Showing papers on "Scintillation published in 1995"

An improved model of equatorial scintillation

Abstract: One of the main limitations of the modeling work that went into the equatorial section of the Wideband ionospheric scintillation model (WBMOD) was that the data set used in the modeling was limited to two stations near the dip equator (Ancon, Peru, and Kwajalein Island, in the North Pacific Ocean) at two fixed local times (nominally 1000 and 2200). Over the past year this section of the WBMOD model has been replaced by a model developed using data from three additional stations (Ascension Island, in the South Atlantic Ocean, Huancayo, Peru, and Manila, Phillipines; data collected under the auspices of the USAF Phillips Laboratory Geophysics Directorate) which provide a greater diversity in both latitude and longitude, as well as cover the entire day. The new model includes variations with latitude, local time, longitude, season, solar epoch, and geomagnetic activity levels. The way in which the irregularity strength parameter C k L is modeled has also been changed. The new model provides the variation of the full probability distribution function (PDF) of log(C k L) rather than simply the average of log(C k L). This permits the user to specify a threshold on scintillation level, and the model will calculate the percent of the time that scintillation will exceed that level in the user-specified scenario. It will also permit calculation of scintillation levels at a user-specified percentile. A final improvement to the WBMOD model is the implementation of a new theory for calculating S 4 on a two-way channel

Scintillation properties of YAP:Ce

Abstract: Yttrium aluminium perovskite activated by cerium (formula YAlO 3 :Ce, abbreviated YAP:Ce) is systematically investigated in view of its use as efficient scintillator for various applications. The basic physical and chemical properties of YAP:Ce monocrystals are summarized in this paper. The luminescence, scintillation and absorption characteristics, the γ-ray detection capability, energy resolution in the range of 30–600 keV, light yield and radiation hardness are presented and compared with existing literature.

Benchmarking the Compton coincidence technique for measuring electron response non-proportionality in inorganic scintillators

Abstract: To study the light yield non-proportionality of inorganic scintillation materials, a Compton coincidence experiment has been designed and implemented. The coincidence technique is used to measure the nearly mono-energetic electron response by recording events only when energetic electrons are produced by gamma rays that are Compton scattered through a specific angle. This technique provides the ability to accurately determine the light yield non-proportionality of scintillation materials as a function electron energy while minimizing the potential effects of surface interaction and X-ray escape. To benchmark the Compton coincidence technique (CCT), the electron response for NaI(Tl) has been measured for electron energies from 2 keV to 450 keV and compared to previously-published analytical and measured light yield response data for electrons. The CCT data represent the most accurate electron response measurement to date on NaI(Tl) for energies below 20 keV. With the CCT benchmarked, the electron response non-proportionality of other scintillators can be analyzed.

A study of inter-crystal scatter in small scintillator arrays designed for high resolution PET imaging

Abstract: Inter-crystal scatter causes mispositioning of scintillation events, which is of particular concern in imaging detectors based on small discrete scintillator elements. Because it is difficult to measure the scatter and its effects on detector intrinsic spatial resolution, a Monte Carlo simulation has been used to study inter-crystal scatter effects for evaluating and optimizing the design of a high resolution animal PET detector based on an array of small scintillator crystals. In this simulation, the authors quantitatively assess the mispositioning of events due to inter-crystal scatter as a function of parameters such as different scintillator materials, crystal geometry, /spl gamma/-ray incident angle and applied energy threshold. In analyzing the tradeoff between the detector efficiency and the position detection accuracy, the authors found that the mispositioning is not sensitive to the energy threshold, however it does change rapidly with the crystal length and the gap between crystals. The authors also compared four different crystal positioning algorithms to provide a theoretical estimate of positioning accuracy and to determine the best algorithm to use. To study how inter-crystal scatter affects detector spatial resolution, the authors analyzed the coincidence line spread function with and without inter-crystal scatter and found that the inter-crystal scatter had very little effect on the FWHM and FWTM of the coincidence line spread function.

Atmospheric-compensation experiments in strong-scintillation conditions.

Abstract: Most atmospheric-turbulence-compensation experiments have been performed under weak-scintillation conditions; conventional phase-conjugate adaptive-optics systems usually provide good correction for these conditions. We have performed an experiment over a 5.5-km horizontal propagation path to explore the efficacy of conventional adaptive optics in strong-scintillation conditions. The experimental results showed a significant degradation in correction as the scintillation increased. The presence of branch points in the phase appears to be the primary reason for the degradation in correction as the scintillation increases.

LuAlO/sub 3/:Ce-a high density, high speed scintillator for gamma detection

Abstract: We present measurements of the scintillation properties of cerium doped lutetium aluminum perovskite, LuAlO/sub 3/:Ce, a new dense (/spl rho/=8.34 g/cm/sup 3/) inorganic scintillator. This material has a 511 keV interaction length and photoelectric fraction of 1.1 cm and 32% respectively, which are well suited to gamma ray detection. In powdered form with 0.5% cerium concentration, the scintillation light output is estimated to be 9,600 photons/MeV of deposited energy, the emission spectrum is a single peak centered at 390 nm, and the fluorescence lifetime is described by the sum of 3 exponential terms, with 60% of the light being emitted with a 11 ns decay time, 26% with a 28 ns decay time, and 13% with a 835 ns decay time. Single crystals contaminated with /spl ap/10% lutetium aluminum garnet (Lu/sub 3/Al/sub 5/O/sub 12/) have significantly altered scintillation properties. The light output is 26,000 photons/MeV (3.2 times that of BGO), but the decay time increases significantly (1% of the light is emitted with a 10 ns decay time, 15% with a 245 ns decay time, and 85% with a 2010 ns decay time) and the emission spectrum is dominated by a peak centered at 315 nm with a secondary peak centered at 500 nm. The short decay lifetime, high density, and reasonable light output of LuAlO/sub 3/:Ce (the perovskite phase) suggest that it is useful for applications where high counting rates, good stopping power, good energy resolution, and fast timing are important. However, it is necessary to grow single crystals that are uncontaminated by the garnet phase to realize these properties. >

Collection of scintillation light from small BGO crystals

Abstract: We propose to develop a high resolution positron emission tomography (PET) detector designed for animal imaging. The detector consists of a 2-D array of small bismuth germanate (BGO) crystals coupled via optical fibers to a multi-channel photomultiplier tube (MC-PMT). Though this approach offers several advantages over the conventional BGO block design, it does require that a sufficient number of scintillation photons be transported from the crystal, down the fiber and into the PMT. In this study we use simulations and experimental data to determine how to maximize the signal reaching the PMT. This involves investigating factors such as crystal geometry, crystal surface treatment, the use of reflectors, choice of optical fiber, coupling of crystal to the optical fiber and optical fiber properties. Our results indicate that using 2/spl times/2/spl times/10 mm BGO crystals coupled to 30 cm of clad optical fiber, roughly 50 photoelectrons are produced at the PMT photocathode for a 511 keV interaction. This is sufficient to clearly visualize the photopeak and provide adequate timing resolution for PET. Based on these encouraging results, a prototype detector will now be constructed. >

The scintillation of CF4 and its relevance to detection science

Abstract: The scintillation properties of CF4 are presented in comparison with those of Xe and CH4. Alpha-particle induced photon emission was measured with vacuum phototubes and with a CsI-based gaseous photomultiplier. The latter method provides an absolute sensitivity of such devices to particle-induced UV-photon background in CF4 and CH4 gaseous Cherenkov radiators. Integrated CF4 scintillation yields over the range of 150–220 nm are, on the average, 315±95 to 242±60 photons/MeV, in the respective pressure range of 0.063 to 0.75 atm, compared to CH4 which emits 0.06±0.01 photons/MeV at 1 atm. The total photon yield, integrated over the full emission spectrum of CF4 (150–500 nm), is of the order of 1200 photons/MeV × 4 π. The primary scintillation photon yield of CF4 is about 16(±5)% of that of Xe. No proportional secondary scintillation was observed in CF4. The avalanche-induced photon yield was measured to be of the order of 0.3 photons per electron. The implications of this considerable photon emission, are discussed.

Origin of the 4.1-eV luminescence in pure CsI scintillator

Abstract: The spectra and decay times of the 4.3- and 3.7-eV luminescence bands associated with the self-trapped excitons (STE's) in CsI have been observed in a wide temperature range (4.5--300 K) by using two-photon spectroscopy. The results reveal that the 4.1-eV luminescence in pure CsI, which is well known as the room-temperature scintillation, appears mainly from the 4.3-eV STE state: the 4.3-eV STE luminescence is dominant below 6 K and is quenched above 6 K, but appears again at 4.1 eV at room temperature by thermal activation of STE's from the 3.7-eV STE state to the 4.3-eV STE state.

Measurement of Fragment Mass Dependent Kinetic Energy and Neutron Multiplicity for Thermal Neutron Induced Fission of Plutonium-239

Abstract: Kinetic energy and neutron multiplicity as a function of fragment mass were measured for the thermal neutron induced fission of 239Pu. By measuring the velocities and energies of two fission fragments simultaneously, both of the pre-neutron emission fragment mass m* and the post-neutron mass m were obtained. The fragment mass dependent neutron multiplicity ν(m*) was deduced by subtracting m from m*. The fragment mass dependent total kinetic energy TKE(m*) was also obtained from this data. The fragment velocity was measured by time-of-flight (TOF) method, for which the start signal was triggered by a very thin plastic scintillation film detector (TFD) and the stop signal was obtained by a silicon surface barrier detector (SSBD) which was also used for the measurement of [he fragment kinetic energy. The present result of ν(m*) is in good agreement with that of Apalin et al. and that in the light fragment region of Fraser et al. The obtained TKE(m*) agrees well with the data of Wagemans et al. A calculation ...

Solar wind velocity and normalized scintillation index from single-station IPS observations

Abstract: The recently refurbished Ooty Radio Telescope in southern India was used in a two-month campaign of interplanetary scintillation (IPS) observations in collaboration with the Cambridge IPS array in England during April–May 1992. The unique feature of this campaign was that, for the first time, scintillation enhancements were predicted in real time by observing solar events on 7–8 May, 1992 and then detected at both Ooty and Cambridge. Also, for the first time, high spatial resolution (∼ 100 sources sr−1) solar wind all-sky velocity maps were obtained at Ooty. Good consistency is found between the IPS observations from both observatories andin-situ shocks detected at Earth by IMP-8.Yohkoh soft X-ray images were used to infer the generation of a coronal mass ejection on 7 May, 1992.

Organic scintillator systems and optical fibers containing polycyclic aromatic compounds

Abstract: A class of polycyclic aromatic compounds containing at least two fused rings, is used to produce very bright liquid or plastic scintillators. When a member of this class, such as pyrene, is added at high concentration to a liquid or polymeric organic solvent, scintillation light output may be increased by between 40 and 100% as compared to existing commercial scintillating compositions. These new bright scintillator compositions can be used in scintillating optical fibers and plates made therefrom. These new scintillating optical fiber plates are particularly useful for diagnostic medical X-ray detection, and superior spatial resolution particle detection and measurement of high energy particles and radiation.

Optimization and Performance of Adaptive Optics for Imaging Extrasolar Planets

Abstract: A recent study by Angel (1994) using simplified analytical models indicated the feasibility of imaging extrasolar planets from the ground, making use of adaptive optics correction of a large telescope We have performed detailed simulations of the method using computer codes that model propagation through atmospheric turbulence, adaptive correction, and broadband imaging We confirm that high-resolution correction at the limit of photon noise errors reduces the halo intensity to 10-6 of the peak star flux Our work shows how to avoid systematic errors Thus, we find that time delays between sensing the wave front and its detection lead to persistent structure in the stellar halo, which uncorrected would prevent rapid averaging of the residual halo speckle structure A local wave front reconstructor that extrapolates ahead in time has been devised to remove this problem We find the chromatic differences in wave front structure are small enough that the signal-to-noise ratio can be improved by wave front sensing and imaging in separate adjacent bands We verify that correction of amplitude scintillation is needed and the optimum level of clipping is derived A simulated image of a twin of the solar system at 8 pc is presented for the new 65 m telescope and the measured turbulence at the Multiple Mirror Telescope site The Jupiter twin shows up at the 5 σ level in a 5 hr integration

Turbulence-induced scintillation on Gaussian-beam waves: theoretical predictions and observations from a laser-illuminated satellite

Abstract: Expressions for the variance and the power spectral density of turbulence-induced log-amplitude fluctuations are derived for Gaussian-beam waves in the regime of weak scattering. This formulation includes effects that are due to turbulence strength variations along the propagation path, offset of the observation point from the beam axis, and sensitivity to focus and beam diameter. Comparison of theoretical results with observed scintillation during experiments with a laser-illuminated satellite reveals good agreement.

Origin of scintillation in cerium‐doped oxide crystals

Abstract: We propose here a model of the scintillation from complex oxide single crystals doped with Ce3+ ions to explain the correlation observed between scintillation light yield and intrinsic luminescence of the matrix.

Attenuation and scintillation of radio waves in the Earth's atmosphere from radio occultation experiments on satellite-to-satellite links

Abstract: A theoretical analysis of refractive loss of radio waves by the Earth's atmosphere in radio occultation measurements along the satellite-to-satellite line for various altitude profiles of the refractive index is given. Experimental results for refractive loss on the orbital spacecraft - geostationary satellite link are presented. Theoretical calculations are compared with experimental data, and a conclusion is drawn that the signal amplitude during radio occultation is strongly dependent on the layered structure of the refractive index profile. Amplitude scintillations of centimeter (λ1 = 2 cm ) and decimeter (λ2 = 32 cm ) radio waves used in radio occultation experiments are described. Dependences of the rms value of the amplitude scintillations versus the minimum altitude of the ray line for the two above wavelength bands are presented. The frequency spectra of the log-amplitude scintillations are analyzed together with the dependence of the rms amplitude on the wavelength. Experimental data are compared to the theory of scintillations in a turbulent atmosphere, and the altitude model of the structure constant of refractivity fluctuations is determined.

Fine-Scale Filamentary Structure in Coronal Streamers

Abstract: Doppler scintillation measurements of a coronal streamer lasting several solar rotations have been conducted by Ulysses in 1991 over a heliocentric distance range of 14-77 R(sub 0). By showing that the solar corona is filamentary, and that Doppler frequency is the radio counterpart of white-light eclipse pictures processed to enhance spatial gradients, it is demonstrated that Doppler scintillation measurements provide the high spatial resolution that has long eluded white-light coronagraph measurements. The region of enhanced scintillation, spanning an angular extent of 1.8 deg in heliographic longitude, coincides with the radially expanding streamer stalk and represents filamentary structure with scale sizes at least as small as 340 km (0.5 sec) when extrapolated to the Sun. Within the stalk of the streamer, the fine-scale structure corresponding to scale sizes in the range of 20-340 km at the Sun and associated with closed magnetic fields amounts to a few percent of the mean density, while outside the stalk, the fine-scale structure associated with open fields is an order of magnitude lower. Clustering of filamentary structure that takes place within the stalk of the streamer is suggestive of multiple current sheets. Comparison with ISEE 3 in situ plasma measurements shows that significant evolution resulting from dynamic interaction with increasing heliocentric distance takes place by the time streamers reach Earth orbit.

Scintillation response of nuclear particle detectors

Abstract: We derive simple algebraic expressions for the ion-induced light output response of most of the popular scintillation detectors used in nuclear and particle physics. The analytical calculation is based on a model for the energy deposition by secondary electrons scattered along the track of the ion, and the subsequent energy transport to luminescence centers. Predictions are compared with published experimental data for various scintillating materials over a wide range of incident ions and energies.

Toward a nuclear medicine with sub-millimiter spatial resolution

Abstract: The HIRESPET Collaboration is developing a new concept of a gamma camera with sub-millimiter spatial resolution. The first prototype consists of a small field size gamma camera based on a Position Sensitive Photo-Multiplier Tube (PSPMT) coupled to a novel scintillation crystal. The intrinsic spatial resolution of the PSPMT is better than 0.3 mm. The scintillation crystal consists of yttrium aluminium perovskit (YAP:Ce). It has a light efficiency of about 40% relative to NaI, a good gamma radiation absorption ( Z = 39) and a high density (5.37 g/cm 3 ). It is inert and not hygroscopic. To match the PSPMT characteristics, a special crystal assembly has been made consisting of a bundle of YAP pillars, where a single crystal has the transversal dimension of 0.6 × 0.6 mm 2 and a thickness ranging between 1 mm and 28 mm. Each scintillation pillar is optically separated from the other by a reflective layer of 5 μm thick. The preliminary results obtained from the gamma camera prototype (YAP camera) show spatial resolution values ranging between 0.6 mm and 1 mm and an intrinsic detection efficiency comparable with a standard Anger camera.

Distribution type detector using scintillation fibers

Abstract: A distribution type detector comprises scintillation fibers (102a,102b) identical in length to each other, an optical delay fiber (103) having a refractive index substantially identical to those of cores and claddings of the scintillation fibers, photosensitive elements (104a,104b), preamplifiers (105a,105b) constant fraction discriminators (106a,106b), a time-to-pulse height converter (107), an analog-to-digital converter (108), and a multichannel pulse-height analyzer (109). A position where a radiation (101) falls on its corresponding scintillation fiber, is detected based on a difference between time intervals necessary for propagation of optical pulses produced in the corresponding scintillation fiber by the radiation. Thus, even if the length of each scintillation fiber is increased, position resolution can be kept high and a measuring circuit system can be simplified.

YAP multi-crystal gamma camera prototype

Abstract: The Anger camera principle has shown a practical limit of a few millimeters spatial resolution. To overcome this limit, a new gamma camera prototype has been developed, based on a position-sensitive photomultiplier tube (PSPMT) coupled with a new scintillation crystal. The Hamamatsu R2486 PSPMT is a 76-mm diameter photomultiplier tube in which the electrons produced in the conventional bi-alkali photocathode are multiplied by proximity mesh dynodes and form a charge cloud around the original coordinates of the light photon striking the photocathode. A crossed wire anode array collects the charge and detects the original position. The intrinsic spatial resolution of PSPMT is better than 0.3 mm. The scintillation crystal consists of yttrium aluminum perovskit (YAP:Ce or YAlO/sub 3/:Ce). This crystal has a light efficiency of about 38% relative to NaI, no hygroscopicity and a good gamma radiation absorption. To match the characteristics of the PSPMT, a special crystal assembly was produced by the Preciosa Company, consisting of a bundle of YAP:Ce pillars where single crystals have 0.6/spl times/0.6 mm/sup 2/ cross section and 3 mm to 18 mm length. Preliminary results from such gamma camera prototypes show spatial resolution values ranging between 0.7 mm and 1 mm with an intrinsic detection efficiency of 37/spl divide/65% for 140 keV gamma energy. >

Scintillation characteristics of GSO single crystal grown under O/sub 2/-containing atmosphere

Abstract: A Ce doped Gd/sub 2/SiO/sub 5/ (GSO) single crystal is an excellent scintillator featuring a large light output, a short decay constant and a high absorption coefficient. We have investigated dependence of the scintillation characteristics on the growth atmosphere. GSO crystal is usually grown in pure N/sub 2/, which due to thermal etching during the growth always roughens the boule surface, inducing some micro-cracks on the surface which often triggers the large crack in the boule. To eliminate this, we first introduced 1 vol% O/sub 2/-containing N/sub 2/ gas for the growth atmosphere. The resultant boule turned to be very smooth in surface, but pale yellow in color and degraded in the scintillation characteristics. To retain these properties, we then introduced an annealing process and found the annealing under pure N/sub 2/ atmosphere quite effective on the recovery both in color and scintillation properties. >

Density fluctuations in different types of solar wind flow at 1 AU and comparison with results from Doppler scintillation measurements near the Sun

Abstract: Density fluctuations with periods in the range 10 min to 1 hour are investigated using ISEE 3 plasma measurements of solar wind flows at 1 AU. Coronal hole, interstream, plasma sheet, coronal mass ejection, and interaction region flow types are considered. The ISEE 3 results are consistent with the interpretation of large-scale variations in density fluctuations observed by Doppler scintillation measurements inside 0.2 AU. The highest absolute and fractional density fluctuations are found ahead of and within the plasma from coronal mass ejections, with the maximum values occurring between the associated interplanetary shocks and the driver gas. For the quasi-stationary solar wind, density and fractional density fluctuations are highest and most variable in the plasma sheet in which the heliospheric current sheet is embedded, and lowest and least variable in the high-speed coronal hole flow. A superposed epoch analysis of heliospheric current sheet crossings shows that the regions of enhanced density fluctuations previously identified in Doppler scintillation measurements near the Sun persist to 1 AU and are broader there. The results of this study confirm the advantages of using density fluctuations rather than density as a tracer of solar wind flows of differing origins at the Sun and as a detector of propagating interplanetary disturbances.

X-ray imaging device

Abstract: An x-ray imaging device One surface of a flat single crystal CsI crystal is supported on an optically transparent support plate The opposite surface, ie an x-ray illumination surface of the crystal is coated with an x-ray transparent optical reflector to provide an x-ray scintillation sandwich having an optical mirror at the x-ray illumination surface of the CsI crystal An optical camera is preferably focused on the illumination surface of the CsI crystal In a preferred embodiment an index of refraction matched optical adhesive is used at the x-ray illumination surface to attach the reflector and to reduce Fresnel reflections

Properties and mechanisms of scintillation in CsGd2F7:Ce3+ and CsY2F7:Ce3+ crystals

Abstract: The scintillation properties of Ce3+ doped CsGd2F7 and CsY2F7 crystals have been studied at various temperatures Upon X-ray excitation CsY2F7:5 mol% Ce3+ shows fast ( tau approximately=32 ns) Ce3+ 5d to 4f luminescence with a yield of 1400 photons MeV-1 at room temperature The most intense Ce3+ luminescence of about 7300 photons MeV-1 is found in CsGd2F7:20 mol% Ce3+ The scintillation decay curve of CsGd2F7:Ce3+ consists of at least two non-exponential components, one of the order of tens of nanoseconds and one in the microsecond region Both CsY2F7 and CsGd2F7 show a broad STE emission band at around 445 nm, especially at low temperatures VUV photon excitation measurements at 300 K and low temperatures revealed the presence of two inequivalent types of Ce3+ centres in both CsY2F7 and CsGd2F7 The presence of Gd3+ appears to have a dominating influence on the luminescence characteristics of CsGd2F7:Ce3+, due to the occurrence of both Ce3+ to Gd3+ and Gd3+ to Ce3+ energy transfer as well as energy migration over the Gd3+ sublattice On the basis of the optical studies, models of the scintillation mechanisms of CsY2F7 and CsGd2F7 are developed which account for the scintillation properties of these crystals

Dual crystal scintillation camera

Abstract: A scintillation camera is provided with a scintillation crystal assembly having multiple crystal layers for interacting with various photon energy levels. The camera performs imaging of conventional nuclear medicine radioisotopes as well imaging of high energy isotopes used in PET (Positron Emission Tomography) applications. The multiple crystal layers have the effect of doubling the sensitivity of the camera to high energy photons, while retaining the performance characteristics needed for conventional low energy photon imaging.

High efficiency CsI(Tl)/HgI/sub 2/ gamma ray spectrometers

Abstract: CsI(Tl)/HgI/sub 2/ gamma-ray spectrometers have been constructed using 0.5" diameter detectors which show excellent energy resolution: 4.58% FWHM for 662 keV /sup 137/Cs gamma-ray photons. Further efforts have been focused on optimization of larger size (/spl ges/1" diameter) detector structures and improvement of low noise electronics. In order to take full advantage of scintillation detectors for high energy gamma-rays, larger scintillators are always preferred for their higher detection efficiencies. However the larger capacitance and higher dark current caused by the larger size of the detector could result in a higher FWHM resolution. Also, the increased probability of including nonuniformities in larger pieces of crystals makes it more difficult to obtain the high resolutions one obtains from small detectors. Thus for very large volume scintillators, it may necessary to employ a photodiode (PD) with a sensitive area smaller than the cross-section of the scintillator. Monte Carlo simulations of the light collection. For various tapered scintillator/PD configuration were performed in order to find those geometries which resulted in the best light collection. According to the simulation results, scintillators with the most favorable geometry, the conical frustum, have been fabricated and evaluated. The response of a large conical frustum (top 2" bottom-1" 2" high) CsI(Tl) scintillator coupled with a 1" HgI/sub 2/ PD was measured. The energy resolution of the 662 keV peak was 5.57%. The spectrum shows much higher detection efficiency than those from smaller scintillators, i.e., much higher peak-to-Compton ratio in the spectrum. >