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Melcher, Charles

Publications and source records attributed to Melcher, Charles.

9 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Crystal growth, physical and optical properties of TlSr 2 Cl 5 and TlSr 2 Br 5

Small diameter (Ø 16 mm) TlSr 2 Cl 5 and TlSr 2 Br 5 crystals were grown by the Vertical Bridgman method. X-ray diffraction measurements show that both have the monoclinic crystal structure with space group P21/c. TlSr 2 Cl 5 and TlSr 2 Br 5 have a density of 4.14 g/cm 3 and 5.03 g/cm 3 , respectively. The effective Z of TlSr 2 Cl 5 and TlSr 2 Br 5 is 63.7 and 58.6, respectively. Radioluminescence spectra of TlSr 2 Cl 5 and TlSr 2 Br 5 feature a broad emission band peaking at 440 and 445 nm, respectively. As a result the light yield of TlSr 2 Cl 5 and TlSr 2 Br 5 was estimated to be 17,000 and 45,000 ph/MeV, respectively.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Serrated flow in $\mathrm{NaI:Tl}$ scintillator crystals

We report the serrated-flow behavior is an important phenomenon that unveils material-deformation mechanisms, as reported for various kinds of materials. NaI doped with Tl (NaI:Tl) is unique among scintillation materials in that the structure contains glide planes that are linked to serration behavior. In the present work, single crystals of NaI:Tl were subjected to room-temperature compression experiments at different strain rates. The serrated flow was observed, and complexity and multifractal analyses were performed to analyze the serration behavior. The findings revealed that the strain rate had a pronounced effect on the complexity and multifractality of the serrated flow, similar to what has been found in other alloy systems. The results also indicate that there may be a strong link between the complexity of the serrated flow behavior and the heterogeneity of the underlying dynamics. It is expected that the present work could be a step toward a better understanding of the deformation behavior and forgeability of NaI:Tl single crystals.

36 MATERIALS SCIENCE↗
DOE OSTI2023

In Situ High-Temperature Structural Analysis of High-Entropy Rare-Earth Sesquioxides

High-entropy rare-earth (RE) sesquioxides (RE 2 O 3 ) containing five cations in equimolar amounts have been investigated for a variety of applications, but little is known about their polymorphic behavior and coefficient of thermal expansion. Here, in this work, we evaluate the effect of the average ionic radius (AIR) on the polymorphism of high-entropy RE 2 O 3 . Powder samples of compositions 1 (Lu,Y,Ho,Nd,La) 2 O 3 (AIR = 0.938 Å) and 2 (Gd,Eu,Sm,Nd,La) 2 O 3 (AIR = 0.982 Å) were synthesized via a wet chemical method, and bead samples were prepared for aerodynamic levitation by melting the powders in a copper hearth. Structural transitions were monitored upon cooling from the melt to 1000 °C via in situ X-ray diffraction on aerodynamically levitated samples. The phase evolution was liquid, hexagonal H-type, and monoclinic B-type for composition 1 and liquid, cubic X-type, H-type, and B-type for composition 2. Based on their AIR, the general polymorphic transformations of the high-entropy RE 2 O 3 follow the trend of single-RE RE 2 O 3 , but the transition temperatures differ from those of single-RE RE 2 O 3 . The coefficient of thermal expansion values of the B-type phase of compositions 1 and 2 are similar to those of Gd 2 O 3 and previously published high-entropy RE 2 O 3 .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Thermal Processing Conditions for the Synthesis of Near Theoretical Density Li 5 La 3 Ta 2 O 12 Ceramics for Ceramic Dual-Mode Detectors

A family of Li-containing garnet compounds, extensively investigated for Li-ion solid electrolyte applications, show great potential to be developed into transparent ceramic dual-mode scintillators. In particular, Li 5 La 3 Ta 2 O 12 (LLTaO) shows promise as an efficient dual-mode scintillator due to its high Li content, high density, non-hygroscopicity, and high effective-Z. Often the synthesis of LLTaO ceramics involves solid state processing techniques starting with oxide and carbonate powders, calcining, sintering for the removal of residual porosity, and post-processing. This study focuses on two objectives: (1) establishing thermal processing boundary conditions for a flexible alkoxide-based sol-gel method for synthesizing fine-grained powders; and (2), consolidating powders via hot-pressing to form high-density ceramics. In this study, near single-phase ceramics were achieved by hot-pressing sol-gel powders with varying amounts of excess Li. Rietveld refinements were used to analyze X-ray diffraction data and provide a quantitative evaluation of the phases present in the hot pressed ceramics. Some of the diffraction data showed large low angle peak asymmetry, suggesting the presence of two cubic Li 5 La 3 Ta 2 O 12 garnet phases with different lattice parameters. Depending on the synthesis and processing conditions, the refinement revealed ~ 90 wt% concentration of a phase with a ≈ 12.8 Å and ~ 10 wt% concentration of a phase with a ≈ 12.9 Å, indicating a remaining excess of Li in the hot pressed ceramics in the large lattice parameter phase. These synthesis experiments allowed for the determination of a baseline for Li loss during hot-pressing. X-ray excited radioluminescence and photoluminescence excitation and emission spectra were obtained from the hot pressed ceramics. In conclusion, the measured emission spectra featured broad tantalate emission and line emissions likely from a rare-earth impurity.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Crystal growth, density functional theory, and scintillation properties of Tl 3 LnCl 6 :Ce 3+ and TlLn 2 Cl 7 :Ce 3+ (Ln = Y, Gd)

In this paper we report on the crystal growth, density functional theory (DFT) calculations and scintillation properties of Tl 3 LnCl 6 :Ce and TlLn 2 Cl 7 :Ce(Ln = Y, Gd). Crystals were grown by the Vertical Bridgman technique up to 16 mm in diameter and 25 mm long. Crystals of Tl 3 YCl 6 :Ce and Tl 3 GdCl 6 :Ce belong to the family of the Nesohalides which have the monoclinic crystal structure. Crystals of TlY 2 Cl 7 :Ce and TlGd 2 Cl 7 :Ce belong to the family of the Phyllohalides which have either the monoclinic or the orthorhombic crystal structure. We report the light yields of these Nesohalides and Phyllohalides are typically on the order of 35,000–40,000 ph/MeV.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2020

Crystal growth and scintillation properties of pure and Tl-doped Cs 3 Cu 2 I 5

Here, the Bridgman crystal growth and scintillation properties of both undoped and Tl-doped Cs 3 Cu 2 I 5 are presented. This material is very attractive for gamma and X-ray detection applications, with a density of 4.53 g/cm 3 and effective atomic number of 51.9. Undoped Cs 3 Cu 2 I 5 had a light yield of 41,500 photons/MeV, with an energy resolution of 4.4% at 662 keV. Thallium doping at 0.5 mol % resulted in a much-improved scintillation response, in which light yield increased to 98,200 photons/MeV and energy resolution reduced to 3.3% at 662 keV. The X-ray excited emission is centered at 442 nm for the undoped and 500 nm for the Tl-doped crystals. The undoped emission is broad, typical of excitonic emission, while thallium doping results in an even broader band with features of both the undoped and thallium defect-mediated emissions.

36 MATERIALS SCIENCE↗
DOE OSTI2020

TlSr 2 I 5 :Eu 2+ - A new high density scintillator for gamma-ray detection

In this paper we report on the scintillation properties of TlSr 2 I 5 doped with Eu 2+ , a novel thallium-containing high-resolution scintillator for gamma-ray spectroscopy. Here, small diameter, good quality crystals of TlSr 2 I 5 :Eu (TSI) with different Eu 2+ concentrations were grown by the vertical Bridgman method. X-ray diffraction measurements show that single crystals of TSI belong to the monoclinic system with space group P2 1 /c. TlSr 2 I 5 has a density of 5.32 g/cm 3 and effective Z of 60. The X-ray excited emission of Eu 2+ doped TlSr 2 I 5 features a broad emission band peaking between 460–470 nm. The light yield of TlSr 2 I 5 crystal doped with 1% Eu 2+ is measured to be ~72,000 ph/MeV with an energy resolution of 2.8% at 662 keV. The scintillation decay time which is characteristic of Eu 2+ shows two components, with 90% of the light in a ~500 ns component and the rest in a longer component of ~3 μs.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2020

Thallium-based scintillators for high-resolution gamma-ray spectroscopy: Ce 3+ - doped Tl 2 LaCl 5 and Tl 2 LaBr 5

In this paper we report on the crystallographic and scintillation properties of Tl 2 LaCl 5 :Ce 3 + and Tl 2 LaBr 5 :Ce 3 + , two novel thallium-containing high-resolution scintillators for gamma-ray spectroscopy. Crystals of Tl 2 LaCl 5 :Ce 3 + and Tl 2 LaBr 5 :Ce 3 + were grown by the Vertical Bridgman method up to 1-inch diameter and 1-inch long. Single crystals of Tl 2 LaCl 5 :Ce 3 + and Tl 2 LaBr 5 :Ce 3 + belong to the orthorhombic system with space group 62 and have a density of 5.16 and 5.98 g/cm 3 , respectively. The scintillators show high light yields of up to 68,000 photons/MeV, excellent gamma-ray energy resolution of ≤ 3% at 662 keV, a fast scintillation decay, and a proportional response over a wide range of energies from 32 keV up to 1275 keV. Density Functional Theory calculations show that the Ce 3 + energy levels are inside the bandgap despite the smaller bandgap of Tl 2 LaCl 5 and Tl 2 LaBr 5 compared to K 2 LaCl 5 and K 2 LaBr 5 . Finally, a systematic Ce 3 + concentration study was performed for Tl 2 LaCl 5 :Ce 3 + and trends observed.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗