Using a unique two-arm detector system for heavy ions (the BRS, binary reaction spectrometer) coincident fission events have been measured from the decay of $^{60}$Zn compound nuclei formed at 88MeV excitation energy in the reactions with $^{36}$Ar beams on a $^{24}$Mg target at $E_{lab}(^{36}$Ar) = 195 MeV. The detectors consisted of two large area position sensitive (x,y) gas telescopes with Bragg-ionization chambers. From the binary coincidences in the two detectors inclusive and exclusive cross sections for fission channels with differing losses of charge were obtained. Narrow out-of-plane correlations corresponding to coplanar decay are observed for two fragments emitted in binary events, and in the data for ternary decay with missing charges from 4 up to 8. After subtraction of broad components these narrow correlations are interpreted as a ternary fission process at high angular momentum through an elongated shape. The lighter mass in the neck region consists dominantly of two or three-particles. Differential cross sections for the different mass splits for binary and ternary fission are presented. The relative yields of the binary and ternary events are explained using the statistical model based on the extended Hauser-Feshbach formalism for compound nucleus decay. The ternary fission process can be described by the decay of hyper-deformed states with angular momentum around 45-52 $hbar$.
This paper presents a selective historical perspective of fission research over the last thirty-five years while Ray Nix has made central contributions to the field. The emphasis is placed on early studies of the shell stabilized secondary minimum in the static fission barrier and on the dynamic properties of fission of hot nuclei, which have recently been the focus of intense study.
Isotopic series of 58 neutron-deficient secondary projectiles (205,206At, 205-209Rn, 208-212,217,218Fr, 211-223Ra, 215-226Ac, 221-229Th, 226-231Pa, 231-234U) were produced by projectile fragmentation using a 1 A GeV 238U beam. Cross sections of fission induced by nuclear and electromagnetic interactions in a secondary lead target were measured. They were found to vary smoothly as a function of proton and neutron number of the fissioning system, also for nuclei with large ground-state shell effects near the 126-neutron shell. No stabilization against fission was observed for these nuclei at low…
We report on a novel experimental approach for studying the dissipative spreading of collective motion in a meta-stable nuclear system, using, for the first time, highly fissile nuclei with spherical shape. This was achieved by fragmentation of 45 radioactive heavy-ion beams at GSI, Darmstadt. The use of inverse kinematics and a dedicated experimental set-up allowed for the identification in atomic number of both fission fragments. From the width of their charge distributions, a transient time of (3.3 +/- 0.7) 10-21 s is deduced for initially spherical nuclei.
A study of photofission on 181Ta nucleus induced by bremsstrahlung photons with endpoint energies of 50 and 3500 MeV has been performed. The fission yields have been measured by using the induced-activity method in an off-line analysis. The absolute photofission cross sections for the tantalum target at 50 and 3500 MeV are found to be 5.4 \pm 1.1 microb and 0.77\pm0.11 mb, respectively, and the corresponding deduced fissilities are (0.23\pm0.05) x 10^{-3} and (2.9 \pm 0.9) x 10^{-3}. Mass- and charge-yield distributions were derived from the data. The results were compared with the simulated r…
The Neutron Induced Fission Fragment Tracking Experiment (NIFFTE) collaboration has performed measurements with a fission time projection chamber (fissionTPC) to study the fission process by reconstructing full three-dimensional tracks of fission fragments and other ionizing radiation. The amount of linear momentum imparted to the fissioning nucleus by the incident neutron can be inferred by measuring the opening angle between the fission fragments. Using this measured linear momentum, fission fragment angular distributions can be converted to the center-of-mass frame for anisotropy measuremen…
Fission fragment angular distributions can provide an important constraint on fission theory, improving predictive fission codes, and are a prerequisite for a precise ratio cross section measurement. Available anisotropy data is sparse, especially at neutron energies above 5 MeV. For the first time, a three-dimensional tracking detector is employed to study fragment emission angles and provide a direct measurement of angular anisotropy. The Neutron Induced Fission Fragment Tracking Experiment (NIFFTE) collaboration has deployed the fission time projection chamber (fissionTPC) to measure nuclea…
Collisions between $^{248}$Cm and $^{48}$Ca are systematically investigated by time-dependent density functional calculations with evaporation prescription. Depending on the incident energy and impact parameter, fusion, deep-inelastic and quasi-fission events are expected to appear. In this paper, possible fission dynamics of compound nuclei is presented.
Multi-pronged tracks have been recorded in the polyethylene terephthalate (C$_{10}$H$_8$O$_4$)$_n$ solid state nuclear track detector by exposure to a $^{252}$Cf fission source. After chemical etching, two-prong to six-pronged tracks along with single tracks have been observed under the optical microscope. We carried out a systematic study to understand the origin of the prongs. The track detectors were coated with metals (Cu, Ag and Pb) and were exposed to $^{252}$Cf source. After chemical etching two-prong to four pronged tracks were observed in each plate. We believe that at this very low e…
The thesis presents investigations on the angular and mass distribution of fission fragments on heavy ion induced fission reactions. The present investigations address current issues in heavy ion induced fission reactions like finding the optimum entrance channel for the synthesis of super heavy elements (SHE). A double arm time of flight spectrometer over long flight path was used to measure the precise masses of complementary fission fragments. Necessary large area position sensitive gas detectors, the method of experiments and data analysis were developed. The first string of measurements w…
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 in…
Fission-fragment mass distributions were measured for 225,227Pa nuclei formed in fusion reactions of 19F + 206, 208Pb around fusion barrier energies. Mass-angle correlations do not indicate any quasi-fission like events in this bombarding energy range. Mass distributions were fitted by Gaussian distribution and mass variance extracted. At below-barrier energies, the mass variance was found to increase with decrease in energy for both nuclei. Results from present work were compared with existing data for induced fission of 224, 226Th and 228U around barrier energies. Enhancement in mass varianc…
A new statistical method for systematizing the nuclei fission fragments, investigation of their mass and charge spectra and neutron fission parameters has been suggested. In proposed method, the mass and charge yields of nucleons are determined from the equilibrium conditions of the nuclear fragments post-fission ensemble. Here we consider protons and neutrons of different fragments as statistically non-equivalent. The abilities of the proposed statistical method for explanation of the U, Pa, Th, Ac and Ra post-fission fragments ordering and neutron emission function calculation have been demo…
Peripheral collisions with radioactive heavy-ion beams at relativistic energies are discussed as an innovative approach for probing the transient regime experienced by fissile systems evolving towards quasi-equilibrium. A dedicated experiment using the advanced technical installations of GSI, Darmstadt, permitted to realize ideal conditions for the investigation of relaxation effects in the meta-stable well. Combined with a highly sensitive experimental signature, it provides a measure of the transient effects with respect to the flux over the fission barrier. Within a two-step reaction proces…
The mass distribution of fission fragments of actinide and superheavy nuclei can be explained if a new state of nuclear matter, a nucleon phase, is created in any fission event.
We employ density-dependent relativistic mean-field theory to study how the triaxiality and pairing interaction affect the inner fission barriers of actinide nuclei. It was found that triaxiality reduced the inner fission barriers and improved agreement with experimental values for many actinides. However, about 1-2 MeV discrepancy to the experimental values still remained for some of the considered nuclei. Such a discrepancy could be made further smaller by increasing the BCS pairing strength parameter. In this work, we demonstrated that adjusting the paring strength was effective to reproduc…
In this work we present an analysis of the yields of fission fragments induced by bremsstrahlung photons with endpoint energies of 50 and 3500 MeV on 232Th and 238U targets using the simulation code CRISP. A multimodal fission option was added to this code and an extension of the calculation to the properties of the fission products is presented. By dividing the fissioning nuclei according to their fissionability, an approach is introduced which accounts for the contribution of symmetric and asymmetric fission. By adopting this procedure, it was possible to calculate the main parameters for th…
A new, rapidly convergent Fourier over spheroid parametrization is developed to describe the shape of a fissioning nucleus: its elongation, non-axiality and left-right asymmetry and neck formation. The 4D Potential Energy Surfaces (PES) of even-even actinide nuclei are evaluated within the macro-micro model. The Langevin trajectories generated on such PESs allow for obtaining the fission fragments' mass and kinetic energy yields. The charge equilibration at the scission configuration and the post-scission neutron emission are also discussed.
We propose to produce neutron-rich nuclei in the range of the astrophysical r-process around the waiting point N=126 by fissioning a dense laser-accelerated thorium ion bunch in a thorium target (covered by a CH2 layer), where the light fission fragments of the beam fuse with the light fission fragments of the target. Via the 'hole-boring' mode of laser Radiation Pressure Acceleration using a high-intensity, short pulse laser, very efficiently bunches of 232Th with solid-state density can be generated from a Th layer, placed beneath a deuterated polyethylene foil, both forming the production t…
An analysis of experimental data from the inverse-kinematics ISODEC experiment on 78Kr+40Ca reaction at a bombarding energy of 10 AMeV has revealed signatures of a hitherto unknown reaction mechanism, intermediate between the classical damped binary collisions and fusion-fission, but also substantially different from what is being termed in the literature as fast fission or quasi fission. These signatures point to a scenario where the system fuses transiently while virtually equilibrating mass asymmetry and energy and, yet, keeping part of the energy stored in a collective shock-imparted and, …
We have studied strength in the BCS pairing force, used as a residual interaction to the relativistic mean-field to reproduce the height of the inner fission barriers for actinide nuclei. It was found that increasing the pairing strength by about 13% makes reproduce the inner fission barriers better over a wide range of actinide nuclei. This result was verified by using the moment-of-inertia of pairing rotational energy, which was introduced to avoid mean-field and odd-mass effects in the pairing interaction to deduce purely the pairing strength. The pairing interaction thus determined could s…
A recent paper has reported the observation of the rotational band of 254No for spins up to I=20, showing that the compound nucleus was formed and survived fission decay at angular momenta I >= 20. We show that this survival is consistent with the leading effects of angular momentum on the barrier height.
The fragment yield ratios were investigated in the fission of 238,233U targets induced by 14 MeV neutrons. The isoscaling behavior was typically observed for the isotopic chains of fragments ranging from the proton-rich to the most neutron-rich ones. The observed high sensitivity of neutron-rich heavy fragments to the target neutron content suggests fission as a source of neutron-rich heavy nuclei for present and future rare ion beam facilities, allowing studies of nuclear properties towards the neutron drip-line and investigations of the conditions for nucleosynthesis of heavy nuclei. The bre…
In highly dissipative collisions between heavy ions, the optimal conditions to investigate different de-excitation channels of hot nuclei such as evaporation, fission or multifragmentation are well known. One crucial issue remains the excitation energy region where fission gives way to multifragmentation. In this paper, the onset of multi-fragment exit channels is investigated in terms of sequential fission. For the first time, the dynamical approach based on solving Langevin transport equations in multidimensional collective coordinate space is used to follow the de-excitation of highly excit…
An analysis of the $^{231}$Pa$(d,3n)$$^{230}$U reaction excitation function at energies around the Coulomb barrier has taken into account the pre-equilibrium and compound-nucleus cross sections corrected for the deuteron-breakup decrease of the total reaction cross section, as well as the inelastic breakup enhancement. The analysis reveals the dominance of the deuteron breakup mechanism unlike a former assessment in this respect of the deuteron-induced fission process.
Fission of atomic nuclei often produces mass asymmetric fragments. However, the origin of this asymmetry was believed to be different in actinides and in the sub-lead region [A. Andreyev {\it et al.}, Phys. Rev. Lett. {\bf 105}, 252502 (2010)]. It has recently been argued that quantum shell effects stabilising pear shapes of the fission fragments could explain the observed asymmetries in fission of actinides[G. Scamps and C. Simenel, Nature {\bf 564}, 382 (2018)]. This interpretation is tested in the sub-lead region using microscopic mean-field calculations of fission based on the Hartree-Fock…
Fission of $^{180}$Hg produces mass asymmetric fragments which are expected to be influenced by deformed shell-effects at N=56 in the heavy fragment and Z=34 in the light fragment [G. Scamps and C. Simenel, arXiv:1904.01275 (2019)]. To investigate both shell-effects and to determine which one has the main influence on the asymmetry in the region of the $^{180}$Hg, we produce a systematic of Constraint-Hartree-Fock calculations in nuclei with similar N/Z ratio than the $^{178}$Pt. It is found that N=56 determines the asymmetry of systems in this region of the nuclear chart.
Dissipative dynamics of nuclear fission is a well confirmed phenomenon described either by a Kramers-modified statistical model or by a dynamical model employing the Langevin equation. Though dynamical models as well as statistical models incorporating fission delay are found to explain the measured fission observables in many studies, it nonetheless shows conflicting results for shell closed nuclei in the mass region 200. Analysis of recent data for neutron shell closed nuclei in excitation energy range 40$-$80 MeV failed to arrive at a satisfactory description of the data and attributed the …
The nuclear fission of very neuron-rich nuclei related to the r-process is essential for the termination of nucleosynthesis flows on the nuclear chart and the final abundances. Nevertheless, most of the available fission data for the r-process calculations are based on theory predictions, including phenomenological treatments. In this study, we calculated a series of nuclear fission distribution for neutron-rich nuclei away from the beta-stability line. As most of these nuclei are experimentally unknown, we are based on theoretical calculations based on the dynamical fission model with the Lan…
The ternary cluster decay of heavy nuclei has been observed in several experiments with binary coincidences between two fragments using detector telescopes (the FOBOS-detectors, JINR, Dubna) placed on the opposite sides from the source of fissioning nuclei. The binary coincidences at a relative angle of 180$^0$ deg. correspond to binary fission or to the decay into three cluster fragments by registration of two nuclei with different masses (e.g.$^{132}$Sn,$^{52-48}$Ca,$^{68-72}$Ni). This marks a new step in the physics of fission-phenomena of heavy nuclei. These experimental results for the co…
Thermal multifragmentation of hot nuclei is interpreted as the nuclear liquid--fog phase transition deep inside the spinodal region. The experimental data for p(8.1GeV) + Au collisions are analyzed. It is concluded that the decay process of hot nuclei is characterized by two size parameters: transition state and freeze-out volumes. The similarity between dynamics of fragmentation and ordinary fission is discussed. The IMF emission time is related to the mean rupture time at the multi-scission point, which corresponds to the kinetic freeze-out configuration.
Measurement of the $^{236}$U fission cross section and angular distributions of fragments from fission of $^{235}$U and $^{236}$U in the neutron energy range of 0.3-500 MeV
The role of spherical quantum shells in the competition between fusion and quasi-fission is studied for reactions forming heavy elements. Measurements of fission fragment mass distributions for different reactions leading to similar compound nuclei have been made near the fusion barrier. In general, more quasi-fission is observed for reactions with non-magic nuclei. However, the $^{40}$Ca+$^{208}$Pb reaction is an exception, showing strong evidence for quasi-fission, though both nuclei are doubly magic. Time-dependent Hartree-Fock calculations predict fast equilibration of $N/Z$ in the two fra…
The generation of fission fragments (FF) spins and of their relative orbital angular momentum has been debated for more than six decades and no consensus has been yet achieved so far. (The published abstract, being too long, can be found in the manuscript.)
In recent years, investigations of angular distributions of fragments in neutron-induced nuclear fission have been extended to intermediate energies, up to 200 MeV, as well as to a wide range of target isotopes. Using as an example the latest data obtained by our group for the reaction 237-Np(n,f), we discuss the specific features of fission fragment angular distribution and present a method for their simulation based on the code TALYS. It is shown that a simplified model reasonably describes energy dependence of the angular distribution in the whole range 1-200 MeV. The ways to improve the mo…
Background: Multi-nucleon transfer reactions have recently attracted attention as a possible path to the synthesis of new neutron-rich heavy nuclei. Purpose: We study transfer reactions involving massive nuclei with the intention of understanding if the semi-classical model GRAZING coupled to an evaporation and fission competition model can satisfactory reproduce experimental data on transfer reactions in which fission plays a role. Methods: We have taken the computer code GRAZING and have added fission competition to it (GRAZING-F) using our current understanding of $Γ_n/Γ_f$, fission barrier…
To disentangle the role of shell effects and dynamics, fission fragment mass distributions of $^{191}$Au, a nucleus in the newly identified island of mass asymmetric fission in the sub-lead region, have been measured down to excitation energy of $\approx$20 MeV above the fission barrier via two different entrance channels, viz. $^{16}$O+$^{175}$Lu and $^{37}$Cl+$^{154}$Sm reactions. Apart from having signature of the shell effects in both the cases, clear experimental evidence of quasifission has been observed in the mass distributions of the Cl induced reaction, that has also been substantiat…
When two fragments are created in a fission decay, any torque due to nuclear and Coulomb interaction can change the fragment's angular momentum. This article explores the character and magnitude of the angular momentum as a function of the initial conditions around the scission point using the time-dependent Hartree-Fock theory. To understand the torque acting on the fragments, the Frozen Hartree-Fock method is also used to determine the collective potential at scission. Two $^{240}$Pu fission channel ( $^{132}$Sn+$^{108}$Ru and $^{144}$Ba+$^{96}$Sr ) are studied. These two channels cover diff…
In this work we present the results for the investigation of intermediate-mass fragment (IMF) production with the proton-induced reaction at 660 MeV on 238U and 237Np target. The data were obtained with the LNR Phasotron U-400M Cyclotron at Joint Institute for Nuclear Research (JINR), Dubna, Russia. A total of 93 isotopes, in the mass range of 30 < A < 200, were unambiguously identified with high precision. The fragment production cross sections were obtained by means of the induced-activation method in an off-line analysis. Mass-yield distributions were derived from the data and compared with…
Several sources of angular anisotropy for fission fragments and prompt neutrons have been studied in neutron-induced fission reactions. These include kinematic recoils of the target from the incident neutron beam and the fragments from the emission of the prompt neutrons, preferential directions of the emission of the fission fragments with respect to the beam axis due to the population of particular transition states at the fission barrier, and forward-peaked angular distributions of pre-equilibrium neutrons which are emitted before the formation of a compound nucleus. In addition, there are …
Fission mode identification in the 180Hg region: derivative analysis approach
University of Bordeaux, CNRS, LP2I Bordeaux, UMR 5797（波尔多大学）；Department of Physics, University of York（约克大学物理系）；Advanced Science Research Center, JAEA（日本原子力研究开发机构先进科学研究中心）
Beta-delayed neutron emission and $β$-delayed fission ($β$df) probabilities were calculated for heavy, neutron-rich nuclei using the Los Alamos coupled Quasi-Particle Random Phase Approximation plus Hauser-Feshbach (QRPA+HF) approach. In this model, the compound nucleus is initially populated by $β$-decay and is followed through subsequent statistical decays taking into account competition between neutrons, $γ$-rays and fission. The primary output of these calculations includes branching ratios along with neutron and $γ$-ray spectra. We find a relatively large region of heavy nuclides where th…
The predictions on the mode of decay of the odd-even and odd-odd isotopes of heavy and superheavy nuclei with Z = 99-129, in the range 228 \leg A \leg 336, have been done within the Coulomb and proximity potential model for deformed nuclei (CPPMDN). A comparison of our calculated alpha half lives with the values computed using other theoretical models shows good agreement with each other. An extensive study on the spontaneous fission half lives of all the isotopes under study has been performed to identify the long-lived isotopes in the mass region. The study reveals that the alpha decay half …
The angular distributions of fission fragments for the $^{32}$S+$^{184}$W reaction at center-of-mass energies of 118.8, 123.1, 127.3, 131.5, 135.8, 141.1 and 144.4 MeV were measured. The experimental fission excitation function is obtained. The fragment angular anisotropy ($\mathcal{A}_{\rm exp}$) is found by extrapolating the each fission angular distributions. The measured fission cross sections of the $^{32}$S+$^{182,184}$W reaction are decomposed into fusion-fission, quasifission and fast fission contributions by the dinuclear system model. The total evaporation residue excitation function…
Experimental studies of fission induced in relativistic nuclear collisions show a systematic enhancement of the excitation energy of the primary fragments by a factor of ~ 2, before their decay by fission and other secondary fragments. Although it is widely accepted that by doubling the energies of the single-particle states may yield a better agreement with fission data, it does not prove fully successful, since it is not able to explain yields for light and intermediate mass fragments. State-of-the-art calculations are successful to describe the overall shape of the mass distribution of frag…
Fission and gamma-emission probabilities induced by transfer or inelastic scattering reactions with light projectile nuclei are very valuable quantities for constraining the models that describe the de-excitation of heavy nuclei. We have developed an experimental set-up that allows us to simultaneously measure fission and gamma-emission probabilities. The measurement of the gamma-emission probability at excitation energies where the fission channel is open is challenging due to the intense background of gamma rays emitted by the fission fragments. We discuss the procedure to subtract such a ba…
We investigate the angular momentum removal from fission fragments (FFs) through neutron and $γ$-ray emission, where we find that about half the neutrons are emitted with angular momenta $\ge 1.5\hbar$ and that the change in angular momentum after the emission of neutrons and statistical $γ$ rays is significant, contradicting usual assumptions. Per fission event, in our simulations, the neutron and statistical $γ$-ray emissions change the spin of the fragment by 3.5 -- 5~$\hbar$, with a large standard deviation comparable to the average value. Such wide angular momentum removal distributions c…
MCNP6 has been Validated and Verified (V&V) against intermediate- and high-energy fission cross-section experimental data. An error in the calculation of fission cross sections of 181Ta and a few nearby target nuclei by the CEM03.03 event generator in MCNP6 and a "bug: in the calculation of fission cross sections with the GENXS option of MCNP6 while using the LAQGSM03.03 event generator were detected during our V&V work. After fixing both problems, we find that MCNP6 using CEM03.03 and LAQGSM03.03 calculates fission cross sections in good agreement with available experimental data for reaction…
Neutron star (NS) merger ejecta offer a viable site for the production of heavy r-process elements with nuclear mass numbers A > 140. The crucial role of fission recycling is responsible for the robustness of this site against many astrophysical uncertainties, but calculations sensitively depend on nuclear physics. In particular the fission fragment yields determine the creation of 110 < A < 170 nuclei. Here we apply a new scission-point model, called SPY, to derive the fission fragment distribution (FFD) of all relevant neutron-rich, fissioning nuclei. The model predicts a doubly asymmetric F…
Lifetimes of super-heavy (SH) nuclei are primarily governed by alpha decay and spontaneous fission (SF). Here we study the competing decay modes of even-even SH isotopes with 108 <= Z <= 126 and 148 <= N <= 188 using the state-of-the-art self-consistent nuclear density functional theory framework capable of describing the competition between nuclear attraction and electrostatic repulsion. The collective mass tensor of the fissioning superfluid nucleus is computed by means of the cranking approximation to the adiabatic time-dependent Hartree-Fock-Bogoliubov approach. Along the path to fission, …
In this work we try to settle down the controversial predictions on the effect of doubly magic nuclei $^{132}$Sn and $^{208}$Pb on the mass distributions of fission fragments of super-heavy nuclei. For this we have calculated the mass distribution of super-heavy nuclei from $^{286}$Cn to $^{306}$122 within the dynamical 4-dimensional Langevin approach. We have found that in "light" super-heavies the influence of $^{208}$Pb on the mass distributions is present but negligible small. In "heavy" super-heavies, Z=120-122, the (quasi)symmetric peaks and strongly asymmetric peaks at fragment mass $A_…
