2. Recent HRIBF Research - Isospin Dependence of Fusion Hindrance
(W. Loveland, Spokesperson RIB-085)
The use of neutron rich projectiles is thought to enhance fusion due to the lowering of the fusion barrier for the neutron rich projectiles. Work done at GSI  in the 1980s, however, suggested a canceling effect in that it was deduced that as the fusing system became more neutron rich (decreasing fissility, x), the fusion hindrance, measured by the "extra push" energy needed to get the nuclei to fuse, increased in contradiction to most theoretical models (see Fig. 2-1). If this were a general trend, the advantage of using radioactive beams in synthesizing new heavy n-rich nuclei would be significantly diminished.
Figure. 2-1. Deduced extra push energies for the 124Sn + 90-96Zr reaction as a function of the fissility of the fused system  and the theoretical predictions for these energies .
Because of the importance of this effect, we directly measured the capture cross-section for the 124Sn + 96Zr reaction. (In , the capture and fusion cross sections had not been directly measured but were deduced from measured evaporation residue cross-sections). The experiment was performed twice with different experimental setups to check the results. In both experiments, coincident fission fragments were detected and distinguished from elastic and inelastic scattered particles by observed angle/energy correlations. In the first experiment, four silicon strip detectors were used while in the second experiment, a larger solid angle, annular segmented strip detector was used to detect the fragments
The data are shown in Fig. 2-2 along with current theoretical estimates of the capture and fusion cross-sections . Our measured data are in agreement with the theoretical predictions . The extra push energy implied by the calculated fusion cross section is very small, in disagreement with the analysis of . (We now believe the analysis used in  was in error because of the assumption that shell effects on level densities are washed out at very low excitation energies).
Figure 2-2. Comparison of our measured capture cross sections for the 124Sn + 96Zr reaction with the theoretical predictions of .
In summary, there appears to be no anomalous fusion hindrance in the 124Sn + 96Zr reaction. We are continuing the studies to characterize the 132Sn + 96Zr reaction.
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3. G. Giardina, et al., (private communication).