L. Grassi

Results of the ASY-EOS experiment at GSI : the symmetry energy at suprasaturation density

Directed and elliptic flows of neutrons and light charged particles were measured for the reaction 197Au+197Au at 400 MeV/nucleon incident energy within the ASY-EOS experimental campaign at the GSI laboratory. The detection system consisted of the Large Area Neutron Detector LAND, combined with parts of the CHIMERA multidetector, of the ALADIN Time-of-flight Wall, and of the Washington-University Microball detector. The latter three arrays were used for the event characterization and reaction-plane reconstruction. In addition, an array of triple telescopes, KRATTA, was used for complementary measurements of the isotopic composition and flows of light charged particles. From the comparison of the elliptic flow ratio of neutrons with respect to charged particles with UrQMD predictions, a value \gamma = 0.72 \pm 0.19 is obtained for the power-law coefficient describing the density dependence of the potential part in the parametrization of the symmetry energy. It represents a new and more stringent constraint for the regime of supra-saturation density and confirms, with a considerably smaller uncertainty, the moderately soft to linear density dependence deduced from the earlier FOPI-LAND data. The densities probed are shown to reach beyond twice saturation.

First Measurement of the 19F(α, p)22Ne Reaction at Energies of Astrophysical Relevance

The observational 19F abundance in stellar environments systematically exceeds the predicted one, thus representing one of the unsolved challenges for stellar modeling. It is therefore clear that further investigation is needed in this field. In this work, we focus our attention on the measurement of the 19F(α, p) 22Ne reaction in the astrophysical energy range, between 0.2 and 0.8 MeV (far below the Coulomb barrier, 3.8 MeV), as it represents the main destruction channel in He-rich environments. The lowest energy at which this reaction has been studied with direct measurements is ∼0.66 MeV, covering only the upper tail of the Gamow window, causing the reaction-rate evaluation to be based on extrapolation. To investigate lower energies, the 19F(α, p) 22Ne reaction has been studied by means of the Trojan horse method, applied to the quasi-free 6Li (19F, p22Ne)2H reaction at Ebeam = 6 MeV. The indirect cross section of the 19F(α, p) 22Ne reaction at energies ≲1 MeV was extracted, fully covering the astrophysical region of interest and overlapping existing direct data for normalization. Several resonances have been detected for the first time inside the Gamow window. The reaction rate has been calculated, showing an increase up to a factor of 4 with respect to the literature at astrophysical temperatures. This might lead to potential major astrophysical implications.

Kinematical coincidence method in transfer reactions

Abstract A new method to extract high resolution angular distributions from kinematical coincidence measurements in binary reactions is presented. Kinematics is used to extract the center of mass angular distribution from the measured energy spectrum of light particles. Results obtained in the case of 10Be+p→9Be+d reaction measured with the CHIMERA detector are shown. An angular resolution of few degrees in the center of mass is obtained. The range of applicability of the method is discussed.

Investigation of the Dependence of CsI(Tl) Scintillation Time Constants and Intensities on Particle's Energy, Charge and Mass Through Direct Fitting of Digitized Waveforms

The dependence of the CsI(Tl) scintillation time constants and intensities on the particle ionization profile is at the basis of the Charge and Mass identification technique for Light Charged Products in Intermediate Energy Nuclear Physics multi-detector arrays. The possibility of storing the digitized waveforms in true beam experiments allows performing further offline analyses and makes available a data set related to the interaction of different particle types in a wide energy range.

LIGHT CLUSTERS EMISSION IN NUCLEAR REACTIONS AT 25 MeV/NUCLEON WITH DIFFERENT N/Z OF ENTRANCE CHANNELS

We discuss results on the emission of light clusters in nuclear reactions 40Ca + 40,48Ca,46Ti at 25 MeV/nucleon. They were detected and identified in mass and charge by means of the CHIMERA 4π array. Inclusive emission at forward angles shows that isotopic yields of Li and Be nuclei are related to the N/Z values of entrance channels. Moreover, structure effects and even-odd staggering in charge distributions of intermediate mass fragments emitted at forward angles will be shown. In agreement with previous experimental findings, odd-even effects are clearly visible in N ≈ Z systems, while are less pronounced in the neutron rich collision 40Ca+48Ca. These experimental findings are compared to preliminary data obtained in 40Ca+48Ca. collision at 25 MeV/nucleon.

Dynamical and thermodynamical properties of incomplete fusion events at 25 MeV/nucleon

Various aspects observed in the reactions 40Ca+40,48 Ca,46Ti at E=25 MeV/nucleon, studied by means of the CHIMERA detector, will be discussed. An isospin effect on the competition between heavy residue production and binary-like channels has been observed in incomplete fusion events. From the thermodynamical point of view, kinetic energy spectra of protons emitted in fusion-evaporation events were fitted with two maxwellian functions. Despite of the different neutron richness of the populated sources, the average temperatures obtained for the quasi-fused systems are similar (T≃5.2MeV). We also performed a preliminary reconstruction of the excitation energy reached by quasi-fusion sources populated in the studied reactions.

Dipolar degrees of freedom and isospin equilibration processes in heavy ion collisions

Background: In heavy ion collision at the Fermi energies Isospin equilibration processes occur- ring when nuclei with different charge/mass asymmetries interacts have been investigated to get information on the nucleon-nucleon Iso-vectorial effective interaction. Purpose: In this paper, for the system 48Ca +27 Al at 40 MeV/nucleon, we investigate on this process by means of an observable tightly linked to isospin equilibration processes and sensitive in exclusive way to the dynamical stage of the collision. From the comparison with dynamical model calculations we want also to obtain information on the Iso-vectorial effective microscopic interaction. Method: The average time derivative of the total dipole associated to the relative motion of all emitted charged particles and fragments has been determined from the measured charges and velocities by using the 4? multi-detector CHIMERA. The average has been determined for semi- peripheral collisions and for different charges Zb of the biggest produced fragment. Experimental evidences collected for the systems 27Al+48Ca and 27Al+40Ca at 40 MeV/nucleon used to support this novel method of investigation are also discussed.

The thick target inverse kinematics technique with a large acceptance silicon detector array

An experimental technique for studying elastic scattering using a thick gas target is described, with a measurement of the α(24Ne,α) reaction used as an example. Advantages such as ease, detector efficiency, and the possibility of measuring the cross section at 180° in the centre-of-mass are discussed. It is shown that a resolution of tens of keV is practical at zero degrees, and that the dominant contribution to the resolution for large angles is angular straggling of the beam in the entrance window. The use of helium gas as the target allows direct measurement of a-cluster states.

Emission of fragments in Ca+Ca reactions at 25 MeV/nucleon

We discuss experimental data concerning 40,48Ca+40,48Ca reactions at 25 MeV/nucleon; the 4π multi-detector Chimera has been used as detection device. Effects that can be attributed to the neutron to proton ratios (N/Z) degree of freedom have been investigated. From the analysis of experimental data it seems that the neutron richness of the interacting system plays an important role on the evolution of fusion-like sources formed in semi-central collisions. In particular, it is observed that the larger is the neutron content and the larger is the emission of heavy residues. Experimental data have been compared with CoMD-II model calculations; a moderately stiff symmetry energy should be used to reproduce satisfactorily the data. A thermodynamical analysis on fusion-like sources has been also performed. In semi-peripheral collisions, isospin diffusion signals have been found. They have been investigated by analyzing isobaric emission (7Li/7Be) of quasi-projectile sources. Experimental data indicate that an incomplete N/Z mixing is reached during the interaction phase.

Probing the Merits of Different Event Parameters for the Identification of Light Charged Particles in CHIMERA CsI(Tl) Detectors With Digital Pulse Shape Analysis

We investigated the merits of different event parameters in the identification of Light Charged Particles (LCPs) with CsI(Tl) scintillators read out by photodiodes at high incident energy (400 MeV/u). This investigation is made possible by digital signal processing the output signals. As in the conventional analogue case, the digitized signals allow the discrimination of light charged particles by computing the fast and slow components. In addition other identification parameters as the rise time of the output pulses of the CsI(Tl) come out nearly for free. Aim of this paper is the investigation of novel identification plots and the probe of their merits, in particular at relativistic energies.