Large area mercuric iodide and lead iodide X-ray detectors for medical and non-destructive industrial imaging

Abstract

Mercuric iodide (HgI 2) and lead iodide (PbI 2) thin polycrystalline films have been under development for several years as direct converter layers for digital X-ray imaging. In this paper, we cover the basic electrical and physical characteristics of these materials and compare to other X-ray sensitive photoconductive materials. Both lead iodide and mercuric iodide were vacuum deposited on a-Si TFT arrays with 127 μm pixel pitch. This coating technology is scalable to sizes required in common X-ray imaging applications, as proved by the recent 10 cm×10 cm and 20 cm×25 cm imager results. A difficult challenge of both lead iodide and mercuric iodide detectors is higher than the desired leakage current. 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 effects on gain. The improvement in dark current correlates with more perfect (single crystalline like) structures as shown by X-ray diffraction data in HgI 2 films. The imagers were evaluated for both radiographic and fluoroscopic imaging. MTF was measured as a function of the spatial frequency and results were compared to values for indirect detectors (CsI). The ability to operate at moderate voltages (∼0.2–1.0 V/μm) provides adequate dark current for most applications and allows low voltage electronics design. Image lag characteristics of mercuric iodide appear adequate for fluoroscopic rates.