▪ Abstract  Scintillators are the primary radiation sensor in many applications such as medical diagnostics, medical radiographs, and industrial component inspection. Some of the limitations in the properties of single-crystal scintillators are discussed for imaging applications, and the advantages of a new class of polycrystalline ceramic scintillators are described in detail. After the important scintillator properties of transparency, X-ray stopping power, light output, primary speed, luminescent afterglow, and radiation damage are described, the processing and performance of ceramic scintillators (Y,Gd)2O3:Eu,Pr; Gd2O2S:Pr,Ce,F; and Gd3Ga5O12:Cr,Ce are discussed. Ceramic scintillator uses and trends are presented in light of issues related to their uses in advanced medical and industrial X-ray detectors for CT imaging applications. Finally, some of the challenges are given for successfully developing a polycrystalline ceramic scintillator for use in photon-counting applications.

Polycrystalline ceramic laser rods, composed of a cubic solid solution of 89 mole% Y2O3, 10% ThO2, and 1% Nd2O3, were made by a conventional ceramic sintering approach. Rods of this material, called Nd-doped Yttralox (NDY) ceramic, have lasing thresholds between 16 and 30 J when employing a 95% output mirror reflectivity and a pump pulse of 150 μsec; the lasing slope efficiencies are approximately 0.1%. As a reference for comparison, a commercially available Nd-doped glass laser exhibits a threshold energy of 9 J and a lasing slope efficiency of 0.44% under the same testing conditions. Active attenuation coefficients for the NDY rods examined were between 5 and 7% cm−1. The lower values are within a factor of 6 of that found for laser glass. The major contribution to the attenuation coefficients for NDY laser rods, as currently produced, is a result of unidentified submicroscopic scattering centers. Direct observation of the pore size distributions in these highly transparent sintered ceramics reveals that (i) the volume fraction of pores is of the order of 1 ppm, (ii) there is a peak in the pore size distribution between 1 and 2 μm, (iii) pore growth to this size most likely occurs during the sintering process, and (iv) pores are not the major scattering centers responsible for high scattering losses.

Extensive grain growth was observed by scanning electron microscopy in very porous AlzO,< compacts, even a t densities <40% of theoretical. After 3% shrinkage a t 17OO0C, the grain size increased from ~0 . 3to 0.51 pm in a compact having a relative green density of 0.31. During grain growth in highly porous compacts, the grains appear initially to be chainlike, then to be oblong, and finally to be equiaxed. The proposed mechanism of initial grain growth involves the filling of necks between adjacent grains followed by the movement of the grain boundary through the smaller grain. Although grain growth in very porous compacts is quite different from coalescence and ordinary grain growth, the kinetics are similar.

In a given batch more than 30%-40% of polycrystalline, MgO-doped Al 2 O 3 tubes were converted into single crystals of sapphire by abnormal grain growth (AGG) in the solid state at 1880°C. Most crystals grew 4 -10-cm in length in tubes with wall thicknesses of 1/2 and 3/4 mm and outer diameters of 5 and 7 mm, respectively, and had their c-axes oriented ϳ 90°a nd 45°to the tube axis. Initiation of AGG was associated with low values of bulk MgO concentration near 50 ppm. The unconverted tubes did not develop centimeter-size crystals but instead exhibited millimeter-size grains. The different grain structures in converted and unconverted tubes may be related to nonuniform concentration of MgO in the extruded tubes. The growth front of the migrating crystal boundary was typically nonuniformly shaped, and the interface between the single crystal and the polycrystalline matrix was composed of many "curved" boundary segments indicative of classical AGG in a single-phase material. The average velocities of many migrating crystal boundaries were quite high and reached ϳ1.5 cm/h. The average grain boundary mobility at 1880°C was calculated as 2 ؋ 10 ؊10 m 3 /(N⅐s), representing the highest value reported so far in Al 2 O 3 and within a factor of 2.5 of the calculated intrinsic mobility. Under similar experimental conditions sapphire crystals did not grow when a codopant of CaO, La 2 O 3 , or ZrO 2 was added in concentrations of several hundred ppm.