Mesures de vies moyennes en ligne sur faisceau d'ions lourds dans la gamme de la microseconde

Life-time measurements were carried out by using time to amplitude conversion methods, the information being classified in a two- or three-dimensional spectrum by means of an on-line IBM 360-50 computer. In the first case, the zero time corresponds to the compound nucleus formation and is defined by a fast coincidence between a prompt γ-ray and the beam pulse detected by a thin plastic foil. In this way, the time distributions of all the γ-rays associated with a given reaction are simultaneously measured and can be extracted from the resulting 1024 (Eγ) × 256 (time) channels spectrum. The random coincidence peaks shown by all the curves with the same repetition rate as that of the cyclotron frequency (150–200 ns) can be eliminated by computing the difference between the contributions of a γ-ray and of the continuous background. However, as it is rather difficult to resolve the components of a complex decay curve, when several half-lives of intermediate levels interfere, we turned to a three-dimensional analysis in order to estimate these components. The pulse starting the TAC is due to the same type of coincidence as before but the γ-rays taken into account are now delayed relative to the compound nucleus formation. The size of this delayed γ spectrum is limited by the computer memory and only eight energy regions can be selected (four peaks and the backgrounds nearby, for example). By treatinf the 8 (Eγ2) × 1024 (Eγ2) × 256 (time) channels spectrum obtained, we can derive the decay curves of all the γ-rays with respect to each of the delayed γ transitions.