arXiv subjects

M. Warda

Publications and source records attributed to M. Warda.

At least 19 recordsLinked to original sources

arXiv2026

Survival of Pairing Correlations and Shell Effects at Scission in Finite-Temperature Nuclear Fission: Implications for Odd-Even Staggering

We investigate the finite-temperature evolution of microscopic free-energy corrections in nuclear fission, focusing on pairing and shell effects near scission. The analysis is based on a finite-temperature BCS treatment combined with the Strutinsky method and is performed for representative deformation points along the fission path. Both pairing and shell contributions exhibit regular thermal attenuation, but their deformation dependencies differ substantially. In particular, pairing remains strongly deformation-dependent in the scission region, and its free-energy contribution differs markedly between the constant and surface-dependent pairing-strength prescriptions. The shell correction near scission is also significant at low temperature and is progressively suppressed with increasing excitation energy. These results support the interpretation of odd-even staggering in fragment charge yields as a manifestation of pairing correlations surviving into the strongly deformed pre-scission configuration. They also show that pairing and shell effects should be treated separately in finite-temperature dynamical calculations, with distinct deformation- and temperature-dependent attenuation laws.

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arXiv2026

Microscopic description of cluster radioactivity fission valleys along isotopic and isotonic chains

Cluster radioactivity has been successfully described as a super-asymmetric fission mode within the microscopic self-consistent Gogny Hartree-Fock-Bogoliubov approximation [Phys. Rev. C 84, 044608 (2011)]. For nuclei preserving the neutron-to-proton $N/Z$ ratio of the doubly magic $^{208}$Pb, a cluster radioactivity fission valley has been identified. Such a valley can also be found both in actinides and super-heavy nuclei. In this paper, chains of isotopes and isotones are examined to determine the limits of existence of the cluster radioactivity fission mode. It is shown that the super-asymmetric valley can be found in a wide range of the nuclear chart. Nevertheless, the valley flattens more and more when diverging from the isospin asymmetry of $^{208}$Pb. For neutron-deficient nuclei with $N/Z <$ 1.41, it is found that the valley diminishes before reaching the scission point, and cluster radioactivity can not be observed.

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arXiv2026

Sensitivity of Isotopic Fission Yields in Actinides to the Macroscopic Liquid-Drop Model: LSD vs ISOLDA

The impact of the macroscopic liquid-drop prescription on isotope-resolved fission-fragment yields in the actinide region is assessed by comparing two alternative parameterizations: the Lublin--Strasbourg Drop (LSD) model and the ISOscalar Liquid Drop Approximation (ISOLDA). The two prescriptions differ primarily in the treatment of isospin dependence in the volume and surface terms; in ISOLDA, an explicit dependence on the isospin square $T(T+1)$, where $T=|N-Z|/2$, is introduced in both coefficients. Using an identical set of fragment-yield observables and the same experimental reference (fission of $^{250}$Cf$^*$ at low and high energies), the propagation of the macroscopic-energy choice into the predicted yields is quantified in terms of (i) the location of the most probable post-neutron isotopes along elemental chains, (ii) the widths and asymmetries of the isotopic distributions, and (iii) the population of neighboring nuclides on the distribution tails. A comparable description of the gross properties of the isotopic yield pattern is obtained with both prescriptions, particularly for light and intermediate fragments, where peak positions and near-maximum curvatures are reproduced similarly. The most discriminating differences are found for heavy-fragment chains, for which the ridge location and isotopic centroids are rendered more sensitive to macroscopic isospin terms. Overall, a closer average agreement with the evaluated data is obtained with LSD, while the LSD--ISOLDA spread is shown to provide a practical estimate of the macroscopic-model uncertainty in isotope-resolved yields.

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arXiv2026

Isotope-Resolved Ba and Xe Yields in Actinide Fission and Correlated Heavy--Light Fragment Systematics

Isotope-resolved post-neutron fission yields in the Ba and Xe chains are calculated and benchmarked against evaluated reference data, with emphasis on element-resolved isotopic chains $Y(N_f)$ at fixed fragment charge $Z$ and on the consistency of heavy--light fragment correlations. Calculations are performed within a four-dimensional (4D) Langevin framework employing Fourier-over-Spheroid shape parametrization. The benchmark covers spontaneous fission of selected Cm and Cf isotopes (including $^{244,246}$Cm and $^{250}$Cf) as well as neutron-induced fission at thermal and 14-MeV energies for representative actinides in the Th--Pu region (including $^{229}$Th, $^{235}$U, $^{239}$Pu, and $^{249}$Cf). The dominant neutron-number maxima are reproduced for a large fraction of the isotopic chains considered, indicating that the mean charge partition and the average neutron content of the main fission channels are described consistently. A systematic residual discrepancy is observed in the isotopic widths: the calculated yields often fall off too rapidly on the distribution tails, producing distributions that are narrower than the evaluated data, most notably for heavy-fragment chains.

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arXiv2025

Search for shape-isomers in the Pt-Hg-Pb region

Potential energy surfaces of nine even-even isotopes of Pt, Hg, and Pb around $^{186}$Pt are evaluated within a macroscopic-microscopic model based on the Lublin-Strasbourg-Drop macroscopic energy and the microscopic energy obtained using the Yukawa-folded mean-field potential to establish the Strutinski shell corrections and the pairing correlation energy through the BCS approach with a monopole pairing force. The rapidly converging Fourier-over-Spheroid shape parametrization is used to describe nuclear deformations. The stability of the identified shape isomeric states with respect to non-axial and higher-order deformations is investigated. It is also found that in the description of non-axial deformations special attention needs to be devoted to the orientation of the triaxial shape. For the example of the $^{186}$Hg nucleus, where three prolate shape-isomeric states are found, it is shown that the potential energy surface obtained in our model is close to the one obtained in the Hartree-Fock-Bogoliubov theory with the Gogny energy-density functional.

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arXiv2025

Microscopic pairing in fission dynamics

Nuclear fission can be modelled as a quantum tunneling process driven by the interplay between the nuclear binding energy and the collective inertia. Within the Wentzel-Kramers-Brillouin formalism, spontaneous fission half-lives can be obtained by minimizing the action integral in the multidimensional space of collective degrees of freedom. Hence, including the relevant collective variables is crucial for properly describing spontaneous fission probabilities. Pairing correlations play an essential role in this evaluation since the collective inertia decreases as the inverse of the square of the pairing gap, and, therefore, they should be considered as a relevant degree of freedom on the same footing as deformation parameters. In this work, we show that the spontaneous fission half-lives in fermium isotopes can be reproduced in a microscopic theory by considering the least-action fission path in a two-dimensional space with constraints on the quadrupole moment and pairing correlations. We consider two microscopic quantities as degrees of freedom associated with pairing: the pairing gap parameter $\Delta$, and the particle number fluctuations $\langle \Delta N^2 \rangle$. Least-action paths, computed using the Dijkstra algorithm, are compared with minimum-energy paths, highlighting the importance of pairing correlations as a dynamical degree of freedom.

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arXiv2024

Pairing in fission: Mean-field and collective inertias study

Pairing plays a crucial role in the microscopic description of nuclear fission. Microscopic methods provide access to three quantities related to pairing, namely, the pairing gap ($\Delta$), the particle number fluctuations ($ \Delta \hat{N}^2 $), and the quenching factor (QF). The aim of this work is to analyse the impact of each of these quantities on the static description of the fission process.

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arXiv2023

Fission Fragment Mass and Kinetic Energy Yields of Fermium Isotopes

A rapidly converging 4-dimensional Fourier shape parametrization is used to model the fission process of heavy nuclei. Potential energy landscapes are computed within the macroscopic-microscopic approach, on top of which the multi-dimensional Langevin equation is solved to describe the fission dynamics. Charge equilibration at scission and de-excitation by neutron evaporation of the primary fragments after scission is investigated. The model describes various observables, including fission-fragment mass, charge, and kinetic energy yields, as well as post-scission neutron multiplicities and, most importantly, their correlations, which are crucial to unravel the complexity of the fission process. The parameters of the dynamical model were tuned to reproduce experimental data obtained from thermal neutron-induced fission of $^{235}$U, which allows us to discuss the transition from asymmetric to symmetric fission along the Fm isotopic chain.

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arXiv2021

Scission configuration in the self-consistent calculations with neck constraint

The calculations of the potential energy surface are essential in the theoretical description of the fission process. In the constrained self-consistent approach, the smooth evolution of nuclear shape is described from the ground state until a very elongated one with a narrow neck. In all microscopic calculations, the rupture of the neck at scission is associated with a substantial change of nuclear matter density distribution and rapid energy decrease. In this paper, we show that there is no discontinuity of the potential energy surface at scission when multi-constrained calculations are applied with the neck constraint. An early rupture of the neck at lower quadrupole and octupole moments is discussed as competitive with the conventional fission path. We discuss the neck properties in the scission configuration. We find that the neck radius in the asymmetric fission mode cannot decrease below 2 fm, and the nuclear matter density cannot decrease below the saturation density. In the compact fission mode, nuclear density may go down to half of the saturation density before the rupture of the neck.

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arXiv2021

Potential energy surfaces and fission fragment mass yields of even-even superheavy nuclei

Potential energy surfaces and fission barriers of superheavy nuclei are analyzed in the macroscopic-microscopic model. The Lublin-Strasbourg Drop (LSD) is used to obtain the macroscopic part of the energy, whereas the shell and pairing energy corrections are evaluated using the Yukawa-folded potential. A standard flooding technique has been used to determine the barrier heights. It was shown the Fourier shape parametrization containing only three deformation parameters reproduces well the nuclear shapes of nuclei on their way to fission. In addition, the non-axial degree of freedom is taken into account to describe better the form of nuclei around the ground state and in the saddles region. Apart from the symmetric fission valley, a new very asymmetric fission mode is predicted in most superheavy nuclei. The fission fragment mass distributions of considered nuclei are obtained by solving the 3D Langevin equations.

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arXiv2020

Description of the multidimensional potential energy surface in fission of $^{252}$Cf and $^{258}$No

The microscopic studies on nuclear fission require the evaluation of the potential energy surface as a function of the collective coordinates. A reasonable choice of constraints on multipole moments should be made to describe the topography of the surface completely within a reasonable amount of computing time. We present a detailed analysis of fission barriers in the self-consistent Hartree-Fock-Bogoliubov approach with the D1S parametrization of the Gogny nucleon-nucleon interaction. Two heavy isotopes representing different spontaneous fission modes - $^{252}$Cf (asymmetric) and $^{258}$No (bimodal) - have been chosen for the analysis. We have shown the existence of complicated structures on the energy surface that can not be fully described in two-dimensional calculations. We analyze apparent problems that can be encountered in this type of calculations: multiple solutions for given constraints and transitions between various potential energy surfaces. We present possible solutions on how to deal with these issues.

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arXiv2018

Cluster Radioactivity in Super Heavy Nuclei

Cluster radioactivity is an exotic nuclear decay observed in actinides where a light nucleus is emitted while the remaining heavy mass residue is the doubly magic $^{208}$Pb or a nucleus in its neighborhood. We have investigated this type of decay in heavier nuclei up to Lv $(Z=116)$ within a microscopic theory. It has been found that super asymmetric fission with $^{208}$Pb as heavy fragment may be dominant decay channel in some super heavy nuclei. This reaction is closely related with cluster radioactivity.

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arXiv2018

First observation of the competing fission modes in the neutron-deficient sub-lead region

Fragment mass distributions from fission of excited compound nucleus $^{178}$Pt have been deduced from the measured fragment velocities. The $^{178}$Pt nucleus was created at the JAEA tandem facility in a complete fusion reaction $^{36}$Ar + $^{142}$Nd, at beam energies of 155, 170 and 180 MeV. The data are indicative of a mixture of the mass-asymmetric and mass-symmetric fission modes associated with higher and lower total kinetic energies of the fragments, respectively. The measured fragment yields are dominated by asymmetric mass splits, with the symmetric mode contributing at the level of $\approx1/3$. This constitutes the first observation of a multimodal fission in the sub-lead region. Most probable experimental fragment-mass split of the asymmetric mode, $A_{L}/A_{H}\approx 79/99$, is well reproduced by nuclear density functional theory using the UNEDF1-HFB and D1S potentials. The symmetric mode is associated by theory with very elongated fission fragments, which is consistent with the observed total kinetic energy/fragment mass correlation.

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arXiv2017

Fission dynamics of $^{252}$Cf

The time-dependent generator coordinate method with the gaussian overlap approximation (TDGCM+GOA) formalism is applied to describe the fission of $^{252}$Cf. We perform analysis of fission from the initial states laying in the energetic range from the ground state to the state located 4 MeV above the fission barrier. The fission fragment mass distributions, obtained for different parity, energy of levels and types of mixed states, are calculated and compared with experimental data. The impact of the total time of wave packet propagation on the final results is studied as well. The weak dependence of obtained mass yields on the initial conditions is shown.

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arXiv2015

Fission barriers and probabilities of spontaneous fission for elements with Z$\geq$100

This is a short review of methods and results of calculations of fission barriers and fission half-lives of even-even superheavy nuclei. An approvable agreement of the following approaches is shown and discussed: The macroscopic-microscopic approach based on the stratagem of the shell correction to the liquid drop model and a vantage point of microscopic energy density functionals of Skyrme and Gogny type selfconsistently calculated within Hartree-Fock-Bogoliubov method. Mass parameters are calculated in the Hartree-Fock-Bogoliubov cranking approximation. A short part of the paper is devoted to the nuclear fission dynamics. We also discuss the predictive power of Skyrme functionals applied to key properties of the fission path of $^{266}$Hs. It applies the standard techniques of error estimates in the framework of a $\chi^2$ analysis.

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arXiv2015

Fission fragment mass yield deduced from density distribution in the pre-scission configuration

Static self-consistent methods usually allow to determine the most probable fission fragments mass asymmetry. We have applied random neck rupture mechanism to the nuclei in the configuration at the end of fission paths. Fission fragment mass distributions have been deduced from the pre-scission nuclear density distribution obtained from the self-consistent calculations. Potential energy surfaces as well as nuclear shapes have been calculated in the fully microscopic theory, namely the constrained Hartree-Fock-Bogolubov model with the effective Gogny D1S density-dependent interaction. The method has been applied for analysis of fission of Fm-256,258, Cf-252 and Hg-180 and compared with the experimental data.

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arXiv2015

On spontaneous fission and alpha-decay half-lives of atomic nuclei

It is shown, that the Gamow-like model with only one adjustable parameter - radius constant is able to reproduce well the alpha-decay half-lives for all even-even nuclei with the proton number larger than 50. The systematics for odd-A and odd-odd isotopes can be also well described when ones introduces an additional hindrance factor. Similar model based on the W. J. Swiatecki idea from 1955 is developed to reproduce the spontaneous fission half-lives of transactinide nuclei. The achieved accuracy of reproduction of the data is better than that obtained in more advanced theories.

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arXiv2014

Excitation energy dependence of fission in the mercury region

Background: Recent experiments on beta-delayed fission reported an asymmetric mass yield in the neutron-deficient nucleus 180Hg. Earlier experiments in the mass region A=190-200 close to the beta-stability line, using the (p,f) and (α,f) reactions, observed a more symmetric distribution of fission fragments. While the beta-delayed fission of 180Hg can be associated with relatively low excitation energy, this is not the case for light-ion reactions, which result in warm compound nuclei. Purpose: To elucidate the roles of proton and neutron numbers and excitation energy in determining symmetric and asymmetric fission yields, we compute and analyze the isentropic potential energy surfaces of 174,180,198Hg and 196,210Po. Methods: We use the finite-temperature superfluid nuclear density functional theory, for excitation energies up to E*=30MeV and zero angular momentum. For our theoretical framework, we consider the Skyrme energy density functional SkM* and a density-dependent pairing interaction. Results: For 174,180Hg, we predict fission pathways consistent with asymmetric fission at low excitation energies, with the symmetric fission pathway opening very gradually as excitation energy is increased. For 198Hg and 196Po, we expect the nearly-symmetric fission channel to dominate. 210Po shows a preference for a slightly asymmetric pathway at low energies, and a preference for a symmetric pathway at high energies. Conclusions: Our self-consistent theory suggests that excitation energy weakly affects the fission pattern of the nuclei considered. The transition from the asymmetric fission in the proton-rich nuclei to a more symmetric fission in the heavier isotopes is governed by the shell structure of pre-scission configurations.

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