AMORPHOUS-SILICON PIXEL LAYERS WITH CESIUM IODIDE CONVERTERS FOR MEDICAL RADIOGRAPHY

Abstract

We describe the properties of evaporated layers of Cesium Iodide (Thallium activated) deposited on substrates that enable easy coupling to amorphous silicon pixel arrays. The CsI(Tl) layers range in thickness from 65 to 220mum. We used the two-boat evaporator system to deposit CsI(Tl) layers. This system ensures the formation of the scintillator film with homogenous thallium concentration which is essential for optimizing the scintillation light emission efficiency. The Tl concentration was kept to 0.1 -0.2 mole percent for the highest light output. Temperature annealing can affect the microstructure as well as light output of the CsI(Tl) film. 200-300-degrees-C temperature annealing can increase the light output by a factor of two. The amorphous silicon pixel arrays are p-in diodes approximately 1 mum thick with transparent electrodes to enable them to detect the scintillation light produced by X-rays incident on the CsI(Tl). Digital radiography requires a good spatial resolution. This is accomplished by making the detector pixel size less than 50mum. The light emission from the CsI(Tl) is collimated by techniques involving the deposition process on patterned substrates. We have measured MTF of greater than 12 line pairs per mm at the 10% level.