Fermi Energy Domain Research Articles

Consistency of nuclear thermometric measurements at moderate excitation

A comparison of various thermometric techniques used for the estimation of nuclear temperature has been made from the decay of hot composite ${}^{32}{\mathrm{S}}^{*}$ produced in the reaction ${}^{20}\mathrm{Ne} (145 \mathrm{MeV}) + {}^{12}\mathrm{C}$. It is shown that the temperatures estimated by different techniques, known to vary significantly in the Fermi energy domain, are consistent with each other within experimental limits for the system studied here.

Neutron-rich rare isotope production in the Fermi energy domain and application to the Texas A&M radioactive beam upgrade

The production and separation of neutron-rich rare isotope beams in peripheral collisions around the Fermi energy is described in the present work. The experiments were performed at the Cyclotron Institute of Texas A&M University employing beams from the K500 Superconducting Cyclotron. Two magnetic separators were used: the MARS recoil separator and the Superconducting Solenoid Line (BigSol Line). Enhanced production cross sections of neutron-rich nuclides in comparison with high-energy fragmentation mechanisms were observed and attributed to the role of the N/Z and the nuclear periphery of the target. From a practical viewpoint, these reactions below or around the Fermi energy offer a promising way to access very neutron-rich rare isotopes. The experience obtained in the production of rare isotope beams (RIB) in this energy range will be applied in the ongoing RIB upgrade of the Cyclotron Institute that will involve production, stopping and re-acceleration of rare isotopes.

Dynamical aspects of isotopic scaling

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.

Production of cold fragments in nucleus–nucleus collisions in the Fermi-energy domain

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 Kr 86 + Sn 124 , 112 at 25 A MeV . 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 Sn 132 where e.g. the reaction Sn 132 + U 238 at 28 A MeV appears as a possible alternative to the use of fragmentation reactions at higher energies.

Investigation of nucleon-induced reactions in the Fermi energy domain within the microscopic DYWAN model

A microscopic investigation of nucleon-induced reactions is addressed within the DYWAN model, which is based on the projection methods of out of equilibrium statistical physics and on the mathematical theory of wavelets. Due to a strongly compressed representation of the fermionic wave functions, the numerical simulations of the nucleon transport in target are therefore able to preserve the quantum nature of the colliding system, as well as a least biased many-body information needed to keep track of the cluster formation. A special attention is devoted to the fingerprints of the phase space topology induced by the fluctuations of the self-consistent mean-field. Comparisons between theoretical results and experimental data point out that ETDHF type approaches are well suited to describe reaction mechanisms in the Fermi energy domain. The observed sensitivity to physical effects shows that the nucleon-induced reactions provide a valuable probe of the nuclear interaction in this range of energy.

The decay time scale for highly excited nuclei as seen from asymmetrical emission of particles

A novel method was developed for the extraction of short emission times of light particles from the projectile-like fragments in peripheral deep-inelastic collisions in the Fermi energy domain. We have taken an advantage of the fact that in the external Coulomb field particles are evaporated asymmetrically. It was possible to determine the emission times in the interval 50–500 fm/c using the backward emission anisotropy of α-particles relative to the largest residue, in the reaction 28Si + 112Sn at 50 MeV/nucleon. The extracted times are consistent with predictions based on the evaporation decay widths calculated with the statistical evaporation model generalized for the case of the Coulomb interaction with the target.

Isospin effects on reaction dynamics from low to relativistic energies

We present several possibilities offered by the reaction dynamics of dissipative heavy ion collisions to study in detail the symmetry term of the nuclear equation of state, EOS. In the Fermi energy domain we discuss isospin effects on the nuclear liquid–gas phase transition, Isospin Distillation , and on mid-rapidity fragment formation in semicentral collisions. The correlation between isospin diffusion and deviations from the Viola systematics will be analysed. Collective flows are studied at intermediate energies in order to extract observables sensitive to the nucleon effective mass splitting at high isospin density.

Reaction dynamics and multifragmentation in Fermi energy heavy ion reactions

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.

Liquid-gas phase transition in hot nuclei studied with INDRA

Thanks to the high detection quality of the INDRA array, signatures related to the dynamics (spinodal decomposition) and thermodynamics (negative microcanonical heat capacity) of a liquid-gas phase transition have been simultaneously studied in multifragmentation events in the Fermi energy domain. The correlation between both types of signals strongly supports the existence of a first order phase transition for hot nuclei.

Neutron-rich rare isotope production in the Fermi energy domain

The production of neutron-rich rare isotopes around the Fermi energy is investigated in the present work. A large enhancement in the production of neutron-rich projectile residues has been observed in the reactions of a 25 MeV/nucleon 86 Kr beam with the neutron rich 64 Ni and 124 Sn 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 112 Sn target. The present data demonstrate the significant influence 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 was found to account for part of the observed large cross sections. The DIT simulation indicates that the neutron skins of the neutron-rich 64 Ni and 124 Sn target nuclei play an important role in the production mechanism. From a practical viewpoint, such reactions between massive neutron-rich nuclei offer a novel synthetic avenue to access extremely neutron-rich rare isotopes towards the neutron-drip line.

Neutron-rich rare isotope production in the Fermi energy domain

The production of neutron-rich rare isotopes around the Fermi energy is investigated in the present work. A large enhancement in the production of neutron-rich projectile residues has been observed in the reactions of a 25 MeV/nucleon 86Kr beam with the neutron rich 64Ni and 124Sn 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 present data demonstrate the significant influence 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 was found to account for part of the observed large cross sections. The DIT simulation indicates that the neutron skins of the neutron-rich 64Ni and 124Sn target nuclei play an important role in the production mechanism. From a practical viewpoint, such reactions between massive neutron-rich nuclei offer a novel synthetic avenue to access extremely neutron-rich rare isotopes towards the neutron-drip line.

Liquid-gas phase transition in hot nuclei studied with INDRA

Thanks to the high detection quality of the INDRA array, signatures related to the dynamics (spinodal decomposition) and thermodynamics (negative microcanonical heat capacity) of a liquid-gas phase transition have been simultaneously studied in multifragmentation events in the Fermi energy domain. The correlation between both types of signals strongly supports the existence of a first order phase transition for hot nuclei.

Isospin effects on reaction dynamics from low to relativistic energies

We present several possibilities offered by the reaction dynamics of dissipative heavy ion collisions to study in detail the symmetry term of the nuclear equation of state, EOS. In the Fermi energy domain we discuss isospin effects on the nuclear liquid–gas phase transition, Isospin Distillation, and on mid-rapidity fragment formation in semicentral collisions. The correlation between isospin diffusion and deviations from the Viola systematics will be analysed. Collective flows are studied at intermediate energies in order to extract observables sensitive to the nucleon effective mass splitting at high isospin density.

New mechanism for the production of extremely fast light particles in heavy-ion collisions in the fermi energy domain

Employing a four-body classical model, various mechanisms responsible for the production of fast light particles in heavy ion collisions at low and intermediate energies have been studied. It has been shown that at energies lower than 50 A MeV, light particles of velocities of more than two times higher than the projectile velocities are produced due to the acceleration of the target light-particles by the mean field of the incident nucleus. It has also been shown that precision experimental reaction research in normal and inverse kinematics is likely to provide vital information about which mechanism is dominant in the production of fast light particles.

Quasiclassical model of intermediate velocity particle production in asymmetric heavy ion reactions

The particle emission at intermediate velocities in mass asymmetric reactions is studied within the framework of classical molecular dynamics. Two reactions in the Fermi energy domain were modelized, $^{58}$Ni+C and $^{58}$Ni+Au at 34.5 MeV/nucleon. The availability of microscopic correlations at all times allowed a detailed study of the fragment formation process. Special attention was paid to the physical origin of fragments and emission timescales, which allowed us to disentangle the different processes involved in the mid-rapidity particle production. Consequently, a clear distinction between a prompt pre- equilibrium emission and a delayed aligned asymmetric breakup of the heavier partner of the reaction was achieved.

Production mechanism of hot nuclei in violent collisions in the Fermi energy domain

A production mechanism of highly excited nuclei formed in violent collisions in the Fermi energy domain is investigated. The collision of two nuclei is decomposed into several stages which are treated separately. Simplified exciton concept is used for the description of pre-equilibrium emission. A modified spectator–participant scenario is used where motion along classical Coulomb trajectories is assumed. The participant and one of the spectator zones undergo incomplete fusion. Excitation energies of both cold and hot fragment are determined. Results of the calculation are compared to recent experimental data in the Fermi energy domain. Data on hot projectile-like, mid-velocity and fusion-like sources are described consistently. Geometric aspects of pre-equilibrium emission are revealed. Explanations to previously unexplained experimental phenomena are given. Energy deposited into non-thermal degrees of freedom is estimated.

Some regularities in the beam-direct production of isotopes with 2⩽ Z⩽11 induced in reactions of 18O (35 [formula omitted]MeV) with Be and Ta

A systematic study of the forward-angle production of nuclei with 2⩽ Z⩽11 induced in nucleus–nucleus collisions of 18O with 9Be and 181Ta in the Fermi energy domain (35 A MeV) has been performed. The reaction products were measured by using the double achromatic fragment-separator COMBAS in the spectrometry mode. The inclusive velocity, the isotopic and element distributions were measured. The dominant role of low-energy reaction mechanisms is observed.

Universal fluctuations in heavy-ion collisions in the Fermi energy domain.

We discuss the scaling laws of both the charged fragments multiplicity n fluctuations and the charge of the largest fragment Z(max) fluctuations for Xe + Sn collisions in the range of bombarding energies between 25A MeV and 50A MeV. We show at E(lab) > or similar to 32 MeV/A the transition in the fluctuation regime of Z(max) which is compatible with the transition from the ordered to disordered phase of excited nuclear matter. The size (charge) of the largest fragment is closely related to the order parameter characterizing this process.

Anomalous radial expansion in central heavy-ion reactions

The expansion velocity profile in central heavy-ion reactions in the Fermi energy domain is examined. The radial expansion is non-hubblean and in the surface region it scales proportional to a higher exponent ($\alpha > 1$) of the radius. The anomalous expansion velocity profile is accompanied by a power law nucleon density profile in the surface region. Both these features of central heavy-ion reactions disappear at higher energies, and the system follows a uniform Hubble expansion ($\alpha \simeq 1$).

Digital filtering: a common analysis for data and mean field theories in heavy-ions collisions at intermediate energy

To select well-defined event configurations from heavy-ion collisions in the Fermi energy domain, a digital filtering technique of the charge density distribution along the deflection axis in the velocity space is presented. Charge density appears as a robust variable and can be used for a quantitative comparison of experimental data obtained with 4π arrays and mean-field transport equation predictions.