Quasi-projectile Sources Research Articles

Isospin dependence of fragment yields in peripheral heavy ion collisions

We have theoretically investigated isospin dependence in peripheral isospin asymmetric $$^{112}$$Sn + $$^{124}$$Sn and $$^{124}$$Sn + $$^{112}$$Sn reaction systems at 50 MeV/nucleon projectile energy. Mass, charge and isotope yields emitted from the excited quasiprojectile sources are obtained within a statistical approach. In order to interpret the experimental data, the N/Z ratio, isotope yield, and isoscaling analyses were carried out for light intermediate mass fragments (with $$Z\le 8$$, at available data range). We show that predicted results produced for fragment yields, are consistent with the experimental data for peripheral isospin asymmetric reaction systems. From the comparison of the statistical model predictions with experimental data, it is concluded that a lower value of the effective symmetry-term coefficient $$\gamma $$ must be considered at low density freeze-out. Statistical analysis of isoscaling shows that the isoscaling parameter $$\alpha $$ is very sensitive to the isospin composition (N/Z ratio) of the the sources, and it is proportional to the strength of the symmetry-term coefficient.

Isoscaling of mass A ≃ 40 reconstructed quasiprojectiles from collisions in the Fermi energy regime

Isoscaling studies of fragments with Z=1-8 from reconstructed quasiprojectiles of mass A ≃40 from the 40Ca + 112,124Sn and 40Ar + 112,124Sn reactions at beam energy of 45 MeV/nucleon were performed. After initial efforts to obtain isoscaling using pairs of systems differing in their neutron-to-proton ratio N/Z, “intra-system” isoscaling for each of these four systems was obtained using fragment sources restricted in two narrow N/Z zones, one neutron rich and one neutron poor. The observed isoscaling behaviour was excellent and the isoscaling parameter α was found to decrease as the excitation energy increases. Corrections due to undetected neutrons were also taken into account in the source N/Z determination by using the theoretical models DIT (Deep Inelastic Transfer) and SMM (Statistical Multifragmentation Model) along with a software replica of the experimental setup. The reduced isoscaling parameter α/∆ was found to decrease as the excitation energy of the quasiprojectile source increases, in good agreement with recent work on reconstructed mass A ≃ 80 quasiprojectiles. This decrease of α/∆ may point to a decrease of the symmetry energy coefficient with increasing excitation energy.

Experimental liquid-gas phase transition signals and reaction dynamics

The experimental liquid-gas phase transition signals are examined for the quasiprojectile (QP) reconstructed from the reactions of $^{40}\mathrm{Ar}+^{27}\mathrm{Al}$, $^{48}\mathrm{Ti}$, $^{58}\mathrm{Ni}$ at 47 MeV/nucleon, using measures of caloric curve, multiplicity derivative, moment parameters, and fluctuation of maximum fragment charge number (NVZ). The QP source is reconstructed, using moving source parametrizations on an event-by-event basis. For the determination of the temperature, a quadrupole fluctuation thermometer is used. Deuterons are chosen for the thermometer to minimize the Coulomb and secondary sequential decay effects. A new event-by-event method is proposed for the thermometer to determine the temperature. All measures show a characteristic signature around the temperature $T=9.0\ifmmode\pm\else\textpm\fi{}0.4$ MeV, which may suggest that the QP system goes into the liquid-gas phase transition at ${T}_{c}=8.3\ifmmode\pm\else\textpm\fi{}0.4$ MeV after the Coulomb correction.

Emission of fragments in Ca+Ca reactions at 25 MeV/nucleon

We discuss experimental data concerning 40,48Ca+40,48Ca reactions at 25 MeV/nucleon; the 4π multi-detector Chimera has been used as detection device. Effects that can be attributed to the neutron to proton ratios (N/Z) degree of freedom have been investigated. From the analysis of experimental data it seems that the neutron richness of the interacting system plays an important role on the evolution of fusion-like sources formed in semi-central collisions. In particular, it is observed that the larger is the neutron content and the larger is the emission of heavy residues. Experimental data have been compared with CoMD-II model calculations; a moderately stiff symmetry energy should be used to reproduce satisfactorily the data. A thermodynamical analysis on fusion-like sources has been also performed. In semi-peripheral collisions, isospin diffusion signals have been found. They have been investigated by analyzing isobaric emission (7Li/7Be) of quasi-projectile sources. Experimental data indicate that an incomplete N/Z mixing is reached during the interaction phase.

Using light charged particles to probe the asymmetry dependence of the nuclear caloric curve

Recently, we observed a clear dependence of the nuclear caloric curve on neutron-proton asymmetry $\frac{N\ensuremath{-}Z}{A}$ through examination of fully reconstructed equilibrated quasiprojectile sources produced in heavy-ion collisions at $E/A=35$ MeV. In the present work, we extend our analysis using multiple light charged particle probes of the temperature. Temperatures are extracted with five distinct probes using a kinetic thermometer approach. Additionally, temperatures are extracted using two probes within a chemical thermometer approach (Albergo method). All seven measurements show a significant linear dependence of the source temperature on the source asymmetry. For the kinetic thermometer, the strength of the asymmetry dependence varies with the probe particle species in a way that is consistent with an average emission-time ordering.

Experimental determination of the quasi-projectile mass with measured neutrons

The investigation of the isospin dependence of multifragmentation reactions relies on precise reconstruction of the fragmenting source. The criteria used to assign free emitted neutrons, detected with the TAMU Neutron Ball, to the quasi-projectile source are investigated in the framework of two different simulation codes. Overall and source-specific detection efficiencies for multifragmentation events are found to be model independent. The equivalence of the two different methods used to assign experimentally detected charged particles and neutrons to the emitting source is shown. The method used experimentally to determine quasi-projectile emitted free neutron multiplicity is found to be reasonably accurate and sufficiently precise as to allow for the study of well-defined quasi-projectile sources. Experimental QP neutron multiplicity distributions for three similar reactions with different isospin content are also presented. An increase in neutron emission is found for more n-rich systems.

Multifragmentation of reconstructed quasi-projectiles in the mass region A ∼ 30

The production of fragments from a broad range of reconstructed quasi-projectiles obtained in the interaction of 32S (45 MeV/nucleon) with 112Sn was studied. A good description of the experimental data on the multifragmentation of these light quasi-projectiles with a model framework based on deep inelastic transfer followed by SMM (statistical multifragmentation model) was obtained. The charge and mass distributions of the observed fragments show that the break-up of a given quasi-projectile source is highly correlated to its isospin composition. The sensitivity of the hot primary fragments to the symmetry energy was investigated in detail. It was found that for the present light systems (A∼30), a good description of the experimental data on the charge distributions, the 〈N/Z〉 distributions and the isotopic distributions of the fragments is obtained by the SMM calculations involving the standard (Csym = 25 MeV), as well as, a reduced value of the symmetry energy (Csym = 8 MeV). However, the reduced value of the symmetry energy appears to provide a better description of the isotopic distributions in most of the cases. This is in agreement with an overall reduction of the symmetry energy of primary fragments indicated by a number of multifragmentation studies of heavier systems.

N/Z effects on40,48Ca+40,48Ca reactions at 25 MeV/nucleon

Effects related to the neutron to proton ratio (N/Z) degree of freedom in 40,48 Ca+ 40,48 Ca reactions at 25 MeV/nucleon have been investigated. Isotopic effect and even-odd staggering characterize the emission of light fragments at forward angles. The study of isobaric ratio 7 Li/ 7 Be for quasi-projectile source in semi-peripheral event of reactions allows moreover to investigate isospin diffusion effects in heavy ion collisions.

Isoscaling of mass [formula omitted] reconstructed quasiprojectiles from collisions in the Fermi energy regime

Isoscaling studies of fragments with Z = 1 – 8 from reconstructed quasiprojectiles of mass A ≃ 40 from the Ar 40 + Sn 124 , 112Sn and Ca 40 + Sn 124 , 112Sn reactions at beam energy of 45 MeV/nucleon were performed. After initial efforts to obtain isoscaling with the “traditional” approach of using pairs of systems differing in their isospin asymmetry (or neutron-to-proton ratio N / Z ), “intra-system” isoscaling for each of these four systems was obtained using fragment sources restricted in two narrow N / Z regions (one neutron-rich and one neutron-poor). The observed isoscaling behavior was excellent and the isoscaling parameter α was found to decrease with increasing excitation energy. Corrections due to undetected neutrons were also taken into account in the source N / Z determination by using the theoretical models DIT (Deep Inelastic Transfer) and SMM (Statistical Multifragmentation Model) along with a software replica of the experimental setup. These corrections were applied to the determination of the parameter Δ (expressing the difference in the isospin asymmetry of the two sources used in the isoscaling). The reduced isoscaling parameter α / Δ was obtained and found to decrease as the excitation energy of the quasiprojectile source increases, in good agreement with recent work on reconstructed mass A ≃ 80 quasiprojectiles. This decrease of α / Δ may point to a decrease of the symmetry energy coefficient with increasing excitation energy.

Fragment properties of fragmenting heavy nuclei produced in central and semi-peripheral collisions

Fragment properties of hot fragmenting sources of similar sizes produced in central and semi-peripheral collisions are compared in the excitation energy range 5 – 10 A MeV . For semi-peripheral collisions a method for selecting compact quasi-projectiles sources in velocity space similar to those of fused systems (central collisions) is proposed. The two major results are related to collective energy. The weak radial collective energy observed for quasi-projectile sources is shown to originate from thermal pressure only. The larger fragment multiplicity observed for fused systems and their more symmetric fragmentation are related to the extra radial collective energy due to expansion following a compression phase during central collisions. A first attempt to locate where the different sources break in the phase diagram is proposed.

Critical behavior in light nuclear systems: Experimental aspects

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

Two-fragment correlation functions for quasiprojectile source and midrapidity component at intermediate energies

Two-fragment reduced-velocity correlation functions of intermediate-mass fragments emitted from midrapidity component and quasiprojectile (QP) sources formed in ${}^{58}\mathrm{Ni}{+}^{12}\mathrm{C}$ and ${}^{58}\mathrm{Ni}{+}^{197}\mathrm{Au}$ reactions at 34.5 MeV/nucleon have been studied. For the midrapidity component, they show a stronger Coulomb suppression at low relative velocities than for the QP source, suggesting a shorter emission time for it than for the QP source. Comparing the experimental correlation functions with the prediction of many-body Coulomb trajectory code, the emission times of a QP source formed in both reactions were extracted as a function of the excitation energy. The variation of emission time of the QP source with the excitation energy is independent of the reaction system. It decreases monotonically with the excitation energy in the range of $(2--6)A$ MeV from several hundred $\mathrm{fm}/c$ to about $100 \mathrm{fm}/c.$ Above an excitation energy of $6A$ MeV, it becomes very short and saturates, suggesting that the QP source undergoes a multifragmentationlike breakup. The influence of the quasitarget fragment Coulomb interaction on QP emission time is also considered.

Fragment emission time from well defined sources in58Ni+197Auat 34.5 MeV/nucleon

We have measured two-fragment correlation functions of the intermediate mass fragments emitted from quasiprojectile sources and midrapidity component formed in ${}^{58}\mathrm{Ni}{+}^{197}\mathrm{Au}$ at 34.5 MeV/nucleon. The two-fragment correlation functions of midrapidity component show a stronger Coulomb suppression than the quasiprojectile source. This Coulomb suppression for midrapidity component changes very little with the excitation energy of the quasiprojectile source deduced event by event by calorimetry method. By comparing the experimental correlation functions with an N-body Coulomb trajectory code calculation, the emission time of quasiprojectile sources has been extracted as a function of the excitation energy. The emission time decreases monotonically with the excitation energy in the range of 2--6 A MeV from 550 $\mathrm{fm}/c$ to about 150 $\mathrm{fm}/c.$ Above excitation energy of $6A$ MeV, the emission time becomes shorter and constant, suggesting that prompt multifragmentation occurs in these quasiprojectile sources.

Negative heat capacity in the critical region of nuclear fragmentation: an experimental evidence of the liquid-gas phase transition

An experimental indication of negative heat capacity in excited nuclear systems is inferred from the event by event study of energy fluctuations in Au quasi-projectile sources formed in Au+Au collisions at 35 A.MeV. Equilibrated events are selected and the excited source configuration is reconstructed through a calorimetric analysis of its de-excitation products. Fragment partitions show signs of a critical behavior at about 4.5 A.MeV excitation energy. Around this value the heat capacity shows a negative branch providing a direct evidence of a first order liquid gas phase transition.