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R. Legrain

Publications and source records attributed to R. Legrain.

17 recordsLinked to original sources
arXiv2007

Source shape determination with directional fragment-fragment velocity correlations

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.

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arXiv2007

Transition from participant to spectator fragmentation in Au+Au reaction between 60 AMeV and 150 AMeV

Using the quantum molecular dynamics approach, we analyze the results of the recent INDRA Au+Au experiments at GSI in the energy range between 60 AMeV and 150 AMeV. It turns out that in this energy region the transition toward a participant-spectator scenario takes place. The large Au+Au system displays in the simulations as in the experiment simultaneously dynamical and statistical behavior which we analyze in detail: The composition of fragments close to midrapidity follows statistical laws and the system shows bi-modality, i.e. a sudden transition between different fragmentation pattern as a function of the centrality as expected for a phase transition. The fragment spectra at small and large rapidities, on the other hand, are determined by dynamics and the system as a whole does not come to equilibrium, an observation which is confirmed by FOPI experiments for the same system.

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arXiv2005

Evaporation residues produced in spallation of 208Pb by protons at 500A MeV

The production cross sections of fragmentation-evaporation residues in the reaction Pb+p at 500A MeV have been measured using the inverse-kinematics method and the FRS spectrometer (GSI). Fragments were identified in nuclear charge using ionisation chambers. The mass identification was performed event-by-event using the B-rho - TOF - Delta-E technique. Although partially-unresolved ionic charge states induced an ambiguity on the mass of some heavy fragments, production rates could be obtained with a high accuracy by systematically accounting for the polluting ionic charge states. The contribution of multiple reactions in the target was subtracted using a new, partly self-consistent code. The isobaric distributions are found to have a shape very close to the one observed in experiments at higher energy. Kinematic properties of the fragments were also measured. The total and the isotopic cross sections, including charge-pickup cross sections, are in good agreement with previous measurements. The data are discussed in the light of previous spallation measurements, especially on lead at 1 GeV.

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arXiv2005

Light Nuclides Produced in the Proton-Induced Spallation of 238U at 1 GeV

The production of light and intermediate-mass nuclides formed in the reaction 1H+238U at 1 GeV was measured at the Fragment Separator (FRS) at GSI, Darmstadt. The experiment was performed in inverse kinematics, shooting a 1 A GeV 238U beam on a thin liquid-hydrogen target. 254 isotopes of all elements in the range from Z=7 to Z=37 were unambiguously identified, and the velocity distributions of the produced nuclides were determined with high precision. The results show that the nuclides are produced in a very asymmetric binary decay of heavy nuclei originating from the spallation of uranium. All the features of the produced nuclides merge with the characteristics of the fission products as their mass increases.

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arXiv2004

Directed and elliptic flow in Au + Au at intermediate energies

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.

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arXiv2004

Isotopic Scaling and the Symmetry Energy in Spectator Fragmentation

Isotopic effects in the fragmentation of excited target residues following collisions of $^{12}$C on $^{112,124}$Sn at incident energies of 300 and 600 MeV per nucleon were studied with the INDRA 4$\pi$ detector. The measured yield ratios for light particles and fragments with atomic number $Z \leq$ 5 obey the exponential law of isotopic scaling. The deduced scaling parameters decrease strongly with increasing centrality to values smaller than 50% of those obtained for the peripheral event groups. Symmetry term coefficients, deduced from these data within the statistical description of isotopic scaling, are near $\gamma =$ 25 MeV for peripheral and $\gamma <$ 15 MeV for central collisions.

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arXiv2004

Isotopic and velocity distributions of Bi produced in charge-pickup reactions of 208Pb at 1 A GeV

Isotopically resolved cross sections and velocity distributions have been measured in charge-pickup reactions of 1 A GeV 208Pb with proton, deuterium and titanium target. The total and partial charge-pickup cross sections in the reactions 208Pb + 1H and 208Pb + 2H are measured to be the same in the limits of the error bars. A weak increase in the total charge-pickup cross section is seen in the reaction of 208Pb with the titanium target. The measured velocity distributions show different contributions - quasi-elastic scattering and Delta-resonance excitation - to the charge-pickup production. Data on total and partial charge-pickup cross sections from these three reactions are compared with other existing data and also with model calculations based on the coupling of different intra-nuclear cascade codes and an evaporation code.

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arXiv2003

Fission-Residues Produced in the Spallation Reaction 238U+p at 1 A GeV

Fission fragments from 1 A GeV 238U projectiles irradiating a hydrogen target were investigated by using the fragment separator FRS for magnetic selection of reaction products including ray-tracing and DE-ToF techniques. The momentum spectra of 733 identified fragments were analysed to provide isotopic production cross sections, fission-fragment velocities and recoil momenta of the fissioning parent nuclei. Besides their general relevance, these quantities are also demanded for applications. Calculations and simulations with codes commonly used and recently developed or improved are compared to the data.

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arXiv2003

Measurement of nuclide cross-sections of spallation residues in 1 A GeV 238U + proton collisions

The production of heavy nuclides from the spallation-evaporation reaction of 238U induced by 1 GeV protons was studied in inverse kinematics. The evaporation residues from tungsten to uranium were identified in-flight in mass and atomic number. Their production cross-sections and their momentum distributions were determined. The data are compared with empirical systematics. A comparison with previous results from the spallation of 208Pb and 197Au reveals the strong influence of fission in the spallation of 238U.

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arXiv2003

Fragmentation in Peripheral Heavy-Ion Collisions: from Neck Emission to Spectator Decays

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.

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arXiv2002

Transverse Velocity Scaling in Au+Au Fragmentation

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.

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arXiv2002

Spallation Neutron Production by 0.8, 1.2 and 1.6 GeV Protons on various Targets

Spallation neutron production in proton induced reactions on Al, Fe, Zr, W, Pb and Th targets at 1.2 GeV and on Fe and Pb at 0.8, and 1.6 GeV measured at the SATURNE accelerator in Saclay is reported. The experimental double-differential cross-sections are compared with calculations performed with different intra-nuclear cascade models implemented in high energy transport codes. The broad angular coverage also allowed the determination of average neutron multiplicities above 2 MeV. Deficiencies in some of the models commonly used for applications are pointed out.

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arXiv2000

Effect of the intermediate velocity emissions on the quasi-projectile properties for the Ar+Ni system at 95 A.MeV

The quasi-projectile (QP) properties are investigated in the Ar+Ni collisions at 95 A.MeV taking into account the intermediate velocity emission. Indeed, in this reaction, between 52 and 95 A.MeV bombarding energies, the number of particles emitted in the intermediate velocity region is related to the overlap volume between projectile and target. Mean transverse energies of these particles are found particularly high. In this context, the mass of the QP decreases linearly with the impact parameter from peripheral to central collisions whereas its excitation energy increases up to 8 A.MeV. These results are compared to previous analyses assuming a pure binary scenario.

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arXiv2000

Cross-sections of spallation residues produced in 1.A GeV 208Pb on proton reactions

Spallation residues produced in 1 GeV per nucleon $^{208}$Pb on proton reactions have been studied using the FRagment Separator facility at GSI. Isotopic produc- tion cross-sections of elements from $_{61}$Pm to $_{82}$Pb have been measured down to 0.1 mb with a high accuracy. The recoil kinetic energies of the produced fragments were also determined. The obtained cross-sections agree with most of the few existing gamma-spectroscopy data. Data are compared with different intra nuclear-cascade and evaporation-fission models. Drastic deviations were found for a standard code used in technical applications.

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arXiv1996

Statistical Binary Decay of $^{35}$Cl + $^{24}$Mg at 8 MeV/nucleon

The properties of the two-body channels in the $^{35}$Cl + $^{24}$Mg reaction at a bombarding energy of 275 MeV have been investigated by using fragment-fragment coincident techniques. The exclusive data show that the majority of events arises from a binary-decay process. The rather large number of secondary light charged-particles emitted from the two excited exit fragments are cnsistent with the expectations of the Extended Hauser-Feshbach Method. No evidence for the occurence of ternary break-up events is observed.

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arXiv1996

$^{35}$Cl+$^{12}$C Asymmetrical fission excitation functions

The fully energy-damped yields from the $^{35}$Cl+$^{12}$C reaction have been systematically investigated using particle-particle coincidence techniques at a $^{35}$Cl bombarding energy of $\sim$ 8 MeV/nucleon. The fragment-fragment correlation data show that the majority of events arises from a binary-decay process with rather large numbers of secondary light-charged particles emitted from the two excited exit fragments. No evidence is observed for ternary break-up events. The binary-process results of the present measurement, along with those of earlier, inclusive experimental data obtained at several lower bombarding energies are compared with predictions of two different kinds of statistical model calculations. These calculations are performed using the transition-state formalism and the Extended Hauser-Feshbach method and are based on the available phase space at the saddle point and scission point of the compound nucleus, respectively. The methods give comparable predictions and are both in good agreement with the experimental results thus confirming the fusion-fission origin of the fully-damped yields. The similarity of the predictions for the two models supports the claim that the scission point configuration is very close to that of the saddle point for the light $^{47}$V mass-asymmetry-dependent fission barriers needed in the transition-state calculation.

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