Fission Of Nuclei Research Articles

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.

Decay properties of undetected superheavy nuclei with Z > 110

A comprehensive study of favoured and unfavoured α-decay, cluster decay, weak-decay along with spontaneous fission in undetected superheavy nuclei within the range for proton number 111 ≤ Z ≤118 and neutron number 161 ≤ N ≤192 is performed. Half-lives for various mentioned decays are estimated with good accuracy on the basis of NUBASE2020 and are found in excellent match with the known half-lives. α-decay mode is found most probable in this wide range and correspondingly potential α-decay chains are reckoned. Peculiarly, the chances of cluster emission, as well as weak-decay, are also anticipated in this region of the periodic chart which open new pathways of detection of superheavy nuclei.

Multiple usage of thorium-based fuel in a VVER-1000 reactor

This paper considers the use of unconventional fuel in nuclear power reactors, using the example of a VVER-type unit, in order to find out the possibility of saving natural fissile uranium nuclei. Saving fissile uranium is one of the important tasks, the solution of which will give time for the development of a two-component nuclear power industry that will have no problems with fuel resources. However, at present, the reserves of cheap uranium can provide the existing level of global nuclear energy for only 80–100 years. The main components of this proposed fuel are 232Th and fissile isotopes of uranium: 235U (loaded) and 233U (produced from thorium). All the uranium isotopes and added 235U nuclei at the beginning of the campaign account for about 6% of the number of thorium nuclei and uranium isotopes. The abbreviated name of this fuel is TORUR-5. To keep fissionable nuclei in the fuel cycle after the spent fuel is unloaded, it is envisaged that all the heavy nuclei will be returned back to the reactor after they have been cleaned from fission fragments, i.e., the fuel cycle will be closed. At the same time, the principle of annual movement of fuel assemblies (as they burn up) is the same as in the existing VVER-1000 reactors. Using the Serpent software, a reactor model was built, the composition and dimensions of which were close to the parameters of the VVER-1000 serial unit. The main results of calculations were the quantitative compositions of isotopes annually loaded into the reactor as well as the amounts of 235U and thorium added also annually. The analysis of the obtained results allowed us to make the following conclusions. The annual reloading of 235U during the computation period is required almost at a constant level and, in comparison with uranium fuel, is about half as much. This is feasible for the following reasons. Part of the fissions of 235U is replaced by the fission of 233U produced from 232Th. In addition, fissionable nuclei are kept in the closed Th-U fuel cycle. This is the first “advantage” of the proposed fuel. TORUR-5 requires uranium enriched to at least 90%, the cost of which is several times higher than that of 3–5% enriched uranium. But since much less highly enriched uranium is required, the cost of fuel for a TORUR-5-fueled VVER-1000 reactor is significantly lower. This is the second “advantage” of the proposed fuel. The negative characteristic of TORUR-5, which requires further investigation, is that, after the initial loading, several uranium isotopes appear in the returned fuel, the total radioactivity of which, according to estimates, exceeds the radioactivity of traditional 3–5% enriched uranium fuel by several thousand times. At the same time, the radioactivity of discharged spent conventional fuel exceeds the radioactivity of fresh fuel by millions of times, and this problem has been solved at NPPs both organizationally and technically. Therefore, it will be necessary to develop a technology for loading TORUR-5, taking into account the estimated radioactivity.

Microscopic calculation of fission product yields for odd-mass nuclei

Fission data are essential inputs to reaction networks involved in nucleosynthesis simulations and nuclear forensics. In such applications as well as in the description of multi-chance fission, the characteristics of fission for odd-mass nuclei are just as important as those for even-even nuclei. The fission theories that aim at explicitly describing fission dynamics are typically based on some variant of the nuclear mean-field theory. In such cases, the treatment of systems with an odd number of particles is markedly more involved, both formally and computationally. In this article, we use the blocking prescription of the Hartree-Fock-Bogoliubov theory with Skyrme energy functionals to compute the deformation properties of odd-mass uranium isotopes. We show that the resulting fission fragment distributions depend quite significantly on the spin of the odd neutron. By direct calculation of the spin distribution of the fissioning nucleus, we propose a methodology to rigorously predict the charge and mass distributions in odd-mass nuclei.

Contributions of quasifission and fusion-fission in the Mg24 + Hf178 reaction at 145 MeV laboratory beam energy using the Boltzmann-Uehling-Uhlenbeck model

Background: Understanding the mechanism of the quasifission reaction is important, because it is an essential competitor to the fusion reaction leading to superheavy elements. However, it is a challenge to separate the quasifission and fusion-fission components.Purpose: This paper provides a dynamics description of the $^{24}\mathrm{Mg}+^{178}\mathrm{Hf}$ reaction at laboratory beam energy of 145 MeV, and studies contributions of quasifission and fusion-fission in the reaction.Method: The Boltzmann-Uehling-Uhlenbeck model is improved to study the heavy-ion collision at incident energy near the Coulomb barrier. A method is developed to determine the window of the dinuclear system during the evolution.Results: It is shown that the fusion occurs with a large percentage in the central collision, and the quasiinelastic scattering occurs mainly in the peripheral collision. The quasifission competes with the fusion in collisions with small impact parameters. It is found that the cross sections of quasifission decrease and those of fusion increase with increasing surface energy or incompressibility. The mass-angular correlation of the quasifission events shows the mass asymmetry. The fragments emitted at the front angle are targetlike fragments and those at the back angle are projectilelike fragments. Furthermore, the differential cross sections at the angles ${0}^{\ensuremath{\circ}}$ and ${180}^{\ensuremath{\circ}}$ are larger than that at ${90}^{\ensuremath{\circ}}$.Conclusions: By comparing the calculations of the mass distribution and mass-angular correlation to the data, it is deduced that the compound nucleus fission plays a main role and accounts for the characteristic of fragment observables in the $^{24}\mathrm{Mg}+^{178}\mathrm{Hf}$ reaction at laboratory beam energy of 145 MeV.

Measurement of the Cross Section for the Neutron-Induced Fission of 238U Nuclei in the Energy Range of 0.3–500 MeV

The cross section for the neutron-induced fission of 238U nuclei has been measured using the time-of-flight spectrometer of the GNEIS neutron complex at the Petersburg Nuclear Physics Institute, National Research Center Kurchatov Institute, in the neutron energy range of 0.3–500 MeV. Fission fragments have been detected by low-pressure position-sensitive multiwire proportional counters. The cross section for 238U(n, f) fission has been measured with respect to the cross section for 235U(n, f) fission, which is an accepted international standard. Data on the energy dependence of the anisotropy of the angular distribution of fragments of neutron-induced 238U nuclei are also presented. The data obtained have been compared to previous experiments carried out using both similar and significantly different methods.

THE URAL PART OF THE BIOGRAPHY OF IGOR VASILYEVICH KURCHATOV, AN OUTSTANDING NUCLEAR SCIENTIST

The name of the outstanding Soviet physicist, Igor Vasilyevich Kurchatov, is known not only in Russia, but also far beyond its borders. Knowledge about him is mainly related to his scientific leadership of the domestic nuclear project. He is one of the nine thrice Heroes of Socialist Labor of the Soviet Union. However, few people know that his main practical results of scientific developments were translated into real industrial production on the Ural land. In addition, his childhood years are connected with the Southern Urals, as well as the years of scientific leadership of the firstborn of the nuclear industry in the Urals - plant No. 817, now the Mayak Production Association. The presented scientific article is prepared for the 120th anniversary of the birth of an outstanding Soviet nuclear scientist, who was the head of the implementation of the uranium problem in the Soviet Union. He was one of the first in the world to identify the possibility of releasing energy of colossal power as a result of fission of the atomic nucleus and implemented them in the creation of the first domestic nuclear weapons, and then participated in the development of technologies for the use of atomic energy for peaceful purposes. I.V.Kurchatov scientific work in the Urals is reflected fragmentally in the published literature. This statement requires more detailed research, systematization and coverage in a scientific publication.

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.

Study of the influence of radiation-induced damage accumulation during the interaction of heavy Xe22+ ions on changes in the thermophysical parameters of zirconia ceramics

The aim of this paper is to evaluate the influence of the processes of radiation-induced damage formation in the form of point defects, dislocations and vacancies, as well as their accumulation and the formation of locally disorderedregions in the near-surface layer of zirconia ceramics under irradiation with heavy Xe22+ions with an energy of 230 MeV, on the change in the thermophysical properties of ceramics. The choice of the ion type for irradiation is due to the possibilities ofmodeling radiation damage processes comparable to the impact of uranium nuclei fission fragments during nuclear reactions in nuclear fuel. The choice of materials for irradiation in the form of ZrO2ceramics is due to the prospects for their use as the main material for inert matrices of dispersed nuclear fuel for new generation reactors. This choice is due to the physicochemical, thermophysical and strength properties of ZrO2ceramics, which are more resistant than other types of oxide ceramics. During research, it was found that the formation of isolated locally heterogeneous regions at low irradiation fluences does not lead to significant changes in the thermophysical properties of the damaged ceramic layer. However, polymorphic transformations of the t-ZrO2 → c-ZrO2type, which occur at irradiation fluences above 1012ion/cm2, lead to a decrease in thermal conductivity and the appearance of heat losses associated with the disruption of the phonon heat transfer mechanisms in the damaged layer.

Three-dimensional potential energy surface for fission of 236U within covariant density functional theory* *Supported by the National Natural Science Foundation of China (11875225, 11790325, 11790320), the Special Fund from the China Nuclear Data Center, the Fundamental Research Funds for the Central Universities, and the Fok Ying-Tong Education Foundation

We calculate the three-dimensional potential energy surface (PES) for the fission of the compound nucleus U using covariant density functional theory with constraints on the axial quadrupole and octupole deformations as well as the nucleon number in the neck . By considering the additional degree of freedom , the coexistence of the elongated and compact fission modes is predicted for . Remarkably, the PES becomes shallow across a large range of quadrupole and octupole deformations for small , and consequently, the scission line in the plane extends to a shallow band, leading to fluctuations of several to ten MeV in the estimated total kinetic energies and of several to approximately ten nucleons in the fragment masses.

The description of dynamical fission process using improved quantum molecular dynamics model incorporated with microscopic potential energy surface

A new method, namely the improved quantum molecular dynamics model incorporated with the microscopic potential energy surface, is proposed to simulate the dynamical evolution from saddle to scission point and to calculate the isotope yields of fission fragments in thermal neutron-induced fission of actinide nuclei near 236U. With this approach, the shell and pairing effects are introduced by the microscopic potential energy, while the isospin effect, the dynamical effects of fluctuations and correlations in fission process are automatically included in the improved quantum molecular dynamics model. These two aspects eventually influence the productions of fission fragments. The calculated both charge and mass distributions of fission fragments for a series of actinide nuclei are in overall agreement with the evaluated data from JEFF-3.3 and ENDF/B-VIII.0. The isotope distributions of fission fragments in the fission of 236U reproduce the data reasonable well, from which we extract the relationship between peak mass number of the isotope distributions and the charge number of fission fragments. We find that this relationship deviates linear relation slightly at certain cases, which indicates the isospin nonequilibrium effect. By tracking back to the fission process on the two dimension plane of elongation and mass asymmetry of prefragments, we find that the necked-in shape and the separation of two parts happen much faster for the 132Sn than 128,136Sn in the time evolution of the distance R between the centers of two parts of the fissioning system. The events producing 128,132,136Sn and 136,140,144Xe present two kinds of average fission paths with bypassing and going through the minimum-energy valley of the microscopic potential energy surface, respectively.

Multi-modal mass-asymmetric fission of 178Pt from simultaneous mass-kinetic energy fitting

The observation of mass-asymmetric fission in neutron-deficient 180Hg dramatically expanded the region of mass-asymmetric fission found across the nuclide chart, and has led to intense experimental and theoretical investigations into the fission of sub-lead nuclei. In particular, two major questions have been raised: how many fission modes are present in the fission of sub-lead nuclides, and which shells dictate these modes?Notably, investigations of the fission modes of 178Pt have led to contrasting results. To solve this disparity, new high-statistics data have been measured at the lowest excitation energy to-date using the CUBE fission spectrometer at The Australian National University. A new fitting procedure was developed to fit the high-statistics two-dimensional mass-kinetic energy distribution without external constraints.The fission of 178Pt can best be described by three fission modes: one mass-symmetric and two mass-asymmetric. Comparisons to previous analyses highlight the necessity of fitting the two-dimensional mass-kinetic energy distribution, rather than fitting slices of individual one-dimensional projections of the full distribution. Systematic studies of high-statistics measurements, combined with a rigorous statistical analysis offer the best chance to determine the shell effects responsible for multi-modal mass-asymmetric fission in this region of the nuclide chart.

The evaluation of the fission mode and fragment yields of neutron-rich nuclei by the dynamical model

Nucleosynthesis by the rapid neutron-capture process (r-process) produces elements heavier than iron via neutron-rich nuclides, observed in the solar system and stars with various metallicities. In the r-process, fission plays a fundamental role by recycling the matter during neutron irradiation and by shaping the final r-abundance distribution. Nevertheless, due to the difficulty of experimental approaches, most of the fission data available for r-process calculations are based on theoretical predictions with phenomenological models. In this study, we focused on the transition of fission mode from asymmetric to symmetric in neutron-rich isotopes, which has been suggested in recent experiments for fermium isotopes. We investigated the fission of neutron-rich nuclei by a theoretical calculation based on the dynamical model and employed Langevin equations.

The Langevin Approach for Fission of Heavy and Super-heavy Nuclei

The Langevin Approach for Fission of Heavy and Super-heavy Nuclei

Fission of Relativistic Nuclei with Electromagnetic Excitation

Fission of Relativistic Nuclei with Electromagnetic Excitation

Statistical Model Analysis of Neutron Multiplicities from Fission of Compound Nuclei in ~ 200 and 250 Mass Region

Statistical Model Analysis of Neutron Multiplicities from Fission of Compound Nuclei in ~ 200 and 250 Mass Region

Адекватность динамической модели деления возбуждённых атомных ядер

The paper presents a model of fission of excited atomic nuclei. The model is based on the stochastic Langevin differential equation. The amplitude of the Langevin source is characterized by white noise. The adequacy of modeling is tested in two ways: by comparing the analytical solution of the Langevin equation with the numerical one in the harmonic oscillator mode and by comparing the results of numerical modeling with the results obtained by other authors. It is shown that in both cases the results of dynamic modeling provide sufficient accuracy.

241Am and 245Cm as candidates for the ROT effect investigation

Shifts of angular distributions of α-particles are calculated for induced fission of 241Am and 245Cm nuclei by slow polarized neutrons. This shift is called the ROT effect, since its manifestation is directly related to the oriented rotation of the nuclei. It is shown that at certain neutron energies, the magnitude of the ROT effect can be two times higher than the maximum effect detected to date. In addition, experiments with these nuclei may allow us to see the predominant rotation of the fissionable nucleus in the opposite direction to that observed in all previously studied isotopes.

Sequential fission and the influence of 208Pb closed shells on the dynamics of superheavy element synthesis reactions

Measured binary quasifission mass spectra in reactions with actinide nuclides show a large peak in yield near the doubly-magic 208Pb. This has generally been attributed to the enhanced binding energy of 208Pb causing a valley in the potential energy surface, attracting quasifission trajectories. To investigate this interpretation, binary quasifission mass spectra and cross-sections have been measured at near-barrier energies for reactions of 50Ti with actinide nuclides from 238U to 249Cf. Cross-sections have also been deduced for sequential fission (a projectile-like nucleus and two fragments from fission of the complementary target-like nucleus). Binary cross-sections fall from ∼70% of calculated capture cross-sections for 238U to only ∼40% for 249Cf, with a compensating increase in sequential fission cross-sections. The data are consistent with the 208Pb peak originating largely from sequential fission of heavier fragments produced in more mass-asymmetric primary quasifission events. These are increasingly suppressed as the heavy quasifission fragment mass increases above 208Pb. The important role of sequential fission calls for re-interpretation of quasifission characteristics and dynamics in superheavy element synthesis reactions.

Fusion-fission analysis of C12+Cm248 and O16+Pu244 nuclear reactions across the Coulomb barrier

By using the symmetric-asymmetric Gaussian barrier distribution (SAGBD) model, Wong formula, and coupled channel approach, heavy ion fusion dynamics is investigated for $^{12}\mathrm{C}+^{248}\mathrm{Cm}$ and $^{16}\mathrm{O}+^{244}\mathrm{Pu}$ reactions at energies lying below and near the Coulomb barrier. Coupling of various channels linked with the structure of participants to the relative motion of the collision partners is done by considering a Gaussian type of weight function in the SAGBD model and cross sections are found to be enhanced relative to the calculations obtained by the simple barrier penetration model. In the SAGBD model, the channel coupling effects are calculated in terms of channel coupling parameter ($\ensuremath{\lambda}$) and percentage reduction in the height of apparent fusion barrier with respect to the Coulomb barrier (${V}_{\mathrm{CBRED}}$). The channel coupling parameter estimates the cumulative influence of dominant intrinsic channels, which are responsible for the sub-barrier fusion enhancement. The SAGBD calculations appropriately explain the dynamics of $^{12}\mathrm{C}+^{248}\mathrm{Cm}$ and $^{16}\mathrm{O}+^{244}\mathrm{Pu}$ reactions at energies lying around the Coulomb barrier. The coupled channel analysis of the present reactions is done by using the code ccfull and the coupled channel calculations unambiguously identify the dominant influences of the rotational states up to ${10}^{+}$ spin states of the ground state rotational band of target isotopes in both reactions. In addition, the couplings to higher order deformation, such as ${\ensuremath{\beta}}_{4}$ for target and low lying quantum states of the projectile, are necessarily required to reproduce the experimental data of $^{12}\mathrm{C}+^{248}\mathrm{Cm}$ and $^{16}\mathrm{O}+^{244}\mathrm{Pu}$ reactions. Apart from the fusion analysis, the dynamical cluster-decay model (DCM) is applied to understand the fission dynamics of the $^{260}\mathrm{No}^{*}$ nucleus formed via the above-mentioned reactions. The decay study is carried out at the center-of-mass energies spread, ${E}_{\mathrm{c}.\mathrm{m}.}$ ($\ensuremath{\approx}79$ to 109 MeV) by including the quadrupole deformations (${\ensuremath{\beta}}_{2}$) and optimum orientations (${\ensuremath{\theta}}_{i}^{\mathrm{opt}}$) of the decaying fragments. According to the experimental observation, the noncompound nucleus (nCN) fission component competes with the compound nucleus (CN) fission processes. Consequently, the possibility of nCN contribution is also explored in the decay of the $^{260}\mathrm{No}^{*}$ compound nucleus. With an aim to have a comprehensive analysis of CN and nCN fission mechanisms, the role of the center-of-mass energy (${E}_{\mathrm{c}.\mathrm{m}.}$) and angular momentum ($\ensuremath{\ell}$) is explored in terms of various parameters of DCM such as fragmentation potential, preformation probability, barrier modification, etc.