Stephanie Lam

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.

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}

The low temperature scintillation properties of LSO:Ce from 4.3 to 300 K were systematically studied and temperature-dependent light yield nonproportionality was measured for the first time. Increasing temperature from 4.3 to 300 K led to increased decay times and light output, an improvement in both the total energy resolution and light yield nonproportionality, and a red shift in the emission. Mechanisms were proposed to explain the complex trends observed in these scintillation properties.

From 295 to 5 K

To better identify the influence of cation impurities on the scintillation performance of SrI2(Eu), SrI2 crystals were grown, each co-doped with 4 mol% Eu^{2 +} and 0.2 mol% of one of the following: Mg^{2 +}, Ba^{2 +}, Cs⁺, Ca^{2 +}, Fe^{2 +}, Cu⁺, Na⁺, and Sn^{2 +}. Four 10 mm diameter crystals were grown at a time by the vertical Bridgman-Stockbarger method. The segregation behavior and the scintillation performance of 10 mm dia.×6 mm cylinders and 7 mm×6 mm×2 mm cuboids were characterized. Mg^{2 +}, Cs⁺, Fe^{2 +}, and Cu⁺ impurities did not adversely affect scintillation properties, and segregated during growth. However, Na⁺, Ba^{2 +}, and Ca^{2 +} did not segregate well and degraded light yield and energy resolution, especially at the larger sample size. Sn^{2 +} proved to be the most detrimental to light yield and produced a secondary emission peak at 600 nm, but did not affect the non-proportionality response. The results of this study suggest that SrI2(Eu) can tolerate a surprisingly large amount of cation impurities. These findings suggest that the purity requirements for starting materials can be relaxed, and purification efforts may be adjusted to target only the most harmful impurities.

Nonproportionality and Scintillation Responses of LSO:Ce From 4.3 to 300 K

We describe the design and operation of a unique hydraulic press for the study of scintillator materials under isostatic pressure. This press, capable of developing a pressure of a gigapascal, consists of a large sample chamber pressurized by a two-stage hydraulic amplifier. The optical detection of the scintillation light emitted by the sample is performed, through a large aperture optical port, by a photodetector located outside the pressure vessel. In addition to providing essential pressure-dependent studies on the emission characteristics of radioluminescent materials, this apparatus is being developed to elucidate the mechanisms behind the recently observed dependency of light-yield nonproportionality on electronic band structure. The variation of the light output of a Tl:CsI crystal under 511-keV gamma excitation and hydrostatic pressure is given as an example.

The Influence of Cation Impurities on the Scintillation Performance of

The scintillation radiation detection community is on the cusp of a major breakthrough with the potential deployment of Europium-activated Strontium Iodide (SrI2:Eu) detectors for medical imaging and homeland security applications. Compared to the traditional scintillators (such as NaI), SrI2 provides much better energy resolution and light output. The crystal growth of SrI2:Eu has been impaired for a long time due to cracking problems, which makes it highly unreliable and nonreproducible. This significantly increases the cost of the material which in turn impedes wide-scale deployment and limits its advantages over other scintillators. In this paper, we demonstrate a technique of growing crack-free SrI2:Eu crystals by monitoring the stoichiometry of the melt atmosphere during processing and crystal growth. Using the feedback information from the in situ monitoring technique, the stoichiometry of the melt was corrected and multiple crack-free SrI2:Eu crystals of diameters 1.5 inches were repeatedly grown using Bridgman configuration with no visible inclusions, bubbles or defects whatsoever.

{{\hbox {SrI}}_2}

To better identify the influence of light anion impurities on the scintillation performance, small boules of SrI₂(Eu) were grown by the vertical Bridgman-Stockbarger method, each co-doped with 0.2% of one of the following: C⁰, CO3^2-, N^3-, O^2-, OH^-, PO4^3-, S^2-, SO4^2-, Cl^- and Br^-. Residual impurity concentrations were measured, and the scintillation performance of resulting detectors was characterized. Oxygen was tolerated up to 0.2% on a molar basis. Sulfur proved to be highly detrimental to both crystallinity and scintillation performance. Nitrogen produced additional emission near 480 nm. This study suggests that SrI₂(Eu) readily incorporates anion impurities, which may substitute for iodine, but these may also be removed before and during growth by volatilization. Purity metrics for starting materials should include sulfur and carbon, as well as oxygen and H₂O.