Laura FrancalanzaCollaborazione EXOCHIMINFN Sezione di Catania - LNS

Competition betweenfusion-evaporation and multifragmentation

in central collisions in 58Ni + 48Ca reactionat 25 AMeV

OUTLINE 58Ni+48Ca reaction at 25 A MeVThe CHIMERA multidetector at INFN-LNS (Catania)Selection of centralityCENTRAL EVENTSCompetition 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 58Ni was accelerated on a thin target 48Ca 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.

58Ni+48Ca a 25 A MeV

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.

CHIMERA

• 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°

TARGETBEAM

1192 telescopes

IDENTIFICATION TECHNIQUES

CsI(Tl)

(Pd)18x18mm2

10cm

300m

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)-TOFA for particles stopped in Si.

Z for particles that punch through Si.

E(Si)-Rise TimeZ for particles

stopped in silicon detector. (NEW)

COMPLETE EVENTS•vPAR = component of velocity parallelto beam direction•A = Mass (amu) of each reaction product

Aproj = 58 umaZproj = 28

Atarget = 48 umaZtarget = 20

vproj ≈ 6.5 cm/nsvCM ≈ 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 ≤ ZTOT / (Zproj + Ztarget) ≤ 1.05

0.7 ≤ pTOT / (pproj + ptarget) ≤ 1.05

The inclination of the main axis of the ellipsoid with respect to beam direction defines the flow angle, ϑflow, that gives information about event shape.NOTE: An increasing in ϑflow values results in a selection of more dissipative events.

SELECTION OF CENTRAL COLLISIONS

TKE = Σi Ekini

CENTRALITYD

ISSIPATION

b)a) c)

ϑflow 60°

vpar (cm/ns)

A (a

mu)

a) b)

c)

CENTRAL EVENTSCentral = 6.16% Complete< MIMF > = 2 (IMF : Z ≥3)< MCP > ≈ 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; vpar spectrum is more and more centered on the CM velocity: strong evidence for a fusion-evaporation residue.

ΘFLOW SELECTION METHOD

MIMF ≈ 1-2MLCP ≈ 4-5

MIMF ≥ 3MLCP ≈ 4

vPARbig (cm/ns)

MIMF

Abig

(am

u) A1 ≥ 50 amuMIMF = 1 : 43.5% amount< MIMF > = 1-2 (IMF : Z ≥3)< MLP > ≈ 4-5 (LP : Z<3)

A1 < 50 amuMIMF up to 6< MIMF > = 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?

QUALITATIVE CHARACTERIZATION

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)

vPARbig (cm/ns)

Abig

(am

u)

A1 ≥ 50 amuMIMF = 1 : 43.5% amount< MIMF > = 1-2 (IMF : Z ≥3)< MLP > ≈ 4-5 (LP : Z<3)

A1 < 50 amuMIMF up to 6< MIMF > = 3

QUALITATIVE CHARACTERIZATION

A1 (amu)

A3 (amu) A2 (amu)

A2 (amu)A3 (amu)

A1 (amu)

A (amu)

MIMF

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

QUALITATIVE CHARACTERIZATION

A (amu) 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 dataBNV – SIMON

MIMF A (amu)

ZIMF ≥ 3

SimulationUNFILTERED

PRELIMINARY RESULTS

0 2 4 6 8 10 12 14 16 18 200

0.2

0.15

0.1

0.05

0.25

0.35

0.45

0.3

0.4

0 20 40 60 80 100 12010-3

10-1

10-2

1

MIMF ≥ 3 MIMF ≥ 4

Abig (amu)

Experimental dataBNV – SIMON b = 0

10-1

1

10-2

10-3

0 4020 1008060 120

Comparison exp.-BNV-SIMON

Experimental dataBNV – SIMON

Comparison between the mass distribution of the heaviest fragment, in each event.Mass distribution for events with MIMF≥3 and MIMF≥4

• SOURCE’S MASS = 94 amu• SOURCE’S CHARGE = 43• E* = 400 MeV (± 50 MeV)• b =0

Experimental dataBNV – SIMON

A (amu)

SimulationUnfiltered

SimulationUnfiltered

SimulationUNFILTERED

CONCLUSIONS

PRELIMINARY RESULTS

• Main criterion for centrality selection: ϑflow.

• Mass and velocity correlations for all emitted fragments have shown the presence of a broad component centered at vCM.

• 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 … )

ACKNOWLEDGEMENTS

Thanks to CHIMERA/EXOCHIM groupfor the help and support during the study

and toM. Colonna for her careful work in calculations.

Thank you all for your attention!