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By codoping Dy3+ into the SrAl2O4:Eu2+ phosphor, long persistent phosphorescence was observed and its afterglow characteristics were investigated. The excitation wavelength suitable for long persistent phosphorescence was 200-250nm, much shorter than the wavelength of 4f7-4f65d absorption of the Eu2+ ions, suggesting that the energy levels of electron traps are higher than the 4f65d level. The phosphorescence intensity and its lifetime increased with increasing temperature. The depth of the electron traps was calculated from the Hoogenstraaten's plot of glow curves at about 0.3 and 0.6eV. From the temperature dependence of decay curves, the deeper trap is considered to contribute to the long-lasting afterglow.
TAKASAKI et al. (Mon,) studied this question.