Theoretical Performance Of Polycrystalline Mercuric Iodide X-Ray Converters Incorporating Pillar-Supported Frisch Grid Structures

For active matrix flat-panel imagers (AMFPIs), relatively high levels of additive noise compared to the average imaging signal per interacting x-ray limits DQE under conditions of low dose per frame - degrading imaging performance in applications such as digital breast tomosynthesis. The development of particle-in-binder, polycrystalline mercuric iodide (PIB HgI2) as an alternative converter is of interest since the material can provide significantly larger imaging signal per x-ray than converters based on CsI:Tl or a-Se. In order to address the high degree of charge trapping exhibited by HgI2 (leading to unacceptably large image lag), incorporation of a Frisch grid structure into the bulk of the converter is under investigation. Earlier theoretical studies demonstrated that such a grid can greatly reduce the contribution of hole charge to imaging signal - which should reduce the effects of charge trapping. In those studies, the grid was assumed to be positioned between two polycrystalline HgI2 layers - a configuration referred to as a floating grid. In this paper, early results from a theoretical examination of the performance of converters containing a grid supported on insulating pillars over an underlying AMFPI array and a single layer of PIB HgI2 material, are reported. The simulation results indicate that, while charge accumulating on the pillars changes the shape of electric fields that allow collection of electron signal, high electron collection efficiency accompanied by substantial suppression of hole signal can nevertheless be maintained through judicious selection of grid design and operational conditions.