A. Chbihi

Surveying the nuclear caloric curve

Abstract The 4π array INDRA was used to detect nearly all charged products emitted in Ar + Ni collisions between 52 and 95 MeV/u. The charge, mass and excitation energy E ∗ of the quasi-projectiles have been reconstructed event by event. Excitation energies up to 25 MeV per nucleon are reached. Apparent temperatures obtained from several double isotopic yield ratios Tr0 show different dependences upon E ∗ . T6Li7Liue5f83Heα0 yields the highest values, as well as the high energy slopes Ts of the kinetic energy spectra. Two statistical models, sequential evaporation and gas in complete equilibrium, taking into account side feeding and discrete excited states population, show that the data can be explained by a steady increase of the initial temperature with excitation energy without evidence for a liquid-gas phase transition.

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

Abstract 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 δ-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.

Response of CsI(Tl) scintillators over a large range in energy and atomic number of ions (Part II): calibration and identification in the INDRA array

Abstract The light output of the 324 CsI(Tl) scintillators of INDRA has been measured over large ranges in energy: 1– 80 AMeV and in atomic number of incident ions: Z =1–60. An analytical expression for the non-linear total light response as a function of the energy and the identity of the ion is developed. It depends on four parameters. For three of them, connected to CsI(Tl) intrinsic characteristics, fixed values are proposed. Two applications are presented: energy calibration and fragment identification in telescopes using a CsI(Tl) crystal as residual energy detector.

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

Abstract The influence of the entrance channel mass 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 4 π 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 compressional (radial flow) component of the excitation energy stored in such nuclear systems.

Excitation energy and angular momentum of quasiprojectiles produced in the Xe+Sn collisions at incident energies between 25 and 50 MeV/nucleon

Abstract The excitation energy and angular momentum transferred to quasiprojectiles have been measured in the 129 Xe+ nat Sn collisions at bombarding energies between 25 and 50xa0MeV/nucleon. The excitation energy of quasiprojectiles has been determined from the kinetic energy of all decay products (calorimetry). It increases with the violence of the collision, approaching 10xa0MeV/nucleon in the most dissipative ones. The angular momentum has been deduced from the kinetic energies and angular distributions of the emitted light charged particles (p, d, t, 3 He and α ). The (apparent) spin value decreases with the violence of the collision. Larger spin values are observed at the lowest bombarding energy. Data are compared with the predictions of dynamical and statistical models. They reproduce the data in a quantitative way indicating that large spin values are transferred to quasiprojectiles during the interaction. The results show that the one-body dissipation formalism still applies at intermediate bombarding energies and low-energy dissipations. With the increase of the energy, the data seem to be better described when the two-body interaction is accounted for.

Fragmentation in peripheral heavy-ion collisions: from neck emission to spectator decays

Abstract Invariant cross sections of intermediate mass fragments in peripheral collisions of 197 Au on 197 Au at incident energies between 40 and 150xa0MeV per nucleon have been measured with the 4 π 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.

Estimate of average freeze-out volume in multifragmentation events

An estimate of the average freeze-out volume for multifragmentation events is presented. Values of volumes are obtained by means of a simulation using the experimental charged product partitions measured by the 4pi multidetector INDRA for 129Xe central collisions on Sn at 32 AMeV incident energy. The input parameters of the simulation are tuned by means of the comparison between the experimental and simulated velocity (or energy) spectra of particles and fragments.

Can we really measure the internal energy of hot nuclei with a 4π detection array

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.

IS REDUCIBILITY IN NUCLEAR MULTIFRAGMENTATION RELATED TO THERMAL SCALING

Abstract Thermal scaling (Arrhenius law for an “elementary” probability p of binomial function) and reducibility in intermediate mass fragments (IMFs) production are examined for data of the reaction 129Xe+ natSn at 50 MeV/u. The study of the longitudinal velocities and of the average transverse energies of the IMFs contradicts the assumption that the total transverse energy of all detected particles Et is related to a well defined temperature. The separation of Et into the total transverse energy of light charged particles (Z=1,2) and that of IMFs elucidates the algorithm which induces a linear behavior of log(1/p) versus 1/ E t . Even in the case of a single thermalized source, calculations based on a sequential statistical model show that the Arrhenius law cannot be observed if Et is taken as an estimation of the thermal energy.