Fission Modes Research Articles

Binary and Ternary Fragmentation Analysis of 252Cf Nucleus using Different Nuclear Radii

Pioneering study reveals that a radioactive nucleus may split into two or three fragments and the phenomena are known as binary fission and ternary fission respectively. In order to understand the nuclear stability and related structure aspects, it is of huge interest to explore the fragmentation behavior of a radioactive nucleus in binary and ternary decay modes. In view of this, Binary and ternary fission analysis of 252Cf nucleus is carried out using quantum mechanical fragmentation theory (QMFT). The nuclear potential and Coulomb potential are estimated using different versions of radius vector. The fragmentation structure is found to be independent to the choice of fragment radius for binary as wellas ternary decay paths. The deformation effect is included up to quadrupole (β2) with optimum cold orientations and their influence is explored within binary splitting mode. Moreover, the most probable fission channels explore the role of magic shell effects in binary and ternary fission modes.

Exploring the fission barrier of U235

In this work we have reexamined the shape isomer in $^{235}\mathrm{U}$. Reanalyzing data from a previous experiment gave a half-life of ${T}_{1/2}=(11\ifmmode\pm\else\textpm\fi{}3)\phantom{\rule{4pt}{0ex}}\mathrm{ms}$ and a cross section for populating the isomer of ${\ensuremath{\sigma}}_{\mathrm{iso}}=(12\ifmmode\pm\else\textpm\fi{}1)\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{b}$. Combining these new results with measured properties of fission fragments from the reaction $^{234}\mathrm{U}(n,f)$ allowed extracting parameters describing the outer fission barrier. The deduced barrier parameters are ${E}_{B}=(5.7\ifmmode\pm\else\textpm\fi{}0.6)$ MeV and $\ensuremath{\hbar}{\ensuremath{\omega}}_{B}=(0.5\ifmmode\pm\else\textpm\fi{}0.1)$ MeV for the height and curvature, respectively, as well as the energy of the superdeformed ground state ${E}_{II}=(2.4\ifmmode\pm\else\textpm\fi{}0.6)$ MeV. Separate barrier parameters for the standard-1 and standard-2 fission modes have been estimated, too. Finally, the results obtained in this work have been successfully tested for their plausibility.

Transformations of actinides fission product yields due to post-scission emission of nuclear particles: 232Th

The “many ensembles” method was proposed to investigate the influence of nuclear particles’ post-scission emission on mass and charge distributions of fission products. The post-scission approximation was used: each of these ensembles consists of the fission fragments after emission of chains of different lengths, both the beta (β±) particles and neutrons. The theory allows one to find the most probable two-fragment clusters of fission products and study their evolution after the post-scission emission of nuclear particles. The isotope 232Th was chosen as an example, the fission fragments of which have been intensively studied experimentally. It is shown that the post-scission emission of nuclear particles eventually leads to the convergence of the asymmetric peaks, which looks like enhanced symmetric fission mode over the asymmetric mode for fission product yields. A comparison of the theoretical results and experimental data for the 232Th fission fragments indicates their satisfactory matching.

Description of the multidimensional potential-energy surface in fission of Cf252 and No258

Microscopic studies of 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, $^{252}\mathrm{Cf}$ (asymmetric) and $^{258}\mathrm{No}$ (bimodal), have been chosen for the analysis. We have shown the existence of complicated structures on the energy surface that cannot 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 overlapping potential-energy surfaces. These issues may be partially solved by the analysis of the potential-energy surface spanned by triple constraints, but even in this case one may find multiple solutions and surface discontinuities. Analysis of the potential-energy surface in two dimensions is often very successful but it must be carried out with special attention to possible discontinuities.

Simultaneous description of charge, mass, total kinetic energy, and neutron multiplicity distributions in fission of Th and U isotopes

For fissioning isotopes of thorium and uranium, the simultaneous description of the charge, mass, total kinetic energy, and neutron multiplicity distributions of fission fragments is presented within the improved scission-point model. Correlations between all these observables are analyzed. The influence of the transition from symmetric to asymmetric fission mode on the shape of neutron multiplicity distribution is studied.

Post-Neutron Mass Yield Distribution in the Spontaneous Fission of 252Cf

Cumulative and independent yields of various fission products within mass ranges of 86 to 119 and 127 to 162 were measured in the spontaneous fission of 252Cf by using an off-line gamma-ray spectrometric technique. From the cumulative yields of the fission products, their mass chain yields were obtained by using the charge distribution correction. Mass yield distribution parameters such as full-width at tenth-maximum of light and heavy mass wings, average light mass <AL> and heavy mass <AH>, and total average neutron multiplicity <ν>expt were obtained. Fission yield data in the 252Cf(SF) reaction were compared with similar data in neutron-induced fission and spontaneous fission of other actinides to examine the role of excitation energy and nuclear structure effect. The role of standard I and standard II asymmetric modes of fission is also discussed.

Post-neutron mass yield distribution in the epi-cadmium neutron induced fission of $${}^{{\mathbf {240}}}$$Pu

In the epi-cadmium neutron induced fission of $${}^{\text {240}}$$ Pu, cumulative yields of fission products within the mass range of 83–117 and 123–157 as well as independent yields of few fission products have been measured by using an off-line $$\gamma $$ -ray spectrometric technique. The post-neutron mass yields were obtained from the cumulative yields after applying the charge distribution correction. From the mass yields data, the mass distribution parameters like full width at tenth maximum (FWTM) of light and heavy mass wing, the peak-to-valley (P/V) ratio, the average light mass ( $$ $$ ) and heavy mass ( $$ $$ ) as well as the average neutrons number ( $$ $$ ) were obtained. Since, the even–odd effect in between the $${}^{\text {240}}$$ Pu(n, f) and $${}^{\text {241}}\hbox {Pu}(\hbox {n}_{\text {th}}$$ , f) reactions are comparable, the mass yield data in between the two fissioning systems were compared to examine the role of small difference in excitation energy on nuclear structure effect. The role of standard I and standard II asymmetric modes of fission was also examined.

Correlation studies of fission-fragment neutron multiplicities

We calculate neutron multiplicities from fission fragments with specified mass numbers for events having a specified total fragment kinetic energy. The shape evolution from the initial compound nucleus to the scission configurations is obtained with the Metropolis walk method on the five-dimensional potential-energy landscape, calculated with the macroscopic-microscopic method for the three-quadratic-surface shape family. Shape-dependent microscopic level densities are used to guide the random walk, to partition the intrinsic excitation energy between the two proto-fragments at scission, and to determine the spectrum of the neutrons evaporated from the fragments. The contributions to the total excitation energy of the resulting fragments from statistical excitation and shape distortion at scission is studied. Good agreement is obtained with available experimental data on neutron multiplicities in correlation with fission fragments from $^{235}$U(n$_{\rm th}$,f). At higher neutron energies a superlong fission mode appears which affects the dependence of the observables on the total fragment kinetic energy.

Role of higher order deformations in fission fragment mass distribution of Astatine isotopes within collective clusterization approach

The fission fragment mass distribution is used as an investigation tool which assists to disentangle between different modes of fission (symmetric fission and asymmetric fission). In the present work, the fission dynamics of various ‘At*’ isotopes with mass number A=191 to 220 formed in 19F-induced reactions at common centre-of-mass energy Ec.m .=78.5 MeV is explored. The calculations are made within the dynamical cluster-decay model (DCM) using three type of fragmentation such as spherical, (β 2-deformed and (β 2 – β 2-deformed with hot compact optimized orientations. The structural effects come into picture when deformation effects are included in the fragmentation potential. The mass distributions of ‘At*’ isotopes gets significantly modified after inclusion of deformation effects of decaying fragments. The preformation yield shows a drift from symmetric to asymmetric fission with increase in mass of compound nuclei from ACN =191 to 220. The clear signature of fine-structure effects is evident in view of single humped to double humped preformation structure in the fissioning region. However, the mass division of At isotopes is symmetric for spherical choice of fragments. Finally, the analysis of hot (compact) and cold (elongated) configurations of β 2-deformed and oriented fragments is also carried out.

A new radioactive decay mode, true ternary fission, the decay of heavy nuclei into three comparable fragments

The ternary cluster decay of heavy nuclei has been observed in several experiments with binary coincidences between two fragments using detector telescopes (the FOBOS-detectors) with very large solid angles and placed on the opposite sides from the source of fissioning nuclei. The binary coincidences at a relative angle of $$180^{\circ }$$ correspond to binary fission or to the decay into three cluster fragments by registration of two of them in coincidences with missing nuclei of different masses (e.g. $$^{132}\hbox {Sn}$$ , $$^{52-48}\hbox {Ca}$$ , $$^{68-72}\hbox {Ni}$$ ). This marks a new step in the physics of fission-phenomena of heavy nuclei. The new decay mode has been observed more then 10 years ago by the FOBOS group in JINR (Dubna) Russia. These experimental results for the collinear cluster tri-partition, refer to the decay into three clusters of comparable masses. The observation of several other fission modes in the same system can be predicted by the potential energy surface, which show pronounced minima and valleys, they correspond to a variety of collinear ternary fission (multi-modal) decays. In the present work we discuss the various aspects of this ternary fission mode, with different mass partitions. The question of collinearity is analyzed on the basis of recent publications.

Mass-asymmetric fission of Bi205,207,209 at energies close to the fission barrier using proton bombardment of Pb204,206,208

Background: Recent observation of mass-asymmetric fission in neutron-deficient Hg and Pt nuclei has reignited interest in fission fragment mass distributions close to Pb. Investigations at energies close to the fission barrier, where mass-asymmetric fission is expected to be most obvious and the sensitivity to shell effects is maximized, are limited in this mass region.Purpose: To measure fission mass distributions for $^{205,207,209}\mathrm{Bi}$ nuclei at the lowest possible excitation energies to determine how the mass distributions change with excitation energy and the neutron number of the compound nucleus.Method: Proton beams bombarding targets of $^{204,206,208}\mathrm{Pb}$ were used to study the fission of $^{205,207,209}\mathrm{Bi}$ at energies from just above to 10 MeV above their fission barriers. Fission fragments were measured using the CUBE fission spectrometer. Fission fragment mass distributions were determined using a newly developed time difference analysis method. Mass distributions were characterized by triple-Gaussian fits to determine the systematic trends across each isotope with excitation energy.Results: Measured mass distributions of all three Bi isotopes exhibit a component of mass-asymmetric fission at all energies studied. The probability of mass-asymmetric fission decreases significantly with increasing excitation energy, from $\ensuremath{\approx}70$ to $\ensuremath{\approx}40%$ over a 10-MeV range. Comparisons between the three Bi isotopes hint at an increase in the mass-symmetric fission yield with increasing neutron number, which could be due to a decrease in the difference between the symmetric and asymmetric fission barriers. The centroids of the mass-asymmetric peaks suggest that several deformed shell gaps in the fission fragments could be contributing to the presence of the mass-asymmetric fission mode with ${Z}_{\mathrm{light}}\ensuremath{\simeq}38$, ${Z}_{\mathrm{heavy}}\ensuremath{\simeq}45$, and ${N}_{\mathrm{light}}\ensuremath{\simeq}56$ all present in the fission fragments.Conclusions: Measurements of fission mass distributions at the lowest possible excitation energies above the fission barrier provide an excellent platform to investigate the origins of the mass-asymmetric fission mode. Further systematic measurements at these energies offer an opportunity to rigorously test new models of fission in this mass region.

Systematics of the mass-asymmetric fission of excited nuclei from 176Os to 206Pb

The competition between the dominant mass-asymmetric and rarer narrow mass-symmetric fission modes in actinide nuclei are controlled by deformed and spherical shell effects. The low energy fission of 80180Hg was recently observed to be strongly mass-asymmetric, indicating that despite spherical shell gaps in fragments around 4090Zr, the system does not fission mass-symmetrically. Several theoretical approaches have been used to explain this unexpected result.To investigate the underlying mechanism, systematic measurements of fission mass distributions for isotopes of Os, Pt, Hg and Pb, formed in fusion reactions with p, 12C, 32S, 40,48Ca projectiles, have been made for excitation energies above the fission saddle-point (Eeff⁎) between 2.8 and 28.2 MeV. Evidence for mass-asymmetric fission is widespread, manifested as flat topped mass distributions or significant deviations from a single Gaussian shape. The systematic trends seen cannot be attributed to quasifission. Comparing two-Gaussian fits at a wide range of E⁎, it is concluded that the fit centroids reflect the low energy character of mass-asymmetric fission in the sub-lead region.Quantitative comparisons were made with microscopic calculations by Scamps and Simenel (2019) [33] of fission mass-asymmetries attributed to the influence of shell gaps in both neutrons (N=52, 56 for compact octuple deformations) and protons (Z=34 and Z=42, 44, 46 with large quadrupole deformations). For the predominant fission mode in the calculations, having one elongated and one compact fragment, the results are in extremely good agreement with all experimental values. This provides strong support for both the calculations, and the exploration of mass-asymmetric fission systematics through heavy ion fusion reactions. The total kinetic energy distributions for 176Pt and 180Pt do not show any evidence of a low TKE mass-symmetric fission mode, as had been reported for 178Pt by Tsekhanovich et al. (2019) [39].

Study of Mass-Asymmetric Fission of 180,190Hg Formed in the 36Ar + 144,154Sm Reactions

t—The mass–energy distributions of fission fragments of excited 180,190Hg nuclei formed in 36Ar + 144,154Sm reactions are measured at incident 36Ar energies of 158, 181, and 222 MeV using the double-arm time-of-flight spectrometer CORSET. The asymmetric fission of 180,190Hg with the most probable masses of light and heavy fragments of 79 and 101 amu, and 84 and 106 amu, respectively, is observed in mass distributions of 180,190Hg at energies of excitation of up to 75 MeV. Two components manifesting the symmetric and asymmetric fission modes are observed in the kinetic energy distributions.

Experimental cross sections and mass distribution of fission products of thorium-232 irradiated with protons in energy range 20–140 MeV

Proton-induced fission of 232Th was studied in a wide proton energy range 20–140 MeV. A large amount of experimental cross sections for individual and cumulative formation of 232Th fission products were determined by means of stacked-foil irradiation followed by high-resolution γ-ray spectrometry. The obtained results were compared with available literature data and theoretical calculations performed with cascade–evaporation–fission model. The fission-product mass distribution in the given proton energy range was described with a three-peak curve representing asymmetric and symmetric fission modes. The fraction of symmetric fission mode increased along with the incident proton energy up to 60–70 MeV reaching a plateau at the value around 0.65.

Anomalous neutron yields confirmed for Ba-Mo and newly observed for Ce-Zr from spontaneous fission of Cf252

We reinvestigated the neutron multiplicity yields of Ba-Mo, Ce-Zr, Te-Pd, and Nd-Sr from the spontaneous fission of $^{252}$Cf; by (i) using both $\gamma$-$\gamma$-$\gamma$-$\gamma$ and $\gamma$-$\gamma$-$\gamma$ coincidence data, (ii) using up to date level scheme structures, and (iii) cross-checking analogous energy transitions in multiple isotopes, we have achieved higher precision than previous analyses. Particular attention was given to the Ba-Mo pairs where our results clearly confirm that the Ba-Mo yield data have a second hot fission mode where 8, 9, 10, and now 11 neutron evaporation channels are observed. These are the first observations of the 11 neutron channel. These 8-11 neutron channels are observed for the first time in the Ce-Zr pairs, but are not observed in other fission pairs. The measured intensities of the second mode in Ba-Mo and Ce-Zr pairs are $\sim$1.5(4)$\%$ and $\sim$1.0(3)$\%$, respectively. These high neutron number evaporation modes can be an indication of hyperdeformation and/or octupole deformation in $^{143-145}$Ba and in $^{146,148}$Ce at scission to give rise to such high neutron multiplicities.

Origin of the Dramatic Change of Fission Mode in Fermium Isotope Investigated Using Langevin Equations

Origin of the Dramatic Change of Fission Mode in Fermium Isotope Investigated Using Langevin Equations

Correlated transitions in symmetric and dominant fission modes, and multipole moments of fission fragments

Systematic and anomalous trends in fragment mass and TKE (total kinetic energy) distributions are investigated in terms of 4D Langevin model developed at Tokyo Tech. We have found that correlated transitions in symmetric components and dominant modes (symmetric v.s. asymmetric) can explain the prominent systematic and anomalous features of fission observables. We have also elucidated that interplay between spherical and deformed magicity at A=132 and A=142 to 144 is important in both observables.

Yields distribution of induced fission with improved scission point model

The charge distributions from induced fission of nuclei 230–234U, 206–212Fr, 214–226Ac and 220–229Th are determined by an improved scission point model. The calculated results can reasonably reproduce the experimental trends, and the asymmetric and symmetric fission modes are anticipated. The order of magnitude of the yield’s distribution is reproduced especially well for the U isotope. The semi-empirical parameters in the scission point model associated with the nuclear structure that can be changed to improve the model. We studied the influences of damping constant ED on the shell correction and the level density parameter a on the calculation results.

Competition between binary fission, ternary fission, cluster radioactivity and alpha decay of 281Ds

The competition between different decay modes such as binary fission, ternary fission, cluster and alpha decay modes finds an important role in the synthesis of superheavy nuclei. The nuclei 281Ds is the most stable among the isotopes of Ds. We have studied the different decay modes in superheavy nuclei 281Ds. It is observed that alpha decay half-lives are smaller compared to other modes. The alpha decay mode of 281Ds is observed with greater probability compared to binary fission.

Role of quantum phase transition in spontaneous fission

A systematic study of quantum phase (i.e., nuclear shape) transition has been carried out from the standpoint of spontaneous fission. The quantum phase transition is defined by the ratio of relative excitation energies of IΠ=4+ to IΠ=2+ states between heavier and lighter fission fragments and its extremum value fixes the most probable pair of fission fragments. This ratio provides an effective order parameter which is not only easy to measure, but also distinguishes between first and second order phase transitions and takes on a special value in the critical region. Its (i.e., the energy ratio) variation is mainly controlled by the relative inertia tensors of the correlated pair of fission fragment and is fully supported by our cranking mass parameter calculations based on the asymmetric two center shell model bases.Once the fission fragment pair is governed by the quantum phase transition, its validity is tested by F-spin selection rule that is based on valence nucleons which are responsible for determining the nuclear shape transition. It implies that if a pair of conjugate fission fragments together with appropriate neutron emission have total F-spin equal to 12 with projections F0=±12 then and only then the fission proceeds, provided both the valence shell nucleons in either of the fission fragments are treated as particles like and is valid for the even-even decay modes. The goodness of F-spin selection rule extends further to extract all the allowed pairs of neutron-rich fission fragments. The validity of F-spin selection rule has been tested in various decay modes of spontaneous fission and seems to be a milestone in deciding the appropriate decay channels. These findings have important consequences in the formation and discovery of drip-line exotic nuclei.