Showing papers on "Scintillation published in 2009"

Scintillators With Potential to Supersede Lanthanum Bromide

Abstract: New scintillators for high-resolution gamma ray spectroscopy have been identified, grown and characterized. Our development efforts have focused on two classes of high-light-yield materials: europium-doped alkaline earth halides and cerium-doped garnets. Of the halide single crystals we have grown by the Bridgman method-SrI2, CaI2, SrBr2, BaI2 and BaBr2-SrI2 is the most promising. SrI2(Eu) emits into the Eu2+ band, centered at 435 nm, with a decay time of 1.2 mus and a light yield of up to 115,000 photons/MeV. It offers energy resolution better than 3% FWHM at 662 keV, and exhibits excellent light yield proportionality. Transparent ceramic fabrication allows the production of gadolinium- and terbium-based garnets which are not growable by melt techniques due to phase instabilities. The scintillation light yields of cerium-doped ceramic garnets are high, 20,000-100,000 photons/MeV. We are developing an understanding of the mechanisms underlying energy dependent scintillation light yield non-proportionality and how it affects energy resolution. We have also identified aspects of optical design that can be optimized to enhance the energy resolution.

Propagation of vector vortex beams through a turbulent atmosphere

Abstract: We numerically study the propagation properties of vector vortex beams through a turbulent atmosphere. The irradiance pattern, degree of polarization, and scintillation index of radially polarized beam are computed for different propagation distances in an atmosphere with weak and strong turbulences. Corresponding properties of a fundamental Gaussian beam and a scalar vortex beam with topological charge of + 1 propagating through the same atmospheric turbulence conditions are calculated for comparison. With the same initial intensity profile, the vector vortex beam shows substantially lower scintillation than the scalar vortex. The existence of the vectorial vortex can be identified with longer propagation distance than the scalar vortex even with vanishing characteristic vortex structure in the irradiance images. This indicates the potential advantages of using a vector vortex beam to mitigate atmospheric effects and enable a more robust free space communication channel with longer link distance.

Climatology of GPS ionospheric scintillations over high and mid-latitude European regions

Abstract: . We analyze data of ionospheric scintillation in the geographic latitudinal range 44°–88° N during the period of October, November and December 2003 as a first step to develop a "scintillation climatology" over Northern Europe. The behavior of the scintillation occurrence as a function of the magnetic local time and of the corrected magnetic latitude is investigated to characterize the external conditions leading to scintillation scenarios. The results shown herein, obtained merging observations from four GISTM (GPS Ionospheric Scintillation and TEC Monitor), highlight also the possibility to investigate the dynamics of irregularities causing scintillation by combining the information coming from a wide range of latitudes. Our findings associate the occurrences of the ionospheric irregularities with the expected position of the auroral oval and ionospheric troughs and show similarities with the distribution in magnetic local time of the polar cap patches. The results show also the effect of ionospheric disturbances on the phase and the amplitude of the GPS signals, evidencing the different contributions of the auroral and the cusp/cap ionosphere.

Scintillation of nonuniformly polarized beams in atmospheric turbulence.

Abstract: We demonstrate, through numerical simulations, that an appropriately chosen nonuniformly polarized coherent optical field can have appreciably smaller scintillation than comparable beams of uniform polarization. This results from the fact that a nonuniformly polarized field acts as an effective two-mode partially coherent field. The results described here are of direct relevance to the development of free-space optical communication systems.

Pr3+-doped complex oxide single crystal scintillators

Abstract: We investigate and compare optical absorption, luminescence and scintillation properties of Pr-doped Y3Al5O12, Lu3Al5O12, Y2SiO5 and Lu2SiO5 single crystals. The processes determining the kinetics of the fast Pr3+ 5d–4f radiative transition are described. Among the studied host materials, only Lu3Al5O12 presents neither any 5d1–4f luminescence state nonradiative quenching nor Pr3+ ionization at room temperature. We evaluate the figure of merit of all materials for application as scintillators. The most promising system appears to be Lu3Al5O12 : Pr, since it combines an elevated density of 6.67 g cm−3, a fast scintillation response dominated by a 21 ns decay time and a spectrally corrected light yield about 160% with respect to that of Bi4Ge3O12 (BGO).

Simulating Ionosphere-Induced Scintillation for Testing GPS Receiver Phase Tracking Loops

Abstract: A simple model is proposed for simulating equatorial transionospheric radio wave scintillation. The model can be used to test Global Positioning System phase tracking loops for scintillation robustness because it captures the scintillation properties that affect such loops. In the model, scintillation amplitude is assumed to follow a Rice distribution, and the spectrum of the rapidly-varying component of complex scintillation is assumed to follow that of a low-pass second-order Butterworth filter. These assumptions are justified, and the model validated, by comparison with phase-screen-generated and empirical scintillation data in realistic tracking loop tests. The model can be mechanized as a scintillation simulator that expects only two input parameters: the scintillation index S 4 and the decorrelation time tau0. Hardware-in-the-loop tests show how the model can be used to test the scintillation robustness of any compatible GPS receiver.

Eu2+-doped Ba2CsI5, a new high-performance scintillator

Abstract: The crystal growth and scintillation properties of Ba 2 CsI 5 :Eu 2+ are reported. Crystals were produced by the vertical Bridgman technique in a sealed quartz ampoule. Ba 2 CsI 5 :Eu 2+ presents excellent scintillation properties. An estimated light yield of 97,000±5,000 photons per MeV (ph/MeV) of absorbed gamma-ray energy was measured. An energy resolution (FWHM over peak position) of 3.8±0.3% was observed for the 662 keV full absorption peak. Pulsed X-ray luminescence measurements show a relatively complex time response with four exponential decay components of 48,383, 1500 and 9900 ns with a contribution to the total light output of 1%, 26%, 68% and 25%, respectively. Under X-ray and UV excitation, the emission corresponds to a broadband centered at 2.85 eV. First principles calculations show strong localization of the excited state on the Eu site. Ba 2 CsI 5 :Eu 2+ has a density of about 5 g/cm 3 . These first reported scintillation properties make Ba 2 CsI 5 :Eu 2+ a very high-performance scintillator.

An analytical model of nonproportional scintillator light yield in terms of recombination rates

Abstract: Analytical expressions for the local light yield as a function of the local deposited energy (−dE/dx) and total scintillation yield integrated over the track of an electron of initial energy E are derived from radiative and/or nonradiative rates of first through third order in density of electronic excitations. The model is formulated in terms of rate constants, some of which can be determined independently from time-resolved spectroscopy and others estimated from measured light yield efficiency as a constraint assumed to apply in each kinetic order. The rates and parameters are used in the theory to calculate scintillation yield versus primary electron energy for comparison to published experimental results on four scintillators. Influence of the track radius on the yield is also discussed. Results are found to be qualitatively consistent with the observed scintillation light yield. The theory can be applied to any scintillator if the rates of the radiative and nonradiative processes are known.

Optical readout tracking detector concept using secondary scintillation from liquid argon generated by a thick gas electron multiplier

Abstract: For the first time secondary scintillation, generated within the holes of a thick gas electron multiplier (THGEM) immersed in liquid argon, has been observed and measured using a silicon photomultiplier device (SiPM). 250 electron-ion pairs, generated in liquid argon via the interaction of a 5.9 keV Fe-55 gamma source, were drifted under the influence of a 2.5 kV/cm field towards a 1.5 mm thickness THGEM, the local field sufficiently high to generate secondary scintillation light within the liquid as the charge traversed the central region of the THGEM hole. The resulting VUV light was incident on an immersed SiPM device coated in the waveshifter tetraphenyl butadiene (TPB), the emission spectrum peaked at 460 nm in the high quantum efficiency region of the device. For a SiPM over-voltage of 1 V, a THGEM voltage of 9.91 kV, and a drift field of 2.5 kV/cm, a total of 62±20 photoelectrons were produced at the SiPM device per Fe-55 event, corresponding to an estimated gain of 150±66 photoelectrons per drifted electron.

Tungstate and Molybdate Scintillators to Search for Dark Matter and Double Beta Decay

Abstract: Results are presented on our latest research, aimed at the development and study of oxide scintillation crystals (ZnWO4, ZnMoO4, PbWO4, PbMoO4, and MgWO4) with high scintillation yield and low intrinsic radioactivity. We report on the improvement of these properties for conventional scintillators, as well as on new promising crystals based on metal tungstates and molybdates. The results are discussed in view of applying these materials in cryogenic experiments searching for dark matter and/or neutrinoless double beta decay.

Outage probability of the Gaussian MIMO free-space optical channel with PPM

Abstract: Atmospheric effects can significantly degrade the reliability of free-space optical communications. One such effect is scintillation, caused by atmospheric turbulence, refers to random fluctuations in the irradiance and phase of the received laser beam. In this paper we investigate the use of multiple lasers and multiple apertures to mitigate scintillation. Since the scintillation process is slow, we adopt a block fading channel model and study the outage probability under the assumptions of orthogonal pulse-position modulation and non-ideal photodetection. Assuming perfect receiver channel state information (CSI), we derive the signal-to-noise ratio (SNR) exponents for the cases when the scintillation is lognormal, exponential and gamma-gamma distributed, which cover a wide range of atmospheric turbulence conditions. Furthermore, when CSI is also available at the transmitter, we illustrate very large gains in SNR are possible (in some cases larger than 15 dB) by adapting the transmitted power. Under a long-term power constraint, we outline fundamental design criteria via a simple expression that relates the required number of lasers and apertures for a given code rate and number of codeword blocks to completely remove system outages.

Development of ZnO:Ga as an ultra-fast scintillator

Abstract: We report on several methods for synthesizing the ultra-fast scintillator ZnO(Ga), and measurements of the resulting products. This material has characteristics that make it an excellent alpha detector for tagging the time and direction of individual neutrons produced by t-d and d-d neutron generators (associated particle imaging). The intensity and decay time are strongly dependent on the method used for dopant incorporation. We compare samples made by diffusion of Ga metal to samples made by solid state reaction between ZnO and Ga2O3 followed by reduction in hydrogen. The latter is much more successful and has a pure, strong near-band-edge fluorescence and an ultra-fast decay time of the x-ray-excited luminescence. The luminescence increases dramatically as the temperature is reduced to 10K. We also present results of an alternate low-temperature synthesis that produces luminescent particles with a more uniform size distribution. We examine possible mechanisms for the bright near-band-edge scintillation and favor the explanation that it is due to the recombination of Ga3+ donor electrons with ionization holes trapped on H+ ion acceptors.

Digital discrimination of neutrons and gamma-rays in liquid scintillators using wavelets

Abstract: A novel algorithm for the digital discrimination of neutrons and gamma-rays in a mixed radiation field is presented. Most of the pulse shape discrimination methods in scintillation detection systems use time-domain features of the signal (e.g. charge comparison method or constant-time discrimination). However, there are no frequency-domain discrimination methods up to date in the literature. Our method employs the wavelet transform to extract frequency-domain features for discrimination. Compared to the pulse gradient analysis (PGA) algorithm, it provides an improvement in reducing the overlap area between neutron and gamma events and also in increasing the Figure of Merit (FoM). Another advantage of this method consists in the removal of the dependency of the discrimination method on timing features. Since the light output in the scintillation process is very noisy, this kind of dependency may degrade the performance of the algorithm.

Exploration of the potential of liquid scintillators for real-time 3D dosimetry of intensity modulated proton beams

Abstract: In this study, the authors investigated the feasibility of using a 3D liquid scintillator (LS) detector system for the verification and characterization of proton beams in real time for intensity and energy-modulated proton therapy. A plastic tank filled with liquid scintillator was irradiated with pristine proton Bragg peaks. Scintillation light produced during the irradiation was measured with a CCD camera. Acquisition rates of 20 and 10 frames per second (fps) were used to image consecutive frame sequences. These measurements were then compared to ion chamber measurements and Monte Carlo simulations. The light distribution measured from the images acquired at rates of 20 and 10 fps have standard deviations of 1.1% and 0.7%, respectively, in the plateau region of the Bragg curve. Differences were seen between the raw LS signal and the ion chamber due to the quenching effects of the LS and due to the optical properties of the imaging system. The authors showed that this effect can be accounted for and corrected by Monte Carlo simulations. The liquid scintillator detector system has a good potential for performing fast proton beam verification and characterization.

Improving the GNSS positioning stochastic model in the presence of ionospheric scintillation

Abstract: Ionospheric scintillations are caused by time- varying electron density irregularities in the ionosphere, occurring more often at equatorial and high latitudes. This paper focuses exclusively on experiments undertaken in Europe, at geographic latitudes between ~50°N and ~80°N, where a network of GPS receivers capable of monitoring Total Electron Content and ionospheric scintillation parameters was deployed. The widely used ionospheric scintillation indices S4 and \({\sigma_{\varphi}}\) represent a practical measure of the intensity of amplitude and phase scintillation affecting GNSS receivers. However, they do not provide sufficient information regarding the actual tracking errors that degrade GNSS receiver performance. Suitable receiver tracking models, sensitive to ionospheric scintillation, allow the computation of the variance of the output error of the receiver PLL (Phase Locked Loop) and DLL (Delay Locked Loop), which expresses the quality of the range measurements used by the receiver to calculate user position. The ability of such models of incorporating phase and amplitude scintillation effects into the variance of these tracking errors underpins our proposed method of applying relative weights to measurements from different satellites. That gives the least squares stochastic model used for position computation a more realistic representation, vis-a-vis the otherwise ‘equal weights’ model. For pseudorange processing, relative weights were com- puted, so that a ‘scintillation-mitigated’ solution could be performed and compared to the (non-mitigated) ‘equal weights’ solution. An improvement between 17 and 38% in height accuracy was achieved when an epoch by epoch differential solution was computed over baselines ranging from 1 to 750 km. The method was then compared with alternative approaches that can be used to improve the least squares stochastic model such as weighting according to satellite elevation angle and by the inverse of the square of the standard deviation of the code/carrier divergence (sigma CCDiv). The influence of multipath effects on the proposed mitigation approach is also discussed. With the use of high rate scintillation data in addition to the scintillation indices a carrier phase based mitigated solution was also implemented and compared with the conventional solution. During a period of occurrence of high phase scintillation it was observed that problems related to ambiguity resolution can be reduced by the use of the proposed mitigated solution.

New Measurement of the Relative Scintillation Efficiency of Xenon Nuclear Recoils Below 10 keV

Abstract: Liquid xenon is an important detection medium in direct dark matter experiments, which search for low-energy nuclear recoils produced by the elastic scattering of WIMPs with quarks. The two existing measurements of the relative scintillation efficiency of nuclear recoils below 20 keV lead to inconsistent extrapolations at lower energies. This results in a different energy scale and thus sensitivity reach of liquid xenon dark matter detectors. We report a new measurement of the relative scintillation efficiency below 10 keV performed with a liquid xenon scintillation detector, optimized for maximum light collection. Greater than 95% of the interior surface of this detector was instrumented with photomultiplier tubes, giving a scintillation yield of 19.6 photoelectrons/keV electron equivalent for 122-keV {gamma} rays. We find that the relative scintillation efficiency for nuclear recoils of 5 keV is 0.14, staying constant around this value up to 10 keV. For higher energy recoils we measure a value of 0.21, consistent with previously reported data. In light of this new measurement, the XENON10 experiment's upper limits on spin-independent WIMP-nucleon cross section, which were calculated assuming a constant 0.19 relative scintillation efficiency, change from 8.8x10{sup -44} cm{sup 2} to 9.9x10{sup -44} cm{sup 2} for WIMPs of mass 100more » GeV/c{sup 2}, and from 4.5x10{sup -44} cm{sup 2} to 5.6x10{sup -44} cm{sup 2} for WIMPs of mass 30 GeV/c{sup 2}.« less

Free Space Optical System Performance for a Gaussian Beam Propagating Through Non-Kolmogorov Weak Turbulence

Abstract: Atmospheric turbulence has been described for many years by Kolmogorov's power spectral density model because of its simplicity. Unfortunately several experiments have been reported recently that show Kolmogorov theory is sometimes incomplete to describe atmospheric statistics properly, in particular in portions of the troposphere and stratosphere. It is known that free space laser system performance is limited by atmospheric turbulence. In this paper we use a non-Kolmogorov power spectrum which uses a generalized exponent instead of constant standard exponent value 11/3 and a generalized amplitude factor instead of constant value 0.033. Using this spectrum in weak turbulence, we carry out, for a Gaussian beam propagating along a horizontal path, analysis of long term beam spread, scintillation, probability of fade, mean signal to noise ratio and mean bit error rate as variation of the spectrum exponent. Our theoretical results show that for alpha values lower than alpha = 11/3, but not for alpha close to alpha = 3, there is a remarkable increase of scintillation and consequently a major penalty on the system performance. However when alpha assumes values close to alpha = 3 or for alpha values higher than alpha = 11/3 scintillation decreases leading to an improvement on the system performance.

The scintillation and ionization yield of liquid xenon for nuclear recoils

Abstract: XENON10 is an experiment designed to directly detect particle dark matter. It is a dual phase (liquid/gas) xenon time-projection chamber with 3D position imaging. Particle interactions generate a primary scintillation signal ( S 1 ) and ionization signal ( S 2 ), which are both functions of the deposited recoil energy and the incident particle type. We present a new precision measurement of the relative scintillation yield L eff and the absolute ionization yield Q y , for nuclear recoils in xenon. A dark matter particle is expected to deposit energy by scattering from a xenon nucleus. Knowledge of L eff is therefore crucial for establishing the energy threshold of the experiment; this in turn determines the sensitivity to particle dark matter. Our L eff measurement is in agreement with recent theoretical predictions above 15 keV nuclear recoil energy, and the energy threshold of the measurement is ∼ 4 keV . A knowledge of the ionization yield Q y is necessary to establish the trigger threshold of the experiment. The ionization yield Q y is measured in two ways, both in agreement with previous measurements and with a factor of 10 lower energy threshold.

An investigation of the digital discrimination of neutrons and γ rays with organic scintillation detectors using an artificial neural network

Abstract: The discrimination of neutron and γ-ray events in an organic scintillator has been investigated by using a method based on an artificial neural network (ANN) Voltage pulses arising from an EJ-301 organic liquid scintillation detector in a mixed radiation field have been recorded with a fast digital sampling oscilloscope Piled-up events have been disentangled using a pile-up management unit based on a fitting method Each individual pulse has subsequently been sent to a discrimination unit which discriminates neutron and γ-ray events with a method based on an artificial neural network This discrimination technique has been verified by the corresponding mixed-field data assessed by time of flight (TOF) It is shown that the characterization of the neutrons and photons achieved by the discrimination method based on the ANN is consistent with that afforded by TOF This approach enables events that are often as a result of scattering or pile-up to be identified and returned to the data set and affords digital discrimination of mixed radiation fields in a broad range of environments on the basis of training obtained with a single TOF dataset

Characteristics of deep gps signal fading due to ionospheric scintillation for aviation receiver design

Abstract: [1] Deep and frequent fading of Global Positioning System (GPS) signals caused by ionospheric scintillation is a major concern for aircraft navigation using GPS in the equatorial region during solar maximum. Aviation receivers use both code and carrier measurements to calculate position solutions. Deep signal fading can break a receiver's carrier tracking lock to a satellite channel. The lost channel cannot be used for position calculation until the receiver reacquires the channel and reestablishes tracking. A solar maximum data set analyzed in this paper demonstrates frequent deep signal fading of almost all satellites in view. This could significantly reduce the number of simultaneous tracked satellites and consequently decrease navigation availability. Forty-five minutes of strong scintillation, which was the worst scintillation period of a 9 day campaign at Ascension Island in 2001, are analyzed in this paper. The importance of short reacquisition time of the receiver is described. In order to design an aviation receiver with short reacquisition time under frequent deep signal fading, the characteristics of signal fading should be well understood. Fading duration and the time between deep fades are two important characteristics for GPS navigation. This paper presents a fading duration model based on real scintillation data. The time between deep fades observed in this data shows very frequent deep fades which can significantly reduce benefit of carrier smoothing filters of aviation receivers.

The timing resolution of scintillation-detector systems: Monte Carlo analysis.

Abstract: Recent advancements in fast scintillating materials and fast photomultiplier tubes (PMTs) have stimulated renewed interest in time-of-flight (TOF) positron emission tomography (PET). It is well known that the improvement in the timing resolution in PET can significantly reduce the noise variance in the reconstructed image resulting in improved image quality. In order to evaluate the timing performance of scintillation detectors used in TOF PET, we use Monte Carlo analysis to model the physical processes (crystal geometry, crystal surface finish, scintillator rise time, scintillator decay time, photoelectron yield, PMT transit time spread, PMT single-electron response, amplifier response and time pick-off method) that can contribute to the timing resolution of scintillation-detector systems. In the Monte Carlo analysis, the photoelectron emissions are modeled by a rate function, which is used to generate the photoelectron time points. The rate function, which is simulated using Geant4, represents the combined intrinsic light emissions of the scintillator and the subsequent light transport through the crystal. The PMT output signal is determined by the superposition of the PMT single-electron response resulting from the photoelectron emissions. The transit time spread and the single-electron gain variation of the PMT are modeled in the analysis. Three practical time pick-off methods are considered in the analysis. Statistically, the best timing resolution is achieved with the first photoelectron timing. The calculated timing resolution suggests that a leading edge discriminator gives better timing performance than a constant fraction discriminator and produces comparable results when a two-threshold or three-threshold discriminator is used. For a typical PMT, the effect of detector noise on the timing resolution is negligible. The calculated timing resolution is found to improve with increasing mean photoelectron yield, decreasing scintillator decay time and decreasing transit time spread. However, only substantial improvement in the timing resolution is obtained with improved transit time spread if the first photoelectron timing is less than the transit time spread. While the calculated timing performance does not seem to be affected by the pixel size of the crystal, it improves for an etched crystal compared to a polished crystal. In addition, the calculated timing resolution degrades with increasing crystal length. These observations can be explained by studying the initial photoelectron rate. Experimental measurements provide reasonably good agreement with the calculated timing resolution. The Monte Carlo analysis developed in this work will allow us to optimize the scintillation detectors for timing and to understand the physical factors limiting their performance.

Novel γ‐ and X‐ray scintillator research: on the emission wavelength, light yield and time response of Ce3+ doped halide scintillators

Abstract: This work reviews the scintillation properties of many Ce3+ doped halide compounds studied during the past two decades with the aim to find new relationships between scintillation performance and materials properties. Three major types of compounds are reviewed; lanthanide trihalides LnX3, (pseudo)-elpasolites M2ALnX6 and ternary halides Am Lnn Xo where M and A are alkali cations, Ln is a rare earth cation and X is a halide anion. We will address the relationship of the emission wavelength of Ce3+ and its Stokes shift with the structure, type of anions, coordination number, band gap etc. A comparison between the observed absolute scintillation yield with the fundamental limits will be made. Scintillation losses due to thermal ionization processes and the relationship between Stokes shift, self absorption, and decay time lengthening are treated. Various trends will be identified that may guide us in the continuing quest for better scintillators. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Temperature behavior of NaI (Tl) scintillation detectors

Abstract: It is a familiar fact that the total measured light yield of NaI(Tl) detectors is a nonlinear function of temperature. Here we present new experimental data for the temperature behavior of doped NaI(Tl) scintillators that instead shows a linear dependence of the light output over a wide temperature range—including that for outdoor applications. The shape of the light pulse shows, in general, two decay processes: a single dominant process (one decay-time constant) above the room temperature and two processes (two decay-time constants) below the room temperature. We show that the redistribution of the intensities between the two processes is temperature dependent; the second (slow) decay component is negligible at room temperatures, but by −20 °C, it contributes up to 40% of the total light and has a duration of several microseconds. We discuss the profound effect this new understanding of the light output has on the pulse-height analysis instrumentation. We introduce a theoretical model to explain the experimental results. In addition, we describe a unique technique for correcting both amplitude and shape temperature changes inside the NaI(Tl) detector package.

Potentials of Digitally Sampling Scintillation Pulses in Timing Determination in PET

Abstract: We investigate the potentials of digitally sampling scintillation pulses techniques for positron emission tomography (PET) in this paper, focusing on the determination of the event time We have built, and continue building, a digital library of PET event waveforms generated with various combinations of photo-detectors and scintillator materials, with various crystal sizes Events in this digital library are obtained at a high sampling of 20 GSps (Giga-samples per second) so that their waveforms are recorded with high accuracy To explore the potential advantages of digitally sampling scintillation pulses, we employ a dataset in the above-mentioned library to evaluate two methods for digitizing the event pulses and linear interpolation techniques to analyze the resulting digital samples Our results show that the two digitization methods that we studied can yield a coincidence timing resolution of about 300 ps FWHM when applied to events generated by a pair of LSO + PMT detector units This timing resolution is comparable with that is achieved by the same detector pair with a constant fraction discriminator (CFD) As a benchmark, regular-time sampling (RTS) method, usually implemented with very fast traditional analog-to-digital converters (ADCs) for digitizing scintillation pulses, is not feasible for a multi-channel system like a PET system Digitizing scintillation pulses with multi-voltage threshold (MVT) method could be implemented at a reasonable cost for a PET system With digitized PET event samples, various digital signal processing (DSP) techniques can be implemented to determine event arrival time Our results have therefore demonstrated the promising potentials of digitally sampling scintillation pulses techniques in PET imaging

Ultra precise timing with SiPM-based TOF PET scintillation detectors

Abstract: The combination of SiPMs with fast and bright scintillators, such as LaBr 3 :Ce, seems very promising for application in time-of-flight (TOF) PET We therefore conducted a series of experiments with the goal of obtaining the best possible timing resolution with SiPM-based scintillation detectors in order to establish a bench mark for future experiments with different detector designs The detectors employed in our measurements consisted of two SiPMs (Hamamatsu MPPC-S10362-33-050C), which were directly coupled to small scintillation crystals, viz LaBr 3 :Ce and LYSO An excellent coincidence resolving time (CRT) for 22Na 511 annihilation photons of 995 ps ± 06 ps FWHM could be achieved at the optimized electronics and digitizer settings with two LaBr 3 :5%Ce crystals A CRT of 1715 ps ± 08 ps FWHM was obtained with L(Y)SO crystals These results compare well to the predictions of a statistical model which was developed to describe the timing performance of SiPM based scintillation detectors

Scintillation Index of Modified Bessel–Gaussian Beams Propagating in Turbulent Media

Abstract: The scintillation index is formulated for modified Bessel-Gaussian beams propagating in weakly turbulent media. Numerical calculations applied directly to the derived triple integral show that, for off-axis positions, the modified Bessel-Gaussian beams of higher than zero order scintillate less than Gaussian beams at large input beam sizes and low beam orders with the increasing width parameter initially contributing positively to this phenomenon of less scintillation. As the beam order exceeds two, this advantage is diminished. The modified Bessel-Gaussian beam of order zero is a special case, however, exhibiting lowest scintillation at small input beam sizes. When considered against the propagation length, higher-order modified Bessel-Gaussian beams continue to offer less scintillation than those of order zero. At various radial positions, the scintillation index of modified Bessel-Gaussian beams with orders higher than zero attains small values toward the beam edges but rises sharply when approaching the beam axis. The effect of inner and outer scales of turbulence is also studied, and it is found that while increasing the inner scale of turbulence seems to cause increases in scintillation, the influence of the outer scale is hardly noticeable.

Scintillation from Eu2+ in Nanocrystallized Glass

Abstract: A Eu 2+ -doped SiO 2 ―Al 2 O 3 ―CaO―CaF 2 glass was prepared and converted into a transparent glass ceramic by heat treatment. The crystalline phase and its size were determined by X-ray diffraction and a transmission electron microscopy. The scintillation of Eu 2+ ions in both glass and ceramic under X-ray excitation was investigated and compared with that in a single-crystal scintillator.

SrI2 scintillator for gamma ray spectroscopy

Abstract: We are working to perfect the growth of divalent Eu-doped strontium iodide single crystals and to optimize the design of SrI2(Eu)-based gamma ray spectrometers. SrI2(Eu) offers a light yield in excess of 100,000 photons/MeV and light yield proportionality surpassing that of Ce-doped lanthanum bromide. Thermal and x-ray diffraction analyses of SrI2 and EuI2 indicate an excellent match in melting and crystallographic parameters, and very modest thermal expansion anisotropy. We have demonstrated energy resolution with SrI2(4-6%Eu) of 2.6% at 662 keV and 7.6% at 60 keV with small crystals, while the resolution degrades somewhat for larger sizes. Our experiments suggest that digital techniques may be useful in improving the energy resolution in large crystals impaired by light-trapping, in which scintillation light is re-absorbed and re-emitted in large and/or highly Eu2+ -doped crystals. The light yield proportionality of SrI2(Eu) is found to be superior to that of other known scintillator materials, such as LaBr3(Ce) and NaI(Tl).

Characterization of large area APDs for the EXO-200 detector

Abstract: EXO-200 uses 468 large area avalanche photodiodes (LAAPDs) for detection of scintillation light in an ultra-lowbackground liquid xenon (LXe) detector. We describe initial measurements of dark noise, gain and response to xenon scintillation light of LAAPDs at temperatures from room temperature to 169 K—the temperature of liquid xenon. We also describe the individual characterization of more than 800 LAAPDs for selective installation in the EXO-200 detector.