Synthesis Of Superheavy Elements Research Articles

Colliding heavy nuclei take multiple identities on the path to fusion

The properties of superheavy elements probe extremes of physics and chemistry. They are synthesised at accelerator laboratories using nuclear fusion, where two atomic nuclei collide, stick together (capture), then with low probability evolve to a compact superheavy nucleus. The fundamental microscopic mechanisms controlling fusion are not fully understood, limiting predictive capability. Even capture, considered to be the simplest stage of fusion, is not matched by models. Here we show that collisions of 40Ca with 208Pb, experience an ‘explosion’ of mass and charge transfers between the nuclei before capture, with unexpectedly high probability and complexity. Ninety different partitions of the protons and neutrons between the projectile-like and target-like nuclei are observed. Since each is expected to have a different probability of fusion, the early stages of collisions may be crucial in superheavy element synthesis. Our interpretation challenges the current view of fusion, explains both the successes and failures of current capture models, and provides a framework for improved models.

Influence of «FLAT–TOP» of the resonant system of the DC-280 accelerator on the beam of charged particles

Experiments on the synthesis of superheavy elements (SHE) which were done at Flerov Laboratory of Nuclear Reactions (FLNR) measure that the probability of occurrence of one SHE event is ranged from one event per month to one event per week. Nowadays, Z = 118 (Oganesson) is the most used chemical element which is synthesized with 48Ca. The synthesis of new SHEs requires heavier bombarding particles: 50Ti, 54Cr, 58Fe, etc. In the experiments with heavier particles, the SHE isotope production cross section is expected to be lower than in the case of using 48Ca. It follows that it is necessary to increase the intensity of bombarding ion beams in order to collect experimental statistics in a reasonable time. Thus, for the synthesis of superheavy elements in the FLNR at the Joint Institute for Nuclear Research (JINR), the Factory of Superheavy Elements was created. The basic equipment of the SHE Factory is the DC-280 cyclotron complex. The accelerator provides high intensity of ion beams. As the intensity in the beam increases, the energy spread increases. That to minimize this process, the DC-280 cyclotron is equipped with an additional FLAT–TOP accelerating system.

Synthesis of superheavy elements

Synthesis of superheavy elements

Experimental investigation of the role of shell structure in quasifission mass distributions

Background: To understand superheavy element synthesis reactions, quantifying the role of quantum shells in quasifission dynamics is important. In reactions with actinide nuclides, a wide peak in the binary quasifission mass yield is seen, centered close to the $^{208}\mathrm{Pb}$ mass. It is generally attributed to the $^{208}\mathrm{Pb}$ spherical closed shells causing a valley in the potential-energy surface, attracting flux to these mass splits. However, an early experiment studying $^{48}\mathrm{Ca}, ^{50}\mathrm{Ti}+^{238}\mathrm{U}$ reactions showed strong evidence that sequential fission plays an important role in generating the observed peak. These conflicting interpretations have not been resolved up to now.Purpose: This work aims to measure quasifission mass spectra for reactions with nuclei lighter than $^{208}\mathrm{Pb}$, having negligible sequential fission, to search for systematic features correlated with the proton shells known to affect low-energy fission mass distributions of the same actinide elements.Methods: Systematic measurements have been made at energies near and below the capture barriers (where quasifission is most prominent) of mass-angle distributions for fission following collisions of $^{48}\mathrm{Ti}$ projectiles with even-even nuclides from $^{154}\mathrm{Sm}$ to $^{200}\mathrm{Hg}$. Mean excitation energies above the ground-states ranged from 51 to 33 MeV, respectively.Results: With increasing compound nucleus atomic number ${\mathrm{Z}}_{CN}$, a rapid transition occurs from fission having characteristics of fusion-fission to fast quasifission. The heaviest reactions form $^{240}\mathrm{Cf}, ^{244}\mathrm{Fm}$, and $^{248}\mathrm{No}$. Low-energy fission of neighboring isotopes is mass asymmetric, correlated with proton number $Z=56$. However, peak quasifission yields are at mass-symmetry for all reactions. There appears to be a very small ($\ensuremath{\approx}3$%) systematic excess of yield correlated with $Z=56$, however this is at the limit of sensitivity of the experiment.Conclusions: No significant ($>3%$) systematic features are seen in the quasifission mass spectra that can be unambiguously identified as resulting from shells. This small influence may result from attenuation of shell effects due to the excitation energy introduced, even in these near-barrier reactions giving low excitation energies typical of superheavy element synthesis reactions.

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

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

Predictions for the synthesis of the Z=120 superheavy element

The possible stable and radioactive beam-induced hot fusion reactions for the production of new superheavy element (SHE) $Z=120$ are investigated within the dinuclear system model. Our investigation shows that the reaction $^{45}\mathrm{Sc}+^{252}\mathrm{Es}$ has a relatively large evaporation residue cross-section (ERCS) for the synthesis of SHE $Z=120$ due to its slightly larger mass asymmetry. In addition, we analyzed the effects of fission barrier and neutron separation energy of superheavy nuclei on ERCS. To synthesize the double-magic $Z=120$ and $N=184$ nucleus predicted by the relativistic mean-field model, some radioactive beam-induced fusion reactions, which can be performed in available experimental equipment, for producing superheavy nucleus $^{304}120$ in neutron evaporation channels are investigated systematically. We find that the hot fusion reaction $^{50}\mathrm{Ca}+^{257}\mathrm{Fm}$ in the $3n$ evaporation channel is optimal for synthesizing the superheavy nucleus $^{304}120$. We hope these predictions will shed new light timely for the recent experiments on the synthesis of $Z=120$ superheavy element and the search for superheavy stability islands.

Development of Micro-Pattern Gaseous Detectors for Nuclear Reaction Studies

One of the frontiers of today’s nuclear physics research is the synthesis of Super Heavy Elements (SHE). Fusion-fission dynamics, namely the competition between quasi fission and fusion is one of the key challenges to optimize the SHE. To have an insight into the dynamics, one requires the study of fission fragment mass and angular distribution near barrier energies for heavy-ion induced fission reactions. Recent successful installation of linear accelerators in India offers a unique opportunity to study the dynamics of nuclear reactions and formation process of SHE. For the effective utilization of these current, as well as upcoming facilities, development of novel detectors to study reaction dynamics, formation process of SHE with heavier projectiles and higher beam energies is needed. Gaseous detectors have undergone a rapid improvement in terms of spatial, temporal and energy resolution, rate capability, radiation hardness, ion feedback etc., ushering in a new genre of micro-structured devices based on semi-conductor technology, commonly known as Micro-Pattern Gaseous Detectors (MPGDs). Although many of the MPGD structures were primarily developed for high-rate tracking of charged particles in high energy physics experiments, stability of operation, simplicity of construction and relatively low cost make these detectors suitable for other applications, such as low-energy nuclear physics experiments. The present activities encompass a detailed evaluation of the operational conditions of Micromesh-Multi Wire and THGEM-Multi Wire hybrid detector operated in low-pressure isobutane gas with a view to optimizing their use in the detection of charged particles and fission fragments.

Synthesis of superheavy elements using krypton and argon beams

Synthesis of superheavy elements using krypton and argon beams

Formation of large chunk of nuclear matter in heavy-ion collisions

In terrestrial laboratory with existing accelerators, the maximum multinucleon system produced so far is limited to that with total nucleon number being less than the sum of two colliding nuclei. Such situation may hinder our studies on the properties of nuclear many-body system. Here a way of producing large multinucleon system via multi-nucleus collision device Collider Plus in terrestrial laboratory is proposed. It is shown that large chunk of nucleonic matter can be produced through nuclear ternary fusion reactions at lower beam energies and also large block of dense matter can be formed via ternary collisions of heavy nuclei at intermediate energies. These investigations may shed lights on the studies of the synthesis of superheavy elements and the properties of bulk superdense nuclear matter.

Systematic study of probable target-projectile combinations for the synthesis of Z = 120 isotopes using the Skyrme energy density formalism

The synthesis of superheavy elements is an interesting field of research for both theoretical as well as experimental physicists. Till now superheavy element from Z=104 to 118 has been synthesized in various laboratories using cold and hot fusion reactions. Recent predictions of various theoretical models state that Z=120 and N=184 seems most probable doubly magic pair in the island of stability. Hence, we aim to the predict most probable target-projectile combinations of 300,302,304120 superheavy nuclei, so that relevant inputs may be imparted for future synthesis of superheavy nuclei. The target-projectile combinations are taken in reference to the minima's of the fragmentation potential calculated using the Quantum Mechanical Fragmentation Theory (QMFT). In addition, the target-projectile combinations from literature are also considered for which attempts were made for the synthesis of Z=120. The barrier height (VB) of the considered target-projectile combinations are calculated using the spherical and deformed choices of decay fragments. The percentage change in the barrier height ΔVB is explored for the hot and cold fusion approaches. The nuclear interaction potential is obtained within the Skyrme energy density formalism (SEDF) using the GSkI force parameters. For the feasible target-projectile (t-p) combinations, the capture cross-sections of these three isotopes 300,302,304120 are calculated using ℓ-summed Wong model and their comparative analysis is worked out. The probability of formation of compound nucleus (PCN) is determined using the energy dependent function. Among the thirty-six considered target-projectile combinations, the Ti-based reactions with Cf targets and also Cr-based reactions with Cm targets are identified as most suitable combinations for the synthesis of the superheavy element with Z=120. For 300120 isotope are 48Ti+252Cf, 50Ti+250Cf, 50Cr+250Cm, 52Cr+248Cm; whereas 50Ti+252Cf, 54Cr+248Cm, and 54Cr+250Cm form most probable target-projectile pair respectively, for 302120 and 304120 superheavy nuclei.

Superconducting Linac Booster for Super-Heavy Element Experiments at RIKEN Radioactive Isotope Beam Factory

The Rikagaku Kenkyūjo (RIKEN) heavy-ion linac was upgraded by introducing a new super-conducting linac-booster to advance the super-heavy elements synthesis program beyond nihonium at the RIKEN Radioactive Isotope Beam Factory (RIBF). The total acceleration voltage was upgraded from 25 MV with 12 room temperature drift-tube-linacs (DTLs) to 39 MV by introducing a superconducting linac booster, SRILAC. The upgrade of the beam intensity is realized by a newly constructed superconducting electron-cyclotron resonance ion source (SC-ECRIS). The construction of SRILAC and the SC-ECRIS started in FY2017. After the hardware installation and commissioning, the first beam acceleration test was successfully conducted at the end of FY2019, and user beam service was started in FY2020. This article provides an overview of the upgrade and its present performance.

New Coulomb barrier scaling law with reference to the synthesis of superheavy elements

The synthesis of superheavy elements based on fusion reactions and multinucleon transfer reactions is sensitive to the reaction energies, where the Coulomb barrier plays a crucial role as it must be overcome for the projectile and target to contact each other. The Coulomb barrier cannot be measured directly, and the synthesis of superheavy elements is sensitive to it. In this study, we systematically extract the barrier information from the experimental fusion excitation functions by the empirical cross section formula, which is based on a single-Gaussian distribution of fusion barrier heights. A total of 403 sets of experimental data are fitted, among which 243 sets have good results with ${\ensuremath{\chi}}^{2}/pt$ less than one. The extracted fusion barrier is a dynamical barrier, which includes the overall effect of coupled channels. Different from the prediction of the WKJ formula, the new scaling law proposed in this work is almost identical to the CW76 Coulomb barrier at the $z=170--300$ region and reproduces well the centroid barrier extracted from quasielastic scattering. The comparison to the other 14 bare potential models and exploration of the very large $z$ region is also discussed. It is also remarkable to find that the predictions of the DP2015 potential, including of the contributions of the macroscopic and shell correction terms, are highly consistent with the extracted results in this work and the CW76 potential. The numerical results demonstrated that the dynamical fusion barrier and the centroid barrier extracted from the quasielastic reaction (the reaction threshold) could be the same physical quantity for superheavy reactions. This study could provide important references for the synthesis of superheavy nuclei based on fusion reactions and very heavy multinucleon transfer reactions.

Investigations on 48Ca and 58Fe-induced heavy ion fusion reactions

In this paper, we have studied all possible [Formula: see text] and [Formula: see text]-induced fusion reactions for the synthesis of superheavy nuclei [Formula: see text]. The semi-empirical formula for fusion barriers of [Formula: see text] and [Formula: see text] is constructed. Fusion and evaporation residue cross-sections are studied using advance statistical model (ASM). The fusion and evaporation residue cross-sections of this work are compared with that of experiments. We have also predicted suitable targets and production cross-sections for the synthesis of superheavy elements using the [Formula: see text] and [Formula: see text]-induced fusion reactions. The [Formula: see text]-induced fusion reactions can be used to synthesize superheavy elements [Formula: see text] and 124 using the targets [Formula: see text]Cm, [Formula: see text]Bk and [Formula: see text]Cf have half-lives greater than one year and produce the cross-sections in terms of pb.

Quasifission in Kr84,86 -induced reactions populating superheavy elements

Background: Quasifission plays a detrimental role in the synthesis of superheavy elements. Understanding the dynamics of quasifission helps in selecting the optimum target projectile combinations for the synthesis of new superheavy elements.Purpose: The influence of various entrance channel parameters on the quasifission dynamics was explored. Three reactions, $^{84}\mathrm{Kr}+^{198}\mathrm{Pt}$, $^{86}\mathrm{Kr}+^{198}\mathrm{Pt}$, and $^{86}\mathrm{Kr}+^{197}\mathrm{Au}$, expected to be dominated by quasifission, were studied near the Coulomb barrier energies.Methods: The binary fission fragments from the three reactions were detected using the double arm time-of-flight spectrometer CORSET. The mass and total kinetic energy distributions, which are sensitive to the quasifission dynamics, were measured and analyzed for the three reactions.Results: The mass and total kinetic energy distributions of all the three reactions have the characteristics that show the dominance of quasifission. For the reaction $^{84}\mathrm{Kr}+^{198}\mathrm{Pt}$, symmetric quasifission has been found to be relatively more dominant compared to $^{86}\mathrm{Kr}+^{198}\mathrm{Pt}$ reaction. The results have been compared with $^{48}\mathrm{Ca}$- and $^{52}\mathrm{Cr}$-induced reactions, populating nearby nuclei, to understand the evolution of quasifission. An analysis of the timescales in these reactions using a dynamical calculation shows that $^{84}\mathrm{Kr}+^{198}\mathrm{Pt}$ takes a longer time to reach a particular dinuclear shape compared to the other two reactions. The driving potential at the barrier radius also shows a shallower pocket for $^{84}\mathrm{Kr}+^{198}\mathrm{Pt}$ compared to $^{86}\mathrm{Kr}+^{198}\mathrm{Pt}$ and $^{86}\mathrm{Kr}+^{197}\mathrm{Au}$ reactions.Conclusion: The entrance channel isospin difference is found to influence the quasifission dynamics. A higher entrance channel isospin difference may have a larger evolution time, thus driving the systems toward a more symmetric mass split.

Possibility to form Z=120 via the Ni64+U238 reaction using the dynamical cluster-decay model

According to different theoretical studies, the next magicity for the proton number should occur at $Z=114$, 120, or 126 and for the neutron number $N=184$. Superheavy nuclei of interest for the forthcoming synthesis of the isotopes with $Z=119$, 120 are investigated. Many reactions have yet to be studied in order to find the possible incoming channels for the formation of $Z=120$. In this work, synthesis of superheavy element 120 in terms of the fission and quasifission cross sections via $^{64}\mathrm{Ni}+^{238}\mathrm{U}$ reaction is evaluated and discussed. We have explored the possibility of formation of $Z=120$ via $^{64}\mathrm{Ni}+^{238}\mathrm{U}$ reaction, using the experimental data from Kozulin , Phys. Lett. B 686, 227 (2010). Some experimental efforts have been made to synthesize $^{302}120$, via Ni-induced and Cr-induced reaction, i.e., $^{64}\mathrm{Ni}+^{238}\mathrm{U}$ reaction at five ${E}^{*}$'s (excitation energies) and the $^{54}\mathrm{Cr}+^{248}\mathrm{Cm}$ reaction at only one energy, respectively. In this work, we have studied the Ni-induced reaction at given energies by including higher multipole deformations ${\ensuremath{\beta}}_{\ensuremath{\lambda}i}$ $(\ensuremath{\lambda}=2,3,4$; $i=1,2)$, and compact orientations ${\ensuremath{\theta}}_{ci}$ using the coplanar degree of freedom $(\mathrm{\ensuremath{\Phi}}={0}^{\ensuremath{\circ}})$ within the framework of the dynamical cluster-decay model (DCM). The neck-length parameter is the only one, which is fixed in reference to the observed data for fission cross section $({\ensuremath{\sigma}}_{\mathrm{ff}})$ calculated for mass region $A/2\ifmmode\pm\else\textpm\fi{}20$, and quasifission cross section $({\ensuremath{\sigma}}_{\mathrm{qf}})$ for the incoming channel of $^{64}\mathrm{Ni}+^{238}\mathrm{U}$ reaction. Our calculations for Ni-induced reaction have shown a good concurrence with the experimental data for quasifission and fission cross sections along with the DCM-calculated estimated and predicted cross sections for evaporation residues (ERs), which can be used for future references. Our results demonstrate the insignificant compound nucleus formation possibility ${P}_{\text{CN}}$ $(\ensuremath{\ll}1)$ for $Z=120$ from Ni-induced reaction because of the very low order of evaporation residues; thus, this work is not supporting this incoming channel for the formation of $Z=120$.

Quasifission and fusion-fission lifetime studies for the superheavy element Z=120

We study the quasifission and fusion-fission lifetimes for a number of fusion reactions used to synthesize the superheavy element $Z=120$ using a statistical method within the framework of the dinuclear system model. In particular, influence of the target orientation and angular momentum on such lifetimes have been investigated. The quasifission and fusion-fission lifetimes have been compared very well with that of available experiments on successful superheavy element synthesis. We notice further the predicted quasifission and fusion-fission lifetimes of the projectile-target combinations used for the synthesis of the superheavy element $Z=120$ also show the similar trend. Furthermore, the fusion-fission lifetime is seen to somewhat decrease with the increase of the angular momentum as well as beam energy. Little effect is seen on fusion barrier of the reaction and fission barrier of superheavy compound nucleus. Variation of quasifission lifetime is also not much with orientation angle, beam energy and fission barrier of the superheavy compound nuclei. Hence, this lifetime study does not provide any good reason why the attempted reactions have failed to synthesize the superheavy nuclei $Z=120$. Furthermore, consideration of the fusion barrier, fissility, mass asymmetry, deformation parameter, fission barrier, etc., leads us to reveal that optimal colliding energy is important to have the largest evaporation residue cross section for any chosen reaction so that it is well within the measurable limit for a specific experimental set up.

A dynamical study of fusion hindrance with the Nakajima–Zwanzig projection method

Abstract A new framework is proposed for the study of collisions between very heavy ions which lead to the synthesis of Super-Heavy Elements (SHE), to address the fusion hindrance phenomenon. The dynamics of the reaction is studied in terms of collective degrees of freedom undergoing relaxation processes with different time scales. The Nakajima–Zwanzig projection operator method is employed to eliminate fast variables and derive a dynamical equation for the reduced system with only slow variables. There, the time evolution operator is renormalised and an inhomogeneous term appears, which represents a propagation of the given initial distribution. The term results in a slip to the initial values of the slow variables. We expect that gives a dynamical origin for the so-called “injection point $s$” introduced by Swiatecki et al. in order to reproduce absolute values of measured cross sections for SHE. A formula for the slip is given in terms of physical parameters of the system, which confirms the results recently obtained with a Langevin equation, and permits us to compare various incident channels.

Investigation of the cold valley paths for the synthesis of isotopes of Ubh in optimum orientations

Cold valley paths have been investigated for the synthesis of isotopes of superheavy element Unbihexium using quantum mechanical fragmentation theory, which suggests the fusion of fission fragments belonging to the cold valleys in the potential energy surfaces with at least one spherical closed shell nucleus as the reaction partner for the nuclear synthesis. For the suitability of the cold valley fission fragments for the synthesis of isotopes of Ubh, first the fragmentation potentials, preformation probabilities and fission barriers of the fission fragments of 314Ubh and 326Ubh compound systems in their ground states have been compared by keeping the fragments either in hot or cold optimum orientations. The comparison suggests 138Ba + 176Yb and 132Sn + 194Os reactions in hot optimum orientations for the synthesis of isotopes of Ubh due to their highest values of preformation probabilities, lowest values of the fragmentation potentials and relatively good values of the fission barriers. Also, the reactions 68Ni + 246Cf and 70Ni + 256Cf have been considered for the comparison with 138Ba + 176Yb and 132Sn + 194Os reactions due to their relatively smaller values of the Coulomb potential, higher mass-asymmetry and fission barrier, and shell closures next to 48Ca which has been used in most of the experiments for the synthesis of superheavy elements. Finally, the excitation functions for the neutron evaporation from the compound systems formed in these reactions, the probabilities of the formation and survival of the compound nuclei have been compared to suggest the suitable cold valley paths energies for the synthesis of the isotopes of Ubh.

Entrance channel dependent hot fusion reactions for superheavy element synthesis

We have studied the compound nucleus probability, the survival probability, and the evaporation residue cross sections for 6645 projectile-target combinations to synthesize the superheavy nuclei with atomic numbers $Z=110$--126 in the light of the entrance channel effects such as mass asymmetry, charge asymmetry, isospin asymmetry, non-compound nuclear fission probability, and Businaro-Gallone mass asymmetry. The role of quasifission in every reaction has also been considered. Decay chains of various superheavy nuclei produced from the reactions are portrayed as well. The most striking results are that the measured evaporation residue cross sections are of the order of picobarns, whereas the proposed reactions can yield up to microbarn cross sections for the superheavy nuclei with $110\ensuremath{\le}Z\ensuremath{\le}126$. We suggest future experiments may utilize these hot fusion reactions to synthesize new elements ($Zg118$) or to study the properties of the known superheavy nuclei in greater detail. In particular, a proposed reaction containing both projectiles and targets that are naturally abundant, for example, $_{36}^{82}\mathrm{Kr}+_{90}^{232}\mathrm{Th}\ensuremath{\rightarrow}^{314}126+0n$, yields evaporation residue cross sections as high as 31 nb. The presently available experimental setup can test the prediction with ease to make a dream come true by putting a footstep into the eighth row of the periodic table.

Investigations on the study of entrance channel effects in synthesis of superheavy elements using Cr-induced fusion reactions

We have investigated the synthesis of superheavy elements using Cr-induced fusion reactions. We have studied all possible Cr-induced fusion reactions for the synthesis of super heavy nuclei [Formula: see text]. We have achieved the semi-empirical formula for fusion barrier heights ([Formula: see text]), positions ([Formula: see text]), curvature of the inverted parabola ([Formula: see text]) of Cr-induced fusion reactions for the synthesis of superheavy nuclei with atomic number range [Formula: see text]. The proposed formula produces fusion barriers of Cr-induced fusion reactions for the synthesis of super heavy nuclei with the simple inputs of mass number ([Formula: see text]) and atomic number ([Formula: see text]) of projectile-targets. We have also identified the targets for Cr-induced fusion reactions to synthesis superheavy elements of [Formula: see text]. We have also studied the entrance channel parameters such as mass asymmetry ([Formula: see text]), charge asymmetry ([Formula: see text]), coulomb interaction parameter ([Formula: see text]’), Businaro–Gallone mass asymmetry parameter ([Formula: see text]) and Isospin asymmetry parameter [[Formula: see text]]. We hope that our predictions may be the guide for the future experiments in the synthesis of more superheavy elements using [Formula: see text]Cr-induced fusion reactions.