Showing papers by "Charles L. Melcher published in 2016"

Quaternary Iodide K(Ca,Sr)I3:Eu2+ Single-Crystal Scintillators for Radiation Detection: Crystal Structure, Electronic Structure, and Optical and Scintillation Properties

Abstract: A mixed crystal strategy is reported as an effective approach to improving the performances of inorganic scintillators for radiation detection applications. The aim of this work is to optimize ternary iodide KCaI3:Eu2+ single crystals via partial Sr2+ substitution, and to provide physical insights and a strategy of designing promising halide scintillators. The ∅22 mm × 50 mm long K(Ca,Sr)I3:Eu2+ single crystals are grown by the Bridgman method. Crystal structure refinements verify the phase purity and the orthorhombic crystal system with a space group of Cmcm (No. 63) in the solid solutions. An energy resolution of 2.5 ± 0.1% at 662 keV and a light yield of 74 000 ± 4000 photons per MeV can be achieved for a 4 mm cube KCa0.835Sr0.165I3:Eu2+. It is for the first time that a halide solid solution is synthesized with a competitive scintillating performance as current state-of-the-art scintillators, such as SrI2:Eu2+ and LaBr3:Ce3+. A combination of first-principles calculations and optical characterization experiments is employed to construct the host material band edges and the relative positions of 5d and 4f energy levels of the Eu2+ activation center. The origins of the improvement of light yield and scintillation response nonproportionality are proposed from experimental and theoretical insights.

Eu2+ concentration effects in KCa0.8Sr0.2I3:Eu2+: A novel high-performance scintillator

Abstract: We report here the effect of Eu2+ concentration in KCa0.8Sr0.2I3:Eu2+ single crystal scintillators. KCa0.8Sr0.2I3:Eu2+ single crystals doped with 0.5, 1, 3, 5, and 7 mol% Eu2+ were grown by the Bridgman method. The effects of varying Eu2+concentration and crystal volume on the scintillation properties, including light yield, energy resolution, nonproportionality, scintillation decay time and afterglow level, were systematically investigated. For 5 mm×5 mm×5 mm samples, the best light yield of 86,000±4000 photons/MeV was achieved with a content of 5 mol% Eu2+; its energy resolution of 2.5% at 662 keV was comparable to that of LaBr3:Ce3+ and SrI2:Eu2+. With larger samples of about 2.2 cm3, the best performances achieved were for 3 mol% Eu2+ concentration, i.e. a light yield of 76,000±4000 photons/MeV and an energy resolution of 3% at 662 keV. A direct correlation between nonproportionality and Eu2+ concentration was found. A continuous lengthening of scintillation decay time and x-ray induced afterglow level with increasing Eu2+ concentration was observed. The self-absorption effect was evaluated by using the Stokes shift and the temperature dependence of the photoluminescence decay (PL) of the Eu2+ centers. The sample with the highest dopant concentration had more severe temperature quenching of the Eu2+5d-4f emission than the sample with the lowest dopant concentration, which could be ascribed to the thermally activated concentration quenching.

Scintillation Properties and Electronic Structures of the Intrinsic and Extrinsic Mixed Elpasolites Cs 2 Na R Br 3 I 3 (R =La , Y)

Abstract: Scintillator materials continue to attract wide interest for radiation detection in high-energy physics and medical diagnostics. Elpasolite halides are among the most promising scintillators, due to their high structural symmetry and excellent performance, and here the authors reveal the unique emission mechanism of two mixed-anion members of this class. This study guides the design of materials with the exceptional energy resolution and light yield needed to improve applications.

Large-Size KCa0.8Sr0.2I3:Eu2+ Crystals: Growth and Characterization of Scintillation Properties

Abstract: The newly developed KCa0.8Sr0.2I3:Eu2+ scintillator with 2.5% energy resolution at 662 keV shows great potential for use in gamma-ray spectroscopy applications, but progress toward large-size crystal growth has been hindered by cracking and inclusion problems. In this paper, we report a detailed study on the role of process variables of Bridgman growth, including grain selector designs, ampule surface treatments, and temperature gradients. It was found that crystal cracking can be effectively prevented by using a carbon-coated quartz ampule with a bent capillary. A high temperature gradient of 45 °C/cm at the solid–liquid interface can positively contribute to the suppression of the formation of visible inclusions caused by constitutional supercooling. By using optimized growth parameters, high quality 22 mm diameter KCa0.8Sr0.2I3 single crystals doped with 0.5, 1, and 3 mol % Eu2+ and 38 mm diameter KCa0.8Sr0.2I3 single crystals doped with 0.5 mol % Eu2+ were grown by the Bridgman method. The scintillati...

Tackling Single Crystal Growth Challenges for Mixed-Elpasolite Scintillators

Abstract: The rising demand for efficient radiation detectors in various applications has led to intense research on single crystal scintillators. The scintillation performance strongly depends on the as-grown crystal quality. Mixed-anion elpasolite single crystals with Ce-doping such as Cs2NaREBr3I3 (RE = La, Y) are among the most promising candidates as next generation scintillators. However, there are multiple challenges to traditional single crystal synthesis using melt–freeze growth techniques due to the complicated composition, as well as impurities, and varying thermal behavior of the starting materials. To improve the as-grown single crystal quality and optimize their scintillation properties, a series of investigations on material treatment and synthesis, ampule design, and crystal growth parameters were conducted. Effective purification of the carbon- and oxygen-based impurities improved the optical transmittance of the crystal. Differential scanning calorimetry and thermal gravimetric analysis results re...

Toward High Energy Resolution in CsSrI3/Eu2+ Scintillating Crystals: Effects of Off-Stoichiometry and Eu2+ Concentration

Abstract: CsSrI3/Eu2+ has very promising scintillation properties for gamma-ray spectroscopy applications, but it has proven difficult to grow high quality single crystals in large sizes. This paper reports a composition-engineering strategy, in this case a combination of off-stoichiometric melts and Eu2+ concentration optimization, to obtain large-size CsSrI3/Eu2+ crystals with excellent energy resolution. Crystals of a series of off-stoichiometric compositions, Cs(1+x)(Sr,Eu)(1–x)I(3–x) (x = 0, 0.05, 0.06, and 0.1), were grown by the Bridgman method. The Cs1.06Sr0.94I2.94/Eu2+ single crystal has the highest optical transmittance between 450 and 800 nm. Cs1.06Sr0.94I2.94 single crystals doped with 0.5, 1, 3, 5, and 7 mol % Eu2+ ions were also grown by the Bridgman method. The effects of Eu2+ concentration on the phase purity and optical and scintillation properties were studied. X-ray diffraction patterns confirmed the phase purity of all samples with the exception of a hydrate phase formed during measurement. Inc...

Effects of melt aging and off-stoichiometric melts on CsSrI3:Eu2+ single crystal scintillators

Abstract: Ternary halide scintillators are commonly prepared from a mixture of commercially available binary halides. The initial binary halides may contain excess halogen ions or have different volatilities, which could lead to loss of stoichiometry of the resulting ternary halide crystals and potentially negatively affect optical and scintillation properties. In this work, the effects of vacuum aging of the melt (melt aging) and use of off-stoichiometric melts via introduction of excess CsI on the crystal quality and scintillation properties of CsSrI3:Eu2+, a promising scintillator for gamma-ray detection applications, are investigated. The phase purity of the grown samples was confirmed by powder X-ray diffraction and differential scanning calorimeter measurements, and the existence of matrix composition variations is revealed by energy-dispersive X-ray spectroscopy analyses. An abnormal relationship between the full energy peak and the shaping time, i.e. full energy peak broadening or existence of two full energy peaks, in the melt-aged and off-stoichiometric samples is observed. It is ascribed to a slow scintillation decay event in a time scale between 15 and 50 μs. For the CsSrI3:Eu2+ single crystal grown from a stoichiometric melt without melt aging treatment, an energy resolution of 5.0% at 662 keV and a light yield of 48 000 ± 2000 photons per MeV can be achieved at a size of 1.4 cm3.

Growth of inch-sized KCa0.8Sr0.2I3:Eu2+ scintillating crystals and high performance for gamma-ray detection

Abstract: A quaternary iodide KCa0.8Sr0.2I3:Eu2+ scintillator with 2.5% energy resolution at 662 keV in a 5 mm3 sample shows great potential for use in gamma-ray spectroscopy applications. In this work, we report the state-of-the-art growth of high quality 25, 38, and 50 mm diameter KCa0.8Sr0.2I3:Eu2+ single crystals by the vertical self-seeding Bridgman method. KCa0.8Sr0.2I3:Eu2+ with a size of ∅25 mm × 25 mm can achieve excellent energy resolutions of 3.15% at 662 keV and 6.8% at 122 keV irradiation, which are superior to that of a commercial NaI:Tl+ of the same size. The nonuniformity of light collection and production in ∅25 mm × 25 mm and ∅38 mm × 38 mm KCa0.8Sr0.2I3:Eu2+ crystals was evaluated by using a technique based on a collimated 137Cs source and coupling the crystal to a photomultiplier tube (PMT) in different directions. The performances of the packaged crystals for practical use were also measured.

Intrinsic radioactivity of KSr 2 I 5 :Eu 2+

Abstract: A current need in nuclear security is an economical, yet high energy resolution (near 2%), scintillation detector suitable for gamma-ray spectroscopy. For current scintillators on the market, there is an inverse relationship between scintillator energy resolution and cost of production. A new promising scintillator, KSr2I5:Eu2+, under development at the University of Tennessee, has achieved an energy resolution of 2.4% at 662 keV at room temperature, with potential growth rates exceeding several millimeters per hour. However, the internal background due to the 40K content could present a hurdle for effective source detection/identification in nuclear security applications. As a first step in addressing this question, this paper reports on a computational investigation of the intrinsic differential pulse height spectrum (DPHS) generated by 40K within the KSr2I5:Eu2+ scintillator as a function of crystal geometry. It was found that the DPHS remains relatively equal to a constant multiplicative factor of the negatron emission spectrum with a direct increase of the 1.46 MeV photopeak relative height to the negatron spectrum with volume. Further, peak pileup does not readily manifest itself for practical KSr2I5:Eu2+ volumes.

Laser etched scintillation detector blocks with internally created reflectors

Abstract: A scintillator element is disclosed where the scintillator element includes a scintillator formed of a scintillation material capable of converting non-visible radiation into scintillation light, wherein the scintillator has a plurality of laser-etched micro-voids within the scintillator, each micro-void having an interior surface, and an intrinsic reflective layer is formed on the interior surface of at least some of the micro-voids, wherein the intrinsic reflective layer is formed from the scintillation material.

Intrinsic and activated a4bx6 scintillators

Abstract: Mixed halide scintillation materials of a first general formula A 4 B (1-y) M y X′ 6(1-z) X″ 6z and a second general formula A (4-y) BM y X′ 6(1-z) X″ 6z are disclosed. In the general formulas, A is an alkali metal, B is an alkali earth metal, and X′ and X″ are two different halogen atoms. Scintillation materials of the first general formula include a divalent external activator M such as Eu 2+ or Yb 2+ or a trivalent external activator M such as Ce 3+ . Scintillation materials of the second general formula include a monovalent external activator M such as In + , Na + , or Tl + or a trivalent external activator such as Ce 3+ .