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Showing papers on "Scintillation published in 2005"


Journal ArticleDOI
Olivier Guyon1
TL;DR: In this paper, the effects of photon noise, aliasing, wave front chromaticity, and scintillation on the point-spread function (PSF) contrast achievable with ground-based adaptive optics (AO) are evaluated for different wave front sensing schemes.
Abstract: The effects of photon noise, aliasing, wave front chromaticity, and scintillation on the point-spread function (PSF) contrast achievable with ground-based adaptive optics (AO) are evaluated for different wave front sensing schemes. I show that a wave front sensor (WFS) based on the Zernike phase contrast technique offers the best sensitivity to photon noise at all spatial frequencies, while the Shack-Hartmann WFS is significantly less sensitive. In AO systems performing wave front sensing in the visible and scientific imaging in the near-IR, the PSF contrast limit is set by the scintillation chromaticity induced by Fresnel propagation through the atmosphere. On an 8 m telescope, the PSF contrast is then limited to 10-4 to 10-5 in the central arcsecond. Wave front sensing and scientific imaging should therefore be done at the same wavelength, in which case, on bright sources, PSF contrasts between 10-6 and 10-7 can be achieved within 1'' on an 8 m telescope in optical/near-IR. The impact of atmospheric turbulence parameters (seeing, wind speed, turbulence profile) on the PSF contrast is quantified. I show that a focal plane wave front sensing scheme offers unique advantages, and I discuss how to implement it. Coronagraphic options are also briefly discussed.

294 citations


Journal ArticleDOI
TL;DR: Nikl et al. as mentioned in this paper showed the structure of a Lu3Al5O12:Ce single crystal, a possible high performance scintillator material, whose scintillation performance is degraded by the occurrence of antisite defects.
Abstract: The cover picture of the current issue refers to the Rapid Research Letter by Martin Nikl et al. [1]. It depicts the structure of a Lu3Al5O12:Ce single crystal, a possible high performance scintillator material. Its scintillation performance is, however, degraded by the occurrence of antisite defects. Such defects arise in the material structure sketched in the figure due to the occurrence of Lu3+ ions at the Al3+ octahedral sites as seen in the middle left octahedron. This defect constitutes a shallow electron trap, which delays energy delivery to the Ce3+ emission centers, and thus slows down the scintillation response of the material. Thermoluminescence measurements appear as sensitive diagnostic tool to evidence these defects in the single crystals grown. Martin Nikl is senior scientist and head of the Laboratory of Luminescence and Scintillation Materials at the Institute of Physics of the Czech Academy of Sciences. His main research activities include the characterization of the luminescence and scintillation properties of wide band-gap materials and the influence of material defects on them.

197 citations


Journal ArticleDOI
TL;DR: In this paper, a GPS receiver in the European high arctic, operating at 1.575 GHz, experienced both phase and amplitude scintillation on several satellite-to-ground links during the 2003 severe ionospheric storms.
Abstract: [1] Severe ionospheric storms occurred at the end of October 2003. During the evening of 30 October a narrow stream of high electron concentration plasma crossed the polar cap in the antisunward ionospheric convection. A GPS scintillation receiver in the European high arctic, operating at 1.575 GHz, experienced both phase and amplitude scintillation on several satellite-to-ground links during this period. Close examination of the GPS signals revealed the scintillation to be co-located with strong gradients in Total Electron Content (TEC) at the edge of the plasma stream. The gradient-drift instability is a likely mechanism for the generation of the irregularities causing some of the scintillation at L band frequencies during this storm. The origin of the high TEC is explored and the possible implications of the work for scintillation forecasting are noted. The results indicate that the GPS scintillation over Svalbard can originate from traceable ionospheric plasma structures convecting from the American sector.

166 citations


Journal ArticleDOI
TL;DR: In this paper, a new scintillator, cerium bromide (CeBr3), has been developed for gamma-ray spectroscopy using the Bridgman process.
Abstract: In this paper, we report on a new scintillator, cerium bromide (CeBr3), for gamma-ray spectroscopy. Crystals of this scintillator have been grown using Bridgman process. In this material Ce 3+ is an intrinsic constituent as well as a luminescence center for the scintillation process. Samples of CeBr3 showed high light output (~68000 photons/MeV) and fast decay constant (~17 ns). Furthermore, they exhibited excellent energy resolution for gamma-ray detection. For example, energy resolution of <4% full width at half maximum (FWHM) has been achieved using this scintillator for 662 keV photons (137Cs source) at room temperature. High timing resolution (<200 ps-FWHM) has been recorded with CeBr3-photomultiplier (PMT) and BaF2-PMT detectors operating in coincidence using 511 keV positron annihilation gamma-ray pairs

140 citations


Journal ArticleDOI
TL;DR: In this paper, a new and inexpensive method of measuring the depth of interaction (DOI) in /spl gamma-ray detectors with large-sized scintillation crystals was proposed.
Abstract: We have studied a new and inexpensive method of measuring the depth of interaction (DOI) in /spl gamma/-ray detectors with large-sized scintillation crystals. This method takes advantage of the strong correlation between the width of the undisturbed light-distribution in continuous crystals and the /spl gamma/-ray's DOI. In order to quantify the dependence of the distribution's width with respect to the DOI, we first studied an analytical model of the light-distribution and tested it by means of Monte Carlo (MC) simulations of the light transport inside the crystal. Further we present an inexpensive modification of the commonly used charge division circuit that allows analog and instantaneous computation of the light-distribution's second moment without affecting the determination of the centroid. This redesigned resistor network is based on the position-sensitive proportional counter (DPC) readout and allows, together with position sensitive photo-detectors, the additional measurement of the light-distribution's standard-deviation /spl sigma/. We tested the proposed circuit using the design-tool OrCAD and found the signal sufficiently large for digitalization. Finally, we conducted MC simulations of a realistic Positron Emission Tomography (PET) detector module setup that mimic a continuous Lu/sub 2/SiO/sub 5/:Ce/sup 3+/ (LSO) crystal of dimensions 40/spl times/40/spl times/10 mm/sup 3/ together with the new large area position-sensitive photo multiplier tube (PSPMT) H8500 from Hamamatsu. The influence of Compton scattering on the DOI determination was also estimated by MC simulations. Altogether, we obtained /spl les/ 5 mm DOI resolution. PACS: 87.57.Ce, 87.58.Fg, 87.62.+n, 07.85.-m.

132 citations



Journal ArticleDOI
TL;DR: In this paper, a copper-based fiber calorimeter was used to measure the electromagnetic shower fraction by comparing the scintillation light and the Cherenkov light generated in this process.
Abstract: Hadronic shower development in a copper-based fiber calorimeter is studied by simultaneously measuring the scintillation light and the Cherenkov light generated in this process. By comparing these two signals, the electromagnetic shower fraction can be measured event by event. Fluctuations in this fraction are the dominant contribution to the hadronic energy resolution. They are also responsible for the signal non-linearity and the non-Gaussian response function typical for hadron calorimeters. The dual-readout technique makes it possible to eliminate the effects of these fluctuations.

111 citations


Journal ArticleDOI
TL;DR: In this article, the authors demonstrated that charge carrier retrapping processes can delay radiative recombination at the luminescence centers in an extended time scale, which makes a substantial part of generated scintillation light technically unexploitable.
Abstract: Luminescence and scintillation dynamics can be extensively influenced by the energy transfer processes, which is demonstrated at three selected scintillator materials: PbWO 4 , Ce-doped aluminium garnets and PrF 3 :Ce single crystals. Charge carrier retrapping processes can delay radiative recombination at the luminescence centers in an extended time scale, which makes a substantial part of generated scintillation light technically unexploitable. Correlated measurements based on time-resolved emission spectroscopy and thermoluminescence are of great help to evaluate these phenomena and to understand their microscopic origin.

109 citations


Journal ArticleDOI
TL;DR: In this article, the role of Vk centers and self trapped excitons (STEs) in the energy transfer from the ionization track to Ce3+ is treated.
Abstract: Last couple of years witnessed the development of various new Ce3+ doped halide scintillators (LaCl3, LaBr3, LuI3, Cs2LiYCl6, Cs2LiYBr6) that possess excellent gamma ray or thermal neutron detection properties. The scintillation pulse in LaBr3:Ce3+ is 20 times faster than in the most commonly used scintillator NaI:Tl. This, combined with a more then two times better energy resolution and higher gamma ray stopping power, makes it ideally suited for many different applications. In this work the scintillation properties and mechanisms of Ce3+ doped inorganic halide (Cl, Br, I) compounds are reviewed; especially the role of Vk centers and self trapped excitons (STEs) in the energy transfer from the ionization track to Ce3+ is treated. Aspects of the lifetime and thermal stability of the self trapped exciton, the migration speed of Vk centers and STEs, spectral overlap between STE emission and Ce3+ absorption, and the influence of the type of anion (Cl, Br, I) are addressed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

100 citations


Journal ArticleDOI
TL;DR: Wavefront control experiments in strong scintillation conditions over a 2.33 km, near-horizontal, atmospheric propagation path are presented and mitigation of atmospheric distortions with a speckle beacon typical for directed energy and free-space laser communication applications is demonstrated.
Abstract: Wavefront control experiments in strong scintillation conditions (scintillation index, ≃1) over a 2.33 km, near-horizontal, atmospheric propagation path are presented. The adaptive-optics system used comprises a tracking and a fast-beam-steering mirror as well as a 132-actuator, microelectromechanical-system, piston-type deformable mirror with a VLSI controller that implements stochastic parallel gradient descent control optimization of a system performance metric. The experiments demonstrate mitigation of atmospheric distortions with a speckle beacon typical for directed energy and free-space laser communication applications.

97 citations


Journal ArticleDOI
TL;DR: An 800l liquid xenon scintillation γ -ray detector was developed for the MEG experiment which will search for μ + → e + γ ǫdecay at the Paul Scherrer Institut as discussed by the authors.
Abstract: An 800 l liquid xenon scintillation γ -ray detector is being developed for the MEG experiment which will search for μ + → e + γ decay at the Paul Scherrer Institut Absorption of scintillation light of xenon by impurities might possibly limit the performance of such a detector We used a 100 l prototype with an active volume of 372 × 372 × 496 mm 3 to study the scintillation light absorption We have developed a method to evaluate the light absorption, separately from elastic scattering of light, by measuring cosmic rays and α sources By using a suitable purification technique, an absorption length longer than 100 cm has been achieved The effects of the light absorption on the energy resolution are estimated by Monte Carlo simulation

Journal ArticleDOI
TL;DR: In this article, core-valence (CV) luminescence characterized by an excitation band above 14 eV and a 2 nns fast and broad emission band was found.
Abstract: Luminescence and scintillation properties of Cs2LiYCl6:Ce were further investigated. Core-valence (CV) luminescence characterized by an excitation band above 14 eV and a 2 ns fast and broad emission band was found. Light yields and decay times under γ excitation were studied for various Ce concentrations. The 0.1% Ce-doped Cs2LiYCl6 was further examined under thermal neutron irradiation. It showed a very high light yield of 70500 photons/neutron. Furthermore the pulse shape for thermal neutrons is different from that for γ-rays which gives the ability of neutron/γ-pulse shape discrimination.

Journal ArticleDOI
TL;DR: A series of comparisons are presented that should provide insight into selection of a scintillator type and volume for development of a medical dosimeter, and it is found that symmetric detectors with nearly the same spatial resolution in each direction could be made with a signal equivalent to those of the more commonly used asymmetricScintillators.
Abstract: Scintillation dosimetry is a promising avenue for evaluating dose patterns delivered by intensity-modulated radiation therapy plans or for the small fields involved in stereotactic radiosurgery. However, the increase in signal has been the goal for many authors. In this paper, a comparison is made between plastic scintillating fibers and plastic scintillator. The collection of scintillation light was measured experimentally for four commercial models of scintillating fibers (BCF-12, BCF-60, SCSF-78, SCSF-3HF) and two models of plastic scintillators (BC-400, BC-408). The emission spectra of all six scintillators were obtained by using an optical spectrum analyzer and they were compared with theoretical behavior. For scintillation in the blue region, the signal intensity of a singly clad scintillating fiber (BCF-12) was 120% of that of the plastic scintillator (BC-400). For the multiclad fiber (SCSF-78), the signal reached 144% of that of the plastic scintillator. The intensity of the green scintillating fibers was lower than that of the plastic scintillator: 47% for the singly clad fiber (BCF-60) and 77% for the multiclad fiber (SCSF-3HF). The collected light was studied as a function of the scintillator length and radius for a cylindrical probe. We found that symmetric detectors with nearly the same spatial resolution in each direction (2 mm in diameter by 3 mm in length) could be made with a signal equivalent to those of the more commonly used asymmetric scintillators. With augmentation of the signal-to-noise ratio in consideration, this paper presents a series of comparisons that should provide insight into selection of a scintillator type and volume for development of a medical dosimeter.

Journal ArticleDOI
TL;DR: In this paper, the authors used multiframe image analysis of received intensity scintillation patterns to optimize the design of free-space optical communication links by using multi-iframe image features.
Abstract: Feature Issue on Optical Wireless Communications (OWC) We have developed a flexible, empirical approach for optimizing the design of free-space optical communication links by using multiframe image analysis of received intensity scintillation patterns. This is a versatile way to perform aperture-averaging analysis. A high-performance digital camera with a frame-grabbing computer interface is used to capture received intensity distributions of a He-Ne laser beam propagating in weak and intermediate turbulence conditions. The aperture-averaging results demonstrate the expected reduction in intensity fluctuations due to increasing the receiver aperture diameter for various strengths of turbulence. Aperture averaging improves the bit error rate.

Journal ArticleDOI
TL;DR: In this paper, the absolute light yield of four different scintillation materials measured with two photomultiplier tubes and two avalanche photodiodes is presented, and the effect of optical coupling between the scintillator and detector is also discussed.
Abstract: The absolute light yield of four different scintillation materials measured with two photomultiplier tubes and two avalanche photodiodes is presented. Commonly, the photoelectron yields and electron–hole pair yields are corrected for the quantum efficiency of the detector, as specified by the manufacturer, to obtain the absolute photon yield. However, the effective quantum efficiency under scintillation measurement conditions is substantially higher due to back reflection. Only when back reflection is properly accounted for, the absolute photon yields obtained with the photomultipliers agree with those obtained with the photodiodes. The effect of optical coupling between the scintillator and detector is also discussed.

Journal ArticleDOI
TL;DR: In this paper, the scintillation efficiency of nuclear recoils with kinetic energy between 10.4 and 56.5 keV relative to that of 122 keV gamma rays was measured.
Abstract: Liquid Xenon (LXe) is expected to be an excellent target and detection medium to search for dark matter in the form of Weakly Interacting Massive Particles (WIMPs). We have measured the scintillation efficiency of nuclear recoils with kinetic energy between 10.4 and 56.5 keV relative to that of 122 keV gamma rays from $^{57}\mathrm{C}\mathrm{o}$. The scintillation yield of 56.5 keV recoils was also measured as a function of applied electric field, and compared to that of gamma rays and alpha particles. The $\mathrm{Xe}$ recoils were produced by elastic scattering of 2.4 MeV neutrons in liquid xenon at a variety of scattering angles. The relative scintillation efficiency is $0.130\ifmmode\pm\else\textpm\fi{}0.024$ and $0.227\ifmmode\pm\else\textpm\fi{}0.016$ for the lowest and highest energy recoils, respectively. This is about $15%$ less than the value predicted by Lindhard, based on nuclear quenching. Our results are in good agreement with more recent theoretical predictions that consider the additional reduction of scintillation yield due to biexcitonic collisions in LXe.

Journal ArticleDOI
TL;DR: The vertical distribution of turbulence over Mauna Kea has been measured on four nights in 2002 October, simultaneously using two different instruments based on stellar scintillation, the generalized SCIDAR (scintillation detection and ranging) and MASS (multi-aperture scINTillation sensor) as discussed by the authors.
Abstract: The vertical distribution of turbulence over Mauna Kea has been measured on four nights in 2002 October, simultaneously using two different instruments based on stellar scintillation—the generalized SCIDAR (scintillation detection and ranging) and MASS (multiaperture scintillation sensor). The turbulence integrals match within 20%, and the low-resolution profiles delivered by MASS correctly reveal the localization of the strongest high-altitude turbulent layers. As deduced from DIMM (differential image motion monitor), MASS, and SCIDAR measurements, optical turbulence in the first 0.7 km above the summit contributed typically half of the total integral, the latter corresponding to a seeing of 0 .5. The ground layer and free atmosphere are not correlated.

Journal ArticleDOI
TL;DR: The CLEAN approach as mentioned in this paper uses a thin film of wavelength-shifting fluor to convert the ultraviolet scintillation light to the visible, thereby allowing detection by conventional photomultipliers.

Journal ArticleDOI
TL;DR: In this article, the luminescence properties of LaI 3 :Ce were investigated by means of optical, X-ray, and γ-ray excitation, and the positions of Ce 3+ levels relative to the host bands were estimated.
Abstract: LaI 3 :Ce 3+ has the smallest band gap in the LaX 3 :Ce 3+ (X=F, Cl, Br, I) series and hence a potentially higher scintillation yield. The luminescence properties of LaI 3 :Ce were investigated by means of optical, X-ray, and γ-ray excitation. Unexpectedly, the compound is a poor scintillator at room temperature but presents good scintillation properties for temperatures below 100 K where Ce 3+ emission is observed peaking at 452 and 502 nm with a yield of ∼16 000 photons/MeV. The drastic thermal quenching of luminescence is explained by the proximity of the Ce 3+ lowest 5d excited state to the host conduction band. Autoionization of Ce 3+ therefore prevents any scintillation effect at room temperature. The positions of Ce 3+ levels relative to the host bands were estimated.

Journal ArticleDOI
TL;DR: In this paper, the evolution of ionospheric plasma density structures at middle and equatorial latitudes during the intense magnetic storms of October 29-31-2003, July 15-2000, and March 30-31, 2001 is investigated.
Abstract: [1] Near simultaneous formation of ionospheric plasma density structures at middle and equatorial latitudes during the intense magnetic storms of October 29–31, 2003, July 15, 2000, and March 30–31, 2001 is investigated. The evolution of these structures is explored by measuring amplitude scintillation of satellite signals at 250 MHz, determining zonal irregularity drifts and by detecting equatorial plasma bubbles with DMSP satellites. During abrupt decreases of SYM-H (1-minute resolution Dst) that signify the penetration of high latitude electric fields, an impulsive onset of scintillation occurs at Hanscom AFB (HAFB), a sub-auroral location, as well as in the equatorial region where the early evening period corresponds to the time of scintillation onset at midlatitudes. The onset of equatorial scintillation is delayed from that at midlatitudes by about 20 minutes which can be accounted for by considering instantaneous electric field penetration and plasma instability growth time of equatorial irregularities.

Journal ArticleDOI
TL;DR: In this article, an independent experiment for measuring ionospheric scintillation effects on GPS signals has been set up at auroral latitudes in the European sector, in order to investigate the occurrence of phase-without-amplitude (SVA) scintillation events, which take place when the fixed filtering window is not appropriate to actual plasma dynamics at small scales.

Journal ArticleDOI
TL;DR: In this paper, the energy resolution and nonproportionality of the light yield versus energy of pure CsI scintillators directly coupled to Large Area Avalanche Photodiodes (LAAPDs) and cooled down to the liquid nitrogen temperatures have been studied.
Abstract: The energy resolution and non-proportionality of the light yield versus γ-ray energy of pure CsI scintillators directly coupled to Large Area Avalanche Photodiodes (LAAPDs) and cooled down to the liquid nitrogen temperatures have been studied. A very high light output above 1 00 000 ph/MeV was measured. One of the samples showed nearly proportional light yield response versus energy of γ-rays yielding energy resolution of 4.3±0.2% for 662 keV γ-rays from a 137 Cs source. The intrinsic resolution of the crystals versus energy of γ-rays has been estimated by measurements of the number of primary electron–hole pairs generated in LAAPD by scintillation light. Inspection of the γ-ray-induced emission spectra of tested samples allows discussing different shapes of non-proportionality curves observed with various samples of CsI.

Journal ArticleDOI
TL;DR: In this article, the impact of scintillation on DGPS and EGNOS users, and on different GPS receiver technologies, was investigated in a Northern European network, covering geographic latitudes from 53° to 70° N approximately, where four GPS Ionospheric Scintillation and Total Electron Content (TEC) Monitor receivers (the NovAtel/AJ Systems GSV4004) have been deployed at stations in the UK and Norway.
Abstract: Extensive ionospheric scintillation and Total Electron Content (TEC) data were collected by the Institute of Engineering Surveying and Space Geodesy (IESSG) in Northern Europe during years of great impact of the solar maximum on GNSS users (2001–2003). The ionospheric TEC is responsible for range errors due to its time delay effect on transionospheric signals. Electron density irregularities in the ionosphere, occurring frequently during these years, are responsible for (phase and amplitude) fluctuations on GNSS signals, known as ionospheric scintillation. Since June 2001 four GPS Ionospheric Scintillation and TEC Monitor receivers (the NovAtel/AJ Systems GSV4004) have been deployed at stations in the UK and Norway, forming a Northern European network, covering geographic latitudes from 53° to 70° N approximately. These receivers compute and record GPS phase and amplitude scintillation parameters, as well as TEC and TEC variations. The project involved setting up the network and developing automated archiving and data analysis strategies, aiming to study the impact of scintillation on DGPS and EGNOS users, and on different GPS receiver technologies. In order to characterise scintillation and TEC variations over Northern Europe, as well as investigate correlation with geomagnetic activity, long-term statistical analyses were also produced. This paper summarises our findings, providing an overview of the potential implications of ionospheric scintillation for the GNSS user in Northern Europe.

16 Sep 2005
TL;DR: In this paper, the effects of ionospheric scintillation on GPS signals, based on a GPS software receiver developed at the University of Calgary, were investigated using an intermediate frequency (IF) GPS signal simulator.
Abstract: Ionospheric scintillations cause RF signal amplitude fading and phase variations as GPS satellite signals pass through the ionosphere. This is a particular concern for GPS operations in high latitude regions, such as Canada, where scintillations are associated with strong aurora – effects which persist even during solar minimum. In general, scintillations can cause degraded receiver tracking performance and, in extreme cases, loss of navigation capabilities entirely. Such effects are an issue for reliable GPS operations in the northern United States and Canada. The University of Calgary currently operates the Canadian GPS Network for Ionosphere Monitoring (CANGIM), which makes scintillations observations at various latitudes in the auroral and sub-auroral regions. These measurements are used to characterize high-latitude scintillation effects, and develop models for assessing GPS receiver performance in the presence of such effects. The focus of this paper is a study of the effects of ionospheric scintillation on GPS signals, based on a GPS software receiver developed at University of Calgary. This study consists of several components: simulating ionospheric scintillation effects on L1 using an intermediate frequency (IF) GPS software signal simulator, investigating phase lock loop (PLL) performance under scintillation conditions using a software receiver, and developing improved tracking loop models to minimize phase errors and loss of signal lock during scintillation events. Results indicate that PLL performance is degraded for moderate to severe scintillations, with loss of lock occurring for narrow bandwidths. By employing a fast adaptive bandwidth approach, reliable signal tracking can be achieved.

Journal ArticleDOI
TL;DR: In this paper, a quenching model based on a biexcitonic diffusion-reaction mechanism was proposed for electronic quenches, which was compared with experimental results reported for nuclear recoils from neutrons.

Journal ArticleDOI
TL;DR: In this article, a simple damped-WKB model was found inadequate, as Landau damping erodes the spectrum faster than is consistent with the observed inner scale, and a turbulent cascade can counteract this damping and push the spectral cutoff back out to the ion inertial scale.
Abstract: [1] Radio scattering and scintillation observations of the near-Sun solar wind are shown to be dominated by effects associated with obliquely propagating Alfven/ion cyclotron waves. We base this on a modeling of structure functions from angular/spectral broadening observations and velocity measurements from interplanetary scintillation (IPS) observations. A simple damped-WKB model was found inadequate, as Landau damping erodes the spectrum faster than is consistent with the observed inner scale. Invoking a turbulent cascade can counteract this damping and push the spectral cutoff back out to the observed inner scale near the ion inertial scale. Adjusting the spectrum amplitude and cascade rate to match observations gives an estimate of the wave dissipation power associated with electron Landau damping and proton cyclotron damping. The implied power levels are substantial, being comparable with levels typically invoked in extended wave heating models. Both the shape and the amplitude of the observed structure functions can be explained by a composite spectrum made up of a power law component of passive or non-Alfvenic density fluctuations and a local flattening associated with the enhanced linear Alfven wave compressibility at small (ion cyclotron) scales. Since IPS is dominated by the enhanced small-scale density fluctuations, the scintillation velocity field should show a strong wave effect. Our modeling of IPS velocities does, in fact, show that the large parallel velocity spread and upward bias to the mean velocity observed near the Sun are a direct result of the density fluctuations associated with Alfven waves along an extended line of sight.

Journal ArticleDOI
TL;DR: The optical and scintillation properties, including the transmittance, emission and excitation spectra and the light output, decay kinetics and light response uniformity, as well as their degradation under gamma-ray irradiation were measured for two long LYSO samples from CPI and Saint-Gobain.
Abstract: Because of their high stopping power and fast bright scintillation, cerium doped silicate based heavy crystal scintillators, such as GSO, LSO, and LYSO, have been developed for medical instruments. Their applications in high energy and nuclear physics, however, are limited by lacking high quality crystals in sufficiently large size. The optical and scintillation properties, including the transmittance, emission and excitation spectra and the light output, decay kinetics and light response uniformity, as well as their degradation under gamma-ray irradiation were measured for two long (2.5times2.5times20 cm) LYSO samples from CPI and Saint-Gobain, and were compared to a BGO sample of the same size from SIC. Possible applications for crystal calorimetry in future high energy and nuclear physics experiments are discussed

Journal ArticleDOI
TL;DR: CCD-based gamma cameras have great potential for applications such as in vivo imaging of gamma emitters, and photon counting improves the intrinsic spatial resolution by a factor of about 3 to 6.
Abstract: Charge-coupled devices (CCDs) coupled to scintillation crystals can be used for high resolution imaging with x-rays and gamma-rays. When the CCD images can be read out fast enough, the energy and interaction position of individual gamma quanta can be estimated by real-time image analysis of scintillation light flashes ('photon counting mode'). We tested a set-up in which an electron-multiplying CCD was coupled to a 1 mm thick columnar CsI crystal by means of a fibre-optic taper. We found that, compared to light integration, photon counting improves the intrinsic spatial resolution by a factor of about 3 to 6. Applying our set-up to Tc-99m and I-125 imaging, we were able to obtain intrinsic resolutions below 60 μm (full width at half maximum). Counting losses due to overlapping of light flashes are negligible for event rates typical for biomedical radio-nuclide imaging and do strongly depend on energy window settings. Energy resolution was estimated to be approximately 35 keV FWHM for a 1:1 taper. We conclude that CCD-based gamma cameras have great potential for applications such as in vivo imaging of gamma emitters.

Journal ArticleDOI
TL;DR: The plans for producing long-term (6-24 hour) forecasts of equatorial plasma structure and radio scintillation for the Communication and Navigation Outage Forecasting System (C/NOFS) program are described in this article.
Abstract: [1] The plans for producing long-term (6–24 hour) forecasts of equatorial plasma structure and radio scintillation for the Communication and Navigation Outage Forecasting System (C/NOFS) program are described. We discuss the calculations and computer models required to represent the physics of the phenomena pertinent to the C/NOFS mission. We describe the means by which the models will be integrated into a system that ingests satellite data and provides forecast maps. The plans for validation of the calculations are outlined briefly, with a few sample results.

Journal ArticleDOI
TL;DR: The multiple photon counting coincidence (MPCC) method as mentioned in this paper is a method for measurements of the scintillation characteristics of crystals that involves recording the sequence of individual photon pulses resulting from an event.
Abstract: We describe a new method for measurements of the scintillation characteristics of crystals. The multiple photon counting coincidence (MPCC) technique involves recording the sequence of individual photon pulses resulting from a scintillation event. The timing of the individual photons allows determination of the scintillation decay time constants. The number of photons recorded per scintillation event is proportional to the scintillation light yield. The decay time constants and the relative light yield of CaWO4 and ZnWO4 scintillators have been investigated in the temperature range 9–350 K. An important advantage of the MPCC method is the possibility to reject spurious events through off-line analysis, taking into account the entire data set of scintillation events. This procedure allows cleaning of the data set from multiple scintillation events (pile-up). The MPCC technique is an excellent complement to conventional characterisation techniques and is particularly suited for investigation of slow scintillation processes.