Effect of ghosting on the modulation transfer function of amorphous selenium based flat panel detectors

Abstract

Direct conversion flat panel detectors (FPD) often experience a loss of sensitivity to x rays caused by previous exposure of the panel to radiation-a phenomenon known as ghosting. Bulk charge trapping and recombination, collectively referred to as incomplete collection of charges, are one of the major causes of ghosting in FPDs. In our investigation, the effects of incomplete charge collection on the modulation transfer function (MTF) of an a -Se direct conversion FPD were studied. The approach used was to repeatedly ghost the panel by uniformly exposing it to a high dose of radiation to force bulk trapping of charges, measure the MTF of the panel after each exposure, and compare this MTF to that of a non-ghosted panel by taking their ratio. MTF ratios allow us to isolate charge collection dependent components of the MTF from all other components. Our results show that ghosting brings about an increase in the MTF of the panel which we interpret as being due to an increase in the incomplete collection of holes. This involves the recombination of free holes with electrons that were trapped from prior exposures, as well as additional trapping of holes caused by the formation of new radiation induced trap states.