Fission of 27Al nucleus by 69 MeV monochromatic photons

Monochromatic photon beams have been used to investigate the photofission of 27Al at 69 MeV with plastic nuclear-track detectors. The measured fissility was found to be (5.7+or-1.1)*10-2. This result, along with the available data for 154Sm and 174Yb nuclei taken at the same energy, has been interpreted within the framework of a two-step model for the photofission reaction. According to this model the incoming photon is considered to be absorbed by a neutron-proton pair, and then followed by an evaporation-fission competition for the excited residual nucleus. The measured fissility values are seen to be in satisfactory agreement with the calculated ones, and the predictions from the model are confirmed, specially in the region of light fissioning systems.