Heavy-ion Reactions Research Articles

Nucleon-nucleon correlation studies in heavy-ion transfer reactions

We present some of the recent experimental results in heavy-ion transfer reactions obtained with the large solid angle magnetic spectrometer PRISMA at energies close to the Coulomb barrier. We focus on a series of experiments that have been carried out to study the nucleon-nucleon correlations for closed shell and superfluid systems. They are discussed together with the newest results concerning the proton transfer channels above and below the Coulomb barrier. The second set of the experiments was performed to study the production mechanism of heavy neutron-rich nuclei and the related effects of secondary processes.

Fission studies using multinucleon transfer reactions

It is shown that the multinucleon transfer reactions is a powerful tool to study fission of exotic neutron-rich actinide nuclei, which cannot be accessed by particle-capture or heavy-ion fusion reactions. Identification of fissioning nuclei and of their excitation energy is performed on an event-by-event basis, through the measurement of outgoing ejectile particle in coincidence with fission fragments. Fission fragment mass distributions (FFMDs) are measured for each transfer channel, in selected bins of excitation energy. It was found that the mass distributions for all the studied nuclides maintain a double-humped shape up to the high energies that the shell effects are expected to smear out. From a comparison with the dynamical calculation based on the fluctuation-dissipation model, this behavior of the mass distributions was unambiguously attributed to the effect of multichance fission.

Background estimate in heavy-ion two-body reactions measured by the MAGNEX spectrometer

The MAGNEX magnetic spectrometer is nowadays used in the experimental measurements of rare quasi-elastic reactions between heavy ions at intermediate energy within the NUMEN project. The small cross sections involved in such processes under the large yields due to competitive reaction channels have motivated an accurate control of the background sources. In such view, the not ideal particle identification could introduce spurious contributions which have been identified and evaluated in the present analysis.

Effects of the QCD phase transition on hadronic observables in relativistic hydrodynamic simulations of heavy-ion reactions in the FAIR/NICA energy regime

We investigate hadronic particle spectra and flow characteristics of heavy-ion reactions in the FAIR/NICA energy range of 1 AGeV ≤ Elab ≤ 10 AGeV within a relativistic ideal hydrodynamic one-fluid approach. The particlization is realized by sampling the Cooper-Frye distribution for a grand canonical hadron gas on a hypersurface of constant energy density. Results of the hydrodynamic calculations for different underlying equations of state are presented and compared with experimental data and microscopic transport simulations. The sensitivity of the approach to physical model inputs concerning the initial state and the particlization is studied.

Nuclear liquid-gas phase transition with machine learning

The machine-learning techniques have shown their capability for studying phase transitions in condensed matter physics. Here, we employ the machine-learning techniques to study the nuclear liquid-gas phase transition. We adopt an unsupervised learning and classify the liquid and gas phases of nuclei directly from the final state raw experimental data of heavy-ion reactions. Based on a confusion scheme which combines the supervised and unsupervised learning, we obtain the limiting temperature of the nuclear liquid-gas phase transition. Its value $9.24\pm0.04~\rm MeV$ is consistent with that obtained by the traditional caloric curve method. Our study explores the paradigm of combining the machine-learning techniques with heavy-ion experimental data, and it is also instructive for studying the phase transition of other uncontrollable systems, like QCD matter.

The total reaction cross section of heavy-ion reactions induced by stable and unstable exotic beams: the low-energy regime

In this review paper we present a detailed account of the extraction and the calculation of the total reaction cross section of strongly bound and weakly bound, stable and unstable, exotic, nuclei. We discuss the optical model and the more general coupled channels model of direct reactions. The effect of long-range absorption due to the coupling to excited states in the target and to the breakup continuum in the projectile is also discussed. The generalized optical theorem for charged particle scattering and the resulting sum-of differences method is then discussed. The so-called "quarter-point recipe" is discussed next, and the quarter-point angle is introduced as a simple and rapid mean to obtain the total reaction cross section. The last topic discussed is the reduction of the total reaction cross section that would allow a large body of data to sit on a single universal function. Such a universal function exists in the case of the fusion data, and the aim of this last topic of the review is to extend the fusion case to the total reaction, by adding the direct reaction contribution. Also discussed is the inclusive breakup cross section and how it can be used to extract the total reaction cross section of the interacting fragment with the target. This method is also known as the Surrogate method and represents a case of hybrid reactions. The sum of the integrated inclusive breakup cross section with the complete fusion cross section supplies the total fusion cross section. The main experimental methods to determine the total reaction cross section are also discussed, with emphasis in recent techniques developed to deal with reactions induced by unstable beams.

Performance results of National Array of Neutron Detectors (NAND) facility at IUAC

The 100 element neutron detector array facility, National Array of Neutron Detectors (NAND) has been installed at Inter University Accelerator Centre (IUAC), New Delhi for experiments in heavy-ion reaction studies. The detector array consists of 100 organic liquid scintillators, each of size 5”×5” coupled to 5 in. diameter photomultiplier. The liquid cells are mounted at 175 cm from the target with full array covering 3.3% of 4π solid angle. We present an overview of the design and implementation of various subsystems of the array. Description of the mechanical structure, development of detectors and nuclear electronics for signal processing, data acquisition, characterization of detectors and test results are presented. Results on the performance characteristics of the array are given based on the offline tests using radioactive sources and commissioning test by in-beam experiments. The system demonstrates good n−γ discrimination and time-of-flight performance.

Fission-fragment excitation energy sharing beyond scission

A simplified, though realistic, model describing two receding and accelerating fission fragments, due to their mutual Coulomb repulsion, shows that fission fragments share excitation energy well after they ceased to exchange nucleons. This mechanism leads to a lower total kinetic energy of the fission fragments, particularly if the pygmy resonances in the fission fragments are excited. Even though the emphasis here is on fission, similar arguments apply to fragments in heavy-ion reactions.

Calibration of the electromagnetic calorimeter ECal of the HADES experiment

HADES is a large acceptance spectrometer operating at SIS18, GSI, Germany. It is aimed at exploration of QCD phase diagram at the ion beam energies of 1-2 AGeV in the region of high baryonic densities. The new segmented electromagnetic calorimeter (ECal) was built to extend experimental opportunities of the HADES detector. The electromagnetic calorimeter will allow to study new reaction channels involving the production of neutral mesons and neu-tral resonances in elementary and heavy-ion reactions via detection of their two photon decay. An additional advantage of such a device is the resulting improvement of the electron-to-pion separation at large momenta.The detector is based on 978 Cherenkov lead glass modules divided into 6 sectors, and it covers forward angles of 12° < θ < 45° and almost full azimuthal angle. Currently four out of six sectors planned are assembled in the experimental area. The first raw beam data obtained with the ECal detector in Ag+Ag reactions at 1.65 AGeV beam are presented.

Role of neutron transfer in the sub-barrier fusion cross section in O18 + Sn116

Background: In heavy-ion-induced fusion reactions, cross sections in the sub-barrier region are enhanced compared to predictions of the one-dimensional barrier penetration model. This enhancement is often understood by invoking deformation and coupling of the relative motion with low-lying inelastic states of the reaction partners. However, effects of nucleon transfer on fusion below the barrier, especially for the systems having positive $Q$ value neutron transfer (PQNT) channels, are yet to be disentangled completely.Purpose: We intend to study the role of the PQNT effect on the sub-barrier fusion of the $^{18}\mathrm{O} + ^{116}\mathrm{Sn}$ system having positive $Q$ value for the two-neutron stripping channel. Also we reflect on the interplay of couplings involved in the system around the Coulomb barrier.Method: The fusion excitation function was measured at energies from $11%$ below to $46%$ above the Coulomb barrier for $^{18}\mathrm{O} + ^{116}\mathrm{Sn}$ using a recoil mass spectrometer, viz., the Heavy-Ion Reaction Analyser (HIRA). Fusion barrier distributions were extracted from the data. Results from the experiment were analyzed within the framework of the coupled-channels model.Results: Fusion cross sections at energies below the Coulomb barrier showed strong enhancement compared to predictions of the one-dimensional barrier penetration model. The fusion process is influenced by couplings to the collective excitations with coupling to single- and two-phonon vibrational states of the target and the projectile respectively. Inclusion of the two-neutron transfer channel in the calculation along with these couplings could reproduce the data satisfactorily.Conclusions: The significant role of PQNT in enhancing the sub-barrier fusion cross section for the chosen system is not observed. It simply reduced the sub-barrier fusion cross section. Therefore, a consistent link between PQNT and sub-barrier fusion enhancement could not be established vividly while comparing the fusion excitation function from this work with the same from other $^{16,18}\mathrm{O}$-induced reactions. This clearly points to the need for more experimental as well as theoretical investigation in this field.

Recent results on heavy-ion direct reactions of interest for 0νββ decay at INFN - LNS

Neutrinoless double beta decay of nuclei, if observed, would have important implications on fundamental physics. In particular it would give access to the effective neutrino mass. In order to extract such information from 0νββ decay half-life measurements, the knowledge of the Nuclear Matrix Elements (NME) is of utmost importance. In this context the NUMEN and the NURE projects aim to extract information on the NME by measuring cross sections of Double Charge Exchange reactions in selected systems which are expected to spontaneously decay via 0νββ. In this work an overview of the experimental challenges that NUMEN is facing in order to perform the experiments with accelerated beams and the research and development activity for the planned upgrade of the INFN-LNS facilities is reported.

Production of neutron deficient isotopes in the multinucleon transfer reaction Ca48(Elab=5.63MeV/nucleon)+Cm248

The yields of products of multinucleon transfer reaction $^{48}\mathrm{Ca}+^{248}\mathrm{Cm}$ at energy ${E}_{\mathrm{lab}}=5.63\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}/\mathrm{nucleon}$ around the corresponding Coulomb barrier are analyzed within the dinuclear system model. The calculation results are compared with the experimental data obtained with the separator for heavy-ion reaction products SHIP at GSI. The model describes rather well the experimental production cross sections of isotopes with $Ng126$ but deviates from the experimental cross sections for the very neutron-deficient Pa and U isotopes.

Study of the surface energy coefficient used in nuclear proximity potential from density-dependent nucleon–nucleon interactions

The nuclear surface tension coefficient, γ, of the proximity potential is systematically investigated from the microscopic double-folding (DF) model accompanied by the realistic density-dependent nucleon–nucleon (NN) interactions. In the present work, we select six versions of density-dependent M3Y–Paris effective NN forces, including DDM3Y1 (with K=176MeV), CDM3Y1 (with K=188MeV), CDM3Y3 (with K=217MeV), CDM3Y5 (with K=241 MeV), CDM3Y6 (with K=252MeV) and BDM3Y1 (with K=270MeV). A total of 70 heavy-ion fusion reactions with condition 48≤Z1Z2≤2460 for the charge product of their reacting nuclei have been analyzed. By fitting the calculated values against the asymmetry parameter As, we present a new analytical formula for the parameter γ which is directly dependent on the nuclear matter (NM) incompressibility constant K. It is shown that the presently obtained formula in proximity potential model can reproduce the fusion data well. In this study, a discussion is also presented about the influence of the incompressibility constant K on the interaction potentials and also on the fusion cross sections resulting from the proximity approach for different heavy-ion reactions.

Direct photon emission and influence of dynamical wave packets in an extended quantum molecular dynamics model

Direct photon produced from first proton-neutron ($p$-$n$) collision during the early stage of heavy ion reaction is a sensitive probe to reflect energy and momentum distribution of nucleons. In this work, we embedded the hard photon production channel in an extended quantum molecular dynamics (EQMD) model, and took the direct photon as a possible probe to improve namely the Fermi motion in the EQMD model. A possible scheme is offered to handle the dynamical wave packet width within incoherent bremsstrahlung process. Direct photons calculated by our modified EQMD were compared with data of $^{14}$N + $^{12}$C at beam energies $E/A$ = 20, 30 and 40 MeV, and it is found that the yield, inverse slope and angular distribution of direct photons could be reasonably reproduced. In addition, asymmetric reaction systems of $^{4}$He + C and $^{4}$He + Zn at $E/A$ = 53 MeV are also simulated in this work. It is found that the symmetric angular distribution in the nucleon-nucleon ($N$-$N$) center-of-mass (c.m.) frame and the velocity of $ the \gamma$-emission source can be reasonably obtained from our method although there is some quantitative differences.

Multinucleon transfer reaction from view point of dynamical dinuclear system method

The dinuclear system (DNS) model has been widely used to explain different heavy-ion reactions such as fusion, quasifission and fission. In previous studies, the DNS is treated as a local N/Z equilibrium system formed after the capture process in the final stage of the reaction. In this work, we generalize the DNS concept to describe the multi-nucleon transfer (MNT) reaction in the dynamical radial process. By treating the ensemble of projectile and target at any distance as the dinuclear system, we solve the semi-classical dynamical radial equation and the master equation for the proton and neutron distributions in the dinuclear system (DyDNS) simultaneously for the first time. In this method, the classical radial movement is coupled to the internal quantum single particle transition between the colliding nuclei. The dynamical Monte Carlo method is engaged to simulate the solutions, and then the GEMINI++ model is adopted to deal with the de-excitations. To illustrate the validity of our approach, the MNT reaction 136Xe + 208Pb has been re-examined. The obtained results demonstrate a better agreement with the available experimental data than the calculations of the GRAZING model, especially for the transfer of many nucleons.

Symmetry energy of super-dense neutron-rich matter from integrating barotropic pressures in neutron stars and heavy-ion reactions

Within the minimum model of neutron stars (NS) consisting of neutrons, protons and electrons, a new approach is proposed for inferring the symmetry energy of super-dense neutron-rich nucleonic matter above twice the saturation density ρ0 of nuclear matter directly from integrating iteratively barotropic pressures in both neutron stars and heavy-ion reactions. Simultaneously, the proton fraction of NSs at β equilibrium is extracted as a function of baryon density from the same procedure. An application of this approach using the NS pressure from GW170817 and the pressure in cold symmetric nuclear matter (SNM) extracted earlier by analyzing nuclear collective flow data in relativistic heavy-ion collisions provides a useful constraining band for the symmetry energy above 2ρ0.

Expanded Empirical Formula for Coulomb Final State Interaction in the Presence of Lévy Sources

Measurements of momentum space correlations in heavy ion reactions are a unique tools to investigate the properties of the created medium. However, these analyses require the careful handling of the final state interactions such as the Coulomb repulsion of the involved particles. In small systems such as $e^+ + e^-$ or $p+p$ the well-known Gamow factor gives an acceptable description but in the case of extended sources like that are created in heavy ion collisions, a more sophisticated approach has to be developed. In this paper we expand our previous work on the investigation of the Coulomb final state interaction in the presence of a L\'evy source. Such sources were shown to be a statistically acceptable assumption to describe the quantumstatistical correlation functions in high energy heavy ion reactions.

Parton-hadron-quantum-molecular dynamics: A novel microscopic n -body transport approach for heavy-ion collisions, dynamical cluster formation, and hypernuclei production

Cluster and hypernuclei production in heavy-ion collisions is presently under active experimental and theoretical investigation. Since clusters are weekly bound objects, their production is very sensitive to the dynamical evolution of the system and its interactions. The theoretical description of cluster formation is related to the n-body problem. Here we present the novel n-body dynamical transport approach PHQMD (Parton-Hadron-Quantum-Molecular Dynamics) which is designed to provide a microscopic description of nuclear cluster and hypernucleus formation as well as of general particle production in heavy-ion reactions at relativistic energies. In difference to the coalescence or statistical models, often used for the cluster formation, in PHQMD clusters are formed dynamically due to the interactions between baryons described on a basis of Quantum Molecular Dynamics (QMD)which allows to propagate the n-body Wigner density and n-body correlations in phase-space, essential for the cluster formation. The clusters are identified by the MST (Minimum Spanning Tree) or the SACA ('Simulated Annealing Cluster Algorithm') algorithm which finds the most bound configuration of nucleons and clusters. Collisions among hadrons as well as Quark-Gluon-Plasma formation and parton dynamics in PHQMD are treated in the same way as in the established PHSD (Parton-Hadron-String Dynamics)transport approach. In order to verify our approach with respect to the general dynamics we present here the first PHQMD results for general 'bulk' observables such as rapidity distributions and transverse mass spectra for hadrons ($\pi, K, \bar K, p, \bar p, \Lambda, \bar \Lambda$) from SIS to RHIC energies. We find a good description of the 'bulk' dynamics which allows us to proceed with the results on cluster production, including hypernuclei.

Systematics of multinucleon transfer in heavy-ion reactions

One-neutron pickup reactions for 52 projectile–target combinations were analysed using a systematics between transfer cross-sections and ground-state Q-values. One-neutron pickup transfer shows a good correlation between reduced transfer cross-sections and ground-state Q-values ( $$Q_{gg}$$ ) if one separate the systems into two groups based on their $$Z_{p}Z_{t}$$ product. Also, similar kind of systematics is applied to 2n, 3n and 4n pickup transfer and a good correlation is obtained between reduced transfer cross-sections and $$Q_{gg}$$ values, where no $$Z_{p}Z_{t}$$ dependence is seen.

Dynamical aspects of Ti48+Fe58,Ni58→Cd*106,Sn*106 reactions at energies near the Coulomb barrier

The study of the heavy-ion reactions at the near- and sub-barrier regimes gives immense information about the nuclear structure and the involved reaction dynamics. It has been observed that a slight difference in the nuclear structure may lead to a significant change in the sub-barrier fusion excitation functions. The studies of different trends of excitation functions below the Coulomb barrier region due to dissimilar structures of the nuclei, which are involved in the reaction, are available in literature. To understand the role of different structures and dynamics involved in such reactions, an investigation of $^{48}\mathrm{Ti}$-induced reactions on $^{58}\mathrm{Fe}$ and $^{58}\mathrm{Ni}$ forming $^{106}\mathrm{Cd}^{*}$ and $^{106}\mathrm{Sn}^{*}$ compound nuclei (CN), respectively, has been made at similar ${E}_{\mathrm{c}.\mathrm{m}.}/{V}_{b}$ values within the framework of the dynamical cluster decay model (DCM). The difference in the structure between the nuclei is quite notable with $^{58}\mathrm{Ni}$ and $^{106}\mathrm{Sn}^{*}$ having a proton shell closure ($Z=28$ and 50, respectively). Within the DCM, the experimental fusion evaporation cross sections are reproduced using deformed configurations effects included up to quadruple deformations (${\ensuremath{\beta}}_{2i}$) for two nuclei having optimum orientations ${\ensuremath{\theta}}^{\mathrm{opt}.}$. The fusion evaporation data at near- and sub-barriers has been explained through the calculated enhanced $\mathrm{\ensuremath{\Sigma}}{P}_{0}$ values of the decaying channels in the case of $^{106}\mathrm{Cd}^{*}$ in comparison to $^{106}\mathrm{Sn}^{*}$. Moreover, it is observed that the quantum tunneling of the fragments is less hindered in the case of $^{106}\mathrm{Cd}^{*}$ as compared to $^{106}\mathrm{Sn}^{*}$ at the lower values of ${E}_{\mathrm{c}.\mathrm{m}.}/{V}_{b}$, i.e., having less barrier modification ($\mathrm{\ensuremath{\Delta}}{V}_{B}$) in the case of the former. The role of magicity has been further explored with the plotted values of the ratio of fusion cross sections (${\ensuremath{\sigma}}_{\mathrm{fus}}$) of CN $^{106}\mathrm{Sn}^{*}$ and $^{112}\mathrm{Xe}^{*}$ (formed in the reaction with both the projectile $^{58}\mathrm{Ni}$ and target $^{54}\mathrm{Fe}$ having proton and neutron magicity, respectively) with respect to $^{106}\mathrm{Cd}^{*}$, which are highly suppressed in the case of the latter in comparison to the former, particularly, below the Coulomb barrier.