Multinucleon Transfer Processes Research Articles

Investigation of multinucleon transfer processes in the Langevin equation model

Investigation of multinucleon transfer processes in the Langevin equation model

Population of heavy neutron-rich nuclei in multinucleon transfer reactions

Selected results recently obtained in studies of heavy ion transfer reactions at energies close to the Coulomb barrier are presented. In particular, the production of neutron-rich heavy nuclei via multinucleon transfer processes in 206Pb+118Sn, 197Au+130Te, and 94Rb+208Pb is discussed.

Reaction mechanism study for multinucleon transfer processes in collisions of spherical and deformed nuclei at energies near and above the Coulomb barrier: The O16+Sm154 reaction

Background: Multinucleon transfer reactions at energies around the Coulomb barrier offer a vital opportunity to study rich physics of nuclear structure and dynamics. Despite the continuous development in the field, we have still limited knowledge about how deformation - one of the representative nuclear structures - affects multinucleon transfer reactions. Purpose: To shed light on the effect of deformation in multinucleon transfer processes, we study the $^{16}$O+$^{154}$Sm reaction at $E_{\rm lab}$=85 MeV (around the Coulomb barrier) and 134 MeV (substantially above the Coulomb barrier), where the target nucleus, $^{154}$Sm, is a well-established, deformed nucleus. Results: Angular distributions for elastic scattering and for various transfer channels were measured over a wide angular range at the BARC-TIFR pelletron-Linac accelerator facility, Mumbai. The $Q$-value- and angle-integrated isotope production cross sections have been extracted from the measured angular distributions. The experimental data are analyzed along with time-dependent Hartree-Fock calculations. For the lower incident energy case, we find a reasonable agreement between the measurements and the TDHF calculations for a-few-nucleon transfer channels; whereas TDHF underestimates cross sections for many-nucleon transfers, consistent with earlier works. On the other side, we find that calculated cross sections for secondary reaction products for the higher incident energy case, qualitatively explains the measured trends of isotopic distributions observed for the lower energy. The latter observation indicates possible underestimation of excitation energies in the present TDHF+GEMINI analysis. Although certain orientation effects were observed in TDHF results, it is difficult to disentangle them from the $Q$-value- and angle-integrated production cross sections. (Shortened due to the arXiv's length limit.)

Unified description of fusion and multinucleon transfer processes within the dinuclear system model

To compare fusion-evaporation (FE) and multinucleon transfer (MNT) approaches and make more reliable predictions for producing superheavy nuclei, the unified description of FE and MNT processes is performed in the reaction $^{48}\mathrm{Ca}+^{238}\mathrm{U}$ within the (DNS) model. We extend the fusion concept in the DNS model based on the deformation degree of freedom. Our calculations support the experimental result [Nishio et al., Phys. Rev. C 77, 064607 (2008)] that compact configuration enhances the fusion probability. The experimental production cross sections of isotopes in FE and MNT processes can be simultaneously reproduced. By comparing the production cross sections in the MNT reaction $^{238}\mathrm{U}+^{248}\mathrm{Cm}$ with the FE reactions $^{48}\mathrm{Ca}+^{238}\mathrm{U}$ and $^{54}\mathrm{Cr}+^{248}\mathrm{Cm}$, we find that the FE process shows great advantages of cross sections for producing Cn isotopes and is the only approach that could produce superheavy elements beyond oganesson.

Ground state and excited state multinucleon transfer channels in interactions of 28Si with 90,94Zr in near barrier region

Multinucleon transfer reactions were measured in interactions of 28Si with 90,94Zr at a near barrier energy using recoil mass separator, Heavy Ion Reaction Analyzer (HIRA) at Inter University Accelerator Centre (IUAC), New Delhi. The effects of shell closure, positive Q-value transfer channels on the relative strengths of ground state and excited state transfer in multinucleon transfer processes were investigated. The kinematic coincidence technique was employed to reduce the background. The m/q ambiguity was resolved by time of flight (TOF) technique. Based on the Q-value considerations, it was observed that pickup channels were predominantly of neutron transfer, whereas, stripping channels were mainly of proton transfer. A 14-element BGO array was used to extract the relative strength of excited state transfer. In interactions of 28Si with 90Zr, it was observed that the ground state and excited states competes for populating all the transfer channels. In reactions of 28Si with 94Zr, nearly similar behavior was observed except for two-neutron pickup channel, where excited state transfer was dominant and four-neutron pickup channel where ground state transfer was dominant.

A Constrained Analysis of the 40Ca(18O,18F)40K Direct Charge Exchange Reaction Mechanism at 275 MeV

The40Ca(18O,18F)40K single charge exchange (SCE) reaction is explored at an incident energy of 275 MeV and analyzed consistently by collecting the elastic scattering and inelastic scattering data under the same experimental conditions. Full quantum-mechanical SCE calculations of the direct mechanism are performed by including microscopic nuclear structure inputs and adopting either a bare optical potential or a coupled channel equivalent polarization potential (CCEP) constrained by the elastic and inelastic data. The direct SCE mechanism describes the magnitude and shape of the angular distributions rather well, thus suggesting the suppression of sequential multi-nucleon transfer processes.

Production mechanism of proton-rich actinide isotopes in fusion reactions and via multinucleon transfer processes

Within the dinuclear system model, unknown neutron-deficient isotopes Np, Pu, Am, Cm, Bk, Cf, Es, Fm are investigated in $^{40}$Ca, $^{36,40}$Ar, $^{32}$S, $^{28}$Si,$^{24}$Mg induced fusion-evaporation reactions and multinucleon transfer reactions with radioactive beams $^{59}$Cu,$^{69}$As,$^{90}$Nb,$^{91}$Tc, $^{94}$Rh, $^{105,110}$Sn, $^{118}$Xe induced with $^{238}$U near Coulomb barrier energies. The production cross sections of compound nuclei in the fusion-evaporation reactions and fragments yields in the multinucleon transfer reactions are calculated within the model. A statistical approach is used to evaluate the survival probability of excited nuclei via the both reaction mechanisms. A dynamical deformation is implemented into the model in the dissipation process. It is found that charge particle channels (alpha and proton) dominate in the decay process of proton-rich nuclides and the fusion-evaporation reactions are favorable to produce the new neutron-deficient actinide isotopes. The total kinetic energies and angular spectra of primary fragments are strongly dependent on colliding orientations.

Quantal diffusion approach for multinucleon transfer processes in theNi58,64+Pb208reactions: Toward the production of unknown neutron-rich nuclei

Background: In recent years, substantial efforts have been made for the study of multinucleon transfer reactions at energies around the Coulomb barrier both experimentally and theoretically, aiming at the production of unknown neutron-rich heavy nuclei. It is crucial to provide reliable theoretical predictions based on microscopic theories with sufficient predictive power.Purpose: This paper aims to clarify the applicability of the quantal diffusion approach based on the stochastic mean-field (SMF) theory for multinucleon transfer processes. Isotope production cross sections are evaluated for the reactions of $^{64}\mathrm{Ni}+^{208}\mathrm{Pb}$ at ${E}_{\mathrm{c}.\mathrm{m}.}$ $=$ 268 MeV and $^{58}\mathrm{Ni}+^{208}\mathrm{Pb}$ at ${E}_{\mathrm{c}.\mathrm{m}.}$ $=$ 270 MeV and are compared with available experimental data.Methods: Three-dimensional time-dependent Hartree-Fock (TDHF) calculations are carried out for a range of initial orbital angular momenta with Skyrme SLy4d functional. Quantal diffusion equations, derived based on the SMF theory, for variances and covariance of neutron and proton numbers of reaction products are solved, with microscopic drift and diffusion coefficients obtained from time evolution of occupied single-particle orbitals in TDHF. Secondary de-excitation processes, both particle evaporation and fission, are simulated by a statistical compound-nucleus de-excitation model, gemini++.Results: Dynamics of a fast isospin equilibration process followed by a slow drift toward the mass symmetry are commonly observed, as expected. Various reaction outcomes are evaluated, including average mass and charge numbers of reaction products, total kinetic energy loss (TKEL), scattering angle, contact time, and production cross sections for primary and secondary products. By comparing with the experimental data, we find that SMF and TDHF quantitatively reproduce experimental data for few-nucleon-transfer channels around the average values. In contrast, for many-nucleon-transfer channels, we find that the SMF approach provides much better description of the experimentally measured isotopic distributions. The results underline the importance of beyond-mean-field effects, especially one-body (mean-field) fluctuations and correlations, in describing multinucleon transfer processes. Moreover, through a combined analysis of SMF with a statistical model, gemini++, we find a significant contribution of transfer-induced fission, which is consistent with the experimental observation. In some cases, the SMF approach overestimates the isotopic width, requiring further improvements of the theoretical description. Possible ways to improve the description are discussed.Conclusions: The SMF approach is designed to describe the quantum many-body problem according to an ensemble of mean-field trajectories, taking into account part of many-body correlations in the description. As it requires feasible computational costs comparable to the ordinary TDHF approach, together with further model improvements, it will be a promising tool in the search for optimal reaction conditions to produce yet-unknown neutron-rich heavy nuclei through the multinucleon transfer reaction.

Production of light neutron-rich nuclei in multinucleon transfer reactions

In order to produce more unknown light neutron-rich nuclei, multinucleon transfer processes at energies slightly above the Coulomb barrier are investigated within the inclusion of the deformations of interacting nuclei as independent variables of the dinuclear system (DNS) model (improved DNS model). Transfer cross sections 58,64Ni+208Pb and 40Ca+208Pb are calculated and compared with the experimental data, we find the improved dinuclear system model describes the transfer cross sections reasonably. The calculated results showed that protons and neutrons are transferred to be strongly influenced by the potential energy surface, the injection point position of the projectile on the potential energy surface and the dynamical properties of the DNS. The cross sections of new neutron-rich isotopes through the transfer reactions 70Ni+208Pb and 32,36S+238U are estimated by the multinucleon transfer reactions.

Analysis of multinucleon transfer reactions with spherical and statically deformed nuclei using a Langevin-type approach

An increasing interest in multinucleon transfer processes in low-energy deep inelastic (damped) collisions of heavy ions appeared in recent years is due, in part, to by the possibility of using them as a method of production of heavy neutron-enriched nuclei. Possible promising projectile-target combinations include nuclei deformed in the ground state (e.g., actinides). Mutual orientations of such the nuclei in the entrance channel of the reaction may significantly influence the reaction dynamics. Available experimental data on multinucleon transfer reactions with statically deformed as well as spherical heavy nuclei 144Sm + 144Sm, 154Sm + 154Sm, 160Gd + 186W, and 208Pb + 208Pb/238U have been analyzed within the developed model. A good agreement of the calculated quantities with the corresponding experimental data is reached. Special attention in the paper is paid to analysis of production possibility of the neutron-enriched isotopes of heavy and superheavy elements in multinucleon transfer processes in the 238U + 238U/248Cm/254Es collisions.

Recent studies of heavy ion transfer reactions using large solid angle spectrometers

We present selected results recently obtained in the study of heavy ion transfer reactions at energies close to the Coulomb barrier by employing the large solid angle magnetic spectrometer PRISMA. We discuss the production of neutron-rich heavy nuclei via multinucleon transfer processes and the related effects of secondary processes, in particular nucleon evaporation, studied in a high resolution kinematic coincidence experiment. We also present the recent results in the studies of neutron-neutron correlations for closed shell and superfluid systems.

Deep-inelastic multinucleon transfer processes in the O16+Al27 reaction

The reaction mechanism of deep-inelastic multinucleon transfer processes in the $^{16}$O+$^{27}$Al reaction at an incident $^{16}$O energy ($E_{\rm lab}=134$ MeV) substantially above the Coulomb barrier has been studied both experimentally and theoretically. Elastic-scattering angular distribution, total kinetic energy loss spectra and angular distributions for various transfer channels have been measured. The $Q$-value- and angle-integrated isotope production cross sections have been deduced. To obtain deeper insight into the underlying reaction mechanism, we have carried out a detailed analysis based on the time-dependent Hartree-Fock (TDHF) theory. A recently developed method, TDHF+GEMINI, has been applied to evaluate production cross sections for secondary products. From a comparison between the experimental and theoretical cross sections, we find that the theory qualitatively reproduces the experimental data. Significant effects of secondary light-particle emissions are demonstrated. Possible interplay between fusion-fission, deep-inelastic, multinucleon transfer and particle evaporation processes are discussed.

Enhanced nucleon transfer in tip collisions of U238+Sn124

Multinucleon transfer processes in low-energy heavy ion reactions have attracted increasing interest in recent years aiming at production of new neutron-rich isotopes. Clearly, it is an imperative task to further develop understanding of underlying reaction mechanisms to lead experiments to success. In this paper, from systematic time-dependent Hartree-Fock calculations for the $^{238}$U+$^{124}$Sn reaction, it is demonstrated that transfer dynamics depend strongly on the orientations of $^{238}$U, quantum shells, and collision energies. Two important conclusions are obtained: (i) Experimentally observed many-proton transfer from $^{238}$U to $^{124}$Sn can be explained by a multinucleon transfer mechanism governed by enhanced neck evolution in tip collisions; (ii) Novel reaction dynamics are observed in tip collisions at energies substantially above the Coulomb barrier, where a number of nucleons are transferred from $^{124}$Sn to $^{238}$U, producing transuranium nuclei as primary reaction products, that could be a means to synthesize superheavy nuclei. Both results indicate the importance of the neck (shape) evolution dynamics, which are sensitive to the orientations, shell effects and collision energies, for exploring possible pathways to produce new unstable nuclei.

Microscopic description of production cross sections including deexcitation effects

Background: At the forefront of the nuclear science, production of new neutron-rich isotopes is continuously pursued at accelerator laboratories all over the world. To explore the currently-unknown territories in the nuclear chart far away from the stability, reliable theoretical predictions are inevitable. Purpose: To provide a reliable prediction of production cross sections taking into account secondary deexcitation processes, both particle evaporation and fission, a new method called TDHF+GEMINI is proposed, which combines the microscopic time-dependent Hartree-Fock (TDHF) theory with a sophisticated statistical compound-nucleus deexcitation model, GEMINI++. Results: The method is applied for multinucleon transfer processes in low-energy heavy ion reactions. It is shown that the inclusion of secondary deexcitation processes, which are dominated by neutron evaporation in the present systems, substantially improves agreement with the experimental data. The magnitude of the evaporation effects is very similar to the one observed in GRAZING calculations. TDHF+GEMINI provides better description of the absolute value of the cross sections for channels involving transfer of more than one protons, compared with the GRAZING results. However, there remain discrepancies between the measurements and the calculated cross sections, indicating a limit of the theoretical framework that works with a single mean-field potential. Possible causes of the discrepancies are discussed. Conclusions: In order to perfectly reproduce experimental cross sections for multinucleon transfer processes, one should go beyond the standard self-consistent mean-field description. Nevertheless, the proposed method will provide valuable information to optimize production mechanisms of new neutron-rich nuclei through its microscopic, non-empirical predictions. (Shortened due to the word limit)

Evolving theoretical descriptions of heavy-ion fusion: from phenomenological to microscopic approaches

We overview the current status of theoretical approaches for heavy-ion fusion reactions at subbarrier energies. We particularly discuss theoretical challenges in the coupled-channels approach, that include i) a description of deep subbarrier hindrance of fusion cross sections, ii) the role of nuclear dissipation, iii) fusion of unstable nuclei, and iv) an interplay between fusion and multi-nucleon transfer processes. We also present results of a semi-microscopic approach to heavy-ion fusion reactions, that combines the coupled-channels approach with state-of-the-art microscopic nuclear structure calculations.

Multinucleon transfer inO16,18,F19+Pb208reactions at energies near the fusion barrier

Background: Nuclear reactions are complex, involving collisions between composite systems where many-body dynamics determines outcomes. Successful models have been developed to explain particular reaction outcomes in distinct energy and mass regimes, but a unifying picture remains elusive. The irreversible transfer of kinetic energy from the relative motion of the collision partners to their internal states, as is known to occur in deep inelastic collisions, has yet to be successfully incorporated explicitly into fully quantal reaction models. The influence of these processes on fusion is not yet quantitatively understood.Purpose: To investigate the population of high excitation energies in transfer reactions at sub-barrier energies, which are precursors to deep inelastic processes, and their dependence on the internuclear separation.Methods: Transfer probabilities and excitation energy spectra have been measured in collisions of $^{16,18}\mathrm{O},\phantom{\rule{0.16em}{0ex}}^{19}\mathrm{F}\phantom{\rule{0.16em}{0ex}}+\phantom{\rule{0.16em}{0ex}}^{208}\mathrm{Pb}$, at various energies below and around the fusion barrier, by detecting the backscattered projectile-like fragments in a $\mathrm{\ensuremath{\Delta}}E\text{\ensuremath{-}}E$ telescope.Results: The relative yields of different transfer outcomes are strongly driven by $Q$ values, but change with the internuclear separation. In $^{16}\mathrm{O}\phantom{\rule{0.16em}{0ex}}+\phantom{\rule{0.16em}{0ex}}^{208}\mathrm{Pb}$, single nucleon transfer dominates, with a strong contribution from $\ensuremath{-}2p$ transfer close to the Coulomb barrier, though this channel becomes less significant in relation to the $\ensuremath{-}2p2n$ transfer channel at larger separations. For $^{18}\mathrm{O}\phantom{\rule{0.16em}{0ex}}+\phantom{\rule{0.16em}{0ex}}^{208}\mathrm{Pb}$, the $\ensuremath{-}2p2n$ channel is the dominant charge transfer mode at all separations. In the reactions with $^{19}\mathrm{F},\phantom{\rule{0.16em}{0ex}}\ensuremath{-}3p2n$ transfer is significant close to the barrier, but falls off rapidly with energy. Multinucleon transfer processes are shown to lead to high excitation energies (up to $\ensuremath{\sim}15$ MeV), which is distinct from single nucleon transfer modes which predominantly populate states at low excitation energy.Conclusions: Kinetic energy is transferred into internal excitations following transfer, with this energy being distributed over a larger number of states and to higher excitations with increasing numbers of transferred nucleons. Multinucleon transfer is thus a mechanism by which energy can be dissipated from the relative motion before reaching the fusion barrier radius.

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.

Synthesis of superheavy nuclei: Obstacles and opportunities

There are only 3 methods for the production of heavy and superheavy (SH) nuclei, namely, fusion reactions, a sequence of neutron capture and beta(-) decay and multinucleon transfer reactions. Low values of the fusion cross sections and very short half-lives of nuclei with Z>120 put obstacles in synthesis of new elements. At the same time, an important area of SH isotopes located between those produced in the cold and hot fusion reactions remains unstudied yet. This gap could be filled in fusion reactions of 48 Ca with available lighter isotopes of Pu, Am, and Cm. New neutron-enriched isotopes of SH elements may be produced with the use of a 48 Ca beam if a 250 Cm target would be prepared. In this case we get a real chance to reach the island of stability owing to a possible beta(+) decay of 291 114 and 287 112 nuclei formed in this reaction with a cross section of about 0.8 pb. A macroscopic amount of the long-living SH nuclei located at the island of stability may be produced by using the pulsed nuclear reactors of the next generation only if the neutron fluence per pulse will be increased by about three orders of magnitude. Multinucleon transfer processes look quite promising for the production and study of neutron-rich heavy nuclei located in upper part of the nuclear map not reachable by other reaction mechanisms. Reactions with actinide beams and targets are of special interest for synthesis of new neutron-enriched transfermium nuclei and not-yet-known nuclei with closed neutron shell N=126 having the largest impact on the astrophysical r-process. The estimated cross sections for the production of these nuclei allows one to plan such experiments at currently available accelerators.

Time-dependent Hartree-Fock calculations for multi-nucleon transfer processes: Effects of particle evaporation on production cross sections

We present a microscopic calculation of multi-nucleon transfer reactions employing the time-dependent Hartree-Fock (TDHF) theory. In our previous publication [Phys. Rev. C 88, 014614 (2013)], we reported our analysis for the multi-nucleon transfer processes for several systems. Here we discuss effects of particle evaporation processes on the production cross sections. Since particle evaporation processes may not be described adequately by the TDHF calculations, we evaluate them using a statistical model. As an input of the statistical model, excitation energies of the final fragments are necessary. We evaluate them from the TDHF wave function after collisions, extending the particle number projection technique. From the calculation, the particle evaporation effects are found to improve descriptions of the production cross sections. However, the production cross sections are still underestimated for processes where a number of protons are transferred. Possible origins of the discrepancy are discussed.

Particle-number projection method in time-dependent Hartree-Fock theory: Properties of reaction products

Background: The time-dependent Hartree-Fock (TDHF) theory has been successful in describing low-energy heavy ion collisions. Recently, we have shown that multinucleon transfer processes can be reasonably described in the TDHF theory combined with the particle-number projection technique. Purpose: In this work, we propose a theoretical framework to analyze properties of reaction products in TDHF calculations. Methods: TDHF calculation in three-dimensional Cartesian grid representation combined with particle number projection method. Results: We develop a theoretical framework to calculate expectation values of operators in the TDHF wave function after collision with the particle-number projection. To show how our method works in practice, the method is applied to $^{24}$O+$^{16}$O collisions for two quantities, angular momentum and excitation energy. The analyses revealed following features of the reaction: The nucleon removal proceeds gently, leaving small values of angular momentum and excitation energy in nucleon removed nuclei. Contrarily, nuclei receiving nucleons show expectation values of angular momentum and excitation energy which increase as the incident energy increases. Conclusions: We have developed a formalism to analyze properties of fragment nuclei in the TDHF theory combined with the particle-number projection technique. The method will be useful for microscopic investigations of reaction mechanisms in low-energy heavy ion collisions as well as for evaluating effects of particle evaporation on multinucleon transfer cross sections.