Importance of lifetime effects in breakup and suppression of complete fusion in reactions of weakly bound nuclei

Complete fusion cross sections in collisions of light, weakly bound nuclei and high Z targets show above-barrier suppression of complete fusion. This has been interpreted as resulting from breakup of the weakly bound nucleus prior to reaching the fusion barrier, reducing the probability of complete fusion. This paper investigates how these conclusions are affected by lifetimes of the resonant states that are populated prior to breakup. If the mean life of a populated resonance is much longer than the fusion timescale, then its breakup cannot suppress complete fusion. For short-lived resonances, the situation is more complex. This work includes the mean life of the short-lived 2+ resonance in 8Be in classical dynamical model calculations to determine its effect on energy and angular correlations of the breakup fragments and on predictions of fusion suppression. Coincidence measurements of breakup fragments produced in reactions of 9Be with 144Sm, 168Er, 186W, 196Pt, 208Pb and 209Bi at energies below the barrier are re-analysed. Predictions of breakup observables and of complete and incomplete fusion at energies above the fusion barrier are made using the classical dynamical simulation code PLATYPUS, modified to include the lifetimes of short-lived resonant states. The agreement of the breakup observables is improved when lifetime effects are included. The predicted suppression of complete fusion due to breakup is nearly independent of Z, with an average value of 9%, below the experimentally determined fusion suppression of 30% in these systems. This more realistic treatment of breakup leads to the conclusion that the suppression of complete fusion cannot be fully explained by breakup prior to reaching the fusion barrier. Other mechanisms that can suppress complete fusion must be investigated. A candidate is cluster transfer that produces the same nuclei as incomplete fusion.This work was supported by ARC Grants No. FL110100098, No. DP130101569, and No. DP140101337

Classical dynamical modelling of near-barrier breakup

The complete fusion of light, weakly-bound nuclides is known to be significantly suppressed with respect to comparable well-bound projectiles or with respect to single barrier penetration model calculations. Strong α-clustering in these light systems mean that they very easily disintegrate into clusters, either via direct excitation of their intrinsic cluster continuum, or via transfer reactions which connect to unbound states in neighbouring nuclides. This breakup is thought to reduce the probability for complete fusion. Here we discuss which processes cause breakup, whether or not breakup happens fast enough, and the interpretation of measurements made at the Australian National University of breakup using classical dynamical models. Understanding the intimate details of breakup, and the resonances through which it proceeds, will be crucial in determining its likely influence on fusion.This work was supported by Australian Research Council Grant Nos. FT120100760 and DP170102423

Resonances in transfer-triggered Breakup of ⁷Li in near-barrier collisions

Above-barrier complete fusion cross sections of weakly-bound 6,7Li and 9Be are known to be suppressed with respect to single-barrier penetration model calculations. Breakup of the projectile - either via direct excitation of continuum states, or by transfer of nucleons - is thought to be the cause, preventing complete capture of the projectile charge. Using the example of 7Li→8Be→ α + α we show how the contributions to breakup from different resonances in 8Be can be identified, and discuss their likely influence on fusionSupport from Australian Research Council grants FL110100098, DP130101569, DE140100784 and DP14101337 is acknowledged. Support for accelerator operations through the NCRIS program is acknowledged

Disintegration locations in ⁷Li-7 -> ⁸Be transfer-triggered breakup at near-barrier energies

Background: At above-barrier energies, complete fusion cross sections in collisions of light weakly bound nuclei with heavy target nuclei are suppressed when compared to well-bound nuclei. Breakup of the projectilelike nucleus was proposed to be the cause. In addition to direct breakup, breakup following transfer was shown to be substantial. Purpose: We investigate breakup in reactions with Li7, triggered by sub-barrier proton pickup to unbound states in Be8, which subsequently separate into two α particles. Method: Measurements of sub-barrier disintegration of Li7 on a Ni58 target were made using the Heavy Ion Accelerator Facility at the Australian National University. Combining the experimental results with classical simulations of post-breakup acceleration, we study the sensitivity of α−α energy and angle correlations to the proximity of disintegration to the target (proton donor) nucleus. Results: The simulations indicate that disintegration as the colliding nuclei approach each other leads to large angular separations θ12 of the α fragments. The detectors allow for a maximum opening angle of θ12=132∘, such that the present experiment is largely insensitive to breakup occurring when the collision partners approach each other. The data are consistent with disintegration of (a) the 0+Be8 ground state far from the targetlike nucleus, and (b) the 2+Be8 resonance near the targetlike nucleus when the Be8 is receding from the targetlike nucleus. Conclusions: The present results shed light on the near-target component of transfer-induced breakup reactions. The distribution of events with respect to the opening angle of the α particles, and the orientation of their relative velocity with respect to the velocity of their center of mass, gives insights into their proximity to the target at the moment of breakup. Further measurements with larger angular coverage and more complete simulations are required to fully understand the influence of breakup on fusion.Support from Australian Research Council Grants No. FL110100098, No. DP130101569, No. DE140100784, and No. DP14101337 is acknowledged. Support for accelerator operations through the NCRIS program is acknowledged

Breakup locations: Intertwining effects of nuclear structure and reaction dynamics

Studies at the Australian National University aim to distinguish breakup of the projectile like-nucleus that occurs when approaching the target from that when receding from the target. Helped by breakup simulations, observables have been found that are sensitive to the breakup location, and thus to the mean-lives of unbound states; sensitivity to even sub-zeptosecond lifetime is found. These results provide insights to understand the reaction dynamics of weakly bound nuclei at near barrier energies.The authors acknowledge operations support for the ANU Heavy Ion Accelerator Facility from NCRIS. Financial support from ARC grants DP130101569, DE140100784, and FL110100098 is acknowledged

Competition between Fusion and Quasi-fission in the Formation of Super-heavy Elements

Quasifission is a non-equilibrium dynamical process resulting in rapid separation of the dinuclear system initially formed after capture and sticking of two colliding heavy nuclei. This can inhibit fusion by many orders of magnitude, thus suppressing the cross section for formation of superheavy elements. Measurements with projectiles from C to Ni, made at the Australian National University Heavy Ion Accelerator Facility, have mapped out quasifission characteristics and systematics using mass-angle distributions (MAD) - the fission mass-split as a function of centre-of-mass angle. These provide information on quasifission dynamics in the least model-dependent way. Quasifission time-scale information in the MAD has been compared with TDHF calculations of the collisions, with good agreement being found. Most significantly, the nuclear structure of the two colliding nuclei has a dramatic effect on quasifission probabilities and characteristics in gentle collisions at near-barrier energies. The effect of static deformation alignment, closed shells and N/Z matching can completely change reaction outcomes. The realization of this strong dependence makes modelling quasifission and superheavy element formation a challenging task, but should ultimately allow more reliable prediction of superheavy element formation cross sections

Evidence for the Role of Proton Shell Closure in Quasifission Reactions from X-Ray Fluorescence of Mass-Identified Fragments

The atomic numbers and the masses of fragments formed in quasifission reactions are simultaneously measured at scission in Ti48+U238 reactions at a laboratory energy of 286 MeV. The atomic numbers are determined from measured characteristic fluorescence x rays, whereas the masses are obtained from the emission angles and times of flight of the two emerging fragments. For the first time, thanks to this full identification of the quasifission fragments on a broad angular range, the important role of the proton shell closure at Z=82 is evidenced by the associated maximum production yield, a maximum predicted by time-dependent Hartree-Fock calculations. This new experimental approach gives now access to precise studies of the time dependence of the N/Z (neutron over proton ratios of the fragments) evolution in quasifission reactions.The authors acknowledge support from the Australian Research Council through Discovery Grants No. FL110100098, No. FT120100760, No. DP130101569, No. DE140100784, No. DP160 101254, and No. DP170102318. Support for accelerator operations through the NCRIS program is acknowledged. Two of us (C. S. and M. A.) acknowledge support from the Scientific Mobility Program of the Embassy of France in Australia. This research was undertaken with the assistance of resources from the National Computational Infrastructure (NCI), which is supported by the Australian Government

Asymptotic and near-target direct breakup of ⁶Li and ⁷Li

Background: Li6,7 and Be9 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 Li6,7 in reactions with nuclei in different mass regions. Method: Charged particle coincidence measurements are carried out with pulsed Li6,7 beams on Ni58 and Zn64 targets at sub-barrier energies and compared with previous measurements using Pb208 and Bi209 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 Li6 with Ni58 and Zn64, direct breakup occurs only asymptotically far away from the target. However, in interactions with Pb208 and Bi209, near-target breakup occurs in addition to asymptotic breakup. Direct breakup of Li7 into α−t is not observed in interactions with Ni58 and Zn64. However, near-target dominated direct breakup was observed in measurements with Pb208 and Bi209. 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 reaction

Interplay of charge clustering and weak binding in reactions of ⁸Li

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 Li8 is therefore puzzling: the lowest breakup threshold yields Li7+n which cannot contribute to fusion suppression because Li7 retains all the projectile charge. In this work, the full characteristics of Li8 breakup in reactions with Bi209 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