Symposium Organizers
Dale L. Perry Lawrence Berkeley National Laboratory
Arnold Burger Fisk University
Larry Franks Special Technologies Laboratory
Michael Schieber The Hebrew University of Jerusalem
O1: Material Development for Security and Defense Applications
Session Chairs
Wednesday PM, November 28, 2007
Republic A (Sheraton)
9:30 AM - **O1.1
Defense Threat Reduction Agency (DTRA): Combating the Radiological/Nuclear Threat.
Robert Harbs 1
1 Nuclear Detection Division, Defense Threat Reduction Agency, Fort Belvoir, Virginia, United States
Show AbstractThe Defense Threat Reduction Agency’s (DTRA) mission is to safeguard the United States, its allies, and coalition partners from Weapons of Mass Destruction (WMD) (chemical, biological, radiological, nuclear, and high yield explosives) by providing capabilities to reduce, eliminate, and counter the threat, and mitigate its effects. As part of this mission, DTRA conducts research and development (R&D) on radiological and nuclear (R/N) detection technologies and assessment tools to improve and expand U.S. forces’ capabilities to detect, locate, identify, tag, and track R/N WMDs and related materials, and to measure warfighter exposure to R/N hazards. The purpose of this presentation is to apprise the R/N detection, materials, instrumentation and hardware/software development community of Department of Defense (DoD) R/N detection R&D objectives, including: DoD’s R/N detection R&D priorities; DoD funded R/N programs and projects; Opportunities for participation in programs and projects; and the process for pursuing these opportunities.DoD’s R/N detection R&D priorities are to: Locate, tag, and track WMD, their delivery systems and related materials; Detect fissile materials at stand-off ranges; Develop interdiction capabilities to stop air, maritime, and ground shipments of WMD, their delivery systems, and related materials; and Develop persistent surveillance capabilities over wide areas to locate WMD capabilities or hostile forces.To meet these priorities, DTRA directs R&D in several technology areas to address major DoD mission capability needs, including Active Interrogation, Imaging Detectors, Gamma Spectrometers, Neutron Detectors/Spectrometers, Signal Processing, Materials, and Alternative Signatures.Each area will be covered in more detail in the presentation, as well as the need for new ideas from the materials research and nuclear science and technology community, particularly in active interrogation for standoff detection, and avenues for collaboration.
10:00 AM - **O1.2
Development of New Scintillators for High Energy Resolution Gamma Radiation Detectors.
Nerine Cherepy 1
1 Chemistry and Materials Science, Lawrence Livermore National Lab, Livermore , California, United States
Show AbstractScintillator radiation detectors for Homeland Security have applications ranging from small handheld detectors to large portal monitors. We are developing new scintillator materials in efforts to achieve higher energy resolution than available from commercial instruments, permitting rapid unambiguous isotope identification. With a goal of 2% resolution at 662 keV, we are exploring transparent ceramic and single crystal candidates that offer low-cost manufacturability and higher-efficiency stopping than commercial scintillator detectors. Recent materials fabrication and performance results achieved with cerium-doped ceramics and iodide single crystals will be discussed. This work was supported by the Domestic Nuclear Detection Office of the Department of Homeland Security and was performed under the auspices of the U.S. Department of Energy by University of California, Lawrence Livermore National Laboratory under Contract W-7405-Eng-48.
10:30 AM - O1.3
Basic Materials Studies of Lanthanide Halide Scintillators.
F. Doty 1 , Douglas McGregor 2 , Mark Harrison 1 2 , Kip Findley 3 , Raulf Polichar 4 , Pin Yang 5
1 Engineered Materials Dept., Sandia National Laboratories, Livermore, California, United States, 2 Dept. of Mechanical & Nuclear Engineering, Kansas State University, Manhattan, Kansas, United States, 3 School of Mechanical and Materials Engineering, Washington State University, Pullman, Washington, United States, 4 , SAIC, San Diego, California, United States, 5 Ceramic & Glass Dept, Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractCerium and lanthanum tribromides and trichlorides form isomorphous alloys with the UCl3 type structure, and have been shown to exhibit high luminosity and proportional response, making them attractive alternatives for room temperature gamma ray spectroscopy. However the fundamental physical and chemical properties of this system introduce challenges for material processing, scale-up, and detector fabrication. In particular, low fracture stress and perfect cleavage along prismatic planes cause profuse cracking during and after crystal growth, impeding efforts to scale this system for production of low cost, large diameter spectrometers. We report recent progress on basic materials science of the lanthanide halides. Studies to date have included thermomechanical and thermogravimetric analyses, hygroscopicity, yield strength, and fracture toughness. Observations including reversible hydrate formation under atmospheric pressure, loss of stoichiometry at high temperature, anisotropic thermal expansion, reactivity towards common crucible materials, and crack initiation and propagation under applied loads will be presented. The observed mechanical properties pose challenging problems for material production and post processing; therefore, understanding mechanical behavior is key to fabricating large single crystals, and engineering of robust detectors and systems. Analysis of the symmetry and crystal structure of this system, including identification of densely-packed and electrically neutral planes with slip and cleavage, and comparison of relative formation and propagation energies for proposed slip systems, suggest possible mechanisms for deformation and crack initiation under stress. The low c/a ratio and low symmetry relative to traditional scintillators indicate limited and highly anisotropic plasticity cause redistribution of residual process stress to cleavage planes, initiating fracture. This proposed failure mechanism and its implications for scale up to large diameter crystal growth will be presented.Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
O2: Materials Theory and Modeling
Session Chairs
Wednesday PM, November 28, 2007
Republic A (Sheraton)
11:00 AM - **O2.1
Applications of First Principles Theory to Inorganic Radiation Detection Materials.
David Singh 1
1 Materials Science and Technlogy Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractRadiation detection poses severe materials requirements, especially in regards to energy transport, electronic structure and carrier recombination. Moreover, in some cases, microscopic mechanisms that affect performance are poorly understood. Here a series of first principles studies of radiation detection materials, mainly scintillators, is presented. A range of materials are discussed, including phosphates, germanates, halides and oxides, with reference to calculated electronic, structural and optical properties as well as defects and their characterization. Validation of results by comparison with experimental data is shown. Finally, some trends that may be useful in seeking improved materials are presented.This work was supported by the Department of Energy, Office of Non-Proliferation Research and Development.
11:30 AM - O2.2
Simulation of Solidification in LP Bridgman Furnace Using Phase Field Method.
Alexandre Tartakovsky 1 , Charles Henager 1 , Andrew Kuprat 1 , John Jaffe 1 , Howard Heinisch 1 , S. Sundaram 1
1 , PNNL, Richland, Washington, United States
Show AbstractRecent advances in computer models for growth processes in the Bridgman systems were useful in understanding the general effects of furnace operating conditions on the growth of crystals for radiation detection applications. As such, computer models became a valuable tool in the furnace design and optimization of the operating conditions. At the same time, most existing models use a continuum approach in the modeling of crystal growth and focus on the study of the macroscopic shape of the solid-liquid interfaces and its dynamics. This approach fails to produce accurate predictions when dendritic crystal growth occurs. We developed a phase-field based model to study the dendritic growth in Bridgman systems. The model takes into account effects of anisotropy in kinetic and interfacial free energy coefficients as well as the effect of the front curvature on the crystal growth. The model was used to study the effect of the orientation of preferred crystal growth directions with respect to the longitudinal axis of the furnace. This analysis was done by randomly placing crystal nuclei with random orientations of the preferred growth directions at the bottom of the furnace. The effect of the furnace geometry and operating conditions on the crystal growth were also investigated.PNNL is operated for the U.S. Department of Energy by Battelle Memorial Institute under Contract DE-AC06-76RLO 1830. This work was funded by the Office of Defense Nuclear Nonproliferation, Office of Nonproliferation Research and Development (NA-22).
11:45 AM - O2.3
Materials for Direct Energy Conversion.
Liviu Popa-Simil 1 , Claudiu Muntele 2
1 , LAVM LLC., Los Alamos, New Mexico, United States, 2 Center for Irradiation of Materials, Alabama A&M University,, Normal, Alabama, United States
Show Abstract12:00 PM - O2.4
Scintillation Response and Transport Properties in CsI:Tl.
Yanwen Zhang 1 , Fei Gao 1 , Yulong Xie 1 , William Weber 1
1 , Pacific Northwest National Laboratory, Richland, Washington, United States
Show AbstractDemands for improved radiation detectors for national security, medical imaging and high-energy nuclear physics applications have prompted research efforts on scintillators with better detection performance. However, research on radiation detection has reached an impasse due to a general lack of understanding of the underlying photon response and charge carrier transport properties. At the current stage, theoretical modeling efforts, which would complement Edisonian approaches to new materials discovery, are hampered in their ability to fully describe radiation detection physics and energy partitioning of charged particles.In this work, fundamental studies on scintillation response to energetic particles and their transport properties in thallium doped cesium iodide (CsI:Tl) have been carried out through both experimental and computational approaches. Charged particles of H+, He+ and O+ have been used to deposit energy in CsI:Tl crystals, and the corresponding scintillation responses have been measured over a continuous energy range. The resulting light yield, nonlinearity, and energy resolution are determined as a function of ion mass, particle energy or electronic energy deposition. The collision cascades and electronic energy deposition of the ions in CsI:Tl are simulated using the SRIM-2006 code, and the spatial distribution of the resulting electron-hole pair production is calculated as a function of particle energy. An advanced Monte Carlo (MC) code, developed at Pacific Northwest National Laboratory, and a radiation transport model are employed to simulate electron interaction with the CsI:Tl in terms of the probabilities of various quantum processes, the trapping/detrapping of the information carriers in defects and impurities, as well as the information carrier collection that is directly related to light output. The computational results provide a detailed interpretation of the experimental observations, as well as the insights into the mechanisms controlling nonlinearity.The use of ions to deposit energy provides additional control and separation of mechanisms related to photon response and transport properties. This work demonstrates a possible pathway to achieve fundamental understanding of the quantum mechanical processes that control energy transfer and affect intrinsic signal variance associated with energy partitioning of charged particles.
12:15 PM - O2.5
3-D Moving Finite Element (MFE) Modeling of Crystal Growth Model and Solution Validation.
Andrew Kuprat 1 , Alex Tartakovsky 2 , Charles Henager 3 , Howard Heinisch 3 , John Jaffe 4 , S Kamakshi Sundaram 5
1 Biological Monitoring/Modeling, PNNL, Richland, Washington, United States, 2 Computational Mathematics, PNNL, Richland, Washington, United States, 3 Materials Sciences, PNNL, Richland, Washington, United States, 4 Interfacial Chem & Engnrg, PNNL, Richland, Washington, United States, 5 Glass Science & Processing, PNNL, Richland, Washington, United States
Show AbstractWe present a Moving Finite Element model of crystal growth in three dimensions. Moving Finite Elements is a Galerkin finite element method where velocity errors in the normal motion of a surface are made to be orthogonal to the set of basis functions spanning allowable normal surface motions. The surface represented is the solid-liquid interface of an evolving crystal and the basis functions correspond to the piecewise-linear interpolated motions of triangles that discretize the surface. Surface physics that determines normal surface velocity is the Gibbs-Thompson law that relates thermal and curvature undercoolings to velocity via a kinetic coefficient. In order to close the scheme, the heat equation is discretized on tetrahedral mesh that moves conformally with the evolving triangular surface mesh. The heat equation is solved with standard Galerkin finite elements, with a compensating term for motion of the grid and a source term at the solid-liquid interface due to release of latent heat. There are many potential advantages to this method. First it is rotationally invariant. There is no Cartesian bias to the discretization and thus crystal axes can be randomly oriented with respect to computational axes. We can thus simultaneously simulate the growth of a collection of randomly oriented crystals. Another advantage is that the method is potentially cheaper than other methods because grid is concentrated where the action is (i.e. close to the solid-liquid surface). Also, since the volume grid is always conformal to the evolving surface grid, discontinuities of physical properties such as density and heat conductivity can be exactly represented. Finally, since the solid-liquid interfaces are being represented by a triangulated surface, it is possible to correctly represent special situations such as when there is a triple intersection line between two crystals and the liquid. We show performance of this method on 3-D crystals evolving under anisotropic surface energy appropriate for crystals with cubic symmetry. We show qualitative agreement with other methods, such as the phase field method. We also show results of a 1-D problem that quantitatively tests the motion of the solid-liquid interface against an analytic solution.___________________________________________PNNL is operated for the U.S. Department of Energy by Battelle Memorial Institute under Contract DE-AC06-76RLO 1830. This work was funded by the Office of Defense Nuclear Nonproliferation, Office of Nonproliferation Research and Development (NA-22).
12:30 PM - O2.6
Te Precipitates in CZT: Experiment and Model.
John Jaffe 1 , Erin Miller 2 , Danny Edwards 5 , Alan Schemer-Kohrn 5 , Mary Bliss 2 , Andrew Kuprat 3 , Alexandre Tartakovsky 4 , Howard Heinisch 5 , Charles Henager 5
1 Fundamental Sci. Directorate, Pacific Northwest Natl. Lab, Richland, Washington, United States, 2 Natl. Security Directorate, Pacific Northwest Natl. Lab, Richland, Washington, United States, 5 Energy Sci. and Tech. Directorate, Pacific Northwest Natl. Lab, Richland, Washington, United States, 3 Enivronmental Tech. Directorate, Pacific Northwest Natl. Lab, Richland, Washington, United States, 4 Comp. and Inf. Sci. Directorate, Pacific Northwest Natl. Lab, Richland, Washington, United States
Show AbstractCadmium zinc telluride (CZT) is useful material for semiconductor gamma-ray spectrometers and other electro-optic devices. It is often grown Te-rich to optimize its electrical characteristics, but this off-stoichiometric growth leads to the formation of semimetallic Te precipitates in the semiconducting host crystal. These precipitates impair device performance and their formation needs to be inhibited if possible during growth. To investigate their formation and effects, we have studied Te precipitates and their surrounding host material in a sample of CZT by orientation imaging microscopy and X-ray diffraction. The precipitates are often roughly tetrahedral in shape, with the faces corresponding to the {111} planes of the cubic CZT host, and with the [0001] direction of trigonal Te preferentially aligned with a <111> direction in CZT. The precipitates are also found to contain large voids accounting for up to about half their total volume. Separately, we have modeled the Te precipitate-host interface in pure CdTe by a first-principles density-functional calculation on a superlattice of nine Te (0001) and six CdTe (111) layers. The calculations suggests that the lattice mismatch is mostly accommodated by straining the Te, and that the Te band gap lies within the CdTe band gap when interfacial polarization is taken into account (a Type I band offset.)PNNL is operated for the U.S. Department of Energy by Battelle Memorial Institute under Contract DE-AC06-76RLO 1830. This work was funded by the Office of Defense Nuclear Nonproliferation, Office of Nonproliferation Research and Development (NA-22).
12:45 PM - O2.7
Molecular Modeling of Gas Adsorption in Scintillating Metal-Organic Frameworks.
Jeffery Greathouse 1 , Tiffany Kinnibrugh 1 , F. Doty 2 , Mark Allendorf 3
1 Geochemistry Department, Sandia National Laboratories, Albuquerque, New Mexico, United States, 2 Engineered Materials Department, Sandia National Laboratories, Livermore, California, United States, 3 Microfluidics Department, Sandia National Laboratories, Livermore, California, United States
Show AbstractMetal organic frameworks (MOFs) are a new class of coordination polymers with unique potential for improved scintillation materials. MOFs are hybrid materials comprised of coordinating metal groups and organic “linker” molecules organized into highly porous crystalline networks capable of accommodating a variety of adsorbates or “guest” molecules. Control of linker moieties gives independent control over pore geometry and chemistry, enabling rational design of materials with specific properties for improved scintillators, such as density, fluorescence, chemical composition, and sorption capacity. For example, MOFs are uniquely suited for development as ultra-low-density scintillating frameworks to minimize dE/dx quenching. Alternatively, MOFs could be used as ultra-high-surface-area scintillating sorbants (up to 6,000 m2/g) for high uptake of particle-conversion gases such as H2, 3He, or Xe, or as a crystalline stabilizing matrix for liquid-phase organic scintillators.Molecular modeling techniques provide atomistic insight into the adsorption process within MOF pores. In a Grand Canonical Monte Carlo (GCMC) simulation, particles are inserted and deleted within the MOF pores until the chemical potential of the adsorbate is in equilibrium with the bulk gas at a given fugacity. The resulting adsorption isotherm can then be compared with experiment. Unlike most GCMC simulation studies of MOFs in which the framework is rigid, our force field contains only nonbonded energy terms between metal-oxygen pairs. Our model allows for framework flexibility, particularly changes in the metal coordination environment upon interaction with adsorbed species. Additionally, the time evolution of molecular properties (structure, diffusion) can be obtained from molecular dynamics (MD) simulations. Both MD and GCMC simulation results will be presented for the adsorption of particle-conversion gases mentioned above within Zn-based MOFs.Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
O3: Other Materials
Session Chairs
Wednesday PM, November 28, 2007
Republic A (Sheraton)
2:30 PM - O3.1
Porous Dielectrics As Active Materials for Nuclear Radiation Detectors.
Martiros Lorikyan 1
1 , YerPhI, Yerevan Armenia
Show Abstract2:45 PM - O3.2
Organic Semiconductors for Detection of Ionizing Radiation.
Tiffany Wilson 1 2 , F. Patrick Doty 2 , Douglas Chinn 3 , Michael King 4 2
1 Chemical and Biomolecular Engineering, The Ohio State University, Columbus, Ohio, United States, 2 , Sandia National Laboratory, Livermore, California, United States, 3 , Sandia National Laboratory, Albuquerque, New Mexico, United States, 4 Nuclear Engineering, University of California, Berkeley, California, United States
Show AbstractOrganic semiconductors are increasingly common in electronics and sensors, and are now under investigation for a novel type of radiation sensors at Sandia National Laboratories. This class of materials can offer wide band gaps, high resistivities, low dielectric constants, and high dielectric strengths, suggesting they may be suitable for solid-state particle counting detectors. A range of solution cast materials have been evaluated for this application, primarily in the family of poly(p-phenylene vinylene)s, or PPVs. The high ratio of hydrogen to carbon offers neutron sensitivity, while the low Z material provides some natural gamma discrimination. Compared to existing detectors, these materials could potentially offer large-scale radiation detection at a substantially reduced cost.We report fundamental studies of the structure and property relations of PPVs. While PPVs hold promise for radiation detection, the mechanical and electrical properties must be optimized and the environmental effects understood. Polymers can offer significantly simplified processing compared to the more common crystals used in solid state detection, which can be size limited and fragile. However, organic semiconductors are very sensitive to processing conditions, and mobility can be affected by orders of magnitude by processing variables, without altering any chemistry. Films prepared with novel mechanical stretching methods show increased bulk order and an increased photoresponse when exposed to a laser stimulus. Additionally, while PPVs generally do not degrade in air, they are affected by environmental conditions including humidity and temperature and these must be well characterized in order to have an effective sensor and to understand packaging requirements. Future work will analyze the feasibility of single particle detection and various geometries for optimization. Additional processing variables will also be investigated for further improvement of mobility and reduction of trap density. Sandia is a multiprogram laboratory operated by Sandia Corporation, a LockheedMartin Company, for the United States Department of Energy’s National NuclearSecurity Administration under contract DE-AC04-94AL85000.
3:00 PM - O3.3
Polymer Composites for Radiation Detection.
Yongsheng Zhao 1 , Haizheng Zhong 1 , Qibing Pei 1
1 , University of California, Los Angeles, California, United States
Show AbstractWe present new advanced materials for the detection of nuclear radiation with high sensitivity and energy resolution. These materials exploit the high optical density and photoluminescent efficiency of conjugated polymers which are largely intact under gamma radiation but can be activated in the presence of high-Z compounds. High-Z compounds are readily mixed with conjugated polymers to form molecular or nanometer-scale composite. Films of various thicknesses are conveniently cast from solution and exposed to gamma radiation. The response of the films to gamma dosage is analyzed by ultraviolet-visible absorption and photoluminescence spectroscopy.
3:15 PM - O3.4
Radiation Hard Diamond Pixel Detectors.
Harris Kagan 1 , Markus Mathus 2 , Markus Cristinziani 2 , Lars Reuen 2 , Shane Smith 1 , William Trischuk 3 , Jaap Velthuis 2 , Norbert Wermes 2
1 Physics, Ohio State University, Columbus, Ohio, United States, 2 Physics, Bonn University, Bonn Germany, 3 Physics, University of Toronto, Toronto, Ontario, Canada
Show AbstractDiamond is a promising sensor material for detectors that can operate in high radiation environments. Its radiation hardness and fast signal collection properties make it an ideal choice for the inner layers of a super Large Hadron Collider (sLHC) detector. Sensor grade polycrystalline material of sufficient size to make a full ATLAS pixel module was produced, metalised, bump-bonded and tested. We present here an ATLAS hybrid pixel module that uses chemical vapor deposition (CVD) diamond as the sensor material. It has an active area of 6 x 2 cm^2, pixelated with 46080 sensitive elements. The module is readout with sixteen radiation hard ATLAS pixel front-end electronic chips that were successfully bump-bonded by IZM in Berlin. The module has been characterized with lab measurements and successfully operated in test beams. The results of these tests will be presented and compared with silicon pixel modules using the same readout electronics. We will also present the first results from a single-crystal CVD diamond pixel detector using the same ATLAS readout electronics. Single-crystal CVD diamond is made by the same process as polycrystalline material but has no grain boundaries. As a result it is full charge collecting. The single chip single-crystal CVD diamond pixel device has been characterized with lab measurements and successfully operated in test beams. The results of these tests will also be presented.
O4: CZT and CdTe
Session Chairs
Wednesday PM, November 28, 2007
Republic A (Sheraton)
4:00 PM - **O4.1
(Cd,Mn)Te as a New Material for X-ray and Gamma-ray Detectors.
Andrzej Mycielski 1 , M. Witkowska-Baran 1 , D. Kochanowska 1 , A. Szadkowski 1 , B. Witkowska 1 , W. Kaliszek 1 , R. Jakiela 1 , V. Domukhovski 1
1 Institute of Physics, Polish Academy of Sciences, Warsaw Poland
Show AbstractThe high-resistivity (Cd,Mn)Te is believed to be suitable to succesfully replace the commonly used (Cd,Zn)Te system as a material for manufacturing large-area X- and γ-ray detectors. The composition homogeneity of large single crystals of the ternary compound used as a material for detectors is one of its most important features. This homogeneity seems to be easier to achieve for (Cd,Mn)Te because the segregation coefficient of Mn in CdTe is negligible with respect to that (approx. 1.3) of Zn. The purpose of our study was to elaborate a method of preparing high quality (Cd,Mn)Te crystal plates as well as a technique of producing good electrical contacts to that material. The (Cd,Mn)Te crystals, doped with vanadium to the level of 1÷5 x 1016 cm-3, were grown using the Bridgman method. The crystals grown with this method are usually twinned in the (111) plane, but by cutting the crystal parallel to the twinning plane we got monocrystalline plates of large area (e.g. 30 x 30 mm2 with the thickens 2÷3 mm). Such plates are essential for application purposes. We obtained semi-insulating material with high (~1010 Ω.cm) resistivity due to doping with vanadium which acts as a compensating centre and due to proper annealing of the crystal plates in cadmium vapours. Annealing reduces the number of cadmium vacancies (acceptors) existing in the crystal after the growth, i.e. – reduces that concentration of vanadium, which is required for compensation. Moreover – annealing of the plates in cadmium vapours allows reduction of the number of tellurium precipitates forming during the growth process The ZnTe:Sb layers (~1 μm thick) were grown on the epi-ready (Cd,Mn)Te:V plates by the MBE technique. The layers were p-type and formed a good electrical contact to the crystal plates. Characterization of the obtained crystals is described. The behaviour of the preliminary detectors is shown.
4:30 PM - **O4.2
Crystal Growth, Post Treatments, and Characterization of CdTe and Cd0.9Zn0.1Te for Nuclear Radiation Detectors.
Krishna Mandal 1 , Sung Kang 1 , Michael Choi 1 , Alket Mertiri 1 , Caleb Noblitt 1 , Anton Smirnov 1
1 Advanced Materials Research Division, EIC Laboratories, Inc., Norwood, Massachusetts, United States
Show AbstractCdTe and Cd0.9Zn0.1Te (CZT) crystals have been studied extensively at EIC Laboratories for various applications including x- and γ-ray imaging and high energy radiation detectors. The crystals were grown from in-house zone refined ultra pure precursor materials using a vertical Bridgman furnace. The growth process has been monitored, controlled and optimized by a computer simulation and modeling program (MASTRAPP). The grown crystals were thoroughly characterized after sequential surface passivations and post-growth annealing treatments with and without component overpressures. The infrared (IR) transmission images of the post-treated CdTe and CZT crystals showed average Te inclusion size of ~10 μm for CdTe crystal and ~8 μm for CZT crystal. The etch pit density was ≤ 5×104 cm-2 for CdTe and ≤ 3×104 cm-2 for CZT. Various planar and Frisch collar detectors were fabricated and evaluated. From the current-voltage measurements, the electrical resistivity was estimated to be ~ 1.5×1010 Ω.cm for CdTe and 2-5×1011 Ω.cm for CZT. The Hecht analysis of electron and hole mobility-lifetime products (μτe and μτh) showed μτe=2×10-3 cm2/V (μτh=8×10-5 cm2/V) and 3-6×10-3 cm2/V (μτh=4-6×10-5 cm2/V) for CdTe and CZT, respectively. Final assessments of the detector performance have been carried out using various radio-isotopic sources in the energy range of 5-662 keV. Detailed characterization results will be correlated to optimum crystal growth conditions.
5:00 PM - **O4.3
X-ray Topography to Characterize CZT Crystals.
David Black 1 , Martine Duff 2 , Douglas Hunter 2 , Arnold Burger 3 , Michael Groza 3
1 , NIST, Gaithersburg, Maryland, United States, 2 , SRNL, Aiken, South Carolina, United States, 3 , Fisk University, Nashville, Tennessee, United States
Show AbstractX-ray topography is an under utilized characterization tool for single-crystal materials. A brief review of synchrotron-radiation based x-ray diffraction topography will be given. The emphasis will be on the practical aspects of the technique applied to study crystal growth and subsequent processing of technologically important materials. The specific application of topography to characterize CdZnTe gamma detector crystals will be discussed and the results of research to correlate detector performance and microstructure will be presented.
5:30 PM - O4.4
Characterization of Spatial Heterogenieties in Detector Grade CdZnTe.
Martine Duff 1 , Doug Hunter 1 , Arnold Burger 2 , Michael Groza 2 , Vladimir Buliga 2 , John Bradley 4 , Giles Graham 4 , Zurong Dai 4 , David Black 3 , Hal Burdette 3 , Antonio Lanzirotti 5
1 , Savannah River National Laboratory, Aiken, South Carolina, United States, 2 , Fisk University, Nashville, Tennessee, United States, 4 , Lawrence Livermore National Laboratory, Livermore, California, United States, 3 , National Inst. of Standards and Technology (NIST), Gaithersburg, Maryland, United States, 5 , University of Chicago, Chicago, Illinois, United States
Show AbstractSynthetic Cd1-xZnxTe or “CZT” crystals are highly suitable for the room temperature-based spectroscopy of gamma radiation. Structural/morphological heterogeneities within CZT, such as secondary phases that are thought to consist of Te metal can have negative impacts on detector performance. In this study, we used transmission and scanning electron microscopy and X-ray based techniques such as X-ray transmission imaging and X-ray topographic imaging to examine the heterogeneous morphological and compositional properties of detector grade CZT crystals. With the use of these characterization techniques, our study identifies two dominant types of secondary phase morphologies on the surface of detector-grade CZT material. The mechanisms for the formation of these types, as well as their individual influence on the detector performance could be different and are yet to be elucidated. A review of prior published work on the characterization of secondary phase morphologies in CZT will also be presented.
5:45 PM - O4.5
Atomistic Simulation of CdTe Solid-liquid Coexistence Equilibria.
Charles Henager 1 , James Morris 2 3 , Lujian Peng 3 , Howard Heinisch 1 , John Jaffe 4 , Andrew Kuprat 5 , Alex Tartakovsky 6 , S Kamakshi Sundaram 7
1 Materials Sciences, PNNL, Richland, Washington, United States, 2 Materials Science & Technology, ORNL, Oak Ridge, Tennessee, United States, 3 Dept. of Materials Science, University of Tennessee, Knoxville, Tennessee, United States, 4 Interfacial Chem & Engnrg, PNNL, Richland, Washington, United States, 5 Biological Monitoring/Modeling, PNNL, Richland, Washington, United States, 6 Computational Mathematics, PNNL, Richland, Washington, United States, 7 Glass Science & Processing, PNNL, Richland, Washington, United States
Show AbstractAtomistic simulations of CdTe using a Stillinger-Weber interatomic potential have been undertaken to model the solid-liquid phase equilibria of this important semiconductor material. Although this potential has been used by others to study liquid CdTe and vapor-liquid interface it is not well-suited for liquid simulations as it is based on fitting parameters optimized for the diamond cubic solid. However, it has not been fully explored as a useful potential for solid-liquid phase equilibria until this work. We report an accurate determination of the melting temperature, TM=1300K near P=0, the heat of fusion at melting and as a function of temperature up to 1700K, and on the relative phase densities with particular emphasis on the melting line. We find that the S-W potential for CdTe, along with not matching known pair correlation data for the liquid, does not simulate a liquid with the proper density for what should be a water-like liquid. We find that the liquid density is less than that of the solid and, hence, the melting line slopes upwards to the right instead of to the left, as it should. We also explore the structure of the liquid-solid interface and techniques for determining the anisotropic interfacial properties for phase-field model calculations.____________________________________________PNNL is operated for the U.S. Department of Energy by Battelle Memorial Institute under Contract DE-AC06-76RLO 1830. This work was funded at PNNL by the Office of Defense Nuclear Nonproliferation, Office of Nonproliferation Research and Development (NA-22) and at ORNL by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy under contract DE-AC05-00OR-22725 with UT-Battelle, LLC.
O5: Poster Session
Session Chairs
Thursday AM, November 29, 2007
Exhibition Hall D (Hynes)
9:00 PM - O5.2
Use of Al2O3 Layer MOS Based Radiation Sensors Fabricated on Si Substrate.
Ercan Yilmaz 1 , Ilker Dogan 2 , Rasit Turan 2
1 Physics, Mustafa Kemal University, Antakya Turkey, 2 Physics, Middle East Technical University, Ankara Turkey
Show Abstract9:00 PM - O5.3
Defects in Rare Earth Sesquioxides.
Mark Levy 1 , Christopher Stanek 2 , Alexander Chroneos 1 , Ken McClellan 2 , Robin Grimes 1
1 Materials, Imperial College London, London United Kingdom, 2 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractOxides with the bixbyite structure have two crystallographically unique cation sites, namely in Wyckoff notation 24d and 8b. Since the symmetries of these two sites are different, C2 and S6 respectively, properties related to solute cations will vary depending on the site preference. Therefore, we have employed atomic scale simulation techniques to systematically investigate the solution site preference of a range of cations.Activated bixbyite oxides (e.g. Eu:Lu2O3) are being considered as radiation detectors. It is well established that these oxide systems exhibit luminescent properties making them ideal in the role as scintillator materials. However, due to the different symmetry of the two cations sites, the 8b is centrosymmetric while the 24d is non-centrosymmetric, the cations that reside at these sites will have different spectroscopic properties. As there are fewer possible transitions from the 8b site, it is expected that slower emission will occur with an increased probability of nonradiative transitions. It is therefore important to determine which lattice site the activator ion will occupy in order to predict efficient detector properties.The scintillator efficiency is limited by the presence of point defects in the lattice which act as nonradiative centres and traps for electrons and holes. It is, however, possible to introduce species into the lattice in order to reduce the concentration of deleterious trap centres and hence optimise optical properties. For example, it has been proposed that oxygen vacancies are the predominant electron trap sites in LuxY1−xAlO3. Having identified the performance limiting defect, it is possible to consider processes that decrease the concentration of these deleterious oxygen vacancies. In particular, Zr4+ has been used because oxygen interstitial defects are simultaneously introduced as charge compensating species. These interstitial defects are then able to recombine with oxygen vacancies through the Frenkel equilibrium, thus suppressing their concentration. Here, simulations have been used to predict the defect processes associated with the solution of extrinsic divalent and tetravalent ions. These calculations provide a mechanistic framework through which it is possible to identify doping schemes that optimise scintillator performance. A change in solution site preference is predicted for both divalent and tetravalent solution as a function of dopant and host lattice cation radii.
9:00 PM - O5.4
Low-Temperature Synthesis of Eu2+ and Dy3+ Doped Strontium Dialuminate (SrAl4O7: Eu2+, Dy3+) Scintillator Materials.
Vanya Uluc 1 , Zachary Beiley 2 , Cem Akatay 1 , Vesna Srot 3 , Cleva Ow-Yang 1
1 Materials Science and Engineering, Sabanci University, Istanbul Turkey, 2 Division of Engineering, Brown University, Providence, Rhode Island, United States, 3 Stuttgart Center for Electron Microscopy, Max-Planck-Institüt für Metallforschung, Stuttgart Germany
Show AbstractAlthough single crystals are widely used as scintillator materials, the use of polycrystalline powder ones is being proposed as a low-cost alternative. In this report, the synthesis of single phase, highly crystalline divalent europium and trivalent dysprosium doped strontium dialuminate (SrAl4O7: Eu2+, Dy3+) was investigated by modifying the Pechini process, which can be conducted at much lower temperatures compared to the conventional solid state diffusion process. These modifications were necessary due to strontium being one of the host elements in the crystal lattice. Because Sr has a high oxygen affinity, the conventional Pechini process promoted the formation of SrCO3, which hindered the material’s ability to form single phase and highly crystalline SrAl4O7. These are the key requirements for the material, when doped with Eu2+, Dy3+, to serve as a model system for studying the luminescence mechanisms. TG/DTA, XRD, and STEM characterizations were used to explore the region of phase stability of doped dialuminate in the SrO-Al2O3 system, between 700-1300°C. The reduction atmosphere and time were carefully controlled in order to achieve fully reduced samples. Oxygen content analysis performed on the samples revealed interesting reduction behavior in the strontium dialuminates not previously reported in the literature. When unreduced, crystalline, doped strontium dialuminate contained 60% oxygen, while after reduction for 4 hours, this amount decreased to and remained at 35%, independent of the reduction time. However, the glow intensity continued to increase in proportion to the change in the reduction time. The luminescence behavior of these compounds would also be presented in order to investigate the evolving electronic structure in the doped dialuminate.
9:00 PM - O5.5
Low-Cost Synthesis of the Scintillator Compound, Eu- and Dy-Doped SrAl12O19 (SA6), as a Model Material for Electronic Structure Characterization.
Cleva Ow-Yang 1 , Omur Isil Aydin 1 , Cahit Benel 1 , Mahmut Aksit 1 , Abdulkadir Yurt 1 , Mehmet Ali Gulgun 1
1 Materials Science & Engineering, Sabanci University, Istanbul Turkey
Show AbstractCrystalline inorganic scintillator materials, such as rare earth-doped strontium aluminate compounds, offer a cost-effective alternative to the widely used single crystal materials in radiation detection. The overall goal of our work is to develop a model for the effect of dopants in such materials on mechanisms of extended afterglow. To this end, the requirements for characterizing the crystal field impose demands for single phase, crystalline powders. We would like to present a solution polymerization process for synthesizing SA6 doped with Eu and Dy. When reduced, these phosphoresce in the green spectral range, and some control over the length of persistence has already been reported. To obtain single phase strontium aluminate compounds, the Pechini process was modified, in order to enable the use of nitrate precursors to form the desired inorganic solid lattices. The key mechanism is based on using a carboxylic acid group and an alcohol to trap metallic cations inside an organic environment, a resin, and igniting the mixture at relatively low temperatures to form the ceramic crystal structures. In contrast to the more commonly used solid state diffusion process (typically at 1900 °C), our modified process allows synthesis at 1100 °C and yields the single phase, crystalline structural properties necessary for characterization. We would present the modified solution polymerization process, which was developed via XRD, STA, and FT-IR, photoluminescence, and electron energy loss spectroscopy analyses, that yielded the model systems for further study of the crystal field in these scintillator materials. The materials characterization techniques implemented would also be broadly applicable in the study of other scintillator materials.
9:00 PM - O5.6
Influence of Gamma-Irradiation on Dielectric Properties of Recycled Polyethylene Composites.
Ulmas Gafurov 1
1 , Instiute of Nuclear Physics, Tashkent Uzbekistan
Show Abstract9:00 PM - O5.7
Unifying Chemical Bonding Models for Boranes.
Mao-Hua Du 1 2 , Shengbai Zhang 2 , Susumu Saito 3
1 , Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States, 3 Physics, Tokyo Institute of Technology, Tokyo Japan
Show AbstractBoron has a high cross section for neutron capture. Semiconducting boron carbide films can be made for neutron detection by depositing carboranes, such as C2B10H12 molecules, on a substrate. In order to optimize the device properties, one needs to understand the basic structural, electronic, and chemical properties of the boron compounds. In this paper, we study the boron chemistry in prototypical systems, boranes. The understanding of the chemical bonding in these systems is useful for the understanding of the properties of boron and boron carbide solids, because the skeletons of the boranes are often the basic building blocks in boron bulk structures. Boron forms a complex series of hydrides, known as boranes. The main families of boranes are closo, nido, and arachno boranes. The closo boranes (BnHn2-) have cage structures, which are the most stable family of boranes. The nido (BnHn+4) and arachno (BnHn+6) boranes have open structures with their skeletons conveniently seen as closo boranes with one and two vertices absent, respectively. Carbon is probably the only other element that also has a rich family of hydrides, i.e., hydrocarbons. However, the chemical bonding in boranes is entirely different from that in hydrocarbons, and the borane structures are also surprisingly complex and less understood compared to hydrocarbons. The origin of the complexity in borane chemistry is that boranes are electron-deficient systems, i.e., boron has more bonding orbitals (4) than electrons (3) such that there are too few electrons to make two-center two-electron (2c2e) σ bonding networks that dominate in hydrocarbon structures. The existing chemical bonding model based on a combination of 2c2e and three-center two-electron (3c2e) bonds can explain the stability of nido and arachno boranes but not the closo boranes. Here, we propose a new model based on the resonant 3c2e bonding (with no localized 2c2e bonding) for the closo boranes, which, together with the previous model, provide a unified chemical bonding model for boranes. Our model explains why all the closo boranes are stable only as dianions. We also show that the delocalized resonant 3c2e bonding in the closo boranes gives rise to 3D σ aromaticity. The electronic shell structure found in B12H122- demonstrates the delocalized nature of the skeletal electrons in the closo boranes and explains the exceptional stability of the B12H122- and C2B10H12 clusters.
9:00 PM - O5.8
Atomistic Simulation of Rare-earth Halide Structures.
Ankoor Patel 1 2 , Mark Levy 1 , Chris Stanek 2 , Kenneth McClellan 2 , Robin Grimes 1
1 Materials Department, Imperial College London, London United Kingdom, 2 Materials Science Division, Los Alamos National Lab, Los Alamos, New Mexico, United States
Show Abstract
Improved radiation detectors are vital to the success of wide ranging activities, notably: medical imaging, high-energy physics research and homeland security. Lanthanide halide scintillators are excellent candidates for new detection materials due to their high densities and high light output. However there are several problems with rare-earth halides; principally they are hygroscopic making single crystal growth difficult.
Transparent polycrystalline ceramics are alternatives to single crystals. However, transparency is difficult to achieve in non-cubic systems due to anisotropic light transmission known as birefringence. Birefringence leads to decreased spatial accuracy of radiation detection, and thus decreases device efficiency. In order to overcome this obstacle for polycrystalline samples, very small grain sizes are required. A ceramic will behave as an isotropic material once the grains are sufficiently small relative to the wavelength of light.
Atomistic scale computer simulations based on interatomic pair potentials have been used to study the phase and structural trends across the lanthanide (La3+ to Lu3+) halide series (F-, Cl-, Br- and I-). We begin our consideration of lanthanide halides with the fluorides, for which the most experimental structure and property data exists. For these compounds we have predicted the phase change from P3-c1 (LaF3) to Pnma (LuF3). We then extended this method to consider lanthanide chlorides, for which less is known experimentally. The ultimate result of our study is a self-consistent set of potentials that can be used to describe lanthanide halide ion-ion attractions and reproduce (or predict) the crystal structures for the entire series of lanthanide halide compounds. Indeed, these new potentials have been used to predict structures of previously uncharacterized compounds, which we have used to systematically predict phase transformations and cell parameters. With this information, it is possible to predict the degree of crystallographic anisotropy, and in-turn the degree of birefringence. Therefore, by understanding structural variations, it may be possible to identify new scintillator compounds as well as general regions of compositional interest.
Symposium Organizers
Dale L. Perry Lawrence Berkeley National Laboratory
Arnold Burger Fisk University
Larry Franks Special Technologies Laboratory
Michael Schieber The Hebrew University of Jerusalem
O6: Scintillators I
Session Chairs
Thursday AM, November 29, 2007
Republic A (Sheraton)
9:30 AM - **O6.1
Alternative Scintillator Materials – Transparent Ceramics and Glass Scintillators.
Lynn Boatner 1
1 CRDMS -MSTD, Oak Ridge National Lab., Oak Ridge , Tennessee, United States
Show AbstractWhile new high-performance scintillator materials like lutetium oxyorthosilicate (LSO) or lanthanum tribromide doped with cerium are currently available, the single crystal growth of these and other important crystalline scintillators continues to be a time-consuming and expensive production method. The elimination of the costly crystal growth process by developing optically transparent ceramics offers a promising alternative approach to the large-scale fabrication of relatively inexpensive scintillators. The technology for forming optically transparent ceramics of cubic materials has recently been well developed for a number of cubic laser hosts, but most of the modern scintillators are optically anisotropic, non-cubic crystals. The fabrication of transparent ceramics of non-cubic systems is a challenging task due to birefringence effects in the differently oriented microstructural grains – effects that can effectively destroy the optical transparency. One approach to this difficult challenge is through the development of highly crystallographically aligned ceramic microstructures or microstructural “texturing”. Here, we will describe the results of studies of the development of novel ceramic scintillators based on selected cubic as well as non-cubic materials including lutetium oxide, ZnO, LSO, and lanthanum tribromide:Ce. The effects of various sintering or hot-pressing conditions, as well as post-densification annealing treatments, will be discussed. Glass scintillators also offer a production path that is an alternative to the growth of single-crystal scintillators, but glass scintillators have traditionally been limited by their low light yields. We are engaged in a program of research in which new families of glass-forming systems are being investigated with the goal of identifying new higher-light-yield glass scintillators that can easily be cast as large-area detectors and to near-net-shape. For example, we are investigating the applicability of a variety of phosphate glasses as host systems for the formation of cerium-activated gamma- and x-ray scintillators. The melting and pouring temperature of ~1050 Centigrade for these phosphate glasses is significantly lower than the processing temperatures generally associated with the formation of silicate glass scintillators. This work on new glass scintillator systems also includes studies of the effects of post-synthesis thermochemical treatments on the scintillator
10:00 AM - **O6.2
Recent Advances in Ceramic Scintillators.
Edgar Van Loef 1 , Yimin Wang 1 , Jarek Glodo 1 , Charles Brecher 2 , Alex Lempicki 2 , Kanai Shah 1
1 , RMD, Watertown, Massachusetts, United States, 2 , ALEM Associates, Watertown, Massachusetts, United States
Show AbstractInorganic scintillators coupled to optical detectors such as photomultiplier tubes (PMTs) or silicon photodiodes, provide detection and spectroscopy of ionizing radiation and charged particles by converting ionizing radiation into optical photons, which are subsequently detected by the PMT or photodiode. These devices are an important part of medical imaging applications such as positron emission tomography and computed tomography, find practical use in nuclear and particle physics experiments, and are indispensable for nuclear non-proliferation.
While most efforts have been directed towards the research and development of novel inorganic single crystals for scintillation detection, the field of ceramic scintillators has only received scant attention since the development of the HiLight™ scintillator by General Electric. This is in part due to the difficult task of fabricating transparent ceramics from non-cubic materials and the manufacturing of large volume devices.
Recently, ceramic scintillators are gaining interest because they may be produced in transparent forms from non-cubic materials using recent advances in nano-technology and alternative chemical synthesis routes. Among the most promising optically transparent ceramic scintillators are SrHfO3, BaHfO3, Lu2Hf2O7, LuAG, and LSO, doped with Ce or Pr. These materials may combine a high density and high atomic number with fast emission and a good light yield.
In this paper we report on the manufacturing and characterization of SrHfO3:Ce, Lu2Hf2O7:Ce, YAG, LuAG, LSO and CeBr3 ceramics. We used conventional and novel chemical methods to synthesize powders, employed hot pressing and hot isostatic pressing to manufacture ceramic scintillators and measured the optical and scintillation properties by means of radioluminescence, pulse height, luminescence decay and timing measurements.
Ceramic forms of SrHfO3:Ce, Lu2Hf2O7:Ce, YAG, LuAG, LSO and CeBr3 appear to be reasonable bright and fast scintillators with light yields of the order of BGO and principle decay time constants of the order of 20 – 100 ns.
Overall, the measurements indicate that these ceramic scintillators are promising candidates for numerous radiation detection applications, including medical imaging modalities such as PET and CT , if they can be produced in large quantities at low cost.
10:30 AM - O6.3
Scintillation Materials Based on Metal Organic Frameworks.
F. Doty 1 , Christina Bauer 3 1 , Andrew Skulan 1 , Patrick Grant 2 , Blake Simmons 1 , Mark Allendorf 1
1 , Sandia National Laboratories, Livermore, California, United States, 3 Dept. of Chemistry, Georgia Institute of Technology, Atlanta, Georgia, United States, 2 , Lawrence Livermore National Laboratory, Livermore, California, United States
Show AbstractExisting scintillation detectors are limited by material properties, such as nonproportional response and low luminosity. For example, conventional organic scintillators (liquid or plastic) used for fast neutron detection suffer from low, and highly nonlinear light yield for recoil protons below 10 MeV, resulting in low signals for neutrons relative to electrons, and an effective threshold for gamma rejection well above the fission spectrum peak. These limitations could be addressed by designing new materials with low dE/dx and high scintillation efficiency.Metal organic framework (MOF) materials are a new class of nanoporous coordination polymers that can be designed with unique structure and properties with potential for improved scintillation materials. The distinguishing features of MOFs are coordinating metal polyhedra causing organic “linker” molecules to self-organize into highly porous networks capable of accommodating a variety of adsorbates or “guest” molecules. The linkers enable rational control over pore geometry and chemistry, affording new degrees of freedom to design materials for specific radiation detection applications, including: independent control of mass density and fluorescence properties; tailorable composition through linker moieties, metal clusters, and guest species; ultra-low density materials to minimize dE/dx quenching; ultra-high surface areas (up to 6,000 m2/g) for high-density storage of particle conversion gases (e.g., H2, 3He, or Xe). We have synthesized and tested new highly fluorescent MOFs based on stilbene dicarboxylic acid as a linker. The crystal structure and porosity of the product are dependent on synthetic conditions and choice of solvent, and a low-density cubic form has been identified by x-ray diffraction. In this work we report experiments demonstrating scintillation properties of these crystals. Bright proton-induced luminescence with large shifts relative to the fluorescence excitation spectra were recorded, peaking near 475 nm. Tolerance to fast proton radiation was evaluated by monitoring this radio-luminescence to absorbed doses of several hundred MRAD. Scintillation and pulse-mode detection were demonstrated in 241Am alpha particle counting experiments, and the luminosity was compared to anthracene samples tested under identical conditions.
O7: Scintillators II
Session Chairs
Thursday PM, November 29, 2007
Republic A (Sheraton)
11:15 AM - **O7.1
The Development of New Scintillator Materials - Challenging From All Aspects.
Michael Squillante 1 , Kanai Shah 1 , Gerald Entine 1
1 , Radiation Monitoring Devices, Inc., Watertown, Massachusetts, United States
Show Abstract11:45 AM - **O7.2
Energy Resolution and Non-proportionality of Scintillation Detectors.
Marek Moszynski 1
1 , Soltan Institute for Nuclear Studies, Otwock-Swierk Poland
Show AbstractThe limitation of energy resolution of scintillation detectors are discussed with a special emphasis on non-proportionality response of scintillators to gamma rays and electrons, which is of crucial importance to an intrinsic energy resolution of the crystals. Examples of the study carried out with different crystals and particularly those of tests of undoped NaI and CsI at liquid nitrogen temperature with the light readout by avalanche photodiodes are presented suggesting strongly that the non-proportionality of the halide crystals are not their intrinsic property. Moreover, the influence of slow components of the light pulses on energy resolution and non-proportionality are discussed.
12:15 PM - O7.3
Electronic Structure Studies of Ce-doped Gamma Detector Materials.
Andrew Canning 1 , Rostyslav Boutchko 1 , Stephen Derenzo 1 , Lin-Wang Wang 1 , Marvin Weber 1
1 CRD, LBNL, Berkeley , California, United States
Show AbstractCerium doped materials such as the lanthanum halides represent some of the brightest known scintillators for the detection of gamma rays. The scintillation process in Cerium doped materials corresponds to the transition from a 5d to 4f state on the Cerium atom where the 4f and 5d states must lie in the gap of the host materials. We have performed electronic structure calculations for many different Cerium doped materials using density functional based methods to determine the positions of the 4f and 5d states relative to the valence and conduction bands of the host materials. We find good agreement with experimental results for the systems studied in particular for the lanthanum halides. Our theoretical calculations will be used as a first step screening for candidate new detector materials.Work supported by the Department of Homeland Security.
12:30 PM - O7.4
Development of Improved Scintillators via Atomistic Simulation Guided Defect Engineering.
Christopher Stanek 1 , Ken McClellan 1 , Mark Levy 2 , Robin Grimes 2
1 Material Science and Technology, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 Materials Dept., Imperial College London, London United Kingdom
Show AbstractSince the discovery that solids are comprised of atoms, researchers have arduously searched for methods to identify the nature of defects in regular atomic structures. Point defects are present in all materials and the effects corresponding to their presence are diverse. In some cases, defects enable materials, such as providing the color of gemstones. However, in other cases, defects limit a material's performance, such as the strength of metals. In either case, it is obvious that the ability to control defect structure and concentration on an atomic scale will lead to significantly improved materials. However, the first hurdle encountered when attempting to defect engineer a material is the identification of defect structure. The determination of defect structure is notoriously difficult, and is generally done by a process of elimination after laborious investigation involving a suite of analytical tools – none of which alone can unequivocally determine defect structure.Gamma-ray spectroscopy is a common technique to detect special nuclear material. This spectroscopic technique most often relies upon a scintillation detector (semiconductor detectors are also used), which in turn must be dense, bright and have a fast response. Significant improvements in material performance can be achieved, predominantly through the understanding and subsequent control of imperfections responsible for non-radiative transitions. For example, the overall scintillator efficiency of YAP:Ce (YAlO3:Ce) has been determined to be only ~ 35%. This marked inefficiency is typical of many promising compounds and can often be traced to defects in the lattice responsible for non-radiative transitions. It therefore follows that if we are able to understand and subsequently control these defects, i.e. defect engineer, significant improvements in scintillator efficiency are possible and worthwhile.In this presentation, we discuss how atomic scale simulations can be used to complement synthesis and characterization activities to improve the performance of a wide range of scintillator materials, such as: REAlO3 perovskites, RE3Al5O12 garnets, RE2O3 bixbyites and REX (where RE denotes a rare earth cation and yttrium and X denotes a halogen anion). These simulations, when properly coupled to synthesis and characterization, are able to reveal non-intuitive phenomena related to defect structure. By adding this tool to the standard defect identification methodology, we can optimize the development of scintillator materials.
12:45 PM - O7.5
Exploring Virtual Candidate Spaces: Design Rules for Ce-activated Scintillators.
Kim Ferris 1 , Bobbie-Jo Webb-Robertson 1 , Dumont Jones 2
1 Computational and Information Sciences, Pacific Northwest National Laboratory, Richland, Washington, United States, 2 , Proximate Technologies, LLC, Columbus, Ohio, United States
Show Abstract The first and most evident goal in new gamma detection material development is the identification of a specific compound or class of materials with optimal targeting of multiple physical property requirements, while facing a large number of potential candidates. We have used an information-based approach to develop design rules that relate structural, chemical, and electronic features to materials and physical properties. Preliminary results (stepwise regression, Table 1 below) on the small set of Ce-activated compounds reported by Balceryk [1], show a R2 = 0.94, a cross-validated R2 = 0.50 using a structure-less basis [2]. The RMS error for the predicted data set is 2930, approximately 18% of the mean scaled luminosity, with the mean RMS error approximately the same upon cross-validation [3]. The primary components in the structure-property type relationship include the band gap and a simple set of physiochemical properties readily calculated from the matrix molecular formula. Suitable luminosity imposes design constraints that are distinct from those for stopping power and efficiency, and depend largely upon matrix valence electron properties and their coupling to activator sites—properties that do not require high atomic masses per se. More importantly, this relationship for Ce-activated luminosity allows for estimation of the luminosity of virtual candidates; compounds yet to be synthesized and measured, developed in silico. [Acknowledgement] The authors gratefully acknowledge financial support from the PNNL Laboratory Directed Research and Development Project. The Pacific Northwest National Laboratory is operated by Battelle Memorial Institute for the US Department of Energy under contract DE-AC06-76-RL1830. [References] 1. M. Balcerzyk, Z. Gontarz, M. Moszynski, and M. Kapusta, J. Lumin. 87-89 (2001) 963. 2. K.F. Ferris, B.M. Webb-Robertson and D. M. Jones, SPIE Special Publication, Vol. 6403 (2007) 64032A-1.3. B.M. Webb-Robertson, K.F. Ferris, and D.M. Jones, IEEE Trans. Nucl. Soc., Proc. SCINT2007, submitted.
O8: Characterization I
Session Chairs
Thursday PM, November 29, 2007
Republic A (Sheraton)
2:30 PM - O8.1
Surface Passivation of Semiconductor Surfaces for Improved Radiation Detectors: X-Ray Photoemission Analysis.
Art Nelson 1 , Adam Conway 2 , Catherine Reinhardt 2 , Cheryl Evans 1 , Jim Ferreira 1 , R. Bliss 1 , Rebecca Nikolic 2 , Steve Payne 1
1 MSTD, LLNL, Livermore, California, United States, 2 ETD, LLNL, Livermore, California, United States
Show AbstractSurface passivation of device-grade radiation detector materials was investigated using x-ray photoelectron spectroscopy in combination with transport property measurements before and after various chemical treatments. Specifically N2H4, NH4F/H2O2, (NH4)2S and NH4OH/thiourea solutions were used to passivate the surface of II-VI and III-VI semiconductor crystals. Scanning electron microscopy was used to evaluate the resultant microscopic surface morphology. Angle-resolved high-resolution photoemission measurements on the valence band electronic structure and core lines were used to evaluate the surface chemistry of the chemically treated surfaces. Metal overlayers were then deposited on these chemically treated surfaces and the I-V characteristics were measured. The measurements were correlated to understand the effect of interface chemistry on the electronic structure at these interfaces with the goal of optimizing the Schottky barrier height for improved radiation detector devices.This work was performed under the auspices of the U.S. Dept. of Energy by the University of California Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48.
3:00 PM - O8.3
Exploring the Use of Orientation Imaging Microscopy in Characterizing Low-Pressure Bridgman Crystal Growth.
Charles Henager 1 , Dan Edwards 1 , Mary Bliss 3 , S Kamakshi Sundaram 2 , Howard Heinisch 1 , Alex Tartakovsky 4 , Andrew Kuprat 5 , John Jaffe 6
1 Materials Sciences, PNNL, Richland, Washington, United States, 3 Radiation Detection and Nuclear Science, PNNL, Richland, Washington, United States, 2 Glass Science and Processing, PNNL, Richland, Washington, United States, 4 Computational Mathematics, PNNL, Richland, Washington, United States, 5 Biological Monitoring/Modeling, PNNL, Richland, Washington, United States, 6 Interfacial Chem & Engnrg, PNNL, Richland, Washington, United States
Show AbstractThe growth-tip region of a high-purity Ge boule grown using low-pressure Bridgman methods was characterized using orientation imaging microscopy, or electron backscatter diffraction (EBSD). The Ge boule, 4.2 cm in diameter, was sectioned and polished in preparation for scanning electron microscopy. The boule had a characteristic conical tip region with cone angle of 40 degrees of a right circular cylinder. The conical tip section was taken from the boule longitudinal centerline and had an approximate surface area of 1 cm2. The majority of this surface area was examined using an EBSD system and the grain structure, grain boundary orientation, twin structure, and overall crystal growth direction were determined. A crystal growth direction of approximately [112] was determined, which was also identified as the growth direction of several prominent twins observed in the tip region. The structure of the tip region appeared to be controlled by the sidewall nucleation of several grains that competed for dominance during crystallization. Grain boundaries were identified as low-energy Σ3 boundary type, which suggests that post-growth annealing will have only minor influence on grain size. The use of EBSD as a tool for characterization of large-scale crystal growth specimens will be discussed.___________________________________________PNNL is operated for the U.S. Department of Energy by Battelle Memorial Institute under Contract DE-AC06-76RLO 1830. This work was funded by the Office of Defense Nuclear Nonproliferation, Office of Nonproliferation Research and Development (NA-22).
3:15 PM - O8.4
Characterization of voids in Lutetium Oxide P.V.D. Coatings.
Stephen Topping 1 , Sridhar Rangan 1 , Chul Park 1 , Vinod Sarin 1
1 Manufacturing Engineering, Boston University, Brookline, Massachusetts, United States
Show AbstractDue to its high density and cubic structure, Lutetium oxide (Lu2O3) has been extensively researched for scintillating applications. Present manufacturing methods, such as hot pressing and sintering, do not provide adequate resolution. Vapor deposition has been investigated as an alternative manufacturing method. Lutetium oxide transparent optical coatings by magnetron sputtering offer a means of tailoring the coating for optimum scintillation and resolution. Structural defects such as creation of voids in the sputtering process adversely affect transparency and scintillation, and are critical in growing stress free, thick coatings. The effect of process parameters on the density of voids is being investigated and techniques to measure and characterize void density via x-ray diffraction (XRD) will be presented and discussed.
O9: Characterization II
Session Chairs
Thursday PM, November 29, 2007
Republic A (Sheraton)
4:00 PM - O9.1
Effect of Processing Parameters on Transparency and Grain Structure of Eu:Y2O3 Scintillator Ceramics.
Stephen Podowitz 1 , Romain Gaume 1 , Robert Feigelson 1
1 Geballe Laboratory for Advanced Materials, Stanford University, Stanford, California, United States
Show AbstractThe need for high efficiency and low cost materials for detector systems is a primary concern for the practical deployment of large scale equipment. Recently, attention in the nuclear detector community has been drawn towards the area of optically transparent ceramics for scintillation detectors. Scintillator ceramics do not undergo component segregation during processing as do single crystals, and can, in general, be more heavily doped, produced in near-net shapes and scaled up to large sizes more readily than single crystal components. However, the fabrication of high optical-grade ceramics is not yet reproducible, and the mechanisms which prevail in their densification have yet to be fully understood.Here, we report on our ceramic processing studies of Eu:Y2O3 transparent scintillators as a model compound for cubic, rare-earth oxides such as Eu:Lu2O3 and Eu:YGO. At Stanford, we have identified several ceramic processing techniques which enable us to control transparency and ceramic microstructure. These involve nanopowder synthesis (size, shape, size distribution, purity, agglomeration), powder casting techniques and sintering techniques (under pressure or pressureless, reactive or non-reactive, by fast sintering). In-situ densification studies during ceramic hot-pressing are discussed to elucidate the mass transport mechanisms that prevail during sintering. SEM, TEM, confocal microscopy and cathodo-luminescence investigations have been carried out in order to visualize the grain size, grain boundary structure, dopant segregation and phase of the Eu:Y2O3 ceramics. Ceramics with attenuation coefficients of less than 1 cm-1 at 633-nm transmission and with typical grain size ranging from 5 μm to 300 μm for maximum hot-press temperatures between 1400C and 1800C have been fabricated. Finally, we discuss our future investigations of the relative scintillation performance of these Eu:Y2O3 ceramics with varied grain structure to identify the true effect of ceramic microstructure on scintillation performance and the extent to which microstructure may be controlled to minimize defects which degrade scintillation performance.
4:15 PM - O9.2
Antisite Defects in Rare Earth Perovskites and Garnets.
Mark Levy 1 , Christopher Stanek 2 , Ken McClellan 2 , Robin Grimes 1
1 Materials, Imperial College London, London United Kingdom, 2 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractRare earth perovskite and garnet based compounds are exciting and promising scintillator radiation detector hosts. Key to the operation and efficiency of these detectors is their defects, a number of which will be cation antisites even in a truly intrinsic material. It has been suggested that the presence of such cation antisite defects decreases efficiency of the detector, with lower light yield and longer emission times. Codoping the host oxide with a similarly sized cation, e.g. Ga in Lu3Al5O12, decreases the emission spectra due to the presence of the cation antisite defects, thereby improving the figure of merit of the detector.Here, atomistic-scale energy-minimisation monte-carlo simulations have been conducted that consider the possible configuration of the cation antisite defects in the supercell of the host lattice, both intrinsically and in the presence of a dopant. The results of these simulations allow for the elucidation of the cation antisite defect concentrations and the host lattice sites at which they reside. These results are then compared to experimental literature data to determine the nature of the increased detector efficiency.It is well known that point defects, either in the form of intrinsic defects or those associated with impurities, are at least partly responsible for nonradiative transition. Since nonradiative processes are a fundamental limitation to scintillator performance, it is imperative to understand these imperfections in order to improve scintillation properties. An important consideration when considering defect concentrations is the sites at which the dopant and the antisite defect reside. The importance of this is that the different cation sites behave differently in terms of allowable electronic transitions, thus limiting the detector efficiency. As the simulations consider the configuration of dopants and defects concentrations in the lattice, these will be used to predict the relative concentration of the different cation antisite defects in relation to their lattice sites. This information will then be able to predict which lattice sites dopants will preferentially reside which in turn will facilitate predictions of improved scintillator chemistry.
4:30 PM - O9.3
Comparison of The Heterogeneous Nucleation and the Coalescence of HgI2 and BiI3 onto Amorphous Substrates.
Laura Fornaro 1 , Ana Noguera 1 , Ivana Aguiar 1 , Maria Perez 1 , Eduardo Quagliata 1
1 Compound Semiconductors Group, Faculty of Chemistry, Montevideo, Montevideo, Uruguay
Show Abstract4:45 PM - O9.4
γ- Irradiation Stimulated Change of Crystalline Structure of Recycled Polypropylene Composites.
Ulmas Gafurov 1
1 , Instiute of Nuclear Physics, Tashkent Uzbekistan
Show AbstractInfluence of γ-irradiation on crystalline structure (morphology) of thermoplastic composites on recycled polypropylene bases has been investigated. The composite samples were γ-irradiated at different dose of the irradiation. It was investigated follows thermoplastic composites 1 - PPR/EPDM- (40/35 wt, % ) and 2- PPR/EPDM/GTR – (40/35/25 wt,%). EPDM(ethylene-propylene diamine)X-Ray diffraction patterns (diffractograms) were obtained using X-ray diffractometer DRON-3M with usual focusing by Bragg-Brentano procedure. At the first stage the samples from all group were irradiated up to doses of 100, 500 and 1000 kGy. But X-ray analysis has shown that all changes of the structure parameters were within experimental error. Thereby doses (and doses step) were increased. The lattice parameters a and b of the polypropylene lattice of the polymeric composite TDV irradiated with various doses of gamma radiation has been defined.Calculation was carried out on usual algorithm: on the measured values of angles of reflection with the help of Woolf – Bragg’s formula.It was shown different character of influence of gamma-irradiation on structure ( lattice parameters a and b ) of the samples with various composition. The dose dependence of crystallinity degree both of composite and polypropylene are analogous; it means that character of these changes is similar. So gamma radiation- induced change of crystallinity degree of the vulcanizates is defined mainly by the recycled polypropylene basis. Some initial increase in crystallinity with a dose is the result both of forming of cross-links and radiation-induced annealing of structure defects. For vulcanizates on the basis of the recycled polypropylene more apparent reduction of crystallinity degree and expansion of a lattice occurs at smaller radiation doses. At doses higher than 2000 kGy polypropylene vulcanizate amorphization was accompanied by decomposition processes (gas outlet). The work was supported by EC (STCU Project U3009).
5:00 PM - O9.5
Effects of Gamma Irradiation on Optical Properties of Colloidal Nanocrystals.
Nathan Withers 1 , Krishnaprasad Sankar 1 , Brian Akins 1 , Tosifa Memon 1 , Shin Bowers 1 4 , Erica Ortega 2 , Melisa Greenberg 1 3 , Gennady Smolyakov 1 , Robert Busch 2 , Marek Osinski 1
1 Center for High Technology Materials, University of New Mexico, Albuquerque, New Mexico, United States, 4 Present address: Division of Engineering, Brown University, Providence, Rhode Island, United States, 2 Chemical and Nuclear Engineering, University of New Mexico, Albuquerque, New Mexico, United States, 3 , Present address: CVI Laser LLC, Albuquerque, New Mexico, United States
Show AbstractColloidal nanocrystals have attracted tremendous interest over the last few years for a wide range of applications. So far, however, their potential has generally eluded the nuclear detector community. Yet, compared to currently used scintillating particles of the micrometer size, NCs offer the prospect of significantly improved performance. Due to their small size, they are expected to have better solubility in organic polymer or inorganic sol-gel host materials and to cause much less scattering, which should result in higher efficiency of the scintillator. Due to three-dimensional confinement and much better overlap of electron and hole wavefunctions, the optical transitions are expected to be much faster than in bulk scintillators, which should eliminate the major problem of relatively slow response of scintillator detectors. So far, only scintillation of commercial CdSe/ZnS core/shell quantum dots under alpha and gamma-ray irradiation has been reported, with no attempt to assess possible degradation effects of large-dose exposure.For future applications of nanocrystals as scintillating materials it is important to know the levels of irradiation that would degrade their optical properties. In this paper we report the results of, to our best knowledge, the very first study of the effects of gamma-irradiation on photoluminescent properties of several types of nanocrystals. Optical degradation of the nanocrystals was evaluated based on the measured dependence of their photoluminescence intensity on the irradiation dose.The initial studies were performed on CdSe, CdSe/ZnS, ZnCdSe, CdS, LaF3:Ce, and LaF3:Eu colloidal nanocrystals synthesized on a Schlenk line using appropriate solvents and precursors. Eberline 1000B multiple source gamma calibrator was used to study the effects of irradiation. In order to accelerate the degradation process, the strongest of the available sources was used, namely a Cs137 source emitting 91.8 rad/hour of 662 keV gamma rays. Using a Horiba Jobin Yvon Fluorolog-3 spectrofluorometer, photoluminescence measurements were performed after various periods of irradiation to check if the nanocrystals exhibited any signs of degradation in their optical characteristics, and compared with baseline samples that were not exposed to radiation.
5:15 PM - O9.6
Bulk Crystal Growth and Scintillation Properties of New Scintillator; Pr:LuAG.
Hiraku Ogino 1 , Kei Kamada 2 3 , Takayuki Yanagida 3 , Akira Yoshikawa 3 , Fumio Saito 3 , Jun-ichi Shimoyama 1 , Kohji Kishio 1
1 , The University of Tokyo, Tokyo Japan, 2 Materials Research Laboratory, Furukawa Co., Ltd., Tsukuba, Ibaraki, Japan, 3 Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, Miyagi, Japan
Show Abstract There are continuous demands for better scintillator crystal with efficient and fast scintillation. In the past decades, scintillators based on a 5d-4f luminescence of Ce3+ were intensively investigated because of their desirable properties. On the other hand, Pr3+ ion also shows the fast 5d-4f emission in the several host materials. Because of its parity/spin allowed nature and shorter wavelength, even faster decay kinetics is expected. However, compared to Ce-doped materials there were no wide studies of Pr- activated materials. Recently our group intensively examined scintillation properties of Pr doped compounds and found out Pr-doped Lu3Al5O12(Pr:LuAG) had higher scintillation efficiency because of better temperature stability compared to Pr:Y3Al5O12 and fast decay time around 20ns. Such properties make it suitable for application to scintillation detectors. From October 2006, national project supported by Japan Science and Technology Agency has started in Tohoku University for the development of Positron Emission Mammography by using the material. Therefore there are demands of mass production of the material. In this study, we will report the growth results of high quality Pr:LuAG bulk single crystals grown by the Czochralski(Cz) method. We will also report their scintillation characteristics such as light yield and decay time.Pr:LuAG single crystals were grown by the Cz method with RF heating system using Ir crucible. Growth conditions such as rotation rate, pulling rate as well as growth orientation were optimized. Segregation coefficient of Pr3+ in LuAG host was determined and relation between Pr content and scintillation properties such as light yield and decay time were investigated. By using these results, transparent and crack-free crystal with homogeneous property could be grown. Light yield and energy resolution were determined by the energy spectra under 662keV γ-ray excitation (137Cs source) with a photomultiplier (Hamamatsu H6521). The crystal showed three times higher light yield than that of Bi4Ge3O12 and energy resolution was 8%. Scintillation decay was also measured and that was 23ns. There are no strong dependence of the scintillation properties with solidification fraction of the grown crystals. These results suggest that the Pr:LuAG is a very promising material for several kinds of scintillator application. Further results of the material will be discussed in the presentation.