Showing papers on "Scintillation published in 1994"

Response of TAPS to monochromatic photons with energies between 45 and 790 MeV

Abstract: The Two Arm Photon Spectrometer TAPS - comprising 384 plastic-BaF2 scintillator telescopes - was tested with monochromatic photons in the energy range between 45 and 790 MeV. The energy resolution for a collimated photon beam hitting the central detector module was determined to sigma/E = 0.59% X E(gamma)-1/2 + 1.9% (E, given in GeV). For the the fast scintillation component alone sigma/E = 0.79% X E(gamma)-1/2 + 1.8% has been measured. The position resolution of the point of impact amounts to DELTAx = 2 cm (FWHM) at the highest energies which corresponds to 30% of the diameter of an individual module. Monte Carlo simulations using the code GEANT3 are in good agreement with the experimental results.

Refractive interstellar scintillation in pulsar dynamic spectra

Abstract: The evolution of the dynamic spectra of eight pulsars over 16 months is reported. Observations were made at Jodrell Bank at 408 MHz from 1984 September to 1986 January. The changing form of the spectra is interpreted as refractive modulation of diffractive interstellar scintillation. The correlation function versus time and frequency was computed for each observation and estimates were made of the apparent decorrelation widths in frequency and time, and the frequency drift rate of scintillation features. A model is developed for how these three parameters vary with epoch under changing angles of refraction. The model successfully describes the variations for four of the pulsars, for which the drift rate shows some sign reversals between epochs. Two pulsars kept a constant sign of drift rate, which we interpret as due to persistent refracting structures in their lines of sight, which are consistent with an intervening H ii region. Studies were made of the variability of the diffractive bandwidth, the visibility of the drifting features in the dynamic spectra, the refractive shifts and the speed estimated for the diffraction pattern. These quantities mostly agree with expectations from the model. The pulsar PSRB2016+28, which has an unusually low proper motion, shows clear modulation of the pattern speed by the Earth's annual motion. In the 11 epochs observed for PSRB2016+28, two showed unmistakable periodic modulation in the dynamic spectra. Several other pulsars exhibited occasional periodic patterns with low modulation. The phenomenon varies substantially between the pulsars, being more common for lower strengths of scattering.

Design of a Compton spectrometer experiment for studying scintillator non-linearity and intrinsic energy resolution

Abstract: A Compton spectrometer experiment has been designed and modeled for the purpose of studying the light yield non-linearities and intrinsic energy resolution of scintillation materials that are used to detect gamma rays. This coincidence method is used to create a nearly monoenergetic internal electron source within the scintillator by recording pulses from the primary detector only when a simultaneous pulse is generated by the coincidence detector. Such an electron source is necessary to accurately quantify the electron response of a scintillator, and has been previously identified as a requirement for quantifying scintillator non-linearity and intrinsic energy resolution. The ability to quantify these scintillator characteristics using this technique and the characterization of the Compton spectrometer geometry, including collimation of primary and scattered gamma rays, using Monte Carlo simulation are discussed.

Atmospheric Modelling and Millimetre Wave Propagation

Abstract: List of symbols. Introduction. Outline of interaction processes. Absorption and dispersion in atmospheric gases. Scintillation. Theory of scattering and absorption. Thermal radiation from hydrometeors and atmospheric gases. Scattering and absorption in sand and dust particle populations. Attenuation by clouds and the melting layer. Attenuation due to rain. Depolarisation by rain. Depolarisation by ice particles and the melting layer. Scattering and interference. Reflection and refraction by atmospheric layers. Physical properties of hydrometeors. Index.

A large air shower array to search for astrophysical sources emitting γ-rays with energies ≥1014 eV

Abstract: We describe the technical details and the performance of a large array which detects both the electron and muon components in extensive air showers with energies ≥ 1014 eV. The array was designed to search for γ-rays from astrophysical sources. The background of cosmic rays is reduced by the selection of muon poor events. The array consists of 1089 scintillation detectors on the surface covering an area of 0.23 km2 and 1024 scintillation counters of 2.5 m2 each, buried 3 m below the surface for muon detection. Each of the surface detectors has its own local electronics and local data storage controlled by a microprocessor. The array is located at Dugway, Utah USA (40.2°N, 112.8°W) where the average atmospheric depth is 870 g/cm2.

Effects of the refractive index spectral model on the irradiance variance of a Gaussian beam

Abstract: The normalized irradiance variance of a Gaussian beam in the weak-fluctuation regime is examined numerically with the use of various spectral models for refractive index fluctuations. The Kolmogorov, Tatarskii, and modified von Karman spectral models are chosen as conventional models, while the Hill numerical spectral model and the Andrews analytic approximation to the Hill model are selected to feature the characteristic bump at high wave numbers. The latter two models are known to predict higher scintillation levels than conventional spectral models when the Fresnel zone and the inner scale are of comparable size. Outer scale effects appear minimal near the centerline of the beam but can reduce off-axis scintillation significantly. Inner scale effects are prominent on axis as well as off axis, although they sometimes tend to diminish near the diffractive beam edge. Analytic approximations are developed for the irradiance variance based on the Kolmogorov, modified von Karman, and Andrews spectral models. These analytic expressions are generally in excellent agreement with numerical results.

On the probability density function of signal intensity in free-space optical communications systems impaired by pointing jitter and turbulence

Abstract: Expressions for the probability density function (pdf) of optical signal intensity in an optical communication channel impaired by motion-induced beam jitter and turbulence are derived. It is assumed that the optical beam possesses a Gaussian profile, generated by a pulsed laser, and that the beam scintillation is governed by either log-normal distribution for weak turbulence or K distribution for moderate to strong turbulence in the saturation region. For extreme propagation distances or very strong turbulence, a negative exponential pdf is used to model turbulence. For the aforementioned beam scintillation statistics, approximate pdf's for the signal intensity are also obtained and the conditions for which these approximations seem to be valid are also discussed.

Irregularity structures in the cusp/cleft and polar cap regions

Abstract: A case study of the temporal behavior of ionospheric scintillations and their frequency spectra in the cusp/cleft and polar cap regions is presented. These measurements were made at Sondrestrom and Thule, Greenland, using the 243-MHz transmissions from quasi-stationary satellites during a coupling energetic and dynamics of atmospheric regions (CEDAR) high-latitude plasma structure (HLPS)/solar terrestrial energy program (STEP) global aspects of plasma structures (GAPS) campaign. During this campaign, the incoherent scatter radar (ISR) observations were also performed at Sondrestrom, which defined the dynamic ionospheric environment in the cusp/cleft region. The availability of the radar results has enhanced this case study. It is found that scintillations at Sondrestrom are abruptly enhanced about an hour before magnetic noon when the propagation path to the satellite entered the cusp/cleft region. Subsequently, a series of enhanced and reduced scintillation activity was detected. The enhanced scintillation structures were found to be asymmetric, with sharp leading edges and diffuse trailing edges. Spaced-antenna scintillation measurements at Sondrestrom detected considerable velocity shear, and the frequency spectra showed flat low-frequency portions, implying the presence of turbulent plasma flows. A comparison with the ISR observations indicates that the temporal variation of scintillation was caused by the poleward convection of alternate regions with high- and low-ionization density, the density depletions being caused by channels of high zonal flows associated with velocity shear. The level of scintillation observed in the low-density regions imply the presence of small-scale irregularities with considerable irregularity amplitude. In contrast to the above behavior, the polar cap scintillations exhibit deep minima between the transit of successive “patches” of ionization, and their frequency spectra imply the absence of turbulent plasma flows. It is postulated that in the cusp/cleft and polar cap regions, the gradient-drift instability mechanism generates the observed small-scale irregularities associated with discrete density enhancements, whereas a shear-driven instability, such as the nonlinear collisional Kelvin-Helmholtz (K-H) instability mechanism, may generate the irregularities in the intervening low-density regions.

Three-dimensional Monte Carlo calculation of the VUV electroluminescence and other electron transport parameters in xenon

Abstract: The paper presents a set of electroluminescence and other transport parameters calculated using a detailed three-dimensional Monte Carlo method, which simulates the drift of electrons in gaseous xenon (p=760 Torr, T=293 K) under reduced electric fields E/N in the 3 to 16 Td range (E/P approximately 1 to 5 V cm-1 Torr-1), which is the region for secondary scintillation production in xenon filled gas proportional scintillation counters. Results are compared with available experimental or theoretical data as well as an earlier one-dimensional Monte Carlo simulation. The calculated parameters are the excitation and scintillation efficiencies and the reduced light yield, together with mean time intervals, mean drift distances and mean number of elastic collisions between xenon excitation collisions. The authors also present electron drift velocities, mobilities and characteristic energies, as well as mean electron energy and electron energy distribution functions.

Qualitative theory of amplitude and phase fluctuations in a medium with anisotropic turbulent irregularities

Abstract: Wave propagation in an anisotropic turbulent medium is studied assuming that the anisotropy coefficient η is constant for all scales of inhomogeneities. A qualitative method of studying the amplitude and phase fluctuations is developed. Formulae have been obtained for the phase structure functions and scintillation index for the medium with spectrum power index 3<μ<5 and anisotropy coefficient η≥1. Two particular cases have been studied in detail—propagation along and across the axis of the stretching of irregularities. During propagation along a short axis of inhomogeneities the fluctuations are decreasing and the phase structure function and scintillation index turn out to be ημ−2 times smaller as compared with isotropic case. At the same time, during propagation along the long axis of inhomogeneities the phase structure function in vertical plane and the scintillation index are η times greater than in the isotropic medium.

Ionospheric Amplitude and Phase Fluctuations at the GPS Frequencies

Abstract: The F-region of the ionosphere at times becomes turbulent and develops small scale (<500 m) irregu- larities of electron density. When sufficiently intense, these irregularities scatter radio waves at L-band fre- quencies and generate amplitude and phase scintilla- tion of GPS signals. Amplitude scintillation causes cycle slips and data losses to occur and phase scintil- lation generates fast variations of frequency with which the receiver has to cope. We have utilized the available data sets to establish the constraints imposed by scin- tillation on the operation of GPS receiving systems. This will help GPS users to correctly distinguish re- ceiver problems from ionospheric scintillation. The worst source of scintillation is the equatorial anomaly region. This region corresponds to two belts, each several degrees wide, of enhanced ionization in the F-region at approximately 15” north and 15O south of the magnetic equator. In this region, during the solar maximum periods, amplitude scintillations at 1.5 GHz may exceed 20 dB for several hours after sunset. Ascension Island in the Atlantic, Diego Garcia in the Indian Ocean, Hong Kong and Taiwan in the Pacific are some of the stations that fall directly under the anomaly region. However, areas within a few degrees of the magnetic equator may show only 5-7 dB fades. Occurrence of scintillation in the equatorial region is a function of longitude. For example the region between 0” and 30” West Longitude can have severe fading from September to May while the region from 110” to 130” East has maximum occurrence in the equinoxes with minimum occurrence from November to January (in solar maximum years). The other potentially active regions are at aurora1 and polar cap latitudes. In the central polar cap in years of solar maximum, GPS receivers may suffer > 10 dB fades during the solar maximum period. Relatively rare effects of magnetic storms noted in Japan will be outlined. GPS satellites offer a unique source for measure ments of amplitude and phase scintillation on a global scale. One receiver can record scintillation magnitudes and spectra at multiple propagation paths in the over- head sky. The data can be used to study ionospheric plasma structures, develop weather models of scintil- lation and can be scaled in frequency to support many operational systems. However the interpretation of the fading structure and scintillation spectra for GPS is not straightforward since both the ionospheric motion and the velocity of the satellite control the fading rate of scintillation. With the use of more data the problem of amplitude fading could be organized for the user’s benefit.

Two dimensional detector array for CT scanners

Abstract: A CT scanner (10) includes a reconstruction processor (82) 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 visible light. An array of photodetectors (24) is mounted beneath the scintillation crystal array for converting light emitted from the scintillation crystals into electrical charge. The combination scintillation crystal, photodetector array is mounted to and preferably integral with an integrated circuit (26) having charge storage devices (32, 34). Electrical charge generated by each photodetector is integrated and stored alternately by a corresponding pair of charge storage devices. The charge storage devices operate as a double buffer in which one charge storage device accumulates charge while the other holds an accumulated charge until read out by downstream circuitry. The downstream circuitry includes a plurality of amplifiers (66) and an analog digital converter (62) integrally formed on the substrate. The analog to digital converter converts analog signals corresponding to the stored charges into digital values. The digital valves created on the detector arrays are transferred in digital form to the reconstruction processor.

Amorphous silicon pixel layers with cesium iodide converters for medical radiography

Abstract: We describe the properties of evaporated layers of cesium iodide (thallium activated) deposited on substrates that enable easy coupling to amorphous silicon pixel arrays. The CsI(Tl) layers range in thickness from 65 to 220 /spl mu/m. We used the two-boat evaporator system to deposit CsI(Tl) layers. This system ensures the formation of the scintillator film with homogenous thallium concentration which is essential for optimizing the scintillation light emission efficiency. The Tl concentration was kept to 0.1-0.2 mole percent for the highest light output. Temperature annealing can affect the microstructure as well as light output of the CsI(Tl) film. 200-360/spl deg/C temperature annealing can increase the light output by a factor of two. The amorphous silicon pixel arrays are p-i-n diodes approximately 1 /spl mu/m thick with transparent electrodes to enable them to detect the scintillation light produced by X-rays incident on the CsI(Tl). Digital radiography requires a good spatial resolution. This is accomplished by making the detector pixel size less than 50 /spl mu/m. The light emission from the CsI(Tl) is collimated by techniques involving the deposition process on patterned substrates. We have measured MTF of greater than 12 line pairs per mm at the 10% level. >

Scintillation in ground-to-space and retroreflected laser beams

Abstract: In the Relay Mirror Experiment (RME), three laser beams were propagated from ground to space and retroreflected from the RME spacecraft. An argon-ion beam at 488-nm wavelength and a Nd:YAG beam at 1064-nm wavelength were projected from a site at an altitude of 3050 m and an argon-ion beam at 514-nm wavelength was projected from a site 20 km away and at an altitude of 100 m. The beams had divergences ranging from 30 to 90 μrad. Uplink irradiances at the spacecraft and the retroreflected irradiances at the ground were measured. The data indicate that the uplink scintillation varies significantly within the Gaussian profile of the beams, and that the uplink scintillation is dominant in the retroreflected beams. The temporal spectrum of the scintillation shows a sharp roll-off beyond a characteristic corner frequency. The corner frequencies observed ranged from 400 to 700 Hz. The implications of the strength and temporal nature of the scintillation on fade statistics of the retroreflected beams are discussed.

Multi-crystal YAP: Ce detector system for position sensitive measurements

Abstract: Yttrium aluminum perovskite (YAP:Ce) scintillation crystal has a light efficiency of about 40% relative to NaI. Because of the yttrium atomic number ( Z = 39) and the relatively high density (5.37 g/cm 3 ) it has a good gamma-ray absorption. Furthermore it is not hygroscopic and is inert. Its peculiarity consists of material processing that provides us with crystal pillars down to 0.3 × 0.3 mm 2 aperture size and up to some centimeters in length. An array consisting of 11 × 22 YAP:Ce elements was made where each crystal has an aperture of 0.6 × 0.6 mm 2 and a length of 7 mm. Each scintillation crystal is optically separated by a reflective material resulting in a separation layer between elements of about 5 μm. The multicrystal detector was optically coupled to a Hamamatsu Position Sensitive Photomultiplier Tube (R2486). The intrinsic spatial resolution of the PSPMT is better than 0.3 mm but it is strongly dependent on the Point Spread Function (PSF) generated on the photocathode. The multicrystal detector very well matched the PSPMT characteristics resulting in a spatial resolution of about 0.7 mm at 140 keV ( 99m Tc) gamma irradiation.

Surface-layer scintillation measurements of daytime sensible heat and momentum fluxes

Abstract: Line-averaged measurements of the structure parameter of refractive index (C 2 ) were made using a semiconductor laser diode scintillometer above two markedly different surfaces during hours of positive net radiation. The underlying vegetation comprised in the first instance a horizontally homogeneous, pasture sward well-supplied with water, and in the second experiment, a sparse thyme canopy in a semi-arid environment. Atmospheric stability ranged between near neutral and strongly unstable (−2≤ζ≤0). The temperature structure parameterC 2 computed from the optical measurements over four decades from 0.001 to 2 K2 m−2/3 agreed to within 5% of those determined from temperature spectra in the inertial sub-range of frequencies. Spectra were obtained from a single fine thermocouple sensor positioned near the midway position of the 100m optical path and at the beam propagation height (1.5m). With the inclusion of cup anemometer measurements, rule-of-thumb assumptions about surface roughness, and Monin-Obukhov similarity theory, path-averaged optical scintillations allow calculation of surface fluxes of sensible heat and momentum via a simple iterative procedure. Excellent agreement was obtained between these fluxes and those measured directly by eddy correlation. For sensible heat, agreement was on average close to perfect over a measured range of 0 to 500 W m−2 with a residual standard deviation of 30 W m−2. Friction velocities agreed within 2% over the range 0–0.9 m s−1 (residual standard deviation of 0.06 m s−1). The results markedly increase the range of validation obtained in previous field experiments. The potential of this scintillation technique and its theoretical foundation are briefly discussed.

Scintillation properties of Lu3Al5-xScxO12 crystals

Abstract: The scintillation properties of Lu3Al5-xScxO12 single garnet crystals doped with different concentrations of Sc3+ were investigated. The best scintillation properties were obtained for the crystal with x=0.2 in the melt. This crystal has a broad (FWHM approximately 1 eV) ultraviolet emission band with a maximum at 275 nm. An energy resolution of 7% is observed for the 662 keV photopeak. The main decay component of the scintillation pulse has an exponential decay time of about 600 ns and accounts for 90% of the total light yield. The total scintillation light yield was found to be about 22500 photons/MeV. Within 10% a linear response was obtained in the energy interval from 8 keV to 1.3 MeV. The energy resolution was found to be approximately inversely proportional to the square root of the excitation energy.

New developments in cadmium tungstate

Abstract: Recent process improvements in the crystal growth of cadmium tungstate at Bicron support the production of scintillation grade material with reduced self absorption. We have reevaluated some of the properties of this material. Of particular note, we find the wavelength of the peak emission intensity to be between 475 and 480 nm and scintillation efficiency relative to NaI to be significantly higher than previously reported. >

Folding-up of the vertical atmospheric turbulence profile using an optical technique of movable observing plane

Abstract: Up to now, most of optical techniques of vertical turbulence profiling were based on the analysis of the scintillation produced at the ground level, preventing from low altitude layer detection. A simple optical combination allows us to explore the atmosphere, wiping out a particular layer while remote layers located beneath or above, are reinforced. A laboratory experiment which involves an optical bench and a simulated turbulent layer is described, and shows the potentialities of the new concept.

Phoswich detectors combining doubly or triply ZnS(Ag), NE102A, BGO and/or NaI(Tl) scintillators for simultaneous counting of α, β and γ rays

Abstract: Phoswich detectors for simultaneous counting of α, β and γ rays have been developed: ZnS(Ag)/Au Mylar/NE102A, ZnS(Ag)/Au Mylar/BGO and ZnS(Ag)/NaI(Tl) for α and β(γ) rays and ZnS(Ag)/Au Mylar/NE102A/BGO and ZnS(Ag)/NE102A/NaI(Tl) for α, β and γ rays. They were prepared by coupling a ZnS(Ag) film scintillator for α counting with a scintillator(s) for β and γ counting having different rise time. In order to adjust each component of pulse height within a given dynamic range, a sheet of Au-coated Mylar (Au Mylar) was used, if necessary, as an optical ND filter for lowering transmittance of scintillation of the ZnS(Ag). Characteristics of these phoswiches were examined by a technique of pulse-shape discrimination. Excellent discrimination among the radiations was attained and small tailings from each other peak were obtained for the prepared phoswiches.

Signal processing in scintillation cameras for nuclear medicine

Abstract: In order to average out thermal noise but to magnify small value photon signals resulting from a gamma ray event in a scintillation camera having an array of photodetectors arranged in rows and column, the photon signals are summed for each row and summed for each column as a first step in the processing of the signals. As a second step the summed signals are weighted in accordance with an approximation to an energy distribution curve, and the weighted signals are then further weighted and summed for determining the position of the gamma ray event.

Radiation detection performance of very high gain avalanche photodiodes

Abstract: Large area silicon avalanche photodiodes (APDs) have been fabricated with maximum avalanche gains exceeding 10 000 and with relatively flat signal-to-noise performance from gains of a few hundred to gains of a few thousand. Gain and noise performance as well as detector speed of response is presented for APDs with active areas of 4 mm2 and 64 mm2. An overview of the performance of these devices as radiation detectors includes pulse height spectra using the APD as a scintillation spectrometer coupled to CsI(Tl) and to plastic scintillator, and also for direct detection of low energy X-rays.

Fast readout of plastic and crystal scintillators by avalanche photodiodes

Abstract: Avalanche photodiodes (APDs) of improved quality and a sensitive area up to cm2 area are becoming commercially available now. In certain applications they can replace photomultipliers for fast readout of scintillation detectors. Expected performances will be discussed and some test measurements presented. Examples of possible applications will be given.

Path-averaged turbulent dissipation measurements using high-frequency acoustical scintillation analysis

Abstract: The path‐averaged turbulent kinetic energy dissipation (per unit mass) is estimated in a shallow tidal channel using acoustical scintillation analysis. The tidal current ensures that fully developed turbulence prevails. In order to make measurements of the turbulence parameters, a high‐frequency (67‐kHz) acoustic propagation experiment was conducted. Our acoustic data is compared to the weak scattering theory of Tatarski assuming a Kolmogorov turbulence model and compared with available oceanographic data. Analysis of log‐amplitude, phase, and phase‐difference spectra shows close agreement with the theory. Comparison of the acoustic data with direct measurement of temperature and salinity fluctuations using in situ sensors allows evaluation of the contribution of turbulent velocity fluctuations to the scintillation signal. The results show that turbulent velocity fluctuations are the dominant (∼95%) component of the observed acoustic scintillation, leading to estimates of the path‐averaged turbulent energ...

Energy-light relation for CsI(T1) scintillators in heavy ion experiments at intermediate energies

Abstract: In this paper an original energy relation for light scintillation in a CsI(T1) detector is established. It is dependent on the charge and mass of the particle (fragment) and is a direct light-to-energy function suitable for the calibration of CsI(T1) detectors used in heavy-ion studies at intermediate energies. Resulting calibration with this function and energy spectra for light ions, obtained from a heavy ion experiment, are presented.

Scintillation decay time and pulse shape discrimination of binary organic liquid scintillators for the Borexino detector

Abstract: The installation of a massive liquid scintillator detector, Borexino, devoted to the real time detection of low energy solar neutrinos, has been recently proposed at the Laboratory of Gran Sasso. The key features of this next generation solar neutrino experiment crucially rely upon the properties of the liquid scintillator used as detection medium. Thus a careful evaluation of these properties has been carried out. In particular we describe here the results of the measurements performed to determine the decay time of the scintillation light pulse for alpha and electron excitation in different solvents and solutes suitable to be used in the detector.