Hydrogenated amorphous silicon (a-Si:H) p-i-n diodes with transparent metallic contacts are shown to be suitable for detecting charged particles, electrons, and X-rays. When coupled to a suitable scintillator using CsI(Tl) as the scintillator we show a capability to detect minimum ionizing particles with SIN similar to 20. We demonstrate such an arrangement by operating a p-i-n diode in photovolaic mode (reverse bias). Moreover, we show that a p-i-n diode can also work as a photoconductor under forward bias and produces a gain yield of 3-8 higher light sensitivity for shaping times of 1 mu s n-i-n devices have similar optical gain as the p-i-n photoconductor for short integrating times (<10 mu S). However, n-i-n devices exhibit much higher gain for a long term integration (10 ms) than the p-i-n ones. High sensitivity photosensors are very desirable for X-ray medical imaging because radiation exposure dose can be reduced significantly. The scintillator CsI layers we made have higher spatial resolution than the Kodak commercial scintillator screens due to their internal columnar structure which can collimate the scintillator light. Evaporated CsI layers are shown to be more resistant to radiation damage than the crystalline bulk CsI(Tl).