Asymptotic and near-target direct breakup ofLi6andLi7

Abstract

Background: 6,7 Li and 9 Be are weakly bound against breakup into their cluster constituents. Breakup location is important for determining the role of breakup in above-barrier complete fusion suppression. Recent works have pointed out that experimental observables can be used to separate near-target and asymptotic breakup. Purpose: Our purpose is to distinguish near-target and asymptotic direct breakup of 6,7 Li in reactions with nuclei in different mass regions. Method: Charged particle coincidence measurements are carried out with pulsed 6,7 Li beams on 58 Ni and 64 Zn targets at sub-barrier energies and compared with previous measurements using 208 Pb and 209 Bi targets. A detector array providing a large angular coverage is used, along with time-of-flight information to give definitive particle identification of the direct breakup fragments. Results: In interactions of 6 Li with 58 Ni and 64 Zn, direct breakup occurs only asymptotically far away from the target. However, in interactions with 208 Pb and 209 Bi, near-target breakup occurs in addition to asymptotic breakup. Direct breakup of 7 Li into α-t is not observed in interactions with 58 Ni and 64 Zn. However, near-target dominated direct breakup was observed in measurements with 208 Pb and 209 Bi. A modified version of the Monte Carlo classical trajectory model code PLATYPUS, which explicitly takes into account lifetimes associated with unbound states, is used to simulate sub-barrier breakup reactions. Conclusions: Near-target breakup in interactions with 6,7 Li is an important mechanism only for the heavy targets 208 Pb and 209 Bi. There is insignificant near-target direct breakup of 6 Li and no direct breakup of 7 Li in reactions with 58 Ni and 64 Zn. Therefore, direct breakup is unlikely to suppress the above-barrier fusion cross section in reactions of 6,7 Li with 58 Ni and 64 Zn nuclei.

Origins of Incomplete Fusion Products and the Suppression of Complete Fusion in Reactions of Li7

“…The idea that breakup capture should peak backward of no-capture breakup is confirmed by classical dynamical model simulations of breakup [24]. The simulation was constrained to reproduce the individual no-capture breakup total cross sections (σ αα , σ αt , σ αd , σ αp ) as well as the measured relative energy distributions of the breakup fragments [20,21,28]. The resulting simulated no-capture breakup double differential cross-section distribution as a function of energy and angle is shown in Fig.…”

“…Some works have suggested that breakup capture fully accounts for the suppression of complete fusion [19,25,26]. However, when the characteristic timescales of breakup are treated explicitly, and model inputs carefully constrained by experimental results for no-capture breakup [22,27,28], the simulations account for only a small fraction of the measured incomplete fusion cross section [29]. There is, therefore, great uncertainty regarding the mechanism producing the majority of the incomplete fusion products, the cause of suppression of complete fusion, and the consequences for weakly bound unstable isotopes.…”

“…Of these, ∼16 mb of breakup occurs via long-lived (≳10 −20 s) resonant states which cannot contribute to breakup capture [21,22,28,29]. For the remaining 20 mb, only a small fraction occurs faster than the fusion process (<10 −21 s) [22,29], and can thus contribute to the suppression of complete fusion.…”

Modelling incomplete fusion dynamics of complex nuclei at Coulomb energies

“…where P BU is the probability of breakup, P L BU (R)dR is the probability of breakup in the interval R to R + dR, and α and β are constants determined from experimental results that vary for different systems [53,54]. This function is sampled to determine the position of breakup in the orbit of the projectile [49].…”

Fusion reaction studies for the Be9+Y89 system at above-barrier energies

“…On the contrary, the coupling effects of breakup and/or transfer channels on the complete fusion (CF) suppress its cross section at energies above the Coulomb barrier and produce some enhancements at sub-barrier energies in comparison to the coupled channels calculations that do not take into account coupling effects [4][5][6][7][8]. Many efforts have been devoted to investigate the systematic of the CF features [9][10][11][12][13][14][15]. In these studies, one conclusion is that the suppression factor of the CF cross section at energies above the Coulomb barrier is independent of the mass of the target, and depends mainly on the projectile breakup threshold [9,10].…”