Projectile-like Fragments Research Articles

Transport description of dissipative heavy ion collisions at Fermi energies

Peripheral collisions of heavy ions at Fermi energies are a powerful tool for producing new isotopes far from stability line. The mechanism of these reactions has features between direct and dissipative reactions, as is seen from the velocity distributions of the projectile-like fragments, which are peaked near the beam velocity and have a long tail to the smaller velocities. In this report we treat these heavy ion collisions in a Boltzmannlike transport approach, which describes the evolution of the phase space distribution function under the influence of a mean field and a two-body collision term. It is used to calculate the primary fragments after the collision phase, which are, however, still highly excited. The de-excitation of the fragments is calculated in a statistical model. We calculate isotope distributions and velocity spectra for the reaction of 40Ar as a projectile on targets of 9Be and 181Ta at E/A = 57 MeV and find reasonably good agreement with experimental data.

Isospin Against Size Effects In Projectile Dynamical Fission For 112,124Sn+58,64Ni and 124Xe+64Zn Reactions At 35 A.MeV

In past experiments, mass asymmetric projectile-target combinations124Sn+64Ni and 112Sn+58Ni were investigated at ELab(112'124Sn)=35 A.MeVbeam energybyusing the 4n multi-detector CHIMERA. From a quantitative comparison of cross sections associated to Statistical and Dynamical Fission of the Projectile-Like Fragments, it resulted that Dynamical Fission process is about two times more probable in the neutron rich 124Sn+64Ni system than in the 112 Sn +58 Ni neutron poor one. In contrast, no sizable difference was found for Statistical Fission mechanism. The observed difference in the strength of the Dynamical effects could arise from the difference in entrance channel Isospin (N/Z) content. In order to disentangle Isospin effects from effects due to the different masses of the two systems, a new experiment 124Xe+64Zn at 35 A.MeV beam energy has been recently carried out.

Aligned ternary partitioning of the 197Au + 197Au system at 23 A MeV beam energy

Ternary partitions of the + system in collisions at 23 A MeV beam energy have been studied. It is observed that the mass number distribution of the lightest fragments, called intermediate mass fragments (IMF), extends up to mass numbers corresponding to partitions into three comparable fragments. The lightest IMFs of show well known features of prompt emission originating from the interaction zone (neck) during reseparation. Heavier IMFs (of ) definitely originate from sequential (but not equilibrated) decay of the primary projectile-like fragments (PLF*) (or target-like fragments (TLF*)). The decaying system evidently keeps memory of the neck configuration and consequently the IMFs are emitted collinearly either from the neck side or from the opposite side along the separation axis (polar emission). The new intriguing process of the polar emission dominates for IMFs of .

Estimates for temperature in projectile-like fragments in geometric and transport models

Projectile like fragments emerging from heavy ion collision have an excitation energy which is often labeled by a temperature. This temperature was recently calculated using a geometric model. We expand the geometric model to include also dynamic effects using a transport model. The temperatures so deduced agree quite well with values of temperature needed to fit experimental data.

Study of the multi-nucleon transfer reactions of136Xe +198Pt for producing exotic heavy nuclei

The feasibility of the multi-nucleon transfer (MNT) reaction in the 136 Xe + 198 Pt system was investigated for synthesizing neutron-rich heavy nuclei around mass number A 200 of astrophysical interest. Identification of the projectile-like fragments (PLF's) was successfully performed by using VAMOS++ spectrometer. The isotopic distributions of the PLF's produced by two-proton stripping and pickup reactions were compared with each other, demonstrating the properties of the MNT reactions observed in the present system. The two-proton pickup channels strongly populated than the two- proton stripping channels were observed. The isotopic distributions for di erent total kinetic energy loss (TKEL) ranges are also discussed. The overall enhancement of the strength of the two-proton pickup compared with the two-proton stripping appears to be related to the MNT reactions with relatively moderate energy dissipation.

Theoretical Calculation of Energies of Projectile like Fragments in 76Ge (635 MeV) + 198Pt Deep-Inelastic Collisions

The theoretical calculation of the energies of projectile like fragments (PLFs) using the heavy-ion reactions between 76Ge and 198Pt are reported in this article. The incident beam energy was 635 MeV. The calculated values of PLFs are compared with the previous experimental results and it is shown that the theoretical calculations of PLFs are consistent with the experimental values. The elastic peak of the projectile is compared theoretically and experimentally. Moreover, the Q-value and binding energy of PLFs were also calculated.

Timescale for equilibration ofN/Zgradients in dinuclear systems

Equilibration of N/Z in binary breakup of an excited and transiently deformed projectile-like fragment (PLF*), produced in peripheral collisions of 64Zn + 27Al, 64Zn, 209Bi at E/A = 45 MeV, is examined. The composition of emitted light fragments (3<=Z<=6) changes with the decay angle of the PLF*. The most neutron-rich fragments observed are associated with a small rotation angle. A clear target dependence is observed with the largest initial N/Z correlated with the heavy, neutron-rich target. Using the rotation angle as a clock, we deduce that N/Z equilibration persists for times as long as 3-4 zs (1zs = 1 x 10^-21 s = 300 fm/c). The rate of N/Z equilibration is found to depend on the initial neutron gradient within the PLF*.

Detection and identification of large fragments from the partitioning of the 197Au + 197Au system at 23A MeV

First results of a study of partitioning of the nonfusing 197Au + 197Au system at an energy of 23A MeV are reported. A variety of light, medium and heavy fragments were detected and identified by using the 4π multidetector system CHIMERA (Charged Heavy Ion Mass and Energy Resolving Array). A new method of identification of low-energy fragments stopped in the silicon detectors, based on a precise time-of-flight analysis, is proposed. The method has been applied in a preliminary analysis of the 197Au + 197Au partitioning processes leading to the neck emission of intermediate mass fragments and sequential breakup of projectile-like or target-like fragments.

Study of collisions of 136Xe + 198Pt for the KEK isotope separator

Multinucleon transfer reactions between two heavy ions are an important tool for production and investigation of exotic neutron-rich nuclei, which are difficult to access by other methods. The 136Xe+198Pt system is a candidate to efficiently produce neutron-rich nuclei around the neutron magic number N=126 for the KEK isotope separation project. In order to confirm this, measurements of the production cross sections with the large acceptance magnetic spectrometer VAMOS++ and de-excitation gamma rays from target-like fragments using the high efficiency germanium array EXOGAM at GANIL are reported. The measured isotopic distributions of the projectile-like fragments are compared with GRAZING calculations. The proton stripping channels show rough agreements between measurements and calculations. For the proton pick-up channels, the measured distributions are shifted toward the heavier masses and show enhanced cross sections in transfers of two or more protons.

(Multi-)nucleon transfer in the reactions 16O, 332S + 208Pb

A detailed analysis of the projectile-like fragments detected at backward angles in the reactions 16O,32S+208Pb at energies below the fusion barrier is presented. Excitation functions corresponding to nucleon transfer with ΔZ = 1 and ΔZ = 2 were extracted, indicating surprisingly large absolute transfer probabilities at sub-barrier energies. A comparison of 2p transfer probabilities with time-dependent Hartree-Fock calculations suggests strong pairing correlations between the two protons. Excitation energies in the projectile-like fragments up to ~ 15MeV for the 16 O and ~ 25MeV for 32S-induced reactions show the population of highly excited states in the residual nuclei, and indicate substantial dissipation of kinetic energy. A new phenomenological framework explores the effect of energy dissipation through these highly inelastic (large excitation energies) and complex (correlated few-nucleon transfer) processes on the probability for fusion in the reaction 16o+208Pb. Preliminary results show that the suppression of fusion at energies above the barrier in this reaction can be explained within the new framework.

Correlations between isospin dynamics and Intermediate Mass Fragments emission time scales: a probe for the symmetry energy in asymmetric nuclear matter

We show new data from the 64Ni+124Sn and 58Ni+112Sn reactions studied in direct kinematics with the CHIMERA detector at INFN-LNS and compared with the reverse kinematics reactions at the same incident beam energy (35 A MeV). Analyzing the data with the method of relative velocity correlations, fragments coming from statistical decay of an excited projectile-like (PLF) or target-like (TLF) fragments are discriminated from the ones coming from dynamical emission in the early stages of the reaction. By comparing data of the reverse kinematics experiment with a stochastic mean field (SMF) + GEMINI calculations our results show that observables from neck fragmentation mechanism add valuable constraints on the density dependence of symmetry energy. An indication is found for a moderately stiff symmetry energy potential term of EOS.

Tracking saddle-to-scission dynamics using N/Z in projectile breakup reactions

Fragments produced in binary splits of an excited projectile-like fragment (PLF*) formed in the reactions 124Xe + 112,124Sn at an incident energy of 50 MeV/A are examined. The dependence of the isotopic composition of light fragments (4≤ ZL ≤8) on rotation angle allows one to explore changes in N/Z on the timescale of the rotational period of the PLF*. Entrance channel effects are examined by comparing the isotopic composition of fragments produced in the binary decay following 64Zn + 27Al, 64Zn, 209Bi at an incident energy of 45 MeV/A.

Asymmetry Energy Effects on Reaction Break-up Mechanisms Near the Fermi Energy

Heavy Ion Collisions play an important role in probing the density dependence of the asymmetry energy by providing a unique tool to probe nuclear interactions away from saturation. Lower energy reactions (~10-15A MeV) provide a means of probing around and below normal nuclear density. In particular, these collisions can provide a sensitive means of probing the interaction and possibly the asymmetry terms of the nuclear Equation of State. Simulations of 124Sn+64Ni using Constrained Molecular Dynamics and Stochastic Mean Field calculations, at different asymmetry energies, have been used to study the asymmetry energy dependence of projectile-like fragment break-up channels as well as the mass correlations of the heaviest fragments. The results of these simulations present the possibility of developing an experimental probe for measuring the density dependence of the asymmetry energy using picosecond time resolution time of flight spectroscopy in a multi-detector array.

Angular distributions and cross-sections of projectile-like fragments in the 19F + 159Tb reaction

The angular distribution of projectile-like fragments (PLFs) in the 19F + 159Tb reaction have been measured at beam energy equal to 98MeV. Angular distributions of PLFs showed a systematic change with increasing mass transfer, starting from the peaking at grazing angle for heavier PLFs to very forward peaked angular distributions for lighter PLFs. Cross-sections of the different PLFs were obtained by integrating their centre-of-mass angular distributions. The PLF cross-sections have been compared with the incomplete fusion cross-sections obtained from the earlier measurement of the evaporation residue cross-section. Reduced cross-sections for lighter PLFs were observed to be higher compared to those observed in 19F + 66Zn reaction at similar values of Ecm/Vb. Also, elastic scattering measurements were carried out to get information about the grazing angle and total reaction cross-section.

Potential Correlations between Unexplained Experimental Observables and Hot Projectile-Like Fragments in Primary Interactions above &amp;lt;i&amp;gt;E&amp;lt;/i&amp;gt;&amp;lt;sub&amp;gt;CM/&amp;lt;i&amp;gt;u&amp;lt;/i&amp;gt;&amp;lt;/sub&amp;gt; ≈ 150 MeV

An enhanced neutron production and an enhanced nuclear destruction due to secondary fragments have been observed in very thick targets irradiated with high energy ions. This enhancement is beyond theoretical calculations and it is an unresolved problem. It is observed only when primary ion interactions exceed an energy threshold (ECM/u ≈ 150 MeV). Investigations using nuclear emulsions for very high-energy nuclear reactions suggest that two distinctly different classes of relativistic projectile-like fragments are emitted in primary interactions: a “cool” channel with a temperature of (T(p)cool ≈ 10 MeV), and a “hot” channel with (T(p)hot ≈ 40 MeV. This second reaction class may induce the above mentioned enhanced reactions of secondary fragments, thus being responsible for unresolved problems. This assumption should be studied in further experiments. Nuclear interactions of secondary particles in thick targets are of interest, in particular in view of radiation protection needs for high energy and high intensity heavy ion accelerators. Many basic ideas of this paper go back to the late Professor E. Schopper (Frankfurt).

Correlations with projectile-like fragments and emission order of light charged particles

Correlations with projectile-like fragments and emission order of light charged particles

Projectile structure effects in multi-nucleon and cluster transfers in 16,18O+164Dy, 208Pb reactions

The yield of projectile like fragments (PLFs) has been measured in the reactions of 16,18O with 164Dy and 208Pb targets at energies above the Coulomb barrier. The experimental data are analyzed to ascertain the role of projectile structure on the multi-nucleon and cluster correlations in transfer reactions. The variation of transfer probability with the number of nucleons transferred from the projectile to the target nucleus at the grazing angle has been investigated for both the systems. In the case of 18O induced reactions the cross sections of the two neutron (2n) stripping as well as the 2n- correlated cluster transfer are strongly enhanced as compared to the 16O induced reactions. These results have been discussed on the basis of the possible influence of the valence di-neutrons in the 18O nucleus on multi-particle transfer reactions.

Spectator fragmentation in nucleus-nucleus collisions: Phase space approach

Dynamics of spectator matter break-up in non-central collisions of 197Au+ 197Au at 1000 MeV/A are explored within the framework of a quantum molecular dynamics (QMD) model. The phase space of nucleons bound in intermediate mass fragments are studied using two clustering subroutines. Backtracking the origin of fragments to the time of the initial contact between the colliding nuclei and applying the simulated annealing clusterization algorithm (SACA) indicates a significant yield of projectile-like and target-like fragments. The simplest clusterization approach based on the spatial correlation technique, however, predicted a much lesser production probability of fragments in the spectator zone.

Tracking saddle-to-scission dynamics usingN/Zin projectile breakup reactions

Fragments produced in binary splits of an excited projectile-like fragment (PLF${}^{*}$) formed in the reactions ${}^{124}$Xe $+$ ${}^{112,124}$Sn at an incident energy of 50 MeV/$A$ are examined. The dependence of the isotopic composition of light fragments ($4\ensuremath{\leqslant}{Z}_{L}\ensuremath{\leqslant}8$) on rotation angle allows one to explore changes in $N/Z$ on the time scale of the rotational period of the PLF${}^{*}$. Changes in the $N/Z$ of the fragments persist for times as long as 2--3 zs (600--900 fm/$c$). The two-component nature of the time dependence of $\ensuremath{\langle}N\ensuremath{\rangle}/Z$ for ${Z}_{L}=4$ and the $Z$ dependence are related to differences in the initial configurations and subsequent decay of the PLF${}^{*}$.

Study of Isotopic Distribution of the Projectile-like Fragments Produced in the 17,18N + 197Au Reactions at 33 MeV/u

The experimental data of the isotopic distribution for projectile-like fragments are presented for the 17,18N + 197Au reaction at 33 MeV/u. The width of the isotopic distributions for 18N projectile is significantly broader than that for 17N projectile, and the average N/Z ratio of the former shifts to higher neutron number side. As long as the realistic nucleon density distribution is used, the isotopic distribution for fragments is reproduced by the simple abrasion-ablation model calculation, which thus provides an independent way to determine the surface distribution of the nuclear matter density for neutron-rich nuclei.