Dark current, sensitivity, and image lag comparison of mercuric iodide and lead iodide x-ray imagers

Abstract

Mercuric iodide (HgI2) and lead iodide (PbI2) materials as direct converter layers for digital x-ray imaging have been studied for several years. This paper present results of basic imaging parameters by comparing dark current, sensitivity and image lag properties of these materials. A difficult challenge of both lead iodide and mercuric iodide photon detectors is higher than desired leakage currents. These currents are influenced by factors such as applied electrical field, layer thickness, layer density, electrode structure and material purity. Minimizing the leakage current must also be achieved without adversely affecting charge transport, which plays a large role in gain and is also influenced by these parameters. New deposition technologies have been developed through which the leakage current has now decreased by more than an order of magnitude while showing no negative affects on gain. Other challenges relate to increasing film thickness without degrading electrical properties. The image lag of the polycrystalline PbI2 is much larger than that of the polycrystalline HgI2 material, however, no significant image lag is observed for single crystal PbI2. Optical microscopy and SEM studies showed that the polycrystalline PbI2 has a low density, randomly oriented morphology with small crystallites while the best HgI2 has a much better oriented (single crystal-like) structure. We believe that the long image lag can be attributed to the large number of deep defect states generated on the surface of the small PbI2 crystallites. The imagers were evaluated for both radiographic and fluoroscopic imaging modes. MTF was measured as a function of the spatial frequency. The MTF data were compared to values published in the literature for indirect detectors (CsI) and direct detectors (a-Se). Resolution tests on resolution target phantoms showed that for both materials resolution is mostly limited by the TFT array Nyquist frequency.