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Ghosting caused by bulk charge trapping in direct conversion flat-panel detectors using amorphous selenium

2005; Wiley; Volume: 32; Issue: 2 Linguagem: Inglês

10.1118/1.1843353

2473-4209

Wei Zhao, Giovanni DeCrescenzo, Safa Kasap, J. A. Rowlands,

Particle Detector Development and Performance

Direct flat-panel detectors using amorphous selenium (a-Se) x-ray photoconductors are gaining wide-spread clinical use. The goal of our investigation is to understand the physical mechanisms responsible for ghosting, i.e., x-ray induced change in sensitivity that results in image persistence, so that the knowledge can be used to consistently minimize ghosting artifacts in a-Se flat-panel detectors. In this paper we will discuss the effect on x-ray sensitivity of charge trapping in a-Se, which is the dominant source for ghosting in a-Se flat-panel detectors. Our approach is to correlate ghosting in electroded a-Se detectors with the trapped charge concentration measured by the "time-of-flight" (TOF) method. All measurements were performed as a function of radiation exposure of up to at electric field strengths of and . The results showed that the x-ray sensitivity decreased as a function of and the amount of ghosting decreased with increasing . The shape of the TOF curves changed as a result of irradiation in a manner indicating trapped electrons in the bulk of a-Se. The density of trapped electrons increases as a function of . A method was developed to determine the values of in the bulk of a-Se from the TOF measurements, and to predict the corresponding change in x-ray sensitivity. Our results showed that a recombination coefficient consistent with that predicted by Langevin produced good agreement between calculated and measured x-ray sensitivity changes. Thus it can be concluded that the trapping of electrons in the bulk of a-Se and their subsequent recombination with x-ray generated free holes is the dominant mechanism for ghosting in a-Se.