Search

The recent observation of the compact star XTE J1814-338 with a mass of $M=1.2^{+0.05}_{-0.05}~{\rm M_{\odot}}$ and a radius of $R=7^{+0.4}_{-0.4}$ km, together with the HESS J1731-347, which has a mass of $M=0.77^{+0.20}_{-0.17}~{\rm M_{\odot}}$ and a radius of $R=10.4^{+0.86}_{-0.78}$ km, they provide evidence for the possible presence of exotic matter in the core of neutron stars and significantly enhance our understanding of the equation of state for the dense nuclear matter. In the present srtudy, we investigate the possible existence of neutral anti-kaons and negative charged kaons in neutron stars by employing the Relativistic Mean Field model with first order kaonic (${K^{-}}$ and ${\bar{K^{0}}}$) condensates. To the best of our knowledge, this represents a first alternative attempt aimed to explain the bulk properties of the XTE J1814-338 object and at the same time the HESS J1731-347 object, using a mixture of kaons condensation in dense nuclear matter. In addition, we compare our analysis approach with the recent observation of PSR J0437-4715 and PSR J1231-1411 pulsars, proposing that to explain all objects simultaneously, it is essential to consider two distinct branches, each corresponding to a different composition of nuclear matter., Comment: 4 pages, 3 figures

The recent observation of a compact star with a mass of $M=0.77^{+0.20}_{-0.17}~{\rm M_{\odot}}$ and a radius of $R=10.4^{+0.86}_{-0.78}$ km, located within the supernova remnant HESS J1731-347, has substantially reinforced the evidence for the presence of exotic matter in neutron stars core. This finding has markedly enhanced our comprehension of the equation of state for dense nuclear matter. In the present work, we investigate the possible existence of a kaon condensation in hadronic neutron stars by employing and comparing two theoretical frameworks: the Relativistic Mean Field model with first order kaon condensate and the Momentum-Dependent Interaction model complemented by chiral effective theory. To the best of our knowledge, this represents a first alternative attempt aimed to explain the bulk properties of the specific event with the inclusion of a kaon condensation in dense nuclear matter. The application of two different models enriches the research, providing insights from the aspect of different theoretical frameworks that accurately predict the existence of HESS J1731-347. In both cases significant insights are extracted for the parameter space of both models, emphasizing to those concerning the nucleon-kaon potential, the threshold density for the appearance of a kaon condensation, as well as the parameter $a_{3}m_{s}$ which is related to the strangeness content of the proton. Concluding, the present research indicates that a more systematic investigation of similar events could offer valuable constraints on the properties of dense nuclear matter., Comment: 9 pages, 7 figures, 1 table

Neutron stars and black holes are the after death remnants of massive stars. However, according to the most recent observations, the neutron stars maximum mass is between $2.0-2.5 M_{\odot}$ while black holes of less than 5 $M_{\odot}$ rarely has been observed. The region between the most massive neutron star and the least massive black hole is called the mass gap. If indeed its existence is confirmed by future observations, that indicates a gap in our understanding which seeks for explanation. In addition, the existence of compact objects within the mass gap should also be supported with the help of possible new theoretical scenarios. In this letter, we propose a possible explanation for the existence of compact objects within the mass gap region. Specifically, we propose that the mass gap region could be bridged by the existence of a hybrid compact object, composed of hadronic and self interacting - non annihilating fermionic dark matter, considering that the interaction between these two fluids its only gravitational. Fundamental questions about how these objects form and how they can be detected are also addressed., Comment: 5 pages, 3 figures. Any comments are welcome

In a remarkable study by M. Gleiser and N. Jiang [Phys. Rev. D {\bf 92}, 044046, 2015], the authors demonstrated that the stability regions of neutron stars, within the framework of the simple Fermi gas model, and self-gravitating configurations of complex scalar field (boson stars) with various self couplings, obtained through traditional perturbation methods, correlate with critical points of the configurational entropy with an accuracy of a few percent. Recently, P. Koliogiannis \textit{et al.} [Phys. Rev. D {\bf 107}, 044069 2023] found that while the minimization of the configurational entropy generally anticipates qualitatively the stability point for neutron stars and quark stars, this approach lacks universal validity. In this work, we aim to further elucidate this issue by seeking to reconcile these seemingly contradictory findings. Specifically, we calculate the configurational entropy of bosonic and fermionic systems, described by interacting Fermi and boson gases, respectively, that form compact objects stabilized by gravity. We investigate whether the minimization of configurational entropy coincides with the stability point of the corresponding compact objects. Our results indicate a strong correlation between the stability points predicted by configurational entropy and those obtained through traditional methods, with the accuracy of this correlation showing a slight dependence on the interaction strength. Consequently, the stability of compact objects, composed of components obeying Fermi or boson statistics, can alternatively be assessed using the concept of configurational entropy., Comment: v1: 9 pages, 3 figures, 1 table. v2: 10 pages, 4 figures, 2 tables; accepted for publication in Physical Review D

Till today, the nature of Dark Matter (DM) remains elusive despite all our efforts. This missing matter of the universe has not been observed by the already operating DM direct-detection experiments, but we can infer its gravitational effects. Galaxies and clusters of galaxies are most likely to contain DM trapped to their gravitational field. This leads us to the natural assumption that compact objects might contain DM too. Among the compact objects exist in galaxies, neutron stars are considered as natural laboratories, where theories can be tested, and observational data can be received. Thus, many models of DM have proposed it's presence in those stars. In particular, in the present study we focus on two types of dark matter particles, namely fermions and bosons with a mass range of [0.01-1.5] GeV and self-interaction strength in the range [10$^{-4}$-10$^{-1}$] MeV$^{-1}$. By employing the two-fluid model, we discovered a stable area in the M-R diagram of a celestial formation consisting of neutron star matter and DM that is substantial in size. This formation spans hundreds of kilometers in diameter and possesses a mass equivalent to 100 or more times the Solar mass. To elucidate, this entity resembles an enormous celestial body of DM, with a neutron star at its core. This implies that a supramassive stellar compact entity can exist without encountering any issues of stability and without undergoing a collapse into a black hole. In any case, the present theoretical prediction can, if combined with corresponding observations, shed light on the existence of DM and even more on its basic properties., Comment: 11 pages, 7 figures. The revised version has also taken into account comments, suggestions and remarks from collaborators while additional references have been included. Any comments are welcome

We investigate the hypothetical X17 boson on neutron stars and Quark Stars (QSs) using various hadronic Equation of States (EoSs) with phenomenological or microscopic origin. Our aim is to set realistic constraints on its coupling constant and the mass scaling, with respect to causality and various possible upper mass limits and the dimensionless tidal deformability $\Lambda_{1.4}$. In particular, we pay special attention on two main phenomenological parameters of the X17, the one is related to the coupling constant $\mathrm{g}$ that it has with hadrons or quarks and the other with the in-medium effects through the regulator $\mathrm{C}$. Both are very crucial concerning the contribution on the total energy density and pressure. In the case of considering the X17 as a carrier of nuclear force in Relativistic Mean Field (RMF) theory, an admixture into vector boson segment was constrained by 20\% and 30\%. In our investigation, we came to the general conclusion that the effect of the hypothetical X17 both on neutron and QSs constrained mainly by the causality limit, which is a specific property of each EoS. Moreover, it depends on the interplay between the main two parameters that is the interaction coupling $\mathrm{g}$ and the in-medium effects regulator $\mathrm{C}$. These effects are more pronounced in the case of QSs concerning all the bulk properties., Comment: 12 pages, 14 figures, 2 tables

An approach to identify medium-mass ejectiles from peripheral heavy-ion reactions in the energy region of 15 MeV/nucleon is developed for data obtained with a large acceptance magnetic spectrometer. This spectrometer is equipped with a focal plane multidetector, providing position, angle, energy loss and residual energy of the ions along with measurement of the time-of-flight. Ion trajectory reconstruction is performed at high order and ion mass is obtained with a resolution of better than 1/150. For the unambiguous particle identification however, the reconstruction of both the atomic number Z and the ionic charge q of the ions is critical and it is suggested, within this work, to be performed prior to mass identification. The new proposed method was successfully applied to MAGNEX spectrometer data, for identifying neutron-rich ejectiles related to multinucleon transfer generated in the 70Zn+ 64Ni collision at 15 MeV/nucleon. This approach opens up the possibility of employing heavy-ion reactions with medium-mass beams below the Fermi energy (i.e., in the region 15-25 MeV/nucleon) in conjunction with large acceptance ray tracing spectrometers, first, to study the mechanism(s) of nucleon transfer in these reactions and, second, to produce and study very neutron-rich or even new nuclides in previously unexplored regions of the nuclear landscape., Comment: 6 pages, 6figures

The nuclear Equation of State (EoS) lies in the center of the nuclear N-body problem as it describes the properties of the Nuclear Matter (NM) and determines the parameters of the nuclear interaction. In this work, we propose a theoretical description of the EoS of both Symmetric (SNM) and Asymmetric (ANM) nuclear matter within the framework of Fermionic Dynamics. With this description we produce several new semi-hard EoS with density dependent effective mass. Finally, we transform the aforementioned theory in order to be consistent with Mean-Field dynamics. We use this approach to accurately calculate the binding energies and charge radii of nuclei in the A= 40--238 region with the Skyrme Hartree Fock (SHF) model., Comment: 21 pages, 3 figures

The synthesis of hyper-heavy elements is investigated under conditions simulating neutron star environment. The Constrained Molecular Dynamics (CoMD) approach is used to simulate low energy collisions of extremely n-rich nuclei. A new type of the fusion barrier due to a "neutron wind" is observed when the effect of neutron star environment (screening of Coulomb interaction) is introduced implicitly. When introducing also a background of surrounding nuclei, the nuclear fusion becomes possible down to temperatures of 10$^{8}$ K and synthesis of extremely heavy and n-rich nuclei appears feasible. A possible existence of hyper-heavy nuclei in a neutron star environment could provide a mechanism of extra coherent neutrino scattering or an additional mechanism, resulting in X-ray burst or a gravitational wave signal and, thus, becoming another crucial process adding new information to the suggested models on neutron star evolution., Comment: 10 pages, 5 figures

This paper presents our recent efforts to study the momentum distributions as well as the production of neutron-rich rare isotopes with heavy-ion beams in the energy region of 15 MeV/nucleon. Experimental cross sections of neutron-rich nuclides from collisions of a 86 Kr (15 MeV/nucleon) beam with 64 Ni and 58 Ni targets are presented. Experimental data were obtained from the previous work of our group with the MARS mass spectrometer at the Cyclotron Institute of Texas A&M University. On that note, detailed calculations of yields and momentum distributions of neutron-rich projectile-like fragments are presented for the interaction of 86 Kr with 64 Ni and compared with the aforementioned experimental data. The calculations were based on a two-step general approach: the dynamical stage of the primary interaction was described with the phenomenological deep-inelastic transfer model (DIT) and the microscopic constrained molecular dynamics model (CoMD); the deexcitation stage of the excited projectile fragments was described with the statistical binary-decay model GEMINI. The experimental data show an enhancement in the production of neutron-rich isotopes close to the projectile, and interestingly of heavier than the projectile neutron-rich nuclei. The behaviour of the data is relative to the predictions of the CoMD/GEMINI calculation. The study of the momentum distributions offers a novel route to study the reaction mechanism that dominates the production of the fragments of interest in peripheral heavy-ion collisions at intermediate energies. In the future, we plan to analyze experimental data that were obtained from the MAGNEX spectometer at the INFN-LNS in Catania, Italy.

We present our recent study of cross sections and angular distributions of projectile fragments from heavy-ion reactions at beam energy of 15 MeV/nucleon. We studied the production cross sections and the angular distributions of neutron-rich nuclides from collisions of a 86 Kr (15 MeV/nucleon) beam with heavy targets ( 64 Ni, 124 Sn and 238 U). Experimental data from our previous work at Texas A & M were compared with model calculations. Our calculations were based on a two-step approach: the dynamical stage of the collision was described with, first, the phenomenological Deep-Inelastic Transfer model (DIT) and, alternatively, with the microscopic Constrained Molecular Dynamics model (CoMD). The de-excitation of the hot heavy projectile fragments was performed with the Statistical Multifragmentation Model (SMM). An overall good discription of the available data was obtained with the models employed. Furthermore, we performed calculations with a radioactive beam of 92 Kr (15 MeV/nucleon) interacting with a target of 238 U. We observed that the multinucleon transfer mechanism leads to extremely neutron-rich nuclides toward and beyond the astrophysical r-process path., Comment: 15 pages, 8 figures

We present a hypothesis that the anomaly in the folding angle distribution of electron-positron pairs, emitted in the decay of the excited levels of nucleus $^{8}$Be and $^{4}$He can be related to the cluster structure of the decaying state. Furthermore, we present a hypothesis that the potentially observed boson with re st mass $m_{X}$=17 MeV can mediate the nucleon-nucleon interaction at the low-energy regime of QCD, in particular in the weakly bound cluster state p+$^{7}$Li,$^{3}$H. We present a possible equations of state of symmetric nuclear matter corresponding to the vector meson mass $m_{v}$=17 MeV, obtained using relativistic mean field theory of nuclear force, QHD-I in particular, with physically relevant incompessibility K$_{0}$=240-260 MeV and the values of couplings $g_{v}$, $g_{s}$ lower than unity. Based on concepts of chiral symmetry breaking, we show that reduction of the rest mass of pseudoscalar particle from physical value $m_{\pi}$=135 MeV to $m_{X}$=17 MeV is equivalent to reduction of the quark mass from dynamical value around 310 MeV down to current quark mass around 5 MeV ($\frac{m_{X}^2} {m_{q,curr}} \simeq \frac{m_{\pi}^2} {m_{q,dyn}}$). Corresponding version of Goldberger-Treiman relation leads to the value of coupling close to the results from relativistic mean field theory of nuclear force. Both model approaches thus point towards apparent restoration of chiral symmetry in nucleon-nucleon interaction at large distances, possibly via bounce into false instanton vacuum. Observation of boson with rest mass $m_{X}$=17 MeV in the decay of high lying excited states of $^{8}$Be and $^{4}$He can possibly resolve one of the longest lasting open questions in nuclear physics., Comment: Version accepted for publication in Journal of Physics G, 10 pages

Fusion dynamics and the onset of quasi-fission in reactions, leading to production of superheavy nuclei are investigated using the constrained molecular dynamics model. Constraints on the parameters of the nuclear equation of state are derived from experimental fusion probabilities. The obtained constraint on the modulus of incompressibility of nuclear matter K0 = 240 - 260 MeV is consistent with the results of previous study using the Boltzmann-Uehling-Uhlenbeck equation and also with constraints derived using the recent neutron star binary collision event GW170817. Unlike the modulus of incompressibility of symmetric nuclear matter, the stiffness of the density-dependence of symmetry energy influences the fusion probability only weakly., Comment: accepted for publication in Nuclear Physics A. arXiv admin note: text overlap with arXiv:1604.01963

We studied the complete dynamics of the proton-induced spallation process with the microscopic framework of the Constrained Molecular Dynamics (CoMD) Model. We performed calculations of proton-induced spallation reactions on 181Ta, 208Pb, and 238U targets with the CoMD model and compared the results with a standard two-step approach based on an intranuclear cascade model (INC) followed by a statistical deexcitation model. The calculations were also compared with recent experimental data from the literature. Our calculations showed an overall satisfactory agreement with the experimental data and suggest further improvements in the models. We point out that this CoMD study represents the first complete dynamical description of spallation reactions with a microscopic N-body approach and may lead to advancements in the physics-based modelling of the spallation process., Comment: 11 pages, 7 figures, submitted to Phys. Rev. C

Thermal and transport properties of hot nuclear matter formed in central $^{129}$Xe + $^{119}$Sn collisions at the Fermi energy are investigated using the isospin-dependent quantum molecular dynamical (IQMD) model. Temperature ($T$), average density ($\rho$), chemical potential ($\mu$), mean momentum ($P$), shear viscosity ($\eta$) and entropy density ($s$) are obtained from the phase-space information. The mean free path ($\lambda_{nn}$) and the in-medium nucleon-nucleon cross section ($\sigma_{nn}$) in the highest compressible stage at different incident energies are deduced and compared with the experimental results from Phys. Rev. C $\bf{90}$ (2014) 064602. The result shows that $\lambda_{nn}$ and $\sigma_{nn}$ have the same trend and similar values as the experimental results when the beam energy is greater than 40 MeV/u at maximum compressed state. Furthermore, the derived shear viscosity over entropy density ($\eta/s$) shows a decreasing behaviour to a saturated value around $\frac{3}{4\pi}$ as a function of incident energy., Comment: 8 pages, 7 figures

We studied the production of neutron-rich nuclides in multinucleon transfer collisions of stable and radioactive beams in the mass range A=40-60. We first presented our experimental cross section data of projectile fragments from the reaction of 40Ar(15 MeV/nucleon) with 64Ni, 58Ni and 27Al. We then compared them with calculations based on either the deep-inelastic transfer (DIT) model or the constrained molecular dynamics (CoMD) model, followed by the statistical multifragmentation model (SMM). An overall good agreement of the calculations with the experimental data is obtained. We continued with calculations of the reaction of 40Ar (15 MeV/nucleon) with 238U target and then with reactions of 48Ca (15 MeV/nucleon) with 64Ni and 238U targets. In these reactions, neutron-rich rare isotopes with large cross sections are produced. These nuclides, in turn, can be assumed to form radioactive beams and interact with a subsequent target (preferably 238U), leading to the production of extremely neutron-rich and even new isotopes (e.g. 60Ca) in this mass range. We conclude that multinucleon transfer reactions with stable or radioactive beams at the energy of around 15 MeV/nucleon offer an effective route to access extremely neutron-rich rare isotopes for nuclear structure or reaction studies., Comment: 28 pages, 11 figures

Thermalization process of nuclear matter in central fireball region of heavy-ion collisions is investigated by employing an extension model of Boltzmann-Uehling-Uhlenbeck, namely the Van der Waals Boltzmann-Uehling-Uhlenbeck (VdWBUU) model. Temperature ($T$) is extracted by the quantum Fermion fluctuation approach and other thermodynamic quantities, such as density ($\rho$), entropy density ($s$), shear viscosity ($\eta$), isospin diffusivity ($D_{I}$) and heat conductivity ($\kappa$), are also deduced. The liquid-like and gas-like phase signs are discussed through the behavior of shear viscosity during heavy-ion collisions process with the VdWBUU model., Comment: 9 pages, 12 figures; accepted by Phys. Rev. C

We investigate the possibilities of producing neutron-rich nuclides in projectile fission of heavy beams in the energy range of 20 MeV/nucleon expected from low-energy facilities. We report our efforts to theoretically describe the reaction mechanism of projectile fission following a multinucleon transfer collision at this energy range. Our calculations are mainly based on a two-step approach: the dynamical stage of the collision is described with either the phenomenological Deep-Inelastic Transfer model (DIT), or with the microscopic Constrained Molecular Dynamics model (CoMD). The deexcitation/fission of the hot heavy projectile fragments is performed with the Statistical Mul- tifragmentation Model (SMM). We compared our model calculations with our previous experimental projectile-fission data of 238U (20 MeV/nucleon)+208Pb and 197Au (20 MeV/nucleon)+197Au and found an overall reasonable agreement. Our study suggests that projectile fission following periph- eral heavy-ion collisions at this energy range offers an effective route to access very neutron-rich rare isotopes toward and beyond the astrophysical r-process path.

Directed and elliptic flows of neutrons and light charged particles were measured for the reaction 197Au+197Au at 400 MeV/nucleon incident energy within the ASY-EOS experimental campaign at the GSI laboratory. The detection system consisted of the Large Area Neutron Detector LAND, combined with parts of the CHIMERA multidetector, of the ALADIN Time-of-flight Wall, and of the Washington-University Microball detector. The latter three arrays were used for the event characterization and reaction-plane reconstruction. In addition, an array of triple telescopes, KRATTA, was used for complementary measurements of the isotopic composition and flows of light charged particles. From the comparison of the elliptic flow ratio of neutrons with respect to charged particles with UrQMD predictions, a value \gamma = 0.72 \pm 0.19 is obtained for the power-law coefficient describing the density dependence of the potential part in the parametrization of the symmetry energy. It represents a new and more stringent constraint for the regime of supra-saturation density and confirms, with a considerably smaller uncertainty, the moderately soft to linear density dependence deduced from the earlier FOPI-LAND data. The densities probed are shown to reach beyond twice saturation., Comment: 25 pages, 22 figures, accepted for publication in Physical Review C

A new technique of elucidating $\beta$-decay schemes of isotopes with large density of states at low excitation energies has been developed, in which a Broad Energy Germanium (BEGe) detector is used in conjunction with coaxial hyper-pure germanium detectors. The power of this technique has been demonstrated on the example of 183Hg decay. Mass-separated samples of 183Hg were produced by a deposition of the low-energy radioactive-ion beam delivered by the ISOLDE facility at CERN. The excellent energy resolution of the BEGe detector allowed $\gamma$ rays energies to be determined with a precision of a few tens of electronvolts, which was sufficient for the analysis of the Rydberg-Ritz combinations in the level scheme. The timestamped structure of the data was used for unambiguous separation of $\gamma$ rays arising from the decay of 183Hg from those due to the daughter decays.

The microscopic description of nuclear fission still remains a topic of intense basic research. Un- derstanding nuclear fission, apart from a theoretical point of view, is of practical importance for energy production and the transmutation of nuclear waste. In nuclear astrophysics, fission sets the upper limit to the nucleosynthesis of heavy elements via the r-process. In this work we initiated a systematic study of intermediate energy proton-induced fission using the Constrained Molecu- lar Dynamics (CoMD) code. The CoMD code implements an effective interaction with a nuclear matter compressibility of K=200 (soft EOS) with several forms of the density dependence of the nucleon-nucleon symmetry potential. Moreover, a constraint is imposed in the phase-space occu- pation for each nucleon restoring the Pauli principle at each time step of the collision. A proper choice of the surface parameter of the effective interaction has been made to describe fission. In this work, we present results of fission calculations for proton-induced reactions on : a) 232 Th at 27 and 63 MeV, b) 235 U at 10, 30, 60 and 100 MeV, and c) 238 U at 100 and 660 MeV. The calculated observables include fission-fragment mass distributions, total fission energies, neutron multiplicities and fission times. These observables are compared to available experimental data. We show that the microscopic CoMD code is able to describe the complicated many-body dynamics of the fission process at intermediate and high energy and give a reasonable estimate of the fission time scale. Sensitivity of the results to the density dependence of the nucleon symmetry potential (and, thus, the nuclear symmetry energy) is found. Further improvements of the code are necessary to achieve a satisfactory description of low energy fission in which shell effects play a dominant role., Comment: 12 pages, 17 figures

The elliptic-flow ratio of neutrons with respect to protons in reactions of neutron rich heavy-ions systems at intermediate energies has been proposed as an observable sensitive to the strength of the symmetry term in the nuclear Equation Of State (EOS) at supra-saturation densities. The recent results obtained from the existing FOPI/LAND data for $^{197}$Au+$^{197}$Au collisions at 400 MeV/nucleon in comparison with the UrQMD model allowed a first estimate of the symmetry term of the EOS but suffer from a considerable statistical uncertainty. In order to obtain an improved data set for Au+Au collisions and to extend the study to other systems, a new experiment was carried out at the GSI laboratory by the ASY-EOS collaboration in May 2011., Comment: Talk given by P. Russotto at the 11th International Conference on Nucleus-Nucleus Collisions (NN2012), San Antonio, Texas, USA, May 27-June 1, 2012. To appear in the NN2012 Proceedings in Journal of Physics: Conference Series (JPCS)

The performance of the LAND neutron detector is studied. Using an event-mixing technique based on one-neutron data obtained in the S107 experiment at the GSI laboratory, we test the efficiency of various analytic tools used to determine the multiplicity and kinematic properties of detected neutrons. A new algorithm developed recently for recognizing neutron showers from spectator decays in the ALADIN experiment S254 is described in detail. Its performance is assessed in comparison with other methods. The properties of the observed neutron events are used to estimate the detection efficiency of LAND in this experiment., Comment: 16 pages, 8 figures

The signal of isospin-asymmetric phase transition in the evolution of the chemical potential was observed for hot quasi-projectiles produced in the reactions 40,48Ca + 27Al confirming an analogous observation in the lighter system 28Si + 112,124Sn. With increasing mass, the properties of hot quasi-projectiles become increasingly influenced by the secondary emission. Thermodynamical observables exhibit no sensitivity to the different number of missing neutrons in the two reactions 40,48Ca + 27Al, thus providing a signal of dynamical emission of neutrons, which can be related to formation of a very neutron-rich low-density region (neck) between the projectile and target., Comment: LaTeX, article expanded, 2 figures added, 17 pages, 7 figures

Thermodynamical properties of the nuclear matter at sub-saturation densities were investigated using a simple van der Waals-like equation of state with an additional term representing the symmetry energy. First-order isospin-asymmetric liquid-gas phase transition appears restricted to isolated isospin-asymmetric systems while the symmetric systems will undergo fragmentation decay resembling the second-order phase transition. The density dependence of the symmetry energy scaling with the Fermi energy satisfactorily describes the symmetry energy at sub-saturation nuclear densities. The deconfinement-confinement phase transition from the quark-gluon plasma to the confined quark matter appears in the isolated systems continuous in energy density while discontinuous in quark density. A transitional state of the confined quark matter has a negative pressure and after hadronization an explosion scenario can take place which can offer explanation for the HBT puzzle as a signature of the phase transition., Comment: 18 pages, 8 figures, LaTeX, minor corrections

Investigation of the effect of the dynamical stage of heavy-ion collisions indicates that the increasing width of the initial isospin distributions is reflected by a significant modification of the isoscaling slope for the final isotopic distributions after de-excitation. For narrow initial distributions, the isoscaling slope assumes the limiting value of the two individual initial nuclei while for wide initial isotopic distributions the slope for hot fragments approaches the initial value. The isoscaling slopes for final cold fragments increase due to secondary emissions. The experimentally observed evolution of the isoscaling parameter in multifragmentation of hot quasiprojectiles at E$_{inc}$=50 AMeV, fragmentation of $^{86}$Kr projectiles at E$_{inc}$=25 AMeV and multifragmentation of target spectators at relativistic energies was reproduced by a simulation with the dynamical stage described using the appropriate model (deep inelastic transfer and incomplete fusion at the Fermi energy domain and spectator-participant model at relativistic energies) and the de-excitation stage described with the statistical multifragmentation model. In all cases the isoscaling behavior was reproduced by a proper description of the dynamical stage and no unambiguous signals of the decrease of the symmetry energy coefficient were observed., Comment: LaTeX, 18 pages, 9 figures, to appear in Phys. Rev. C

The reaction mechanism of nucleus-nucleus collisions at projectile energies around the Fermi energy is investigated with emphasis on the production of fragmentation-like residues. The results of simulations are compared to experimental mass distributions of elements with Z = 21 - 29 observed in the reactions 86Kr+124,112Sn at 25 AMeV. The model of incomplete fusion is modified and a component of excitation energy of the cold fragment dependent on isospin asymmetry is introduced. The modifications in the model of incomplete fusion appear consistent with both overall model framework and available experimental data. A prediction is provided for the production of very neutron-rich nuclei using a secondary beam of 132Sn where e.g. the reaction 132Sn+238U at 28 AMeV appears as a possible alternative to the use of fragmentation reactions at higher energies., Comment: LaTeX, 15 pages, 5 figures, minor modifications, accepted for publication in Nuclear Physics A

The sizes, temperatures and free neutron to proton ratios of the initial interaction zones produced in the collisions of 40 MeV/nucleon $^{40}$Ar + $^{112}$Sn and 55 MeV/nucleon$^{27}$Al + $^{124}$Sn are derived using total detected neutron plus charged particle multiplicity as a measure of the impact parameter range and number of participant nucleons. The size of the initial interaction zone, determined from a coalescence model analysis, increases significantly with decreasing impact parameter. The temperatures and free neutron to proton ratios in the interaction zones are relatively similar for different impact parameter ranges and evolve in a similar fashion., Comment: 7 pages, 8 figures

The evolution of the symmetry energy coefficient of the binding energy of hot fragments with increasing excitation is explored in multifragmentation processes following heavy-ion collisions below the Fermi energy. In this work, high-resolution mass spectrometric data on isotopic distributions of projectile-like fragments from collisions of 25 MeV/nucleon 86Kr and 64Ni beams on heavy neutron-rich targets are systematically compared to calculations involving the Statistical Multifragmentation Model. The study reveals a gradual decrease of the symmetry energy coefficient from 25 MeV at the compound nucleus regime (E*/A < 2 MeV) towards 15 MeV in the bulk multifragmentation regime (E*/A > 4 MeV). The ensuing isotopic distributions of the hot fragments are found to be very wide and extend towards the neutron drip-line. These findings may have important implications to the composition and evolution of hot astrophysical environments, such as core-collapse supernova., Comment: 5 pages, 4 figures, submitted to Phys. Rev. C

The isoscaling properties of isotopically resolved projectile residues from peripheral collisions of 86Kr (25 MeV/nucleon), 64Ni (25 MeV/nucleon) and 136Xe (20 MeV/nucleon) beams on various target pairs are employed to probe the symmetry energy coefficient of the nuclear binding energy. The present study focuses on heavy projectile fragments produced in peripheral and semiperipheral collisions near the onset of multifragment emission E*/A = 2-3 MeV). For these fragments, the measured average velocities are used to extract excitation energies. The excitation energies, in turn, are used to estimate the temperatures of the fragmenting quasiprojectiles in the framework the Fermi gas model. The isoscaling analysis of the fragment yields provided the isoscaling parameters "alpha" which, in combination with temperatures and isospin asymmetries provided the symmetry energy coefficient of the nuclear binding energy of the hot fragmenting quasiprojectiles. The extracted values of the symmetry energy coefficient at this excitation energy range (2-3 MeV/nucleon) are lower than the typical liquid-drop model value ~25 MeV corresponding to ground-state nuclei and show a monotonic decrease with increasing excitation energy. This result is of importance in the formation of hot nuclei in heavy-ion reactions and in hot stellar environments such as supernova., Comment: 11 pages, 9 figures, submitted to Phys. Rev. C

A comparison of experimental heavy residue cross sections from the reactions 86Kr+64Ni,112,124Sn with the model of deep-inelastic transfer (DIT) is carried out. A modified expression for nucleon transfer probabilities is used at non-overlapping projectile-target configurations, introducing a dependence on isospin asymmetry at the nuclear periphery. The experimental yields of neutron-rich nuclei close to the projectile are reproduced better and the trend deviating from the bulk isospin equilibration is explained. For the neutron-rich products further from the projectile, originating from hot quasiprojectiles, the statistical multifragmentation model reproduces the mass distributions better than the model of sequential binary decay. In the reaction with proton-rich target 112Sn the nucleon exchange appears to depend on isospin asymmetry of nuclear periphery only when surface separation is larger than 0.8 fm due to the stronger Coulomb interaction at more compact di-nuclear configuration., Comment: LaTeX, 13 pages, 7 figures, to appear in Nuclear Physics A

Calorimetric and coalescence techniques have been employed to probe equilibration for hot nuclei produced in heavy ion collisions of 35 to 55 MeV/u projectiles with medium mass targets. Entrance channel mass asymmetries and energies were selected in order that very hot composite nuclei of similar mass and excitation would remain after early stage pre-equilibrium particle emission. Inter-comparison of the properties and de-excitation patterns for these different systems provides evidence for the production of hot nuclei with decay patterns relatively independent of the specific entrance channel., Comment: 7 pages, 2 figures

The critical behavior for the light nuclei with A$\sim 36$ has been investigated experimentally by the NIMROD multi-detectors. The wide variety of observables indicate the critical point has been reached in the disassembly of hot nuclei at an excitation energy of 5.6$\pm$0.5 MeV/u., Comment: 4 pages, 2 figures; Proceeding of 18th Nuclear Physics Division Conference of the Euro. Phys. Society (NPDC18) "Phase transitions in strongly interacting matter", Prague, 23.8.-29.8. 2004. To be published in Nuclear Physics A

An extensive experimental survey of the features of the disassembly of a small quasi-projectile system with $A \sim$ 36, produced in the reactions of 47 MeV/nucleon $^{40}$Ar + $^{27}$Al, $^{48}$Ti and $^{58}$Ni, has been carried out. Nuclei in the excitation energy range of 1-9 MeV/u have been investigated employing a new method to reconstruct the quasi-projectile source. At an excitation energy $\sim$ 5.6 MeV/nucleon many observables indicate the presence of maximal fluctuations in the de-excitation processes. The fragment topological structure shows that the rank sorted fragments obey Zipf's law at the point of largest fluctuations providing another indication of a liquid gas phase transition. The caloric curve for this system shows a monotonic increase of temperature with excitation energy and no apparent plateau. The temperature at the point of maximal fluctuations is $8.3 \pm 0.5$ MeV. Taking this temperature as the critical temperature and employing the caloric curve information we have extracted the critical exponents $\beta$, $\gamma$ and $\sigma$ from the data. Their values are also consistent with the values of the universality class of the liquid gas phase transition. Taken together, this body of evidence strongly suggests a phase change in an equilibrated mesoscopic system at, or extremely close to, the critical point., Comment: Physical Review C, in press; some discussions about the validity of excitation energy in peripheral collisions have been added; 24 pages and 32 figures; longer abstract in the preprint

The kinetic energy variation of emitted light clusters has been employed as a clock to explore the time evolution of the temperature for thermalizing composite systems produced in the reactions of 26A, 35A and 47A MeV $^{64}$Zn with $^{58}$Ni, $^{92}$Mo and $^{197}$Au. For each system investigated, the double isotope ratio temperature curve exhibits a high maximum apparent temperature, in the range of 10-25 MeV, at high ejectile velocity. These maximum values increase with increasing projectile energy and decrease with increasing target mass. The time at which the maximum in the temperature curve is reached ranges from 80 to 130 fm/c after contact. For each different target, the subsequent cooling curves for all three projectile energies are quite similar. Temperatures comparable to those of limiting temperature systematics are reached 30 to 40 fm/c after the times corresponding to the maxima, at a time when AMD-V transport model calculations predict entry into the final evaporative or fragmentation stage of de-excitation of the hot composite systems. Evidence for the establishment of thermal and chemical equilibrium is discussed., Comment: 9 pages, 5 figures

Production cross sections and excitation functions of neutron deficient isotopes of U, Pa and Th have been measured in the reaction of 22Ne + 208Pb. The comparison of experimental production cross sections of uranium isotopes with the neutron number 126 <= N <= 134 to the values calculated using statistical model of nuclear deexcitation has been carried out. There is shown, that none of the commonly used methods of introduction of shell effect influence on the production cross sections of evaporations residues allows to reproduce unambigiously used set of experimental data. Simple semiempirical method of introduction of shell effect influence in fission channel is proposed and good agreement with production cross sections of neutron deficient isotopes in the region from Bi to U is reached., Comment: 10 pages, 4 figures. We re-publish a contribution to Proc. of Int. Conf. DANF'96 (Casta-Papiernicka, Slovakia), Dubna 1996, p. 129 (with 2 extra figures), Part of M.V.'s PhD thesis. A fission barrier systematics is introduced which was further used in the model of fusion-quasifission competition (see nucl-ex/0302012)

Recently, the fragment yield ratios were investigated in the fission of 238,233U targets induced by 14 MeV neutrons [1]. The effect of the deformed shell structure around N=64 on the dynamics of scission was suggested as an explanation of the observed breakdown of the isoscaling behavior. As an alternative explanation, the effect of binding energy of final fragments, in particular the neutron shell closure of the complementary fragment around N=82, was suggested by Friedman [2]. When applying that concept consistently, the observed two breakdowns of the isoscaling behavior around N=62 and N=80 signal the effect of two shell closures (N=82 and N=64) on the dynamics of scission. To separate the two effects, we examine the isoscaling plots obtained using fission fragment yield data from the spontaneous fission of the heavier nuclei 248,244Cm and find that the breakdown of isoscaling around N=64 persists even if it can not be related to shell closure of the complementary fragment and thus it manifests the effect of the deformed shell structure in the scission configuration., Comment: 3 pages, 2 figures, LaTeX

The reaction systems, 64Zn + 58Ni, 64Zn + 92Mo, 64Zn + 197Au, at 26A, 35A and 47A MeV, have been studied both in experiments with a 4$\pi$ detector array, NIMROD, and with Antisymmetrized Molecular Dynamics model calculations employing effective interactions corresponding to soft and stiff equations of state (EOS). Direct experimental observables, such as multiplicity distributions, charge distributions, energy spectra and velocity spectra, have been compared in detail with those of the calculations and a reasonable agreement is obtained. The velocity distributions of $\alpha$ particles and fragments with Z >= 3 show distinct differences in calculations with the soft EOS and the stiff EOS. The velocity distributions of $\alpha$ particle and Intermediate Mass Fragments (IMF's) are best described by the stiff EOS. Neither of the above direct observables nor the strength of the elliptic flow are sensitive to changes in the in-medium nucleon-nucleon (NN) cross sections. A detailed analysis of the central collision events calculated with the stiff EOS revealed that multifragmentation with cold fragment emission is a common feature predicted for all reactions studied here. A possible multifragmentation scenario is presented; after the preequilibrium emission ceases in the composite system, cold light fragments are formed in a hotter gas of nucleons and stay cold until the composite system underdoes multifragmentation. For reaction with 197Au at 47A MeV a significant radial expansion takes place. For reactions with 58Ni and 92Mo at 47A MeV semi-transparency becomes prominent. The differing reaction dynamics drastically change the kinematic characteristics of emitted fragments. This scenario gives consistent explanations for many existing experimental results in the Fermi energy domain., Comment: 34 pages, 43 figures

The fragment yield ratios were investigated in the fission of 238,233U targets induced by 14 MeV neutrons. The isoscaling behavior was typically observed for the isotopic chains of fragments ranging from the proton-rich to the most neutron-rich ones. The observed high sensitivity of neutron-rich heavy fragments to the target neutron content suggests fission as a source of neutron-rich heavy nuclei for present and future rare ion beam facilities, allowing studies of nuclear properties towards the neutron drip-line and investigations of the conditions for nucleosynthesis of heavy nuclei. The breakdowns of the isoscaling behavior around N=62 and N=80 manifest the effect of two shell closures on the dynamics of scission. The shell closure around N=64 can be explained by the deformed shell. The investigation of isoscaling in the spontaneous fission of 248,244Cm further supports such conclusion. The Z-dependence of the isoscaling parameter exhibits a structure which can be possibly related to details of scission dynamics. The fission isoscaling studies can be a suitable tool for the investigation of possible new pathways to synthesize still heavier nuclei., Comment: 7 pages, 3 figures, RevTex, final version, to appear in Phys. Rev. C as a regular article

The isotopic and isobaric scaling behavior of the yield ratios of heavy projectile residues from the collisions of 25 MeV/nucleon 86Kr projectiles on 124Sn and 112Sn targets is investigated and shown to provide information on the process of N/Z equilibration occurring between the projectile and the target. The logarithmic slopes $\alpha$ and $\beta^{'}$ of the residue yield ratios with respect to residue neutron number N and neutron excess N--Z are obtained as a function of the atomic number Z and mass number A, respectively, whereas excitation energies are deduced from velocities. The relation of the isoscaling parameters $\alpha$ and $\beta^{'}$ with the N/Z of the primary (excited) projectile fragments is employed to gain access to the degree of N/Z equilibration prior to fragmentation as a function of excitation energy. A monotonic relation between the N/Z difference of fragmenting quasiprojectiles and their excitation energy is obtained indicating that N/Z equilibrium is approached at the highest observed excitation energies. Simulations with a deep-inelastic transfer model are in overall agreement with the isoscaling conclusions. The present residue isoscaling approach to N/Z equilibration offers an attractive tool of isospin and reaction dynamics studies in collisions involving beams of stable or rare isotopes., Comment: 15 pages, 4 figures, submitted to Phys. Lett. B

The isoscaling is investigated using the fragment yield data from fully reconstructed quasi-projectiles observed in peripheral collisions of 28Si with 124,112Sn at projectile energies 30 and 50 MeV/nucleon. The excitation energy dependence of the isoscaling parameter beta_prime is observed which is independent of beam energy. For a given quasi-projectile produced in reactions with different targets no isoscaling is observed. The isoscaling thus reflects the level of N/Z-equilibration in reactions with different targets represented by the initial quasi-projectile samples. The excitation energy dependence of the isoscaling parameter beta_prime, corrected for the trivial 1/T temperature dependence, does not follow the trend of the homogeneous system above 4 MeV/nucleon thus possibly signaling the onset of separation into isospin asymmetric dilute and isospin symmetric dense phase., Comment: 4 pages, 4 figures, RevTeX, to appear in Physical Review C

The scaling of the yields of heavy projectile residues from the reactions of 25 MeV/nucleon 86Kr projectiles with 124Sn,112Sn and 64Ni, 58Nitargets is studied. Isotopically resolved yield distributions of projectile fragments in the range Z=10-36 from these reaction pairs were measured with the MARS recoil separator in the angular range 2.7-5.3 degrees. The velocities of the residues, monotonically decreasing with Z down to Z~26-28, are employed to characterize the excitation energy. The yield ratios R21(N,Z) for each pair of systems are found to exhibit isotopic scaling (isoscaling), namely, an exponential dependence on the fragment atomic number Z and neutron number N. The isoscaling is found to occur in the residue Z range corresponding to the maximum observed excitation energies. The corresponding isoscaling parameters are alpha=0.43 and beta=-0.50 for the Kr+Sn system and alpha=0.27 and beta=-0.34 for the Kr+Ni system. For the Kr+Sn system, for which the experimental angular acceptance range lies inside the grazing angle, isoscaling was found to occur for Z<26 and N<34. For heavier fragments from Kr+Sn, the parameters vary monotonically, alpha decreasing with Z and beta increasing with N. This variation is found to be related to the evolution towards isospin equilibration and, as such, it can serve as a tracer of the N/Z equilibration process. The present heavy-residue data extend the observation of isotopic scaling from the intermediate mass fragment region to the heavy-residue region. Such high-resolution mass spectrometric data can provide important information on the role of isospin in peripheral and mid-peripheral collisions, complementary to that accessible from modern large-acceptance multidetector devices., Comment: 8 pages, 6 figures, submitted to Phys. Rev. C

A wide variety of observables indicate that maximal fluctuations in the disassembly of hot nuclei with A ~ 36 occur at an excitation energy of 5.6 +- 0.5 MeV/u and temperature of 8.3 +- 0.5 MeV. Associated with this point of maximal fluctuations are a number of quantitative indicators of apparent critical behavior. The associated caloric curve does not appear to show a flattening such as that seen for heavier systems. This suggests that, in contrast to similar signals seen for liquid-gas transitions in heavier nuclei, the observed behavior in these very light nuclei is associated with a transition much closer to the critical point., Comment: v2: Major changes, new model calculations, new figures

The mechanism of fusion hindrance, an effect observed in the reactions of cold, warm and hot fusion leading to production of the superheavy elements, is investigated. A systematics of transfermium production cross sections is used to determine fusion probabilities. Mechanism of fusion hindrance is described as a competition of fusion and quasi-fission. Available evaporation residue cross sections in the superheavy region are reproduced satisfactorily. Analysis of the measured capture cross sections is performed and a sudden disappearance of the capture cross sections is observed at low fusion probabilities. A dependence of the fusion hindrance on the asymmetry of the projectile-target system is investigated using the available data. The most promising pathways for further experiments are suggested., Comment: 8 pages, 7 figures, talk presented at 7th International School-Seminar on Heavy-Ion Physics, May 27 - June 1, 2002, Dubna, Russia

A large enhancement in the production of neutron-rich projectile residues is observed in the reactions of a 25 MeV/nucleon 86Kr beam with the neutron rich 124Sn and 64Ni targets relative to the predictions of the EPAX parametrization of high-energy fragmentation, as well as relative to the reaction with the less neutron-rich 112Sn target. The data demonstrate the significant effect of the target neutron-to-proton ratio (N/Z) in peripheral collisions at Fermi energies. A hybrid model based on a deep-inelastic transfer code (DIT) followed by a statistical de-excitation code appears to account for part of the observed large cross sections. The DIT simulation indicates that the production of the neutron-rich nuclides in these reactions is associated with peripheral nucleon exchange. In such peripheral encounters, the neutron skins of the neutron-rich 124Sn and 64Ni target nuclei may play an important role. From a practical viewpoint, such reactions between massive neutron-rich nuclei offer a novel and attractive synthetic avenue to access extremely neutron-rich rare isotopes towards the neutron-drip line., Comment: 4 pages, 3 figures, submitted to Phys. Rev. Lett

The cross sections and velocity distributions of heavy residues from the reaction of 20 AMeV 124Sn + 27Al have been measured at forward angles using the MARS recoil separator at Texas A&M in a wide mass range. A consistent overall description of the measured cross sections and velocity distributions was achieved using a model calculation employing the concept of deep-inelastic transfer for the primary stage of peripheral collisions, pre-equilibrium emission and incomplete fusion for the primary stage of more violent central collisions and the statistical model of multifragmentation (SMM code) for the deexcitation stage. An alternative calculation employing the sequential binary decay (GEMINI code) could not reproduce the observed yields of the residues from violent collisions (A<90) due to different kinematic properties. The success of SMM demonstrates that the heavy residues originate from events where a competition of thermally equilibrated fragment partitions takes place rather than a sequence of binary decays., Comment: 17 pages, 15 figures, LaTeX, to appear in NPA

The cross sections and velocity distributions of projectile-like fragments from the reaction of 25 MeV/nucleon 86Kr + 64Ni have been measured using the MARS recoil separator at Texas A&M, with special emphasis on the neutron rich isotopes. Proton-removal and neutron pick-up isotopes have been observed with large cross sections. A model of deep-inelastic transfer (DIT) for the primary interaction stage and the statistical evaporation code GEMINI for the deexcitation stage have been used to describe the properties of the product distributions. The results have also been compared with the EPAX parametrization of high-energy fragmentation yields. The experimental data show an enhancement in the production of neutron-rich isotopes close to the projectile, relative to the predictions of DIT/GEMINI and the expectations of EPAX. We attribute this enhancement mainly to the effect of the extended neutron distribution (neutron ``skin'') of the 64Ni target in peripheral interactions of 86Kr with 64Ni. The large cross sections of such reactions near the Fermi energy, involving peripheral nucleon exchange, suggest that, not only the N/Z of the projectile and the target, but also the N/Z distribution at the nuclear surface may properly be exploited in the production of neutron-rich rare isotopes. This synthesis approach may offer a fruitful pathway to extremely neutron-rich nuclei, towards the neutron-drip line., Comment: 14 pages, 4 figures, submitted to Phys. Lett B

An overview of recent results on reaction dynamics in the energy region 20 - 50 A.MeV is given. The results of the study of projectile multifragmentation using the detector array FAUST are presented. Reaction mechanism is determined and thermodynamical properties of the hot quasiprojectile are investigated. Preliminary results on fragment isospin asymmetry obtained using the 4$\pi$ detector array NIMROD are given. Procedure for selecting centrality in two-dimensional multiplicity histograms is described. Possibility to extract thermodynamical temperature from systematics of isotope ratios is investigated. Reaction mechanism leading to production of hot sources is discussed. Furthermore, the possibilities for production of rare isotopes are discussed and recent experimental results obtained using recoil separator MARS are presented., Comment: 18 pages, 11 figures, contribution to proceedings of the conference DANF'2001, Casta-Papiernicka, Slovakia published by World Scientific