Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session U3: Invited Session: Application of the First-Principles and Atomistic Methods to Nuclear Detection Materials |
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Sponsoring Units: DCOMP DCMP Chair: David Beach, Department of Energy, National Nuclear Security Administration Room: Ballroom III |
Thursday, March 21, 2013 11:15AM - 11:51AM |
U3.00001: Point Defect Properties of Cd(Zn)Te and TlBr for Room-Temperature Gamma Radiation Detectors Invited Speaker: Vincenzo Lordi The effects of various crystal defects in CdTe, Cd$_{1-x}$Zn$_x$Te (CZT), and TlBr are critical for their performance as room-temperature gamma radiation detectors. We use predictive first principles theoretical methods to provide fundamental, atomic scale understanding of the defect properties of these materials to enable design of optimal growth and processing conditions, such as doping, annealing, and stoichiometry. Several recent cases will be reviewed, including (i) accurate calculations of the thermodynamic and electronic properties of native point defects and point defect complexes in CdTe and CZT; (ii) the effects of Zn alloying on the native point defect properties of CZT; (iii) point defect diffusion and binding related to Te clustering in Cd(Zn)Te; (iv) the profound effect of native point defects---principally vacancies---on the intrinsic material properties of TlBr, particularly electronic and ionic conductivity; (v) tailored doping of TlBr to independently control the electronic and ionic conductivity; and (vi) the effects of metal impurities on the electronic properties and device performance of TlBr detectors. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:27PM |
U3.00002: First-principles calculations of self-trapping of carriers and excitons in NaI and SrI$_2$ Invited Speaker: Daniel Aberg While the general potential of scintillators as radiation detectors has been demonstrated, one of the current goals is to develop materials with improved energy resolution sufficient to detect fissile materials with a low probability of errors at ports, borders, and airports. The poor resolution has been linked to the non-linear response to the gamma ray energy. Fundamental understanding of this requires detailed knowledge of elementary electronic excitation processes. In particular, in most metal halide scintillators charge carriers and excitations localize and create self-trapped species associated with large effective masses and slow diffusivities. First-principles modeling is essential for providing quantitative understanding of the involved microscopic processes. Here, we present comprehensive ab-initio calculations, with techniques ranging from hybrid DFT+exact exchange to self-consistent GW and Bethe-Salpeter approach, for modeling the electronic structure and mobilities of self-trapped carriers and excitons in metal halides with particular attention given to sodium and strontium iodide. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 1:03PM |
U3.00003: Multiscale Modeling of Crystal Growth and Microstructural Evolution of CdZnTe Invited Speaker: Charles Henager, Jr. Crystal growth models and modeling tools for CdTe and CZT along with experimental melt-growth data will be presented and discussed. The emphasis will be on creating a multiscale-modeling framework that can be applied to solve portions of the crystal quality and reproducibility problem of CZT crystals grown for high-resolution radiation detectors. The growth models and methods include ab initio models of CdTe, ab initio molecular dynamics (MD) models CdTe, MD of solidification of CdTe, equilibrium growth defects in CdTe, and development of coarser-scale microstructural evolution models using phase field methods. These model and theory results will be discussed in terms of designing a multiscale approach to two relevant problems in CZT crystal growth, namely solid-liquid interface (SLI) stability and concurrent defect generation in the hot but cooling CZT solid. This dovetails with recent experimental research focused on the growth of CdTe from Te-rich melts with an emphasis on SLI instability. Experimental data on SLI instabilities will be featured as well as results of transmitted IR data on Te-particle distributions in as-grown CZT. A new mechanism of Te-particle genesis and spatial arrangement in CdTe and CZT is discussed in terms of a Rayleigh instability mechanism coupled with crystallographic SLI instabilities during growth. However, there are gaps in our capabilities at every length and time scale, plus gaps in building coarse-grained models from fine-scale models, in statistical representations of complex equilibria, and in understanding the complexities of solidification in ternary alloy systems where coupled thermal, concentration, stress, liquid flow, and SLI morphological fields exist. The talk concludes with an assessment of methods and approaches to address desired models and simulations of CZT solidification from the melt. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:39PM |
U3.00004: First-principles Electronic Structure Calculations for Scintillation Phosphor Nuclear Detector Materials Invited Speaker: Andrew Canning Inorganic scintillation phosphors (scintillators) are extensively employed as radiation detector materials in many fields of applied and fundamental research such as medical imaging, high energy physics, astrophysics, oil exploration and nuclear materials detection for homeland security and other applications. The ideal scintillator for gamma ray detection must have exceptional performance in terms of stopping power, luminosity, proportionality, speed, and cost. Recently, trivalent lanthanide dopants such as Ce and Eu have received greater attention for fast and bright scintillators as the optical 5d to 4f transition is relatively fast. However, crystal growth and production costs remain challenging for these new materials so there is still a need for new higher performing scintillators that meet the needs of the different application areas. First principles calculations can provide a useful insight into the chemical and electronic properties of such materials and hence can aid in the search for better new scintillators. In the past there has been little first-principles work done on scintillator materials in part because it means modeling f electrons in lanthanides as well as complex excited state and scattering processes. In this talk I will give an overview of the scintillation process and show how first-principles calculations can be applied to such systems to gain a better understanding of the physics involved. I will also present work on a high-throughput first principles approach to select new scintillator materials for fabrication as well as present more detailed calculations to study trapping process etc. that can limit their brightness. This work in collaboration with experimental groups has lead to the discovery of some new bright scintillators. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 2:15PM |
U3.00005: DFT Studies of Semiconductor and Scintillator Detection Materials Invited Speaker: Koushik Biswas Efficient radiation detection technology is dependent upon the development of new semiconductor and scintillator materials with advanced capabilities. First-principles based approaches can provide vital information about the structural, electrical, optical and defect properties that will help develop new materials. In addition to the predictive power of modern density functional methods, these techniques can be used to establish trends in properties that may lead to identifying new materials with optimum properties. We will discuss the properties of materials that are of current interest both in the field of scintillators and room temperature semiconductor detectors. In case of semiconductors, binary compounds such as TlBr, InI, CdTe and recently developed ternary chalcohalide Tl6SeI4 will be discussed. Tl6SeI4 mixes a halide (TlI) with a chalcogenide (Tl2Se), which results in an intermediate band gap (1.86 eV) between that of TlI (2.75 eV) and Tl2Se (0.6 eV). For scintillators, we will discuss the case of the elpasolite compounds whose rich chemical compositions should enable the fine-tuning of the band gap and band edges to achieve high light yield and fast scintillation response. [Preview Abstract] |
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