Competition between binary reactions and fusion in heavy-ion collisions at the Coulomb barrier

Abstract

Mass and charge distributions for binary reaction channels have been measured for the reactions 86Kr with 76Ge, I~ and 13~ at the Coulomb barrier using chemical separations and 7-ray spectroscopy. These systems span the region where dynamical hindrance to complete fusion sets in. The binary reactions can be subdivided into two components associated with i) reflection from the outer potential barrier (quasielastic), and ii) reseparation after passing the barrier (complex reactions). The sum of complex-reaction channels and evaporation residues from complete fusion can be reproduced by a barrier passing calculation. The fraction of the barrier passing flux leading to reseparation increases from 26 +_ 10% for the lightest system to more than 90% for the heaviest system. The data indicate that fusion hindrance is primarily caused by reseparation shortly after passage of the barrier before Swiatecki's conditional saddlepoint is overcome, resulting in partitions close to the entrance channel configuration. In addition, for the heaviest system, a quasifission component representing somewhat less than 20% of the barrier-passing flux was observed. From the missing masses of fragment pairs we can deduce that the reseparating complex-reaction products have kinetic energies well below the fusion barrier and share the excitation energy in a way similar to the sawtooth-like curve known from low-energy fission. The quasielastic, predominantly one-and two-nucleon transfer channels, have strongly varying cross sections for the three systems despite similar effective Q-values. A systematics of one-neutron transfer cross sections at the Coulomb barrier is established and shown to differ considerably from the smooth behaviour observed at energies 20-30% above the barri- er. The connection to nuclear polarization phenomena and orbit matching is pointed out.

Formation of the Compound Nucleus in Fusion of Heavy Nuclei

“…This causes the fusion hindrance for 110 Pd+ 110 Pd which is consistent with the experimental conclusions drawn in Ref. [1]. The final population probabilities in the fusion region (λ 1.3 in Fig.…”

“…The competition between fusion and quasifission (reseparation before CN formation) can inhibit fusion by many orders of magnitude, e.g. [1]. Understanding this inhibition may be the key to forming more SHE.…”

“…For computational reasons the calculations are done using the coordinates λ and ξ that are the relevant ones in the current models for CN formation. The non-linear set of equations (1)(2)(3)(4)(5) are solved by sucessive iterations using a small time step ∆t = 10 −23 s. The master equation ( 1) is solved on a grid (1 λ 1.8,|ξ| 0.7 where ∆λ = 0.02 and ∆ξ = 0.1) with appropriate boundary conditions to follow the continuous split of the initial population probabilities p(q, 0) into fusion and quasifission. Values of |ξ| > 0.7 are not included in the calculation because the TCSM used [22] is not appropriate for large ξ.…”

αdecay ofAu176

“…1(b [24] predict the cross-section ratio of σ (5n)/σ (α,5n) > 20-30 at these beam energies. Therefore, essentially all 172 Ir present in our data was produced by α decay of the parent 176 Au.…”

Long-termα- and spontaneous fission measurement of aRf/Dressler1, Eichler2, Schumann3 et al. 2009Phys. Rev. C

“…A second model proposed by [26] was evaluated. In a first step the isotopic cross sections and in a second step the quadratic dependence of the obtained fit parameters on the atomic number were fitted.…”