Search for friction precursors in fusion barrier distributions

Fusion barrier distributions, determined by taking the second differential of the product Eσ fus, have been measured for the reactions of 16 O and 28 Si beams on 144 Sm and 208 Pb targets. The shape of the barrier distribution reflects the important processes influencing fusion. It is know that the 28 Si + 208 Pb reaction results in quasi-fission, and heavier projectiles (Z>20) require an extra-push energy before quasi-fission occurs. By comparing results for the 144 Sm and 208 Pb targets, signatures of the microscopic processes responsible for the extra-push energy may be seen in differences in the barrier distributions. However, preliminary analysis of the new data presented here show no evidence for such friction precursors. The barrier distributions for the 144 Sm and 208 Pb targets show significant differences, which are interpreted in terms of single (144 Sm) and double (208 Pb) octupole phonon-excitations. 17 refs., 5 figs