The Role of Core Levels in Scintillation Processes
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*Synchrotron Radiation Laboratory, Physics Department, Moscow State University, Moscow, 117234, Russia "**Institute for Single Crystals, Kharkov, Ukraine tLaboratoire de Physico-Chimie des Materiaux Luminescents, University Lyon I,
Batiment 205, 43, boulevard du 11 Novembre 1918, 69622 Villeurbanne Cedex, France ttLaboratoire pour rUtilisation du Rayonnement Electromagnetique, CNRS, Batiment 209D, Centre Universitaire Paris-Sud, 91405 Orsay Cedex, France ABSTRACT The Auger decay of a core hole results in appearance of several strongly correlated
excitations. This excited region strongly polarizes the lattice and thus the defect creation is possible. In all cases the core hole causes the strong local perturbation of electronic and lattice subsystems. The creation of such excited region with mutual relaxation of correlated electrons
and holes can result inthe increase of the efficiency of energy transfer to activators, acceleration of the luminescence kinetics, and the appearance of radiation-induced luminescence centers. These effects have been studied using VUV and soft X-ray synchrotron radiation, when the selective excitation of different core levels is possible.
INTRODUCTION The absorption of -y-and X-rays results in creation of primary electronic excitations (EEs), i.e. hot electrons and core holes. Their energies depend on the energy of the excitation. Inelastic scattering of a primary electron by valence and core electrons and Auger relaxation of a core hole produce large number (n=E/(2+3)Eg, Eg is the forbidden gap) of secondary low-energy EEs, which transfer energy to emission centers. The spatial distribution of secondary EEs is non-homogeneous and depends on their energies and mechanism of the relaxation of hot EEs. Local regions with high density of electronic excitations can be created during the relaxation. Secondary EEs are produced at distances from I to 5 nm after the relaxation of core holes. In such region the creation of multiply ionized ions and groups of EEs is possible. The aim of the present report is the discussion of some effects after the relaxation of core holes on the scintillation properties of crystals. Fast electrons after the absorption of a y-quantum with the energy about megaelectronvolts excite mainly electrons from the highest lying levels with energies 10 to 100 eV, since the energy loss function is large in this energy region. They can be excited separately by VUV photons as well. Therefore the usage of VUV spectroscopy gives the opportunity to study the important stage of the energy relaxation in the scintillation process.
241 Mat. Res. Soc. Symp. Proc. Vol. 348. 01994 Materials Research Society
ENERGY TRANSFER TO ACTIVATORS ABOVE CORE LEVEL IONIZATION THRESHOLD Figure I shows the luminescence excitation spectra of BaS, SrS and LSO doped with Ce 3+. Each of these excitation spectra has a peak at the energies of the exciton creation. The excitation efficiency decreases with photon energies above the exciton energy. The dip is sharp for BaS, moderate for SrS and smooth fo
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