Effects of the thickness of CsI(Tl) scintillator on its image qualities

Abstract

The scintillating layer coupled to amorphous silicon photodiode with thin film routing electronics have a good prospect as a substitute of conventional X-ray film. There is a need to fabricate the large-area CsI(Tl) thin film with high scintillation efficiency and high spatial resolution for this purpose. In this study, the thickness dependency on the light output and the spatial resolution of CsI(Tl) layer was investigated by calculation and measurement. The result can be used to determine the optimum thickness of CsI(Tl) scintillation layer for any specific application. Thin, homogeneous CsI(Tl) scintillation layers (area as large as 30㎠) have been produced by vacuum evaporation of the mixture of CsI and TlI powder. The scintillation performance of evaporated CsI(Tl) layer was proved to be comparable to that of the commercially available phosphors. The evaporated CsI(Tl) layers with various thickness ranging from 25㎛ to 200㎛ were prepared. The light output and spatial resolution of evaporated layers were measured in the unit of electrons equivalent to the scintillation photons and line-pair per mm respectively. Light collection efficiency of ideal photosensor was calculated with the assumption of the isotopic emission of light from x-ray absorption. Using the light collection efficiency, we obtained the light output and the spatial resolution of ideal CsI(Tl) layers. It was revealed that the light output and spatial resolution have compromise in term of the thickness of scintillation layer. As the thickness increases, the light output increases and the spatial resolution decreases. A curve illustrating the correlation between the relative efficiency and the spatial resolution of the CsI(Tl) scintillator layer was obtained from the measurement as well as the calculation.