Romain M. Gaume

High-Throughput Combinatorial Database of Electronic Band Structures for Inorganic Scintillator Materials

For the purpose of creating a database of electronic structures of all the known inorganic compounds, we have developed a computational framework based on high-throughput ab initio calculations (AFLOW) and an online repository (www.aflowlib.org). In this article, we report the first step of this task: the calculation of band structures for 7439 compounds intended for the research of scintillator materials for γ-ray radiation detection. Data-mining is performed to select the candidates from 193,456 compounds compiled in the Inorganic Crystal Structure Database. Light yield and scintillation nonproportionality are predicted based on semiempirical band gaps and effective masses. We present a list of materials, potentially bright and proportional, and focus on those exhibiting small effective masses and effective mass ratios.

Mechanisms of nonstoichiometry in Y3Al5O12

Currently, Y2O3–Al2O3 phase diagrams do not show the technologically significant yttrium aluminum garnet (Y3Al5O12,YAG) phase as deviating from the stoichiometric ratio, i.e., YAG is always expressed as a line compound. In this paper, we not only report the synthesis of nonstoichiometric YAG, but also the use of atomistic simulation to predict the defect structure associated with the deviation. By comparing the experimental variation in the lattice parameter as a function of deviation from stoichiometry with the defect volume changes predicted by atomistic simulation, we predict that nonstoichiometry in YAG proceeds via cation antisite defects.

Comparative Study of Nonproportionality and Electronic Band Structures Features in Scintillator Materials

The origin of nonproportionality in scintillator materials has been a long standing problem for more than four decades. In this manuscript, we show that, with the help of first principle modeling, the parameterization of the nonproportionality for several systems, with respect to their band structure curvature suggests a correlation between carrier effective mass and energy response. We attribute this correlation to the case where free electrons and holes are the major energy carriers. Excitonic scintillators do not show such a definitive trend. This model suggests a potential high-throughput approach for discovering novel proportional scintillators in the former class of materials.

Nonproportionality and Scintillation Studies of

The low temperature scintillation properties of 5 atomic % Eu:SrI2 from ambient temperature down to 5 K were studied for the first time. With decreasing temperature, a shift in emission wavelength and a shortening of decay time were observed. Light yield and energy resolution exhibited notable changes with temperature, and were maximized as temperature was decreased. A degradation of light yield proportionality with decreasing temperature was observed.

{\hbox{Eu:}} {\hbox{SrI}}_{\rm 2}

Preparation of SrI2 and Eu:SrI2 ceramics is reported on for the first time. Translucent ceramics were sintered by hot-pressing. Thermal analysis on SrI2 and EuI2 starting powders was used to identify dehydration temperatures and informed the choice of sintering conditions. Optical transmission was found to decrease with increasing temperature and time. Microstructure studies (grain size and texture) were used to evaluate variations in optical transmission between samples. Scintillation characterization was carried out on 5 wt% Eu:SrI2 ceramics. X-ray radio-luminescence spectra showed both the characteristic Eu2+ emission band and a lower-intensity, broad emission in the visible. Scintillation lifetimes were shorter than those previously reported for single-crystal samples, and afterglow was substantially lower. A light yield of 21 000 ph/MeV was calculated from the photopeaks under γ-irradiation. Like single-crystal samples, Eu:SrI2 ceramics were found to have a very proportional response with γ -ray energy.

From 295 to 5 K

X-ray radioluminescence microscopy (XRLM), a novel fluorescence microscopy technique under focused x-ray excitation, was used to characterize micro-scale luminescence of Eu:Y2O3 and Ce:YAG transparent ceramics and bicrystals. The diffusion length of a known semiconductor measured by XRLM was found to be in agreement with previously measured values, illustrating its use for characterizing charge carrier transport. Emission intensity was found to drop at the boundaries in both Eu:Y2O3 and Ce:YAG ceramics and bicrystals. The depletion in emission at grain boundaries was ultimately found to be related to charge carrier depletion (through either deep trapping or non-radiative recombination). A charge carrier diffusion model was used to understand the effect of grain boundaries on charge carrier transport in these scintillators. The diffusion model was found to accurately predict the spatial distribution of emission in a Ce:YAG single-crystal as a function of x-ray excitation energy. Structural and chemical cha...

Fabrication and Properties of Translucent SrI

We demonstrate an optical technique, called laser-gain scanning microscopy (LGSM), to map dopant concentration profiles in engineered laser gain-media. The performance and application range of this technique are exampled on a Nd(3+) concentration profile embedded in a YAG transparent ceramic sample. Concentration profiles measured by both LGSM and SIMS techniques are compared and agree to within 5% over three-orders of magnitude in Nd(3+) doping level, from 0.001 at.% to 0.9 at.%. One of the unique advantages of LGSM over common physical methods such as SIMS, XPS and EMPA, is the ability to correlate optical defects with the final doping profile.

_2

When properly designed, optical-ceramics can yield high performance lasers and scintillators. Controlling the defects in these materials is essential to these applications. Systematic composition studies in YAG-ceramics, investigated by novel optical characterization techniques, will be presented. Article not available.

and Eu:SrI

Edge-pumping Nd:YAG laser gain media is a convenient method to couple pump power into a laser cavity. A difficulty with this geometry is that for uniformly doped materials, pump power deposited near the edge of the gain medium cannot be efficiently extracted by a diffraction-limited beam. However, ceramic Nd:YAG with smooth changes in neodymium doping level (doping profiles) can now be fabricated to ameliorate this problem. A slab engineered with a doping profile that has a higher concentration of Nd in the center, and less at the edges, would allow more pump power to be efficiently extracted by a diffraction-limited laser beam. Yet this solution poses its own problem because variations in Nd concentration introduce optical path length distortions that can significantly reduce beam quality. The variations in optical path length are predominantly from changes in the refractive index of the host due to Nd doping and spatially varying temperatures. A genetic-algorithm-based approach is presented that balances improvement in mode-overlap between excited state distribution and the signal laser beam against optical path length distortions. A doping profile was found for an edge-pumped, zig-zag slab amplifier that is expected to yield a 39% improvement in extracted power delivered into a diffraction-limited spot compared to a uniformly doped slab.

_2

Laser driven particle accelerators require submicroncontrol of the laser field as well as precise electron-beam guiding so fabrication techniques that allow integrating both elements into an accelerator-on-chip format become critical for the success of such next generation machines. Micromachining technology for silicon has been shown to be one such feasible technology in PAC2003[1] but with a variety of complications on the laser side. However, fabrication of transparent ceramics has become an interesting technology that could be applied for laser-particle accelerators in several ways. We discuss the advantages such as the range of materials available and ways to implement them followed by some different test examples we been considered. One important goal is an integrated system that avoids having to inject either laser or particle pulses into these structures.