Rare Earth Phosphors and Scintillators

Luminescent rare earth compounds are receiving renewed attention as a result of their successful application as phosphors for color television and lighting and as crystals for solid state lasers. The compounds of practical interest are hardy materials, frequently simple or complex oxides in which the rare earth activator occurs in the trivalent state. We report here the results of a partially completed survey of electron- and x-ray-stimulated luminescence of the rare earths in selected oxide hosts, on the basis of which it is possible to draw preliminary conclusions regarding the suitability of such materials for scintillation and other radiation detection and measurement applications. The luminescent compounds fall into two categories. Materials of the first category exhibit the sharp line spectra characteristic of transitions within the 4f electron shell. Efficiencies of materials in this category are high but decay times range from microseconds to milliseconds. As a result, their greatest utility would seem to lie in imaging and energy conversion applications. Materials of the second category exhibit broad-band spectra. These spectra arise from either 5d to 4f transitions of the activator ions or from emission of the host itself. Decay times fall in the range from tens of nanoseconds to microseconds. Consequently, these materials have the greatest potential for scintillation counting application. The best materials from each of the classes are discussed.