l-distributions in fusion of 64Ni + 92Zr and 12C + 144Sm measured with the Darmstadt-Heidelberg crystal ball

The authors have previously observed that in fusion of 64Ni and 92Zr there is a suppression of neutron emission, whereas in fusion of 12C and 144Sm into the same compound nucleus the observed neutron multiplicity distribution is close to that expected. Possible explanations for the suppression in terms of anomalously energetic neutrons, unusually large gamma decay widths or an unknown yrast line have been ruled out. It has been suggested that the explanation may lie in a tail to very high l-values arising from, for instance, zero point fluctuations in the near-barrier 64Ni-induced reaction. Therefore, it is germane to directly measure the l-distributions, which in addition, are of crucial interest in understanding the enhancement of sub-barrier fusion cross-sections. It is found that with 225 MeV 64Ni, which is near the barrier, there is significantly larger yield at higher l-values than predicted by an empirical model due to Kailas and Gupta. This observation is consistent with an enhancement of sub-barrier fusion. In contrast, at 239 MeV, l/sub max/ approx. 37 is close to the Kailas-Gupta value. Furthermore, no tail to unexpectedly high-l values is observed, ruling out this possibility as an explanation for the neutron suppression. The l-distributions for the 12C-induced reactions are rather similar to the expected empirical ones