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Experimental results concerning the dynamical fission of quasiprojectiles in semiperipheral collisions for the system Kr80+Ca48 at 35 MeV/nucleon are presented. Data have been collected with four blocks of the FAZIA setup in the first physics experiment of the FAZIA Collaboration. The degree of isospin equilibration between the two fission fragments and its dependence on their charge asymmetry is investigated. The data are compared with the results of the AMD model coupled to GEMINI as an afterburner, in order to get hints about the timescale of the process.

International audience; In nuclear thermodynamics, the determination of the excitation energy of hot nuclei is a fundamental experimental problem. Instrumental physicists have been trying to solve this problem for several years by building the most exhaustive 4π detector arrays and perfecting their calorimetry techniques. In a recent paper, a proposal for a new calorimetry, called “3D calorimetry”, was made. It tries to optimize the separation between the particles and fragments emitted by the quasiprojectile and the other possible contributions. This can be achieved by determining the experimental probability for a given nucleus of a nuclear reaction to be emitted by the quasiprojectile. It has been developed for the INDRA data. In the present work, we wanted to dissect and validate this new method of characterization of a hot quasiprojectile. So we tried to understand and control it completely to determine these limits. Using the Heavy Ion Phase Space Exploration (HIPSE) event generator and a software simulating the functioning of INDRA, we were able to achieve this goal and provide a quantitative estimation of the quality of the quasiprojectile characterization.

Background: The isovector part of the nuclear equation of state remains partly unknown and is the subject of many studies. The degree of equilibration between the two main collision partners in heavy ion reactions may be used to study the equation of state since it is connected to isospin (N/Z) transport properties of nuclear matter. Purpose: We aim to test chemical equilibrium attainment by measuring isotopic characteristics of emitted elements as a function of impact parameter. Method: We study four Xe136,124+Sn124,112 reactions at 32 MeV/nucleon. The data were acquired with the INDRA detector at the GANIL (Caen, France) facility. Combined (projectile+target) systems are identical for two studied reactions, therefore it is possible to study the path towards chemical equilibrium from different neutron to proton ratio (N/Z) entrance channels. The study is limited to identified isotopes detected in the forward part of the center of mass in order to focus on the evolution of projectile-like fragment isotopic content and the benefit of excellent detection performances of the forward part of the apparatus. Results: Light charged particle productions, multiplicities, and abundance ratios dependence against impact parameter are studied. It is measured to almost identical mean characteristics for the two Xe124+Sn124 and Xe136+Sn112 systems for central collisions. Comparing all four studied systems it is shown that mean values evolve from projectile N/Z to projectile+target N/Z dependence. Those identical mean characteristics concern all light charged particles except He3 whose mean behavior is strongly different. Conclusions: Our inclusive analysis (no event selection) shows that N/Z equilibration between the projectile-like and the target-like is realized to a high degree for central collisions. The light charged particle production mean value difference between Xe124+Sn124 and Xe136+Sn112 systems for central collisions is of the order of a few %. This slight difference could be explained by pre-equilibrium particle emission whose intensity may differ for the two reactions. This point is demonstrated using He3 mean characteristics whose production takes place before chemical equilibrium attainment. The realized N/Z balance between projectile-like and target-like does not imply a pure two-body mechanism. Indeed a midrapidity production of light charged particle does exist and its N/Z is different as compared to the projectile-like one: it is n enriched. This point is touched using He6 midrapidity production which is favored by the drift phenomenon.

Background: We study heavy-ion reactions from barrier up to Fermi energy. The data were acquired with the INDRA detector at the GANIL (Caen, France) facility.Purpose: We aim to determine the reaction and fusion cross sections for the reactions induced by $^{129}\mathrm{Xe}$ projectiles on $^{\mathrm{nat}}\mathrm{Sn}$ targets for incident energies ranging from $8A$ to $35A$ MeV. In particular, the evaluation of the fusion and incomplete fusion cross sections is the main purpose, altogether with the comparison with the systematics of Eudes et al. [Europhys. Lett. 104, 22001 (2013)].Method: The reaction cross sections are evaluated at each beam energy with data acquired thanks to the INDRA $4\ensuremath{\pi}$ array. The events are sorted with the help of the observable ${E}_{\text{iso,max}}$. We focus therefore our study on a selected sample of events, in such a way that the fusion and incomplete fusion cross sections can be estimated.Results: We present the excitation function of reaction and fusion cross sections for the heavy and nearly symmetric system $^{129}\mathrm{Xe}+^{\mathrm{nat}}\mathrm{Sn}$ from $8A$ to $35A$ MeV. For the fusion excitation function the comparison with the systematics of Eudes et al. seems to be in a fair agreement starting from the beam energy $20A$ MeV. For the lower beam energies ($8A$ and $12A$ MeV) discrepancies are observed.Conclusions: The evaluated fusionlike cross sections show a good agrement with a recent systematics for beam energies greater than $20A$ MeV. For low beam energies the cross-sectional values are lower than the expected ones. A probable reason for these low values is in the fusion hindrance at energies above or close to the barrier.

Background: In heavy-ion collisions at intermediate energies the production of light charged particles and intermediate mass fragments (IMFs) is due to different reaction mechanisms and different time scales, ranging from fast dynamical processes to statistical emission from the equilibrated system.Purpose: We compare the IMF statistical and dynamical emission probabilities in collisions of a neutron-rich $^{124}\mathrm{Sn}+^{64}\mathrm{Ni}$ system and a neutron-poor $^{112}\mathrm{Sn}+^{58}\mathrm{Ni}$ system at the laboratory energy of 35 $A$ MeV.Method: The IMFs production mechanism in semiperipheral reactions has been investigated in our previous works. In this paper, the analysis is expanded for the same set of data and production cross sections have been evaluated for dynamical and statistical emission in a coherent way for light and heavy fragments.Results: The data analysis has evidenced a strong competition between dynamical and statistical emission mechanisms. Probability of the dynamical emission of IMFs is strongly influenced by the ($N/Z$) ratio of the colliding system.Conclusions: It is demonstrated that the statistical emission is equally probable for the two systems, while the dynamical emission is enhanced for the neutron-rich system, especially for heavy fragments ($Z\ensuremath{\ge}6$). The observed effect points at a high sensitivity of the prompt-dynamical emission to the ($N/Z$) ratio of a given system.

Isotopic ratios for light fragments ($Z\ensuremath{\le}4$) emitted by the quasiprojectile (QP) and the mid-rapidity (MR) sources are investigated by the use of a slightly asymmetric system ($^{36}\mathrm{Ar}+^{58}\mathrm{Ni}$) and a symmetric one ($^{58}\mathrm{Ni}+^{58}\mathrm{Ni}$) for six energies between 32A and 84A MeV and three semiperipheral centrality range selections. Experimental data come from the INDRA $4\ensuremath{\pi}$ multidetector. The results show a clear neutron-rich isotope production from the MR region as compared to the QP source. The beam energy and the centrality also show interesting different trends depending on the charge of the fragments and the emission source. Experimental results are compared to antisymmetrized molecular dynamics simulations.

The odd-even staggering of the yield of final reaction products has been studied as a function of proton (Z) and neutron (N ) numbers for the collisions 84 Kr + 112 Sn and 84 Kr + 124 Sn at 35 MeV/nucleon in a wide range of elements (up to Z ≈ 20). The experimental data show that staggering effects rapidly decrease with increasing size of the fragments. Moreover the staggering in N is definitely larger than the one in Z. Similar general features are qualitatively reproduced by the GEMINI code. Concerning the comparison of the two systems, the staggering in N is in general rather similar, being slightly larger only for the lightest fragments produced in the n-rich system. In contrast the staggering in Z, although smaller than that in N , is sizably larger for the n-poor system with respect to the n-rich one.

Isospin transport phenomena in dissipative heavy ion collisions have been in- vestigated at Fermi energies with a beam of 84 Kr at 35AMeV. A comparison of theh Ni /Z of light and medium products forward-emitted in the centre of mass frame when the beam impinges on two different targets, the n-poor 112 Sn and the n-rich 124 Sn, is presented. Data were collected by means of a three-layer telescope with very good performances in terms of mass identification (full isotopic resolution up to Z�20 for ions punching through the first detector layer) built by the FAZIA Collaboration and located just beyond the grazing angle for both reactions. Theh Ni /Z of the products detected when the n-rich target is used is always higher than that associated to the n-poor one; since the detector was able to measure only fragments coming from the QuasiProjectile decay and/or neck emission, the observed behaviour can be ascribed to the isospin diffusion process, driven by the isospin gradient between QuasiProjectile and QuasiTarget. Moreover, for light fragments theh Ni /Z as a function of the lab velocity of the fragment is observed to increase when we move from the QuasiProjectile velocity to the centre of mass (neck zone). This effect can be interpreted as an evidence of isospin drift driven by the density gradient between the QuasiProjectile zone (at normal density) and the more diluted neck zone.

Sideward flow values have been determined with the INDRA multidetector for Ar+Ni, Ni+Ni and Xe+Sn systems studied at GANIL in the 30 to 100 A.MeV incident energy range. The balance energies found for Ar+Ni and Ni+Ni systems are in agreement with previous experimental results and theoretical calculations. Negative sideward flow values have been measured. The possible origins of such negative values are discussed. They could result from a more important contribution of evaporated particles with respect to the contribution of promptly emitted particles at mid-rapidity. But effects induced by the methods used to reconstruct the reaction plane cannot be totally excluded. Complete tests of these methods are presented and the origins of the ``auto-correlation'' effect have been traced back. For heavy fragments, the observed negative flow values seem to be mainly due to the reaction plane reconstruction methods. For light charged particles, these negative values could result from the dynamics of the collisions and from the reaction plane reconstruction methods as well. These effects have to be taken into account when comparisons with theoretical calculations are done., Comment: 27 pages, 15 figures

We present a new experimental method to correlate the isotopic composition of intermediate mass fragments (IMF) emitted at midrapidity in semiperipheral collisions with the emission timescale: IMFs emitted in the early stage of the reaction show larger values of $\ensuremath{\langle}N/Z\ensuremath{\rangle}$ isospin asymmetry, stronger angular anisotropies, and reduced odd-even staggering effects in neutron to proton ratio $\ensuremath{\langle}N/Z\ensuremath{\rangle}$ distributions than those produced in sequential statistical emission. All these effects support the concept of isospin ``migration'', that is sensitive to the density gradient between participant and quasispectator nuclear matter, in the so called neck fragmentation mechanism. By comparing the data to a stochastic mean field (SMF) simulation we show that this method gives valuable constraints on the symmetry energy term of nuclear equation of state at subsaturation densities. An indication emerges for a linear density dependence of the symmetry energy.

Decay modes of excited nuclei are investigated in {sup 78,82}Kr+{sup 40}Ca reactions at 5.5 MeV/nucleon. Charged products were measured by means of the 4{pi} INDRA array. Kinetic-energy spectra and angular distributions of fragments with atomic number 3 {le} Z {le} 28 indicate a high degree of relaxation and are compatible with a fissionlike phenomenon. Persistence of structure effects is evidenced from elemental cross sections ({sigma}{sub Z}) as well as a strong odd-even staggering (o-e-s) of the light-fragment yields. The magnitude of the staggering does not significantly depend on the neutron content of the emitting system. Fragment-particle coincidences suggest that the light partners in very asymmetric fission are emitted either cold or at excitation energies below the particle emission thresholds. The evaporation residue cross section of the {sup 78}Kr+{sup 40}Ca reaction is slightly higher than the one measured in the {sup 82}Kr+{sup 40}Ca reaction. The fissionlike component is larger by {approx}25% for the reaction having the lowest neutron-to-proton ratio. These experimental features are confronted to the predictions of theoretical models. The Hauser-Feshbach approach including the emission of fragments up to Z = 14 in their ground states as well as excited states does not account for the main features of {sigma}{submore » Z}. For both reactions, the transition-state formalism reasonably reproduces the Z distribution of the fragments with charge 12 {le} Z {le} 28. However, this model strongly overestimates the light-fragment cross sections and does not explain the o-e-s of the yields for 6 {le} Z {le} 10. The shape of the whole Z distribution and the o-e-s of the light-fragment yields are satisfactorily reproduced within the dinuclear system framework which treats the competition among evaporation, fusion-fission, and quasifission processes. The model suggests that heavy fragments come mainly from quasifission while light fragments are predominantly populated by fusion. An underestimation of the cross sections for 16 {le} Z {le} 22 could signal a mechanism in addition to the capture process.« less

A quantitative comparison is made between the absolute cross sections associated with statistical and dynamical emission of heavy fragments in the {sup 124}Sn+{sup 64}Ni and {sup 112}Sn+{sup 58}Ni collisions experimentally investigated at 35A MeV beam energy using the multidetector CHIMERA. The result shows that the dynamical process is about twice as probable in the neutron-rich {sup 124}Sn+{sup 64}Ni system as in the {sup 112}Sn+{sup 58}Ni neutron-poor one. This unexpected and significant difference indicates that the reaction mechanism is strongly dependent on the entrance-channel isospin (N/Z) content.

Isospin diffusion in semi-peripheral collisions is probed as a function of the dissipated energy by studying two systems $^{58}Ni$+$^{58}Ni$ and $^{58}Ni$+$^{197}Au$, over the incident energy range 52-74\AM. A close examination of the multiplicities of light products in the forward part of phase space clearly shows an influence of the isospin of the target on the neutron richness of these products. A progressive isospin diffusion is observed when collisions become more central, in connection with the interaction time.

Signatures of isospin effects were investigated for neutron-rich ({sup 124}Sn+{sup 64}Ni) and neutron-poor ({sup 112}Sn+{sup 58}Ni) systems at 35 MeV/nucleon for noncentral collisions. The centrality dependence of these signatures was tested for several impact parameter estimators. Our main observations are (i) the yields of {sup 1}H and {sup 3}He particles in the neutron-poor system are strongly enhanced with respect to the neutron-rich system, and the yields of {sup 3}H, {sup 6}He, and {sup 7,8}Li are suppressed at all impact parameters, (ii) the yields of {sup 2}H, {sup 4}He, and {sup 6}Li particles are almost the same for both systems, (iii) the N/Z ratio of intermediate mass fragments is correlated with the neutron richness of the system and is weakly dependent on the centrality of the collision, and (iv) the neutron richness of the detected fragments increases strongly with decreasing rapidity in the range from that of the projectile-like fragment to the c.m. region. The gross features of experimental data are reproduced by quantum molecular dynamics model calculations. A comparison between model calculations and the data indicates that the fragments produced in the c.m. regions are weakly excited.

Multi-breakup processes for the {sup 124}Sn+{sup 64}Ni system at 35 MeV/nucleon have been studied with the forward part of the CHIMERA detector. An extensive comparison between experimental data corresponding to almost complete ternary events and constrained molecular dynamics (CoMD-II) calculations suggests different characteristic times in the selected processes. This is in agreement with previous studies of the same reaction already published concerning the prompt intermediate-mass-fragment emission. Stimulated by CoMD-II calculations, we investigate the existence of more complex dynamical multi-breakup processes occurring on the same time scale. A detailed study of the rotational dynamics leading to slower dynamical fission processes is also presented.

Some properties of fast, nonequilibrium splitting of projectiles in the $^{124}\mathrm{Sn}+^{64}\mathrm{Ni}$ reaction at 35A MeV were determined using the $4\ensuremath{\pi}$ CHIMERA detector system. In particular the charge distributions, in- and out-of-plane angular distributions, and relative velocities of projectilelike fragments were measured. The time scale of the process was estimated and it turned out that the process is sequential but much faster than the ordinary, equilibrated fission.

Simultaneous measurement of both neutrons and charged particles emitted in the reaction {sup 64}Zn + {sup 64}Zn at 45 MeV/nucleon allows comparison of the neutron to proton ratio at midrapidity with that at projectile rapidity. The evolution of N/Z in both rapidity regimes with increasing centrality is examined. For the completely reconstructed midrapidity material one finds that the neutron to proton ratio is above that of the overall {sup 64}Zn + {sup 64}Zn system. In contrast, the reconstructed ratio for the quasiprojectile is below that of the overall system. This difference provides the most complete evidence to date of neutron enrichment of midrapidity nuclear matter at the expense of the quasiprojectile.

Characteristics of the primary fragments produced in central collisions of ${}^{129}\mathrm{Xe}{+}^{\mathrm{nat}}\mathrm{Sn}$ from 32 to 50 A MeV have been obtained. By using the correlation technique for the relative velocity between light charged particles (LCP) and fragments, we were able to extract the multiplicities and average kinetic energy of secondary evaporated LCP. We then reconstructed the size and excitation energy of the primary fragments. For each bombarding energy a constant value of the excitation energy per nucleon over the whole range of fragment charge has been found. This value saturates at 3A MeV for beam energies 39A MeV and above. The corresponding secondary evaporated LCP represent less than 40% of all produced particles and decreases down to 23% for 50A MeV. The experimental characteristics of the primary fragments are compared to the predictions of statistical multifragmentation model (SMM) calculations. Reasonable agreement between the data and the calculation has been found for any given incident energy. However SMM fails to reproduce the trend of the excitation function of the primary fragment excitation energy and the amount of secondary evaporated LCP's.

The fragmentation of the quasi-projectile is studied with the INDRA multidetector for different colliding systems and incident energies in the Fermi energy range. Different experimental observations show that a large part of the fragmentation is not compatible with the statistical fragmentation of a fully equilibrated nucleus. The study of internal correlations is a powerful tool, especially to evidence entrance channel effects. These effects have to be included in the theoretical descriptions of nuclear multifragmentation., 13 pages, 26 figures, submitted to Physical Review C

Invariant transverse-velocity spectra of intermediate-mass fragments were measured with the 4-pi multi-detector system INDRA for collisions of Au on Au at incident energies between 40 and 150 MeV per nucleon. Their scaling properties as a function of incident energy and atomic number Z are used to distinguish and characterize the emissions in (i) peripheral collisions at the projectile and target rapidities, and in (ii) central and (iii) peripheral collisions near mid-rapidity. The importance of dynamical effects is evident in all three cases and their origin is discussed.

Angular distributions of recoil nuclei were measured in coincidence with low-energy intermediate-mass fragments emitted in $^{14}\mathrm{induced}$ reactions on Ag at E/A=35 MeV. The measured correlations are inconsistent with a purely binary reaction mechanism. The incomplete linear momentum transfer mechanism alone does not reproduce the data, which show evidence for emission of additional matter to the same side of the beam axis as the intermediate-mass fragment.