Interplay of charge clustering and weak binding in reactions of Li8

Abstract

In collisions of light, stable, weakly bound nuclides, complete fusion (capture of all of the projectile charge) has been found to be suppressed by ∼30% at above-barrier energies. This is thought to be related to their low thresholds for breakup into charged clusters. The observation of fusion suppression in the neutron-rich radioactive nucleus 8 Li is therefore puzzling: the lowest breakup threshold yields 7 Li + n which cannot contribute to fusion suppression because 7 Li retains all the projectile charge. In this work, the full characteristics of 8 Li breakup in reactions with 209 Bi are presented, including, for the first time, coincidence measurements of breakup into charged clusters. Correlations of cluster fragments show that most breakup occurs too slowly to significantly suppress fusion. However, a large cross section for unaccompanied α particles was found, suggesting that charge clustering, facilitating partial charge capture, rather than weak binding is the crucial factor in fusion suppression, which may therefore persist in exotic nuclides.

Theoretical analysis of 8Li + 208Pb reaction and the critical angular momentum for complete fusion

“…[41] for 6,7 Li projectiles, providing numerical support to results reported in Ref. [26]. The artificial variation of the ground-state binding energy in that reference, revealed another interesting aspect: in the case of 7 Li projectile, for binding energies weaker than the experimental values, the complete fusion cross section displays an insignificant dependence on this energy variation, while the opposite was observed when considering the 6 Li projectile (see Fig.…”

“…Nevertheless, one should point out that if the projectile breaks up when is moving apart of the target, this so-called delayed breakup will affect the reaction cross section, but not the fusion cross section [1]. Complete fusion is regarded as the absorption of the whole charge of the projectile, and not necessarily its whole mass, while incomplete fusion refers to the absorption of only a part of the projectile charge [1,8,17,26]. On the other hand, complete fusion is also interpreted as the absorption from bound states, with the incomplete fusion as the absorption from breakup states [2].…”

“…For example, while it is widely believed that the suppression of complete fusion has a stronger dependence on the projectile breakup threshold (see, for instance, Refs. [9,40] and references therein), recent experimental results in 7,8 Li + 209 Bi reactions have suggested charge clustering rather than weak projectile binding (i.e., breakup prior to reaching the fusion barrier) as the crucial factor in complete fusion suppression [26,27]. It is further asserted in these references that weak projectile binding energy leads to incomplete fusion enhancement, while strong charge clustering leads to complete fusion suppression.…”

“…Therefore, to some extend, these results clarify the fact that weak binding energy alone is not a sufficient condition to warrant the suppression of the complete fusion cross section, in accordance to Refs. [26,27]. Since the complete fusion process can also be regarded as the absorption of the whole projectile charge, one would as well expect this process to exhibit an insignificant dependence on variations of the ground-state biding energy in the case of 8 Li ( 7 Li+n), regardless the target mass.…”

“…Such study would serve not only as a further numerical proof of the conclusions drawn in Refs. [26,27], but to some extend can be useful to establish a connection between different definitions for complete and incomplete fusion processes.…”