S. Kowalski

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.

Mass and charge identification of fragments detected with the Chimera Silicon-CsI(Tl) telescopes

Abstract Mass and charge identification of charged products detected with Silicon–CsI(Tl) telescopes of the Chimera apparatus are presented. An identification function, based on the Bethe–Bloch formula, is used to fit empirical correlations between Δ E and E ADC readings, in order to determine, event by event, the atomic and mass numbers of the detected charged reaction products prior to energy calibration.

Correlations between emission timescale of fragments and isospin dynamics in

We present a new experimental method to correlate the isotopic composition of intermediate mass fragments (IMF) emitted at mid-rapidity in semi-peripheral collisions with the emission timescale: IMFs emitted in the early stage of the reaction show larger values of

Sn+

isospin asymmetry, stronger angular anisotropies and reduced odd-even staggering effects in neutron to proton ratio

Ni and

distributions than those produced in sequential statistical emission. All these effects support the concept of isospin migration, that is sensitive to the density gradient between participant and quasi-spectator nuclear matter, in the so called neck fragmentation mechanism. By comparing the data to a Stochastic Mean Field (SMF) simulation we show that this method gives valuable constraints on the symmetry energy term of nuclear equation of state at subsaturation densities. An indication emerges for a linear density dependence of the symmetry energy.

^{112}

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.

Sn+

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.

^{58}

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.