Comparison of Mercuric Iodide and Lead Iodide X-Ray Detectors for X-Ray Imaging Applications

Mercuric iodide (HgI₂) and lead iodide (PbI₂) materials have been investigated for several years as direct converter layers for digital x-ray imaging applications. A difficult challenge of both lead iodide and mercuric iodide is the higher than desired leakage currents. These currents are influenced by different factors such as applied electrical field, layer thickness, layer density, electrode structure, material purity and by the deposition parameters. Minimizing the leakage current must also be achieved without adversely affecting charge transport, which plays a large role in gain and is influenced by these parameters. Other challenges relate to increasing film thickness without degrading electrical properties. This paper compares some imagers as the result of optimization process. We deposited the above materials on flat panel thin film transistor (TFT) arrays with 127 um pixel pitch. The imagers were evaluated for both radiographic and fluoroscopic imaging. Modulation Transfer Function (MTF) was measured as a function of the spatial frequency. The MTF data were compared to values published in the literature for indirect detector (CsI). Image lag characteristics of mercuric iodide appear adequate for fluoroscopic rates. The structure and x-ray diffraction data of the two materials were compared to explain the difference in image lag between them