Zachary M. Seeley

Transparent ceramic scintillators for gamma spectroscopy and radiography

Transparent ceramics combine the scintillation performance of single crystals with the ruggedness and processability of glass. We have developed a versatile, scaleable fabrication method, wherein nanoparticle feedstock is consolidated at temperatures well below melting to form inch-scale phase-pure transparent ceramics with optical scatter of α <0.1 cm-1. We have fabricated Cerium-doped Gadolinium Garnets with light yields of ~50,000 Ph/MeV and energy resolution of <5% at 662 keV. We have also developed methods to form sheets of the high-Z ceramic scintillator, Europium-doped Lutetium Oxide Bixbyite, producing ~75,000 Ph/MeV for radiographic imaging applications.

Comparative gamma spectroscopy with SrI 2 (Eu), GYGAG(Ce) and Bi-loaded plastic scintillators

We are developing new scintillator materials that offer potential for high resolution gamma ray spectroscopy at low cost. Single crystal SrI<inf>2</inf>(Eu) offers ∼3% resolution at 662 keV, in sizes of ∼1 in³. We have developed ceramics processing technology allowing us to achieve cubic inch scale transparent ceramic scintillators offering gamma spectroscopy performance superior to NaI(Tl). Our bismuth-loaded plastic scintillator demonstrates energy resolution of ∼8% at 662 keV, for samples of ∼0.5 cm³.

Development of Transparent Ceramic Ce-Doped Gadolinium Garnet Gamma Spectrometers

Transparent polycrystalline ceramic scintillators based on the garnet structure and incorporating gadolinium for high stopping power are being developed for use in gamma spectrometers. Optimization of energy resolution for gamma spectroscopy involves refining the material composition for high stopping and high light yield, developing ceramics fabrication methodology for material homogeneity, as well as selecting the size and geometry of the scintillator to match the photodetector characteristics and readout electronics. We have demonstrated energy resolution of 4% at 662 keV for 0.05 cm3 GYGAG(Ce) ceramics with photodiode readout, and 4.9% resolution at 662 keV for 18 cm 3 GYGAG(Ce) ceramics and PMT readout. Comparative gamma spectra acquired with GYGAG(Ce) and NaI(Tl) depict the higher resolution of GYGAG(Ce) for radioisotope identification applications. Light yield non-proportionality of garnets fabricated following different methods reveal that the fundamental shapes of the light yield dependence on energy are not intrinsic to the crystal structure, but may instead depend on trap state distributions. With exposure to 9 MeV Brehmsstrahlung radiation, we also find that GYGAG(Ce) ceramics exhibit excellent radiation hardness.

High energy resolution with transparent ceramic garnet scintillators

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 in3 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.

Two-step sintering of Gd 0.3 Lu 1.6 Eu 0.1 O 3 transparent ceramic scintillator

Transparent ceramic scintillators with the composition Gd0.3Lu1.6Eu0.1O3 (GLO:Eu) have been prepared by different sintering profiles: a traditional profile consisting of a slow ramp followed by a dwell, and a two-step profile consisting of a fast ramp and short dwell followed by a long dwell at a lower temperature. A subsequent Hot Isostatic Press (HIP) step was used to achieve full density and transparency. Two-step sintering allowed full transparency to be achieved after HIPing at 1525°C, while traditionally sintered samples required 1850°C in the HIP to achieve high transparency indicating that two-step sintering is successful in maintaining a small grain size and therefore allowing densification to be decoupled from grain growth during the low temperature HIP step. HIPing at elevated temperatures between 1525 and 1850°C resulted in rapid grain growth from sub-micron to ~300 µm grains. Radioluminescence spectra show negligible difference between samples with sub-micron grain size and those with 300 µm grains.

New High Stopping Power Thin Scintillators Based on

X-ray computed tomography devices and X-ray diffraction techniques are powerful tools: the former provide volumetric data of samples during a non-destructive examination for biology and material science, and the latter measure grain orientation and strain, as well as crystalline phase identification and structure refinement. Today, the European Synchrotron Radiation Facility (ESRF) provides increasingly higher energy beams, up to 150 keV combined with higher brilliance (1013 X-ray photons/sec). This means that detectors suffer from low X-ray absorption at high spatial resolution (1-10 μm) and from radiation damage in tomography and diffraction applications. In addition, more and more experiments in medicine require the absorbed dose by the sample to be reduced. In this context, more efficient scintillators are developed and evaluated at the ESRF. In order to perform sub-micrometer and micrometer resolution imaging scintillators 1 μm to 500 μm thin are required. Single Crystal Film scintillators (SCF), 1 μm to 100 μm can be obtained via Liquid Phase Epitaxy for sub-micrometer resolution. Transparent ceramics, 100 μm to 500 μm thick are promising candidates for X-ray imaging requiring high X-ray absorption and good contrast with micrometer resolution. Commonly available scintillators, such as CdWO4 and YAG:Ce suffer from low efficiency, therefore new scintillators with higher light yield and stopping-power are required. A first test was carried out to evaluate an Europium doped Lutetium Oxide ceramic for micrometer resolution and new SCFs of Lu3Ga5-xInxO12:Eu for sub-micrometer resolution are investigated. Performance of Lu2O3 and LuInGG, i.e absorption, light yield, afterglow, spatial resolution will be presented and compared to standard screens (YAG, GGG). First results will be illustrated with X-ray images and will demonstrate the absorption efficiency improvement at high spatial resolution.

{\rm Lu}_{2}{\rm O}_{3}

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/cm3, Zeff = 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)2O3, or “GLO,” offers excellent x-ray stopping, with ρ = 9.1 g/cm3 and Zeff = 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.

and

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.

{\rm Lu}_{3}{\rm Ga}_{5-{\rm x}}{\rm In}_{\rm x}{\rm O}_{12}

Breakthrough energy resolution, R(662keV) <4%, has been achieved with an oxide scintillator, Cerium-doped Gadolinium Yttrium Gallium Aluminum Garnet, or GYGAG(Ce), by optimizing fabrication conditions. Here we describe the dependence of scintillation light yield and energy resolution on several variables: (1) Stoichiometry, in particular Gd/Y and Ga/Al ratios which modify the bandgap energy, (2) Processing methods, including vacuum vs. oxygen sintering, and (3) Trace co-dopants that influence the formation of Ce4+ and modify the intra-bandgap trap distribution. To learn about how chemical composition influences the scintillation properties of transparent ceramic garnet scintillators, we have measured: scintillation decay component 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. Optimized GYGAG(Ce) provides R(662 keV) =3.0%, for 0.05 cm3 size ceramics with Silicon photodiode readout, and R(662 keV) =4.6%, at 2 in3 size with PMT readout.

for High Resolution X-ray Imaging

Results are presented of investigations into the composition, uniformity and gamma-ray imaging performance of new ceramic scintillators with synthetic garnet structure. The ceramic scintillators were produced by a process that uses flame pyrolysis to make nanoparticles which are sintered into a ceramic and then compacted by hot isostatic compression into a transparent material. There is concern that the resulting ceramic scintillator might not have the uniformity of composition necessary for use in gamma-ray spectroscopy and gamma-ray imaging. The compositional uniformity of four samples of three ceramic scintillator types (GYGAG:Ce, GLuGAG:Ce and LuAG:Pr) was tested using an electron microprobe. It was found that all samples were uniform in elemental composition to the limit of sensitivity of the microprobe (few tenths of a percent atomic) over distance scales from ~ 1 cm to ~ 1 um. The light yield and energy resolution of all ceramic scintillator samples were mapped with a highly collimated 57Co source (122 keV) and performance was uniform at mapping scale of 0.25 mm. Good imaging performance with single gamma-ray photon detection was demonstrated for all samples using a BazookaSPECT system, and the imaging spatial resolution, measured as the FWHM of a LSF was 150 um.