Showing papers on "Scintillation published in 1999"

Theory of optical scintillation

Abstract: A heuristic model of irradiance fluctuations for a propagating optical wave in a weakly inhomogeneous medium is developed under the assumption that small-scale irradiance fluctuations are modulated by large-scale irradiance fluctuations of the wave. The upper bound for small turbulent cells is defined by the smallest cell size between the Fresnel zone and the transverse spatial coherence radius of the optical wave. A lower bound for large turbulent cells is defined by the largest cell size between the Fresnel zone and the scattering disk. In moderate-to-strong irradiance fluctuations, cell sizes between those defined by the spatial coherence radius and the scattering disk are eliminated through spatial-frequency filtering as a consequence of the propagation process. The resulting scintillation index from this theory has the form σI2=σx2+σy2+σx2σy2, where σx2 denotes large-scale scintillation and σy2 denotes small-scale scintillation. By means of a modification of the Rytov method that incorporates an amplitude spatial-frequency filter function under strong-fluctuation conditions, tractable expressions are developed for the scintillation index of a plane wave and a spherical wave that are valid under moderate-to-strong irradiance fluctuations. In many cases the models also compare well with conventional results in weak-fluctuation regimes. Inner-scale effects are taken into account by use of a modified atmospheric spectrum that exhibits a bump at large spatial frequencies. Quantitative values predicted by these models agree well with experimental and simulation data previously published. In addition to the scintillation index, expressions are also developed for the irradiance covariance function of a plane wave and a spherical wave, both of which have the form BI(ρ)=Bx(ρ)+By(ρ)+Bx(ρ)By(ρ), where Bx(ρ) is the covariance function associated with large-scale fluctuations and By(ρ) is the covariance function associated with small-scale fluctuations. In strong turbulence the derived covariance shows the characteristic two-scale behavior, in which the correlation length is determined by the spatial coherence radius of the field and the width of the long residual correlation tail is determined by the scattering disk.

Images and spectra from the interior of a relativistic fireball

Abstract: The power-law decay of gamma-ray burst (GRB) afterglow can be well described by synchrotron emission from a relativistic spherical blast wave, driven by an expanding fireball. We calculate the spectrum and the light curve expected from an adiabatic blast wave which is described by the Blandford-McKee self-similar solution. These calculations include emission from the whole blast wave and not just from the shock front. We provide numerical corrections that can be used to modify simple analytic estimates of such emission. We find that the expected light curve and spectra are flat near the peak. This rules out the interpretation of the sharp optical peak observed in GRB 970508 as the peak of the light curve. We also calculate the observed image of an afterglow. This image could be resolved in future VLBI observations, and its structure could influence microlensing and scintillation. The observed image is ringlike: brighter near the edge and dimmer at the center. The image depends on the observed frequency. The contrast between the edge and the center increases and the ring becomes narrower at higher frequencies.

Prospects for time-of-flight PET using LSO scintillator

Abstract: We present measurements of the timing properties of lutetium orthosilicate (LSO) scintillator crystals coupled to a photomultiplier tube (PMT) and excited by 511 keV photons. These crystals have dimensions suitable for use in PET cameras (3/spl times/3/spl times/30 mm/sup 3/). Coincidence timing resolution of 475 ps fwhm is measured between detectors utilizing two such crystals, significantly worse than the 300 ps fwhm predicted based on first principles for small crystals and measured in 3 mm cubes. This degradation is found to be caused by the scintillation light undergoing multiple reflections at quasi-random angles within the scintillator crystal, which has two effects. First, it slows down the effective information propagation speed within the crystal (to an effective n/spl circ/=3.9-5.3). Since the incident annihilation photon travels with n=1, information from interactions at different depths arrives at the PMT with different time delays. Second, the random nature of the reflection angles (and path lengths) introduce dispersion and so a 10%-90% rise time of 1 ns to the optical signal.

Discrimination between nuclear recoils and electron recoils by simultaneous detection of phonons and scintillation light

Abstract: We have developed a detector, consisting of a cryogenic calorimeter with a scintillating crystal as an absorber, and a second calorimeter for the detection of scintillation light, both operated at 12 mK Using a CaWO4 crystal with a mass of 6 g as the scintillating absorber, we have achieved a discrimination between nuclear recoils and electron recoils with a suppression factor of 997% at energies above 15 keV This method will be applied for background rejection in the CRESST dark matter search (Cryogenic Rare Event Search with Superconducting Thermometers)

Comparisons of measured and predicted acoustic fluctuations for a 3250-km propagation experiment in the eastern North Pacific Ocean

Abstract: During the Acoustic Engineering Test (AET) of the Acoustic Thermometry of Ocean Climate (ATOC) program, acoustic signals were transmitted from a broadband source with 75-Hz center frequency to a 700-m-long vertical array of 20 hydrophones at a distance of 3252 km; receptions occurred over a period of six days. Each received pulse showed early identifiable timefronts, followed by about 2 s of highly variable energy. For the identifiable timefronts, observations of travel-time variance, average pulse shape, and the probability density function (PDF) of intensity are presented, and calculations of internal-wave contributions to those fluctuations are compared to the observations. Individual timefronts have rms travel time fluctuations of 11 to 19 ms, with time scales of less than 2 h. The pulse time spreads are between 0 and 5.3 ms rms, which suggest that internal-wave-induced travel-time biases are of the same magnitude. The PDFs of intensity for individual ray arrivals are compared to log-normal and exponential distributions. The observed PDFs are closer to the log-normal distribution, and variances of log intensity are between (3.1 dB)2 (with a scintillation index of 0.74) for late-arriving timefronts and (2.0 dB)2 (with a scintillation index of 0.2) for the earliest timefronts. Fluctuations of the pulse termination time of the transmissions are observed to be 22 ms rms. The intensity PDF of nonidentified peaks in the pulse crescendo are closer to a log-normal distribution than an exponential distribution, but a Kolmogorov–Smirnov test rejects both distributions. The variance of the nonidentified peaks is (3.5 dB)2 and the scintillation index is 0.92. As a group, the observations suggest that the propagation is on the border of the unsaturated and partially saturated regimes. After improving the specification of the ray weighting function, predictions of travel-time variance using the Garrett–Munk (GM) internal-wave spectrum at one-half the reference energy are in good agreement with the observations, and the one-half GM energy level compares well with XBT data taken along the transmission path. Predictions of pulse spread and wave propagation regime are in strong disagreement with the observations. Pulse time spread estimates are nearly two orders of magnitude too large, and Λ–Φ methods for predicting the wave propagation regime predict full saturation.

Spherical Neutral Detector for VEPP-2M collider

Abstract: The Spherical Neutral Detector (SND) operates at VEPP-2M collider in Novosibirsk studying e^+e^- annihilation in the energy range up to 1.4 GeV. Detector consists of a fine granulated spherical scintillation calorimeter with 1632 NaI(Tl) crystals, two cylindrical drift chambers with 10 layers of sense wires, and a muon system made of streamer tubes and plastic scintillation counters. The detector design, performance, data acquisition and processing are described.

Present status of liquid rare gas scintillation detectors and their new application to gamma-ray calorimeters

Abstract: Scintillation from liquid rare gases has a fast decay component with a high photon yield which is comparable to that of NaI(Tl), although the wavelength is in the vacuum ultraviolet. On the basis of these properties, many investigators have tried to realize liquid rare gas scintillation detector. In particular, liquid Xe is expected to be an excellent detector medium for γ-rays because of its fast response, its large atomic number, and high density. However, it is very difficult to achieve sufficient light collection as scintillator, because the light emission are in vacuum ultraviolet. Recently, some groups have reported liquid Xe/Kr homogeneous scintillation calorimeters for high-energy γ-rays using reflectors method, but their energy resolution was not as high as expected. In this situation it seems that the realization of liquid rare gas detectors may not be easy. In this paper, we review the physical properties of scintillation from liquid rare gas, the scintillation yields, the decay time constants and attenuation lengths. We summarize the results obtained with liquid rare gas scintillation detector. The present status of development of liquid Kr/Xe homogeneous scintillation calorimeters by using reflected light is described. A new device which observes only direct light is proposed as high-energy γ-ray detector or homogeneous electromagnetic shower calorimeters for γ-rays.

Discrimination between Nuclear Recoils and Electron Recoils by Simultaneous Detection of Phonons and Scintillation Light

Abstract: We have developed a detector, consisting of a cryogenic calorimeter with a scintillating crystal as absorber, and a second calorimeter for the detection of the scintillation light, both operated at 12 mK. Using a CaWO4 crystal with a mass of 6g as scintillating absorber, we have achieved a discrimination of nuclear recoils against electron recoils with a suppression factor of 99.7% at energies above 15 keV. This novel method will be applied for background rejection in the CRESST (Cryogenic Rare Event Search with Superconducting Thermometers) experiment looking for dark matter Weakly Interacting Massive Particles (WIMPs).

Improved models for long-term prediction of tropospheric scintillation on slant paths

Abstract: The prediction models for tropospheric scintillation on Earth-satellite paths from Karasawa, Yamada, and Allnutt (1988) and the ITU-R are compared with measurement results from satellite links in Europe, the United States, and Japan at frequencies from 7 to 30 GHz and elevation angles of 3 to 33/spl deg/. The existing prediction models relate the long-term average scintillation intensity to the wet term of refractivity at ground level. The comparison shows that the seasonal variation of scintillation intensity is well predicted by this relation, but for the annual average some additional meteorological information is needed. A much better agreement with measurement results is found when a parameter representing the average water content of heavy clouds is incorporated. This confirms the assumption that scintillation is, at least partly, associated with turbulence inside clouds. The asymmetry between the distributions of signal fade and enhancement can also be explained by turbulence inside clouds. The asymmetry depends on the intensity of the scintillation, which is consistent with the theory assuming a thin layer of cloudy turbulence. A new model based on this theory predicts the distributions of signal fade and enhancement significantly better.

Long-Term Scintillation Studies of Pulsars. I. Observations and Basic Results

Abstract: We report long-term scintillation observations of 18 pulsars in the dispersion measure range 3-35 pc cm-3 carried out from 1993 January to 1995 August using the Ooty Radio Telescope at 327 MHz. These observations were made with the aim of studying refractive effects in pulsar scintillation and obtaining reliable estimates of diffractive and refractive scintillation properties. Dynamic scintillation spectra of pulsars were regularly monitored at 10-90 epochs spanning 100-1000 days. Significant changes are observed in the dynamic spectra over timescales as short as a few days. Large-amplitude fluctuations are observed in quantities such as decorrelation bandwidth, scintillation timescale, drift rate, and flux density. Several pulsars show organized features, such as drifting bands in a highly pronounced manner. For some pulsars, gradual and systematic variations are seen in the drift rate of patterns that undergo several sign reversals during the observing time spans. Anomalous behavior, such as persistent drifts lasting over many months, is seen for PSRs B0834+06 and B1919+21. Four pulsars were studied for 2-4 well-separated observing sessions, each lasting over ~100 days. In some cases, significant variations are seen in the average scintillation properties and/or flux densities between successive observing sessions. From our data, we have been able to obtain more accurate and reliable estimates of scintillation properties and flux densities than those from the earlier observations by averaging out the fluctuations due to refractive scintillation effects. These measurements are used to derive parameters such as the strength of scattering and scintillation speeds. The scintillation speed estimates are found to be reasonably good indicators of proper-motion speeds of pulsars. The present measurements are compared with earlier measurements and the long-term stability of scintillation properties and flux densities is discussed.

Prediction of tropospheric scintillation on satellite links from radiosonde data

Abstract: On the basis of radiosonde data, a new method is proposed for predicting tropospheric scintillation effects on slant paths. It stems from a rigorous statistical development and consists of two steps. First, statistical features of tropospheric turbulence responsible for scintillation are extracted from the analysis of a large amount of radiosonde ascents. Second, long-term scintillation statistics are inferred from these turbulence characteristics, using the theory of propagation through a turbulent medium. The method is applied to a complete year of radiosonde data measured in Belgium and the predicted scintillation results are compared with measurements carried out on the same year near to the meteorological station. An agreement better than with any other usual prediction method is found. The method yields very accurate predictions of scintillation annual statistics and also adequately represents the seasonal and monthly variability of scintillation. Unlike the current prediction models, the proposed radiosonde-based method does not rest on empirical relationships derived from particular propagation experiments and could, therefore, be applied more widely.

Development of a Gd Loaded Liquid Scintillator for Electron Anti-Neutrino Spectroscopy

Abstract: We report on the development and deployment of 11.3 tons of 0.1% Gd-loaded liquid scintillator used in the Palo Verde reactor neutrino oscillation experiment. We discuss the chemical composition, properties, and stability of the scintillator elaborating on the details of the scintillator preparation crucial for obtaining a good scintillator quality and stability. ( 1999 Elsevier Science B.V. All rights reserved. PACS: 14.60.P; 29.40.M

Comparison of the scintillation properties of LSO:Ce manufactured by different laboratories and of LGSO:Ce

Abstract: We measured photoelectron yield, light output, decay times of the light pulses, cerium concentration, energy resolution and time resolution of LSO:Ce manufactured by different laboratories and LGSO:Ce. The LSO samples show excellent scintillation properties: high light output, close to 30,000 ph/MeV and good energy resolution of 7.3% FWHM for /sup 137/Cs /spl gamma/- source full energy peak. Time resolution measured in geometry fulfilling the PET scanners requirements is equal to 450 ps. We also present results from the measurements with LGSO:Ce by Hitachi Chemical Co., which is of similar chemical composition to LSO. LGSO, at present stage of development, shows about 20% lower light output than LSO and energy resolution of 12.4% FWHM for 662 KeV /spl gamma/-rays. LSO crystals used in our studies possess similar in scintillation properties, although we suppose that the details of the productions method are different due to the differences in Ce concentration. LGSO is a new and very promising scintillator due to lower background radiation in comparison to LSO, but it features worse energy resolution and smaller number of photoelectrons.

Measurement of scintillation for free-space laser communication at 785 nm and 1550 nm

Abstract: An experiment comparing atmospheric scintillation for 785 nm and 1550 nm laser beam transmission is presented. Fluctuations in received optical power were recorded for both wavelengths at terrestrial ranges of 1.2 km and 2.2 km. The number of transmit apertures was also varied. The results indicated that scintillation fades are more of a problem at 1550 nm compared to 785 nm. This will require more scintillation fade margin built into the design of free-space laser communication systems operating at 1550 nm. As well, any advantage in decreased atmospheric attenuation margin at 1550 nm could be lost because of the need for greater fade margin. The overall reduction in scintillation with an increased number of transmit apertures was verified. A possible physical explanation will explain why more scintillation was observed at 1550 nm as compared to 785 nm.

Long-Term Scintillation Studies of Pulsars. II. Refractive Effects and the Spectrum of Plasma Density Fluctuations

Abstract: Refractive scintillation effects in pulsars are powerful techniques for discriminating between different models proposed for the electron density fluctuation spectrum in the interstellar medium. Data from our long-term scintillation study of 18 pulsars in the dispersion measure range 3-35 pc cm-3 (Paper I) are used to investigate two important observable effects of refractive scintillation, viz., (1) modulations of diffractive scintillation observables and flux density, and (2) drifting bands in dynamic scintillation spectra. Our data provide simultaneous measurements of decorrelation bandwidth, scintillation timescale, flux density, and drift rate of patterns. The observed modulations of the first three are compared with the available theoretical predictions, and constraints are placed on the power spectrum of plasma density fluctuations. The measured modulation indices are found to be larger than predicted by a Kolmogorov form of density spectrum. The properties of the drift rate of patterns along with the diffractive scintillation parameters have been used to estimate independently the slope of the density power spectrum, which is found to be consistent with a Kolmogorov form for several pulsars. The contradictory results from these two independent methods of constraining the electron density spectrum are not reconcilable with the simple theoretical models based on power-law forms of density spectrum. Our observations show anomalous scintillation behavior such as persistent drifting bands for some pulsars. This can be interpreted as an excess power in the low wavenumber range (~10-12 to 10-13 m-1) compared to the Kolmogorov expectations, or the existence of localized density structures. The results from our observations are discussed in combination with those from earlier studies in an attempt to understand the overall nature of the density spectrum. The emerging picture is a Kolmogorov-like spectrum (α≈11/3) in the wavenumber range ~10-6 m-1 to ~10-11 m-1, which either steepens or has a bump near ~10-12 to 10-13 m-1. The accumulated data also suggest the existence of discrete density structures along some lines of sight. We also discuss the possible implications of our results for the theoretical models.

Detection of scintillation light in coincidence with ionizing tracks in a liquid argon time projection chamber

Abstract: A system to detect light from liquid argon scintillation has been implemented in a small, ICARUS-like, liquid argon time projection chamber. The system, which uses a VUV-sensitive photomultiplier to collect the light, has recorded many ionizing tracks from cosmic-rays in coincidence with scintillation signals. Our measurements demonstrate that scintillation light detection can provide an effective method for absolute time measurement of events and eventually a useful trigger signal. (C) 1999 Elsevier Science B.V. All rights reserved.

Advanced radiation imaging of low-intensity gamma-ray sources

Abstract: Imaging gamma-ray sources and distributions of low intensity is difficult using current commercially available radiation imagers. Radiation Monitoring Devices, Inc. has carried out the research and development necessary to construct a novel, compact radiation-imaging device, RADCAM, for low-intensity applications. The device consists of a position-sensitive photomultiplier tube (PSPMT) coupled to a CsI(Na) scintillation crystal, which is an excellent candidate for such applications due to its high light output. A tungsten coded-aperture mask, placed in front of the scintillator, creates a gamma-ray intensity pattern across the face of the crystal. The PSPMT detects the resulting scintillation pattern and the analog output signals are captured and converted to digital signals by the RMD PSPMT interface card. The digital data is stored and processed by a portable personal computer. The gamma-ray “shadowgram” is then mathematically decoded to yield the original source image. The pseudo-color radiation-source image is overlaid on a video picture of the same area captured by a high-resolution CCD. The combined image is displayed on screen as an accurate map of radioactive gamma-ray sources in the physical environment. Data acquisition and image display are controlled by the IMager Acquisition and Graphical-user-interface Environment (IMAGE), a Windows-NT program developed for the imager.

Suppression of the radiation damage in lead tungstate scintillation crystal

Abstract: Methods to suppress damage centres in lead tungstate (PWO) crystals under irradiation are discussed. It is shown that in large size scintillation elements (23 cm in length) the minimization of both electron and hole centers created under irradiation by recharge of structural defects is achieved by proper stoichiometry tuning and additional simultaneous doping of crystals by Y and Nb. The different aspects of the suppression of the recharge process in PWO scintillation crystals are also discussed.

Liquid Helium and Liquid Neon - Sensitive, Low Background Scintillation Media For the Detection of Low Energy Neutrinos

Abstract: The use of liquid helium and neon as scintillators for neutrino detection is investigated. Several unique properties of these cryogens make them promising candidates for real-time solar neutrino spectroscopy: large ultraviolet scintillation yields from ionizing radiation, transparency to their own scintillation light, and low levels of radioactive impurities. When neutrinos scatter from electrons in liquid helium or neon, ultraviolet light is emitted. The ultraviolet scintillation light can be efficiently converted to the visible with wavelength shifting films. In this way the neutrino-electron scattering events can be detected by photomultiplier tubes at room temperature. We conclude that the solar neutrino flux from the $\rm p+p\to e^{+}+d+ u_{e}$ reaction could be characterized and monitored versus time using a 10 ton mass of liquid helium or neon as a scintillation target.

Prospects for first-principle calculations of scintillator properties

Abstract: Several scintillation processes can be modeled from first principles using quantum chemistry cluster calculations and recently available high-performance computers. These processes include the formation of excitons and trapping centers, the diffusion of ionization energy (electrons and holes) through a host crystal, and the efficient capture of these carriers by an activator atom to form a luminous, non-quenched excited state. As examples of such calculations, results are presented for (1) hole transport in the known scintillator host crystal CsI, (2) hole trapping in the non-scintillator PbF 2 , (3) hole transport in the experimentally unexplored PbF 4 , and (4) the electronic nature of excited states of CsI : Tl and CsI : Na.

Interstellar Scintillation od Pulsar B0809+74

Abstract: Weak interstellar scintillations of pulsar B0809+74 were observed at two epochs using a 30m EISCAT antenna at 933 MHz. These have been used to constrain the spectrum, the distribution and the transverse velocity of the scattering plasma with respect to the local standard of rest (LSR). The Kolmogorov power law is a satisfactory model for the electron density spectrum at scales between 20 megameters and 1 gigameter. We compare the observations with model calculations from weak scintillation theory and the known transverse velocities of the pulsar and the Earth. The simplest model is that the scattering is uniformly distributed along the 310 pc line of sight (l=140 deg, b=32 deg) and is stationary in the LSR. With the scattering measure as the only free parameter, this model fits the data within the errors and a range of about +/-10 km/s in velocity is also allowed. The integrated level of turbulence is low, being comparable to that found toward PSR B0950+08, and suggests a region of low local turbulence over as much as 90 deg. in longitude including the galactic anti-center. If, on the other hand, the scattering occurs in a compact region, the observed time scales require a specific velocity-distance relation. In particular, enhanced scattering in a shell at the edge of the local bubble, proposed by Bhat et al. (1998), near 72 pc toward the pulsar, must be moving at about ~ 17 km/s; however, the low scattering measure argues against a shell of enhanced scattering in this direction. The analysis also excludes scattering in the termination shock of the solar wind or in a nebula associated with the pulsar.

Single crystal scinitillator

Abstract: The present invention relates to single crystal scintillators and includes a transparent single crystal of cerium-activated lutetium yttrium oxyorthosilicate having the general formula Lu(2-x-z)YxCezSiO5, wherein 0.05≦x≦1.95 and 0.001≦z≦0.02. The crystal scintillator of the present invention is dense, bright, rugged, and non-hygroscopic and has a relatively short decay time for luminescence. The invention also includes a scintillation detector using the crystal scintillator, which produces an electrical signal in response to light received from the crystal scintillator upon exposure to gamma rays, x-rays, and the like.

Temporal broadening and scintillations of ultrashort optical pulses

Abstract: The purpose of this paper is to study the temporal broadening and temporal scintillations of ultrashort optical pulses We present the general formulation of the temporal broadening and scintillation index of ultrashort (femtosecond) space–time Gaussian pulses propagating through weak optical turbulence and derive analytic approximations for the near- and far-field zones We then apply the results to cross-link, uplink and downlink communication channels in a laser satellite communication system We show that both the temporal broadening and scintillations depend mainly on the strength of optical turbulence and the outer scale and increase with these parameters We also show that 10–30 fs pulses broaden significantly more and have greater scintillations than wide pulses Furthermore, the horizontal and vertical/slant path temporal broadening are identical and the temporal scintillation indices exhibit similar behaviour

Update on 4x2.5-Gb/s 4.4-km free-space optical communications link: availability and scintillation performance

Abstract: We present new performance results for a multi-gigabit terrestrial free-space laser communications system. The measured laser communication link performance parameters include: link bit-error-rate, received optical power, scintillation characteristics and atmospheric visibility.