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 scintillator materials was modeled under X-ray excitation conditions, similar to those used in imaging techniques. The model describes the light emission efficiency, 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 materials, like the intrinsic X-ray to light conversion efficiency, the spectrum of the emitted light, and the light attenuation coefficients. The performances of a high-efficiency material (Gd2O2S) combined with either Tb3+ or Eu3+ activators were compared. Results showed that the terbium-activated material exhibited high emission efficiency (number of emitted photons per incident X-ray) and modulation transfer function (spatial resolution and image contrast) while the europium activated material showed slightly better signal-to-noise ratio properties at low spatial frequencies. Both materials were found to exhibit high spectral compatibility with currently used modern optical detectors. In conclusion, the choice of activator may improve spectral compatibility, but care must be taken because it may also alter emission efficiency and image quality.