Gadolinium Garnet transparent ceramics doped with Ce, ((Gd,Y,Ce)3(Ga,Al)5O12), for gamma-ray spectroscopy provide high density, high light yield, high energy resolution , high Z, mechanical robustness, and they are unreactive to air and water. Gadolinium garnet single crystals are costly to grow, due to their high melting points, and suffer from non-uniform light yield, due to Ce segregation. In contrast, transparent polycrystalline ceramic Garnets are never melted, and therefore are less costly to produce and provide the uniform light yield required to achieve high energy resolution with a scintillator.
GYGAG(Ce) transparent ceramics offer energy resolution as good as R(662 keV) = 3.5%, in a pixelated detector utilizing Silicon photodiode array readout. We have developed a modular handheld detector based on pixelated GYGAG(Ce) on a photodiode array, that offers directional detection for point source detection as well as gamma spectroscopy. Individual modules can be assembled into detectors ranging from pocket-size to large panels, for a range of applications.
Large GYGAG(Ce) transparent ceramics in the 2-5 in3 size range have been fabricated at LLNL. Instrumentation of these ceramics with Silicon photomultipliers (SiPMs) and super bi-alkali PMTs has been explored and energy resolution as good as R(662 keV) = 5% has been obtained. Further improvements with SiPM readout will leverage their high quantum efficiency in the 500-650 nm range where GYGAG(Ce) emits, and implement electronics that minimize the effect of SiPM dark current and capacitance on the pulse height spectra.
This work was performed under the auspices of the U.S. DOE by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344, and has been supported by the US Department of Homeland Security, Domestic Nuclear Detection Office, under competitively awarded IAA HSHQDC-12-X-00149 under Contract No. DE-AC03-76SF00098. LLNL-ABS-724480.
We report on the development of two new mechanically rugged, high light yield transparent ceramic scintillators: (1) Ce-doped Gd-garnet for gamma spectroscopy, and (2) Eu-doped Gd-Lu-bixbyite for radiography. GYGAG(Ce) garnet transparent ceramics offer ρ = 5.8g/cm<sup>3</sup>, Z<sub>eff</sub> = 48, principal decay of <100 ns, and light yield of 50,000 Ph/MeV. Gdgarnet ceramic scintillators offer the best energy resolution of any oxide scintillator, as good as R(662 keV) = 3% (Si-PD readout) for small sizes and typically R(662 keV) < 5% for cubic inch sizes. For radiography, the bixbyite transparent ceramic scintillator, (Gd,Lu,Eu)<sub>2</sub>O<sub>3</sub>, or “GLO,” offers excellent x-ray stopping, with ρ = 9.1 g/cm<sup>3</sup> and Z<sub>eff</sub> = 68. Several 10” diameter by 0.1” thickness GLO scintillators have been fabricated. GLO outperforms scintillator glass for high energy radiography, due to higher light yield (55,000 Ph/MeV) and better stopping, while providing spatial resolution of >8 lp/mm.
Breakthrough energy resolution, R(662keV) < 4%, has been achieved with an oxide scintillator, Cerium-doped Gadolinium Yttrium Gallium Aluminum Garnet, or GYGAG(Ce). Transparent ceramic GYGAG(Ce), has a peak emission wavelength of 550 nm that is better matched to Silicon photodetectors than to standard PMTs. We are therefore developing a spectrometer based on pixelated GYGAG(Ce) on a Silicon photodiode array that can provide R(662 keV) = 3.6%. In comparison, with large 1-2 in<sup>3</sup> size GYGAG(Ce) ceramics we obtain R(662 keV) = 4.6% with PMT readout. We find that ceramic GYGAG(Ce) of a given stoichiometric chemical composition can exhibit very different scintillation properties, depending on sintering conditions and post-anneal treatments. Among the characteristics of transparent ceramic garnet scintillators that can be controlled by fabrication conditions are: scintillation decay components and their amplitudes, intensity and duration of afterglow, thermoluminescence glow curve peak positions and amplitudes, integrated light yield, light yield non-proportionality - as measured in the Scintillator Light Yield Non-Proportionality Characterization Instrument (SLYNCI), and energy resolution for gamma spectroscopy. Garnet samples exhibiting a significant fraction of Cerium dopant in the tetravalent valence also exhibit: faster overall scintillation decay, very low afterglow, high light yield, but poor light yield proportionality and degraded energy resolution.
Nanocerox produces oxide nanopowders via flame spray pyrolysis that have proven effective in the processing of a host of high quality optical ceramic materials. In order to produce LWIR windows to compete with ZnS, however, oxide materials are not suitable. Nanocerox has therefore developed aqueous synthesis techniques for the production of zinc sulfide nanopowders. The proprietary processing technique allows control of primary particle size, high purity, low levels of agglomeration, and cost effective synthesis. Crystallinity, particle size, and purity of the powders will be presented. Characterization of parts fabricated from these powders via sinter/HIP processing will also be discussed, including optical performance and microstructural characterization.
Optical quality ceramic Yttrium Aluminum Garnet (YAG, Y<sub>3</sub>Al<sub>5</sub>O<sub>12</sub>) materials for high power solid state lasers are being developed at Raytheon. The remaining challenge for ceramic gain materials is elimination of residual absorption and scattering centers. At Raytheon, significant progress has been achieved in the optical quality improvement, scale-up, and demonstration of laser quality Yb, Nd, and Er doped ceramic YAG materials. This communication presents Raytheon's current development status in ceramic YAG fabrication and doped ceramic YAG material characteristics.
Currently available IR transparent materials typically exhibit a trade-off between optical performance and mechanical strength. For instance, sapphire domes are very strong, but lack full transparency throughout the 3-5 micron mid-wave IR band. Yttria is fully transparent from 3-5 microns, but lacks sufficient strength, hardness, and thermal shock resistance for the most demanding aero-thermal applications. Missile system designers must limit system performance in order to accommodate the shortcomings of available window and dome materials. Recent work in the area of nanocomposite ceramics may produce new materials that exhibit both excellent optical transparency and high strength, opening the door to improved missile performance. The requirements for optical nanocomposite ceramics will be presented and recent work in producing such materials will be discussed.
Optical quality polycrystalline yttrium aluminum garnet (YAG) materials suitable for laser gain application have been under development at Raytheon Advanced Materials Laboratory since late 2003. Significant progress has been achieved in the optical quality improvement, scale-up, Yb and Nd dopant incorporation, and various characterizations. This communication discusses Raytheon's ongoing developments in laser quality ceramic YAG fabrication and its characteristics in comparison to the current state of the art ceramic YAG made by Konoshima Chemical in Japan.
New materials with improved mechanical properties and high optical transmission in the full 3-5 micron MWIR region wavelength are required. Commercially available polycrystalline transparent Yttria, with >100 micron average grain size, does not perform satisfactorily in demanding applications because of its modest strength. One way to improve strength is to develop an ultra-fine grained material with acceptable optical transmission properties. To realize fine grains it is necessary to use other phases to inhibit grain growth during fabrication. A promising processing method comprises: (a) synthesis of an extended metastable solid solution by plasma melting and quenching, and (b) consolidation of the metastable ceramic powder to form dense submicron-grained (<1 micron) composites. Two ceramic composites containing 20 and 50 vol% of second phase are evaluated in this study. Optical transmission, hardness, and indentation fracture toughness are measured and correlated with structure.