Quasifission Process Research Articles

Mechanisms Suppressing Superheavy Element Yields in Cold Fusion Reactions.

Superheavy elements are formed in fusion reactions which are hindered by fast nonequilibrium processes. To quantify these, mass-angle distributions and cross sections have been measured, at beam energies from below-barrier to 25% above, for the reactions of ^{48}Ca, ^{50}Ti, and ^{54}Cr with ^{208}Pb. Moving from ^{48}Ca to ^{54}Cr leads to a drastic fall in the symmetric fission yield, which is reflected in the measured mass-angle distribution by the presence of competing fast nonequilibrium deep inelastic and quasifission processes. These are responsible for reduction of the compound nucleus formation probablity P_{CN} (as measured by the symmetric-peaked fission cross section), by a factor of 2.5 for ^{50}Ti and 15 for ^{54}Cr in comparison to ^{48}Ca. The energy dependence of P_{CN} indicates that cold fusion reactions (involving ^{208}Pb) are not driven by a diffusion process.

Entrance channel effects on the quasifission reaction channel in Cr + W systems

Background: Formation of a fully equilibrated compound nucleus is a critical step in the heavy-ion fusion reaction mechanism but can be hindered by orders of magnitude by quasifission, a process in which the dinuclear system breaks apart prior to full equilibration. To provide a complete description of heavy-ion fusion it is important to characterize the quasifission process. In particular, the impact of changing the neutron-richness of the quasifission process is not well known. A previous study of Cr + W reactions at a constant 13 % above the Coulomb barrier concluded that an increase in neutron-richness leads to a decrease in the prominence of the quasifission reaction channel. Purpose: The interplay between the fusion-fission and quasifission reaction channels, with varying neutron-richness, was explored at a constant excitation energy, closer to the interaction barrier than the previous work, to see if the correlation between neutron-richness and quasifission is valid at lower energies. Methods: Mass distributions were determined for eight different combinations of Cr + W reactions at the Australian National University at 52.0 MeV of excitation energy in the compound nucleus. Results: A curvature parameter was determined for the fission-like fragment mass distributions and compared to various reaction parameters known to influence quasifission. Conclusions: The present work demonstrates that at energies near the interaction barrier the deformation effects dominate over the neutron-richness effects in the competition between quasifission and compound nucleus formation in these Cr + W systems and is an important consideration for future with heavy and superheavy element production reactions.

Role of quasifission in fission fragment mass distributions for the Si28+Au197 reaction

Fission fragment mass distribution has been measured for $^{28}\mathrm{Si}+^{197}\mathrm{Au}$ reaction at bombarding energies around the Coulomb barrier (${V}_{b}=126.5\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$). The experimental variance of the mass distribution shows a sudden change from monotonic decrease to a saturation at energies close to the Coulomb barrier and again a decreasing trend at sub-barrier energies. The variance of the mass distribution was also obtained from dynamical and statistical model calculations for the fission of a fully equilibrated compound nucleus. Both of the theoretical calculations show a monotonic change of the mass variance around the Coulomb barrier. The anomalous dependence of fission fragment mass width at energies around the barrier may be due to the dominance of the quasifission process as compared to compound nuclear fission.

Possibilities for synthesis of new neutron-deficient isotopes of superheavy nuclei * * Supported by the National Natural Science Foundation of China (11705055), Hunan Provincial Natural Science Foundation of China (2018JJ3324) and Excellent Youth Fund of Hunan Provincial Education Department (17B154)

This study investigates the optimal projectile/target combination for the production of new neutron-deficient isotopes of superheavy nuclei (SHN). To this end, the dependence of the evaporation residue cross-section (ERCS) used to synthesize SHN on the mass asymmetry and the isospin of colliding nuclei are analyzed within the dinuclear system (DNS) concept. The predicted ERCSs for the production of new neutron-deficient isotopes of SHN were found to be quite large with the 36S projectile, and the cross-section of SHN decreases slowly with the charge of compound nuclei owing to the increase in their survival probability, . Wsur is not canceled by the decreasing probability, PCN, that the system will evolve from a touching configuration to the compound nucleus in competition with the quasifission process.

Fusion probability of massive nuclei in reactions leading to heavy composite nuclear systems

Reactions between massive nuclei show a considerable reduction in fusion-evaporation cross-sections at the Coulomb barrier according to the comparison of experimental values with those calculated by barrier passing (BP) and statistical model (SM) approximations. Reduced fusion cross-sections corresponding to fusion probability PCN<1 are accompanied by a high probability of deep-inelastic and quasi-fission processes arising on the way to fusion. At the same time, the excitation functions for evaporation residues (ERs) obtained in very mass-asymmetric projectile-target combinations are well described in the framework of the BP model (assuming PCN=1) and SM approximations. In the framework of SM, the survivability of produced heavy nuclei can be described with the use of adjusted macroscopic fission barriers. Fusion suppression appears in less asymmetric combinations, for which PCN values can be estimated using survivability obtained for very asymmetric ones leading to the same CN. An attempt was made to systemize the PCN data derived from different projectile-target combinations leading to ERs in the range from Pb to the most heavies, which are compared withPCN values obtained in fission experiments.

Connecting the nuclear equation of state to the interplay between fusion and quasifission processes in low-energy nuclear reactions

Within the Time Dependent Hartree Fock (TDHF) approach, we investigate the impact of several ingredients of the nuclear effective interaction, such as incompressibility, symmetry energy, effective mass, derivative of the Lane potential and surface terms on the exit channel (fusion vs quasifission) observed in the reaction $^{238}$U+$^{40}$Ca, close to the Coulomb barrier. Our results show that all the ingredients listed above contribute to the competition between fusion and quasifission processes, however the leading role in determining the outcome of the reaction is played by incompressibility, symmetry energy and the isoscalar coefficient of the surface term. This study unravels the complexity of the fusion and quasifission reaction dynamics and helps to understand the microscopic processes responsible for the final outcome of low energy heavy ion collisions in terms of relevant features of the nuclear effective interaction and associated equation of state (EoS).

Sub-barrier quasifission in heavy element formation reactions with deformed actinide target nuclei

Background: The formation of superheavy elements (SHEs) by fusion of two massive nuclei is severely inhibited by the competing quasifission process. Low excitation energies favor SHE survival against fusion-fission competition. In ``cold'' fusion with spherical target nuclei near $^{208}\mathrm{Pb}$, SHE yields are largest at beam energies significantly below the average capture barrier. In ``hot'' fusion with statically deformed actinide nuclei, this is not the case. Here the elongated deformation-aligned configurations in sub-barrier capture reactions inhibits fusion (formation of a compact compound nucleus), instead favoring rapid reseparation through quasifission.Purpose: To determine the probabilities of fast and slow quasifission in reactions with prolate statically deformed actinide nuclei, through measurement and quantitative analysis of the dependence of quasifission characteristics at beam energies spanning the average capture barrier energy.Methods: The Australian National University Heavy Ion Accelerator Facility and CUBE fission spectrometer have been used to measure fission and quasifission mass and angle distributions for reactions with projectiles from C to S, bombarding Th and U target nuclei.Results: Mass-asymmetric quasifission occurring on a fast time scale, associated with collisions with the tips of the prolate actinide nuclei, shows a rapid increase in probability with increasing projectile charge, the transition being centered around projectile atomic number ${Z}_{\mathrm{P}}=14$. For mass-symmetric fission events, deviations of angular anisotropies from expectations for fusion fission, indicating a component of slower quasifission, suggest a similar transition, but centered around ${Z}_{\mathrm{P}}\ensuremath{\sim}8$.Conclusions: Collisions with the tips of statically deformed prolate actinide nuclei show evidence for two distinct quasifission processes of different time scales. Their probabilities both increase rapidly with the projectile charge. The probability of fusion can be severely suppressed by these two quasifission processes, since the sub-barrier heavy element yield is likely to be determined by the product of the probabilities of surviving each quasifission process.

Inverse quasifission in the reactions Gd156,160+W186

Background: Low-energy multinucleon transfer reactions may be used for production of new neutron-enriched heavy nuclei.Purpose: Our aim is to investigate the influence of proton ($Z=82$) and neutron ($N=82$, 126) shells as well as orientation effects on the formation of reaction products in the inverse quasifission process in the reactions $^{156,160}\mathrm{Gd}+^{186}\mathrm{W}$.Methods: Mass, energy, and angular distributions of primary binary fragments formed in the reactions $^{156}\mathrm{Gd}+^{186}\mathrm{W}$ at an energy of 878 MeV, and $^{160}\mathrm{Gd}+^{186}\mathrm{W}$ at 860 and 935 MeV, have been measured using the double-arm time-of-flight spectrometer CORSET at the U400 cyclotron of the Flerov Laboratory of Nuclear Reactions (FLNR) at the Joint Institute for Nuclear Research (JINR), Dubna.Results: Enhancement in the yield of products with masses 200--215 u has been found for both reactions. The cross sections of the formation of trans-target fragments with masses around 208 u are found to be about 10 \ensuremath{\mu}b at the Coulomb barrier energy and reach the level of 0.5 mb at the energy above the barrier for side-to-side collision.Conclusions: The enhanced yield of products with masses heavier than the target mass confirms the important role of the closed shells at $Z=82$ and $N=82$, 126 in the inverse quasifission process in low-energy damped collisions. The orientation effect caused by the strong deformation of colliding nuclei can result in a gain in the yield of heavy target-like fragments.

Angular momentum dependence of quasifission dynamics in the reaction 48Ca+244Pu

The quasifission dynamics in the reaction $^{48}$Ca+$^{244}$Pu is investigated in the framework of time-dependent Hartree-Fock (TDHF) theory.The calculations are performed in three-dimensional Cartesian coordinate without any symmetry restrictions.The full Skyrme energy functional is incorporated in our TDHF implementation.The quasifission dynamics is quite sensitive to the angular momentum of colliding system.The contact time of quasifission decreases as a function of angular momentum and then forms a plateauwith small oscillations. The quasifission process is accompanied by an important multi-nucleon transfer.The quantum shell effect plays a crucial role in the mass and charge of quasifission fragments.The mass-angle distribution of the fragments is calculated, which can be compared directly with future experiments.

Fusion-fission of superheavy nuclei and clustering phenomena

Results of the study of mass-energy distributions of binary fragments for a wide range of nuclei with Z = 82-122 produced in reactions with heavy ions at energies close and below the Coulomb barrier are reported. The role of the shell effects, the influence of the entrance channel asymmetry and the deformations of colliding nuclei on the mechanism of the fusion-fission and quasifission processes are discussed. The observed peculiarities of the mass and energy distributions of reaction fragments are determined by the shell structure of the formed fragments.

Time-dependent mean-field investigations of the quasifission process

We demonstrate that the microscopic Time-dependent Hartree-Fock (TDHF) theory provides an important approach to shed light on the nuclear dynamics leading to the formation of superheavy elements. In particular, we discuss studying quasifission dynamics and calculating ingredients for compound nucleus formation probability calculations. We also discuss possible extensions to TDHF to address the distribution of observables.

Time-dependent Hartree-Fock calculations for multinucleon transfer and quasifission processes in theNi64+U238reaction

Background: Multinucleon transfer (MNT) and quasifission (QF) processes are dominant processes in low-energy collisions of two heavy nuclei. They are expected to be useful to produce neutron-rich unstable nuclei. Nuclear dynamics leading to these processes depends sensitively on nuclear properties such as deformation and shell structure. Purpose: We elucidate reaction mechanisms of MNT and QF processes involving heavy deformed nuclei, making detailed comparisons between microscopic time-dependent Hartree-Fock (TDHF) calculations and measurements for the $^{64}$Ni+$^{238}$U reaction. Methods: Three-dimensional Skyrme-TDHF calculations are performed. Particle-number projection method is used to evaluate MNT cross sections from the TDHF wave function after collision. Results: Fragment masses, total kinetic energy (TKE), scattering angle, contact time, and MNT cross sections are investigated for the $^{64}$Ni+$^{238}$U reaction. They show reasonable agreements with measurements. At small impact parameters, collision dynamics depends sensitively on the orientation of deformed $^{238}$U. In tip (side) collisions, we find a larger (smaller) TKE and a shorter (longer) contact time. In tip collisions, we find a strong influence of quantum shells around $^{208}$Pb. Conclusions: It is confirmed that the TDHF calculations reasonably describe both MNT and QF processes in the $^{64}$Ni+$^{238}$U reaction. Analyses of this system indicates the significance of the nuclear structure effects such as deformation and quantum shells in nuclear reaction dynamics at low energies.

Non-coplanar compact configurations of nuclei and non-compound-nucleus contribution in the fusion cross section of theC12+Nb93reaction

Background: In our earlier study of the $^{12}\mathrm{C}+^{93}\mathrm{Nb}\ensuremath{\rightarrow}^{105}\mathrm{Ag}^{*}$ reaction at three near- and below-barrier energies (${E}_{\mathrm{c}.\mathrm{m}.}=41.097$, 47.828, and 54.205 MeV), using the dynamical cluster-decay model (DCM) with various nuclear interaction potentials (the Blocki et al. pocket formula and others derived from the Skyrme energy density formalism) for compact, coplanar $({\mathrm{\ensuremath{\Phi}}}_{c}={0}^{0})$ nuclei, we found a large non-compound-nucleus (nCN) contribution in the measured fusion cross section of this reaction.Purpose: In the present work, we look for the effect of using non-coplanar, compact configurations $({\mathrm{\ensuremath{\Phi}}}_{c}\ensuremath{\ne}{0}^{0})$, in the Blocki et al. pocket formula of the nuclear proximity potential, on the non-compound-nucleus (nCN) contribution, using the DCM.Methods: Allowing the $\mathrm{\ensuremath{\Phi}}$ degree of freedom in the DCM formalism, we calculate the compound-nucleus (CN) and nCN cross sections. The only parameter of the DCM is the neck-length parameter $\mathrm{\ensuremath{\Delta}}R$, which also fits the empirically determined nCN cross section nearly exactly, under the assumption of considering it like a quasifission process where the fragment preformation factor ${P}_{0}=1$.Results: With the $\mathrm{\ensuremath{\Phi}}$ degree of freedom included, at the higher two energies the nCN cross section gets enhanced, and hence the pure CN cross section is decreased, since the calculated (total) fusion cross section is fitted to experimental data. The parameter $\mathrm{\ensuremath{\Delta}}R$ for the nCN contribution is smaller, and hence the reaction time larger, than for the CN decay process. Also, the contributing angular momentum ${\ensuremath{\ell}}_{\mathrm{max}}$ value increases in going from ${\mathrm{\ensuremath{\Phi}}}_{c}={0}^{0}$ to ${\mathrm{\ensuremath{\Phi}}}_{c}\ensuremath{\ne}{0}^{0}$ for both the CN and nCN processes. The intermediate mass fragments (IMFs), measured up to mass 13 in this reaction, are shown extended up to mass 16, and the fusion-fission $(ff)$ region is identified as $A/2\ifmmode\pm\else\textpm\fi{}16$, the same as for the ${\mathrm{\ensuremath{\Phi}}}_{c}={0}^{0}$ case.Conclusions: As a result of enhanced nCN cross section due to ${\mathrm{\ensuremath{\Phi}}}_{c}\ensuremath{\ne}{0}^{0}$, the CN fusion probability ${P}_{\mathrm{CN}}$ for $^{105}\mathrm{Ag}^{*}$ changes its behavior from an increasing to a decreasing function of center-of-mass energy ${E}_{\mathrm{c}.\mathrm{m}.}$, and hence belongs to the group of weakly fissioning nuclei, instead of the strongly fissioning superheavy nuclei for ${\mathrm{\ensuremath{\Phi}}}_{c}={0}^{0}$. On the other hand, with measured IMFs taken to represent the $ff$ component, non-coplanarity simply increases the magnitude of CN survival probability ${P}_{\mathrm{surv}}$, although its functional dependence on ${E}_{\mathrm{c}.\mathrm{m}.}$ remains the same as for weakly fissioning nuclei. In other words, on adding the $\mathrm{\ensuremath{\Phi}}$ degree of freedom, the inconsistent result of ${P}_{\mathrm{CN}}$ behaving like strongly fissioning superheavy nuclei and ${P}_{\mathrm{surv}}$ like the weakly fissioning nuclei for ${\mathrm{\ensuremath{\Phi}}}_{c}={0}^{0}$ changes to both ${P}_{\mathrm{CN}}$ and ${P}_{\mathrm{surv}}$ behaving consistently like those of weakly fissioning nuclei. Thus, our calculations advocate for ${\mathrm{\ensuremath{\Phi}}}_{c}$ as an important degree of freedom, like deformations of nuclei themselves.

Effect of entrance channel on dynamics of heavy ions collision

A combined dynamical model using concept of dinuclear systems (DNS) and one-dimensional (1D) Langevin equations was applied to investigate the effect of entrance channel on dynamics of heavy ions collision. The [Formula: see text]Si+[Formula: see text]Er, [Formula: see text]O+[Formula: see text]W and [Formula: see text]F+[Formula: see text]Ta reactions which formed the compound nucleus [Formula: see text]Pb have been considered to study this effect. We studied these reactions dynamically and calculated the ratio of evaporation residue cross-section to fusion cross-section [Formula: see text] as a tool for investigation of entrance channel effect. Results of combined model are compared with available experimental data and results of 1D Langevin equations. Obtained results based on combined model are in better agreement with experimental data in comparison with results of Langevin equations. We concluded for [Formula: see text]Si+[Formula: see text]Er and [Formula: see text]F+[Formula: see text]Ta reactions the results of combined model that support the quasi-fission process are different relative to Langevin dynamical approach, whereas for [Formula: see text]O+[Formula: see text]W system the two models give similar results.

Fusion and quasi-fission dynamics in nearly-symmetric reactions

Some nearly-symmetric fusion reactions are systematically investigated with the improved quantum molecular dynamics (ImQMD) model. By introducing two-body inelastic scattering in the Fermi constraint procedure, the stability of an individual nucleus and the description of fusion cross sections at energies near the Coulomb barrier can be further improved. Simultaneously, the quasi-fission process in $^{154}$Sm+$^{160}$Gd is also investigated with the microscopic dynamics model for the first time. We find that at energies above the Bass barrier, the fusion probability is smaller than $10^{-5}$ for this reaction, and the nuclear contact-time is generally smaller than $1500$ fm/c. From the central collisions of Sm+Gd, the neutron-rich fragments such as $^{164,165}$Gd, $^{192}$W can be produced in the ImQMD simulations, which implies that the quasi-fission reaction could be an alternative way to synthesize new neutron-rich heavy nuclei.

Fusion and fission of heavy and superheavy nuclei (experiment)

The scope of this review is to summarize the main advancements in the search of signatures of the compound nucleus fission and quasifission processes in heavy and superheavy systems. The understanding of fusion and fission in heavy and superheavy elements is needed for tracing paths aimed at reaching the island of stability situated near Z=114–122 and N=184. With increasing charge of the interacting nuclei other processes, like quasifission, emerge and compete against fusion. Hence also their study must be pursued. After a brief look at the experimental techniques, the behavior of several observables is extracted from the most recent data to aid in the disentanglement of the various competing processes which hinder the production of superheavy elements.

Quasifission mass distributions in the synthesis of 274Hs with 26Mg and 36S projectiles

The potential energy landscape for the [Formula: see text] synthesis is evaluated in the framework of the macroscopic–microscopic model. The fragmentation potential calculated in a configuration space characterized by five degrees of freedom associated to elongation, mass asymmetry, necking, and left/right deformations reveals the existence of several minima and some pronounced valleys. The valleys correspond mainly to the [Formula: see text] and [Formula: see text] quasifission channels, but also for some paths in the formation of the compound nuclear system. In this context, two different paths are obtained for the [Formula: see text] and [Formula: see text] entrance channels. The [Formula: see text] path leads to the formation of an isomeric minimum that decays by fission. The [Formula: see text] path evidences a larger probability for the formation of the compound nucleus. Therefore, the two distributions obtained for the associated quasifission processes are very different.

Some aspects of fission and quasifission processes

The discovery of nuclear fission in 1938–1939 had a profound influence on the field of nuclear physics and it brought this branch of physics into the forefront as it was recognized for having the potential for its seminal influence on modern society. Although many of the basic features of actinide fission were described in a ground-breaking paper by Bohr and Wheeler only six months after the discovery, the fission process is very complex and it has been a challenge for both experimentalists and theorists to achieve a complete and satisfactory understanding of this phenomenon. Many aspects of nuclear physics are involved in fission and it continues to be a subject of intense study even three quarters of a century after its discovery. In this talk, I will review an incomplete subset of the major milestones in fission research, and briefly discuss some of the topics that I have been involved in during my career. These include studies of vibrational resonances and fission isomers that are caused by the second minimum in the fission barrier in actinide nuclei, studies of heavy-ion-induced fission in terms of the angular distributions and the mass–angle correlations of fission fragments. Some of these studies provided evidence for the importance of the quasifission process and the attendant suppression of the complete fusion process. Finally, some of the circumstances around the establishment of large-scale nuclear research in India will be discussed.

Experimental study of the quasifission, fusion-fission, and de-excitation of Cf compound nuclei

Background: The fusion-evaporation reaction at energies around the Coulomb barrier is presently the only way to produce the heaviest elements. However, formation of evaporation residues is strongly hindered due to the competing fusion-fission and quasifission processes. Presently, a full understanding of these processes and their relationships has not been reached.Purpose: This work aims to use new fission measurements and existing evaporation residue and fission excitation function data for reactions forming Cf isotopes to investigate the dependence of the quasifission probability and characteristics on the identities of the two colliding nuclei in heavy element formation reactions.Method: Using the Australian National University's 14UD electrostatic accelerator and CUBE detector array, fission fragments from the ${}^{12}\mathrm{C}+{}^{235}\mathrm{U}$, ${}^{34}\mathrm{S}+{}^{208}\mathrm{Pb}$, ${}^{36}\mathrm{S}+{}^{206}\mathrm{Pb}$, ${}^{36}\mathrm{S}+{}^{208}\mathrm{Pb}$, and ${}^{44}\mathrm{Ca}+{}^{198}\mathrm{Pt}$ reactions were measured. Mass and angle distributions of fission fragments were extracted and compared to investigate the presence and characteristics of quasifission.Results: Mass-angle-correlated fission fragments were observed for the ${}^{44}\mathrm{Ca}+{}^{198}\mathrm{Pt}$ reaction; no correlation was observed in the other reactions measured. Flat-topped fission-fragment mass distributions were observed for ${}^{12}\mathrm{C}+{}^{235}\mathrm{U}$ at compound-nucleus excitation energies from 28 to 52 MeV. Less pronounced flat-topped distributions were observed, with very similar shapes, for all three sulfur-induced reactions at excitation energies lower than 45 MeV.Conclusions: A high probability of long-time-scale quasifission seems necessary to explain both the fission and evaporation residue data for the ${}^{34}\mathrm{S}+{}^{208}\mathrm{Pb}$ and ${}^{36}\mathrm{S}+{}^{206}\mathrm{Pb}$ reactions. Flat-topped mass distributions observed for ${}^{12}\mathrm{C}$- and ${}^{34,36}\mathrm{S}$-induced reactions are suggested to originate both from late-chance fusion-fission at low excitation energies and the persistence of shell effects at the higher energies associated with quasifission.

Reduced quasifission competition in fusion reactions forming neutron-rich heavy elements

Measurements of mass-angle distributions (MADs) for Cr + W reactions, providing a wide range in the neutron-to-proton ratio of the compound system, ${(N/Z)}_{\mathrm{CN}}$, have allowed for the dependence of quasifission on the ${(N/Z)}_{\mathrm{CN}}$ to be determined in a model-independent way. Previous experimental and theoretical studies had produced conflicting conclusions. The experimental MADs reveal an increase in contact time and mass evolution of the quasifission fragments with increasing ${(N/Z)}_{\mathrm{CN}}$, which is indicative of an increase in the fusion probability. The experimental results are in agreement with microscopic time-dependent Hartree-Fock calculations of the quasifission process. The experimental and theoretical results favor the use of the most neutron-rich projectiles and targets for the production of heavy and superheavy nuclei.