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Flat-panel detectors using amorphous silicon p-i-n diodes have been developed for chest and therapeutic portal imaging. In radiation therapy, an imaging device is exposed to a high radiation dose. The radiation damage affects the leakage current of diode and may decrease the whole system performance. In this research, we have investigated the radiation damage to the leakage current of a-Si:H diode made by plasma-enhanced chemical vapor deposition (PECVD) and ion-shower doping method, and studied the effect of a copper plate generally used in portal imaging to increase the efficiency of detector. We measured the transient current of diodes between radiation exposures, and obtained the leakage current of diodes as a function of absorbed dose. The two diodes show similar behavior when used with a copper plate. The leakage current shows a linear dependence on the absorbed dose, and threshold dose was observed when a copper plate was placed on the top. The ion-shower diode shows higher rate of current increase, which is considered to originate from the intrinsic junction properties.