Charge trapping at high doses in an active matrix flat panel dosimeter

Active matrix flat panel dosimeters (AMFPDs) have a high density of charge trapping centers (/spl sim/3/spl times/10/sup 15//cm/sup 3/) which lead to both lag and ghosting in reconstructed dose measurements. Lag is due to charge release within the detector and is a function of the lifetime of the trapping centers within the detector. Ghosting, defined as excess charge not produced by charge release, is a phenomenon believed to be caused by reduced charge trapping probability due to the trapped charge population density being greater after exposure to a radiation field than previous to the exposure. This paper examines the characteristics of a large AMFPD (26 cm /spl times/ 26 cm) detector for long exposure times (up to 1 minute) and high doses (up to 3 Gy). This detector was operated in a direct detection mode. Nonlinear response functions were mapped as a function of exposure time and dose for an X-ray beam where the accelerating potential of the electrons was 6 megavolts (MV). Additionally, the impulse response functions (IRFs) were measured at 6 MV for the same set of exposure times and doses used for the measurement of the nonlinear mapping. These IRFs are shown to be functions of both exposure time and dose delivered to the detector. The reproducibility of dose measurements using procedures derived from this paper is discussed. The temporal transfer function (TTF), defined as the absolute value of the Fourier transform of the IRF, is introduced and several examples of the TTFs are shown and discussed.