Search

We have developed a computational model of energy transfer processes in scintillators using the kinetic Monte Carlo (KMC) approach. In this publication, we focus on the alkali halide compound CsI both pure and doped with a range of thallium concentrations. The KMC model makes use of an explicit atomistic representation of the crystal lattice, activator sites, defect sites, and individual electron-hole pairs. The probability of individual diffusion, recombination, and scintillation events is calculated from rate equations parameterized with data published in the literature. Scintillation decay curves, relative intensities of emission peaks, and light yields are computed and found to be in good agreement with experimental data for a range of temperatures and thallium concentrations. This demonstrates that the KMC scintillation model is capable of reproducing both the kinetics and the efficiency of the scintillation process in CsI. In addition, novel predictions emerge from our simulations such as the diffusion distance distributions of serf-trapped holes and excitous. Finally, the KMC scintillation model provides a framework for probing possible physical processes responsible for the nonlinear relationship between scintillation light yield and incident gamma-ray energy. Index Terms--Alkali halides, cesium iodide, emission spectra, gamma-ray radiation, kinetic Monte Carlo, light yield, nonlinearity, scintillation decay curves, scintillation mechanisms, thallium.

This paper presents new development methods for property-screening design rules, using structure-property relationships for two fundamental properties of activated scintillating based gamma radiation detection-luminosity and stopping power. The first and most evident goal in developing screening models of luminosity and stopping power, as indicated by the weight and electron densities, is to obtain new candidate cerium scintillating materials. However, a second and more strategic goal is to extract design rules, which define the structural limitations on materials consistent with desirable detector properties. These design rules are based on our capability to predict the luminescence and stopping power of a material from a set of structural descriptors. Predictive models are generated using statistical multiple linear regression over a set of 24 descriptors. We find that within a set of ten cerium-doped scintillator materials we can quantitatively predict luminosity with a correlation coefficient of ~ 0.94 based on 4 of the 24 descriptors, improving to ~ 0.99 with 6 descriptors; and electron density to ~ 0.99 with 3 descriptors. Furthermore, we show in this circumstance that the luminosity and stopping power are only nominally related. In particular, luminosity depends largely on matrix valence electron properties and their coupling to activator sites-properties that do not require high atomic masses or atomic numbers per se, requirements for high stopping power. Index Terms--Density, efficiency, informatics, luminescence, scintillation detectors.

Effect of Ga substitution in the [(Lu, Pr).sub.3][Al.sub.5][O.sub.12](Pr : LuAG) scintillator was examined at the crystals grown by the micropulling down ([mu]-PD) method. Host luminescence due to an exciton localized around the Lu-Al antisite defect was suppressed by Ga admixture. Concentration depenendence was investigated and suppression was observed even at the lowest Ga concentration of 5 mol%. Faster decay time as well as less intense slower components were obtained upon increasing the amount of Ga. In the thermostimulated luminescence measurement, the glow curve peaks are gradually shifted to lower temperatures with the gallium admixture, and noticeable intensity decrease was observed only for the [(Lu, Pr).sub.3][([Ga.sub.0.2][Al.sub.0.8]).sub.5][O.sub.12]. The undesired quenching effect of Ga on the [Pr.sup.3+] luminescence seems to be reasonably low up to [(Lu, Pr).sub.3][(Ga.sub.0.2][Al.sub.0.8]).sub.5][O.sub.12] sample. It is estimated to introduce nonradiative losses of less than 10% with respect to Ga-free Pr:LuAG. Bulk crystals were also grown by the Czochralski (Cz) method and these effects were confirmed. Index Terms--Gamma-ray detectors, garnets, nuclear imaging, praseodymium compounds, radioluminescence.

By means of a photomultiplier tube, a Si-photodiode, and a Si-avalanche photodiode, the absolute scintillation yield of recently developed La[Br.sub.3]:Ce, La[Cl.sub.3]:Ce, and [(Lu, Y).sub.2]Si[O.sub.5]:Ce scintillators and traditional [Lu.sub.2]Si[O.sub.5]:Ce, [Bi.sub.4][Ge.sub.3][O.sub.12], NaI:Tl CsI:Tl, and CsI:Na scintillators were determined. These are all well known scintillators that cover emission wavelengths from 250 nm to 750 nm. By comparing the scintillation yield independently measured with the three different photon detectors reliable yield values are obtained. Index Terms--Absolute scintillation yield, gamma detectors, photodiode readout, scintillators.

The authors constructed a prototype Time-Encoded Signature (TES) system, complete with automated detection algorithms that can be used to detect point-like gamma-ray sources in search applications where detectors observe large variability in background count rates beyond statistical (Poisson) noise. The person-carried system consists of two cesium iodide scintillators placed on opposite sides of a lead shield. This geometry mitigates systematic background variation and induces a unique signature upon encountering point-like sources. This manuscript focuses on the development of detection algorithms that identify point-source signatures while remaining computationally simple. The latter constraint derives from the instrument's mobile (and thus low power) operation. The authors evaluated algorithms using both simulated and field data. The results of this analysis demonstrate the capability to detect sources at a wide range of source-detector distances using computationally simple algorithms. Index Terms--Detection statistic, gamma ray detection, handheld detector, matched filter, point source detection, standoff distance.

The assessment of the durability of candidate space components and materials requires the use of ground laboratory testing facilities when no reliable information exists in the published literature. Accelerated tests simulating the radiation/temperature/vacuum environment need to be performed to investigate possible synergistic degradation effects. An irradiation chamber combining high vacuum and thermal cycling with a large range of gamma dose rates has been designed and constructed for this purpose at SCK.CEN, the Belgian Nuclear Research Center. The irradiation chamber module with actively-controlled vacuum is designed to fit in our gamma irradiation facilities. The vacuum irradiation module operates in the high-vacuum regime even in the presence of moderate outgassing from polymer materials. A pressure of [10.sup.-5] mbar can be routinely achieved but lower pressure can be obtained in the case of inert and mineral materials. In addition to the facility description, our paper discusses preliminary results of radiation-assisted outgassing of common polymers obtained using this new vacuum irradiation module. Index Terms--Gamma radiation, hardness assurance, irradiation facility, radiation assessment, vacuum.

We have investigated the variation of the optical absorption induced by pulsed (dose rate > [10.sup.8] rad/s) and continuous ( Index Terms--Color centers, optical fibers, pulsed X-rays, radiation-induced effects, [gamma]-rays.

The spatial contributions of electrons to gamma values rising gamma density analysis were examined to investigate the structure-property relation in gamma for D25 and L25. Fractal antenna dendrimers are expected to be promising novel nonlinear optical (NLO) substances with a controllability of the magnitude and spatial contribution of the third-order NLO properties.

Short-duration γ-ray bursts are intense flashes of cosmic γ-rays, lasting less than about two seconds, whose origin is unclear (1,2). The favoured hypothesis is that they are produced by a [...]

Starburst galaxies exhibit in their central regions a highly increased rate of supernovae, the remnants of which are thought to accelerate energetic cosmic rays up to energies of ~[10.sup.15] electron volts. We report the detection of gamma rays--tracers of such cosmic rays--from the starburst galaxy NGC 253 using the High Energy Stereoscopic System (H.E.S.S.) array of imaging atmospheric Cherenkov tetescopes. The gamma-ray flux above 220 billion electron volts is F = (5.5 [+ or -] [1.0.sub.stat] [+ or -] [2.8.sub.sys]) X [10.sup.-13] [cm.sup.-2] [s.sup.-1], implying a cosmic-ray density about three orders of magnitude larger than that in the center of the Milky Way. The fraction of cosmic-ray energy channeled into gamma rays in this starburst environment is five times as large as that in our Galaxy. 8 July 2009; accepted 15 September 2009 Published online 24 September 2009; 10.1126/science.1178826 Include this information when citing this paper.

Cerium-doped gadolinium pyrosilicate single crystals Ce : [Gd.sub.2][Si.sub.2][O.sub.7] (Ce:GPS) with various Ce concentrations of 2.5-30 mol% with respect to the total rare-earth sites were prepared by the floating zone method. Their scintillation performances were investigated under irradiation of gamma rays of [sup.137]Cs. The Ce concentration dependence of scintillation characteristics and high-performance scintillation characteristics were obtained: 3-6 times greater light output than that of BGO single crystals, rapid decay, and good energy resolutions of 5.1-8.4%. Furthermore, X-ray diffraction patterns of Ce:GPS revealed Ce concentration dependence of the crystal structure of Ce:GPS. Index Terms--Crystal growth, gadolinium compounds, [Gd.sub.2][Si.sub.2][O.sub.7], neutron detectors, radiation detectors, scintillators.

An analytic solution for the fluence rate of primary (non-scattered) gamma rays emerging from a spherical radioactive source is presented. The solution takes self attenuation into account and is valid at any distance from the source. It is derived using elementary integral calculus and does not require special functions. The key to the relatively simple solution is a shift of the center of the sphere along the z-axis by a distance equal to the radius, so that the surface of the sphere passes through the origin of the coordinate system. [DOI: 10.1119/1.2348891]

Author(s): Richard Bowtell (corresponding author) [1] Magnetic resonance imaging (MRI) provides beautiful images of the inside of the human body that can depict spatial detail at the submillimetre level. However, [...]

Byline: Tran Dai Nghiep, Nguyen Thanh Cong The incoherent scattering of 661.6 keV gamma rays in a wide range of atomic numbers was studied for 6C, 13Al, 26Fe, 29Cu, 47Ag, 48Cd and 82Pb. It was shown that for heavy elements, the scattering on binding electrons is important. Meanwhile, for light elements, the scattering on free electrons is prominent. The differential Compton scattering cross-section as well as the incoherent scattering functions S (E, θ, Z) of atomic number Z for the studied subject was defined by the exponential saturated functions of the energy transfer model. The experimental values agree with the theoretical ones within experimental errors.

Researchers have used raw computing power to hunt down a 'black widow' pulsar that is evaporating its companion star. Pulsars are stellar remnants that emit lighthouse-like beams of radiation. They [...]