Showing papers on "Scintillation published in 1980"

Scintillation study of ZnWO4 single crystals

Abstract: Scintillation characteristics of the Czochralski‐grown ZnWO4 single crystal were studied. This crystal, with light red‐brown color, exhibits a maximum luminescence wavelength of 480 nm under excitation at 254 nm. The total luminescence output under x‐ray excitation at 254 nm. The total luminescence output under x‐ray excitation was found to be 2.3 times larger than that of the best Bi4Ge3O12 at room temperature with small afterglow.

Coordinated study of equatorial scintillation and in situ and radar observations of nighttime F region irregularities

Abstract: A coordinated set of Atmospheric Explorer E (AE-E) satellite in situ, VHF radar backscatter, and scintillation measurements performed during 1977 over a common ionospheric volume is used to study the relationship between the plasma depletions or bubbles, the extended 3-m irregularity structures known as plumes, and bursts of scintillation activity or patches in the nighttime equatorial F region. The implications of the observed spatial structures and the level of ambient concentration on the generation of 3-m irregularities and scintillation modeling are discussed.

The gamma ray spectrometer for the Solar Maximum Mission.

Abstract: The paper describes an actively shielded, multicrystal scintillation spectrometer for measurement of the solar gamma ray flux used by the Solar Maximum Mission Gamma Ray Experiment. The instrument provides a 476-channel pulse height spectrum every 16.38 s over the 0.3-9 MeV energy range; the gamma ray spectral analysis can be extended to at least 15 MeV on command. The instrument is designed to measure the intensity, energy, and Doppler shift of narrow gamma ray lines, the intensity of extremely broadened lines, and the photon continuum.

Spectral characteristics of medium-scale equatorial F region irregularities

Abstract: : The spectral characteristics of equatorial F-region irregularities with scale sizes from a few kilometers to several hundred kilometers have been measured using data from the Atmospheric Explorer-E (AE-E) satellite. The spectra admit the expected power-law characterization with a mean spectral index slightly less than 2. The spectral index, p1, however, decreases with increasing perturbation strength. The same data have been compared with nearly simultaneous phase scintillation data from the Wideband satellite. The power-law index of the phase scintillation data varies with perturbation strength in exactly the same manner as does p1. With realistic propagation model parameters, the scintillations-inferred perturbation levels can be made to match those measured in-situ. However, the long-accepted unity separation between the in-situ and phase spectral indices is not observed. This discrepancy is attributed either to shortcomings in the theory or to lack of temporal/spatial comparability of the two measurements. (Author)

Estimates of surface heat flux from sodar and laser scintillation measurements in the unstable boundary layer

Abstract: Measurements of acoustic backscatter in the lower planetary boundary layer and optical line-of-sight scintillation in the surface layer are each used to compute sensible heat fluxes in the unstable surface layer. Comparisons with simultaneous low-level point measurements by eddy correlation show good agreement, indicating that remote-sensing methods can be successful over less homogeneous terrain where conventional surface layer measurement techniques are less accurate. Corrections to take into account the effects of humidity fluctuations are found necessary in order to achieve accuracies within 10%. Free convection is assumed to permit interpretation of the sodar data, while either forced or free convection is assumed for the scintillation data. A systematic overestimate of heat fluxes is found from sodar measurements made during the morning, when the height of the convectively mixed layer is increasing rapidly.

Simultaneous rocket probe, scintillation, and incoherent scatter radar observations of irregularities in the auroral zone ionosphere

Abstract: Two rocket payloads carrying plasma density probes with high spatial resolution have been flown in the auroral zone during active conditions. Simultaneous Wideband satellite scintillation and Chatanika incoherent scatter radar observations were made in order to study the properties of high-latitude irregularities and their effects on radio wave transmission. Unlike barium cloud striations and bottomside equatorial spread F, the observed power law dependence of the irregularities does not seem to be due to steepening of kilometer-scale structures, rather, a turbulent process seems to occur. In addition the power law indexes determined both from the probe and from the scintillation measurements indicates an in situ one-dimensional spectrum less steep than the k−2 value often reported. Both the probe and the scintillation data indicate absolute electron density fluctuations (Δn2e)½ of several times 109 m−3 during the expansion phase of an auroral substorm, with a layer thickness of several hundred kilometers. The observed S4 levels at VHF were in the range of 0.1–0.4. This level of scintillation, as well as the absolute density fluctuation levels and the power spectral density at the kilometer scale, are shown to be comparable with bottomside equatorial spread F. It is suggested that differences between the power spectral index in the present data set and the other ionospheric experiments mentioned above may be due to a highly conductive E layer and its effects upon the nonlinear evolution of irregularities. During another flight with lower magnetic activity but several bright auroral areas in the trajectory, much lower levels of absolute and relative density fluctuations were observed with a corresponding lower value for S4. Two very sharp changes in electron density were observed (e-folding scales of 1.45 and 0.7 km) near the field line projected position of the auroral arcs. The associated density spectra were peaked at short wavelengths. The detection of very structured plasma within minutes of the poleward expansion phase of a substorm suggests that the F layer irregularities were formed in the precipitation event. On the other hand, evidence is also presented for production or enhancement of irregularities in the presence of horizontal density gradients which suggests that plasma instabilities also play a role in the production of auroral zone irregularities.

On the morphology of auroral zone radio wave scintillation

Abstract: This paper describes the morphology of midnight sector and morning sector auroral zone scintillation observations made over a two-year period using the Wideband satellite, which is in a sun-synchronous, low-altitude orbit. No definitive seasonal variation was found. The nighttime data showed the highest scintillation occurrence levels, but significant amounts of morning scintillation were observed. For the most part the scintillation activity followed the general pattern of local magnetic activity. The most prominent feature in the nighttime data is a localized amplitude and phase scintillation enhancement at the point where the propagation vector lies within an L shell. A geometrical effect due to a dynamic slab of sheetlike structures in the F region is hypothesized as the source of this enhancement. The data have been sorted by magnetic activity, proximity to local midnight, and season. The general features of the data are in agreement with the accepted morphology of auroral zone scintillation.

Cesium Fluoride: A New Detector for Positron Emission Tomography

Abstract: The ideal scintillation detector for positron tomography would be a very high Z material, nonhygroscopic, have a fast decay time, and copious light output. NaI(Tl) has good light output, and even though it is hygroscopic has found acceptance as a good scintillator. Recently BGO has been shown to satisfy the first two requirements for tomography, and has replaced NaI(Tl) as the detector of choice in tomography. However, its low light yield and poor coincidence timing make it unsuitable for fast scanners. Cesium fluoride (CsF) has been investigated by us as a possible scintillation detector for positron tomography, and shows great promise even though it is extremely hygroscopic and exhibits low scintillation efficiency as compared to NaI(Tl). Its very fast decay time, good detection efficiency, and fast coincidence timing make it an ideal detector for tomographs designed for fast dynamic studies. Moreover, it permits the incorporation of time of flight information with conventional tomography for improved signal to noise ratio in the image.

The Photo-Ionization Proportional Scintillation Chamber

Abstract: A krypton-filled gas scintillation proportional counter has been coupled, through an ultraviolet transparent LiF window, to a multi-anode proportional chamber operated in a gas mixture (argon-triethylamine-methane) having a large quantum efficiency for the emitted photons. The first experimental results suggest that, in the detection of soft X-rays, one can obtain the good energy resolution characteristics of gas scintillation counters, together with the bi-dimensional coordinate localization typical of multiwire proportional chambers (MWPCs). The name Photo-Ionization Proportional Scintillation chamber (PIPS) is proposed for the device.

Long‐term 1.5 GHz amplitude scintillation measurements at the magnetic equator

Abstract: The diurnal and seasonal behavior of long-term (approximately 20 months) 1.54 GHz amplitude scintillation measurements under low sunspot conditions observed at Huancayo, Peru are presented. The measurements refer to a low elevation geometry in the E-W plane at the magnetic equator. The data show clear equinoctial maxima of occurrence and confinement to the pre-midnight hours. There is a total absence of scintillations during the May-July period. The present results are critically compared to previously reported GHz measurements in the context of geometry of observations, equatorial irregularity strength variation and spatial structure information obtained from in-situ data.

Model computations of radio wave scintillation caused by equatorial ionospheric bubbles

Abstract: In situ data measured on board AE satellites and rockets reveal spiky and wedgelike electron density structures inside the equatorial ionospheric bubbles. Two models are constructed to simulate the initial stage and fully developed stage of a bubble. Effects of radio propagation through such bubbles are simulated by solving the parabolic equation numerically. The results show that even though the amplitude scintillation at 136 MHz appears to be stationary, such is not the case at gigahertz frequencies. Instead, the amplitude at gigahertz frequencies shows outbursts with large excursions whenever the direct ray intersects the spicky ionization structure. Both the peak-to-peak excursion and the amplitude distribution cannot be predicted by the scintillation theory that assumes the medium to be random.

The time structure of transionospheric radio wave scintillation

Abstract: The time structure of a representative set of weakly and strongly scintillating transionospheric beacon signals is analyzed. Under conditions of weak scatter, the coherence time of the signal intensity is a monotonic function of the Fresnel radius divided by the effective scan velocity. The shape of the function, however, is controlled by the power law index. Data from a Peruvian station show evidence of a slightly steeper spectral distribution than do data from Kwajalein in the Marshall Islands. Under conditions of strong scattering, the intensity coherence time depends only on the perturbation strength. The strong scatter data show remarkably little dispersion when they are plotted against the perturbation strength. The data are all consistent with a phase spectral slope somewhat less than 3, which is independently verified by using phase scintillation data.

Numerical computations for a one‐dimensional power law phase screen

Abstract: In this paper, numerical computations of the intensity spectral density function of a wave field scattered by a one-dimensional, power law phase screen are presented. The computations verify theoretically derived asymptotic results showing that when the power law index is greater than three, the scintillation index saturates at a value larger than unity. Moreover, strong focusing, a local maximum in the scintillation index variation with increasing perturbation strength, only occurs in a one-dimensional medium when the spectral index is greater than or equal to 3. The application of these results to the interpretation of radiowave scintillation data is discussed.

Comparison of multifrequency equatorial scintillation: American and Pacific sectors

Abstract: In this paper we examine the severity of radio wave amplitude scintillation measured at two stations near the equator but far separated in longitude: Kwajalein, Marshall Islands (167°E), and Ancon, Peru (−77°E). The data used are long-term observations of the Defense Nuclear Agency (DNA) Wideband satellite signal intensity at VHF, UHF, and L band frequencies. The seasonal behavior of the scintillation at the two stations is similar; each shows a broad 8- to 9-month disturbed season centered about local summer. There is short-term variability in the scintillation occurrence statistics but no clear equinoctial maxima. Little difference is observed in the occurrence or severity of L band scintillation at the two stations, although a systematic difference in the frequency dependence of the scintillation produces significantly stronger VHF and UHF scintillation at Ancon. The VHF and UHF latitudinal distributions of scintillation are asymmetric about the geomagnetic equator at both stations.

Remote Investigation of Tropospheric Turbulence by Two-Dimensional Analysis of Stellar Scintillation

Abstract: Several geophysical parameters can be deduced from multidimensional statistical analysis of stellar scintillation. In particular, the motion of the various atmospheric turbulent layers can be determined both in magnitude and direction, in addition to their turbulence strength (as measured by CN2 or Cr2) as a function of attitude. Until now, sequential analysis methods took ∼20 min, during which time stationary could not be guaranteed due to intermittency. Here we present a system which is faster due to better use of available information. It consists of a very light-sensitive image receiver (television camera), followed by a specialized numerical computer (two-dimensional correlator) which utilizes in real time the spatial distribution of the irradiance of a large telescope entrance pupil. This apparatus allows us to identify fast temporal variations of the vertical profiles of turbulence strength, using soundings only 10 s apart. By tracking the double-star γ Andromedae, which has a wider angle ...

A High Resolution Gas Scintillation Proportional Counter for Studying Low Energy Cosmic X-Ray Sources

Abstract: A gas scintillation proportional counter has been constructed for use in detecting subkiloelectron-volt X-rays from cosmic sources. Over a sensitive area of 5 cm2, this instrument has a measured energy resolution of 85 eV (FWHM) at 149 eV. This is significantly better than similarly sized solid state detectors. Rise-time discrimination is used both to reject space background and to reduce low energy noise below 50 eV. The development of an imaging capability for this instrument is discussed.

Detection and imaging of the spatial distribution of visible or ultraviolet photons

Abstract: A gas scintillation proportional counter, with a photosensitive layer, is coupled, through a UV transparent window, to a multi-anode proportional chamber filled with a gas mixture (for instance an argon triethylamine-methane mixture) having a large quantum efficiency for the UV photons. When detecting incident photons, there is obtained the good efficency of photosensitive layers and the satisfactory two-dimensional coordinate localization of multiwire proportional chambers.

The morphology of high-latitude VHF scintillation near 70°W

Abstract: The long-term VHF scintillation data from ATS 3 obtained at three stations situated in the North Atlantic sector at auroral and subauroral locations during the period 1968–1974 are used to determine the morphology of high-latitude scintillations near the 70°W longitude sector. The variation of the average level of scintillation at each observatory is studied as a function of time of day, season, and magnetic activity in a manner suitable for incorporation into statistical models of scintillation occurrence. The most prominent feature of the data is a seasonal dependence of scintillations with a 2:1 variation from northern summer to winter under quiet magnetic conditions. This also causes a large variation in the latitudinal gradient of scintillations from 2 dB per degree in summer to 1 dB per degree in winter for latitudes >60° invariant. The observed seasonal control of scintillations is related to the variation of the tilt angle of the earth's magnetic dipole and consequent modulation of the particle precipitation in the North Atlantic sector of the auroral oval.

The low-temperature scintillation properties of bismuth germanate and its application to high-energy gamma radiation imaging devices

Abstract: Bismuth germanate is a scintillation material with very high z, and high density (7.13 g/cm3). It is a rugged, nonhygroscopic, crystalline material with room-temperature scintillation properties described by previous investigators as having a light yield approximately 8% of that of NaI(Tl), emission peak at approximately 480 nm, decay constant of 0.3 microsec, and energy resolution congruent to 15% (FWHM) for Cs-137 gamma radiations. These properties make it an excellent candidate for applications involving the detection of high-energy gamma photons and positron annihilation radiation, particularly when good spatial resolution is desired. At room temperature, however, the application of this material is somewhat limited by low light output and poor energy resolution. This paper presents new data on the scintillation properties of bismuth germanate as a function of temperature from -- 196 degrees C to j0 degrees C. Low-temperature use of the material is shown to greatly improve its light yield and energy resolution. The implications of this work to the design of imaging devices for high-energy radiation in health physics and nuclear medicine are discussed.