It is reported that natural background radiation and X-ray radiation for the baggage check in an airport affect the photographic properties of silver halide imaging materials. Since the mechanism of latent image formation for these ionizing radiations has not yet been established precisely, it was investigated here for ⁶⁰Co-γ ray and 80kVp X-ray with octahedral AgBr emulsions. As a result, the sensitivities for these ionizing radiations could be quantitatively predicted from the characteristic curve for visible-light.