Model independent reconstruction of impact parameter distributions for intermediate energy heavy ion collisions

14 pages, 7 figures. Revised version accepted for publication in Physical Review CInternational audienceWe present a model-independent method to reconstruct the impact parameter distributions of experimental data for intermediate energy heavy ion collisions, adapted from a recently proposed approach for ultra-relativistic heavy ion collisions. The method takes into account the fluctuations which are inherent to the relationship between any experimental observable and the impact parameter in this energy range. We apply the method to the very large dataset on heavy ion collisions in the energy range 20-100 MeV/nucleon obtained with the INDRA multidetector since 1993, for two observables which are the most commonly used for the estimation of impact parameters in this energy range. The mean impact parameters deduced with this new method for "central" collisions selected using typical observable cuts are shown to be significantly larger than those found when fluctuations are neglected, and as expected the difference increases as bombarding energy decreases. In addition, we will show that this new approach may provide previously inaccessible experimental constraints for transport models, such as an estimation of the extrapolated mean value of experimental observables for b = 0 collisions. The ability to give more realistic, model-independent, estimations of the impact parameters associated to different experimental datasets should improve the pertinence of comparisons with transport model calculations which are essential to better constrain the equation of state of nuclear matter

In-medium effects in central heavy ion collisions at intermediate energies

Energy dissipation and cluster production in central nuclear collisions in the Fermi energy range have been investigated using the large INDRA database. A study of the in-medium nucleon-nucleon cross section, σNN∗, has been performed by comparing the results of a model with the measured isotropy ratio of the kinetic energy distributions of the detected protons. This analysis has been done for a large variety of systems at various incident energies from 20 to 100 MeV/nucleon and confirms nicely previous results with enhanced statistics in a full Monte Carlo treatment. Most importantly, it is found that σNN∗ is drastically reduced as compared to the vacuum values, due partly to Pauli blocking but also to in-medium effects for which mean values and uncertainties are given. Concerning the cluster production, a coalescence model has been developed to study the parallel and perpendicular velocity distributions of the light clusters (A≤4). For AC=3 (H3, He3), it is found that the mean internal kinetic energy of the nucleons in the cluster is reduced as compared to the vacuum value. These two results shows that any comparison between experimental and theoretical predictions in heavy-ions induced reactions around and above the Fermi energy must take into account these in-medium effects

Reconstructing the impact parameter dependence of experimental observables from intermediate energy heavy-ion collision data

22 pages, 15 figures. Submitted to Physical Review CPrecise constraints on the equation of state (EoS) of dense matter can be obtained through comparison of data from heavy-ion collisions (HIC) with transport models employing different effective interactions. An essential input for such comparisons is a reliable estimate of the impact parameter distributions P(b) which are representative of the data. For HIC in the intermediate energy range (20-150 MeV/A), there was no way up to now to extract such distributions from data in a model-independent way and it is well known that the only existing method for experimental impact parameter estimation underestimates those of the most central collisions, but not by how much. We adopt a method first developed for ultra-relativistic HIC in which a monotonic relationship is assumed between the mean value of a given observable X and b, whose parameters are adjusted in order to reproduce the b-integrated inclusive distribution P(X), taking into account fluctuations of X around . Using Bayes' theorem, the resulting conditional probability distribution P(X|b) can then be used to deduce both the impact parameter dependence of the observable X and the impact parameter distributions P(b|S) of any subset of events S represented by the corresponding experimental distribution P(X|S). We perform a survey of the bombarding energy, total mass and mass asymmetry dependence of the deduced impact parameter dependence for the most common observables used for centrality estimation and/or selections. A consistent picture of the evolution of reaction mechanisms in this energy range towards the participant-spectator regime emerges. Evaluating the effective centrality of commonly-used selections of "central collisions" we show that it is largely independent of the colliding system, decreasing in a very similar way with the available energy in the center of mass frame

In-medium effects in central heavy ion collisions at intermediate energies

International audienceEnergy dissipation and cluster production in central nuclear collisions in the Fermi energy range have been investigated using the large INDRA database. A study of the in-medium nucleon-nucleon cross section, σNN*, has been performed by comparing the results of a model with the measured isotropy ratio of the kinetic energy distributions of the detected protons. This analysis has been done for a large variety of systems at various incident energies from 20 to 100 MeV/nucleon and confirms nicely previous results with enhanced statistics in a full Monte Carlo treatment. Most importantly, it is found that σNN* is drastically reduced as compared to the vacuum values, due partly to Pauli blocking but also to in-medium effects for which mean values and uncertainties are given. Concerning the cluster production, a coalescence model has been developed to study the parallel and perpendicular velocity distributions of the light clusters (A≤4). For AC=3 (H3, He3), it is found that the mean internal kinetic energy of the nucleons in the cluster is reduced as compared to the vacuum value. These two results shows that any comparison between experimental and theoretical predictions in heavy-ions induced reactions around and above the Fermi energy must take into account these in-medium effects

Isospin diffusion measurement from the direct detection of a quasiprojectile remnant

The neutron-proton (n-p) equilibration process in Ca48+Ca40 at 35 MeV/nucleon bombarding energy is experimentally estimated by means of the isospin transport ratio. Experimental data are collected with a subset of the FAZIA telescope array, which permits us to determine the Z and N of detected fragments. For the first time, the quasiprojectile (QP) evaporative channel is compared with the QP breakup one in a homogeneous and consistent way, pointing to comparable n-p equilibration, which suggests a close interaction time between projectile and target independently of the exit channel. Moreover, in the QP evaporative channel n-p equilibration is compared with the prediction of the antisymmetrized molecular dynamics model coupled with the Gemini statistical model as an afterburner, showing a higher probability of proton and neutron transfers in the simulation with respect to the experimental data

FAZIA: A new performing detector for charged particles

The FAZIA apparatus is a new detector designed for the Fermi energy domain for charged particles based on three stages telescopes: Silicon detector (300 μm thick), Silicon detector (500 μm thick) and CsI(Tl) (10 cm). Using the ΔE-E technique and the Pulse Shape Analysis (PSA) it permits the charge and mass discrimination up to more than Z=20. In the following, some details about the FAZIA detectors and electronics, their performance and the frst experimental campaigns already performed will be discussed

First results from the INDRA-FAZIA apparatus on isospin diffusion in Ni 58,64 + Ni 58,64 systems at Fermi energies

An investigation of the isospin equilibration process in the reactions Ni58,64+Ni58,64 at two bombarding energies in the Fermi regime (32 and 52 MeV/nucleon) is presented. Data have been acquired during the first experimental campaign of the coupled INDRA-FAZIA apparatus in GANIL. Selecting from peripheral to semicentral collisions, both the neutron content of the quasiprojectile residue and that of the light ejectiles coming from the quasiprojectile evaporation have been used as probes of the dynamical process of isospin diffusion between projectile and target for the asymmetric systems. The isospin transport ratio technique has been employed. The relaxation of the initial isospin imbalance with increasing centrality has been clearly evidenced. The isospin equilibration appears stronger for the reactions at 32 MeV/nucleon, as expected due to the longer projectile-target interaction time than at 52 MeV/nucleon. Coherent indications of isospin equilibration come from the quasiprojectile residue characteristics and from particles ascribed to the quasiprojectile decay

FAZIA: a new performing detector for charged particles

International audienceThe FAZIA apparatus is a new detector designed for the Fermi energy domain for charged particles based on three stages telescopes: Silicon detector (300 μm thick), Silicon detector (500 μm thick) and CsI(Tl) (10 cm). Using the ΔE-E technique and the Pulse Shape Analysis (PSA) it permits the charge and mass discrimination up to more than Z=20. In the following, some details about the FAZIA detectors and electronics, their performance and the frst experimental campaigns already performed will be discussed

Isospin diffusion measurement from the direct detection of a quasiprojectile remnant

International audienceThe neutron-proton (n-p) equilibration process in Ca48+Ca40 at 35 MeV/nucleon bombarding energy is experimentally estimated by means of the isospin transport ratio. Experimental data are collected with a subset of the FAZIA telescope array, which permits us to determine the Z and N of detected fragments. For the first time, the quasiprojectile (QP) evaporative channel is compared with the QP breakup one in a homogeneous and consistent way, pointing to comparable n-p equilibration, which suggests a close interaction time between projectile and target independently of the exit channel. Moreover, in the QP evaporative channel n-p equilibration is compared with the prediction of the antisymmetrized molecular dynamics model coupled with the Gemini statistical model as an afterburner, showing a higher probability of proton and neutron transfers in the simulation with respect to the experimental data