Showing papers on "Scintillation published in 2001"

Laser Beam Scintillation with Applications

Abstract: Optical Wave Propagation In Random Media - Background Review Optical Scintillation Modelling Theory Of Scintillation - Plane Wave Model Theory Of Scintillation - Spherical Wave Model Theory Of Scintillation - Gaussian-Beam Wave Model Aperture Averaging Optical Communication Systems Fade Statistics For Lasercom Systems Laser Radar Systems - Scintillation Of Return Waves Laser Radar Systems - Imaging Through Turbulence.

Mathematical model for the irradiance probability density function of a laser beam propagating through turbulent media

Abstract: We develop a model for the probability density function (pdf) of the irradiance fluctuations of an optical wave propagating through a turbulent medium. The model is a two-parameter distribution that is based on a doubly stochastic theory of scintillation that assumes that small-scale irradiance fluctuations are modulated by large-scale irradi- ance fluctuations of the propagating wave, both governed by indepen- dent gamma distributions. The resulting irradiance pdf takes the form of a generalized K distribution that we term the gamma-gamma distribution. The two parameters of the gamma-gamma pdf are determined using a recently published theory of scintillation, using only values of the refractive-index structure parameter C n (or Rytov variance) and inner scale l 0 provided with the simulation data. This enables us to directly calculate various log-irradiance moments that are necessary in the scaled plots. We make a number of comparisons with published plane wave and spherical wave simulation data over a wide range of turbu- lence conditions (weak to strong) that includes inner scale effects. The gamma-gamma pdf is found to generally provide a good fit to the simu- lation data in nearly all cases tested. © 2001 Society of Photo-Optical Instrumen-

System and method for x-ray fluoroscopic imaging

Abstract: A system for x-ray fluoroscopic imaging of bodily tissue in which a scintillation screen and a charge coupled device (CCD) is used to accurately image selected tissue. An x-ray source generates x-rays which pass through a region of a subject's body, forming an x-ray image which reaches the scintillation screen. The scintillation screen re-radiates a spatial intensity pattern corresponding to the image, the pattern being detected by the CCD sensor. In a preferred embodiment the imager uses four 8×8-cm three-side buttable CCDs coupled to a CsI:T1 scintillator by straight (non-tapering) fiberoptics and tiled to achieve a field of view (FOV) of 16×16-cm at the image plane. Larger FOVs can be achieved by tiling more CCDs in a similar manner. The imaging system can be operated in a plurality of pixel pitch modes such as 78, 156 or 234-μm pixel pitch modes. The CCD sensor may also provide multi-resolution imaging. The image is digitized by the sensor and processed by a controller before being stored as an electronic image. Other preferred embodiments may include each image being directed on flat panel imagers made from but not limited to, amorphous silicon and/or amorphous selenium to generate individual electronic representations of the separate images used for diagnostic or therapeutic applications.

Limitations in GPS receiver tracking performance under ionospheric scintillation conditions

Abstract: GPS receiver tracking performance can be degraded during periods of enhanced ionospheric activity when small-scale scintillation effects (phase and amplitude variations) may be observed in the high latitude auroral region and the low latitude equatorial anomaly region. During periods of intense scintillation the availability of carrier phase observations may be limited through loss of signal lock, with a significant impact on precise positioning applications. Such effects have a larger impact on the L2 tracking performance where codeless and semi-codeless technologies are employed to extract the encrypted L2 signal. The tracking performance of a given receiver depends not only on the magnitude of scintillation activity observed, but also on the receiver tracking capabilities. Tracking performance can vary significantly between receivers under identical scintillation conditions. In this paper the impact of scintillation effects on GPS receiver tracking performance is investigated in both the equatorial and high latitude auroral regions. A comparison of receiver technologies is also conducted using both codeless and semi-codeless GPS receivers. Performance comparisons and tracking statistics are established and interpreted with respect to GPS network applications and the availability of GPS observations at solar maximum.

Fading timescales associated with GPS signals and potential consequences

Abstract: The effect of equatorial ionospheric scintillations on the operation of GPS receivers is investigated, with special attention given to the effect of scintillation timescales on the code division multiple access (CDMA) protocol used by GPS. We begin by examining the timescales of scintillation fades modeled as a horizontally drifting pattern whose timescales are determined by the Fresnel length and the drift speed. The model is tested by comparing the speed, determined by dividing the Fresnel length by the autocorrelation time (width), with the speed estimated using spaced receivers, and the two independent estimates of speed are shown to possess a linear relationship. Next we show that the scintillation pattern drift speed is given by the difference of the ionospheric drift and the speed of the GPS signal F region puncture point. When the ionosphere and GPS signal puncture point speeds match, the fade timescales lengthen. Additionally, if the fade depth is adequate, during periods of longer fade times the loss of receiver lock on GPS signals is more likely, as shown in several examples; that is, both larger fade depths and longer fade timescales are required to produce loss of tracking. We conclude by demonstrating that speed matching or resonance between the ionosphere and receiver is most likely when the receiver is moving from west to east at speeds of 40–100 m/s (144–360 km/h). This is in the range of typical aircraft speeds.

Theory of optical scintillation: Gaussian-beam wave model

Abstract: A scintillation model previously developed by the authors is extended in this paper to the case of a propagating Gaussian-beam wave. As in the previous model, we account for the loss of spatial coherence as the optical wave propagates through atmospheric turbulence by eliminating effects of certain turbulent scale sizes that exist between the scale size of the spatial coherence radius of the beam and that of the scattering disc. These mid-range scale-size effects are eliminated through the formal introduction of spatial frequency filters that continually adjust spatial cut-off frequencies as the optical wave propagates. Unlike the previous model, in this paper we include the effect of a finite outer scale in addition to the inner scale. With a finite outer scale, the scintillation index can be substantially lower in strong turbulence than that predicted by a model with an infinite outer scale. This particular behaviour of scintillation in strong turbulence, mostly associated with horizontal paths...

Let Dependence of Scintillation Yields in Liquid Argon

Abstract: Scintillation yields (scintillation intensity per unit absorbed energy) in liquid argon for ionizing particles are reviewed as a function of LET for the particles. The maximum scintillation yield, which is obtained for relativistic heavy ions from Ne to La, is about 1.2 times larger than that for gamma rays in NaI(Tl) crystal. In the low LET region, the scintillation yields for relativistic electrons, protons and He ions are 10–20% lower than the maximum yield. This tendency can be explained by taking into account the existence of the electrons which have escaped from their parent ions. In the high LET region, a quenching effect due to high ionization density is observed for alpha particles, fission fragments and relativistic Au ions.

Annual modulation in IDV of quasar 0917+624 due to Interstellar Scintillation

Abstract: The quasar 0917+624 has been one of the best studied intraday variable (IDV) radio sources. However, debate continues as to whether the underlying cause is intrinsic or extrinsic. Much previous work has assumed the IDV to be intrinsic and which implies an extraordinarily compact source for the radio emission; in contrast, an extrinsic variation due to interstellar scintillation (ISS) implies a relatively larger source diameter, though at the smaller end of the range expected for relativistic jet models. Kraus et al. (1999) reported a marked slowing of the IDV at 6cm wavelength in September 1998, and suggested a change in the source was responsible. However, here we show that the slowing is consistent with the annual modulation in scintillation time-scale expected for ISS, under the assumption that the scattering medium moves with the local standard of rest (LSR). The ISS time scale is governed by the ISS spatial scale divided by the Earth's velocity relative to the scattering plasma. It happens that in the direction of 0917+624 the transverse velocity of the Earth with respect to the LSR varies widely with a deep minimum in the months of September to November. Hence the slowing of the IDV in September 1998 strongly suggests that ISS, rather than intrinsic variation of the source is the dominant cause of the IDV.

Development and use of a GPS ionospheric scintillation monitor

Abstract: Global Positioning System (GPS) satellite signals provide convenient radio beacons for ionospheric studies. Among other propagation phenomena, the ionosphere affects GPS signal propagation through amplitude scintillations that develop after radio waves propagate through ionospheric electron density irregularities. This paper outlines the design, testing, and operation of a specialized GPS receiver to monitor L-band amplitude scintillations: the Cornell scintillation monitor. The Cornell scintillation monitor consists of a commercial GPS receiver development kit with its software modified to log signal strength from up to 12 channels at a high data rate (50 samples/s). Other features of the receiver include the optional assignment of a channel to monitor the receiver noise level in the absence of signal tracking and the means to synchronize measurements between nearby independent receivers to perform drift measurements and correlation studies. The Cornell scintillation monitor provides characterization of the operational L-band scintillation environment and additionally permits study of the multipath environment of a static antenna, GPS scintillation monitors can provide information about the state of ionospheric irregularities for pure research purposes as well. Their strength lies in the fact that they are inexpensive and compact and therefore can he readily proliferated. Even a single scintillation monitor can supplement radar spatial coverage of irregularities in a limited way because it monitors several satellite lines of sight simultaneously.

Back-illuminated photodiodes for computed tomography detectors

Abstract: A CT scanner (10) includes a reconstruction processor (42) for reconstructing an image from digital signals from detector arrays (20). Each detector array includes scintillation crystals (22) arranged in an array for converting x-ray radiation into light. An array of back-illuminated photo diodes (24) is mounted beneath the scintillation crystal array for converting the light emitted from the scintillation crystals into electrical charge. The electrical charge from the back-illuminated photodiodes is transmitted via a path (54, 56) orthogonal to the detector array (20) to signal processing circuitry (66). The back-illuminated photodiode has a side (28) which is in optical communication with the crystal array (22) and which is optically transmissive to photons of light emanating from the crystal. The converted electrical charge leaves the photodiode via electrical connections (32, 38) or bump bonds (62, 72) on the photodiode (24). This arrangement allows a plurality of paths (54, 56) through the substrate (52) supporting the photodiode to provide electrical connectivity from the array (20) to processing circuitry (66), reducing or eliminating the bottleneck of electrical leads from conventional arrays.

Gamma-ray spectrometry

Abstract: Different geometries of scintillation spectrometer are disclosed which provide improved resolution over prior art scintillation spectrometers. By ensuring that photons generated by scintillation events occurring in different locations within the scintillation material generate similar light profiles on the photo-detector, the output signal is made less sensitive to the initial interaction site. This can be achieved in a number of ways, such as: by limiting the exit window of the scintillation crystal to a smaller detector, by introducing an optical spacer (94) between the scintillation crystal and detector (99), and/or by making the crystal longer than necessary to stop the gamma rays. A principal advantage of these new geometries is that deconvolution of the raw-data is more effective, thus improving resolution.

Dynamics of equatorial F region irregularities from spaced receiver scintillation observations

Abstract: Spaced receiver observations of amplitude scintillations on a 244 MHz signal, at an equatorial station, have been used to study random temporal changes associated with the scintillation-producing irregularities and the variability of their motion. The computed drift of the scintillation pattern shows the presence of velocity structures associated with equatorial bubbles in the early phase of their development. On magnetically quiet days, after 22:00 LT, the estimated drifts fall into a pattern which is close to that of the ambient plasma drift. There is considerable decorrelation between the two signals until 22:00 LT. The power spectra of the most highly correlated scintillations recorded by spaced receivers indicate that the associated irregularities are confined to a thin layer on the bottomside of the equatorial F region. This suggests that the convection pattern associated with bottomside irregularities is stable due to the dominance of ion-neutral collisions over ion inertia.

Intra-day variability and the interstellar medium towards 0917+624

Abstract: The intra-day variable source 0917+624 displays annual changes in its timescale of variability. This is explained in terms of a scintillation model in which changes in the variability timescale are due to changes in the relative velocity of the scintillation pattern as the Earth orbits the sun.

Scintillation modeling for GPS‐Wide Area Augmentation System receivers

Abstract: A scintillation signal model and a Global Positioning System (GPS)-Wide Area Augmentation System (WAAS) receiver model are developed. The scintillation signal model is based on a Nakagami-m distribution for intensity and a Gaussian distribution with zero mean for phase. The GPS-WAAS receiver model includes Link 1 (L1) GPS and WAAS carrier-and C/A-code-tracking loops, as well as semicodeless Link 2 (L2) carrier and Y-code tracking capabilities. The results show that noncoherent delay locked loops (DLLs) typically used for code tracking are very robust to both amplitude and phase scintillation. Carrier-phase-tracking loops are much more susceptible to scintillation, and the signal-to-noise threshold for reliable carrier tracking is very dependent on the scintillation strength. Fortunately, it appears that the worst case scintillation encountered at midlatitudes, including the United States, does not significantly impact L1 carrier-tracking performance. Semicodeless tracking of the L2 carrier is shown to be very fragile. Even weak scintillation can cause loss of L2 carrier lock for low-elevation satellites.

Development of portable gas proportional scintillation counters for x-ray spectrometry

Abstract: A review on gas proportional scintillation counters (GPSCs) is presented. Recent achievements towards the portability of simple, inexpensive and compact GPSCs are discussed. Compensation of solid angle effects with the curved grid technique can be used to produce non-focused GPSCs with medium-sized radiation windows, at least up to 80% of the photosensor active diameter, without degradation of detector performance. Low power-consuming and compact vacuum UV photosensors that can operate in direct contact with the scintillation gas, as an alternative to photomultiplier tubes, are now available. Small gettering devices with a low-power built-in heating elements have been shown to be sufficient for the required gas purification in GPSCs assembled with simple and inexpensive techniques, such as the use of epoxies for ceramic-to-metal joints.

Measurements of Scintillation Efficiency and Pulse-Shape for Low Energy Recoils in Liquid Xenon

Abstract: Results of observations of low energy nuclear and electron recoil events in liquid xenon scintillator detectors are given. The relative scintillation efficiency for nuclear recoils is 0.22 +/- 0.01 in the recoil energy range 40 keV - 70 keV. Under the assumption of a single dominant decay component to the scintillation pulse-shape the log-normal mean parameter T0 of the maximum likelihood estimator of the decay time constant for 6 keV < Eee < 30 keV nuclear recoil events is equal to 21.0 ns +/- 0.5 ns. It is observed that for electron recoils T0 rises slowly with energy, having a value ~ 30 ns at Eee ~ 15 keV. Electron and nuclear recoil pulse-shapes are found to be well fitted by single exponential functions although some evidence is found for a double exponential form for the nuclear recoil pulse-shape.

Intra-Day Variability and the Interstellar Medium Towards 0917+624

Abstract: The intra-day variable source 0917+624 displays annual changes in its timescale of variability. This is explained in terms of a scintillation model in which changes in the variability timescale are due to changes in the relative velocity of the scintillation pattern as the Earth orbits the sun. (see also astro-ph/0102050)

Measuring Ionospheric Scintillation Effects from GPS Signals

Abstract: GPS signals provide an excellent means for measuring ionospheric scintillation effects on a global basis because the signals are continuously available and can be measured through many points of the ionosphere simultaneously. GPS signals are themselves affected. However, tracking through disturbances with a GPS receiver is usually possible with reasonably wide bandwidth tracking loops. Because of this, ionospheric scintillation can be monitored, and is currently being monitored around the world. This was not widely possible during the last solar activity peak. The importance of the wide bandwidth is that scintillation parameters, such as spectral content, can be computed, not just the effects of the scintillation on GPS receiver performance. The majority of the current wide bandwidth monitoring is being done using a commercially off-the-shelf GPS receiver implemented with special software -- the GSV4000 GPS Ionospheric Scintillation Monitor (GISM) and predecessor prototype units. Now, GPS Silicon Valley is pleased to offer the new GSV4004 GPS Ionospheric Scintillation and TEC Monitor (GISTM) receiver. This receiver, a NovAtel EURO4 dual-frequency receiver with special firmware, comprises the major component of a GPS signal monitor, specifically configured to measure amplitude and phase scintillation from the L1 frequency GPS signals, and the ionosphere's TEC from the L1 and L2 frequency GPS signals. This scintillation and TEC monitoring receiver is housed in a NovAtel GPStation4E housing with a low phase noise oscillator, and provides true amplitude, single frequency carrier phase measurements and TEC measurements of up to 11 GPS satellites in view. It will also track one SBAS (WAAS, EGNOS or MSAS) satellite, providing L1 measurements and data, as the 12th satellite. The unit comes with complete software that allows the automatic measurement and computation of all the major scintillation parameters and TEC. A variety of antennae, with or without choke rings and cables, are offered as options. In this paper, the wide bandwidth monitoring capabilities of these receivers are described. This is followed by the presentation of data collected from a selection of recorded scintillation events and TEC calibration results.

Absorption dose measuring apparatus for intensity modulated radio therapy

Abstract: An intensity modulated radio therapy (IMRT) dosimeter measuring a three-dimensional absorption dose distribution in a transparent plastic phantom readily and accurately. In the IMRT dosimeter, radiation beams are directed to a phantom assembly having a scintillation fiber block sandwiched between two blocks of the phantom. As an intensity of light proportional to the radiation beam is emitted from one side of the scintillation fiber block, its profile is measured by an image sensor. Then, the three-dimensional absorption dose distribution is obtained from the light intensity distribution data. By summing the three-dimensional absorption dose distribution data measured with the radiation beam at different angles, an integrated three-dimensional absorption dose distribution in the phantom can be calculated readily and accurately.

Study of the turbulence and inner waves in the stratosphere based on the observations of stellar scintillations from space: A model of scintillation spectra

Abstract: Stellar scintillations observed from space through the atmosphere show that density inhomogeneities in the stratosphere are stretched along the Earth's surface. This is true for vertical scales above dozens of metres. The observations reveal the existence of locally isotropic small-scale structure with fluctuation sizes up to fractions of a metre. The subject of this paper is to find out how the rotation of inhomogeneities with respect to the passing ray affects scintillations. Another subject of this study is chromatic aberration in the atmosphere which distorts the scintillation spectra. Numerical modelling within the weak-fluctuation approximation showed that the characteristic value of the anisotropy parameter is equal to the square root of the Earth's radius divided by the atmospheric scale. After the anisotropy exceeds this value, the growth of scintillation variance quickly becomes saturated. Chromatic aberration suppresses the high-frequency branch of the scintillation spectrum. However, ...

Semiconductor radiation detector with internal gain

Abstract: An avalanche drift photodetector (ADP) incorporates extremely low capacitance of a silicon drift photodetector (SDP) and internal gain that mitigates the surface leakage current noise of an avalanche photodetector (APD). The ADP can be coupled with scintillators such as CsI(Tl), NaI(Tl), LSO or others to form large volume scintillation type gamma ray detectors for gamma ray spectroscopy, photon counting, gamma ray counting, etc. Arrays of the ADPs can be used to replace the photomultiplier tubes (PMTs) used in conjunction with scintillation crystals in conventional gamma cameras for nuclear medical imaging.

Backside illuminated photodiode array

Abstract: An improved semiconductor position-sensitive radiation detection device based on a photodiode array formed in a substrate. In one embodiment, the substrate has a first surface and a second surface opposing the first surface. The first surface is electrically conducting to provide a common bias potential to the photodiodes and is optically transparent to receive input photons to be detected. The device includes a grid of conducting wires formed over and in electrical contact with the first surface and configured to define an array of pixels corresponding to the array of photodiodes. A scintillation array of scintillation elements can be coupled to match the pixels defined by the grid of conducting wires and to convert incident radiation at a first wavelength outside the characteristic spectral response range of the substrate into secondary photons at a second wavelength within the spectral response range of the substrate. The scintillation array includes optically reflective surfaces disposed between the scintillation elements to optically isolate one scintillation element from another.

Electromagnetic scintillation. I. Geometrical optics

Abstract: Electromagnetic scintillation describes the phase and amplitude fluctuations imposed on signals that travel through the atmosphere. These volumes provide a modern reference and comprehensive tutorial for this subject, treating both optical and microwave propagation. Measurements and predictions are integrated at each step of the development. This first volume deals with phase and angle-of-arrival measurement errors. These are accurately described by geometrical optics. Measured properties of tropospheric and ionospheric irregularities are reviewed first. Electromagnetic fluctuations induced by these irregularities are estimated for a wide range of applications and compared with experimental results in each case. These fluctuations limit the resolution of astronomical interferometers and large single-aperture telescopes. Synthetic-aperture radars and laser pointing/tracking systems are also limited by such effects. Phase errors ultimately limit the accuracy of laser metrology and GPS location. Similar considerations will become important as terrestrial and satellite communications move to higher frequencies. Scintillation measurements now provide an accurate and economical way to establish atmospheric properties. Amplitude and intensity fluctuations are addressed in the second volume. This volume will be of particular interest to astronomers, applied physicists and engineers developing instruments and systems at the frontier of technology. It also provides a unique reference for atmospheric scientists and scintillation specialists. It can be used as a graduate textbook and is designed for self-study. Extensive references to original work in English and Russian are provided.

Electron traps and scintillation mechanism in LuAlO3:Ce

Abstract: In this paper we report measurements of thermoluminescence in the temperature range of 20–370 K, isothermal decays, pulsed vacuum ultraviolet and γ-excited luminescence time profiles at various temperatures on cerium-activated orthoaluminate (LuAlO3:Ce, LuAP), a new and promising scintillator material. We demonstrate that results of all these experiments can be consistently explained by assuming a recombination mechanism of scintillation light production in the LuAP scintillator. Using a simple first-order kinetic model that includes Ce3+ ions as recombination centres and a number of electron traps, we extract from experimental data the basic trap parameters (energy depths and frequency factors). Consequently we identify nine traps that are responsible for undesired features of the LuAP scintillator, such as a reduced scintillation light output, a relatively long scintillation rise time and slow scintillation components (afterglow) at room temperature. We demonstrate that some of these traps are responsible for large variations of the scintillation light yield with temperature as reported earlier. Although the deepest traps do not alter scintillation time profiles, they are responsible for a significant scintillation light loss and are, therefore, detrimental to scintillation performance of the material. We observe that there is an apparent correlation between trap depths and frequency factors for at least five of the traps that may fit some more general pattern involving various groupings of all the traps. This, in turn, would indicate that traps in LuAP are not unrelated and are due, most likely, to a series of native defects in the LuAP crystal structure. Although the specific identity of traps remains unknown, the performance of the LuAP scintillator is now, in practical terms, fully understood and can be described numerically at any temperature using a model and a set of parameters given in this paper. It is clear that any major improvement of the material would require that traps are eliminated or that their influence on the scintillation process is minimized.