Asymmetric Fission Research Articles

Construction of Continuous Collective Energy Landscapes for Large Amplitude Nuclear Many-Body Problems.

Several protocols are proposed to build continuous energy surfaces of many-body quantum systems, regarding both energy and states. The standard variational principle is augmented with constraints on state overlap, ensuring arbitrary precision on continuity. As an illustration, the lowest energy and excited state paths relevant for the ^{240}Pu asymmetric fission are studied. The scission is clearly signed, with a neutron excess in the neck, the ultimate glue before its rupture. Our approach can potentially connect any couple of Hilbert space states, which opens up new horizons for various applications.

Fission fragment spectroscopy of 235U(nth,f)

Simultaneous measurements of the relative fission fragment charge and mass yield distributions have been performed for the even-even fission fragments produced from the reaction, 235U(nth,f). The measurements have been carried out using the conventional fission fragment spectroscopic technique. The extracted results are interpreted on the basis of the Multi-Modal Random Neck Rupture Model (MM-RNRM). The results from the analysis bring out the necessary experimental evidence for the influence of the deformed proton shell closures at Z = 52 and ∼ 56 along with the neutron shell closures at N = 82 (spherical) and 88 (deformed) in controlling the respective S1 and S2 fission modes occurring in the heavier group of asymmetric fission fragments. The new findings from the present investigation provide the crucial inputs for understanding the features of different fission modes that persist in the low-energy fission dynamics of the lighter actinides. The pair wise neutron multiplicity distribution profiles for the five correlated fission fragment pairs have been presented, and the corresponding extracted neutron multiplicity values are also reported.

Mass distributions in 12C + 232Th fission: role of shell effects and excitation energy

This article reports the measurement of cross sections of charge and mass identified fission products and the mass distributions in 232Th(12C,f) reaction at 62.5, 70.7 and 102.9 MeV beam energies to investigate the role of single particle effects. The study was carried out using the recoil catcher technique followed by off-line γ-ray spectrometry. Cross sections of 32, 54 and 64 fission products were measured for 62.5, 70.7 and 102.9 MeV beam energies, respectively. The mass distributions obtained at 62.5 and 70.7 MeV show a flat-top nature indicating significant asymmetric fission contribution (heavy mass peak corresponding to Z range of 54–56), whereas, a nearly Gaussian behaviour was observed at 102.9 MeV indicating dominant contribution from symmetric fission. The experimental mass distributions were in gross agreement with the “GEF, 2021/1.1” calculations which predicted significant asymmetric fission contribution dominated by Z ≈ 55 (‘Standard 2’ mode) at 62.5 and 70.7 MeV. The present study shows the significant role of charge polarization resulting in the deviation of the most probable charge (ZP) values from those obtained using UCD hypothesis. “GEF, 2023/2.1” calculations were unable to explain the observed mass distributions with significant asymmetric fission contribution due to comparatively lower contribution from higher chance fission resulting from the lower saddle point energies. An attempt has been made to estimate the mass distributions arising from the complete fusion fission and α-transfer induced fission. The trend shows an increase in the α-transfer induced fission contribution with increasing beam energy which is in qualitative agreement with the sum-rule model calculations.

Exploring asymmetric fission in 180Hg⁎ compound nucleus using dynamical cluster-decay model

The observed asymmetric fission of the 180Hg⁎ compound nucleus challenges conventional expectations of symmetric fission, which are attributed to the presence of shell closures at Z=40 (semi-magic) and N=50 (magic). To comprehend this novel phenomenon, the dynamical cluster-decay model has been employed. For the first time, this model incorporates the bulk and neutron-proton asymmetry coefficients of the nuclear shape-dependent mass excess formula which are tuned recently to the ground state mass excess data of AME2020 and/or FRDM(2012) along with the temperature dependence for the nuclear shape and the surface energy coefficient of the nuclear proximity potential. The calculations have considered nuclear shapes as both spherical and deformed (quadrupole), with and without temperature dependence for the quadrupole deformation. A new minimum appears in the symmetric mass region of the fragmentation potential for masses (80, 100), when the fragments are deformed and optimally oriented, at an energy lower than that obtained for masses (90, 90) where the fragments are assumed to be spherical or nearly spherical at higher temperatures. This new minimum seems equivalent to the appearance of a new shell gap with deformation and is responsible for the asymmetric fission of 180Hg⁎. The most probable fission channel and the transition from asymmetric to symmetric mass distribution at higher excitation energies are consistent with experiments for the current temperature dependence assigned to the quadrupole deformation parameter.

Theoretical investigation of heavy cluster decay from Z=118 and 120 isotopes: A search for an empirical formula in superheavy region

Various decay modes in superheavy nuclei have been of significant interest among which cluster radioactivity has recently gained sizable attention. The α-decay being a predominant decay mode in the superheavy region, the accurate determination of cluster decay half-lives is also crucial in this region as it has tremendous potential to be explored as one of the major decay channels. The usability of the Royer analytical formula [Nuclear Physics A 683 (2001) 182], which is based on the asymmetric fission model, has been investigated for the cluster and α-decay in superheavy region, by comparing it with several other (semi)empirical/analytical formulas. After fitting the formula on around 100 cluster-decay data and around 423 α-decay data, the refitted Royer formula (RRF) is found to be very robust which is able to estimate the cluster decay and α-decay half-lives with good accuracy. In fact, a comparison of the half-lives of both the decay modes using the same formula points towards a substantial chance of heavy cluster (Kr and Sr) decay from various isotopes of Z=118 and 120. Hence, the formula proposed in this study works fairly well for the estimation of cluster decay half-lives in superheavy regions where most empirical formulas fail to match with the half-lives from the various established theories.

Nearly Symmetric, Asymmetric, Very Asymmetric, and Super Asymmetric Fission Half-Lives of Heavy and Superheavy Elements Using a Modified Geiger-Nuttall Law

Nearly Symmetric, Asymmetric, Very Asymmetric, and Super Asymmetric Fission Half-Lives of Heavy and Superheavy Elements Using a Modified Geiger-Nuttall Law

β-delayed fission studies at ISOLDE-CERN

This contribution provides an overview of the systematic β-delayed fission (βDF) studies performed by our collaboration at the ISOLDE facility at CERN. Measurements of βDF properties of eight isotopes, including the first identification of this process for six of them, were carried out over the years. Systematic studies of fission fragment mass distributions, which led to the discovery of the new region of asymmetric fission and to identification of multimodal fission in several isotopes, are revisited. Motivation and description of the most recent experiments aimed at 178Au and neutron rich actinium and francium isotopes are introduced. The detection system used until recently as well as the two newly-developed detection systems are described.

Shell effects in fission and quasi-fission

Quantum shell effects are responsible for asymmetric fission. They are also expected to affect the formation of fission fragments in quasi-fission reactions occurring in heavy-ion collisions. Shell effects in fission are studied with the single-particle energy level density near the Fermi level. In particular, shell effects in the pre-fragments and their role in fixing the final mass asymmetry of the fission fragments are discussed. Systematic time-dependent Hartree-Fock simulations of heavy-ion collisions show that quasi-fission fragment properties share strong similarities with fragments formed in fission of the compound nuclei. This is an indication that similar shell effects are responsible for the final asymmtery in both mechanisms.

Excitation energy dependency of the low-energy fission dynamics: Probing through prompt gamma-ray spectroscopy

Fission Fragment Spectroscopy (FFS) technique has been utilized to make simultaneous measurements of both the relative charge and mass yield distributions of even-even fission fragments produced from the fission of 236U at two different excitation energies, Eex. The necessary analysis has been carried out on the measured yield distributions by employing the Multimodal Random Neck Rupture Model (MM-RNRM). The relative contribution of two different types of asymmetric mode of fission – Standard I and Standard II has been extracted at Eex = 6.5 and 21.5 MeV. An attempt has been made to study the evolution of these two asymmetric fission modes with respect to the values of Eex of the concerned fissioning system at low-excitation regime. The probable evidence, although at the primitive stage, for an oscillatory behavior of the relative contributions of different low-energy fission modes with increasing values of Eex has been presented.

Shell effects in fission and quasi-fission reactions

Quantum shell effects are responsible for asymmetric fission. They are also expected to affect the formation of fission fragments in quasi-fission reactions occurring in heavy-ion collisions. Systematic time-dependent Hartree-Fock simulations of 40–56Ca+176Yb collisions show that quasi-fission fragment properties share strong similarities with fragments formed in fission of the compound nuclei. This is an indication that similar shell effects are responsible for the final asymmtery in both mechanisms.

Deformed shell stabilities influencing the fragment formation in sub-lead nuclei

The fragment mass distributions from fission in the sub-lead nuclei gave traces of the co-existence of symmetric and asymmetric fission modes. To investigate the major drivers behind the observed asymmetry, we have accumulated the fission data of 19 neutron-deficient lead island nuclei from the last few years and compared them with the corresponding predictions made by the GEF (General description of fission observables) model. Consequently, both experimental and theoretical analysis performed to extract asymmetric fission properties led to similar results, showing the deformed proton shell gaps, at Z = 34 for light fragments and Z = 42, 44, and 46 for heavier fragments, associated with large quadrupole deformations as the possible origin for the asymmetric split for all isotopes ranging from 178−198Hg, 176−186Pt, and 179−191Au. These observations contradicted the earlier theoretical report where neutron numbers were declared to play a leading role in deciding asymmetric fission. This disagreement motivated us to get better insight by extending the examination over a large domain around Pb covering Hg, Pt, Au, Pb, and Po nuclides. Although protons in the light fragments are still spotted to be constant and influenced by the deformed shell gaps at Z = 34, protons in the heavier fragments are found increasing with rising ZCN and gave evidence of negligible influence from Z = 42, 44, and 46 shell gaps. Interestingly, the excellent compatibility of GEF results with the experimental outcomes proved it an efficient model for extracting fragment properties. However, it could only reproduce the experimental symmetric fission fraction (SFF) within the 30–52 MeV energy window. The GEF's inadequate predictability of the multichance fission contributions might be considered as one of the possible reasons behind the observed disagreements between the GEF and experimental SFF above 52 MeV. Moreover, the other reason might be the inadequate modeling of the evolution of the fission fragment mass/charge distribution with excitation energy.

Influence of the transition from symmetric to asymmetric fission mode on the average total kinetic energy and neutron multiplicity

Influence of the transition from symmetric to asymmetric fission mode on the average total kinetic energy and neutron multiplicity

Shell effect on fission fragment mass distribution at Ecn* up to 70 MeV: Role of multichance fission

Mass distributions of fission fragments produced from the $^{249}\mathrm{Bk}$ compound nucleus (CN), populated by complete fusion of $^{11}\mathrm{B}$ with $^{238}\mathrm{U}$, were measured for the excitation energies in the range of ${E}_{\mathrm{cn}}^{*}\ensuremath{\approx}36.7--69.7$ MeV. Nearly flat tops observed for these distributions indicate the presence of asymmetric fission, contrary to the pure Gaussian mass distributions expected at such high excitation energies. A fit to the mass distribution using three Gaussian functions provides an estimate of individual contributions from symmetric and asymmetric modes of fission. A significant contribution of the asymmetric component observed at CN excitation energies above 40 MeV can be understood only by invoking ``multichance fission'' in the calculations using the semiempirical model code gef. A systematic analysis of the mass distributions for several heavy transuranic nuclei reveals that the manifestation of the fragment shell effect in the integral mass distributions are visible even up to an initial compound nucleus excitation of ${E}_{\mathrm{cn}}^{*}\ensuremath{\approx}70$ MeV due to their influence on the distributions of higher chance fissions.

Time-dependent Hartree-Fock study of quasifission trajectories in reactions forming Og294

Background: Fission modes in superheavy nuclei are expected to be impacted by quantum shell effects. Similar shell effects may be present in quasifission reactions, acting to hinder the mass equilibration process in heavy-ion collisions.Purpose: To investigate quasifission mechanisms in five different reactions forming $^{294}\mathrm{Og}$ as a compound nucleus and compare quasifission trajectories with predicted fission modes.Methods: The potential energy surface (PES) of $^{294}\mathrm{Og}$ is calculated using the static Hartree-Fock approach with BCS pairing correlations. Quasifission trajectories for central collisions at various energies are studied with the time-dependent Hartree-Fock theory.Results: The exit channel strongly depends on initial mass asymmetry and orientation, but it only exhibits small dependencies in the reaction energy. The $^{48}\mathrm{Ca}+^{246}\mathrm{Cf}$ reaction is affected by the PES topography, leading to either fusion or asymmetric fission. $^{126}\mathrm{Sn}+^{168}\mathrm{Er}$ reactions exhibit large total kinetic energies and compact scission configurations, which could be interpreted as an effect of the $Z=50$ spherical magic gap.Conclusions: Quasifission trajectories can be interpreted in terms of the underlying PES for low excitation energies. Future investigations of quasifission with temperature- and angular-momentum-dependent PES could be considered.

Shell effect driven fission modes in fragment mass and total kinetic energy distribution of Hg*192

Background: Asymmetric fission in sublead nuclei, especially mercury and platinum isotopes, has generated keen interest in studying fission in $A<200$ mass region. The role of proton and neutron shells on asymmetric fission in these nuclei is still under test and requires more investigation.Purpose: This paper presents measurements aimed at studying the fission modes of $^{192}\mathrm{Hg}$ formed in the reaction $^{32}\mathrm{S}+^{160}\mathrm{Gd}$ in the excitation energy range 54--74 MeV.Method: Mass distributions have been determined from the fission fragments' (FF) time of flight (TOF). Two multiwire proportional chambers were placed symmetrically on opposite sides of the beam to cover the folding angle for symmetric fission. The measured TOF of the fission fragments was used to obtain their velocities, and after clean separation of compound nuclear fission events, the velocities were further used to obtain the fission fragment mass and total kinetic energy (TKE) distribution.Results: The fission fragment mass distributions at all beam energies were found to be characteristically flat topped, which deviates significantly from a single Gaussian behavior. The distinctive features of mass and energy distributions happen to be explained by the presence of a symmetric fission mode and three asymmetric fission modes, manifested by different total kinetic energies in varying mass regions. These asymmetric fission modes are a consequence of the stabilization role of proton shells at $Z\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}$ 36, $Z\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}$ 46, and $Z$ = 28/50. The most probable mass of light and heavy fragments are found to be around 86 and 106 u, respectively. The symmetric yield is found to increase with the excitation energy.Conclusions: The present study indicates a mixture of symmetric and asymmetric modes in the fission of $^{192}\mathrm{Hg}$ nuclei and points out the relevance of deformed proton shells at $Z\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}$ 36 and 46.

Hartree–Fock–Bogoliubov study of quantum shell effects on the path to fission in ^{180}Hg, ^{236}U and ^{256}Fm

Quantum shell effects stabilising fission fragments with various shapes have been invoked as a factor determining the distribution of nucleons between the fragments at scission. Shell effects also induce asymmetric shapes in the nucleus on its way to fission well before the final fragments are (pre)formed. These shell effects are studied in fission of ^{180}Hg, ^{236}U and ^{256}Fm with constrained Hartree-Fock-Bogoliubov calculations using the D1S parametrisation of the Gogny interaction. Strutinsky shell energy correction and single-particle energy level density near the Fermi surface are computed. Several neutron and proton shell effects are identified as drivers towards asymmetric fission. Shell effects are also used to identify the preformation of the fragments in the later stage of fission.

Microscopic study of neutron-induced fission process of 239Pu via zero- and finite-temperature density functional theory* *Supported by the National Natural Science Foundation of China (11790325, 12275081, 11790320, 11790321, 11961131010, 11605054, 12105369, 12147219, 12047568) and the Continuous Basic Scientific Research Project (WDJC-2019-09)

To study the neutron-induced fission of Pu, potential energy surface (PES) calculations were performed using zero and finite-temperature density functional theory (FT-DFT) with the Skyrme force. The energy of the incident neutron was simulated by the temperature of the FT-DFT. The variations of the least-energy fission path, fission barrier, total kinetic energy, scission line, and mass distribution of fission fragments with the incident neutron energy were analyzed. It was learned that an increase in the temperature lowers the barrier height, the isomeric-state energy, and the ridge between symmetric and asymmetric fission valleys. Additionally, the gaps of the single particle levels become smaller with an increase in the temperature. As the temperature increases, the pre-fission region shrinks, and the scission occurs at smaller deformation around the symmetric fission channel. At low temperatures, the pairing correlations in the collective space are similar to those in zero-temperature DFT, and when the temperature is 0.3 MeV, the pairing gaps decrease rapidly. Two different methods were used to calculate the fission yields of the neutron-induced fission Pu (n, f) with different incident neutron energies, in the framework of time-dependent generator coordinate method (TDGCM). One way to calculate the fission yield of Pu (n, f) is to solve the collective equation of the TDGCM by using the PES from the FT-DFT with the corresponding temperature. The other involves using the PES from the zero-temperature DFT and adjusting the initial collective energy of the wave packet in the TDGCM according to the incident neutron energy. For the cases of the lower incident neutron energies, these two methods gave similar results and reproduced the experimental peak and width of fission fragment distribution. However, for the highest incident neutron energy considered in this study, the results from the TDGCM using the PES from zero-temperature DFT deviated explicitly from the experimental data, whereas those obtained by using the PES from FT-DFT remained close to the experimental data. This indicated that, with the increase in the incident neutron energy, the shell structure of the compound nuclei changed explicitly; thus, it may not be effective to use the PES from zero-temperature to perform the fission dynamic calculation.

Mitochondrial temporal network tracking for 4D live-cell fluorescence microscopy data.

Mitochondria form a network in the cell that rapidly changes through fission, fusion, and motility. This four-dimensional (4D, x, y, z, time) temporal network has only recently been made accessible through advanced imaging methods such as lattice light-sheet microscopy. Quantitative analysis tools for the resulting datasets however have been lacking. Here we present MitoTNT, the first-in-class software for Mitochondrial Temporal Network Tracking in 4D live-cell fluorescence microscopy data. MitoTNT uses spatial proximity and network topology to compute an optimal tracking. Tracking is >90% accurate in dynamic spatial mitochondria simulations and are in agreement with published motility results in vitro. Using MitoTNT, we reveal correlated mitochondrial movement patterns, local fission and fusion fingerprints, asymmetric fission and fusion dynamics, cross-network transport patterns, and network-level responses to pharmacological manipulations. MitoTNT is implemented in Python with a JupyterLab interface. The extendable and user-friendly design aims at making temporal network tracking accessible to the wider mitochondria community.

Four-dimensional Langevin approach to fission with Cassini shape parameterization

We apply the Cassini shape parameterization to the dynamical study of actinide fission using the multi-dimensional Langevin equation. The fragment mass distributions for fission of 236U and 256, 258 Fm are calculated with the 4D Langevin equation. The 4D collective space is formed by adding either α3 or α6 to the basic 3D {α, α1, α4} space. We investigate the role of α3 and α6 by comparing the results of 4D (α3 or α6) with that of 3D. For 236U and 256Fm, where asymmetric fission is predominant, the fragment mass distributions calculated in the 4D (α3) space are in good agreement with experimental data. On the other hand, for 258Fm, which is characterized by symmetric fission, 4D (α6) is effective for describing the shape of compact symmetric fragments. We demonstrate the advantages of the Cassini shape parameterization for the dynamical study of actinide fission.

Fission Properties of Neutron-Rich Nuclei around the End Point of r-Process

We studied the fission properties of neutron-rich nuclei 278, 286Cf around the end point of r-process by microscopic self-consistent approaches. The fission barriers and potential energy surfaces are obtained by constrained static Skyrme Hartree–Fock-BCS calculations. Fission fragments are studied by dynamical time-dependent Hartree–Fock+BCS calculations. Results show that 286Cf has an octupole deformation at ground state, which can increase the fission barrier height by 1.1 MeV and enhance significantly the spontaneous fission half-life. To search possible fission channels, dynamical calculations with a broad coverage of initial deformations result in two slightly asymmetric peaks around A = 128 and 150 for 278Cf, and A = 133 and 153 for 286Cf. Very asymmetric fission channels as given by semi-empirical models are not found in our results.