Formation Of Superheavy Elements Research Articles

Observation of suppression of heavy-ion fusion by slow quasifission

The formation of superheavy elements (SHEs) by nuclear fusion can be conceptually divided into two steps: capture, and compound nucleus formation. Once captured, the two nuclei may reseparate before fusing to form a compact compound nucleus. This outcome is called quasifission. Fusion-fission, in many cases, leads to reactions outcomes inseparable from quasifission. Evaporation residue (ER) measurements are therefore the most reliable, direct experimental signature of fusion. ER cross section measurements forming the same compound nucleus, 220Th, using 16O, 40Ar, 48Ca, 82Se and 124Sn-induced reactions [1–4] revealed [1] that fusion was severely suppressed for the more symmetric reactions relative to the 16O-induced reaction. Here two new reactions forming 220Th using 28Si, 34S projectiles provide conclusive evidence that the ER cross section is exponentially suppressed as a function of ZpZt. The fission characteristics show no mass-angle correlation, demonstrating that here ER cross sections are suppressed by slow quasifission.

Systematic evidence for quasifission in Be9−, C12−, and O16 -induced reactions forming No258,260

Background: Cross sections for the formation of superheavy elements (SHE) by heavy ion fusion are suppressed by the competing quasifission process. This results in a fissionlike decay after capture but before formation of a compact compound nucleus. Fast quasifission is evident from very mass-asymmetric fission, focused in angle. In contrast, slow quasifission shows no significant mass-angle correlation, and a mass distribution peaked at symmetry. However, it shows angular distributions more anisotropic than those calculated for fission following fusion. Following fusion, low excitation energies should increase SHE survival through reduced competition from fission. However, in reactions with deformed actinide target nuclei, subbarrier fusion is highly suppressed by both fast and slow quasifission.Purpose: To investigate the threshold for quasifission by investigating signatures of slow quasifission in both fission angular and mass distributions, as a function of beam energy with respect to the capture barrier, for the projectiles $^{9}\mathrm{Be}$, $^{12}\mathrm{C},$ and $^{16}\mathrm{O}$ that form the neighboring compound nuclei $^{258,260}\mathrm{No}$.Methods: Fission mass and angular distributions have been measured from below to above-barrier energies using the kinematic coincidence method for the reactions $^{9}\mathrm{Be}+^{249}\mathrm{Cf}$, $^{12}\mathrm{C}+^{248}\mathrm{Cm}$, and $^{16}\mathrm{O}+^{244}\mathrm{Pu}$. Fission following transfer reactions can significantly contaminate fission events that follow capture, and must be rejected. Existing methods to reject transfer-induced fission have been refined to allow quantitative subtraction of the transfer fission component.Results: The capture-fission mass-angle distributions show no evidence for fast quasifission, as might be expected. However, measured fission fragment angular anisotropies are larger than transition state model (TSM) calculations for fusion fission. The deviations increase with larger projectile charge and for bombarding energies below the mean capture barrier energy. Even for the $^{9}\mathrm{Be}+^{249}\mathrm{Cf}$ reaction, the subbarrier angular anisotropy significantly exceeds the TSM calculation. Fission mass distributions measured at the same excitation energies also show a consistent dependence on the projectile charge.Conclusions: New refined analysis techniques have been developed to enable reliable separation of fission following capture from sequential fission following transfer reactions. For fission following capture at above-barrier energies, the $^{9}\mathrm{Be}$ angular anisotropies are close to the TSM predictions, supporting the validity of TSM calculations of fusion-fission for such heavy elements. At subbarrier energies the angular anisotropy data indicate a component of slow quasifission even for $^{9}\mathrm{Be}$, and a probability that increases rapidly with projectile charge. It is concluded that the probability of slow quasifission changes smoothly with projectile charge, having no sharp threshold.

Chemical Elements in Space

Astrophysical observations give scientists the most essential information about the processes and “rates of production” of elements in nature and their abundance in the Universe. The lightest elements of the Periodic Table—hydrogen, helium, and, partially, lithium—appeared in the first seconds after the birth of the Universe. The first stars “gathered” from them; they enriched the Universe with heavier elements during the evolutionary process, with the participation of which “modern” stars subsequently formed. These stars, in turn, serve as natural factories for the synthesis of heavy elements not only throughout their lives but also during their deaths in supernova outbursts. The recent discovery of the fusion of neutron stars and subsequent studies of their afterglow revealed the features of the formation of superheavy elements in the Universe up to gold and uranium.

Systematic Study of Quasifission in 48Ca-induced reactions

The production of superheavy elements through the fusion of two heavy nuclei is severely hindered by the quasifission process, which results in the fission of heavy systems before an equilibrated compound nucleus (CN) can be formed. The heaviest elements have been synthesised using 48Ca as the projectile nucleus. However, the use of 48Ca in the formation of new superheavy elements has been exhausted, thus a detailed understanding of the properties that made 48Ca so successful is required. Measurements of mass-angle distributions allow fission fragment mass distribution widths to be determined. The effect of the orientation of prolate deformed target nuclei is presented. Closed shells in the entrance channel are also shown to be more important than the stability of the formed CN in reducing the quasifission component, with reduced mass widths for reactions with the closed shell target nuclei 144Sm and 208Pb. Comparison to mass widths for 48Ti-induced reactions show a significant increase in the mass width compared to 48Ca-induced reactions, highlighting the difficulty faced in forming new superheavy elements using projectiles with higher atomic number than 48Ca.

Stars from Craddle to Grave: Powerful Factories of Chemical Elements

The first elements of the Periodic Table – hydrogen, helium and partly lithium - appeared in the first seconds after the birth of the Universe. The first stars “gathered” from these materials are the natural factories of the synthesis of heavier elements, not only throughout their lives, but even during their death process, during Supernova explosions. Supernova explosions, in their turn, are powerful factories for the production of heavy elements. Modern instruments allow scientists not only to register such events, but also to determine how many different chemical elements were formed during such events. The recent discovery of the merging neutron stars and subsequent studies of their afterglow allowed us to clarify the process of formation of superheavy elements in the Universe up to the gold and uranium. Thus, astrophysical observations give scientists the most important information about the “production rates” of elements in the nature, and their abundance in the Universe.

Influence of the shell effects on evaporation residue cross section of superheavy nuclei

In order to study the influence of the shell effects on the formation and fission of superheavy elements, we applied multidimensional Langevin equations. The evaporation residue cross sections have been calculated for 3n, 4n, and 5n evaporation channels using three ( $$K=0$$ )- and four ( $$K \ne 0$$ )-dimensional Langevin equations. Calculations were done for 48Ca + 238U and 48Ca + 244Pu hot fusion reactions with 3n, 4n evaporation channels and 70Zn + 208Pb, and 54Cr + 209Bi cold fusion reactions with 1n and 2n evaporation channels. The calculations were performed for 4n and 5n evaporation channels of the 26Mg + 238U reaction, as well. Our results show that with increasing dimension of Langevin equations the residue cross section increases, whereas the fission cross section decreases. The obtained results with four-dimensional Langevin and considering shell effects are in better agreement with experimental data in comparison with three- and four-dimensional Langevin equations without shell effects.

Erratum to: “Formation of Superheavy Elements in Nature”

The list of affiliations should read: Institute for Theoretical and Experimental Physics, National Research Center Kurchatov Institute, Bol’shaya Cheremuskinskaya ul. 25, Moscow, 117218 Russia Sternberg Astronomical Institute (GAISh), Moscow State University, Universitetskii pr. 13, Moscow, 119992 Russia National Research Center “Kurchatov Institute”, pl. Akademika Kurchatova 1, Moscow, 123182, Russia

Formation of Superheavy Elements: Study Based on Dynamical Approach

Using multi-dimensional Langevin equations for the probability distribution of the distance between the surfaces of two approaching nuclei, we have studied the formation of superheavy elements via calculation of evaporation and fission cross sections of these elements. Evaporation residue cross sections have been calculated for the 1n, 2n, 3n, 4n, and 5n evaporation channels using one and four dimensional Langevin equations for the 48Ca+226Ra, 232Th, 238U, 237Np, 239,240,242,244Pu, 243Am, 245,248Cm, 249Bk, and 249Cf reactions. Our results show that with increasing dimension of Langevin equations the evaporation residue cross section is increased. Also, obtained results based on fourdimensional Langevin are in better agreement with experimental data in comparison with one-dimensional model.

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.

Formation of Superheavy Elements in Nature

It is shown that superheavy elements may also be formed in the main r process responsible for the formation of the heaviest elements observed in nature. Under conditions of a high neutron density, the nucleosynthesis region lies close to the neutron drip line, so that the r process may circumvent the region where nuclei undergo spontaneous fissions and therefore have short lifetimes. However, a high induced-fission rate, which increases with the charge number, may prevent the nucleosynthesis wave from overcoming the region of isotopes heavier than curium, and the beta-decay chain leading to an increase in the charge number of product elements inevitably results in the spontaneous fission of the majority of product nuclei. Calculations of nucleosynthesis that were performed with available nuclear data within the scenario of a neutron-star merger reveal that only Z < 106 superheavy elements are formed. Their abundance at the end of the r process is commensurate with the abundance of uranium, but their lifetime does not exceed several years, so that they fast undergo decay.

Quasifission Dynamics in the Formation of Superheavy Elements

Superheavy elements are created through the fusion of two heavy nuclei. The large Coulomb energy that makes superheavy elements unstable also makes fusion forming a compact compound nucleus very unlikely. Instead, after sticking together for a short time, the two nuclei usually come apart, in a process called quasifission. Mass-angle distributions give the most direct information on the characteristics and time scales of quasifission. A systematic study of carefully chosen mass-angle distributions has provided information on the global trends of quasifission. Large deviations from these systematics at beam energies near the capture barrier reveal the major role played by the nuclear structure of the two colliding nuclei in determining the reaction outcome, and thus implicitly in hindering or favouring superheavy element synthesis.

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.

The cross sections of fusion-evaporation reactions: the most promising route to superheavy elements beyond Z=118

The synthesis of superheavy elements beyond oganesson (Og), which has atomic number Z = 118, is currently one of the main topics in nuclear physics. An absence of sufficient amounts of target material with atomic numbers heavier than californium (Z = 98) forces the use of projectiles heavier than 48 Ca (Z = 20), which has been successfully used for the discoveries of elements with Z = 114 - 118 in complete fusion reactions. Experimental cross sections of 48 Ca with actinide targets behave very differently to “cold” and “hot” fusion-evaporation reactions, where doubly-magic lead and deformed actinides are used as targets, respectively. The known cross sections of these reactions have been analysed compared to calculated fission barriers. It has been suggested that observed discrepancies between the cross sections of 48 Ca-induced and other fusionevaporation reactions originate from the shell structure of the compound nucleus, which lies in the island of the stability. Besides scarcely known data on other reactions involving heavier projectiles, the most promising projectile for the synthesis of the elements beyond Og seems to be 50 Ti. However, detailed studies of 50 Ti, 54 Cr, 58 Fe and 64 Ni-induced reactions are necessary to be performed in order to fully understand the complexities of superheavy element formation.

ERRATUM: The cross sections of fusion-evaporation reactions: the most promising route to superheavy elements beyond Z=118

The synthesis of superheavy elements beyond oganesson (Og), which has atomic number Z = 118, is currently one of the main topics in nuclear physics. An absence of sufficient amounts of target material with atomic numbers heavier than californium (Z = 98) forces the use of projectiles heavier than 48 Ca (Z = 20), which has been successfully used for the discoveries of elements with Z = 114 - 118 in complete fusion reactions. Experimental cross sections of 48 Ca with actinide targets behave very differently to “cold” and “hot” fusion-evaporation reactions, where doubly-magic lead and deformed actinides are used as targets, respectively. The known cross sections of these reactions have been analysed compared to calculated fission barriers. It has been suggested that observed discrepancies between the cross sections of 48 Ca-induced and other fusionevaporation reactions originate from the shell structure of the compound nucleus, which lies in the island of the stability. Besides scarcely known data on other reactions involving heavier projectiles, the most promising projectile for the synthesis of the elements beyond Og seems to be 50 Ti. However, detailed studies of 50 Ti, 54 Cr, 58 Fe and 64 Ni-induced reactions are necessary to be performed in order to fully understand the complexities of superheavy element formation.

Fission and quasifission of composite systems with Z=108−120 : Transition from heavy-ion reactions involving S and Ca to Ti and Ni ions

Background: Suppression of compound nucleus formation in the reactions with heavy ions by a quasifission process in dependence on the reaction entrance channel.Purpose: Investigation of fission and quasifission processes in the reactions $^{36}\mathrm{S},^{48}\mathrm{Ca},^{48}\mathrm{Ti}$, and $^{64}\mathrm{Ni}+\phantom{\rule{0.16em}{0ex}}^{238}\mathrm{U}$ at energies around the Coulomb barrier.Methods: Mass-energy distributions of fissionlike fragments formed in the reaction $^{48}\mathrm{Ti}+\phantom{\rule{0.16em}{0ex}}^{238}\mathrm{U}$ at energies of 247, 258, and 271 MeV have been measured using the double-arm time-of-flight spectrometer CORSET at the U400 cyclotron of the Flerov Laboratory of Nuclear Reactions and compared with mass-energy distributions for the reactions $^{36}\mathrm{S},^{48}\mathrm{Ca},^{64}\mathrm{Ni}+\phantom{\rule{0.16em}{0ex}}^{238}\mathrm{U}$.Results: The most probable fragment masses as well as total kinetic energies and their dispersions in dependence on the interaction energies have been investigated for asymmetric and symmetric fragments for the studied reactions. The fusion probabilities have been deduced from the analysis of mass-energy distributions.Conclusion: The estimated fusion probability for the reactions S, Ca, Ti, and Ni ions with actinide nuclei shows that it depends exponentially on the mean fissility parameter of the system. For the reactions with actinide nuclei leading to the formation of superheavy elements the fusion probabilities are of several orders of magnitude higher than in the case of cold fusion reactions.

Beta-delayed fission probabilities of transfermium nuclei, involved in the r-process

For the nucleosynthesis of heavy and superheavy nuclei fission becomes very important when the r-process runs in a very high neutron density environment. In part, fission is responsible for the formation of heavy nuclei due to the inclusion of fission products as new seed nuclei (fission cycling). More than that, beta-delayed fission, along with spontaneous fission, is responsible in the late stages of the r-process for the suppression of superheavy element yields.For beta-delayed fission probability calculations a model description of the beta-strength- functions is required. Extended theoretical predictions for astro-physical applications were provided long ago, and new predictions also for superheavy nuclei with uptodate nuclear input are needed. For the further extension of data to heavier transactinides the models of strength- functions should be modified, taking into account more complicated level schemes. In our present calculations the strength-function model is based on the quasi-particle approximation of Finite Fermi Systems Theory.The probabilities of beta-delayed fission and beta-delayed neutron emission are calculated for some transfermium neutron-rich nuclei, and the influence of beta-delayed fission upon superheavy element formation is discussed.

Quasifission in heavy and superheavy element formation reactions

Superheavy elements are created in the laboratory by the fusion of two heavy nuclei. The large Coulomb repulsion that makes superheavy elements decay also makes the fusion process that forms them very unlikely. Instead, after sticking together for a short time, the two nuclei usually come apart, in a process called quasifission. Mass-angle distributions give the most direct information on the characteristics and time scales of quasifission. A systematic study of carefully chosen mass-angle distributions has provided information on the global trends of quasifission. Large deviations from these systematics reveal the major role played by the nuclear structure of the two colliding nuclei in determining the reaction outcome, and thus implicitly in hindering or favouring superheavy element production.

Time-dependent HF approach to SHE dynamics

We employ the time-dependent Hartree–Fock (TDHF) method to study various aspects of the reactions utilized in searches for superheavy elements. These include capture cross-sections, quasifission, prediction of PCN, and other interesting dynamical quantities. We show that the microscopic TDHF approach provides an important tool to shed some light on the nuclear dynamics leading to the formation of superheavy elements.

How signatures of quasifission evolve in reactions forming Curium

Quasifission, a fission-like reaction outcome in which no compound nucleus forms, is an important competitor to fusion in reactions used for super-heavy element formation. The precise mechanisms driving the competition between quasifission and fusion are poorly understood. To explore the influence reaction parameters have on quasifission probabilities, an investigation into the evolution of quasifission signatures as a function of entrance channel parameters is required. Using the Australian National University’s 14UD tandem accelerator and CUBE detector for two-body fission studies, measurements were made for a diverse range of reactions forming isotopes of Curium. Observables known to reveal signs of quasifission—namely mass ratio spectra, mass-angle distributions, and angular anisotropies—were extracted. Evidence of quasifission was observed in all reactions, but the observables showing evidence of quasifission were not the same for all reactions. A link between this evolution and reaction timescales will be discussed.

Dissipative dynamics in quasifission

Quasi-fission is the primary reaction mechanism that prevents the formation of superheavy elements in heavy-ion fusion experiments. Employing the time-dependent density functional theory approach we study quasi-fission in the systems $^{40,48}$Ca+$^{238}$U. Results show that for $^{48}$Ca projectiles the quasi-fission is substantially reduced in comparison to the $^{40}$Ca case. This partly explains the success of superheavy element formation with $^{48}$Ca beams. For the first time, we also calculate the repartition of excitation energies of the two fragments in a dynamic microscopic theory. The system is found in quasi-thermal equilibrium only for reactions with $^{40}$Ca. The differences between both systems are interpreted in terms of initial neutron to proton asymmetry of the colliding partners.