| Περίληψη: | Scintillators are materials used in ionizing radiation detector systems. In x – ray imaging they usually constitute the first component of an ionizing radiation detector system, responsible for absorbing the incident X – ray photons and for each X – ray photon absorbed they produce a large number of optical photons, incident on a sensitive optical sensor. Thus the dose required to obtain a certain image is reduced, resulting in reduced patient and personnel dose.
One of the commonly used scintillator in x – ray imaging, is the structured CsI:Tl. Due to its columnar growth it guides the optical photon beams to the optical sensor more effective than other type scinitllators. This scintillator has been studied in literature through Monte Carlo models and through analytical models.
This thesis aims to develop an analytical model for the optical photon propagation inside columnar phosphors that resolves the weaknesses of previous analytical models. This analytical model is based on geometrical and physical properties of the crystalline material. It accounts for the physical dimensions such as the length and the height of the crystal column, the absorption of x – rays inside the crystal bulk, the site of the creation of the optical photon beams and the production of optical photon beams inside the crystal bulk, the angle of emission and consequently the direction of propagation of these optical photon beams, their attenuation inside the crystal bulk, the reflections on the front, back and lateral sides of the crystal column, the lateral propagated optical photon beams after multiple refractions from the neighbor columns, the angle of refraction at the exit side of the crystal column, the refraction coefficient of the crystal with respect to the surrounding medium and the light attenuation coefficient of the optical photon beams.
The novelty of this work is that the developed model accounts for all the parameters referred above and not only one exit of each optical photon beam produced, but for multiple exits taking into account the multiple forward and backward propagation of each optical photon beam.
This model was used to predict parameters useful for the characterization of the scintillator, such as Light Collection (LC), Energy Absorption Efficiency (EAE), Detector Optical Gain (DOG), Angular Distribution and Modulation Transfer Function (MTF). It was applied to structured CsI:Tl scntillator, it was validated against published results and good agreement was observed.
It was found that the DOG is affected by the length of the columns, as well as the incident x-ray energy spectrum and the results of DOG are in accordance with the use of short crystal columns for lower energies (mammography) and the use of long crystal columns for higher energies (general radiology). The results of the angular distribution are in accordance with the theory that the longer crystal columns have more directional light distribution and that the angular distribution depends on the medium attached to the scintillator.
The results of MTF are in good accordance with experimental results found in literature and they also show that decreasing the pixel size the frequency dependent MTF is increased.
Even if the model was applied and validated for columnar CsI:Tl, the optical part of it may be applied to single crystal CsI:Tl and other columnar scintillators provided that the geometrical and optical properties of the crystal are known.
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