Abstract:
A steeper fall of fusion excitation function, compared to the predictions of
coupled-channel (CC) models, at energies below the lowest barrier between
the reaction partners, is termed as deep sub-barrier fusion hindrance. This
phenomenon has been observed in many symmetric and nearly symmetric
systems. Different physical origins of the hindrance have been proposed
though a complete understanding is yet to be achieved. This work reports the
measurement of the fusion (evaporation residue) cross sections for the system
19F+181Ta, from above the barrier down to the energies where fusion hindrance is expected to come into play. CC calculation with standard Woods–
Saxon potential gives a fair description of the fusion excitation function down to energies ;14% below the barrier. This is in contrast with the observation of
increasing fusion hindrance in asymmetric reactions induced by increasingly
heavier projectiles, viz. 6,7Li, 11B, 12C and 16O. The most likely reason for
non-observation of fusion hindrance in the present system is the ‘softness’ of
the collision partners. The ‘threshold’ for the onset of fusion hindrance in
reactions involving well-deformed nuclei is expected to be at an energy lower
than that estimated from the systematics. It is also noted that the asymmetric
reactions, which have not shown any signature of fusion hindrance within the
measured energy range, are induced by projectiles with low α-particle breakup threshold and have positive Q-values for most light particle pick-up
channels. Since CC calculation reproduces the fusion excitation functions for
these systems, probable roles of projectile break-up and particle transfer in
fusion deep below the barrier are not conclusively proven. Further measurements and inclusion of break-up and transfer channels within the framework of
CC formalism would be of interest.
