Properties of projectile-fragments in the 40^{40}Ar + 27^{27}Al reaction at 44 A MeV. Comparison with a multisequential decay model

GANIL-EXPResults on projectile fragment–fragment coincidences in the forward direction and for the reaction 40Ar + 27Al at 44 A MeV are presented and compared with the predictions of two different entrance channel models, a two-body and a three-body mechanism both followed by a binary multisequential decay including fission. This analysis shows that many features of the projectile decay products are well accounted for by the binary multisequential decay model. However the results depend critically upon the initial masses and excitation energies of the primary projectile fragments. In this respect, the three-body approach underestimates the excitation energy imparted to the primary fragments whereas the two-body scenario overestimates it. The present data put strong constraints on the initial excitation energy imparted to the primary fragments which appears to be intermediate between the predictions of the two models

Correlation functions and emission time sequence of light charged particles from projectile-like fragment source in E/A = 44 and 77 MeV 40Ar + 27Al collisions

Two-particle correlation functions, involving protons, deuterons, tritons, and alpha-particles, have been measured at very forward angles (0.7 deg < theta_lab < 7 deg), in order to study projectile-like fragment (PLF) emission in E/A = 44 and 77 MeV 40Ar + 27Al collisions. Peaks, originating from resonance decays, are larger at E/A = 44 than at 77 MeV. This reflects the larger relative importance of independently emitted light particles, as compared to two-particle decay from unstable fragments, at the higher beam energy. The time sequence of the light charged particles, emitted from the PLF, has been deduced from particle-velocity-gated correlation functions (discarding the contribution from resonance decays). Alpha-particles are found to have an average emission time shorter than protons but longer than tritons and deuterons.Comment: 18 pages, 5 figures, submitted to Nuclear Physics

Multifragmentation process for different mass asymmetry in the entrance channel around the Fermi energy

The influence of the entrance channel asymmetry upon the fragmentation process is addressed by studying heavy-ion induced reactions around the Fermi energy. The data have been recorded with the INDRA 4pi array. An event selection method called the Principal Component Analysis is presented and discussed. It is applied for the selection of central events and furthermore to multifragmentation of single source events. The selected subsets of data are compared to the Statistical Multifragmentation Model (SMM) to check the equilibrium hypothesis and get the source characteristics. Experimental comparisons show the evidence of a decoupling between thermal and compresional (radial flow) degrees of freedom in such nuclear systems.Comment: 28 pages, 15 figures, article sumitted to Nuclear Physics

Multiplicity correlations of intermediate-mass fragments with pions and fast protons in 12C + 197Au

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.Comment: 9 pages, 11 figures, subm. to EPJ

Multifragmentation in Xe(50A MeV)+Sn Confrontation of theory and data

We compare in detail central collisions Xe(50A MeV) + Sn, recently measured by the INDRA collaboration, with the Quantum Molecular Dynamics (QMD) model in order to identify the reaction mechanism which leads to multifragmentation. We find that QMD describes the data quite well, in the projectile/target region as well as in the midrapidity zone where also statistical models can be and have been employed. The agreement between QMD and data allows to use this dynamical model to investigate the reaction in detail. We arrive at the following observations: a) the in medium nucleon nucleon cross section is not significantly different from the free cross section, b) even the most central collisions have a binary character, c) most of the fragments are produced in the central collisions and d) the simulations as well as the data show a strong attractive in-plane flow resembling deep inelastic collisions e) at midrapidity the results from QMD and those from statistical model calculations agree for almost all observables with the exception of ${d^2 \sigma \over dZdE}$. This renders it difficult to extract the reaction mechanism from midrapidity fragments only. According to the simulations the reaction shows a very early formation of fragments, even in central collisions, which pass through the reaction zone without being destroyed. The final transverse momentum of the fragments is very close to the initial one and due to the Fermi motion. A heating up of the systems is not observed and hence a thermal origin of the spectra cannot be confirmed.Comment: figures 1 and 2 changed (no more ps -errors

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

Response of CsI(Tl) scintillators over a large range in energy and atomic number of ions (Part I): recombination and delta -- electrons

A simple formalism describing the light response of CsI(Tl) to heavy ions, which quantifies the luminescence and the quenching in terms of the competition between radiative transitions following the carrier trapping at the Tl activator sites and the electron-hole recombination, is proposed. The effect of the delta rays on the scintillation efficiency is for the first time quantitatively included in a fully consistent way. The light output expression depends on four parameters determined by a procedure of global fit to experimental data.Comment: 28 pages, 6 figures, submitted to Nucl. Inst. Meth.

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.Comment: 5 pages, 5 eps figures, RevTeX4, submitted to Phys. Lett.

Fission time measurements: a new probe into super-heavy element stability

Accepted for publication in Physical Review LettersReaction mechanism analyses performed with a 4 $\pi$ detector for the systems $^{208}$Pb+Ge, $^{238}U+Ni and$^{238}$U+Ge, combined with analyses of the associated reaction time distributions, provide us with evidence for nuclei with Z=120 and 124 living longer than$10^−18$s and arising from highly excited compound nuclei. By contrast, the neutron deficient nuclei with Z=114 possibly formed in$^{208}$Pb+Ge reactions have shorter lifetimes, close to or below the sensitivity limit of the experiment

Study of intermediate velocity products in the Ar+Ni collisions between 52 and 95 A.MeV

Intermediate velocity products in Ar+Ni collisions from 52 to 95 A.MeV are studied in an experiment performed at the GANIL facility with the 4$\pi$ multidetector INDRA. It is shown that these emissions cannot be explained by statistical decays of the quasi-projectile and the quasi-target in complete equilibrium. Three methods are used to isolate and characterize intermediate velocity products. The total mass of these products increases with the violence of the collision and reaches a large fraction of the system mass in mid-central collisions. This mass is found independent of the incident energy, but strongly dependent on the geometry of the collision. Finally it is shown that the kinematical characteristics of intermediate velocity products are weakly dependent on the experimental impact parameter, but strongly dependent on the incident energy. The observed trends are consistent with a participant-spectator like scenario or with neck emissions and/or break-up.Comment: 37 pages, 13 figure