On the mechanism leading to afterglow in Gd2O2S:Pr

Abstract

The performance of scintillator materials and phosphors for x-ray imaging is adversely affected by afterglow, giving rise to ghost images and thus compromising image quality. Here, the afterglow of the well-known computed tomography (CT) phosphor Gd2O2S:Pr is investigated with time-delayed afterglow spectroscopy. It is shown that these afterglow spectra mainly show Eu3+ emission. This points to a mechanism, during excitation across the bandgap, in which holes are captured by Pr3+ ions, resulting in Pr4+ and electrons are captured by Eu3+ ions, creating Eu2+. Afterglow is due to thermal excitation of electrons from the valence band to Pr4+ ions (detrapping of holes from Pr4+), restoring Pr3+ ions. Capture of the resulting holes in the valence band by Eu2+ ions leads to excited Eu3+ ions that generate Eu3+ emission thus causing the undesired afterglow. Time delayed Eu3+ afterglow spectra are observed already for very low Eu3+ concentrations (<10 ppm). These observations provide insight in the dominant afterglow mechanism in Gd2O2S:Pr3+ phosphors and allow to optimize the performance and improve the CT image quality by selection of starting materials with the lowest possible Eu3+ contamination levels to effectively reduce afterglow without loss of light yield.