STUDY OF TRANSFER CHANNEL COUPLING AND ENTRANCE CHANNEL EFFECTS FOR THE NEAR AND SUBBARRIER FUSION OF TI-46-64, TI-50+NI-60 AND F-19+NB-93 SYSTEMS(NI)

The fusion excitation functions for the Ti-46+Ni-64, Ti-50+Ni-60 and F -19+Nb-93 systems have been measured from similar to 10% below to 15% above the nominal Coulomb barrier, utilizing the recoil mass separator , HIRA at the Nuclear Science Centre, New Delhi. Mean spins have been deduced from the fusion cross sections by two different methods, one u sing the statistical model and the second by a fit to the fusion cross sections, Good agreement is found between them. It is seen thar the s implified coupled-channel calculations, with couplings to the lowest s urface inelastic excitations alone, do not explain both the observed f usion cross sections and mean spins in the low energy domain for all t he three systems. A systematic comparison is made between the systems Ti-46+Ni-64 and Ti-50+Ni-60 which have quite different ground-state tr ansfer Q-values (Q(gg)) for the transfer channels. The Ti-46+Ni-64 sys tem shows a significant enhancement of the sub-barrier fusion cross se ction and mean spin as compared with the Ti-50+Ni-60 system, indicatin g the importance of the transfer channel coupling, Fusion cross sectio n and mean spin have been studied for the near and sub-barrier fusion of the highly mass asymmetric system F-19+Nb-93 and compared with thos e for the Ti-48+Ni-64 system leading to the same compound nucleus via the nearly symmetric entrance channel, Since the F-19+Nb-93 system has a large positive Q(gg) for one-proton pick-up, the fusion data have b een complemented by one-nucleon transfer measurements at energies arou nd the Coulomb barrier. A simultaneous analysis of the fusion excitati on functions and mean spin data indicates no significant effects that may be related to the entrance channel mass asymmetry.