Laura Francalanza Collaborazione EXOCHIM INFN Sezione di Catania - LNS

OUTLINE  58 Ni+ 48 Ca reaction at 25 A MeV  The CHIMERA multidetector at INFN-LNS (Catania)  Selection of centrality  CENTRAL EVENTS  Competition between reaction mechanisms: “fusion-evaporation” and “multifragmentation”.  Comparison with dynamical + sequential evaporative model.  Conclusions and perspectives.

The experiment was performed by the ISOSPIN NUCL-EX collaboration and it was realized by means of the CHIMERA apparatus, located at LNS – INFN (Catania). An ion beams of 58 Ni was accelerated on a thin target 48 Ca by the LNS Superconducting Cyclotron, and the reaction products were collected by the 1192 telescopes of CHIMERA multidetector. It was devoted to study the competition between reaction mechanisms for central collisions in the Fermi energy domain. RELEVANT ISSUES: Enhancement in the number of the emitted IMFs (fragments with Z ≥3 ) with respect to low energy domain; predominance of fusion mechanism for small impact parameter and competition with prompt multifragmentation.

9 rings in the angular range 1°≤θ≤30° (forward part) 17 rings between 30°≤θ≤176 °( sphere ) High granularity Efficiency in angular coverage up to 94% 4π 1° 30° 176° TARGET BEAM 1192 telescopes

CsI(Tl) (Pd)18x18mm 2 10cm 300  m Si PSD in CsI(Tl) Z and A for light particles  E(Si)-E(CsI) Z for charged particles that punch through silicon detector. Z and A for ions up to Z <9.  E(Si)-TOF A for particles stopped in Si. Z for particles that punch through Si. E(Si)-Rise Time Z for particles stopped in silicon detector. (NEW)

v PAR = component of velocity parallel to beam direction A = Mass (amu) of each reaction product A proj = 58 uma Z proj = 28 A target = 48 uma Z target = 20 v proj ≈ 6.5 cm/ns v CM ≈ 3.8 cm/ns Relevant presence of fragments with properties like PLF and TLF. Contribution of reaction products with parallel velocity close to centre of mass velocity. 11.5% Minimum value : 70% Maximum value : 105% 0.7 ≤ Z TOT / (Z proj + Z target ) ≤ ≤ p TOT / (p proj + p target ) ≤ 1.05

The inclination of the main axis of the ellipsoid with respect to beam direction defines the flow angle, ϑ flow, that gives information about event shape. NOTE: An increasing in ϑ flow values results in a selection of more dissipative events. TKE = Σ i Ekin i CENTRALITY DISSIPATION

b)b) a)a) c)c) vpar (cm/ns) A (amu) a)a) b)b) c)c) CENTRAL EVENTS Central = 6.16% Complete = 2 (IMF : Z ≥3) ≈ 6-7 (LP : Z<3) a)Large number of PLF and TLF, indicative for peripheral collisions. b)PLF and TLF contributions are progressively reduced. c)Emissions from peripheral collisions vanished; v par spectrum is more and more centered on the CM velocity: strong evidence for a fusion- evaporation residue.

M IMF ≈ 1-2 M LCP ≈ 4-5 M IMF ≥ 3 M LCP ≈ 4 v PAR big (cm/ns) M IMF Abig (amu) A 1 ≥ 50 amu M IMF = 1 : 43.5% amount = 1-2 (IMF : Z ≥3) ≈ 4-5 (LP : Z<3) A 1 < 50 amu M IMF up to 6 = 3 a) b) Two classes of emitted fragments, concerning their mass. Event by event analysis, in order to disentangle between events that show this heavy residue (fusion-evaporation meachanism) and ones that don’t show this feature: good candidates for prompt multifragmentation process? Mass and longitudinal velocity of the heaviest fragment

EVENTS a) The biggest fragment, is accompanied with lighter reaction products. There is a strong difference in mass value between the first three emitted fragments, ordered according to decreasing masses. EVENTS b) Completely different nature of the emission pattern: the three biggest fragments show very similar mass values. a) b) v PAR big (cm/ns) Abig (amu) A 1 ≥ 50 amu M IMF = 1 : 43.5% amount = 1-2 (IMF : Z ≥3) ≈ 4-5 (LP : Z<3) A 1 < 50 amu M IMF up to 6 = 3 A1 (amu) A3 (amu) A2 (amu) A3 (amu) A1 (amu)

A (amu) M IMF We can notice the sensitivity of the shape of mass distribution to M IMF : Heavy residue, with A>50 amu, strongly decreases with increasing of M IMF. The intermediate mass component is dominant for high values of M IMF. A (amu)

We have compared mass distributions, multiplicity … for selected central events with those predicted by a two step mechanism: dynamical BNV calculation followed by the sequential de-excitation of a composite source (SIMON). The source information were obtained from a BNV calculation, including a pre-equilibrium emission: SOURCE’S MASS = 94 amu SOURCE’S ATOMIC NUMBER = 43 E* = 400 MeV (± 50 MeV) Comparison exp.-BNV-SIMON Experimental data BNV – SIMON M IMF A (amu) Z IMF ≥ 3 Simulation UNFILTERED PRELIMINARY RESULTS

M IMF ≥ 3M IMF ≥ 4 Abig (amu) Experimental data BNV – SIMON b = Comparison exp.-BNV-SIMON Experimental data BNV – SIMON Comparison between the mass distribution of the heaviest fragment, in each event. Mass distribution for events with M IMF ≥3 and M IMF ≥4 SOURCE’S MASS = 94 amu SOURCE’S CHARGE = 43 E* = 400 MeV (± 50 MeV) b =0 Experimental data BNV – SIMON A (amu) Simulation Unfiltered Simulation Unfiltered Simulation UNFILTERED

CONCLUSIONS PRELIMINARY RESULTS Main criterion for centrality selection: ϑ flow. Mass and velocity correlations for all emitted fragments have shown the presence of a broad component centered at v CM. Low MIMF values are associated with emission of a heavy residue, while events with high values of MIMF show relevant presence of fragments with mass of intermediate values. Preliminary comparisons of experiment with reaction simulation show quite good agreement with the assumption of theoretical sequential emission. However further analysis are needed in order to obtain definitive conclusions (IMF- IMF correlations, SMM calculations … )