Publications (16)16.72 Total impact
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Article: Bimodality - a general feature of heavy ion reactions
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ABSTRACT: Recently, is has been observed that events with the {\it same} total transverse energy of light charged particles (LCP) in the quasi target region, $E_{\perp 12}^{QT}$, show two quite distinct reaction scenarios in the projectile domain: multifragmentation and residue production. This phenomenon has been dubbed "bimodality". Using Quantum Molecular Dynamics calculations we demonstrate that this observation is very general. It appears in collisions of all symmetric systems larger than Ca and at beam energies between 50 A.MeV and 600 A.MeV and is due to large fluctuations of the impact parameter for a given $E_{\perp 12}^{QT}$. Investigating in detail the $E_{\perp 12}^{QT}$ bin in which both scenarios are present, we find that neither the average fragment momenta nor the average transverse and longitudinal energies of fragments show the behavior expected from a system in statistical equilibrium, in experiment as well as in QMD simulations. On the contrary, the experimental as well as the theoretical results point towards a fast process. This observation questions the conjecture that the observed bimodality is due to the coexistence of 2 phases at a given temperature in finite systems. Comment: accepted PRC09/2009; -
Article: Source shape determination with directional fragment-fragment velocity correlations
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ABSTRACT: Correlation functions, constructed from directional projections of the relative velocities of fragments, are used to determine the shape of the breakup volume in coordinate space. For central collisions of 129Xe + natSn at 50 MeV per nucleon incident energy, measured with the 4pi multi-detector INDRA at GSI, a prolate shape aligned along the beam direction with an axis ratio of 1:0.7 is deduced. The sensitivity of the method is discussed in comparison with conventional fragment-fragment velocity correlations.Physics Letters B 01/2008; 659(4):807. · 3.95 Impact Factor -
Article: Yield scaling, size hierarchy and fluctuations of observables in fragmentation of excited heavy nuclei
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ABSTRACT: Multifragmentation properties measured with INDRA are studied for single sources produced in Xe+Sn reactions in the incident energy range 32-50 A MeV and quasiprojectiles from Au+Au collisions at 80 A MeV. A comparison for both types of sources is presented concerning Fisher scaling, Zipf law, fragment size and fluctuation observables. A Fisher scaling is observed for all the data. The pseudo-critical energies extracted from the Fisher scaling are consistent between Xe+Sn central collisions and Au quasi-projectiles. In the latter case it also corresponds to the energy region at which fluctuations are maximal. The critical energies deduced from the Zipf analysis are higher than those from the Fisher analysis.10/2007; -
Article: Model-independent tracking of criticality signals in nuclear multifragmentation data
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ABSTRACT: We look for signals of criticality in multifragment production in heavy-ion collisions using model-independent universal fluctuations theory. The phenomenon is studied as a function of system size, bombarding energy, and impact parameter in a wide range of INDRA data. For very central collisions (b/b_max<0.1) we find evidence that the largest fragment in each event, Z_max, plays the role of an order parameter, defining two different regimes at low and high incident energy, respectively, according to the scaling properties of its fluctuations. Data for a wide range of system masses and incident energies collapse on to an approximately universal scaling function in each regime for the most central collisions. The forms of the scaling functions for the two regimes are established, and their dependence on the total mass and the bombarding energy is mapped out. Data suggest that these regimes are linked to the disappearance of heavy residues in central collisions.Physical Review C 03/2005; 71(3):034607. · 3.31 Impact Factor -
Article: Directed and elliptic flow in Au + Au at intermediate energies
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ABSTRACT: Directed and elliptic flow for the Au + Au system at incident energies between 40 and 150 MeV per nucleon has been measured using the INDRA 4 pi multi-detector. For semi-central collisions, the elliptic flow of Z <= 2 particles switches from in-plane to out-of-plane enhancement at around 100 MeV per nucleon, in good agreement with the result reported by the FOPI Collaboration. The directed flow changes sign at a bombarding energy between 50 and 60 MeV per nucleon and remains negative at lower energies. The conditions for the appearance and possible origins of negative flow are discussed.11/2004; -
Article: Multiplicity correlations of intermediate-mass fragments with pions and fast protons in C-12+Au-197
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ABSTRACT: Low-energy π+ ( Eπ≲35 MeV) from 12 C + 197 Au collisions at incident energies from 300 to 1800 MeV per nucleon were detected with the Si-Si(Li)-CsI(Tl) calibration telescopes of the INDRA multidetector. The inclusive angular distributions are approximately isotropic, consistent with multiple rescattering in the target spectator. The multiplicity correlations of the low-energy pions and of energetic protons ( Ep≳150 MeV) with intermediate-mass fragments were determined from the measured coincidence data. The deduced correlation functions 1+R≈1.3 for inclusive event samples reflect the strong correlations evident from the common impact parameter dependence of the considered multiplicities. For narrow impact parameter bins (based on charged-particle multiplicity), the correlation functions are close to unity and do not indicate strong additional correlations. Only for pions at high particle multiplicities (central collisions) a weak anticorrelation is observed, probably due to a limited competition between these emissions. Overall, the results are consistent with the equilibrium assumption made in statistical multifragmentation scenarios. Predictions obtained with intranuclear-cascade models coupled to the Statistical Multifragmentation Model are in good agreement with the experimental data.European Physical Journal A 08/2004; 21(2):293. · 2.19 Impact Factor -
Article: Model-independent tracking of criticality signals in nuclear multifragmentation data
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ABSTRACT: We look for signals of criticality in multifragment production in heavy-ion collisions using model-independent universal fluctuations theory. The phenomenon is studied as a function of system size, bombarding energy, and impact parameter in a wide range of INDRA data. For very central collisions (b/b_max05/2004; -
Article: Multiplicity correlations of intermediate-mass fragments with pions and fast protons in 12C + 197Au
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ABSTRACT: Low-energy pi+ (E < 35 MeV) from 12C+197Au collisions at incident energies from 300 to 1800 MeV per nucleon were detected with the Si-Si(Li)-CsI(Tl) calibration telescopes of the INDRA multidetector. The inclusive angular distributions are approximately isotropic, consistent with multiple rescattering in the target spectator. The multiplicity correlations of the low-energy pions and of energetic protons (E > 150 MeV) with intermediate-mass fragments were determined from the measured coincidence data. The deduced correlation functions 1 + R \approx 1.3 for inclusive event samples reflect the strong correlations evident from the common impact-parameter dependence of the considered multiplicities. For narrow impact-parameter bins (based on charged-particle multiplicity), the correlation functions are close to unity and do not indicate strong additional correlations. Only for pions at high particle multiplicities (central collisions) a weak anticorrelation is observed, probably due to a limited competition between these emissions. Overall, the results are consistent with the equilibrium assumption made in statistical multifragmentation scenarios. Predictions obtained with intranuclear cascade models coupled to the Statistical Multifragmentation Model are in good agreement with the experimental data.03/2004; -
Article: Fragmentation in Peripheral Heavy-Ion Collisions: from Neck Emission toSpectator Decays
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ABSTRACT: Invariant cross sections of intermediate mass fragments in peripheral collisions of Au on Au at incident energies between 40 and 150 AMeV have been measured with the 4-pi multi-detector INDRA. The maximum of the fragment production is located near mid-rapidity at the lower energies and moves gradually towards the projectile and target rapidities as the energy is increased. Schematic calculations within an extended Goldhaber model suggest that the observed cross-section distributions and their evolution with energy are predominantly the result of the clustering requirement for the emerging fragments and of their Coulomb repulsion from the projectile and target residues. The quantitative comparison with transverse energy spectra and fragment charge distributions emphasizes the role of hard scattered nucleons in the fragmentation process.Physics Letters B 01/2003; 566(1-2):76. · 3.95 Impact Factor -
Article: Transverse Velocity Scaling in Au+Au Fragmentation
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ABSTRACT: 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.Physical Review C 08/2002; 66(6):064606. · 3.31 Impact Factor -
Article: Transverse velocity scaling in Au plus Au midrapidity emissions
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ABSTRACT: The emission of intermediate-mass fragments in collisions of Au-197 on Au-197 was systematically studied over the range of incident energies from 40 to 150 A MeV using the 4pi-multidetector INDRA and beams from the heavy-ion synchrotron SIS at GSI Darmstadt. The analysis was performed as a function of incident energy and of impact parameter, defined through the total transverse energy of light charged particles (Etrans12, Zless than or equal to2). This observable was found to scale linearly with the collision energy up to a certain limit above which it saturates. Strong forward-backward asymmetries in the emission pattern with respect to the rapidities of the projectile and target residua were observed. Their possible relation to equilibration is discussed and interpreted in the framework of molecular dynamics and statistical multifragmentation models. Transverse velocities at midrapidity for peripheral collisions were found to show a surprisingly weak dependence on the incident energy. On the contrary, transverse velocities at midrapidity for central collisions showed a strong dependence on incident energy indicating an increasing importance of collective flow with increasing bombarding energy.AIP Conference Proceedings 01/2002; 610:711. -
Article: Nuclear multifragmentation and the onset of radial flow: A study of Au plus Au collisions between 40 and 100 MeV/A
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ABSTRACT: The influence of radial flow on nuclear multifragmentation is studied using Au+Au central collisions for incident energies between 40 and 100 MeV/A. Central collisions are selected via an impact parameter estimator defined by the transverse energy of light charged particles. At all energies, the fragment size distributions are studied using the Statistical Multifragmentation Model (SMM) in order to extract thermal excitation energies. The excess kinetic energies are analyzed via the hypothesis of a radial, self-similar, but non-isotropic flow. At the lowest energy, this flow is small with respect to the thermal energy whereas, at 100MeV/A thermal and radial energies are of the same order. The nature and origin of the radial flow is discussed. A comparison with the predictions of the Quantum Molecular Dynamics code is presented.AIP Conference Proceedings 01/2002; 610:716. -
Article: Excitation energy and angular momentum of quasiprojectiles produced in the Xe + Sn collisions at incident energies between 25 and 50 MeV/nucleon
Nuclear Physics A. 01/2001; 686:537-567. -
Article: A study of nuclear stopping in central symmetric nuclear collisions at intermediate energies
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ABSTRACT: Nuclear stopping has been investigated in central symmetric nuclear collisions at intermediate energies. Firstly, it is found that the isotropy ratio, Riso, reaches a minimum near the Fermi energy and saturates or slowly increases depending on the mass of the system as the beam energy increases. An approximate scaling based on the size of the system is found above the Fermi energy suggesting the increasing role of in-medium nucleon-nucleon collisions. Secondly, the charge density distributions in velocity space, dZ/dvk and dZ/dv?, reveal a strong memory of the entrance channel and, as such, a sizeable nuclear transparency in the intermediate energy range. Lastly, it is shown that the width of the transverse velocity distribution is proportional to the beam velocity. -
Article: Intranuclear cascade+percolation+evaporation model applied to the $^{12}$C+$^{197}$Au system at 1 GeV/nucleon
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Article: Can we really measure the internal energy of hot nuclei with a 4π detection array?
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ABSTRACT: The second generation of high-quality detection arrays gave hope to nuclear physicists to finally obtain an experimental equation of state of nuclear matter. In spite of this progress, the measurement of the internal energy of a hot nucleus remains a very difficult task. This paper illustrates this difficulty by a methodological study of a classical technique of excitation energy measurement used in the Fermi energy range. The aim of this study is to verify the validity, the accuracy and the experimental limits of these measurements. It is shown that it is difficult to have a real experimental mastery of the source reconstruction and calorimetry at least for limited bombarding energies and violent collisions.Nuclear Physics A.
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Institutions
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2009
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GSI Helmholtzzentrum für Schwerionenforschung
Jena, Thuringia, Germany
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