Sensitivity of x-ray photoconductors: Charge trapping and absorption-limited universal sensitivity curves

Direct conversion flat panel x-ray sensors that are currently under development are based on stabilized a-Se (a-Se alloyed with a small percentage of As and doped with Cl in the ppm amount). There are also other potential x-ray photoconductors such as PbO, PbI2, HgI2, CdTe, etc. Indeed, good x-ray images have been reported from PbI2, HgI2, and CdTe based x-ray sensors. The present article considers the x-ray sensitivity of photoconductors in terms of the following combined effects: (i) Absorption of x-rays, controlled by the linear attenuation coefficient α(E) and energy absorption coefficient αen(E), both x-ray photon energy E dependent. (ii) Electric field F and x-ray photon energy dependent ionization of the medium, that is, in terms of the electron and hole creation energy W±(E,F). (iii) The transport and trapping of charges across the photoconductor as they drift to the collecting electrodes. (iv) The electron and hole pairs are generated with an exponentially decaying distribution across the thickness of the photoconductor. We analytically solve the continuity equation by considering the drift of electrons and holes in the presence of deep traps. We derive an expression for the amount of collected charge per unit incident radiation, defined as the x-ray sensitivity S, in terms of W±, α, αen, and the normalized parameters: normalized attenuation depth and electron and hole schubwegs per unit thickness. We obtain two- and three-dimensional universal sensitivity curves that allow x-ray sensitivity of any potential x-ray photoconductor material to be determined from the normalized parameters.