Investigating columnar scintillators through analytical modeling. A semiempirical approach

Abstract

X-ray digital imaging systems often employ scintillator materials as part of the radiation detector (indirect detection). Recently CsI:Tl columnar phosphor is used extensively as an X-ray converter due to its higher spatial resolution compared to granular phosphors like Gd2O2S:Tb. The efficiency and the imaging performance of CsI:Tl has been already studied either experimentally or by Monte Carlo simulation. The aim of this work is to provide a semi empirical analytical model for determining CsI:Tl optical photon propagation related parameters like photon attenuation per elementary thickness and spread to the output. Hence the calculation of Detective Quantum Efficiency (DQE) of CsI:Tl for different column thicknesses (50μm up to 300μm) was achieved. The developed model considers the absorption of X-rays in the scintillator, the production of optical photons, the propagation and the reflections of the optical photons to the input and output interfaces. Finally it acknowledges the spread of the optical photons to the output. The optical photon propagation is studied by considering that the fraction of the optical photons passing through an elementary screen layer is constant. The optical spread to the output is assumed to result in a Gaussian shaped LSF. Published Monte Carlo data for 85 μm, 100 μm and 300 μm CsI:Tl columns were used to fit the analytical model equations. The optical photon fraction transmitted through each layer was found approximately 98.6%. The LSF shape was found to be depended by column thickness. Higher DQE at 0 lp/mm exhibited the 300μm column thickness CsI:Tl, while at 10lp/mm the 50μm column thickness CsI:Tl. It was interesting to notice that at approximately 4lp/mm the DQE curves of all thicknesses were comparable.