Showing papers on "Scintillation published in 2004"

Model for a partially coherent Gaussian beam in atmospheric turbulence with application in lasercom

Abstract: Analytic expressions for the mutual coherence function (MCF) and the scintillation index of a partially coherent lowest order Gaussian beam wave propagating through the atmosphere (based on Kolmogorov spectrum model) are developed for the pupil plane of a receiving system. Partial coherence of the beam is modeled as a thin (complex) phase screen with Gaussian spectrum (Rytov theory and ABCD ray matrices are applied). The relation between the second- and fourth-order statistics for a beam with any degree of coherence in the atmosphere is introduced with the help of ''effective'' beam parameters, deduced from the free-space MCF. In particular, the scintillation (in weak and strong atmospheric conditions), based on these parameters, is stud- ied as a function of the diffuser's strength and that of the atmosphere. The model is applied for the calculation of the SNR and bit error rates (OOK modulation) of the communication link with diffuser at the trans- mitter and slow detection system. The improvement of bit error rates is observed in weak and strong atmospheric turbulence. In the weak re- gime, the optimal diffuser can be found. © 2004 Society of Photo-Optical Instru-

Field guide to atmospheric optics

Abstract: The material in this Field Guide is a condensed version of similar material found in two textbooks: Laser Beam Propagation through Random Media (SPIE Vol. PM53) and Laser Beam Scintillation with Applications (SPIE Vol. PM99). Topics chosen for this concise presentation include a review of classical Kolmogorov turbulence theory, Gaussian-beam waves in free space, and atmospheric effects on a propagating optical wave. These atmospheric effects have great importance in a variety of applications like imaging, free space optical communications, laser radar, and remote sensing. This Guide presents tractable mathematical models from which the practitioner can readily determine beam spreading, beam wander, spatial coherence radius (Fried's parameter), angle of arrival fluctuations, scintillation, aperture averaging effects, fade probabilities, bit error-rates, and enhanced backscatter effects, among others.

Scintillation and Beam-Wander Analysis in an Optical Ground Station-Satellite Uplink

Abstract: In an optical communication link between an optical ground station and a geostationary satellite the main problems appear in the uplink and are due to beam wander and to scintillation Reliable methods for modeling both effects simultaneously are needed to provide an accurate tool with which the robustness of the communication channel can be tested Numerical tools, especially the split-step method (also referred to as the fast-Fourier-transform beam propagation method), have demonstrated their ability to deal with problems of optical propagation during atmospheric turbulence However, obtaining statistically significant results with this technique is computationally intensive We present an analytical-numerical hybrid technique that provides good information on the variance in optical irradiance with an important saving of time and computational resources

Scintillation: mechanisms and new crystals

Abstract: The physical mechanisms active in inorganic scintillators used for medical imaging are reviewed briefly. These include relaxation of electronic excitation following initial absorption of high-energy radiation, thermalization of electrons and holes, formation of excitons, charge carrier trapping on defects and self-trapping, transfer of excitation to luminescence centers, and emission of detectable light. Materials include intrinsic and activated insulating crystals and semiconductors involving several different luminescent centers and radiative processes. Fundamental limitations of scintillator performance and nonradiative processes arising from native defects and impurities that can limit scintillation light output are discussed. The properties of several recently reported scintillating crystals are also presented.

Size, shape, orientation, speed, and duration of GPS equatorial anomaly scintillations

Abstract: [1] GPS L1 C/A signal scintillation data were collected at the equatorial anomaly over a period of three months using five receivers spaced on magnetic east-west and north-south axes to examine the speed, orientation, shape, width, and duration of GPS scintillation fade patterns. The nighttime speeds were primarily eastward in the range of 100–200 m/s with a significant spread to both larger values and negative (westward) values as expected, given the known behavior of ionospheric drifts and GPS signal path movement. The characteristic velocity was found to be small so that the true velocity was equal to the apparent velocity to a very good approximation. The orientation of the scintillation fade patterns was organized by a simple projection model of the magnetic field along the GPS signal path onto the horizontal plane when the signal paths were aligned no closer than 60° from the magnetic field. The shape of the scintillation fade pattern was greatly elongated in the magnetic north-south direction, and no change could be detected over a distance of 1 km. The east-west widths of the scintillation fade patterns were variable, but after normalizing to the elevation angle, accounting for the fade orientation, and eliminating signal paths within 60° of the magnetic field, an organized scale length of about 450 m was determined. The duration of the scintillation fade patterns was examined using the optimal cross-correlation amplitude as a measure of change. For a 5 s duration, 49% of the optimal cross-correlation amplitudes exceed a value of 0.8.

MCP-PMT timing property for single photons

Abstract: We have measured the performance, especially the timing properties, of micro-channel plate photo-multiplier tubes (MCP-PMTs) by irradiating with single photons with/without a magnetic field. A time resolution of σ=30– 35 ps was obtained for single photons under 1.5 T . With an MCP-PMT, a small time-of-flight counter, by means of Cherenkov light radiation instead of scintillation light has been prepared, and a time resolution σ∼10 ps was attained for a high-energy π-beam by multiple photons.

Validation of the GATE Monte Carlo simulation platform for modelling a CsI(Tl) scintillation camera dedicated to small-animal imaging.

Abstract: Monte Carlo simulations are increasingly used in scintigraphic imaging to model imaging systems and to develop and assess tomographic reconstruction algorithms and correction methods for improved image quantitation. GATE (GEANT4 application for tomographic emission) is a new Monte Carlo simulation platform based on GEANT4 dedicated to nuclear imaging applications. This paper describes the GATE simulation of a prototype of scintillation camera dedicated to small-animal imaging and consisting of a CsI(Tl) crystal array coupled to a position-sensitive photomultiplier tube. The relevance of GATE to model the camera prototype was assessed by comparing simulated 99m Tc point spread functions, energy spectra, sensitivities, scatter fractions and image of a capillary phantom with the corresponding experimental measurements. Results showed an excellent agreement between simulated and experimental data: experimental spatial resolutions were predicted with an error less than 100 µm. The difference between experimental and simulated system sensitivities for different source-to-collimator distances was within 2%. Simulated and experimental scatter fractions in a [98–182 keV] energy 9 All authors are members of the OpenGATE collaboration.

Scintillation response of liquid Xe surrounded by PTFE reflector for gamma rays

Abstract: The scintillation response of liquid xenon for gamma rays ranging from 30 to 4000 keV has been measured. The experiment was performed with a double phase Xe detector of 0.3 l fiducial volume with PTFE reflector. The light collection efficiency for the detector was calculated by Monte Carlo simulation. The energy resolution and scintillation efficiency were compared with other results.

Development of ultra-fast semiconducting scintillators using quantum confinement effect

Abstract: The concept of a "quantum scintillator", satisfying both a large light output and a quick response, is proposed. The temporal behavior of scintillation from (n-C6H13NH3)2PbI4, a natural multiple quantum well structure provided by the lead-halide-based perovskite-type organic-inorganic hybrid compound, was investigated using a short-pulsed electron beam and a streak camera. A decay component of 390 ps was efficiently observed even at room temperature. This response is notably quicker than conventional Ce3+-activated scintillators because of a quantum confinement effect that increases the overlapping region of electron and hole wavefunctions in the low-dimensional system. This achievement would be the next breakthrough in the development of ultra-fast inorganic scintillators.

Annular Gaussian beams in turbulent media

Abstract: A laser beam with a different irradiance profile may have favorable scintillation and diffraction characteristics, which are important qualities in the area of optical wireless communication. The propagation in random media (atmosphere) of a laser beam with an annular, donut-shaped, irradiance cross section was examined. Annular beams are created by unstable optical resonators, which are used as resonant cavities in high power lasers, and by beam expanders that consist of telescopes where the second mirror obstruct the central portion of the circular aperture. The annular beam was modeled as the difference of two collimated Gaussian beams, with different spot sizes. Diffraction and scintillation characteristics of the annular beam, for horizontal near ground propagation (i.e. constant refractive-index structure parameter Cn2), were compared to one collimated and one focused Gaussian beam. The Rytov approximation was used to calculate the on-axis scintillation index in weak atmospheric turbulence for both the annular and the Gaussian beams. The extended Huygens-Fresnel integral was solved to obtain the mean irradiance, from which the fraction of power in half aperture diameter was calculated.

Scintillation detector array for encoding the energy, position and time coordinates of gamma ray interactions

Abstract: A scintillation detector which includes a plurality of discrete scintillators composed of one or more scintillator materials. The discrete scintillators interact with incident radiation to produce a quantifiable number of photons with characteristic emission wavelength and decay time. A light guide is operatively associated with the scintillation crystals and may be either active or non-active and segmented or non-segmented depending upon the embodiment of the design. Photodetectors are provided to sense and quantify the scintillation light emissions. The process and system embodying various features of the present invention can be utilized in various applications such as SPECT, PET imaging and simultaneous PET systems. In accordance with the present invention, the detector array of the present invention incorporates either a single scintillator layer of discrete scintillators or discrete scintillators composed of two stacked different layers that can be the same scintillator material or of two different scintillator materials. In either case the different layers are composed of materials that have distinctly different decay times. The variants in these figures are the types of optical detectors which are used, i.e. photomultipliers and/or photodiodes, whether or not a segmented optical light guide is used, and whether the light guide is active or non-active. If a segmented optical light guide is used then the variant is whether the configuration is inverted or non-inverted.

Performance measurements of a depth-encoding PET detector module based on position-sensitive avalanche photodiode read-out.

Abstract: We are developing a high-resolution, high-efficiency positron emission tomography (PET) detector module with depth of interaction (DOI) capability based on a lutetium oxyorthosilicate (LSO) scintillator array coupled at both ends to position-sensitive avalanche photodiodes (PSAPDs). In this paper we present the DOI resolution, energy resolution and timing resolution results for complete detector modules. The detector module consists of a 7 x 7 matrix of LSO scintillator crystals (1 x 1 x 20 mm3 in dimension) coupled to 8 x 8 mm2 PSAPDs at both ends. Flood histograms were acquired and used to generate crystal look-up tables. The DOI resolution was measured for individual crystals within the array by using the ratio of the signal amplitudes from the two PSAPDs on an event-by-event basis. A measure of the total scintillation light produced was obtained by summing the signal amplitudes from the two PSAPDs. This summed signal was used to measure the energy resolution. The DOI resolution was measured to be 3-4 mm FWHM irrespective of the position of the crystal within the array, or the interaction location along the length of the crystal. The total light signal and energy resolution was almost independent of the depth of interaction. The measured energy resolution averaged 14% FWHM. The coincidence timing resolution measured using a pair of identical detector modules was 4.5 ns FWHM. These results are consistent with the design goals and the performance required of a compact, high-resolution and high-efficiency PET detector module for small animal and breast imaging applications.

Measuring Ionospheric Scintillation in the Equatorial Region Over Africa, Including Measurements From SBAS Geostationary Satellite Signals

Abstract: A GPS Ionospheric Scintillation and TEC Monitor (GISTM) has been deployed in a near-equatorial location in Africa (Douala, Camaroon -- 4o01’N, 9o43’E). The new feature of this receiver is that it can also measure the amplitude and phase scintillation indices on the SBAS signal received from a geostationary satellite. This observation, with its fixed link geometry, is particularly interesting for the analysis of temporal variation in the scintillation behavior of the equatorial ionosphere. At the same time it allows a direct assessment of the threat posed by scintillation to the SBAS signal. This paper presents the experimental setup, the principle behind the measurements and the first results.

Measurement of the refractive index and attenuation length of liquid xenon for its scintillation light

Abstract: The attenuation length and refractive index of liquid xenon for its intrinsic scintillation light (λ=178 nm ) have been measured in a single experiment. The value obtained for the attenuation length is 364±18 mm . The refractive index is found to be 1.69±0.02. Both values were measured at a temperature of 170±1 K .

The recent development and study of silicon photomultiplier

Abstract: Recent developments and results from the study of a Silicon Solid State Photomultiplier (Si-PM) are presented. The basis of this new type of photodetector is a fine structure of microcells operating in the Geiger mode with an internal gain greater than 10 6 . Common signal output allows for the detector to be operated in the proportional mode, and to reach a dynamic range of 1.5 × 10 3 . Such photodetectors have shown single photon response at room temperature with a fast timing of ∼ 100 ps. They are compact, robust and non-sensitive to magnetic fields. Results show the detection of low-intensity light in single photon mode and the detection of minimal ionizing particles using a scintillation tile for hadron calorimetry. The silicon photomultiplier is suitable for wide application in scintillation calorimetry, medical application, etc.

Implementation of fast-Fourier-transform-based simulations of extra-large atmospheric phase and scintillation screens

Abstract: Fast-Fourier-transform-based simulations of single-layer atmospheric von Karman phase screens and Kolmogorov scintillation screens up to hundreds of meters in size were implemented and tested for applications with percent range accuracy. The tests included the expected and the observed structure and pupil variance functions; for the phase, the tests also included the Fried turbulence parameter r0 measured by the seeing and by a simulated differential image motion monitor. The standard compensations used to correct the undersampling at low spatial frequencies were improved, and those needed for the high spatial frequencies were determined analytically. The limiting ratios of the screen sampling step to r0 and of the screen size to the pupil aperture were estimated by means of the simulated data. Sample results are shown that demonstrate the performances of the simulations for single-layer Kolmogorov and von Karman phase screens up to 200 m in size and for Kolmogorov scintillation screens for pupils up to 50 m of aperture.

Plastic scintillation dosimetry for radiation therapy: minimizing capture of Cerenkov radiation noise

Abstract: Over the last decade, there has been an increased interest in scintillation dosimetry using small water-equivalent plastic scintillators, because of their favourable characteristics when compared with other more commonly used detector systems. Although plastic scintillators have been shown to have many desirable dosimetric properties, as yet there is no successful commercial detector system of this type available for routine clinical use in radiation oncology. The main factor preventing this new technology from realizing its full potential in commercial applications is the maximization of signal coupling efficiency and the minimization of noise capture. A principal constituent of noise is Cerenkov radiation. This study reports the calculated capture of Cerenkov radiation by an optical fibre in the special case where the radiation is generated by a relativistic particle on the fibre axis and the fibre axis is parallel to the Cerenkov cone. The fraction of radiation captured is calculated as a function of the fibre core refractive index and the refractive index difference between the core and the cladding of the fibre for relativistic particles. This is then used to deduce the relative intensity captured for a range of fibre core refractive indices and fibre core–cladding refractive index differences. It is shown that the core refractive index has little effect on the amount of radiation captured compared to the refractive index difference. The implications of this result for the design of radiation therapy plastic scintillation dosimeters are considered.

Ionospheric irregularities, scintillation and its effect on systems

Abstract: An essential component of the ionosphere is a small-scale electron density structure that causes scintillation of radio waves on transionospheric links. We briefly review the morphology of irregularities and physical mechanisms of their formation. Results of the scintillation theory relating statistical characteristics of irregularities and scintillation are also outlined. Examples of the scintillation effect on the system performance are given. Finally, we will present recent attempts to model the scintillation.

Radio tomography and scintillation studies of ionospheric electron density modification caused by a powerful HF-wave and magnetic zenith effect at mid-latitudes

Abstract: Observations of the ionospheric electron density modified by a powerful wave of the Sura HF heating facility were carried out in Russia at middle latitudes in August 2002. Amplitude scintillations and variations of the phase of VHF signals from Russian orbiting satellites passing over the heated region along the chain of three satellite receivers have been recorded. The experimental data were converted to electron density maps using a stochastic inversion. Tomographic measurements conducted during a low magnetic activity revealed that HF powerful waves can produce significant electron density disturbances up to heights significantly exceeding altitudes of the F layer peak. Both large-scale plasma enhancements and small-scale density irregularities can be generated by the HF radiation. Wavy density structures were also observed within a sector which is much wider than the area covered by the main lobe of the heating antenna. Small-scale density irregularities are mostly field-aligned although large-scale structures can be detected within a much larger area. A distinctive peculiarity of electron density changes occurred during heating is producing a zone of low density inside the area illuminated by the antenna beam. The results indicate that satellite radio tomography and scintillation measurements are effective diagnostic techniques giving a valuable information to studies of effects induced by HF modification. The complete system of plasma density disturbances describing by the theory of “the magnetic zenith effect” has been for the first time studied in this Letter. A good agreement between the theory and experimental data has been obtained.

Equatorial spread F irregularity characteristics over São Luís, Brazil, using VHF radar and GPS scintillation techniques

Abstract: [1] Multitechnique observations may considerably improve our understanding of factors responsible for the generation, growth, and dynamics of the destabilized nighttime equatorial F region plasma irregularities. In order to investigate the dynamics of plasma density irregularities of different scale sizes, a campaign of observations was conducted during 11–20 November 2001 at the Brazilian magnetic equatorial station Sao Luis (2.57°S, 44.21°W, dip latitude 1.73°S). We carried out observations using VHF coherent backscatter radar, two spaced GPS-based scintillation monitors, and one digisonde. Range type spread F on ionograms and radar plume signatures on range-time-intensity maps of the VHF radar started at similar times. In order to compare GPS L1 (1.575 GHz) scintillations and radar plumes we used the scintillation S4 index computed for the signal transmitted by the highest elevation satellite. GPS scintillations were not observed during the initial bottom-type layer shown by the radar; however, stronger scintillations (higher S4 values) were observed concurrently to stronger radar echoes. Although the time duration of GPS scintillation is longer than the duration of the plumes observed by the radar, ionosonde spread F is still much longer than scintillation occurrence, confirming that smaller scale-size irregularities decay faster. Zonal and vertical velocities of 5-m irregularities measured by the radar were analyzed jointly with the apparent zonal velocity of ∼400-m irregularities measured by the spaced-receiver scintillation method. Larger values of the zonal velocity measured by the scintillation technique were found during the explosive growth phase of radar plumes associated with large values of vertical drifts measured by the radar.

Ionospheric scintillation monitoring and mitigation using a software GPS receiver

Abstract: [1] Scintillations are caused by ionospheric irregularities and affect the amplitude, phase, dispersion, and related parameters of GPS signals Both L1 and L2 are affected in a somewhat uncorrelated fashion Adequate understanding of the effects that ionospheric scintillation has on GPS signals is essential in order to produce a GPS receiver that is immune to the degradation of signal caused by these scintillation-related effects Access to the internal workings of commercial GPS receivers is not provided to users However, in order to understand, control, and mitigate scintillation effects, it is necessary to have access to tracking loops and associated components such as local oscillator The software GPS receiver developed by the Center for Remote Sensing, Inc allows the user to access its internal workings; hence facilitating advanced development of GPS receivers under different conditions In this paper we present the results of analysis of GPS signals under scintillating conditions Raw signals under conditions of ionospheric scintillation at Ascension Island have been processed using our software GPS receiver in order to derive the scintillation parameters The receiver has been configured to provide stable operation during scintillation We describe the receiver architecture, particularly the portion involving the tracking filters and related components, and demonstrate their performances under scintillating conditions The software-based approach used by us allows receiver implementations which are suitable for ionospheric monitoring as well as for reliable operation during scintillating conditions The improved architecture may be used for several other applications

Proportional light in a dual-phase xenon chamber

Abstract: A dual-phase xenon chamber was tested to measure the yield of electrons extracted from the liquid to the gas and their subsequent amplification via proportional light. The light yield was studied as a function of electric field and gas pressure. A UV-sensitive compact metal channel photomultiplier tube, placed in the gas above the liquid xenon, was used for these measurements. An electron lifetime of at least 1.5 ms was inferred from a measurement of the ionization signal, triggered by the primary scintillation light, with the same chamber operated in single phase.

Optical properties and energy levels of Ce3+ in lutetium pyrosilicate scintillator crystal

Abstract: Cerium doped lutetium pyro-silicate Ce3+:Lu2Si2O7 (LPS) is an inorganic scintillator that displays particularly promising performance. The spectroscopic properties of pure and Ce3+-doped Lu2Si2O7 are investigated. X-ray-excited luminescence, time-resolved excitation and emission spectroscopy, as well as decay time spectra are presented. For better understanding of the scintillation mechanism, several approaches were undertaken to locate the Ce3+ energy levels relative to levels of LPS host. To estimate the energy position of Ce3+ 4f levels with respect to the valence band, x-ray photoemission spectroscopy (XPS) was used. Scintillation properties of LPS: Ce are explained in terms of the relative positions of Ce3+-5d levels and the conduction band.