Energy Localization and Decay in Highly Ionic Crystals
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Some of the most widely used standard scintillators are impurity-activated ionic crystals offering high efficiency, often at the price of speed. The ongoing quest for optimized fast scintillators now includes undoped crystalline insulators among the leading contenders. Indeed if speed of response were simply related to the distance from point of excitation to the nearest luminescent entity, and if efficiency were directly proportional to the concentration of luminescent entities, then intrinsic luminescence in pure crystals should define the extrapolation goal of ultimate scintillator performance. While both of the above
considerations are true in part, we know that translational symmetry in a pure crystal has crucial ramifications for electronic excitations and their luminescent decay. The interband electronic excitations of a pure crystalline insulator are excitons, whether bound or ionized, and formally occupy the whole crystal. This is true in every perfect rigid lattice with translational symmetry, although intersite transfer rate and consequent band dispersion vary greatly from molecular crystals, rare-earth intra-atomic excitations, and core excitations at the low end, up to semiconductor valence-to-conduction transitions at the high end. Alkali halides and similar ionic insulators would be somewhere in the middle with regard to their rigid-lattice exciton transport properties, if vibrational interactions did not exert the strong localizing influence to be discussed below. Energy migration, as a consequence of the excitonic nature of pure-crystal excitations, can severely limit the efficiency of scintillation if those excitations quench at crystal surfaces and impurities or defects. Equally serious for efficient performance of a pure crystalline scintillator is the question of re-absorption. That is, if nothing happens to shift recombination luminescence away from the exciton absorption bands (typically broadened into an Urbach edge filling the phonon-assisted exciton spectral range), the luminescence will be strongly re-absorbed. A good illustration of these consequences can be found in the exciton luminescence spectrum of GaAs at 2 K, shown in the upper left of Fig. 1. The free exciton luminescence is the very weak peak at 1.5155 eV. Even though these data 1 were acquired on a high 331 Mat. Res. Soc. Symp. Proc. Vol. 348. 01994 Materials Research Society
purity crystal with the intention of revealing the exciton luminescence as clearly as possible, it can be seen that peaks due to defect-trapped excitons, such as the Do-X line at 1.5141 eV dominate, because the mobile exciton easily finds imperfections throughout the crystal within its radiative lifetime. There is an overall reduction of intensity because of excitons
reaching the surface, which acts as a quenching site. The n= 1 direct exciton absorption peak is resonant with the free exciton luminescence peak in this spectrum, so re-absorption
is serious. For a variety of reasons, the free exciton luminescence peak is not even resolvable in such a sa
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