Quasifission Research Articles

Investigation of decay mechanisms and associated aspects of exotic Nobeliumisotopes using the Skyrme energy density formalism

Abstract Persistent theoretical and experimental attempts have been made to investigate the
heavy ion induced reactions and their subsequent decay mechanisms in superheavy mass region.
In addition, the region of transfermium elements is itself of great interest because of the neutron
/ proton shell effects. Here, we aim to study the subsequent decay mechanisms of two isotopes of
Z = 102 nucleus, i.e. 248No and 250No.
The dynamical cluster-decay model (DCM) based on Quantum Mechanical Fragmen-
tation Theory (QMFT), is employed to conduct a comprehensive analysis of compound nucleus
(CN) and non-compound nucleus (nCN) mechanisms such as fusion-fission (ff), Quasi fission (QF)
and fast fission (FF), the role of centre of mass energy (Ec.m.) and angular momentum (ℓ) for
248No and 250No isotopes. The nuclear interaction potential is obtained using the Skyrme energy
density formalism (SEDF) in the domain of GSkI force parameters. The probability of compound nucleus formation
(PCN ) is determined using a function that depends on the centre of mass energy. The lifetimes for
the fusion-fission (ff) quasi fission (QF) channels are explored.
Here, CN and nCN decay mechanisms for two isotopes of Z = 102 nobelium are studied
over the wide range of centre-of-mass (Ec.m.) by including the quadrupole deformation (β2) and
optimum orientations (θopt.) of decaying fragments. The fragmentation potential, preformation
probability, neck length parameter and reaction cross-sections are explored. Further, the calcula-
tions are done for PCN in order to identify the decay modes of 248No and 250No isotopes. The
fusion-fission lifetimes and quasi fission lifetimes are compared with the dinuclear system (DNS)
approach. The most probable fragments such as 122Sn and 128T e are observed near to the magic shell closure Z = 50 and
N = 82. The ff and qf lifetime decreases with increase in the excitation energy.

Exploring fission and quasi-fission in 31Al + 197Au: probing energies near and beyond Coulomb barrier

An idea to understand the fragmentation of a formed nuclear entity (i.e. compound nucleus), induced via heavy-ion reactions, is an interesting task especially at incident energies near, above and far away from the Coulomb barrier. In the present work, we are studying the decay dynamics of a formed nuclear entity i.e. 228U* compound nucleus (CN), via the radioactive beam of Aluminium-31 (31Al) striking on the stable Gold-197 (197Au) target nucleus, for incident energies E c.m. = 106.2–163.2 MeV. A comprehensive knowledge regarding the probable decaying fragments from 228U* has been accumulated using the collective clusterization approach of the Dynamical Cluster-decay Model (DCM) in terms of mass and charge dispersion, which built up on the Quantum Mechanical Fragmentation Theory (QMFT). Through both of these dispersion cases, we have identified the decaying fragments in the fission and heavy-mass regions, at different scale of incident energies. Further, the fission cross-sections ( σfisTheo ) have been obtained for 31Al + 197Au → 228U* reaction at the above said E c.m. (MeV) and compared with the experimental data. This study has been carried out using two different sets of Skyrme force parameters (SIII and SSk), which are employed within the semiclassical approach of Skyrme Energy Density Formalism. Unlike for energies below and near the barrier, a significant difference has been observed between calculated ( σfisTheo ) and experimental ( σfisExpt ) cross-sections for above barrier energies, with independent choice of Skyrme forces. Such discrepancy noticed has been addressed with the presence of quasi-fission (QF) event. Similar phenomena has also been seen for 19F-induced reaction forming the same CN (228U*).

Entrance channel dependence of quasi fission in reactions leading to 206Po compound nucleus

The evaporation residue (ER) cross sections for the reaction F19+187Re→206Po⁎ are measured, in the excitation energy range of 86.1 to 118.4 MeV. The measured cross sections are compared with that of 30Si + 176Yb reaction populating the same compound nucleus. Theoretical calculations are performed using the coupled - channels calculations for the capture cross sections and statistical model calculations for the ER cross sections. The dependence of quasi fission on entrance channel parameters such as charge product (Z1Z2), mass asymmetry (α), target deformation (β2) and effective fissility (χeff) are studied.

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.

Fragmentation analysis of Z=112 –116 nuclei using a Skyrme energy density formalism within the collective clusterization approach

The Skyrme energy density formalism is applied to address the capture cross sections of $_{112}^{286}\mathrm{Cn}^{*}$, $_{114}^{292}\mathrm{Fl}^{*}$, and $_{116}^{296}\mathrm{Lv}^{*}$ superheavy nuclei formed in $^{48}\mathrm{Ca}\phantom{\rule{0.16em}{0ex}}+\phantom{\rule{0.16em}{0ex}}^{238}\mathrm{U},^{244}\mathrm{Pu},^{248}\mathrm{Cm}$ reactions. The dynamics of $^{48}\mathrm{Ca}$-induced reactions is investigated by including GSkI and SSk forces for the hot-optimum configurations of decay fragments. The neutron evaporation $(3n \mathrm{and} 4n)$, compound nucleus fission (CN fission) and quasifission (QF) cross sections are calculated with the application of both forces, but the GSkI force seems to be appropriate only for the neutron evaporation channels. However, by including SSk force various decay processes can be handled by including the only parameter of the model, called the neck length parameter $(\mathrm{\ensuremath{\Delta}}R)$. The fragment mass distribution of $Z=112$--116 nuclei shows considerable modifications in the fission, symmetric quasifission $({\mathrm{QF}}_{\mathrm{sym}.})$ and asymmetric quasifission $({\mathrm{QF}}_{\mathrm{asym}.})$ regions with the inclusion of the SSk force as compared to analysis based on the density independent potentials. In addition to this, the role of ${\ensuremath{\beta}}_{2}$ deformations and magic shell effects (around the Pb isotope) is scrutinized through the fragmentation behavior and preformation probability of $_{112}^{286}\mathrm{Cn}^{*}$, $_{114}^{292}\mathrm{Fl}^{*}$, and $_{116}^{296}\mathrm{Lv}^{*}$ nuclei. Finally, the distribution of the average total kinetic energy of fragments is calculated with the density dependent SSk force and compared with experimental data and earlier work based on the proximity potential.

Effect of nuclear structure and fissility on quasifission

In order to investigate the interplay between nuclear structure (shell closure) and dynamics, fission fragment mass distributions have been measured for reactions $^{35,37}\mathrm{Cl}+^{206,208}\mathrm{Pb}$. These reactions have similar entrance channel parameters except for the number of closed shell configuration in the entrance channel. Fission fragment mass distributions have been measured for these reactions in the energy region close to the Coulomb barrier. No mass-angle correlation was observed in any of the four reactions, indicating the absence of fast quasifission (QF), but mass distributions were wider than that of compound nucleus (CN) fission, which indicates the presence of slow QF. With comparable fissility and deformation values, extra stability against QF was observed for reactions which have more closed shell configurations in the entrance channel. Mass-angle distributions of these reactions do not follow the systematics with respect to mean fissility. A new systematics has been obtained by considering quadrupole deformation of the heavy reaction partner along with the fissility, which explains the absence of mass-angle correlation in the presently measured reactions.

Investigation of various decay mechanisms for Th*216 following the S32+W184 reaction in the range Ec.m.=118–196 MeV

Various decay possibilities of the $^{216}\mathrm{Th}^{*}$ nuclear system formed via interaction of a $^{32}\mathrm{S}$ projectile on a $^{184}\mathrm{W}$ target are investigated within the collective clusterization approach of the dynamical cluster-decay model (DCM). The study is carried out at center-of-mass energies spread across the Coulomb barrier ($118.8\ensuremath{\le}{E}_{\mathrm{c}.\mathrm{m}.}\ensuremath{\le}195.9\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$) by including the quadrupole deformations (${\ensuremath{\beta}}_{2}$ static) and optimum orientations (${\ensuremath{\theta}}_{i}^{\mathrm{opt}.}$) of the decay fragments. According to experimental observation, the noncompound nucleus (nCN) component competes with the compound nucleus (CN) processes [evaporation residue (ER) and fusion-fission (ff)]. The anomalous behavior of calculated fission anisotropies ($A$) for the $^{216}\mathrm{Th}^{*}$ nucleus indicates the presence of nCN contributions, such as quasifission (QF) and fast fission (FF). With an aim to have comprehensive analysis of CN and nCN fission mechanisms (ff, QF, FF), the role of center-of-mass energy (${E}_{\mathrm{c}.\mathrm{m}.}$) and angular momentum ($\ensuremath{\ell}$) is explored in terms of various observables of DCM such as fragmentation potential, preformation probability, scattering potential, and penetrability. The fragmentation potential of the $^{216}\mathrm{Th}^{*}$ nucleus shows asymmetric fission mass distribution, independent of ${E}_{\mathrm{c}.\mathrm{m}.}$ and $\ensuremath{\ell}$ values. The capture excitation functions ${\ensuremath{\sigma}}_{\text{Cap.}}$ are obtained by adding the DCM-calculated CN (${\ensuremath{\sigma}}_{\mathrm{CN}}={\ensuremath{\sigma}}_{\mathrm{ER}}+{\ensuremath{\sigma}}_{\mathrm{ff}}$) and nCN (${\ensuremath{\sigma}}_{\mathrm{nCN}}={\ensuremath{\sigma}}_{\mathrm{QF}}+{\ensuremath{\sigma}}_{\mathrm{FF}}$) contributions. The calculated cross sections find nice agreement with the experimental data, and the evaporation residue contribution is predicted to be negligibly small. The compound nucleus fusion/formation probability ${P}_{\mathrm{CN}}$ is estimated as a function of ${E}_{\mathrm{c}.\mathrm{m}.}$, which in turn suggests that the maximum contribution from the CN channel is $\ensuremath{\approx}66%$. Finally, the effect of ${\ensuremath{\beta}}_{2}$-dynamic deformations on the various decay mechanisms of the $^{216}\mathrm{Th}^{*}$ nucleus is explored at highest center-of-mass energy (195.9 MeV), where nCN processes start competing with the CN decay mechanism.

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.

Quasifission dynamics in TDHF

For light and medium mass systems the capture cross-section may be considered to be the same as that for complete fusion, whereas for heavy systems leading to superheavy formations the evaporation residue cross-section is dramatically reduced due to the quasifission (QF) and fusion-fission processes thus making the capture cross-section to be essentially the sum of these two cross-sections, with QF occurring at a much shorter time-scale. Consequently, quasifission is the primary reaction mechanism that limits the formation of superheavy nuclei. Within the last few years the time-dependent Hartree-Fock (TDHF) approach has been utilized for studying the dynamics of quasifission. The study of quasifission is showing a great promise to provide insight based on very favorable comparisons with experimental data. In this article we will focus on the TDHF calculations of quasifission observables for the $^{48}$Ca+$^{249}$Bk system.

Production of evaporation residues in the vicinity of the N =126 closure

The shell structure and fission-barrier heights of nuclei involved in the compound- nucleus (CN) de-excitation chains leading to observable evaporation residues (ER) determine essentially fusion-evaporation cross sections for heavy fissile nuclei produced in the vicinity of the N=126 neutron shell in heavy ion (HI) reactions. The entrance-channel effect of quasifission (QF) lowering the fusion cross section in reactions with massive nuclei and the effect of the collective enhancement in the nuclear level density (CENLD), corresponding to the exit channel, should strongly reduce production cross sections for ER and increase the observed fission cross section in the vicinity of N=126. These effects were not evidently manifested in the analysis of cross sections obtained in different CN reactions leading to Po isotopes produced in a wide region of N. This analysis has been performed in the framework of standard statistical model (SSM) approximations. As a whole, production cross sections for neutron deficient Po nuclei produced in HI reactions can be reproduced in the framework of SSM with the reduced liquid-drop fission barriers [1]. We present a similar analysis of ER and fission cross section data obtained in the reactions induced by 3,4He on Pb and Bi targets, which lead to Po and At compound nuclei [2], which proved to be a good test of the preceding results [1]. Such reactions are characterized by much lower angular momenta transferred to a CN that allows to suggest more pronounced manifestation of CENLD effects in the CN decay. These reactions also do not reveal any indications of QF. This simplifies the data analysis and makes it more unambiguous. The results of the analysis of the 3,4He reactions performed in the present work demonstrate the same changes in the fission barrier heights for Po and At nuclei as obtained earlier with HI reactions data [1].

Processes in massive nuclei reactions and the way to complete fusion of reactants. What perspectives for the synthesis of heavier superheavy elements?

By using the dinuclear system (DNS) model we determine the capture of reactants at the first stage of reaction, the competition between the DNS decay by the quasifission (QF) and the complete fusion (CF) process up to formation of the compound nucleus (CN) having compact shape. Further evolution of the CN is considered as its fission into two fragments or formation of evaporation residues (ER) by its cooling after emission of neutrons or/and charged light particles. Disappearance of the CN fission barrier due to its fast rotation leads to the fast fission (FF) by formation of fissionlike fragments. The results of calculations for the mass symmetric 136 Xe+ 136 Xe reaction, almost mass symmetric 108 Mo+ 144 Ba reaction, and mass asymmetric like 24 Mg+ 238 U and 34 S+ 248 Cm reactions are discussed. The fusion probability P CN calculated for many massive nuclei reactions leading to formation of superheavy nuclei have been analyzed. The reactions which can lead in perspective to the synthesis of superheavy elements in the Z = 120 − 126 range and, eventually, also to heaviest nuclei, are discussed.

Fusion probability and survivability in estimates of heaviest nuclei production

Production of the heavy and heaviest nuclei (from Po to the region of superheavy elements close to Z =114 and N =184) in fusion-evaporation reactions induced by heavy ions has been considered in a systematic way within the framework of the barrier-passing model coupled with the statistical model (SM) of de-excitation of a compound nucleus (CN). Excitation functions for fission and evaporation residues (ER) measured in very asymmetric combinations can be described rather well. One can scale and fix macroscopic (liquid-drop) fission barriers for nuclei involved in the calculation of survivability with SM. In less asymmetric combinations, effects of fusion suppression caused by quasi-fission (QF) are starting to appear in the entrance channel of reactions. QF effects could be semi-empirically taken into account using fusion probabilities deduced as the ratio of measured ER cross sections to the ones obtained in the assumption of absence of the fusion suppression in corresponding reactions. SM parameters (fission barriers) obtained at the analysis of a very asymmetric combination leading to the production of (nearly) the same CN should be used for this evaluation.

Competing fusion and quasifission reaction mechanisms in the production of superheavy nuclei

Within the framework of a dinuclear system model, a new master equation is constructed and solved, which includes the relative distance of nuclei as a new dynamical variable in addition to the mass asymmetry variable so that the nucleon transfer, which leads to fusion and the evolution of the relative distance, which leads to quasifission (QF) are treated simultaneously in a consistent way. The QF mass yields and evaporation residual cross sections to produce superheavy nuclei are systematically investigated under this framework. The results fit the experimental data well. It is shown that the Kramers formula gives results of QF, which agree with those by our diffusion treatment, only if the QF barrier is high enough. Otherwise some large discrepancies occur.

Capture cross sections for very heavy systems

In intermediate-mass systems, collective excitations of the target and projectile can greatly enhance the subbarrier capture cross section σ cap by giving rise to a distribution of Coulomb barriers. For such systems, capture essentially leads directly to fusion [formation of a compound nucleus (CN)], which then decays through the emission of light particles (neutrons, protons, and alpha particles). Thus, the evaporation-residue (ER) cross section is essentially equal to σ cap. For heavier systems, the experimental situation is significantly more complicated owing to the presence of quasifission (QF) (rapid separation into two fragments before the CN is formed) and by fusion-fission (FF) of the CN itself. Thus, three cross sections need to be measured in order to evaluate σ cap. Although the ER essentially recoil along the beam direction, QF and FF fragments are scattered to all angles and require the measurement of angular distributions in order to obtain the excitation function and barrier distribution for capture. Two other approaches to this problem exist. If QF is not important, one can still measure just the ER cross section and try to reconstruct the corresponding σ cap through use of an evaporation-model code that takes account of the FF degree of freedom. Some earlier results on σ cap obtained in this way will be reanalyzed with detailed coupled-channels calculations, and the “extra-push” phenomenon discussed. One may also try to obtain σ cap by exploiting unitarity, that is, by measuring instead the flux of particles corresponding to quasielastic (QE) scattering from the Coulomb barrier. Some new QE results obtained for the 86Kr + 208Pb system at iThemba LABS in South Africa will be presented.

Unified consideration of deep inelastic, quasi-fission and fusion–fission phenomena

A new approach is proposed for a unified description of strongly coupled deep inelastic (DI) scattering, fusion, fission and quasi-fission (QF) processes of heavy-ion collisions. The standard (most important) degrees of freedom of the nuclear system, unified driving potential, and a unified set of dynamic equations of motion are used in this approach. This makes it possible to perform a full (continuous) time analysis of the evolution of heavy nuclear systems, starting from the approaching stage, moving up to the formation of the compound nucleus and eventually emerging into two final fission fragments. The calculated mass, charge, energy and angular distributions of the reaction products agree well with the corresponding experimental data. It gives us hope to obtain rather accurate predictions for the probabilities of superheavy element formation in near-barrier fusion reactions.

Excitation energy division in 51V + 197Au collisions at and near the barrier

Mass- and charge-yield distributions for 19 < Z < 84 were determined radiochemically for the binary collision products of 51V + 197Au collisions at a bombarding energy corresponding exactly to the Bass-model barrier, E cm = B, and at E cm = B + 25 MeV. The average excitation energies as a function of Z are determined by comparing the centroids of the experimental, secondary mass distributions for given values of Z with the calculated primary centroids from minimization of the potential energy of the di-nuclear system, i.e. from the missing masses. At the barrier, in striking contrast to a thermal equilibrium, we find an extreme donor-acceptor asymmetry in the excitation-energy division reminiscent of the “sawtooth” phenomenon in low-energy nuclear fission. Here, the excitation energy sharing is apparently dominated by shape fluctuations at scission. At the slightly higher bombarding energy, E cm = B + 25 MeV, we observe a rapid change toward equipartition of the excitation energy indicating that, here, the excitation energy division due to shape fluctuations is already covered up by the dissipative exchange of nucleons. Also, the balance of integral cross sections for fusion fission, deep-inelastic scattering, and quasi fission is investigated and is shown to contain important information about the dynamical evolution of the 51V + 197Au system after having passed the entrance channel barrier.

Fusion-fission of heavy systems

The influence of the entrance channel on fission processes was studied by forming the same composite system by two different target-projectile combinations (40Ar+209Bi and56Fe+187Re, respectively). Compound nucleus fission and quasi fission were observed and the analysis was performed in the framework of the extra-extra-push model, which provides a qualitative interpretation of the results; limits for the extra-extra-push threshold are given, but problems with quantitative predictions for the extra-push are noted.

Angular distributions in quasi-fission reactions: Evidence for incomplete relaxation of the tilting mode

Angular distributions of fission-like fragments have been measured for50Ti+208Pb and56Fe+208Pb collisions. Z-dependent asymmetries around Θincm= 90° preclude their interpretation in terms of compound nucleus fission with the transition state theory. Fits of the data with a simple ansatz for statistical angular momentum fluctuations (tilting) give evidence for an incomplete relaxation of the tilting mode in quasi fission reactions.

Deep inelastic collisions between 280 MeV 63Cu ions and 93Nb nuclei

Mass and kinetic energy distributions have been obtained at several angles for products from 280 MeV 63Cu bombardments of 93Nb. Data were decomposed into quasi-fission (QF) and fusion-fission components. The QF angular distribution is peaked forward of the grazing angle and the QF kinetic energies are fission-like. The mass distributions are peaked near the projectile mass at all angles and their width increases as the angle is further removed from the grazing angle. The nature of the angular distribution favors the interpretation that QF events are due to relatively high I-waves.