Fission and quasifission modes in heavy-ion-induced reactions leading to the formation of Hs *

Abstract. Production of the heavy and heaviest nuclei (from Po to the region of superheavy elements close to Z=114 and N=184) in fusion-evaporation reactions induced by heavy ions has been considered in a systematic way within the framework of the barrier-passing model coupled with the statistical model (SM) of de-excitation of a compound nucleus (CN). Excitation functions for fission and evaporation residues (ER) measured in very asymmetric combinations can be described rather well. One can scale and fix macroscopic (liquid-drop) fission barriers for nuclei involved in the calculation of survivability with SM. In less asymmetric combinations, effects of fusion suppression caused by quasi-fission (QF) are starting to appear in the entrance channel of reactions. QF effects could be semi-empirically taken into account using fusion probabilities deduced as the ratio of measured ER cross sections to the ones obtained in the assumption of absence of the fusion suppression in corresponding reactions. SM parameters (fission barriers) obtained at the analysis of a very asymmetric combination leading to the production of (nearly) the same CN should be used for this evaluation.

Section: Cnr*11supporting

confidence: 72%

“…As one can see in Figure 8 these values do not contradict to the ones derived from fission data. Similar fits were used to estimate the asymptotic values of P fus for the 48 Ca + 238 U and 36 S+ 238 U data obtained in the fission experiments [7,60,63]. The value extracted from the data [7] is included into the systematics shown in Figure 8.…”

Section: Cnr*11mentioning

confidence: 99%

Fusion probability and survivability in estimates of heaviest nuclei production

Sagaidak

2012

“…To define the smooth trends in quasifission, a large number of MAD measurements have been selected, for beam energies somewhat above the capture barrier (typically by ∼ 6%). Here the known effects of deformation alignment [10,11,17,19] and shell structure observed in measurements at below-barrier energies [14,20] are much reduced [11,21,22]. However the beam energies should not be too far above the capture barriers, otherwise high angular momenta would be introduced in the collisions, which would then not be representative of heavy element formation reactions.…”

Section: Introductionmentioning

confidence: 99%

Systematic study of quasifission characteristics and timescales in heavy element formation reactions

Hinde

Williams

Mohanto

et al. 2016

Superheavy elements can only be created in the laboratory by the fusion of two massive nuclei. Mass-angle distributions give the most direct information on the characteristics and time scales of quasifission, the major competitor to fusion in these reactions. The systematics of 42 mass-angle distributions provide information on the global characteristics of quasifission. Deviations from the systematics reveal the major role played by the nuclear structure of the two colliding nuclei in determining the reaction outcome, and in hindering or favouring heavy element production.To form very heavy and superheavy elements (SHE), heavy-ion fusion reactions are used. Their cross sections can be significantly suppressed [1] by quasifission [2]. This dynamical non-equilibrium process results when the combined system formed after capture separates into two (fission-like) fragments in times ∼10 −20 s, before a compact compound nucleus is formed. The probability of quasifission (P QF ) can be very large, with the corresponding probability of compound nucleus formation (P CN = 1 -P QF ) being lower than 10 −3 in unfavourable cases. Understanding the competition between quasifission and fusion is thus very important in predicting the best fusion reactions to use to form new isotopes of heavy and super-heavy elements.A key quantity characterizing quasifission is its "sticking time" between capture of the two nuclei inside the entrance channel potential barrier [4] and breakup (scission). Quasifission mass-angle distributions (MAD) first measured at GSI in the 1980s [2,5] showed that quasifission timescales could often be shorter than the rotation time of ∼10 −20 s. However, subsequently only a few measurements [6,7] were made until recent years, when an extensive series of experiments (using the Australian National University Heavy Ion Accelerator Facility and CUBE spectrometer) were carried out [3,[8][9][10][11][12][13][14][15][16]. The kinematic coincidence technique used in the measurements [2,3,17] provides direct information on the mass-ratio of the fragments at scission; thus, the data are represented in terms of mass ratio M R , rather than pre-or According to the characteristics of the MAD (minimum mass yield at symmetry, mass-angle correlation with peak yield at symmetry, and no significant mass-angle correlation), they are assigned as type MAD1, MAD2 and MAD3 respectively [3]. There is a clear correlation between the MAD class and the entrance channel charge product. Other entrance channel characteristics are important in determining the sticking times and MAD characteristics, including neutron richness [14,18], and shell structure including static deformation [11] and magic numbers [14].To improve our quantitative understanding of the role of shell structure in the dynamics of quasifission, we make an analogy with the liquid drop model approach to nuclear masses, in which localized shell effects can be quantified when the underlying smooth (liquid drop) trends are well defined. To define the smooth trends in ...

“…Usual experimental observables to investigate the quasifission process include, in addition to the fragment mass distribution, the kinetic energy of the fragments [13] and the scattering angle [6,51]. Figure 10 shows the evolution of the scattering angle in 40 Ca+ 238 U at E c.m.…”

Section: Quasi-fission Within the Tdhf Approachmentioning

confidence: 99%

“…Note that lighter systems may also exhibit quasi-fission, although with a smaller probability. For instance, quasi-fission has been observed in 16 O, 32 S+ 238 U [12][13][14], and in 32 S+ 208 Pb [14].…”

Section: Introductionmentioning

confidence: 99%

Effects of nuclear structure on quasi-fission

Simenel

Wakhle

Avez

et al. 2012

Abstract. The quasi-fission mechanism hinders fusion of heavy systems because of a mass flow between the reactants, leading to a re-separation of more symmetric fragments in the exit channel. A good understanding of the competition between fusion and quasi-fission mechanisms is expected to be of great help to optimize the formation and study of heavy and superheavy nuclei. Quantum microscopic models, such as the time-dependent Hartree-Fock approach, allow for a treatment of all degrees of freedom associated to the dynamics of each nucleon. This provides a description of the complex reaction mechanisms, such as quasi-fission, with no parameter adjusted on reaction mechanisms. In particular, the role of the deformation and orientation of a heavy target, as well as the entrance channel magicity and isospin are investigated with theoretical and experimental approaches.