Z. Majka

Particle identification method in the CsI(Tl) scintillator used for the CHIMERA 4π detector

Abstract The charged particle identification obtained by the analysis of signals coming from the CsI(Tl) detectors of the CHIMERA 4 π heavy-ion detector is presented. A simple double-gate integration method, with the use of the cyclotron radiofrequency as reference time, results in low thresholds for isotopic particle identification. The dependence of the identification quality on the gate generation timing is discussed. Isotopic identification of light ions up to Beryllium is clearly seen. For the first time also the identification of Z =5 particles is observed. The identification of neutrons interacting with CsI(Tl) by ( n , α ) and ( n , γ ) reactions is also discussed.

Isotope correlations as a probe for freeze-out characterization: central124Sn+64Ni,112Sn+58Ni collisions

Abstract 124 Sn+ 64 Ni and 112 Sn+ 58 Ni reactions at 35 AMeV incident energy were studied with the forward part of CHIMERA multi-detector. The most central collisions were selected by means of a multidimensional analysis. The characteristics of the source formed in the central collisions, as size, temperature and volume, were inspected. The measured isotopes of light fragments (3 ⩽ Z ⩽ 8) were used to examine isotope yield ratios that provide information on the free neutron to proton densities.

RE-SEPARATION MODES OF 197Au + 197Au SYSTEM AT SUB-FERMI ENERGIES

Collisions of a very heavy nuclear system, 197Au + 197Au, were studied at the energy of 15 MeV/nucleon with the aim to identify and investigate main macroscopic modes of re-separation of such a heavy nuclear system. The experiment was performed at the INFN LNS laboratory in Catania by using the multidetector array CHIMERA. Along with binary strongly damped collisions, a strong component of a new re-separation mode: the fast ternary fission in characteristic nearly co-linear configuration was observed and the time scale of this process was determined.

DYNAMICAL EVOLUTION OF THE 197Au + 197Au SYSTEM AT 15 MeV/NUCLEON

Collisions of a very heavy 197Au + 197Au system have been studied at an energy of 15 MeV/nucleon with the aim to investigate a possible change of the energy dissipation mechanism from one-body dissipation (presumably dominating at low excitation energies) to two-body dissipation. The experiment was carried out in 4π geometry using the CHIMERA multidetector array. A class of ternary events satisfying nearly complete balance of mass numbers was selected. Preliminary results suggest that the selected ternary events represent mostly prompt ternary partitions of the colliding system, in which the lightest fragment is quite massive. In the light of theoretical predictions of Cârjan, Sierk and Nix this result may imply that the observed ternary reactions are driven by the two-body dissipation mechanism already at 15 MeV/nucleon.

TERNARY REACTIONS IN 197AU + 197AU COLLISIONS REVISITED

Ternary events originating from the 197Au + 197Au reaction at 15 MeV/nucleon, studied by using the multidetector array CHIMERA at LNS Catania, have been collected in almost complete 2π range of the forward hemisphere of CHIMERA thus extending the angular range of our previously reported preliminary data. Energy- and time calibrations for heavy fragments at large angles were supplemented with an analysis of coincidence events of binary strongly damped reactions. Contrary to the previous preliminary results, the new complete data clearly show presence of sequential fission processes in which one of Au-like nuclei undergoes fission after re-separation of the primary binary system. Mechanism of these two types of reactions observed in our data, sequential fission reactions and prompt ternary processes, are presently studied using comparisons with predictions of the stochastic BNV model of Baran, Colonna and Di Toro and the QMD model of Łukasik.

Freeze-out configuration properties in theAu197+Au197reaction at23AMeV

Data from the experiment on the 197Au + 197Au reaction at 23 AMeV are analyzed with an aim to find signatures of exotic nuclear configurations such as toroid-shaped objects. The experimental data are compared with predictions of the ETNA code dedicated to look for such configurations and with the QMD model. A novel criterion of selecting events possibly resulting from the formation of exotic freeze-out configurations, the efficiency factor, is tested. Comparison between experimental data and model predictions may indicate for the formation of flat/toroidal nuclear systems.

Isospin effects in projectile‐like dynamical fission for 112,124Sn+58,64Ni reactions at 35 AMeV

A quantitative comparison regarding absolute cross sections associated to Statistical and Dynamical Fission processes in the 124Sn+64Ni and 112Sn+58Ni collisions experimentally investigated at 35 AMeV beam energy using multi‐detector CHIMERA is presented. It results that Dynamical Fission process is about two times more probable in the neutron rich 124Sn+64Ni system than in the 112Sn+58Ni neutron poor one. The observed difference could indicate an influence of the entrance channel Isospin (N/Z) content on the reaction mechanisms.

Centrality dependence of isospin effect signatures in {sup 124}Sn+{sup 64}Ni and {sup 112}Sn+{sup 58}Ni reactions

Signatures of isospin effects were investigated for neutron-rich ({sup 124}Sn+{sup 64}Ni) and neutron-poor ({sup 112}Sn+{sup 58}Ni) systems at 35 MeV/nucleon for noncentral collisions. The centrality dependence of these signatures was tested for several impact parameter estimators. Our main observations are (i) the yields of {sup 1}H and {sup 3}He particles in the neutron-poor system are strongly enhanced with respect to the neutron-rich system, and the yields of {sup 3}H, {sup 6}He, and {sup 7,8}Li are suppressed at all impact parameters, (ii) the yields of {sup 2}H, {sup 4}He, and {sup 6}Li particles are almost the same for both systems, (iii) the N/Z ratio of intermediate mass fragments is correlated with the neutron richness of the system and is weakly dependent on the centrality of the collision, and (iv) the neutron richness of the detected fragments increases strongly with decreasing rapidity in the range from that of the projectile-like fragment to the c.m. region. The gross features of experimental data are reproduced by quantum molecular dynamics model calculations. A comparison between model calculations and the data indicates that the fragments produced in the c.m. regions are weakly excited.