Using the transient behavior of the photoconductive-gain mechanism, a signal gain in radiation detection with a-Si : H devices may be possible. The photoconductive gain mechanism in two types of hydrogenated amorphous silicon devices, p-i-n and n-i-n configurations, was investigated in connection with applications to radiation detection. Photoconductive gain was measured in two time scales: one for short pulses of visible light (< 1 mu s) which simulate the transit of energetic charged particles or gamma-rays, and the other for rather long pulses of light (greater than or equal to 1 ms) which simulate X-ray exposure in medical imaging. The photoconductive gain in our devices could be calculated by comparing the photo-induced signals from n-i-n photoconductors and forward biased p-i-n photodiodes to the maximum signals from corresponding reverse biased p-i-n photodiodes. We used two definitions of photoconductive gain: current gain and charge gain which is an integration of the current gain. We obtained typical charge gains of 3-9 for short pulses and a few hundreds for long pulses at a dark current density level of 10 mA/cm(2). Various gain results are discussed in terms of the device structure, applied bias and dark-current density.