Experimental and Monte Carlo analyses of a-Se and CsI:Na/a-Se for X-ray imaging

In this paper, the pertinence of a solution using thin coplanar selenium as a photosensitive converter requiring only a few tens of volts of bias, associated with a thick columnar coating of sodium-doped cesium iodide scintillator, was evaluated. A new evaporation process for direct CsI:Na deposition on an a-Se layer substrate was developed. The photoluminescence characterization of this scintillator shows a light emission peak centered at 420 nm, as expected, which matches the sensitivity spectrum of selenium. In addition, to optimize the thickness of the CsI:Na/a-Se structure, the X-ray absorption was investigated at various diagnostic X-ray energy levels using the MCNP 4C code. The Monte Carlo simulation results showed that the absorption fraction of 500 μm-Se film and 180 μm-CsI(Na) film is about 80% at 60 kVp. The simulation results were verified by comparing these with experimental measurements and results. The measured dark currents were below 300 pA/cm² at an electric field of 10 V/μm for a-Se and CsI:Na/a-Se. The preliminary sensitivity measurements give a signal in the range of about 7.31 and 3.95 nC/cm²/mR for 180 μm-CsI:Tl/a-Se and 500 μm-Se at the exposure conditions of 60 kVp and 2-mm aluminum filter, respectively.