Scintillating fiber optic screens: A comparison of MTF, light conversion efficiency, and emission angle with Gd2O2S:Tb screens

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Abstract
The widespread effort in developing digital imaging systems has led to large area high pixel density photodetectors such as charge coupled devices (CCDs), amorphous silicon photodiode arrays, and complementary metal oxide semiconductor (CMOS) imagers. These photodetectors have different capabilities, characteristics, and requirements than conventional silver-halide-based film, and this fact had led to a new generation of exotic scintillators, including fiber optic screens made from scintillating glass. The scintillator performance characteristics of five different scintillating fiber optic screens and two conventional Gd2O2S:Tb screens (one 34 mg/cm2 and the other 60 mg/cm2) were measured and compared. The measurements that were made included the angular dependence of light emission relative to the normal, the modulation transfer function (MTF), and the absolute effective conversion efficiency (light photons per absorbed x-ray photon). It was found that the light emission of scintillating fiber optic screens is markedly forward peaked (depending on the sample) compared to conventional screens or Lambertian emitters. The MTFs of the five scintillating fiber optic screens measured were comparable and fell approximately midway between the two conventional screen MTFs. One of the scintillating fiber optic screens demonstrated light efficiency similar to the thick (60 mg/cm2) conventional screen, another had light output capabilities similar to the thin (34 mg/cm2) conventional screen, and the three others were less efficient than the thin screen. The non-Lambertian characteristics of the fiber optic scintillators will cause errors of up to 75% in lens efficiency calculations if a Lambertian source is assumed. The conventional screens were found to conform within about 5% of an ideal Lambertian emitter.