DaleL. Perry Lawrence Berkeley National Laboratory
Kelvin Lynn Washington State University
Arnold Burger Fisk University
Larry Franks Special Technologies Laboratory
Kazuhito Yasuda Nagoya Institute of Technology
Michael Fiederle Albert-Ludwigs-Universitaet Freiburg
U7: In-Room Poster Session
Wednesday PM, April 27, 2011
Golden Gate Foyer (Marriott)
2:30 PM - U7.1
Luminescent Materials on the Basis of Zinc Oxide.
Nikolai Starzhinskiy 1 , Oleg Sidletskiy 1 , Konstantin Katrunov 1 , Igor Zenya 1 , Vladimir Ryzhikov 1 , Alexandr Zhukov 1 , Olga Trubaeva 1 Show Abstract
1 NAS of Ukraine, Institute of Scintillation Materials, Kharkov Ukraine
Recently zinc oxide has attired renewed attention because of possibilities of creation of new efficient materials for optoelectronic and luminescent instruments. Data are presented on preparation features and properties of luminophores based on ZnO doped with chalcogenides (S, Se, Te) and sulfides of rare-earth metals (Gd, Eu, Dy). In the first case, doping of ZnO was realized both by individual components and by admixtures of ZnS, ZnSe, ZnTe. Thermodynamical analysis has been carried out of possible solid-phase reactions in systems ZnO-S, ZnO-Se, ZnO-Te, ZnO-ZnS, ZnO-ZnSe, ZnO-ZnTe in the 1000-1300 K temperature range at dopant concentrations within 0,1- 5%. It has been shown that kinetics of the synthesis and homogeneity of the luminescent material are substantially affected by the particle size of initial components, as well as duration of annealing in neutral medium (Ar, P=0,1 MPa ), which is strongly dependent on the component composition of the initial mixture. Physico-chemical and optical characteristics of the synthesized materials were determined by amplitude-spectral and kinetic characteristics, as well as from data of diffuse reflection and atomic absorption analysis, with special attention to the content of chalcogenide activator dopants in the obtained luminophores.The studied luminophores on the basis of isovalently activated zinc oxide show high emissive ability both under X-ray and UV excitation. The luminescence spectra are broad bands in the 480-610 nm region, with their maximums dependent upon the type and concentration of the dopant and regimes of thermal treatment of initial components in neutral medium. The emission kinetics is presented both by long and very short components in millisecond, nanosecond and subnanosecond ranges.The obtained materials can be used both in low-energy detectors for multi-energy radiography and as highly efficiency converters of UV radiation into visible.
2:30 PM - U7.10
Photoemission and Cathodoluminescence of Doped Lithium Tetraborate Crystals Being Developed for Neutron Detection.
Christina Dugan 1 , Robert Hengehold 1 , Stephen McHale 1 , Yaroslav Losovyj 3 2 , John McClory 1 , James Petroksy 1 Show Abstract
1 Engineering Physics, Air Force Institute of Technology, Wright Patterson AFB, Ohio, United States, 3 Center for Advanced Microstructures and Devices, Louisiana State University, Baton Rouge, Louisiana, United States, 2 Physics, University of Nebraska, Lincoln, Nebraska, United States
Lithium tetraborate (Li2B407) crystals are being developed for possible use in solid state neutron detectors. Already used in thermoluminescent dosimeters, enriched Li2B407 is of interest due to its large cross section for neutron capture by 6Li and10B. The 6Li(n,α)3H and 10B(n,α)7Li reactions are the basis for neutron detection, and a Li2B407crystal enriched with Mn should show improved efficiency for neutron detection. Li2B407 crystals doped with Mn significantly increases the light output in dosimeter applications. There is, however, a lack of fundamental characterization information regarding this useful material, particularly with regard to its electronic configuration. Photoemission spectroscopy was used to determine the energy level structure of the Mn doped Li2B407 crystals. Measurements were made using ultraviolet photoemission spectroscopy (UPS) at the Louisiana State University (LSU) Center for Advanced Microstructures and Devices (CAMD), using synchrotron radiation on two different beamlines. The first was the 3 m toroidal grating monochromator (TGM) beam line equipped with a 50 mm hemispherical electron energy analyzer providing a resolution of 70 mev and a photon energy range up to 95 eV. The TGM confirmed the presence of Mn. The second was the normal incident monochromator (NIM) beamline equipped with a Scienta SES200 electron energy analyzer with a resolution of approximately 10 mev and a photon energy range from 24 to 32 eV to study the band gap states. Measurements were made at elevated sample temperatures to reduce charging effects. Cathodoluminescence studies were also made across the band gap of undoped and Mn doped Li2B407 using excitation from a Kimball Physics EMG-12 electron gun and a Model 234 McPherson Vacuum Ultraviolet Spectrometer. Measurements were made at various beam energies from 1 to 10 KeV and sample temperatures ranging from liquid helium to room temperature.
2:30 PM - U7.11
Informatics Model for Hygroscopy in Radiation Detection Materials.
Daniel Strano 1 , Kim Ferris 1 , Kunal Shah 1 , Dumont Jones 2 Show Abstract
1 , Pacific Northwest National Laboratory, Richland, Washington, United States, 2 , Proximate Technologies, LLC., Columbus, Ohio, United States
A hygroscopy informatics model has been constructed to provide partial design guidance for radiation-detection materials. Hygroscopy is an important processing design factor for detector materials, reflecting the difficulty of preserving a dry detector environment throughout fabrication and deployment. Detector hygroscopy combines both physical surface adsorption and the absorption (solubility) of water driven by thermodynamically-favored interactions of water with detector atoms. The latter interactions inflict true detector damage, and are the focus of the present model. This hygroscopy model provides categorical solubility guidance for common classes of scintillating and semiconducting detector materials. To create a model suitable for general screening, only inputs readily available for a unsynthesized material have been deployed. In particular, because it is difficult to develop crystal-structure information prior to synthesis, the model uses only predictors that do not require 3-D structural information. To develop this model, a knowledge base of relevant materials has been constructed from standard reference sources (CRC, NIST…) and open literature publications. Descriptors include thermodynamic and particle chemistry (surface charge density) effects as used in past works by Smith  and Topping . Finally, additional Villars-type descriptors supplement the descriptor set. Analysis of the model suggests that the distinct but related pictures offered by classic thermodynamic and particle descriptors both contribute to hygroscopy. Design rules relating the descriptors to hygroscopy and to each other can be created, providing design guidance in terms of chemical structure or energetic criteria. The authors gratefully acknowledge financial support from U.S. Department of Homeland Security, Domestic Nuclear Detection Office under Contract No. HSHQDC-08-X-00872. D.M. Smith, M.P. Neu, E. Garcia, and L.A. Morales, “Hydration of Plutonium Oxide and Process Salts, NaCl, KCl, CaCl2, MgCl2: Effect of Calcination on Residual Water and Rehydration”, Los Alamos Report LA-UR-99-261. D. O. Topping, G. B. McFiggans, and H. Coe, “A curved multi-component aerosol hygroscopicity model framework: Part 1 – Inorganic compounds”, Atmos. Chem. Phys., 5, 1205–1222, 2005.
2:30 PM - U7.13
Spectroscopic Characteristics of HfO2:Eu,M (M=V, Nb, Ta, Li) X-ray Phosphors.
Aneta Wiatrowska 1 , Eugeniusz Zych 1 Show Abstract
1 University of Wroclaw, Faculty of Chemistry, Wroclaw Poland
HfO2 is an matrix for scintillators and X-ray phosphors. Yet, doping hafnia with trivalent lanthanides perturbs the metal site symmetry due to a charge divergence and also the doped materials tend to crystallize in different structures, mostly the cubic one. The charge discrepancy between the dopant and Hf(IV) forces the host into incorporation of defects to keep the electric neutrality. Consequently, we chose such co-dopants as V(V), Nb(V), Ta(V) or Li+ to compensate the lower charge of Eu3+ vs. Hf(IV). Previously, we reported on photo- and radioluminescence properties of HfO2:Eu. The aim of the present work was to determine whether the co-dopants are capable of modifying spectroscopic behavior of Eu-activated hafnia and especially if they may improve its performance as X-ray phosphor. We employed the Pechini route followed by a thermal treatment, up to 1700C, to prepared monoclinic undoped hafnia and HfO2:Eu,M (M=V, Ta, Nb, Li) powders. Undoped HfO2 produces a broad band X-ray excited luminescence (XEL) covering almost the whole visible part of the electromagnetic spectrum and a significant afterglow is observed after stopping the material stimulation. Room temperature XEL spectra of the various Eu-doped compositions will be presented and discussed together with some structural data. The Eu3+ 5D0-7F2 emission intensity increases systematically with preparation temperature up to 1500C. Changes in the luminescence related to the different co-dopants will be discussed in detail. Results of experiments upon excitation down to 50nm with synchrotron radiation at room and 10K temperatures will also be presented and discussed.
2:30 PM - U7.14
Surface Modification of TlBr Crystals for Gamma Spectroscopy.
Lars Voss 1 , Adam Conway 1 , Patrick Beck 1 , Robert Graff 1 , Rebecca Nikolic 1 , Art Nelson 1 , Steve Payne 1 , Kanai Shah 2 , Len Cirignano 2 , Hadong Kim 2 Show Abstract
1 , Lawrence Livermore National Laboratory, Livermore, California, United States, 2 , Research Monitoring Devices, Watertown, Massachusetts, United States
Thallium bromide is a material undergoing intense development for room temperature gammadetection. It possesses a range of attractive properties for this application, including its wide bandgap(2.7 eV), high average atomic number (58), and large electron mobility-lifetime product (> 10-3 cm2/V-s). Previous researchers have demonstrated energy resolutions of 1.2% at 662 keV, indicating the greatpotential for TlBr. Several problems remain to be resolved, however, including polarization phenomenawhich decrease the energy resolution during continuous operation.It has previously been observed that the processing of the surface has a large effect on the longterm operation of these detectors. In this work, we explore a variety of wet chemical treatments tomodify the surface of TlBr crystals with the intent of altering their behavior. These treatments includechemicals to modify the surface termination through the substitution of alternate halogens for Br(HF, HCl, and HI), oxidizers (H2O2 and NH4OH), and removal of the surface layer to expose fresh TlBr(Br2, HBr). The effect of these chemicals on the surface has been studied using X-ray PhotoelectronSpectroscopy (XPS), Atomic Force Microscopy (AFM), current-voltage (IV), current-time (I-t), and gammaray measurements.It was found that the surface atomic composition and morphology can be controlled through theuse of wet chemical treatments. Use of halogen-type etches result in an altered surface stoichiometryand resulting change in crystal at the surface, such as TlBrCl. Final morphology is also dependent on thetype of chemical treatment employed. In addition, polishing damage layer removal has been shown topositively affect the long term performance of planar detectors fabricated on thin (0.6 mm) substrates.Detectors which have been etched to remove the damage layer display stable performance for theduration of the measurement, which is greater than 72 hours. In contrast, buildup of the internalelectric field and decreased signal collection are observed soon after testing is begun on detectorsfabricated without a final wet chemical step.This work performed under the auspices of the U.S. Department of Energy by Lawrence LivermoreNational Laboratory under Contract DE-AC52-07NA27344, LLNL-ABS-461317. This work was supportedby the Domestic Nuclear Detection Office in the Department of Homeland Security.
2:30 PM - U7.15
ZnCdSeTe Semiconductor Compounds: Preparation and Properties.
Vello Valdna 1 , Maarja Grossberg 1 , Jaan Hiie 1 , Urve Kallavus 1 , Valdek Mikli 1 , Taavi Raadik 1 , Rainer Traksmaa 1 Show Abstract
1 Dept. of Materials Science, Tallinn Univ. of Technology, Tallinn Estonia
Group II-VI narrow band gap compounds CdTe, ZnCdTe and CdSeTe are known as the most suitable semiconductor materials for the room temperature gamma- and X-ray radiation detectors. In this work, we investigated electronic properties of a quadrupole compound ZnCdSeTe. Chlorine, copper and oxygen doped host material was synthesized from the grinded mixture of ZnTe, CdTe and CdSe by the help of CdCl2 flux. Precautions were applied to achieve an uniform doping and high quality of the crystal surfaces. Residue phases after the thermal treatments were removed by the help of a vacuum annealing. Fabricated polycrystalline samples showed a high performance from NIR via VIS and UV to X-ray band, with a sharp donor-acceptor pair peak from red to NIR band, depending on the composition. High stability, good linearity and performance of samples was measured using X-ray source with Cu-anode, at 40 kV.
2:30 PM - U7.16
Bridgman Growth of SrI2.
Michael Fiederle 1 , Leonard Alaribe 1 Show Abstract
1 FMF Dept, University Freiburg, Freiburg, BW, Germany
SrI2 crystals have been grown by using vertical Bridgman methods. The crystals have been doped with europium to obtain scintillator crystals. The feed material war pre-synthesized powder from a commercial supplier. the feed material was additionally purified. The crystals were grown in sealed silica ampoules. Several modifications have been made to increase the single crystalline areas and reduce the cracking of the material. The material properties and the detector performance will be discussed.
2:30 PM - U7.17
Mobility Variation in Electron Irradiated Gated and Ungated AlGaN/GaN High Electron Mobility Transistors.
Helen Jackson 1 , Gary Farlow 3 , Timothy Cooper 4 , James Petrosky 2 , John McClory 2 Show Abstract
1 Materials and Manufactoring Directorate, Air Force Research Lab, Wright Patterson AFB, Ohio, United States, 3 Physics Dept., Wright State University, Dayton, Ohio, United States, 4 Semiconductor Research Center, Wright State University, Dayton, Ohio, United States, 2 Engineering Physics, Air Force , Wright Patterson AFB, Ohio, United States
The Hall mobility, carrier concentration and conductivity of AlGaN/GaN High Electron Mobility Transistors (HEMTS) were subjected to 0.8 and 0.5 MeV electron irradiations at progressive total dose to 1016 cm-2. They were examined post-situ via temperature dependent Hall measurements using both a gated and ungated Van Der Pauw configuration. Typical room temperature phonon scattering is observed. However, the overall mobility of the gated configurations increases until a threshold dose is reached, and then decreases. The reason for this for this behavior is hypothesized as: (1) dynamic annealing due to the electron irradiation after initial defect creation and /or (2) parallel conduction in the layers. Device simulations using physical models that include variations in irradiation energies, irradiation dose, device layer structure, temperature dependent scattering cross sections for likely defects, and variations in the Fermi energy are used to explain the nature of the observed behavior. Examples of how this mobility behavior can be used for both lifetime reliability studies and for design of a fine tuned device layer geometry parameter set to optimize device performance in radiation intense environments are presented.
2:30 PM - U7.2
Peculiar Features of Preparation of Scintillators Based on ZnS-ZnTe Compounds.
Nikolai Starzhinskiy 1 , Alexandr Zhukov 1 , Konstantin Katrunov 1 , Vladimir Ryzhikov 1 , Sergej Galkin 1 , Alexandr Lalayants 1 , Leonid Gal’chinetskii 1 , Igor Zenya 1 Show Abstract
1 NAS of Ukraine, Institute of Scintillation Materials, Kharkov Ukraine
Conditions for obtaining a new type of chalcogenide scintillators based on zinc sulphide were studied. A thermodynamic analysis was carried out of possible chemical reactions taking place during formation of solid solution from powders of ZnS and ZnTe at concentrations of tellurium up to 10%. It is shown that significant effect on sintering of the components is exerted by the oxide phase (especially zinc oxide), which is taken into account when determining the optimal technological conditions of synthesis of solid solutions. It is shown that sintering under hydrogen atmosphere leads to more efficient and homogeneous reaction as compared to argon due to chemical and thermal etching of the ZnO layer. Introduction of elemental sulfur in the process of sintering under hydrogen leads to enhancement of the oxide film etching effects on the boundary of the interacting components ZnS and ZnTe. The formation of solid solution was monitored by the shift and slope change of the absorption edge in the spectral dependence of diffuse reflectance and compared with data of atomic absorption analysis of the content of tellurium and weight loss as a result of the synthesis. It was shown that the actual process of sintering of ZnS and ZnTe powders obtained at hydrogen atmosphere is a complex heterogeneous reaction, which is affected by factors other than purely thermodynamic and kinetic. Determined The solubility limit of Te in the lattice of ZnS was determined (about 2%). X-ray spectra of the obtained solid solutions are superpositions of several bands, the main ones being the band with λmax =460 nm and λmax=520 nm . Upon changes in concentration of Te, redistribution of the intensities of these bands is observed. Additional thermal treatment in an inert atmosphere at T =900 C gives rise to a more intense band (above 20 times) with λmax =470 nm and a decrease in the afterglow level (up to 2% after 20 ms), which is accompanied by rearrangement of the lattice (wurtzite → sphalerite). Subsequent annealing in zinc vapor is accompanied by an increase in light output of scintillation material ZnS (Te) by more than 2-3 times.
2:30 PM - U7.3
Preparation and Properties of Nano-dimensional Zinc and Cadmium Tungstates.
Nikolai Starzhinskiy 1 , Konstantin Katrunov 1 , Anna Yakubovskaya 1 , Ludmila Nagornaya 1 , Irina Tupitsyna 1 , Igor Zenya 1 , Alexandr Zhukov 1 , Oleg Vovk 1 Show Abstract
1 NAS of Ukraine, Institute of Scintillation Materials, Kharkov Ukraine
Results are presented on preparation and studies of nanocrystalline zinc tungstate (ZWO) and cadmium tungstate (CWO) scintillators obtained by liquid-phase method. The following two-step synthesis scheme was used: Zn2 + + WO2- > amorphous ZnWO4 + LiNO3 (as solvent) > nanocrystalline ZnWO4 .At the first stage, the starting compounds were prepared by co-deposition of 0.1 M aqueous solutions of zinc (cadmium) and tungsten salts at room temperature. At the second stage, the amorphous precipitate of the starting material was mixed with lithium nitrate in weight ratio of 1:10 and was sintered at 270 ° C for 8 h. The reaction product was washed, filtered and dried at 70 ° C in air for 3 hours Peculiar features of this method are that it does not require special expensive equipment, requires only short-time heat treatment at relatively low temperatures and is characterized by purity and monophasic structure of the product with the unique crystal morphology. Correlation between sintering time and the weight ratio of the melting salt and amorphous source product allows one to control the morphology and size of nanoparticles. The electron microscopy data showed that the materials obtained are nanostructured crystals in the form of elongated grains of 30-40 nm width and length of up to 80 nm (ZWO) and 100-150 nm (CWO). X-ray diffraction analysis identified the lattice as monoclinic with the structure of wolframite. For determination of optical-luminescence characteristics and their comparison with those of melt-grown crystals, we used samples of synthesized nanopowder polymerized in a transparent silicone rubber in a weight ratio of 3:5. It has been found that the nanocrystalline samples of ZWO show intense X-ray luminescence (light output ~ 0.9 with respect to the milled crystal) with spectrum similar to that of single-crystal sample (λmax = 480 nm). For CWO, the light output (with respect to fragmented crystal) is lower (~ 0,2-0,3), and X-ray luminescence spectrum is shifted to longer wavelengths by ~20 nm. The afterglow in the 10- 20 ms range both for ZWO and CWO is comparable to single-crystal samples (0,05-0.01%), but in the initial time interval (1-10ms) the time decay is more steep. Diffuse reflectance spectra show high level of reflectance in the short wavelength range (200-350nm), which is not typical for melt-grown crystals and is probably related to the absence of impurity or intrinsic defects. The obtained dispersed scintillation materials can be used as scintillation screens in introscopic systems.
2:30 PM - U7.4
X-ray Radioluminescence Microscopy for Scintillation Characterization across Boundaries in Transparent Ceramics.
Stephen Podowitz 1 , Romain Gaume 1 , Robert Feigelson 1 Show Abstract
1 Materials Science and Engineering, Stanford University, Stanford, California, United States
While it is known that point defects in single-crystal scintillators significantly affect performance by creating carrier traps or introducing non-radiative recombination pathways, the true effect of grain boundaries in ceramics on scintillation performance is not well-characterized. Many studies of ceramic scintillators point to "defects at the boundaries" as a possible source of degradation of scintillator performance and an origin of deep trapping. Bulk characterization of Eu:Y2O3 transparent ceramics with varied grain sizes and processing conditions showed a relationship between processing-induced defect concentrations and grain size that led to enhancements in light yield, but showed no direct dependence of light yield on grain size. In order to elucidate the results of previous studies focused on macro-scale scintillation characterization of ceramics, we have used fluorescence microscopy techniques under focused, ionizing radiation to investigate local variations in scintillator properties near and away from grain boundaries in ceramic scintillators. By coupling these techniques with micro-scale structural and chemical characterization, the true effect of the presence of sharp interfaces on scintillator performance within ceramic materials may be more directly investigated. X-Ray radioluminescence microscopy (XRLM), a novel technique for characterizing micro-scale luminescence using a focused beam of monochromatic hard X-rays from a synchrotron source, was used to characterize variations in scintillation luminosity across a grain boundary in Eu:Y2O3 and Ce:YAG transparent ceramic samples. Transmission electron microscopy (TEM) was used to characterize the structure of boundaries. A charge carrier transport model for the emission profile within a grain with a Robin-type boundary condition at an abrupt boundary and experimentally-derived parameters was used to understand the observed variation in scintillation across a boundary. The solutions of this model were also used to predict the dependence of the overall degradation in light yield on grain size from the presence of boundaries alone in these ceramics.
2:30 PM - U7.5
Phosphate Glasses for Detection of Penetrating Radiation via the Cherenkov Effect.
Jason Hayward 1 2 , Lynn Boatner 2 , Zane Bell 2 , Joane Ramey 2 , Rose Johnson 1 , Clint Hobbs 1 , Gerald Jellison 2 , James Kolopus 2 Show Abstract
1 , The University of Tennessee, Knoxville, Tennessee, United States, 2 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Cherenkov detectors are widely used for particle identification and threshold detectors in high-energy physics. Glass Cherenkov detectors that are sensitive to beta emissions originating from neutron activation have been demonstrated recently. In our work, glass samples have been loaded with various moderate and high Z constituents in order to fabricate them for sensitivity to MeV photons. Cherenkov glasses are advantageous because very good efficiency can be achieved at low cost, time response is limited only by photosensor speed, and an inherent discrimination threshold exists for radiation background. Out of a number of glass forming families investigated, we have found that phosphate glasses samples have the greatest response to isotopic gamma ray sources. Physical, optical, and radiation response properties of phosphate glass Cherenkov samples are described.
2:30 PM - U7.6
Characterization of Lead Iodide Films Using Raman Scattering, Electrical Resistivity and Mammographic Energy Detection.
Jose Condeles 2 , Marcelo Mulato 1 Show Abstract
2 Physics, Universidade Federal do Triângulo Mineiro, Uberaba, Minas Gerais, Brazil, 1 Department of Physics and Mathematics, University of São Paulo, Ribeirão Preto-SP, São Paulo, Brazil
Polycrystalline lead iodide (PbI2) thin films have been deposited by spray pyrolysis method using N.N-dimethylformamide (DMF) as an alternative solvent under varying deposition parameters. Due to its high atomic number and intrinsic band gap (2.3 eV), lead iodide (PbI2) is a very promising semiconductor candidate for room temperature X-ray detector and has attracted special attention in the past decades. In this work the solution rate during the deposition time of 3h was varied in the range of 0.11 cm3/min up to 0.30 cm3/min. A growth rate varying from 19 Ås-1 up to 47 Ås-1 was obtained as a function of solution rate. The drawback is a final morphology with increasing porosity. The electrical resistivity was obtained in the dark at room temperature for all the samples. A non-linear dependence was observed with solution rate and the maximum value was obtained for the smallest deposition rate of about 19 Ås-1. For larger growth rates a drastic decrease is observed and a possible saturation might even occur. A deeper investigation of the electrical properties of the samples was done by measuring the dark current as a function of temperature for: i) solution rate of 0.11 and 0.30 cm3/min and ii) temperature range of 170oC up to 250oC with a fixed solution rate of 0.16 cm3/min. Dark current as a function of temperature for the final films reveals that for larger solution rates smaller values of electrical resistivity is obtained. For a solution rate of 0.30 cm3/min, an activation energy (Ea) of about 0.65 eV was measured for the whole temperature range. On the other hand, for the sample deposited with a solution rate of 0.11 cm3/min, two main transport mechanisms can be observed with an activation energy of about 1.23 eV for temperatures above 50oC. The effect of substrate temperature is also discussed. We performed Raman scattering experiments at room temperature with illumination at 647.1 nm. The results suggest that the presence of 4H-PbI2 polytype. The sample with larger activation energy was tested under mammography diagnosis energy range, showing a linear response. The current density as a function of exposure was recorded from 360 mR up to 4100 mR, using two values of equivalent photon energies. The response is very linear with a slope of 40 µA/ cm2 R. This illustrates the potential application of the technique and the material for future use in medical imaging.
2:30 PM - U7.7
Comparative Study of HgI2, PbI2 and TlBr Films Aimed for Ionizing Radiation Detection in Medical Imaging.
Marcelo Mulato 1 , Jose Condeles 2 , Julio Ugucioni 1 , Ademar Caldeira-Filho 1 , Natalia Destefano 1 Show Abstract
1 Department of Physics and Mathematics, University of São Paulo, Ribeirão Preto-SP, São Paulo, Brazil, 2 Physics, Universidade Federal do Triângulo Mineiro, Uberaba, Minas Gerais, Brazil
Several semiconductors have been investigated and applied as X- and γ-ray detectors. Apart from the important decision concerning adequate material, many techniques for the preparation of semiconductors pose several difficulties because they are usually very expensive and involve very sophisticated apparatus. Therefore, there has been intensive search for an inexpensive, simple technique for crystal growth. Digital radiography has emerged as an alternative medical imaging technique in the last decades. The final medical set-up should be made of a square matrix of individually addressed voxels. The total apparatus size would be determined by the specific medical applications. For instance, a breast or mammography exam would require a 40 x 40 cm2 detector area. Thus, large detector areas are needed and the detector must be fabricated in the film form. Lots of works have focused on crystal fabrication for many decades. However the limited final crystal size is not technologically appropriate for the coverage of large area flat panel imagers. Mercuric iodide (HgI2) has gained considerably importance in last decades because of its detection properties for high-energy radiation. Its principal properties are high atomic number (Hg = 80, I = 53), high mass density (6.4 g/cm3), wide optical band-gap (2.13 eV) and high photon absorption coefficient for X- and gama- radiation. Lead iodide (PbI2) is also a very promising candidate. The bulk crystalline phase has a melting point at 408oC, a dielectric constant equal to 21, a mass density of 6.2 g/cm3 and forbidden band gap of 2.34 eV. Its atomic numbers are Pb = 82 and I = 53. Thallium bromide (TlBr) is another semiconductor material that can be used for ionizing radiation detection. It presents a simple cubic structure, as CsCl, with lattice parameter a =3.97 A. It has also a large optical band gap of 2.68 eV and a high stopping power for X-rays and g-rays, mainly due to its high atomic number (Tl=81 and Br=35) and its mass density of 7.56 g/cm3. Due to these properties, these compound semiconductors are promising candidates for the fabrication of medical imagers using the direct detection method. This work presents a comparative study between the three candidates. Over the last ten years our research group has focused on the fabrication of films aimed for large area medical imagers. We present structural, morphological, optical and electrical properties of theses materials when fabricated using deposition techniques such as thermal evaporation, solvent evaporation, and spray pyrolysis. The photoresponse of the devices when submitted to irradiation under mammographic energy range is also presented and discussed.
2:30 PM - U7.8
Effect of Irradiation on Vitreous Silica for Fiber Optics Cables in Sensor Applications.
Harish Govindarajan 1 , Wolfgang Windl 1 Show Abstract
1 Materials Science and Engineering, The Ohio State University, Columbus, Ohio, United States
Vitreous silica finds extensive use in fiber optic cables, which are currently considered to be used for sensor applications under high radiation fields in nuclear reactors. The darkening of the optical fibers in high radiation environments, resulting in the attenuation of signals, is a major concern in the performance of fiber optic sensors used in the reactor. This project utilizes molecular dynamics (MD) simulations and first-principles electronic structure calculations to model and analyze the structural changes in glass fibers at high-temperature in radiation environments. Besides the characterization of optically active defect structures in terms of vacancies or miscoordinated atoms, we examine specific approaches for defining and computing quantities that provide useful information about structural changes and crystallization effects in the amorphous system upon heating and irradiation. The Tersoff interatomic potential as parameterized by Munetoh et al. is utilized for the calculations. We find that this potential predicts the structural properties of various silica polymorphs in good agreement with experimental results, besides being well suited for large Si-O systems due to its computational efficiency primarily caused by the absence of long-range electrostatic interaction terms. MD simulations using the GULP MD code were used to model the temperature evolution of the structure within the typical operating range of existing and future nuclear reactors (300K – 1400K). For the irradiation effects, a range of primary knock-on events with varying energies was simulated and the effect of PKA energy on defect distribution was analyzed and correlated to TRIM calculations. In post-processing, a number of structural parameters were used to characterize the structure, including ring-order distribution, coordination number variation in correlation to potential energy, pair-correlation (radial distribution) functions and structure factor. The possibility of crystallization pockets in the irradiated, amorphous structure was established using heat flux calculations (which represent the results of Differential Scanning Calorimetry (DSC) measurements) and simulation of x-ray diffraction patterns which displayed characteristic peaks in addition to the amorphous signal as found from comparisons to experimental data. First-principles electronic structure calculations are used to correlate the modeled structures to the resulting optical activity and to the resulting total attenuation.
2:30 PM - U7.9
Cu-α-In2Se3 Schottky-Mott Diodes.
S. Drapak 1 Show Abstract
1 , Frantsevich Institute of Material Sciences Problems, National Academy of Sciences of Ukraine, Chernivtsi Ukraine
In2Se3 is a semiconducting compound of the A2IIIB3VI family, the structure of which are defective with respect to their metal atoms: only two thirds of the sites in the cation sublattice are occupied. Materials with such structure are suitable for sensors of small particles and they could be used for the manufacture of detectors of the ionising radiation. There are some relatively well-know structures of In2Se3, which include layered structure (α-), rhombohedral structure (β-), defect Wurtzite structure (γ-), anisotropic structure (κ-phase) and so called “vacancy ordered in screw form” structure. Often for investigation multiphase samples are used that results in the rather discordant experimental data.Layered n-type α-In2Se3 single crystals (Eg = 1.42 eV, n =10^17 cm^-3 at 300 K) were grown by the Bridgman method from a stoichiometric melt. Due to a high density of surface states the Cu (Ni, Al, Ag)- α-In2Se3 structures prepared by vacuum deposition of the metal onto the cleaved surface (0001) of semiconducting substrate demonstrate linear I-V dependences. At the same time Cu-α-In2Se3 structures prepared by chemical deposition of Cu demonstrate good rectifying characteristics. It is shown that I-V curves of such Cu-α-In2Se3 diodes agree well with the Schottky theory (at low forward biases) and with the trap-controlled space charge limited transport theory (at higher forward biases), which is attributed to the formation of thin (40-60 nm) high resistivity layer at the interface Cu/In2Se3. The formation of such a layer is caused by the diffusion of Cu-ions into the bulk of In2Se3 due to a large number of intrinsic defects in the semiconducting substrate. According to the experimental data Cu doping of α-In2Se3 single crystals leads to the increasing of their resistivity mainly because of the decreasing of electron mobility. It is shown that an irradiation of Cu-α-In2Se3 diodes with gamma quanta from a continuous source based on 60Co isotope (total dose up to 10^5 R) does not lead to changing neither electrical properties nor the main photoelectrical parameters (values of volt/watt and ampere/watt photosensitivity, spectral distribution of photocurrent). The possibility to use Cu - α-In2Se3 diodes as the detectors of ionising radiation is also under consideration.
DaleL. Perry Lawrence Berkeley National Laboratory
Kelvin Lynn Washington State University
Arnold Burger Fisk University
Larry Franks Special Technologies Laboratory
Kazuhito Yasuda Nagoya Institute of Technology
Michael Fiederle Albert-Ludwigs-Universitaet Freiburg
U11: Scintillators III
Thursday PM, April 28, 2011
Golden Gate C1 (Marriott)
2:30 PM - **U11.1
New Experimental Technique for the Isotopic Determination of Boron in Neutron-related Nuclear Radiation Detection Materials.
Richard Russo 1 2 , Alexander Bol'shakov 2 , Xianglei Mao 1 , Dale Perry 1 , Osman Sorkhabi 1 Show Abstract
1 , Lawrence Berkeley National Laboratory, Berkeley, California, United States, 2 , Applied Spectra, Inc., Fremont, California, United States
A new method of performing optical isotopic analyses of condensed samples in air at ambient pressure has been developed. The technique involves the use of obtaining spectra of materials with no sample preparation. The new method can determine not only chemical composition but also isotopic ratios of key elements in the sample in real time. Isotopic measurements are enabled by obtaining significantly larger elemental isotopic shifts of the sample than isotopic shifts previously reported for similar techniques. Analyses of samples can be performed from a distance with no sample preparation. The detection and analysis of isotopes of boron, used in radiation detection materials, are discussed to illustrate the technique.
3:00 PM - U11.2
UV Emitting Single Crystalline Film Scintillators Grown by LPE Method: Current Status and Perspective.
Yuriy Zorenko 1 , Vitaliy Gorbenko 1 , Volodymyr Savchyn 1 , Taras Voznyak 1 , Miroslaw Batentschuk 2 , Albrecht Winnacker 2 , Qi Xia 2 , Christoph J. Brabec 2 Show Abstract
1 Electronic Department, Ivan Franko National University of Lviv, Lviv Ukraine, 2 Department of Materials Science, University of Erlangen-Nuremberg, Erlangen Germany
The work is dedicated to the development of new type of phosphors based on single crystalline films (SCF) of oxide compounds (aluminum garnets and perovskites, sapphire, silicates, tungstates, i.e.) grown by liquid phase epitaxy (LPE). The field of applications of such phosphors is rapidly extending and includes α– and β-scintillators, screens for visualization of x-ray images, cathodoluminescent light sources, laser media as well as converters of LED radiation [1, 2].The main goal of this talk is to report on the development of UV emitting SCF scintillators and screens based on aluminum garnet compounds. The shift of emission spectra of SCF to the UV range with respect to recently developed YAG:Ce and LuAG:Ce films with emission in the visible (450-750 nm) range  in principle can result in an increase of the light yield (LY) and energy resolution of scintillators. Specifically, using the UV-emitting SCF scintillating screens can significantly improve the spatial resolution of detectors for visualization of X-ray images . Development of the raster scanning optical microscope technique also requires different UV cathodoluminescent light sources for the control of biological objects. The development of two types of UV emitting SCF scintillators is reported in this work: 1) Pr-doped SCF of Y-Lu-Al garnet having the intensive Pr3+ f-d luminescence in the 300-400 nm and 230-300 nm spectral ranges with a decay time of about 13-18 ns. 2) SCF of Y-Lu-Al-garnet doped with Sc3+ isoelectronic impurity emitting in the 290-400 nm range due to the formation of the ScY,Lu and ScAl centers with a luminescence decay time in the order of 1 μs. The main problem in the development of these types of scintillators by LPE from the traditional PbO-B2O3 flux is related to significantly larger influence of Pb dopant on UV luminescence of Pr3+ and Sc3+ ions than on the Ce3+ emission in the visible range in SCF of garnets . That is the main reason for lower light yield of Pr - and Sc-doped SCFs (up to 30 % and 50-70 %, respectively) in comparison with the single crystals analogues of these garnets. Therefore, the future development of UV emitting scintillators based on Pr3+ and Sc3+ doped SCF of garnets strongly demands the usage of alternative lead-free fluxes for LPE. Therefore, this work contains also some new “hot” results related to the usage of the BaO-B2O3-BaF2 and Li2WO4-Li2MO4 fluxes for crystallization of the SCF based on the Pr3+- and Sc3+ doped garnets. The improvement of properties of SCF scintillators grown from BaO-based flux with respect to the SCF analogues grown from the traditional PbO-based flux is presented.1. Y. Zorenko, V. Gorbenko, E. Mihokova, M. Nikl, K. Nejezchleb, A. Vedda, V. Kolobanov, D. Spassky, 2007. Rad. Measur., 42, 521-527.2. Y. Zorenko, V. Gorbenko, T. Voznyak, M. Batentschuk, A. Osvet, A. Winnacker, 2008. J. Luminescence, 128, 652-660.3. T. Martin, A. Koch, 2006. J. Synchrotron Rad. 13, 180-199.
3:15 PM - U11.3
Transparent Gd-based Ceramic Scintillators for Nuclear Radiation Detection.
Joshua Kuntz 1 , Zachary Seeley 1 , Nerine Cherepy 1 , Scott Fisher 1 , Stephen Payne 1 Show Abstract
1 , Lawrence Livermore National Laboratory, Livermore, California, United States
Transparent ceramics are fully dense polycrystalline optical materials, typically formed from cubic oxide or fluoride structures. The rare earth aluminum garnets, in particular, are materials of great interest for use as optical components, laser amplifiers, and scintillators. Yttrium aluminum garnet (YAG) has seen the widest study and even commercialization for both scintillation and laser applications. At Lawrence Livermore National Laboratory, highly transparent ceramic oxide scintillators on the cubic inch scale are being developed for nuclear radiation detection applications. In single crystal growth, slightly off-stoichiometry feedstock can result in good single crystals, with rejected materials left at the end of the boule. In contrast, transparent ceramics are polycrystalline materials with micron-scale grain size, and their fabrication requires careful control of elemental composition, otherwise secondary phases tend to accumulate at grain boundaries significantly degrading transparency, even at levels less than 1%. In fact, the largest challenge in obtaining highly transparent ceramic optics is the narrow compositional range that leads to the presence of a single phase in the final ceramic part. Certain garnets are essentially line compounds (YAG, LuAG, TAG), while others exhibit large compositional ranges (GGG, GSAG, YSAG). YAG:Ce has been used as a scintillator for gamma ray spectroscopy but its low Zeff results in poor photopeak efficiency and extreme attention to composition is required to produce it as a single phase ceramic. Gadolinium gallium garnet (GGG), on the other hand, has broad range of compositions resulting in a single ceramic phase and a very high Zeff. Unfortunately, GGG does not activate with Ce. To create a phase-stable, high Zeff, Ce-activated scintillator, we have combined YAG and GGG into GYGAG:Ce.We identified a broad compositional range of single phase GYGAG:Ce by intentionally varying the ratio of Gd, Y and Ce to Ga and Al. The ratio bracketed the ideal garnet ratio of 3 to 5. We found a compositional range that resulted in highly transparent ceramics spanning greater than 1% variation in composition, allowing us flexibility in feedstock production and ceramics fabrication processing. The transparent ceramics were then evaluated for both light yield and energy resolution. The best composition resulted in a light yield more than 50% higher than YAG:Ce and an energy resolution of ~5% at 662 keV.Thanks to Thomas Hurst and Owen Drury for light yield/energy resolution characterizations of the scintillators. This work was supported by the Department of Homeland Security Domestic Nuclear Detection Office and performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS-461472.
3:30 PM - U11.4
A Novel, High Conversion Efficiency, Large-area Scintillator Film.
Vladimir Gelfandbein 1 , Steven Cool 1 , Harish Bhandari 1 , Vivek Nagarkar 1 Show Abstract
1 Advanced Imaging Technology, Radiation Monitoring Devices, Inc, Watertown, Massachusetts, United States
We are now working with a new inorganic scintillator, with properties of key importance for the most demanding static and dynamic radiation detection, measurement and imaging applications.Recently, the growth of high quality crystals of this material, about 10 mm in diameter, by the Bridgman method, was reported. However, this excellent scintillator is still relatively expensive and in limited supply.Therefore, we have developed and are now refining a new low-cost method to fabricate thick, large-area films of this remarkable material. So far, we have deposited films up to 70 mm in diameter and 10 mm thick, with a polycrystalline columnar structure that provides light piping and improves light collection efficiency. We now measure high scintillation conversion efficiencies for these new films that are even higher than the reported values for commercially available crystals of LaBr3:Ce, one of the most desirable scintillators.The method that we have developed is a special hot-wall evaporation (HWE) technique that shows significant promise for use in fabricating scintillators for gamma-ray spectroscopy applications, as well as some new areas such as room temperature radioisotope identification, quantitative molecular imaging, X-ray CT, and more. Here we provide some details of fabrication as well as emission and performance data from the first samples made by this method.