Showing papers on "Scintillation published in 1998"

Diffractive Interstellar Scintillation Timescales and Velocities

Abstract: We derive general relationships between the observed timescale of diffractive interstellar scintillations and the physical velocities of the observer, the source, and the scattering medium. Our treatment applies exclusively to saturated scintillations of point sources in the strong scattering regime. We show how scintillation observations may be combined with other observables (proper motion and dispersion measure) to yield (1) improvements in galactic models for the free-electron density and (2) estimates of the distance and transverse velocity of individual pulsars. We explicitly consider cases of current astrophysical interest, including hypervelocity pulsars too far above the Galactic plane to allow distance estimates from dispersion measures alone. We also briefly consider scintillations of extragalactic sources, including gamma-ray burst sources at great distances from the Galaxy.

Heliospheric tomography using interplanetary scintillation observations. 1. Combined Nagoya and Cambridge data

Abstract: We have produced a computer assisted tomography program that optimizes a three-dimensional model to fit observational data. We have used this program with interplanetary scintillation data from Nagoya, Japan, and Cambridge, England. The program iterates to a least squares solution fit of observed data using solar rotation and solar wind motion to provide perspective views of each point in space accessible to the observations. We plot the optimized model as Carrington maps in velocity V and density N e for the two data sets with resolutions of 10° in heliographic longitude and latitude. We map the model to 1 AU and compare this to in situ observations from the IMP spacecraft. From this comparison we find ΔN e N e 0.3 . We plot Carrington maps extrapolated to the solar surface to compare with Yohkoh Soft X ray Telescope (SXT), Sacramento Peak green line, and Mark III K-coronameter observations. High velocities modeled at the solar surface for individual rotations trace coronal holes (including polar ones) observed in SXT data. Regions of high density modeled from the Cambridge scintillation level data generally show a high correlation with regions of high solar activity observed as bright in Yohkoh SXT and green line observations. There is also a general correspondence of the regions of high density and the areas which are bright in K-coronameter observations.

Interstellar scintillation of compact extragalactic radio sources

Abstract: The recent discovery of radio variability of a quasar on short time-scales (hours) prompts us to examine what is expected in respect of the interstellar scintillation of very compact, extragalactic radio sources. We find that large-amplitude, rapid, variability is predicted at commonly observed radio frequencies (1–20 GHz) over the vast majority of the extragalactic sky. As a guide to assist observers in understanding their data, we demonstrate simple techniques for predicting the effects of interstellar scintillation on any extragalactic source.

Atmospheric Intensity Scintillation of Stars. III. Effects for Different Telescope Apertures

Abstract: Stellar intensity scintillation in the optical was extensively studied at the astronomical observatory on La Palma (Canary Islands). Atmospheric turbulence causes "flying shadows" on the ground, and intensity fluctuations occur both because this pattern is carried by winds and is intrinsically changing. Temporal statistics and time changes were treated in Paper I, and the dependence on optical wavelength in Paper II. This paper discusses the structure of these flying shadows and analyzes the scintillation signals recorded in telescopes of different size and with different (secondary-mirror) obscurations. Using scintillation theory, a sequence of power spectra measured for smaller apertures is extrapolated up to very large (8 m) telescopes. Apodized apertures (with a gradual transmission falloff near the edges) are experimentally tested and modeled for suppressing the most rapid scintillation components. Double apertures determine the speed and direction of the flying shadows. Challenging photometry tasks (e.g., stellar microvariability) require methods for decreasing the scintillation "noise." The true source intensity I(l) may be segregated from the scintillation component in DI(t,l,x,y) postdetection computation, using physical modeling of the temporal, chromatic, and spatial properties of scintillation, rather than treating it as mere "noise." Such a scheme ideally requires multicolor high-speed (&10 ms) photometry on the flying shadows over the spatially resolved ( &10 cm) telescope entrance pupil. Adaptive correction in real time of the two-dimensional intensity excursions across the telescope pupil also appears feasible, but would probably not offer photometric precision. However, such "second-order" adaptive optics, correcting not only the wavefront phase but also scintillation effects, is required for other critical tasks such as the direct imaging of extrasolar planets with large ground-based telescopes.

Scintillation parameters for 49 pulsars

Abstract: We report on multi-epoch, multifrequency observations of 64 pulsars with high spectral and time resolution. Scintillation parameters were obtained for 49 pulsars, including 13 millisecond pulsars. Scintillation speeds were derived for all 49, which doubles the number of pulsars with speeds measured in this way. There is excellent agreement between the scintillation speed and proper motion for the millisecond pulsars in our sample using the simple assumption of a mid-placed scattering screen. This indicates that the scaleheight of scattering electrons is similar to that of the dispersing electrons. In addition, we present observations of the Vela pulsar at 14 and 23 GHz, and show that the scintillation bandwidth scales as ν3.93 over a factor of 100 in observing frequency. We show that for PSR J0742−2822, and perhaps PSR J0837−4135, the Gum nebula is responsible for the high level of turbulence along their lines of sight, contrary to previous indications. There is a significant correlation between the scintillation speeds and the product of the pulsar's period and period derivative for the ‘normal’ pulsars. However, we believe this to be caused by selection effects both in pulsar detection experiments and in the choice of pulsars used in scintillation studies.

Two-Deformable-Mirror Concept for Correcting Scintillation Effects in Laser Beam Projection Through the Turbulent Atmosphere

Abstract: A two-deformable-mirror concept for correcting scintillation effects in laser beam projection through the turbulent atmosphere is presented. This system uses a deformable mirror and a Fourier-transforming mirror to adjust the amplitude of the wave front in the telescope pupil, similar to kinoforms used in laser beam shaping. A second deformable mirror is used to correct the phase of the wave front before it leaves the aperture. The phase applied to the deformable mirror used for controlling the beam amplitude is obtained with a technique based on the Fienup phase-retrieval algorithm. Simulations of propagation through a single turbulent layer sufficiently distant from the beacon observation and laser beam transmission aperture to cause scintillation shows that, for an ideal deformable-mirror system, this field-conjugation approach improves the on-axis field amplitude by a factor of approximately 1.4 to 1.5 compared with a conventional phase-only correction system.

Characteristics of plasma structuring in the cusp/cleft region at Svalbard

Abstract: Satellite scintillation, all-sky optical imager, and digisonde observations were coordinated during a cusp campaign conducted at Ny Alesund, Svalbard (78.9°N, 11.8°E 75.7°N corrected geomagnetic latitude, over the period January 4–15, 1997. This paper is focused on a study of the distribution and dynamics of mesoscale (tens of kilometers to tens of meters) electron density irregularities in the dayside auroral region. This study has been performed at Ny Alesund, Svalbard, by measuring the effects of these irregularities on the amplitude scintillation of 250-MHz transmissions from a quasi-stationary polar satellite as well as the amplitude and phase scintillation of 1.6-GHz signals from Global Positioning System (GPS) satellites. These GPS scintillation measurements were augmented by the use of dual-frequency (1.2 and 1.6 GHz) GPS phase data acquired at the same station by the Jet Propulsion Laboratory for the International GPS Geodynamic Service (IGS). The continuous 250-MHz scintillation observations explored the daytime auroral ionosphere 2° poleward of Ny Alesund and showed that the scintillation spectra are often broad, as may be expected for irregularities in a turbulent flow region. Such irregularity dynamics were detected poleward of the nominal cusp region over the interval of 0600–1500 magnetic local time. The period of observations included the magnetic storm of January 10–11, 1997, when GPS observations of the IGS detected polar cap patches with total electron contents of 3×1016 m−2 and large-scale (tens of kilometers) phase variations at the GPS frequency of 1.6 GHz that corresponded to temporal gradients of 2×1016 m−2 min−1. However, amplitude scintillations at the GPS frequency of 1.6 GHz could not be detected in association with these large-scale phase variations, indicating that the irregularities with wavelengths less than the Fresnel dimension of 400 m were below the detectable limit. This is shown to be consistent in the context of enhanced ionospheric convection determined by digisonde and scintillation spectra.

Energy resolution of scintillation detectors readout with large area avalanche photodiodes and photomultipliers

Abstract: The energy resolution of small NaI(Tl), CsI(Tl), BGO, GSO, YAP and LSO crystals has been studied using 16 mm diameter large area avalanche photodiodes (LAAPD) and a 52 mm diameter photomultiplier. The best result of 4.8% for 662 keV /spl gamma/-rays from a /sup 137/Cs source was obtained with a 9 mm in diameter by 9 mm high CsI(Tl) scintillator coupled to an LAAPD. Measuring the number of primary electron-hole pairs produced in the LAAPD and photoelectrons in the photomultiplier, as well as the noise contribution of the LAAPD, allowed a quantitative discussion of the results. The energy resolutions measured with LAAPDs are comparable to, or significantly better (at certain emission wavelengths) than, those obtained with the photomultiplier. At energies above 100 keV the energy resolution measured with the majority of crystals and the LAAPD was weakly affected by the photodiode noise contribution. The advantages and limitations of LAAPDs in energy spectrometry with scintillation detectors are also discussed.

Model-based prediction of amplitude scintillation variance due to clear-air tropospheric turbulence on Earth-satellite microwave links

Abstract: A statistical method to predict tropospheric amplitude scintillation parameters along Earth-space microwave links from meteorological data is proposed. The evaluation of the mean value and the variance of the refractive-index structure constant and of the scintillation power (i.e. the variance of the log-amplitude fluctuations of the received electromagnetic field) is carried out from conventional radio-sounding measurements. A large radio-sounding data set, collected in Northern Italy over ten years is utilized to simulate clear-air amplitude scintillation variance at microwaves and millimeter-waves on slant paths. Scintillation statistics of interest for link-budget design are also derived from the radio-sounding data set for short and long-term applications. Scintillation prediction formulas, based on measurements of surface temperature and relative humidity, are also derived and regression coefficient tables are given on an hourly and a monthly basis. Comparisons of short-term and long-term prediction results with Olympus down-link measurements at 19.8 GHz are shown and discussed. A model investigation about the statistical correlation between scintillation power and brightness temperature is performed, deriving an extension of the estimation methods to include integrated water vapor measurements from ground-based microwave radiometers.

Scintillation properties of LSO:Ce boules

Abstract: We investigated the scintillation characteristics of 1880 LSO crystals that were cut from 76 crystal boules. We measured the light output and energy resolution of all 1880 crystals, and also measured the decay times of 1169 of the crystals. We observed trends in light output and energy resolution that were not previously evident from studies of small numbers of crystals, and in addition, a correlation between light output and decay time became evident for the first time. The results may be interpreted in terms of the properties of the two Ce scintillation centers in LSO, the effect of quenching centers, and the effects of Ce segregation during crystal growth.

Pulsar Scintillation and the Local Bubble

Abstract: We present the results from an extensive scintillation study of 20 pulsars in the dispersion measure range 3-35 pc cm-3, carried out using the Ooty Radio Telescope at 327 MHz, to investigate the distribution of ionized material in the local interstellar medium (LISM). Observations were made during the period 1993 January-1995 August, in which the dynamic scintillation spectra of these pulsars were regularly monitored over 10-90 epochs spanning ~100 days. Reliable and accurate estimates of strengths of scattering have been deduced from the scintillation parameters, averaged out for their long-term fluctuations arising from refractive scintillation effects. Our analysis reveals several anomalies in the scattering strength, which suggest that the distribution of scattering material in the solar neighborhood is not uniform. We have modeled these anomalous scattering effects in terms of inhomogeneities in the distribution of electron density fluctuations in the LISM. Our model suggests the presence of a low-density bubble surrounded by a shell of much higher density fluctuations. We are able to put some constraints on geometrical and scattering properties of such a structure and find it to be morphologically similar to the Local Bubble known from other studies.

Optical and scintillation properties of large crystals

Abstract: The system has been shown to be a promising scintillator for medical imaging devices. Recently, efforts were focused on the improvement of its scintillating properties. Several large crystals with various cerium concentrations were grown. Absorption and excitation spectra were measured in a range extending from the visible to the vacuum ultraviolet (VUV). Emission spectra, fluorescence decay times and light yields, both under -ray and x-ray excitation, were measured under various experimental conditions. A reabsorption process is shown to take place in this material. This process is responsible for the observed decrease of the light yield when increasing the size of the sample.

Pulse-shape discrimination with CdWO4 crystal scintillators

Abstract: The properties of light emission from a CdWO 4 crystal scintillator were investigated with different kinds of sources (α’s, protons, cosmic muons and γ’s). At least three components of scintillation signal were recognized with decay time of ≈1, 6 and 14 μs. By using the dependence of the relative amplitudes of these components on the nature of incoming radiation, the pulse-shape discrimination method based on the optimal digital filter was developed to process the scintillation pulses from the CdWO 4 crystal. The clear discrimination between γ-rays and α-particles was achieved, that permits the use of this technique in double-β-decay research and in other low-background measurements.

Recombination and scintillation processes in

Abstract: This paper reports spectroscopic and scintillation studies of the well established scintillator material Standard measurements of luminescence emission and excitation spectra have been accompanied by investigations of thermoluminescence and scintillation light yield over a wide temperature range, and by decay measurements under pulsed gamma and VUV excitations at various temperatures These measurements are interpreted in the framework of a model that includes a recombination centre and a number of electron traps We demonstrate that some unusual and largely unexplained features of the scintillator, such as a substantial disparity between scintillation and radiative decay times, the presence of slow components in scintillation decays, and a strong temperature variation of scintillation light yield between 150 and 300 K, have their origin in the processes of charge carrier capture and emission by electron traps Although the nature of these traps remains elusive, most of the trap parameters, such as frequency factors, energy depths, and relative populations, have been estimated This makes it possible to predict the characteristics of trap-free material and thereby the potential improvements that could be achieved

Segmented scintillation detector for encoding the coordinates of photon interactions

Abstract: Light guides (1) capable of encoding the transverse and longitudinal coordinates of light emission induced by the interaction of photons in an array of a plurality of the light guides. Each light guide has at least two discrete crystal segments (4) adjacently disposed along a common longitudinal axis of the light guide (1). Between adjacent segments is a boundary layer (7) having less light transmission than the light transmission of the crystal segments (4). A light absorbing mask (8) increases light adsorption in a segment (4). Photons enter the light guide (1) and cause the emission of scintillation light which is delivered in different and resolvable quantities to light sensing devices. The differences in quantity of delivered light is caused by successive decreases in light in part by the boundary layers (7). The differences in quantity of light establish the segment from which the light emission took place.

Measurement of scintillation and link margin for the TerraLink laser communication system

Abstract: AstroTerra's TerraLinkTM 8-155 laser communications equipment is designed for a clear weather range of 8 km and a data rate of 230 Mb/s, and TerraLink 4-155 is designed for a 2 km range. The TerraLink equipment achieves a reduction in scintillation-induced intensity fluctuations by using large receive apertures and multiple transmit apertures. We present measurements of received intensity fluctuations at different ranges through 4 inch and 8 inch receive apertures. We also present link margin data, with its implications for use of lasercom equipment in various weather conditions. Scintillation measurements were made while a communications link was operating by placing a second receive telescope with a PIN photodiode next to one of the lasercom transceivers. By plotting the probability of intensity vs. intensity, the necessary link margin to achieve a desired burst error rate can be calculated. At the longest ranges, the TerraLink equipment requires a scintillation fade margin of about 10 dB to achieve a 10-9 bit error rate. The equipment is designed with an additional margin of 4 - 5 dB for atmospheric attenuation.

Asymmetric signal level distribution due to tropospheric scintillation

Abstract: Short term distribution of signal fluctuations due to troposheric scintillation (in decibels) is usually modelled as Gaussian. For long term distribution, this results in a symmetrical distribution function. However, various experiments have shown a significant asymmetry in long term distribution for strong scintillations. It is shown that the observed asymmetry follows directly from theory, if a different modelling approach is applied.

The study of α/γ ratio for inorganic scintillation detectors

Abstract: To optimize characteristics of α-radiation detectors as well as those intended to stabilize γ spectrometers, the α/γ ratio has been studied for inorganic scintillation detectors within a wide temperature range (from −20 to +80°C) and at various signal formation time values (from 0.2 to 20 μs). Inorganic scintillators with high atomic numbers have been studied, namely, alkali halide and oxide crystals as well as a semiconducting single-crystal ZnSe(Te). Though the scintillators on the base of the oxide materials have satisfactory resolution and light output, they cannot be considered as efficient detectors of α-radiation due to a low α/γ ratio (∼0.20). The most appropriate detector of α-radiation remains single crystalline CsI(Tl) having the best scintillation characteristics, nonhygroscopic, convenient to use and comparatively cheap.

Characterisation of tropospheric turbulent layers from radiosonde data

Abstract: Thin cloudy turbulent layers, responsible for tropospheric scintillation on earth-space paths, are proved to be efficiently detected and characterised from radiosonde ascents, Based on one complete year of radiosonde data, statistical distributions of turbulent layer height, thickness and intensity are analysed, as is their seasonal variability.

Beta-gamma counting system for Xe fission products

Abstract: A beta-gamma coincidence counting system has been developed for automated analysis of Xe gas samples separated from air. The Xe gas samples are contained in a cylindrical plastic scintillator cell located between two NaI(T1) scintillation detectors. The X-ray and gamma spectra gated by coincident events in the plastic scintillator cell are recorded for each NaI(T1) crystal. The characteristic signatures of the131mXe,133gXe,133mXe, and135gXe isotopes of interest for nuclear test-ban verification as well as the procedures and results of absolute efficiency measurements are described. A NaI(T1) crystal with provision for 4 sample cells has been implemented for the system to be deployed in the field. Examples of data on ambient air samples in New York City obtained with the field prototype are presented.

Scintillation light emission studies of LSO scintillators

Abstract: UV and /spl gamma/-ray excited luminescence and nuclear spectroscopy were used to study the relationship between the scintillation mechanisms of LSO and the spectroscopic characteristics obtained with PMT and APD readouts at room temperature. No correlation was found between scintillation decay time and light output. Like other investigators, we observed the existence of two distinct luminescence centers, Ce1 and Ce2, that mainly give rise to short (420 nm) and long (440 nm) emission wavelengths. Our measurements showed that different LSO crystals excited by /spl gamma/-rays have emission spectra with largely different shapes and maxima depending on the relative population and luminescence efficiency of these centers. It was also found that the poor energy resolution of LSO and YSO scintillators is well correlated with the coexistence of the two competing luminescence mechanisms. The prevalence of either Ce1 or Ce2 luminescence tends to reduce the variance of light emission and, thus, to improve energy resolution. Inversely, the coexistence of the two centers increases variance and degrades energy resolution.

Lu2S3:Ce3+, A new red luminescing scintillator

Abstract: Scintillation properties of Lu2S3:Ce3+ crystals are presented. The studied crystals of ∼1 × 1 × 1 mm3 have the corundum structure (density 6.25 g/cm3, effective atomic number 66.8). The linear attenuation length of a 511 keV photon is 1.35 cm. The light yield is 25 000–30 000 photons per MeV and the decay time is rather short, i.e. 32 ns. The emission spectrum is between 550 and 700 nm, peaking at 592 nm. Consequently the scintillation light can be efficiently detected by means of silicon diodes.

Scintillation measurements performed during the limited-visibility lasercom experiment

Abstract: The effects of scintillation on the performance of free-space laser communication (lasercom) systems can be significant. Atmospheric scintillation can introduce burst errors which can be the major contributor to the laser link's overall bit error rate (BER). As part of the limited-visibility lasercom experiment, independent measurements of scintillation were taken to determine how much of the overall lasercom system's performance degradation was due to scintillation as opposed to atmospheric attenuation. The scintillation measurements were also correlated to the suite of meteorological measurements (total attenuation coefficient, total backscatter, temperature, relative humidity, and wind speed and direction) recorded concurrently. Scintillation variation over the time of day was measured. Indications of a reduction of scintillation in conditions of poorer visibility (rain) were observed. All of these measurements will assist in providing more accurate predictions of lasercom performance where these routinely measured meteorological variables are available.

Radiation detector system and method with stabilized system gain

Abstract: A closed-loop stabilization system and method for a radiation detector system including a scintillator and a light sensing device for detecting the scintillation light produced in the scintillator through interaction with impinging radiation. A spectrally stable gamma ray measurement for downhole and other non-isothermal gamma ray measurement applications is obtained by use of a light source for emitting photons, preferably ultra-violet photons, into the scintillator that are not detectable by the light sensing device. The light source causes the scintillator to fluoresce at a wavelength detectable by the light sensing device, and the change in measured fluorescence is used to adjust system gain to account for variations in scintillator output attributable to environmental conditions. In this manner, the stabilization system adjusts the system gain to maintain consistency among radiation measurements over a wide temperature range.

A thermal-neutron scintillator with n/γ discrimination LiBaFCe,Rb

Abstract: A new method to discriminate thermal neutrons against γ-rays using a radiation damaged LiBaF3 : Ce, Rb scintillation crystal is presented. Discrimination is based on the scintillation pulse shape which for thermal neutrons is very different than that for γ-rays because different scintillation mechanisms are involved. Efficient suppression of γ-rays with an energy from a few keV to a few MeV is possible.

Influence of Mo Impurity on the Spectroscopic and Scintillation Properties of PbWO4 Crystals

Abstract: The influence of molybdenum doping on the spectroscopic and scintillation properties of lead tungstate crystals has been investigated. From the results the slow scintillation component as well as the afterglow are found to be due to the Mo impurity. In addition the blue luminescence from the excited (WO 4 ) 2- complex seems to be increasingly suppressed as the doping concentration increases. Possible mechanisms for the effects have been discussed.