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

We describe the properties of evaporated layers of Cesium Iodide (Thal lium activated) deposited on substrates that enable easy coupling to a morphous silicon pixel arrays. The CsI(Tl) layers range in thickness f rom 65 to 220mum. We used the two-boat evaporator system to deposit Cs I(Tl) layers. This system ensures the formation of the scintillator fi lm with homogenous thallium concentration which is essential for optim izing the scintillation light emission efficiency. The Tl concentratio n was kept to 0.1 -0.2 mole percent for the highest light output. Temp erature annealing can affect the microstructure as well as light outpu t of the CsI(Tl) film. 200-300-degrees-C temperature annealing can inc rease the light output by a factor of two. The amorphous silicon pixel arrays are p-in diodes approximately 1 mum thick with transparent ele ctrodes to enable them to detect the scintillation light produced by X -rays incident on the CsI(Tl). Digital radiography requires a good spa tial resolution. This is accomplished by making the detector pixel siz e less than 50mum. The light emission from the CsI(Tl) is collimated b y techniques involving the deposition process on patterned substrates. We have measured MTF of greater than 12 line pairs per mm at the 10% level.