques. We begin by describing our own study of the feasibility of making a practical positron scanning system, and consider the problems of scintillation decay time, linearity, efficiency, purity, and electric-field amplitifcation. The prospects for a practical instrument are considered. (AIP)

Calculations of equatorial scintillations at VHF and gigahertz frequencies based on a new model of the disturbed equatorial ionosphere

Abstract: The weak scattering, thin screen theory of scintillation of radio stars and satellites has been applied, in a mathematically rigorous way, to the observations of the plasma density structure deduced from rocket and radar measurements during equatorial spread F conditions. It is shown that bottomside spread F is capable of producing moderate scintillation at VHF, but not at gigahertz frequencies. Upwelling regions of depleted plasma density however, such as those described in several recent publications, are shown to have properties that could cause intense VHF scintillations as well as scintillations in the gigahertz range.

Simultaneous 1.5- and 4-GHz ionospheric scintillation measurement

Abstract: Analysis of simultaneously recorded 1.5- and 4-GHz scintillations at Tangua, Brazil, has shown that the scintillation index varies approximately as the wavelength between 1.5 and 4 GHz under weak scattering conditions. The amplitude distributions, especially those of the 1.5-GHz data, are somewhat irregular in shape and cannot be represented by a simple mathematical expression. The power spectra of the amplitude fluctuations on a log-log plot show a slope of −3 and have low wave number rolloffs near 10−2 m−1. This paper discusses some possible implications of these findings. Half of the 4-GHz power spectra show the minima which are predicted by the thin-screen weak scattering theory. The l/e falloff of the autocorrelation corresponds to irregularities between 100 and 200 m. The cross correlation between the 1.5- and 4-GHz scintillations is about 0.3.

Compton scatter scintillation camera system

Abstract: A method and apparatus for producing tomographic or cross-sectional radiographic images of a portion of a patient's anatomy, or of another object, by means of a planar-collimated gamma-ray source, from which the radiation is substantially confined to a single plane, and a conventional scintillation camera located to detect gamma radiation scattered from the object being examined, the scintillation camera including a collimator arranged to detect scattered radiation only from a direction perpendicular to the plane being examined.

Correlated measurements of scintillations and in‐situ F‐region irregularities from OGO‐6

Abstract: Scintillation estimates obtained from in-situ irregularity measurements by OGO-6 are compared with simultaneous 137 MHz and 6 GHz scintillations recorded at equatorial stations in the American and African sectors. In this first such correlated study, it is found that the equatorial irregularity amplitudes in these sectors are so large that for a moderately thick irregularity layer the commonly observed monotonic power law spectrum with outer scale dimensions much larger than the Fresnel dimension at VHF can explain the observed GHz scintillations and can locate the VHF band in the saturated scintillation regime. This finding is in contrast to other numerical modelling attempts which usually call for artificial tailoring of irregularity spectra to account for the observed GHz scintillations.

Laser wind sensing: the effects of saturation of scintillation.

Abstract: We have developed a physically based extension of the first-order perturbation theory of optical scintillation that accounts for the observed variance and covariance of the amplitude fluctuations in strong integrated turbulence. We use this model to analyze the experimentally observed changes in the operation of our laser wind sensor. The theory suggests a transmitter–receiver configuration that can nearly eliminate the performance-degrading effects of strong turbulence. Based on this analysis, we have developed a saturation-resistant optical wind sensor that maintains its calibration and wind-weighting function throughout the observed range of integrated-turbulence values.

Correction of data loss in gamma ray scintillation cameras

Abstract: A method of and circuitry for correcting gamma ray camera data signals for signal processing circuitry is disclosed. Pulses are artificially generated which replicate the data signals produced at the output of photomultiplier tubes of the gamma ray scintillation camera and are inserted in the gamma ray camera internal processing circuitry in such a manner as to remain distinguishable from the data signals generated by the photomultiplier tubes. By monitoring the number of artificial pulses produced artificially and the number of such pulses actually counted by the data processing circuitry of the gamma scintillation camera, a factor indicative of the losses of scintillation count in the processing circuitry may be determined.

Interplanetary scintillation of radio sources during August 1972

Abstract: The observations of interplanetary scintillation of radio sources in early August 1972 are reviewed. Three-site measurements of solar wind speed were made at University of California, San Diego (73.8 MHz) and at Nagoya University, Toyokawa (69.3 MHz). Single-site measurements of scintillation index were made at Mullard Radio Astronomy Observatory, Cambridge (81.5 MHz) and at University of Adelaide (111.5 MHz). The enhancements in solar wind speed and scintillation index associated with three shock waves were detected. The extent in both longitude and latitude of the shock wave associated with the solar flare on 7 August, the anisotropic expansion of shock waves and the detection of the corotating high-speed streams are main results deduced from the observations.

High‐latitude observations of solar wind streams and coronal holes

Abstract: Interplanetary scintillation observations of the solar wind velocity during 1973 and the first part of 1974 reveal several corotating high-speed streams. These streams, of heliographic latitudes from +40 deg to -60 deg, have been mapped back to the vicinity of the sun and have been compared with coronal holes identified in wide band XUV solar images taken during the manned portions of the Skylab mission. There is some evidence that the high-speed streams are preferentially associated with coronal holes and that they can spread out from the hole boundaries up to about 20 deg in latitude. However, this association is not one to one; streams are observed which do not map back to coronal holes, and holes are observed which do not lie at the base of streams. To the extent that a statistical interpretation is possible the association is not highly significant, but individual consideration of streams and holes suggests that the statistical result is biased somewhat against a strong correlation.

Some new results on the statistics of radio wave scintillation 1. Empirical evidence for Gaussian statistics

Abstract: This paper presents an analysis of ionospheric scintillation data which shows that the underlying statistical structure of the signal can be accurately modeled by the additive complex Gaussian perturbation predicted by the Born approximation in conjunction with an application of the central limit theorem. By making use of this fact, it is possible to estimate the in-phase, phase quadrature, and cophased scattered power by curve fitting to measured intensity histograms. By using this procedure, it is found that typically more than 80% of the scattered power is in phase quadrature with the undeviated signal component. Thus, the signal is modeled by a Gaussian, but highly non-Rician process. From simultaneous UHF and VHF data, only a weak dependence of this statistical structure on changes in the Fresnel radius is deduced. The signal variance is found to have a nonquadratic wavelength dependence. It is hypothesized that this latter effect is a subtle manifestation of locally homogeneous irregularity structures, a mathematical model proposed by Kolmogorov (1941) in his early studies of incompressible fluid turbulence.

Influence of hole shape on collimator performance.

Abstract: The imaging properties of triangular and hexagonal hole shapes were compared for low energy scintillation camera collimators. The response of both hole shapes were calculated using a ray-tracing computer program, corresponding collimators were constructed and their performance was evaluated by measuring line spread functions and imaging bar patterns. The triangular hole shape was found to give better results above a spatial frequency of 1.5 cycles cm-1, below that frequency the hexagonal hole shape is superior. The differences, however, are small and the superior performance of the hexagonal hole collimator at clinically significant spatial frequencies is barely visible in bar pattern images.

Thallium-201: Scintillation Camera Imaging Considerations

Abstract: Modulation transfer functions, line spread functions, and energy spectra were obtained for an Anger scintillation camera using three different collimators and commercially produced $sup 201$Tl. Images of a thyroid phantom were obtained using these collimators. For each of the three collimators, line spread functions and modulation transfer functions were obtained for both the 75-keV x- ray and 167-keV gamma photon of $sup 201$Tl. Although the intrinsic resolution of the scintillation camera is superior when imaging with the 167-keV gamma photon, system performance was superior when the 75-keV x-ray was imaged. Contamination from $sup 202$Tl, which emits abundant 439-keV gamma photons, degraded images taken at 167 keV because of septum penetration. At 75 and 167 keV the converging collimator yielded the best system performance. Imaging time was significantly shorter using the 75-keV x-rays.

Well-type scintillation assembly

Abstract: A scintillation detector assembly is described which employs a scintillation phosphor such as a thallium activated sodium iodide crystal, wherein a blind hole is machined in the crystal to improve the efficiency of measuring the degradation rate of a radioactive material placed therein. The performance of the assembly is defined by the energy resolution of the radiation emanating from the radioactive source. This performance is improved by balancing the collection of light from areas of low and high light collection. This is achieved by suitably preparing the surfaces of the crystal and by controlling the reflectivity of these surfaces using controlled and fixed reflectivity material such as optical filters, cellulose ester-type fibrous paper and like materials. Improvements in resolution of 30 - 50% over prior art well assemblies have been measured for radioactive sources such as 129 I 53 .

Low Elevation Angle Measurement Limitations Imposed by the Troposphere: An Analysis of Scintillation Observations Made at Haystack and Millstone

Abstract: : Tropospheric angle-of-arrival and amplitude scintillation measurements were made at X-band (7.3 GHz) and at UHF (0.4 GHz). The measurements were made using sources on satellites with 12-day orbits. The angle of arrival of the ray path to a satellite changed slowly allowing observations of fluctuations caused by atmospheric irregularities as they slowly drifted across the ray path. The fluctuations were characterized by the rms variations of elevation angle and the logarithm of received power (log power). Over a one- year period, 458 hours of observation were amassed spanning every season, time of day, and weather conditions. The results show strong scintillation occurrences below 1 to 2 degree elevation angles characterized by a number of random occurrences of multipath events that produce deep fades, angle-of-arrival fluctuations, and depolarization of the received signal. The log power fluctuations ranged from 1 to 10 dB rms at elevation angles below 2 degrees to less than 0.1 dB at elevation angles above 10 degrees. The elevation angle fluctuations ranged from 1 to 100 mdeg at elevation angles below 2 degrees to less than 5 mdeg at a 10 degree elevation angle. Comparable fluctuations in elevation angle are expected for bias refraction correction models based upon the use of surface values of the refractive index.

Nuclear reactor ex-core startup neutron detector

Abstract: A scintillation type neutron detector is provided herein for the measurement of neutrons with optimum neutron sensitivity and minimum gamma sensitivity A large diameter nuclear radiation insensitive photomultiplier tube is optically connected to a scintillating medium which is responsive to thermalized neutrons The neutrons available for detection are thermalized by a neutron moderating material adjacent to the scintillator medium Enclosing and shielding the photomultiplier, the scintillator medium, and the moderator is a combined lead and borated silicone resin housing

Minicomputer enhancement of scintillation camera images using fast Fourier transform techniques.

Abstract: Minicomputer methods were developed to enhance lesions in scintillation camera images. This study was directed towards improving the diagnostic quality of liver images. A PDP-12 digital computer was interfaced to a Pho/Gamma HP III scintillation camera and programmed to carry out two-dimensional frequency-domain analysis and processing as an on-line operation. A two-dimensional frequency spectrum is produced. An interactive program allows the operator to construct graphically a frequency-domain filter and apply it to the data matrix. The filter is optimized using the image of a known phantom and then applied unchanged to the clincial liver image. An inverse Fourier transform produces an enhances image in the spatial domain. Significant enhancement of both phantom and liver images has been obtained.

Detection of optical pulses: the effect of atmospheric scintillation

Abstract: Refractive index inhomogeneities within the path traversed by a pulse of optical energy cause a nonuniform energy distribution across the wavefront of the pulse known as scintillation. It has been shown that this energy variation has a log normal distribution, characterized by a variance σE2, which is a function of the degree of atmospheric turbulence. In this paper, we derive an expression for the detection probability of an optical pulse that has scintillation with a log normal variance σE2 in the presence of normally distributed optical noise characterized by variance σN2. The result is a convolution of scintillation and receiver detection statistics. The results of several calculations using different SNRs are presented graphically.