Role of the activator in the performance of scintillators used in X-ray imaging

The aim of this study was to investigate how the activator type affects the performance of X-ray scintillators. To this aim the behavior of scintillat or materials was modeled under X-ray excitation conditions, similar to thos e used in imaging techniques. The model describes the light emission effici ency, the spectral compatibility with optical detectors (films, photodiodes , and photocathodes), and the imaging capabilities of a scintillating layer . Using the model equations the role of the activator type in scintillator performance was examined. Activators affect some important properties of ma terials, like the intrinsic X-ray to light conversion efficiency, the spect rum of the emitted light, and the light attenuation coefficients. The perfo rmances of a high-efficiency material:(Gd2O2S) combined with either Tb3+ or EU3+ activators were compared. Results showed that the terbium-activated m aterial exhibited high emission efficiency (number of emitted photons per i ncident X-ray) and modulation transfer function (spatial resolution and ima ge contrast) while the europium activated material showed slightly better s ignal-to-noise ratio properties at low spatial frequencies. Both materials were found to exhibit high spectral compatibility with currently used moder n optical detectors. In conclusion, the choice of activator may improve spe ctral compatibility, but care must be taken because it may also alter emiss ion efficiency and image quality. (C) 2001 Elsevier Science Ltd. All rights reserved.