First-principles calculations of hole trapping and transport: Effects on scintillator luminescence

An ab initio molecular orbital program is applied to clusters to compute en ergies and electron densities that describe the formation and transport of holes produced by ionizing radiation and their effects on luminescence of i norganic scintillator crystals. The resulting electron density plots for Cs I show that a relaxed hole is shared equally by two bound I atoms (the clas sic V-k center). The calculated energy barrier for motion of the self-trapp ed hole is in excellent agreement with that expected from experimental meas urements of the temperature dependence of the slow scintillation rise time of Tl-activated CsI. For CaF2 initial V-k center formation is again predict ed. The calculated energy barrier against hole migration is in reasonable a greement with the experimental value, however the scintillation rise time o f Eu2+-activated CaF2 is very fast. In this case the Eu2+ activator promptl y captures a hole before it can become self-trapped, (C) 2000 Elsevier Scie nce B.V. All rights reserved.