Abstract
Experimental results on the onset of incomplete fusion at slightly above barrier energies are dis- cussed in this paper. Spin-distributions of evaporation residues populated via complete and/or incomplete fusion of 12 C, 16 O (Elab�4-7 MeV) with 169 Tm have been measured to probe associated` -values. Particle (Z=1,2) - - coincidence technique has been used for channel selection. Entirely different entry state spin populations have been observed during the de-excitation of complete and incomplete composites. The complete fusion residues are found to be strongly fed over a broad spin range. While, a narrow range feeding for only high spin states has been observed in case of incomplete fusion residues. In the present work, incomplete fusion is shown to be a promising tool to populate high spin states in final reaction products. For better insight into the onset and strength of incomplete fusion, the relative contributions of complete and incomplete fusion have been deduced from the analysis of excitation functions and forward recoil ranges. A significant fraction of ICF has been observed even at energy as low as�7% above the barrier. The relative strengths of complete and incomplete fusion deduced from the analysis of forward-recoil-ranges and excitation functions complement each other. All the available results are discussed in light of the Morgenstern's mass-asymmetry systematics. Incomplete fusion fraction is found to be large for more mass-asymmetric systems for individual projectiles, which points towards the projectile struc- ture effect on incomplete fusion fraction. Experimentally measured forward ranges of recoils complement the existence of incomplete fusion at slightly above barrier energies, where more than one linear-momentum-transfer components associated with full- and/or partial-fusion of projectile(s) have been observed. Present results con- clusively demonstrate the possibility to selectively populate high spin states via incomplete fusion.
Highlights
The spin-distribution(s) associated with ICF are found to be arrised from the narrow spin population, localized near and/or above to the critical angular momentum for component: fusion-evaporation (CF)
The trend of spindistributions indicates the competition from successively opened ICF channels for each value of l above lcrit for normal fusion (CF)
At slightly above barrier energies, the ICF is found to be originated from non-central interactions (l ≥ lcrit) due
Summary
The ICF reaction products achieve relatively low E* (due to the involvement of partial degrees of excitations) and high angular-momenta (relatively higher values of impact parameters contribute to the high spin states) at a given projectile energy. In this case, the number of ‘yrast’-like transitions are much higher than that of statistical ones, where less or no feeding is expected. As the residual nucleus de-excites, the feeding intensity decreases gradually with available excitation energy and/or angular momenta This clearly shows the absence of feeding to the lowest members of the ‘yrast’ band in case of ICF, or the low spin states are less populated in ICF-channels. Detailed discussion on FIPs of different CF/ICF- channels can be found in our recent papers [1,2,3]
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