arXiv subjects

Yasuhisa Abe

Publications and source records attributed to Yasuhisa Abe.

11 recordsLinked to original sources

arXiv2020

A Dynamical Study of Fusion Hindrance with Nakajima-Zwanzig Projection Method

A new framework is proposed for the study of collisions between very heavy ions which lead to the synthesis of Super-Heavy Elements (SHE), to address the fusion hindrance phenomenon. The dynamics of the reaction is studied in terms of collective degrees of freedom undergoing relaxation processes with different time scales. The Nakajima-Zwanzig projection operator method is employed to eliminate fast variable and derive a dynamical equation for the reduced system with only slow variables. There, the time evolution operator is renormalised and an inhomogeneous term appears, which represents a propagation of the given initial distribution. The term results in a slip to the initial values of the slow variables. We expect that gives a dynamical origin of parameter "injection point $s$" introduced by Swiatecki et al in order to reproduce absolute values of measured cross sections for SHE. Formula for the slip is given in terms of physical parameters of the system, which confirms the results recently obtained with a Langevin equation.

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arXiv2012

Resonances in 28Si+28Si. I

A molecular model developed for resonances observed in medium light heavy-ion collisions is described. At high spins in 28Si+28Si (oblate-oblate system), a stable dinuclear configuration is found to be equator-equator touching one. The normal modes around the equilibrium are investigated. These modes are expected to be the origin of a large number of resonances observed. Furthermore, due to the axially asymmetric shape of the stable configuration of 28Si+28Si, the system rotates preferentially around the axis with the largest moment of inertia, which gives rise to wobbling motion ($K$-mixing). Energy spectra for the normal modes and for the extended model including the wobbling motion are given.

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arXiv2012

Resonances in 28Si+28Si. II

Resonances observed in the 28Si+28Si collision are studied by the molecular model. In the preceding paper, it is clarified that at high spins in 28Si+28Si (oblate-oblate system), the stable dinuclear configuration of the system is equator-equator touching one, and that the axially asymmetric shape of the stable configuration of 28Si+28Si gives rise to a wobbling motion ($K$-mixing). There, the normal modes around the equilibrium have been solved and various excited states have been obtained. Those states are expected to be the origin of a large number of resonances observed. Hence their physical quantities are analyzed theoretically. The results are compared with the recent experiment performed in Strasbourg and turn out to be in good agreement with the data. Disalignments between the orbital angular momentum and the spins of the constituent 28Si nuclei in the resonance state are clarified. Moreover the analyses of the angular correlations indicate characteristic features for each normal-mode excitation. Thus it is possible to identify the modes, and a systematic experimental study of angular correlation measurements is desired.

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arXiv2011

Fusion Hindrance in the Heavy Ion Reactions -- Border Between the Normal and Hindered Fusions

The fusion hindrance in heavy ion collisions is studied in the framework of the two-center liquid drop model. It appears that the neck and the radial degrees of freedom might both be hampered by an inner potential barrier on their path between the contact configuration to the compound nucleus. Heavy ion reactions with and without the two kinds of fusion hindrance are classified through systematic calculations. It is found that the number of reactions without radial fusion hindrance is much smaller than that without neck fusion hindrance, and for both kinds of fusion hindrance the number of reactions without fusion hindrance at small mass-asymmetry parameter $α$ is smaller than that at large $α$. In the formation of a given compound nucleus, if a reaction with $α_c$ is not hindered, then other reactions with $α> α_c$ are also not hindered as it is well known experimentally.

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arXiv2009

Analysis of the Fusion Hindrance in Mass-symmetric Heavy Ion Reactions

The fusion hindrance, which is also denominated by the term extra-push, is studied on mass-symmetric systems by the use of the liquid drop model with the two-center parameterization. Following the idea that the fusion hindrance exists only if the liquid drop barrier (saddle point) is located at the inner side of the contact point after overcoming the outer Coulomb barrier, the reactions in which two barriers are overlapped with each other are determined. It is shown that there are many systems where the fusion hindrance does not exist for the atomic number of projectile or target nucleus $Z\leq43$, while for $Z>43$, all of the mass-symmetric reactions are fusion-hindered.

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arXiv2008

Isospin dependence of reactions $^{48}$Ca+$^{243-251}$Bk

The fusion process of $^{48}$Ca induced reactions is studied with the two-step model. In this model, the fusion process is devided into two stages: first, the sticking stage where projectile and target come to the touching point over the Coulomb barrier from infinite distance, and second, the formation stage where the di-nucleus formed with projectile and target evolve to form the spherical compound nucleus from the touching point. By the use of the statistical evaporation model, the residue cross sections for different neutron evaporation channels are analyzed. From the results, optimum reactions are given to synthesize $Z$ = 117 element with $^{48}$Ca induced reactions.

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arXiv2003

On Theoretical Problems in Synthesis of Superheavy Elements

Towards precise predictions of residue cross sections of the superheavy elements, recent theoretical developments of reaction mechanisms are presented, together with the remaining problems which give rise to ambiguities in absolute values of cross sections predicted.

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arXiv2003

KEWPIE: a dynamical cascade code for decaying exited compound nuclei

A new dynamical cascade code for decaying hot nuclei is proposed and specially adapted to the synthesis of super-heavy nuclei. For such a case, the interesting channel is the tiny fraction that will decay through particles emission, thus the code avoids classical Monte-Carlo methods and proposes a new numerical scheme. The time dependence is explicitely taken into account in order to cope with the fact that fission decay rate might not be constant. The code allows to evaluate both statistical and dynamical observables. Results are successfully compared to experimental data.

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arXiv2003

Theory of fusion hindrance and synthesis of the superheavy elements

The two-step model for fusion reactions of massive systems is briefly reviewed.By the use of fusion probabilities obtained by the model and of survival probabilities obtained by the new statistical code, we predict residue cross sections for 48Ca+actinide systems leading to superheavy elements with Z=114, 116 and 118.

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arXiv2003

Predictions of the residue cross-sections for the elements Z=113 and Z=114

An extremely good reproduction of experimental excitation function of the 1n reactions producing Z=110,Z=111 and Z=112 is obtained by the two-step model and the statistical decay code KEWPIE. Thus, an extension of the recipe permits us to predict reliable values of the residue cross-sections of the elements Z=113 and Z=114 which will be a useful guide for planning of experiments.

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arXiv2002

Two-Step Model of Fusion for Synthesis of Superheavy Elements

A new model is proposed for fusion mechanisms of massive nuclear systems where so-called fusion hindrance exists. The model describes two-body collision processes in an approaching phase and shape evolutions of an amalgamated system into the compound nucleus formation. It is applied to $^{48}$Ca-induced reactions and is found to reproduce the experimental fusion cross sections extremely well, without any free parameter. Combined with the statistical decay theory, residue cross sections for the superheavy elements can be readily calculated. Examples are given.

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