Showing papers on "Scintillation published in 2000"

High-energy-resolution scintillator: Ce3+ activated LaBr3

Abstract: The scintillation properties of LaBr3 doped with 0.5% Ce3+ are presented. Under optical and gamma ray excitation, Ce3+ emission is observed to peak at 356 and 387 nm. The scintillation light output is 61 000±5000 photons/MeV at 662 keV. More than 90% is emitted with a decay time of 35 ns. An energy resolution (full width at half maximum over the peak position) of 2.85%±0.05% was observed for the 662 keV full absorption peak. A time resolution of 385 ps was obtained using BaF2 as second scintillator and gamma rays of 60Co.

Wide Band Gap Scintillation Materials: Progress in the Technology and Material Understanding

Abstract: Luminescence and scintillation characteristics of four selected material systems, namely CsI:Tl(Na), CeF 3 , PbWO 4 and Ce-doped aluminium perovskites XAIO 3 :Ce (X=Y, Lu, Y-Lu) are reviewed. The progress in their physical understanding and related optimisation of their characteristics and technology are demonstrated. The important role of various defect states in the scintillator performance of these materials is stressed, which has led to the need for a deeper study of the processes of energy transfer and storage to achieve their intrinsic limits and full exploitation in scintillation detectors.

Scintillation crystals for PET.

Abstract: In PET, inorganic scintillator crystals are used to record gamma-rays produced by the annihilation of positrons emitted by injected tracers. The ultimate performance of the camera is strongly tied to both the physical and scintillation properties of the crystals. For this reason, researchers have investigated virtually all known scintillator crystals for possible use in PET. Despite this massive research effort, only a few different scintillators have been found that have a suitable combination of characteristics, and only 2 (thallium-doped sodium iodide and bismuth germanate) have found widespread use. A recently developed scintillator crystal, cerium-doped lutetium oxyorthosilicate, appears to surpass all previously used materials in most respects and promises to be the basis for the next generation of PET cameras.

Traps and Timing Characteristics of LuAG:Ce3+ Scintillator

Abstract: This note provides a direct comparison between YAP:Ce{sup 3+} and LuAG:Ce{sup 3+} scintillators grown in the same laboratory as for radioluminescence light output, scintillation decay and thermoluminescence characteristics. A correlation between the occurence of thermo-luminescence (TSL) peaks round RT and of very slow components in the scintillation decay of LuAG:Ce{sup 3+} is revealed. Importance of understanding of the nature of such TSL active trapping states and their removal for further optimisation of LuAG:Ce{sup 3+} scintillator is concluded. (orig.)

The Discovery of a Microarcsecond Quasar:J1819+3845

Abstract: We report on the discovery of a source that exhibits over 300% amplitude changes in radio flux density on the period of hours. This source, J1819+3845, is the most extremely variable extragalactic source known in the radio sky. We believe these properties are due to interstellar scintillation and show that the source must emit at least 55% of its flux density within a radius of fewer than 16 μas at 5 GHz. The apparent brightness temperature is greater than 5 × 1012 K, and the source may be explained by a relativistically moving source with a Doppler factor of ~15. The scattering occurs predominantly in material only a few tens of parsecs from the Earth, which explains its unusually rapid variability. If the source PKS 0405-385 is similarly affected by local scattering material, Doppler factors of ~1000 are not required to explain this source. The discovery of a second source whose properties are well modeled by interstellar scintillation strengthens the argument for this as the cause for much of the variation seen in intraday variables.

Estimating the Refractive Index Structure Parameter () over the Ocean Using Bulk Methods.

Abstract: Infrared scintillation measurements were obtained along a 7-km path over San Diego Bay concurrently with meteorological measurements obtained from a buoy at the midpoint of the path. Bulk estimates of the refractive index structure parameter were computed from the buoy data and compared with scintillation-derived values. The bulk estimates agreed well with the scintillation measurements in unstable conditions. In stable conditions the bulk estimates became increasingly higher than the scintillation values as the air–sea temperature difference increased. This disagreement may be due to enhanced wave-induced mixing of the lower atmosphere that decreases the vertical temperature and humidity gradients in stable conditions from the assumed Monin–Obukhov similarity (MOS) theory forms, resulting in bulk values that are too high. The bulk estimates decrease rapidly when the absolute air–sea temperature difference approaches small positive values. These predicted decreases in were not observed in either ...

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.

Ionospheric Scintillation Effects in the Equatorial and Auroral Regions

Abstract: Ionospheric scintillation effects on GPS signals and ultimately on satellite navigation and communication is a growing concern. This concern is primarily due to the challenges that intense scintillation may pose on these systems during periods of high solar activity. The most extreme scintillation activity is expected to occur in the equatorial anomaly regions during the evening hours at high solar activity levels. The auroral and polar cap regions are also expected to experience significant scintillation activity during geomagnetically disturbed periods. Scintillation is expected to be rare in the mid- latitudes, limited to periods of extreme geomagnetic activity. The FAA's National Satellite Test Bed (NSTB) has been monitoring scintillation activity at Fairbanks, Alaska since mid-1998, intermittently from Santiago, Chile since February 1999 and more recently from Mauna Loa, Hawaii. The Air Force Research Laboratory (AFRL) has also initiated long term scintillation measurements near the magnetic equator at Ancon, Peru and at several locations in the equatorial anomaly region. These measurements have been made with a commercially available Ionospheric Scintillation Monitor (ISM) developed by GPS Silicon Valley. The ISM is a modified NovAtel GPS single-frequency receiver that measures amplitude and phase scintillation parameters in real-time. In this paper, we submit a summary of the scintillation activity recorded by the NSTB at Fairbanks and by AFRL at Ancon. In addition, we correlate the scintillation activity with receiver performance of nearly co-located dual-frequency codeless L2 GPS receivers. The results of this study may begin to quantify the frequency and magnitude of scintillation at GPS frequencies at these two dynamic locations. It will also illuminate the effect that scintillation can have on the performance of co-located codeless L2 dual-frequency receivers and ultimately on GPS based satellite navigation for civilian receivers without access to the encrypted L2 code. Finally, it will serve as a test for the ISM receiver to monitor amplitude and phase scintillation in real-time.

Scintillation model for a satellite communication link at large zenith angles

Abstract: A scintillation model is developed for uplink-downlink optical communication channels applicable in moderate to strong fluctuation conditions that may arise under large zenith angles between transmitter and receiver. The model developed here is an extension of a recently published theory that treats irradiance fluctuations along a horizontal path as a modulation of small-scale scintillation by large-scale scintilla- tion. For a downlink path the scintillation index is modeled like that of an infinite plane wave, and for an uplink path we consider a spherical wave model. In both cases the scintillation index agrees with conventional weak-fluctuation-theory results out to zenith angles of 45 to 60 deg. The covariance function of irradiance fluctuations is also developed under the same conditions as assumed for the scintillation index. On a downlink path under small zenith angles the implied correlation length is propor- tional to the Fresnel-zone scale. For zenith angles exceeding 85 deg, the downlink correlation length varies directly with the spatial coherence ra- dius weighted by a factor that depends on changes in C n the refractive index structural parameter with altitude. © 2000 Society of Photo-Optical Instru- mentation Engineers. (S0091-3286(00)02412-0)

Scintillation efficiency of nuclear recoil in liquid xenon

Abstract: We present the results of a test done with a Liquid Xenon (LXe) detector for “Dark Matter” search, exposed to a neutron beam to produce nuclear recoil events simulating those which would be generated by WIMP's elastic scattering. The aim of the experiment was to measure directly the scintillation efficiency of nuclear recoil. The nuclear recoil considered in the test was in the tens of keV range. The ratio of measured visible energy over the true recoil energy was evaluated to be about 20%, in good agreement with the theoretical predictions.

Progress in the development of LuAlO/sub 3/ based scintillators

Abstract: LuAlO/sub 3/:Ce/sup 3+/ (LuAP) and Lu/sub x/Y/sub 1/-xAlO/sub 3/:Ce/sup 3+/ (LuYAP) crystals are used as scintillation materials for positron emission tomography. The actual study of these scintillators develops in three directions: (i) growth of large size LuAP crystals with stable properties, (ii) the relationship between the composition of LuYAP crystals and scintillation properties, and (iii) scintillation mechanisms in lutetium compounds. After improving of growth conditions a large size samples (length >40 mm) have been prepared. Crystals show a good correlation between growth parameters, light yield and transmission spectra. We studied a series of samples with calibrated size (2/spl times/2/spl times/10 mm3) and compare the light yield with standard BGO and LSO samples. Mixed crystals with composition of 0.6

Lutetium yttrium orthosilicate single crystal scintillator detector

Abstract: A single crystal having the general composition, Ce 2x (Lu 1-y Y y ) 2(1-x) SiO 5 where x=approximately 0.00001 to approximately 0.05 and y=approximately 0.0001 to approximately 0.9999; preferably where x ranges from approximately 0.0001 to approximately 0.001 and y ranges from approximately 0.3 to approximately 0.8. The crystal is useful as a scintillation detector responsive to gamma ray or similar high energy radiation. The crystal as scintillation detector has wide application for the use in the fields of physics, chemistry, medicine, geology and cosmology because of its enhanced scintillation response to gamma rays, x-rays, cosmic rays and similar high energy particle radiation.

Large Area Avalanche Photodiodes in X-rays and scintillation detection

Abstract: The performance of 10 and 16 mm diameter beveled edge Large Area Avalanche Photodiodes (LAAPD) was studied in X-rays and scintillation detection. A good linearity of the LAAPD response to X-rays was observed up to energies of about 20 keV. The ratio of the APD gain for X-rays and light pulses remained constant (within 1%) for both the 5.9 and 16.6 keV photopeaks in the APD gain range up to 100. This allowed for use of soft X-rays as an accurate reference in APD scintillation detection study. The study covered measurements of the number of generated e}h pairs and energy resolution for several scintillators. Particularly, the energy resolution of 4.8$0.2% was measured with a small CsI(Tl) crystal for 662 keV c-rays from a 137Cs source and 4.3$0.2% with YAP:Ce. The measured energy resolutions were comparable or better than those measured with a photomultiplier. ( 2000 Elsevier Science B.V. All rights reserved.

Radioluminescence and recombination processes in BaF2:Ce

Abstract: In this paper we report measurements of x-ray- and vacuum UV-excited luminescence, luminescence excitation spectra and time profiles, low temperature thermoluminescence and isothermal decays as well as x-ray- and gamma-excited scintillation time profiles and scintillation light yields at various temperatures on BaF2:Ce and, for reference, on undoped BaF2, two well known scintillator materials. For the first time we find that all these results can be consistently interpreted in the frame of a model that includes Ce-related recombination centres and several charge `traps'. The charge trapping at most of these `traps' has its origin in self-trapping and trapping of holes at regular (Vk) and interstitial (H) fluorine sites. We have identified and characterized two different modes of thermally activated Vk release that precede radiative recombination at Ce sites. These two modes are responsible for a 7 ns rise time and a slower 114 ns components in the scintillation time profile at room temperature (297 K) that produce about 67% of the scintillation light detected within a 0.5 µs time window. The remaining 33% is due to a prompt component decaying with the Ce3+ radiative lifetime of about 30 ns that originates from the direct recombination of charge carriers at Ce3+ ions. We also estimate that scintillation light loss due to even slower components, at 23.1 µs (H centres), 1.1 ms and 7 ms (VkA and VkA' centres), exceeds at least threefold the amount of light emitted in the 0.5 µs time window. Therefore in addition to their well known role as defect centres actively participating in the formation of stable radiation damage centres these species are also involved in the radiative recombination process itself. The perspectives of improvements in performance of the BaF2:Ce scintillator are also briefly 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 p+p→e++d+ν e reaction could be characterized and monitored versus time using a 10 ton mass of liquid helium or neon as a scintillation target.

Modeling the Effects of Ionospheric Scintillation on GPS/SBAS Availability

Abstract: Ionospheric scintillation is a rapid change in the amplitude and phase of a radio signal as it passes through small-scale plasma density irregularities in the ionosphere. These scintillations not only can reduce the accuracy of GPS/SBAS receiver pseudorange and carrier phase measurements, but can result in a complete loss of lock on a satellite. In a worst-case scenario, lock on enough satellites could result in lost positioning service. Scintillation has not had a major effect on mid-latitude regions (e.g., CONUS) since most severe scintillation occurs in a band approximately 20 degrees (deg) on either side of the magnetic equator and to a lesser extent in the polar and auroral regions. Most scintillation occurs for a few hours after sunset during the peak years of the solar cycle. Typical delay locked loop (DLL) / phase locked loop (PLL) designs of GPS/SBAS receivers enable them to handle moderate amounts of scintillation. Consequently, any attempt to determine the effects of scintillation on GPS/SBAS must consider both predictions of scintillation activity in the ionosphere, and the residual effect of this activity after processing by a receiver. This paper estimates the effects of scintillation on the availability on GPS and SBAS for L1 C/A and L2 semicodeless receivers. These effects are described in terms of loss of lock and degradation of accuracy and are related to different times, ionospheric conditions, and positions on the Earth. Sample results are presented using WAAS in the Western Hemisphere.

A xenon gas proportional scintillation counter with a UV-sensitive large-area avalanche photodiode

Abstract: The performance characteristics of a xenon gas proportional scintillation counter instrumented with an large-area avalanche photodiode with enhanced UV-sensitivity were evaluated. By integrating the photodiode within the xenon gas envelope of the scintillator, the intervening quartz window was eliminated. Energy resolutions of 7.8% and 4.4% were measured for 5.9-keV and 22.1-keV X-rays, respectively. The results demonstrate that large-area avalanche photodiodes may replace photomultiplier tubes without compromising the performance of the gas proportional scintillation counter.

Radio Scintillation due to Discontinuities in the Interstellar Plasma Density

Abstract: We develop the theory of interstellar scintillation as caused by an irregular plasma having a power-law spatial-density spectrum with a spectral exponent of β = 4 corresponding to a medium with abrupt changes in its density. An "outer scale" is included in the model that represents the typical scale over which the density of the medium remains uniform. Such a spectrum could be used to model plasma shock fronts in supernova remnants or other plasma discontinuities. We investigate and develop equations for the decorrelation bandwidth of diffractive scintillations and the refractive scintillation index and compare our results with pulsar measurements. We consider both a medium concentrated in a thin layer and an extended irregular medium. We conclude that the β = 4 model gives satisfactory agreement for many diffractive measurements, in particular the VLBI measurements of the structure function exponent between 5/3 and 2. However, it gives less satisfactory agreement for the refractive scintillation index than does the Kolmogorov turbulence spectrum. The comparison suggests that the medium consists of a pervasive background distribution of turbulence embedded with randomly placed discrete plasma structures such as shocks or H II regions. This can be modeled by a composite spectrum following the Kolmogorov form at high wavenumbers and steepening at lower wavenumbers corresponding to the typical (inverse) size of the discrete structures. Such a model can also explain the extreme scattering events. However, lines of sight through the enhanced scattering prevalent at low Galactic latitudes are accurately described by the Kolmogorov spectrum in an extended medium and do not appear to have a similar low-wavenumber steepening.

A Comparison of GPS Performance in a Scintillation Environment at Ascension Island

Abstract: : Post-sunset disturbances in the equatorial ionosphere routinely cause rapid phase and amplitude fluctuations (i.e., scintillation) of radio waves propagating through the disturbed regions. The intensity of scintillations is positively correlated with the solar cycle and the associated signal fades will often exceed 20 dB at L-band frequencies during solar maximum. The effect of such an environment on the performance of GPS navigation systems is poorly understood. In March 2000 AFRL conducted a campaign at Ascension Island to test the performance of several GPS receivers under potentially severe scintillation conditions. Ascension Island is located at approximately 16 S magnetic latitude, a region of intense ionospheric disturbances. The systems tested included a Plessey GPS Builder, a Novatel-based prototype GPS Silicon Valley (GSV) Ionospheric Scintillation Monitor (ISM) modified specifically for scintillation applications, a custom High Gain Advanced GPS Receiver (HAGR) developed for AFRL by NAVSYS Corporation and an Ashtech Z-12. Overall, the Ashtech proved to be very robust at tracking the carrier signal amplitude and phase, but it experienced scintillation-induced navigation outages on four of the eight nights of observations. The responses of the different receivers during severe scintillation varied significantly, suggesting that models to simulate ionospheric effects on GPS performance must be receiver-specific.

Interstellar Scintillation of Pulsar B0809+74

Abstract: Weak interstellar scintillations of pulsar B0809+74 were observed at two epochs using a 30 m 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 LSR. The Kolmogorov power law is a satisfactory model for the electron density spectrum at scales between 2 × 107 and 109 m. 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°, b = 32°) 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-1 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° in longitude, including the Galactic anticenter. If, on the other hand, the scattering occurs in a compact region, the observed timescales 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. in 1998, near 72 pc toward the pulsar, must be moving at about ~17 km s-1; 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.

Wave-front sensing and deformable-mirror control in strong scintillation

Abstract: Recent studies of coherent wave propagation through turbulence have shown that under conditions where scintillation is significant a continuous phase function does not in general exist, owing to the presence of branch points in the complex optical field. Because of branch points and the associated branch cuts, least-squares approaches to wave-front reconstruction and deformable-mirror control can have large errors. Branch-point reconstructors are known to provide superior performance to least-squares reconstructors, but they require that branch points be explicitly detected. Detecting branch points is a significant practical impediment owing to spatial sampling and measurement noise in real wave-front sensors. Branch points are associated with real zeros in an optical field, and hence information about the phase of the field is encoded in the amplitude of the wave. We present a new wave-front-sensor processing algorithm that exploits this observation in the wave-front-reconstruction and deformable-mirror-control process. This algorithm jointly processes three intensity measurements by using light from the beacon field to develop a set of deformable-mirror actuator commands that are maximally consistent with three intensity measurements: (1) the entire wave-front-sensor image, (2) a pupil intensity image, and (3) a conventional image. Owing to the nonlinear nature of the resulting algorithm, we have used a simulation to evaluate performance. We find that in a focused laser beam projection paradigm that uses a point-source beacon, the new algorithm provides significantly improved performance over that of conventional Hartmann sensor least-squares deformable-mirror control based on centroid processing of wave-front-sensor outputs. The performance of the new algorithm approaches, the performance of an idealized branch-point reconstructor that requires pointwise phase differences for operation.

New water equivalent liquid scintillation solutions for 3D dosimetry.

Abstract: Despite recent advances in radiochromic film and gel dosimetry techniques, radiation therapy still lacks an efficient, accurate, and convenient dose measurement method capable of measuring the dose simultaneously over a plane or a volume (3D). A possibility for creating such a 3D method based on observing scintillation photons emitted from an irradiated volume was recently reported [A. S. Kirov et al., Med. Phys. 26, 1069 (1999)]. In the present article, we investigate the potential to use a liquid scintillation solution (LS) as a dose sensitive media and, simultaneously, as a water equivalent phantom material which fills the measurement volume. We show that matching water density in addition to energy absorption properties is important for using the LS solution as a phantom. Through a parametric study of the LS attenuation and absorption coefficients as well as Monte Carlo dose calculations and scintillation efficiency measurements we developed novel LS materials. For the new solutions, the calculated dose in LS is within 8% of the dose to water for depths up to 5 cm for photons having energies between 30 keV and 2 MeV. The new LS solutions, which are loaded with a Si containing compound, retain more than 85% of the scintillation efficiency of the unloaded solutions and exhibit high localization of the scintillation process. The new LS solutions are superior with respect to efficiency and water equivalence to plastic scintillator materials used in dosimetry and may be used apart from the mentioned 3D method.

Modeling scintillation from an aperiodic Kolmogorov phase screen

Abstract: We propose a technique for the accurate modeling and simulation of scintillation patterns that are due to Kolmogorov statistics without assuming periodic boundary conditions. We show how the more physically justifiable assumption of smoothness results in a propagation kernel of finite extent. This allows the phase screen dimensions for an accurate simulation to be determined, and truncation can then be used to eliminate the unwanted spectral leakage and diffraction effects usually inherent in the use of finite apertures. A detailed outline of the proposed technique and comparison of simulations with analytic results are presented.

Self aligning inter-scintillator reflector x-ray damage shield and method of manufacture

Abstract: A scintillator pack including an x-ray damage shield. The scintillator pack has an array of scintillator pixels. A scintillation light reflecting layer that reflects scintillation light from the pixels is included at least between the scintillator pixels in inter-scintillator regions. An x-ray absorbing layer acts as the x-ray damage shield to protect the portions of the scintillation light reflecting layer from x-rays. The x-ray absorbing layer is formed selectively and in a self aligned manner in regions over the inter-scintillator regions.

Small-size pulsed X-ray source for measurements of scintillator decay time constants

Abstract: Summary form only given. The main scintillator characteristics such decay time, light output, emission spectrum, are usually measured at excitation by gamma quanta or particles (electrons, protons, neutrons). Rather recently, subnanosecond X-ray pulses have been used for study of scintillators. In this work, we describe the small-size short-pulsed X-ray source. The basic elements of the source are three-electrode X-ray tube, which operates at 30 kV, 0.5 A, and fast GaAs modulator coaxial matched with grid electrode of the tube. The source has the following limiting parameters pulse duration /spl sim/0.5 ns, pulse frequency 100 kHz, required power /spl sim/50 W. The source is constructed in form of cylinder with 15 cm length and 10 cm diameter (together with lead radiation protection). Such system, like PMT, is easily matched with cryostat. The advantages of the tube with thermocathode over light-excited X-ray tube is higher anode current (0.5 A compared with 50 mA in last case) and lower cost. The decay curve of conventional scintillator BaF/sub 2/ with fast (0.88/spl plusmn/0.02 ns) and slow (850/spl plusmn/5 ms) components was measured to illustrate the capabilities of the set-up. Some traditional and new scintillating materials have been also investigated.

An argon gas proportional scintillation counter with UV avalanche photodiode scintillation readout

Abstract: An uniform-field argon-filled gas proportional scintillation counter, using a large-area avalanche photodiode, is presented. The excitation and ionization thresholds for argon were found to be approximately 0.7 and 3.7 V cm/sup -1/ torr/sup -1/, respectively. An upper limit of 0.30/spl plusmn/0.04 was measured for the argon Fano factor for 5.9-keV X-rays. An energy resolution of 12.5% full-width-at-half-maximum was achieved for 5.9-keV X-rays. The ultraviolet large-area avalanche photodiode proved to be suitable for reading the scintillation light of an argon-filled gas proportional scintillation counter, providing a quantum efficiency of about 50% at 128 nm. Good photodiode performance is already achieved for gains about 100.