The energies of the ground 4f~(n) levels of tri- and divalent rare-earth ions with respect to the conduction and valence bands of Gd_(2)O_(2)S crystal has been determined. It is shown that the Pr~(3+), Tb~(3+), and Eu~(3+) ions can be luminescence centers in Gd_(2)O_(2)S. The levels of the Nd~(3+), Dy~(3+), Er~(3+), Tm~(3+), Sm~(3+), and Ho~(3+) ions lie in the valence band; therefore, these ions cannot play the role of activators. The ground 4f level of the Ce~(3+) ion is near the midgap, due to which Ce~(3+) effectively captures holes from the valence band and electrons from the conduction band and significantly decreases the afterglow level of the Gd_(2)O_(2)S:Pr and Gd_(2)O_(2)S:Tb phosphors.
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