FISSION OF AL-27 NUCLEUS BY 69 MEV MONOCHROMATIC PHOTONS

Monochromatic photon beams have been used to investigate the photofiss ion of Al-27 at 69 MeV with plastic nuclear-track detectors. The measu red fissility was found to be (5.7 +/- 1.1) x 10(-2). This result, alo ng with the available data for Sm-154 and Yb-174 nuclei taken at the s ame energy, has been interpreted within the framework of a two-step mo del for the photofission reaction. According to this model the incomin g photon is considered to be absorbed by a neutron-proton pair, and th en followed by an evaporation-fission competition for the excited resi dual nucleus. The measured fissility values are seen to be in satisfac tory agreement with the calculated ones, and the predictions from the model are confirmed, specially in the region of light fissioning syste ms.