arXiv subjects

Y. Aritomo

Publications and source records attributed to Y. Aritomo.

At least 19 recordsLinked to original sources

arXiv2026

Theoretical estimates for the synthesis of $Z=119$ superheavy nuclei with Ca, Ti, V, and Cr projectiles: effects of reaction $Q$ values and mass-model dependence

Fusion reactions with 48Ca beams, which have been used for synthesis of $Z \le 118$ nuclei, face practical limitations for the synthesis of nuclei with $Z \ge 119$ because of the limited availability of suitable target nuclei. We estimate evaporation-residue (ER) cross sections for the reactions 48Ca + 254Es, 50Ti + 249Bk, 51V + 248Cm, and 54Cr + 243Am and examine the role of nuclear-mass-model uncertainties. We employ a hybrid framework for the three stages of the fusion reaction. The capture stage is described by the coupled-channels method, the formation stage by a Langevin approach, and the de-excitation stage by a statistical model. Using the nuclear properties from the FRDM2012 mass model, the maximum values of ER cross section summed over all xn channels are calculated to be 233, 206, 33, and 38 fb for the 48Ca + 254Es, 50Ti + 249Bk, 51V + 248Cm, and 54Cr + 243Am reactions, respectively. The relationship between the reaction Q value and the Coulomb-barrier height is found to be a key factor in comparing reactions leading to the same atomic number. In particular, the relatively small Q value magnitude of the 51V + 248Cm reaction leads to a higher excitation energy and a reduced survival probability, giving the smallest ER cross section among the reactions considered. We also find a significant mass-model dependence on the survival probability. Using the nuclear properties predicted by several mass tables yields differences in the survival probability ranging from about one to several orders of magnitude. This difference mainly originates from the neutron binding energy and shell-correction energy predicted by the nuclear mass models. The ER cross sections for the synthesis of Z = 119 nuclei are governed by both the relative relationship between the reaction Q value and the Coulomb-barrier height and nuclear-mass-model uncertainties that strongly affect the survival probability.

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arXiv2025

Kinetic energy of fission fragments within a dynamical model

Kinetic energy of individual fission fragment for actinide nuclei is, for example, important for evaluating the prompt-neutron spectrum in the laboratory system. It is experimentally known that kinetic energy for each fragment is constant at about 100 MeV for light fragments and that for heavy fragments decreases linearly with mass number. Most of the theoretical studies carried out so far attempted to calculate the total kinetic energy of both fragments, i.e. sum of the energies of two fragments, but the kinetic energy of each fragment was not analyzed in detail as far as we recognize. We have calculated them in thermal-neutron induced fission of $^{239}\mathrm{Pu}$ with a dynamical model using Langevin equations within a three-dimensional two-center parametrization. Also fission of $^{258}\mathrm{Fm}$ was investigated. It is calculated from the Coulomb energy at the scission point and the pre-scission kinetic energy. It is found that the pre-scission kinetic energy has about 2-4% contribution in the kinetic energy. The calculated results reproduce the trend of the experimental data.

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arXiv2025

Neutron emission during fission and its impact on fission-fragment mass distribution studied by Langevin model

Actinide nuclei exhibit mass-asymmetric fission at low energy due to shell structure. The fission-fragment mass distributions produced at high energy tend to have a symmetric shape due to smearing of shell effects. On the other hand, the distribution can be changed by neutron emission occurring before fission, as this decreases the excitation energy of the fissioning nucleus, and thus revives the shell structure. In so called multichance fission, neutron emission is considered prior to fission at the initial nuclear shape, and competition between fission and neutron emission is determined with the framework of the statistical model. In the present work, we describe fission in the Langevin equations, and neutron emission is treated throughout the fission process. The calculation reproduces experimentally observed mass distributions, and for a wide range of initial compound-nucleus excitation energy up to 60 MeV. The results show that, while neutron emission dominates at the ground-state shape, it occurs along the shape evolution path to the scission point.

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arXiv2023

Modes of massive nucleon transfer appearing in quasifission processes for collisions of superheavy nuclei

It is challenging to distinguish between fusion-fission and quasifission experimentally. To determine the characteristics of quasifission processes associated with dominant phenomena in heavy-ion collisions is important for estimating precisely the fusion cross section, which is relevant to the synthesis of new elements. We classified fusion-fission and quasifission processes theoretically in the past for an accurate assessment of the fusion cross section. However, no detailed analysis focused on each process was performed. In this work, we aimed to analyze the dynamical characteristics of quasifission processes in terms of the Langevin equation model. We specify the quasifission processes, and analyze the scission configuration. Finally, we clarify the origin of several modes included in quasifission. The calculation framework is the multidimensional dynamical model of nucleus-nucleus collisions based on the Langevin equations. It is shown that several quasifission modes exist leading to different fragment deformations. The time scale of the quasifission process differs for several different modes. Each scission configuration and total kinetic energy also differ. The different quasifission modes are caused by the neck relaxation controlling the mass drift toward symmetry. This means that it is possible to discuss the time-dependent functional form of the neck parameter $ε$ for the quasifission process in the framework of the dynamical model based on Langevin equations.

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arXiv2023

Modeling of nuclear reactions with Langevin calculations

The mass angle distribution shows a strong correlation between mass and angle when quasifission events are dominant. Therefore, as long as quasifission events are dominant, the mass angle distribution is characterized in that a diagonal correlation appears. This diagonal correlation could not be reproduced in our previous model that is before introducing $f_\text{ina}$ and $γ_\text{t}^{0}$ model parameters. In this study, we clarify the indeterminate parameters included in the model to reproduce the diagonal correlation appearing in the mass angle distribution in the $^{48}$Ti + $^{186}$W reaction system. As a result, $f_\text{ina}$ and $γ_\text{t}^{0}$ of model parameters were found to be key parameters for MAD. it was also found that the balance between $f_\text{ina}$ and $γ_\text{t}^{0}$ parameter values is important for the strong correlation between mass and angle.

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arXiv2023

Dynamical mechanism of fusion hindrance in heavy ion collisions

In the fusion process, the investigation of the reaction dynamics in the time evolution of the nuclear configuration is necessary. The neck parameter $ε$ which is one of the parameters representing the nuclear configuration in the two center shell model is important in fusion owing to the nucleons transferring through the neck. The time evolution of the neck has not been discussed in detail, but is crucial for fusion cross section in the assessment of new elements synthesis. The dynamical analysis for the fusion hindrance under the neck formation on the nuclear deformation space has been done. The fusion probability $P_\text{CN}$ considering the different denecking motion and the fusion hindrance are discussed. The calculations were performed using the dynamical model of nucleus-nucleus collisions based on the multidimensional Langevin equations.The formation of the neck bridge at the approaching stage is found to be crucial to the fusion hindrance. It is clarified that the inner barrier appears owing to the change in the degree of mass asymmetry $α$ with the relaxation of $ε$.The fusion hindrance occurs because the inner barrier is formed by the early neck formation. The role of the neck parameter $ε$ is critically important for the fusion dynamics.

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arXiv2023

Nuclear viscosity estimated by dynamics of neck formation in the early stage of nuclear collision

The very early stage of the coalescence of two nuclei is studied and used to estimate the nuclear viscosity. The time evolution of the neck region has been simulated by the unified Langevin equation method, which is used in the analysis of heavy-ion collisions from the approaching stage to the fusion-fission stage. It is found that the transition from viscous to inertial coalescence that appeared in the neck growth of macroscopic drops can also be seen in the present simulation in nucleus-nucleus collisions. The dynamics of neck growth is analyzed in terms of the hydrodynamical formula and the viscosity coefficient of nuclear matter is estimated using the analogy of macroscopic drops.

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arXiv2023

Fusion cross section and total kinetic energy of fission fragments by the dynamical dissipative surface-friction model

The capture cross section, the fusion cross section, and the quasi-fission yield producing symmetric fragments ($A_{CN}/2\pm20u$) in the $^{48}$Ca+$^{238}$U reaction are analyzed by the multidimensional Langevin equation taking account of the surface friction effect. From the experimental data, the strength of the tangential friction has been determined. It is presented that tangential friction increases in proportional to the power of the relative velocity of the projectile and the target.

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arXiv2021

Fission fragment distributions of neutron-rich nuclei based on Langevin calculations: toward r-process simulations

The nuclear fission of very neuron-rich nuclei related to the r-process is essential for the termination of nucleosynthesis flows on the nuclear chart and the final abundances. Nevertheless, most of the available fission data for the r-process calculations are based on theory predictions, including phenomenological treatments. In this study, we calculated a series of nuclear fission distribution for neutron-rich nuclei away from the beta-stability line. As most of these nuclei are experimentally unknown, we are based on theoretical calculations based on the dynamical fission model with the Langevin method. We performed fission distribution calculations for neutron-rich actinoid nuclei, applicable to the r-process nucleosynthesis simulations. In the present paper, we compared the obtained mass and charge distributions with experimental data. We also show the results of the systematic behaviour of mass distribution for neutron-rich U and Fm isotopes.

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arXiv2014

Fission dynamics at low excitation energy. 2

The mass asymmetry in the fission of U-236 at low excitation energy is clarified by the analysis of the trajectories obtained by solving the Langevin equations for the shape degrees of freedom. It is demonstrated that the position of the peaks in the mass distribution of fission fragments is determined mainly by the saddle point configuration originating from the shell correction energy. The width of the peaks, on the other hand, results from the shape fluctuations close to the scission point caused by the random force in the Langevin equation. We have found out that the fluctuations between elongated and compact shapes are essential for the fission process. According to our results the fission does not occur with continuous stretching in the prolate direction, similarly to that observed in starch syrup, but is accompanied by the fluctuations between elongated and compact shapes. This picture presents a new viewpoint of fission dynamics and the splitting mechanism.

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arXiv2014

The Scission-Point Configuration within the Two-Center Shell Model Shape Parameterization

Within the two-center shell model parameterization we have defined the optimal shape which fissioning nuclei attain just before the scission and calculated the total deformation energy (liquid drop part plus the shell correction) as function of the mass asymmetry and elongation at the scission point. The three minima corresponding to mass symmetric and two mass asymmetric peaks in the mass distribution of fission fragments are found in the deformation energy at the scission point. The calculated deformation energy is used in quasi-static approximation for the estimation of the total kinetic and excitation energy of fission fragments and the total number of emitted prompt neutrons. The calculated results reproduce rather well the experimental data on the position of the peaks in the mass distribution of fission fragments, the total kinetic and excitation energy of fission fragments. The calculated value of neutron multiplicity is somewhat larger than experimental results.

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arXiv2013

Fission dynamics at low excitation energy

The origin of mass asymmetry in the fission of uranium at a low excitation energy is clarified by a trajectory analysis of the Langevin equation. The positions of the peaks in the mass distribution of fission fragments are mainly determined by fission saddle points originating from the shell correction energy. The widths of the peaks, on the other hand, result from a shape fluctuation around the scission point caused by the random force in the Langevin equation. We found that a random vibration in the oblate direction of fissioning fragments is essential for the fission process. According to this picture, fission does not occur with continuous stretching in the prolate direction, similarly to that observed in starch syrup. This is expected to lead to a new viewpoint of fission dynamics and the splitting mechanism.

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arXiv2013

Fission process of low excited nuclei with Langevin approach

Fragment mass distributions from the fission of U and Pu isotopes at low excitation energies are studied using a dynamical model based on the fluctuation-dissipation theorem formulated as Langevin equations. The present calculations reproduced the overall trend of the asymmetric mass distribution without parameter adjustment for the first time using the Langevin approach. The Langevin trajectories show a complicated time evolution on the potential surface, which causes the time delay of fission, showing that dynamical treatment is vital. It was found that the shell effect of the potential energy landscape has a dominant role in determining the mass distribution, although it is rather insensitive to the strength of dissipation. Nevertheless, it is essential to include the effect of dissipation, since it has a crucial role in giving "fluctuation" to Langevin trajectories as well as for explaining the multiplicities of pre-scission neutrons as the excitation energy increases. Therefore, the present approach can serve as a basis for more refined analysis.

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arXiv2012

Dynamical approach to heavy-ion induced fission using actinide target nuclei at energies around the Coulomb barrier

In order to describe heavy-ion fusion reactions around the Coulomb barrier with an actinide target nucleus, we propose a model which combines the coupled-channels approach and a fluctuation-dissipation model for dynamical calculations. This model takes into account couplings to the collective states of the interacting nuclei in the penetration of the Coulomb barrier and the subsequent dynamical evolution of a nuclear shape from the contact configuration. In the fluctuation-dissipation model with a Langevin equation, the effect of nuclear orientation at the initial impact on the prolately deformed target nucleus is considered. Fusion-fission, quasi-fission and deep quasi-fission are separated as different Langevin trajectories on the potential energy surface. Using this model, we analyze the experimental data for the mass distribution of fission fragments (MDFF) in the reactions of $^{34,36}$S+$^{238}$U and $^{30}$Si+$^{238}$U at several incident energies around the Coulomb barrier. We find that the time scale in the quasi-fission as well as the deformation of fission fragments at the scission point are different between the $^{30}$Si+$^{238}$U and $^{36}$S+$^{238}$U systems, causing different mass asymmetries of the quasi-fission.

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arXiv2010

A dynamical model of surrogate reactions

A new dynamical model is developed to describe the whole process of surrogate reactions; transfer of several nucleons at an initial stage, thermal equilibration of residues leading to washing out of shell effects and decay of populated compound nuclei are treated in a unified framework. Multi-dimensional Langevin equations are employed to describe time-evolution of collective coordinates with a time-dependent potential energy surface corresponding to different stages of surrogate reactions. The new model is capable of calculating spin distributions of the compound nuclei, one of the most important quantity in the surrogate technique. Furthermore, various observables of surrogate reactions can be calculated, e.g., energy and angular distribution of ejectile, and mass distributions of fission fragments. These features are important to assess validity of the proposed model itself, to understand mechanisms of the surrogate reactions and to determine unknown parameters of the model. It is found that spin distributions of compound nuclei produced in $^{18}$O+$^{238}$U $\rightarrow ^{16}$O+$^{240*}$U and $^{18}$O+$^{236}$U $\rightarrow ^{16}$O+$^{238*}$U reactions are equivalent and much less than 10$\hbar$, therefore satisfy conditions proposed by Chiba and Iwamoto (PRC 81, 044604(2010)) if they are used as a pair in the surrogate ratio method.

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arXiv2006

Pre-scission neutron multiplicity associated with the dynamical process in superheavy mass region

The fusion-fission process accompanied by neutron emission is studied in the superheavy-mass region on the basis of the fluctuation-dissipation model combined with a statistical model. The calculation of the trajectory or the shape evolution in the deformation space of the nucleus with neutron emission is performed. Each process (quasi-fission, fusion-fission, and deep quasi-fission processes) has a characteristic travelling time from the point of contact of colliding nuclei to the scission point. These dynamical aspects of the whole process are discussed in terms of the pre-scission neutron multiplicity, which depends on the time spent on each process. We have presented the details of the characteristics of our model calculation in the reactions $^{48}$Ca+$^{208}$Pb and $^{48}$Ca+$^{244}$Pu, and shown how the structure of the distribution of pre-scission neutron multiplicity depends on the incident energy.

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arXiv2006

Fusion hindrance and roles of shell effects in superheavy mass region

We present the first attempt of systematically investigating the effects of shell correction energy for a dynamical process, which includes fusion, fusion-fission and quasi-fission processes. In the superheavy mass region, for the fusion process, shell correction energy plays a very important role and enhances the fusion probability when the colliding partner has a strong shell structure. By analyzing the trajectory in three-dimensional coordinate space with the Langevin equation, we reveal the mechanism of the enhancement of the fusion probability caused by `cold fusion valleys'. The temperature dependence of shell correction energy is considered.

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arXiv2006

Possibility of synthesizing doubly closed superheavy nucleus

The possibility of synthesizing a doubly magic superheavy nucleus, $^{298}114_{184}$, is investigated on the basis of fluctuation-dissipation dynamics. In order to synthesize this nucleus, we must generate more neutron-rich compound nuclei because of the neutron emissions from excited compound nuclei. The compound nucleus $^{304}114$ has two advantages to achieving a high survival probability. First, because of small neutron separation energy and rapid cooling, the shell correction energy recovers quickly. Secondly, owing to neutron emissions, the neutron number of the nucleus approaches that of the double closed shell and the nucleus obtains a large fission barrier. Because of these two effects, the survival probability of $^{304}114$ does not decrease until the excitation energy $E^{*}= 50$ MeV. These properties lead to a rather high evaporation reside cross section.

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