Ghosting and its recovery mechanisms in multilayer selenium detectors for mammography

Abstract

The ghosting and its recovery mechanisms in multilayer Selenium detectors for mammography are experimentally and theoretically investigated. The theoretical model considers accumulated trapped charges and their effects (trap filling, recombination, electric field profile, electric field dependent electron-hole pair creation), the carrier transport in the blocking layers, X-ray induced metastable deep trap center generations, and the effects of charge injection. The time dependent carrier detrapping and structural relaxation (recovery of meta-stable trap centers) are also considered. We simultaneously solve the continuity equations for both holes and electrons, trapping rate equations, and the Poisson's equation across the photoconductor for a step X-ray exposure by the Backward Euler finite difference method. The amount of ghosting strongly depends on the applied electric field and the initial carrier lifetimes. The dark current increases significantly with accumulated exposures. The sensitivity in a rested sample is recovered mainly by the carrier detrapping and the recombination of the injected carriers with the existing trapped carriers. The electric fields at the metal contacts increses with time in ghosting recovery process which leads to the initial increase of the dark current. The sensitivity is expected to recover almost fully by resting the sample longer than the recovery time constant of the meta-stable trap centers (the structural relaxation time constant), which is more than 24 hours. The theoretical model shows a very good agreement with the experimental relative sensitivity versus time and accumulative X-ray exposure characteristics.