Peripheral Heavy Ion Reactions Research Articles

Identification of medium mass (A=60–80) ejectiles from 15 MeV/nucleon peripheral heavy-ion collisions with the MAGNEX large-acceptance spectrometer

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.

Compact initial state for peripheral heavy ion reactions and its consequences

An initial state is created based field dominance and on flux-tubes or streaks in the transverse plane with local momentum and angular momentum conservation and local shear. The global dynamical consequences as well as polarization and correlations of produced particles will be discussed.

Λ polarization in peripheral collisions at moderately relativistic energies

The polarization of $\Lambda$ hyperons from relativistic flow vorticity is studied in peripheral heavy ion reactions at FAIR and NICA energies, just above the threshold of the transition to the Quark-Gluon Plasma. Previous calculations at higher energies with larger initial angular momentum, predicted significant $\Lambda$ polarization based on the classical vorticity term in the polarization, while relativistic modifications decreased the polarization and changed its structure in the momentum space. At the lower energies studied here, we see the same effect namely that the relativistic modifications decrease the polarization arising from the initial shear flow vorticity.

Λpolarization in an exact rotating and expanding fluid dynamical model for peripheral heavy ion reactions

We calculate the Lambda polarization in an exact analytical, rotating model based on parameters extracted from a high resolution (3+1)D Particle-in-Cell Relativistic hydrodynamics calculation. The polarization is attributed to effects from thermal vorticity and for the first time the effects of the radial and axial acceleration are also studied separately

Isoscaling of heavy projectile residues andN/Zequilibration in peripheral heavy-ion collisions below the Fermi energy

The isoscaling of heavy projectile residues from peripheral heavy-ion reactions at 15--25 MeV/nucleon is employed to obtain information on the process of $N/Z$ equilibration. Recent mass spectrometric data of projectile residues from the reactions of $^{86}\mathrm{Kr}$ (15 MeV/nucleon) with $^{64,58}\mathrm{Ni}$ and $^{124,112}\mathrm{Sn}$ were first analyzed. The isotopically resolved yield distributions of the fragments in the range $Z=26--39$ were employed for the isoscaling analysis. The yield ratios ${R}_{21}(N,Z)$ of the fragments from each pair of systems exhibit isoscaling (i.e., an exponential dependence on the fragment neutron number $N$ for each atomic number $Z$) with the isoscaling parameter $\ensuremath{\alpha}$ increasing with decreasing (or increasing) $Z$ away from the projectile. This variation is related to the evolution toward $N/Z$ equilibration with increasing energy dissipation estimated from the residue velocities. In parallel to the new heavy-residue isoscaling data of $^{86}\mathrm{Kr}$ at 15 MeV/nucleon, our previous data at 25 MeV/nucleon for the reactions $^{86}\mathrm{Kr}+^{124,112}\mathrm{Sn}$ and $^{64}\mathrm{Ni}+^{64,58}\mathrm{Ni}$, $^{64}\mathrm{Ni}+^{124,112}\mathrm{Sn}$, as well as our data at 15 MeV/nucleon of the lighter system $^{40}\mathrm{Ar}+^{64,58}\mathrm{Ni}$, were analyzed in a similar way. Calculations with the stochastic nucleon-exchange model DIT (deep inelastic transfer) and the microscopic many-body model CoMD (constrained molecular dynamics) provided an overall fair description of data and valuable guidance for their interpretation. Interestingly, the data of the $^{86}\mathrm{Kr}+\mathrm{Ni},\mathrm{Sn}$ reactions at 15 MeV/nucleon show a retardation of the process of $N/Z$ equilibration which, as suggested by the CoMD calculations, is indicative of the collective character of the process. This retardation is not present in the investigated systems at 25 MeV/nucleon (and the light $^{40}\mathrm{Ar}+\mathrm{Ni}$ systems at 15 MeV/nucleon), whose behavior is found to be consistent with stochastic nucleon exchange.

Study of rotating high energy systems with the differential HBT method

Peripheral heavy-ion reactions at ultra relativistic energies have large angular momentum that can be studied via two particle correlations using the Differential Hanbury Brown and Twiss method. In the present work, we analyze the possibilities and sensitivity of the method in rotating, few source systems. Analytic results provide insight in the advantages of this method.

Unfolding the response of a zero-degree magnetic spectrometer from measurements of the [formula omitted] resonance

The magnetic spectrometer FRagment Separator at GSI has been used to investigate the in-medium Δ-resonance excitation in peripheral heavy-ion reactions. The resolving power of this spectrometer makes it possible to disentangle the longitudinal-momentum loss induced by the excitation of the Δ resonance in the projectile residues produced in isobaric charge-exchange collisions. However, beam emittance, electromagnetic interactions of projectile and residual nuclei in the target, and the accuracy of the tracking detectors limit the final resolution. The characterization of the Δ resonance requires then to unfold the measured longitudinal-momentum distribution from the response of the spectrometer. In this work, we use an unfolding procedure based on the Richardson–Lucy method with a regularization technique to optimize the stability of the solution against statistical fluctuations. The method is validated using measurements of isobaric charge-changing collisions with a 136Xe beam at 500AMeV.

Dissipative processes in peripheral heavy ion collisions at Fermi energies

Peripheral heavy ion reactions are of interest for the production of new isotopes. In this contribution we present an investigation of reactions of light projectiles O and Ne on Be and Ta targets at Fermi energies in the framework of transport theory. Transport theory describes dissipative (deep-inelastic) processes, where considerable amounts of mass and energy are exchanged. The data, on the other hand, also seem to include a more direct component with small energy loss. We separate the two components on the basis of the velocity distribution and compare the dissipative component to the transport calculations. The primary fragments of the transport calculation still have considerable excitation energies. For the comparison with experiment we take into account the secondary evaporation in a statistical model. This improves the qualitative agreement with the data.

Excitation energy sharing in binary peripheral heavy ion reactions at Fermi energies

Evidence for the dependence of excitation energy sharing between two heavy remnants on the difference in the lost mass in two-body peripheral heavy ion reactions at Fermi energy is presented, based on experimental results for the reactions $^{93}\mathrm{Nb}$$+$$^{116}\mathrm{Sn}$, $^{116}\mathrm{Sn}$$+$$^{93}\mathrm{Nb}$, and $^{93}\mathrm{Nb}$$+$$^{93}\mathrm{Nb}$ at $38A$ MeV. An observable based on the experimental velocities of the heavy residues is used to select reactions with equal preevaporative masses of projectile-like fragments and target-like fragments. The excitation energy, evaluated by means of a complete average calorimetry, is found to be larger for the nucleus that finally retains a larger part of the hot interaction region.

Pre-equilibrium dipole strength in charge asymmetric peripheral heavy-ion reactions

We report on the results obtained from the study of the 32S + 64Ni and 32S + 58Ni peripheral reactions at incident energies E lab = 288 MeV and E lab = 320 MeV, respectively. High-energy γ-rays were detected in an array of 8 seven-pack BaF2 clusters. Coincidence with complex fragments detected in 12 three-stage telescopes ensured the selection of peripheral reaction events. All of the relevant reaction parameters were kept constant with the exception of the different initial dipole moment caused by the different entrance channel charge asymmetry. While for quasi-elastic events no N/Z effect was observed in the differential γ-ray multiplicities of the two reactions, for deep-inelastic events a larger dipole γ-ray emission occurs during the more N/Z asymmetric reaction. A theoretical interpretation based on a collective Bremsstrahlung analysis of the reaction dynamics is presented.

Midrapidity charge distribution in peripheral heavy ion collisions

The charge density distribution with respect to the velocity of matter produced in peripheral heavy ion reactions around Fermi energy is investigated. The experimental finding of enhancement of mid-rapidity matter shows the necessity to include correlations beyond BUU which was performed in the framework of nonlocal kinetic theory. Different theoretical improvements are discussed. While the in-medium cross section changes the number of collisions, it leaves the transferred energy almost unchanged. In contrast the nonlocal scenario changes the energy transferred during collisions and leads to an enhancement of mid-rapidity matter. The renormalisation of quasiparticle energies can be included in nonlocal scenarios and leads to a further enhancement of mid-rapidity matter distribution. This renormalisation is accompanied by a dynamical softening of the equation of state seen in longer oscillation periods of the excited compressional collective mode. We propose to include quasiparticle renormalisation by using the Pauli-rejected collisions which circumvent the problem of backflows in Landau theory. Using the maximum relative velocity of projectile and target like fragments we associate experimental events with impact parameters of the simulations. For peripheral collisions we find a reasonable agreement between experiment and theory. For more central collisions, the velocity damping is higher in one-body simulations than observed experimentally, because of missing cluster formations in the kinetic theory used.

Dynamical analysis of isospin and angular momentum effects in peripheral heavy-ion reactions

Abstract The semi-classical Landau-Vlasov model has been used to investigate the decay modes of peripheral Pb+Au reactions at 29 MeV/n. Statics and dynamics of these very massive nuclei are analyzed especially through the isospin dependence of the effective nuclear force. The degree of dissipation of the collisions is studied for different bins of impact parameter pointing out the influence of the nucleon-nucleon cross section. The appearance of intermediate mass fragments from neck-like structures is evidenced, the effects of angular momentum transfers are shown to play a fundamental role in this phenomenon. The theoretical results are compared with experimental data, showing the importance of the dynamical and out-of-equilibrium effects on the observables.

Approaching peripheral collisions in the reaction Ar+Au at 27.2 MeV/u by the associated neutron multiplicities

A 4π detector has been used to measure the neutron multiplicity associated with peripheral heavy ion reactions induced at 27.2 MeV/u. A strong correlation is observed between the number of emerging neutrons and the mass of the projectile-like-fragment. However, the large energy dissipation which is observed is still too small to support the idea of a pure massive transfer process.

Inclusive observables calculated with the nuclear random walk model: Application to the 40Ar+68Zn reaction at 14.6 and 27.6 MeV/nucleon.

A model for peripheral heavy ion reactions which includes, in a consistent formalism, both primary and secondary phases of the reaction mechanism is described. The model is based on a ``nuclear random walk'' in the projectile-like fragment mass which depends on the number of collisions between projectile and target nucleons along the classical trajectory of relative motion. Emphasis is placed on simple evaluation of experimental observables. An application to the $^{40}$Ar${+\mathrm{}}^{68}$Zn reaction, at 14.6 and 27.6 MeV/nucleon, is made and differences between model predictions and measurements, especially at the higher energy, are discussed.

Partition of excitation energy in peripheral heavy-ion reactions

The partition of excitation energy between target-like and projectile-like primary fragments from 11 MeV/nucleon 20 Ne + 197 Au quasi-elastic reactions was determined from kinematic analyses of three-body final states. Projectile breakup following stripping, pickup, and inelastic scattering was studied, and the excitation-energy partition was found to be strongly correlated with the direction of the mass transfer. Results are in quantitative agreement with optimum- Q-value calculations.

Angular evolution of peripheral heavy ion reactions at intermediate energies

Energy spectra and angular distributions of projectile-like fragments have been measured in the vicinity of the grazing angle for the 40Ar + 40Ca and 40Ar + 208Pb reactions at 44 MeV/nucleon. Measurements of the 40Ar + 40Ca system at 27 MeV/nucleon and 20Ne + 208Pb reaction at 44 MeV/nucleon at one angle have also been performed. For fragments with charge and mass close to the projectile numerous deviations from the standard fragmentation model have been observed including rapidly changing shapes of the angular distributions with the fragment mass. Moreover the isotopic distributions and mean fragment velocities are strongly dependent on detection angle. A surface transfer reaction component dominant at the grazing angle can be separated from a second component which cannot be entirely accounted for by a simple fragmentation mechanism.

Projectile-like fragment production in heavy ion reactions

A model for peripheral heavy ion reactions is proposed in which the reaction mechanism is governed by the number of nucleon-nucléon collisions taking place during the interaction. A general random walk process leads to simple expressions for the mass yields. Angular distributions are obtained as the convolution of distributions due to deflection by the ion-ion potential and recoil effects due to the change in mass.

On emission of light particles following peripheral heavy-ion reactions

A simple theoretical approach is presented for the equilibrium and nonequilibrium components of the angular correlations between light particles and outgoing fragments in paripheral heavy-ion collisions. Semi-classical formulae for the in-plane differential multiplicities are given. In particular, the double forward-peaked shape in the nonequilibrium angular-correlation pattern is explained in terms of emission of fast light particles from a «l-window» and it is out-lined that different angular correlations are expected depending on the values of the quantal deflection angle involved in this process.

Alpha particle emission in peripheral heavy ion reactions at 20 MeV/u

The reaction $^{197}\mathrm{Au}(^{16}\mathrm{O}, \mathrm{HI}\ensuremath{\alpha})$ has been investigated at 310 MeV. The angular correlations of alpha particles measured in coincidence with projectile-like fragments are very strongly forward peaked; the reaction is dominated by large negative values of the three-body $Q$ value. Most of the observed alpha particles are emitted from projectile-like fragments. Within a factor of 2, no anisotropy of emission in the center-of-mass frame of the coincident projectile residue and alpha particle was found.

Simple model for nuclear alignment in peripheral nuclear collisions

A nucleon exchange model incorporating both the effects of the relative motion of the target and projectile and the internal motion of the constituent nucleons is developed to calculate the alignment and magnitude of the transferred angular momentum in peripheral heavy ion reactions. The model leads in a natural way to unequal components or angular momentum along the two directions perpendicular to the normal to the reaction plane, and explains the decrease in alignment for small energy losses in a physically transparent manner.NUCLEAR REACTIONS Nucleon exchange model distinguishing between effects of relative motion and Fermi motion. Magnitude and alignment of angular momentum transfer calculated for heavy ion reaction.