P. Pawlowski

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.

New isospin effects in central heavy-ion collisions at Fermi energies

Isospin e ffects on multifragmentation properties were studied thanks to nuclear collisions between di fferent isotopes of xenon beams and tin targets. It is shown that, in central collisions leading to multifragmentation, the mean number of fragments and their mean kinetic energy increase with the neutron-richness of the total system. Comparisons with a stochastic transport model allow to attribute the multiplicity increase to the multifragmentation stage, before secondary decay. The total charge bound in fragments is proposed as an alternate variable to quantify preequilibrium emission and to investigate symmetry energy e ffects.

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.

Isospin effects and symmetry energy studies with INDRA

Abstract.The equation of state of asymmetric nuclear matter is still controversial, as predictions at subsaturation as well as above normal density widely diverge. We discuss several experimental results measured in heavy-ion collisions with the INDRA array in the incident energy range 5-80A MeV. In particular an estimate of the density dependence of the symmetry energy is derived from isospin diffusion results compared with a transport code: the potential part of the symmetry energy linearly increases with the density. We demonstrate that isospin equilibrium is reached in mid-central collisions for the two reactions Ni+Au at 52A MeV and Xe + Sn at 32A MeV. New possible variables and an improved modelization to investigate symmetry energy are discussed.

Nuclear multifragmentation time-scale and fluctuations of largest fragment size

Distributions of the largest fragment charge, Zmax, in multifragmentation reactions around the Fermi energy can be decomposed into a sum of a Gaussian and a Gumbel distribution, whereas at much higher or lower energies one or the other distribution is asymptotically dominant. We demonstrate the same generic behavior for the largest cluster size in critical aggregation models for small systems, in or out of equilibrium, around the critical point. By analogy with the time-dependent irreversible aggregation model, we infer that Zmax distributions are characteristic of the multifragmentation time scale, which is largely determined by the onset of radial expansion in this energy range.

Constrained caloric curves and phase transition for hot nuclei

Simulations based on experimental data obtained from multifragmenting quasi-fused nuclei produced in central

Experimental evidence for spinodal decomposition in multifragmentation of heavy systems

^{129}

Improving isotopic identification with INDRA Silicon–CsI(Tl) telescopes

Xe +

Evidence for Spinodal Decomposition in Nuclear Multifragmentation

^{nat}

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

Sn collisions have been used to deduce event by event freeze-out properties in the thermal excitation energy range 4-12 AMeV [Nucl. Phys. A809 (2008) 111]. From these properties and the temperatures deduced from proton transverse momentum fluctuations, constrained caloric curves have been built. At constant average volumes caloric curves exhibit a monotonic behaviour whereas for constrained pressures a backbending is observed. Such results support the existence of a first order phase transition for hot nuclei.