Interplay between Quantum Shells and Orientation in Quasifission

Abstract

The quasi-fission mechanism hinders fusion in heavy systems through breakup within zeptoseconds into two fragments with partial mass equilibration. Its dependence on the structure of both the collision partners and the final fragments is a key question. Our original approach is to combine an experimental measurement of the fragments' mass-angle correlations in 40 Ca+ 238 U with microscopic quantum calculations. We demonstrate an unexpected interplay between the orientation of the prolate deformed 238 U with quantum shell effects in the fragments. In particular, calculations show that only collisions with the tip of 238 U produce quasi-fission fragments in the magic Z = 82 region, whilst collisions with the side are the only one which may result in fusion.In the late 70's, Heusch and collaborators measured fission characteristics in heavy-ion collisions which could not be reconciled with the statistical decay of a compound nucleus [1]. Later, the angular anisotropy of the fission fragments was found to be much larger than that predicted by the statistical model in some reactions [2,3], which was taken as a clear signature for an out-ofequilibrium process.The origin of these characteristics is understood to be a process known as quasi-fission. Here the dinuclear system fissions before reaching the stage of an equilibrated compound nucleus [3]. Quasi-fission thus results in fusion hindrance in reactions forming heavy nuclei [4][5][6]. In fact, this is by far the dominant mechanism suppressing the formation of super-heavy elements. The understanding of this process is thus crucial in order to optimise the formation of new heavy and superheavy nuclei.Since the discovery of quasi-fission, important progress has been made thanks to extensive experimental studies [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]. Correlations between the mass and the angles of the fragments show that quasi-fission often takes place before a full rotation of the di-nuclear system, that is, with typical contact times between the fragments of 5 to 10 zs [7,8,17]. The characteristics of the entrance channel -in particular, the deformation [10,11,[13][14][15] and shell structure [21] of the collision partners as well as the fissility of the system [19,22] and its energy [15,20] were shown to play an important role. Shell effects could also favor the production of fragments in the vicinity of magic nuclei [12,15,16,[24][25][26].The complex interplay of all these variables that have been identified by experiments dictates quasi-fission characteristics and probability, and hence the suppression of fusion. To understand the dynamics at play, in particular the inter-dependency of these variables, it is necessary to perform theoretical calculations. Classical dynamical models have been developed where the system is described as a viscous fluid evolving through a family of parametrised shapes [27][28][29][30][31][32]. Despite their ability to reproduce some experimental observables, these approaches require parameters, such as t...

Shape evolution and collective dynamics of quasifission in the time-dependent Hartree-Fock approach

“…Although this approach has been widely used, it could have limitations due to the fact that quasifission is not a statistical decay but a dynamical process, and that fusion-fission could be asymmetric due to, e.g., late-chance fusion-fission at low excitation energies [28] as well as shell structure of pre-scission configurations [29]. Correlations between mass and scattering angle of the fragments have also been measured extensively [25,[30][31][32][33]. In particular, they can be used to disentangle fast quasi-fission processes (few zeptoseconds) to longer reaction mechanisms associated with contact times between the fragments exceeding 10 − 20 zs (i.e., long-time quasifission and fusion-fission).…”

“…The Hartree-Fock approximation and its time-dependent generalization, the time-dependent Hartree-Fock theory, have provided a possible means to study the diverse phenomena observed in low energy nuclear physics [41,42]. In general modern TDHF calculations provide a useful foundation for a fully microscopic many-body description of large amplitude collective motion including collective surface vibrations and giant resonances [48][49][50][51][52][53][54][55][56][57][58][59][60][61] nuclear reactions in the vicinity of the Coulomb barrier, such as fusion [36,39,[62][63][64][65][66][67][68][69], deepinelastic reactions and transfer [70][71][72][73][74][75][76], and dynamics of (quasi)fission fragments [33,[43][44][45][46][47].…”

“…In particular, the time-dependent Hartree-Fock theory [40], has been recognized for its realistic description of several low-energy nuclear reaction mechanisms [41,42]. It has been recently utilized for studying the dynamics of quasifission [25,33,43,44] and scission [45][46][47]. The study of quasifission is showing a great promise to provide insight based on very favorable comparisons with experimental data [25,33].…”

“…It has been recently utilized for studying the dynamics of quasifission [25,33,43,44] and scission [45][46][47]. The study of quasifission is showing a great promise to provide insight based on very favorable comparisons with experimental data [25,33]. As a dynamical microscopic theory, TDHF provides us with the complete shape evolution of the nuclear densities which can be used to compute the time evolution of deformation and inertia parameters.…”

“…These are indeed the relevant quantities for fragment mass-angle distributions and angular distribution analyses. These quantities are computed for collisions between calcium isotopes and actinides for which the TDHF approach has been shown to provide a deep insight into the quasifission reaction mechanisms [33,43,44].…”