Publications

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TlBr has the properties to become the leading radiation detection semiconductor. It has not yet been deployed due to a short lifetime of only hours to weeks. While the rapid structural deteriorations must come from ionic conduction under operating electrical fields, detailed aging mechanisms have not been understood. As a result, progress to extend lifetime has been limited despite extensive studies in the past. We have developed new atomistic simulation capabilities to enable study of ionic conduction under electrical fields. Our combined simulations and experiments indicate that dislocations in TlBr climb under electrical fields. This climb is the root cause for structural deterioration. Hence, we discovered new strengthening methods to reduce aging. Our new atomistic simulation approach can have broader impact on other Sandia programs including battery research. Our project results in 4 publications, a new invention, new LAMMPS capabilities, solution to mission relevant materials, and numerous presentations.

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TlBr crystals have superior radiation detection properties; however, their properties degrade in the range of hours to weeks when an operating electrical field is applied. To account for this rapid degradation using the widely-accepted vacancy migration mechanism, the vacancy concentration must be orders of magnitude higher than any conventional estimates. The present work has incorporated a new analytical variable charge model in molecular dynamics (MD) simulations to examine the structural changes of materials under electrical fields. Our simulations indicate that dislocations in TlBr move under electrical fields. This discovery can lead to new understanding of TlBr aging mechanisms under external fields.

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TlBr is promising for g- and x- radiation detection, but suffers from rapid performance degradation under the operating external electric fields. To enable molecular dynamics (MD) studies of this degradation, we have developed a Stillinger-Weber type of TlBr interatomic potential. During this process, we have also addressed two problems of wider interests. First, the conventional Stillinger-Weber potential format is only applicable for tetrahedral structures (e.g., diamond-cubic, zinc-blende, or wurtzite). Here we have modified the analytical functions of the Stillinger-Weber potential so that it can now be used for other crystal structures. Second, past modifications of interatomic potentials cannot always be applied by a broad community because any new analytical functions of the potential would require corresponding changes in the molecular dynamics codes. Here we have developed a polymorphic potential model that simultaneously incorporates Stillinger-Weber, Tersoff, embedded-atom method, and any variations (i.e., modified functions) of these potentials. As a result, we have implemented this polymorphic model in MD code LAMMPS, and demonstrated that our TlBr potential enables stable MD simulations under external electric fields.

Elpasolite scintillators are a large family of halides which includes compounds reported to meet the NA22 program goals of <3% energy resolution at 662 keV1. This work investigated the potential to produce quality elpasolite compounds and alloys of useful sizes at reasonable cost, through systematic experimental and computational investigation of crystal structure and properties across the composition space. Discovery was accelerated by computational methods and models developed previously to efficiently identify cubic members of the elpasolite halides, and to evaluate stability of anion and cation exchange alloys.

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An evaluation of calcium tungsten oxide (CaWO4) nanoparticles' properties was conducted using the powders generated from an all-alkoxide solvothermal (SOLVO) route. The reaction involved a toluene/pyridine mixture of tungsten(V) ethoxide ([W(OEt)5]) with calcium bis(trimethyl silyl) amide ([Ca(N(Si(CH3)3)2]) modified in situ by a series of alcohols (H-OR) including neo-pentanol (H-OCH2C(CH 3)3 or H-ONep) or sterically varied aryl alcohols (H-OC6H3R2-2,6 where R = CH3 (H-DMP), CH(CH3)2 (H-DIP), C(CH3)3 (DBP))]. Attempts to identify the intermediates generated from this series of reactions led to the crystallographic identification of [(OEt) 4W(μ-OEt)2Ca(DBP)2] (1). Each different SOLVO generated "initial" powder was found by transmission electron microscopy (TEM) and powder X-ray diffraction (PXRD) to be nanomaterials roughly assigned as the scheelite phase (PDF 00-041-1431); however, these initial powders displayed no luminescent behavior as determined by photoluminescence (PL) measurements. Thermal processing of these powders at 450, 650, and 750 C yielded progressively larger and more crystalline scheelite nanoparticles. Both PL and cathodoluminescent (CL) emission (422-425 and 429 nm, respectively) were observed for the nanomaterials processed at 750 C. Ion beam induced luminescence (IBIL, 478 nm) appeared to be in agreement with these PL and CL measurements. Further processing of the materials at 1000 C, led to a coalescence of the particles and significant improvement in the observed PL (445 nm) and CL measurements; however, the IBIL spectrum of this material was significantly altered upon exposure. These data suggest that the smaller nanoparticles were more stable to radiation effects possibly due to the lack of energy deposits based on the short track length; whereas the larger particles appear to suffer from radiation induced structural defects. © 2013 American Chemical Society.

An ideal ³He detector replacement for the near- to medium-term future will use materials that are easy to produce and well understood, while maintaining thermal neutron detection efficiency and gamma rejection close to the ³He standard. Toward this end, we investigated the use of standard alkali halide scintillators interfaced with ⁶Li and read out with photomultiplier tubes (PMTs). Thermal neutrons are captured on ⁶Li with high efficiency, emitting high-energy and triton (³H) reaction products. These particles deposit energy in the scintillator, providing a thermal neutron signal; discrimination against gamma interactions is possible via pulse shape discrimination (PSD), since heavy particles produce faster pulses in alkali halide crystals. We constructed and tested two classes of detectors based on this concept. In one case ⁶Li is used as a dopant in polycrystalline NaI; in the other case a thin Li foil is used as a conversion layer. In the configurations studied here, these systems are sensitive to both gamma and neutron radiation, with discrimination between the two and good energy resolution for gamma spectroscopy. We present results from our investigations, including measurements of the neutron efficiency and gamma rejection for the two detector types. We also show a comparison with Cs₂LiYCl₆:Ce (CLYC), which is emerging as the standard scintillator for simultaneous gamma and thermal neutron detection, and also allows PSD. We conclude that ⁶Li foil with CsI scintillating crystals has near-term promise as a thermal neutron detector in applications previously dominated by ³He detectors. The other approach, ⁶Li-doped alkali halides, has some potential, but require more work to understand material properties and improve fabrication processes.