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

Active matrix flat panel dosimeters (AMFPD´s) have a high density of charge trapping centers (∼3×10¹⁵/cm³) 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 work examines the characteristics of a large AMFPD (26 cm × 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 both exposure time and dose at a beam energy of 6 MV. Additionally, the impulse response functions (IRF´s) were also measured at 6 MV for the same set of exposure times and doses used for the measurement of the nonlinear mapping. These IRF´s are shown to be functions of both exposure time and dose delivered to the detector. The reproducibility of the dose measurements is discussed.