Transparent ceramic scintillators for gamma-ray spectroscopy and radiography
Transparent ceramics have become available in useful sizes for a variety of applications within the past 15 years thanks to improved understanding of sintering phenomena on the nanoscale.1,2 While traditional ceramics may comprise single or multiple crystalline and amorphous mineral phases and are typically translucent or opaque to light, modern optically transparent ceramics are fully dense monoliths of micron-scale (or smaller) crystallites, in most cases formed from pure-phase cubic crystal structures. Fabrication of transparent ceramics begins with high-purity ceramic nanopowders, which are consolidated into a ‘green body’ by pressing or casting. The green body is sintered to near-full density, and the residual porosity is removed by processing the samples in a pressure vessel with high-pressure argon gas (hot isostatic pressing). A few commercial transparent ceramics are Lumicera lenses (a barium-based oxide made by Murata Mfg.), laser gain media—Nd:YAG (neodymium-doped yttrium aluminum garnet) is available from Konoshima—and transparent armor (aluminum oxynitride and spinel are available from Surmet and other vendors). As the manufacturing processes for transparent ceramics become better understood, industrial optical uses for these transparent materials with excellent mechanical robustness are likely to expand.
At Lawrence Livermore National Laboratory (LLNL), transparent ceramic oxide scintillators are being developed for gamma ray spectrometers and for high-energy (MeV) radiographic imaging devices.3–5 Scintillators are optical materials that emit pulses of visible photons when excited with high-energy radiation. These pulses may be read out with a photodetector and processed to provide gamma-ray spectroscopy. Gamma-ray spectrometers providing high sensitivity and effective isotope identification require high energy resolution, high effective atomic number, and scintillators that can be produced in large volumes. The scintillators currently offering the best energy resolution for gamma spectroscopy are cerium-doped lanthanum bromide, LaBr3(Ce), and europium-doped strontium iodide, SrI2(Eu). However, these materials are available only as single crystals. In harsh environments, such as nuclear-fuel-rod pools or for oil-well logging, a rugged material, such as a transparent ceramic, is desirable. Among the garnets, the density and effective atomic number of YAG are too low, while lutetium (Lu)-based garnets have excellent stopping power but possess intrinsic radioactivity due to the presence of Lu-176, resulting in an undesirable background for low-rate counting.
We have developed a cerium-doped gadolinium yttrium gallium aluminum garnet, with the chemical formula (Gd,Y)3(Ga,Al)5O12, referred to hereafter as GYGAG(Ce). Figure 1 shows a 1in3 fully dense ceramic of GYGAG(Ce) fabricated at LLNL. This material exhibits excellent light yield proportionality, the intrinsic limit to scintillator energy resolution.6 Gamma-ray spectra acquired with a barium (Ba)-133 source, using four different scintillators (see Figure 2), show that GYGAG(Ce) resolves complex lines better than thallium-doped sodium iodide—NaI(Tl)—though still not as well as SrI2(Eu) or LaBr3(Ce). We are carefully engineering a GYGAG(Ce) gamma spectrometer using a high-quantum-efficiency photodetector, highly reflective cladding, and digital pulse readout to realize its optimal performance.
For MeV radiography, moderate light yield terbium-doped scintillating glass sheets, such as that produced by Industrial Quality Inc. (IQI glass), may be coupled to imaging cameras.7 An alternative is the General Electric radiography scintillator ceramic, HiLight, a Eu-doped bixbyite—(Gd,Y)2O3—with better light yield and stopping power about 2× that of scintillating glass.8 We are developing fabrication methods for extremely low optical scatter Lu2O3(Eu) ceramic sheets that offer 4× better stopping power and 3.5× higher light yield than scintillating glass, thus potentially improving imaging throughput by more than 12×. Figure 3 compares the light yields of a IQI scintillating glass and a Lu2O3(Eu) ceramic scintillator fabricated at LLNL, at 20,000 and 75,000 photons/MeV, respectively.
Transparent ceramics are a promising class of materials for scintillator and other optical materials applications where single-crystal performance accompanied by robust mechanical properties is desirable. Some challenges and limitations remain. Unfortunately, relatively few cubic crystalline phases are thermodynamically and environmentally stable enough to achieve the degree of phase purity required to form transparent ceramics, thus limiting candidate lists for new materials development efforts. The preparation of sufficiently uniform green bodies for sintering to the required transparency, especially for large or unusual-sized optics, can be difficult. Polishing costs can become a significant cost center when compared with glasses but are typically less than for single-crystal optics.
For transparent ceramic scintillators, widespread use will depend on demonstrated performance enhancements such as high light yields and uniformity of response, and on the economics of industrial fabrication. In addition to gamma-ray spectroscopy and MeV radiography, we are interested in extending our fabrication expertise to develop improved scintillators for medical applications (keV imaging and positron-emission tomography), particle physics (hadron calorimetry, rare-event detectors), and well logging for oil and mineral exploration.
This work was supported by the Department of Homeland Security Domestic Nuclear Detection Office (Alan Janos) and the US Department of Energy (DOE), Office of the National Nuclear Security Administration, Enhanced Surveillance Subprogram (Patrick Allen). This work was performed under the auspices of DOE by LLNL under contract DE-AC52-07NA27344.
Nerine Cherepy works on scintillator materials and detector development, including single-crystal SrI2(Eu), transparent ceramics for gamma spectroscopy and radiographic imaging, and ‘high-Z’ polymer scintillators.