E. De Filippo (INFN Catania) for the REVERSE / ISOSPIN collaboration Time sequence and isoscaling in neck fragmentation  Fragments production in peripheral collisions: isospin dependence in neck formation  The Reverse experiment with CHIMERA detector  Characterization of dynamical emitted light fragments in ternary events: time scale and time sequence  Comparison with BNV calculations  Isoscaling in “neck” fragmentation ?  CONCLUSIONS AND OUTLOOK (Chimera upgrading) Pisa, February

Fragments can have several origin: they can be emitted sequentially from (eventually equilibrated) projectile-like or target-like source or promptly ( dynamical emission ) during the first stage of the reaction. At the Fermi energy, in binary dissipative collisions, an emission component of fragments and light particles is centered between quasi- projectile and quasi-target-velocity. Evolution of the density contour plot at 6 fm in the reaction 124 Sn + 64 Ni at 35 A.MeV: the formation of a neck-like structure brought after fm/c to a ternary event with the appearance of dynamical emitted IMFs. V. Baran et al. Nucl. Phys. A730 (2004) 329

Neck fragmentation and isospin degree of freedom Asymmetry Looking for a constraint to the density dependence of EOS asymmetry term Depending upon the shape of symmetry potential around  0 neutron/proton diffusion effects and a neutron enrichment of the neck region could be induced ( isospin fractionation ). NEUTRONS PROTONS Asy-soft Asy-stiff Nucl Phys. A703, 603 (2002) 124 Sn I=0.2

The CHIMERA detector and Reverse experiment 1192 Si-CsI(Tl) Telescopes 1m 1° 30° TARGET 176° Beam REVERSE Experiment: 688 Telescopes, forward part. 2002/2003- CHIMERA-Isospin 1192 telescopes REVERSE Experiment: 688 Telescopes, forward part. 2002/2003- CHIMERA-Isospin 1192 telescopes Experimental Methods:  E(Si)-E(CsI(tl)): CHARGE, ISOTOPES E(Si) –TOF(Si) VELOCITY - MASS PULSE SHAPE in CsI(Tl) p,d,t, 3 He, 4 He,. 6,7,.. Li,… Z light <5 124 Sn + 64 Ni, 27 Al 112 Sn + 58 Ni 1° 30° 35 A.MeV

TERNARY EVENTS SELECTION To get insight the different mechanisms of IMFs production we have selected in the Vpar-Charge bi-dimensional plot three regions where PLFs, TLFs and IMFs can be easily separated: p/p beam > 0.6 Z1+Z2+Z3 ~ Z TOT PLF TLF IMF

BASIC CHARACTERISTICS OF SELECTED EVENTS Parallel velocity distribution for Z=4,6,12,18 IMFs in coincidence with projectile-like fragment (PLF) and target-like fragment in ternary events. BNV

IMFs mechanism production: REDUCED VELOCITY PLOT* TLF PLF Vr1 Vr2 IMF We have constructed event-by-event the relative velocity of IMF respect to TLF (ry) and of IMF respect to the PLF (rx). Relative velocities were normalized to the relative velocity for a Coulomb repulsion between fragments of charge Z 1,Z 2 (V viola ) Plotting the two reduced relative velocity (rx) versus (ry) in a bi- dimensional plot different scenarios can be disentangle: for example sequential decay from PLF (TLF) should be represented by a distribution around rx=1 (ry=1). On the contrary simultaneous values of rx and ry larger than one can support a non-statistical origin for these fragments. * E. De Filippo, A. Pagano, J. Wilczyński et al. (Isospin collaboration), to be published Phys. Rev. C

Events close to diagonal correspond to a prompt ternary division while those approaching a ratio ~ 1 correspond to a sequential emission from PLF or TLF respectively. Prompt fm/c 2 80 fm/c fm/c Points are calculated in a simple kinematical simulation assuming that IMFs separate from projectile (square) or from target (circle) after a time interval of 40, 80 and 120 fm/c elapsed from the primary binary separation of the projectile from the target at t=0. Results of BNV transport model for IMFs emission probability from neck region for different impact parameters (V. Baran et al. Nucl. Phys. A , 2004).

REDUCED VELOCITY PLOTS: BNV Note: BNV model accounts only for the “prompt” component of IMF’s

Angular distributions: alignment characteristics  plane is the angle, projected into the reaction plane, between the direction defined by the relative velocity of the CM of the system PLF- IMF to TLF and the direction defined by the relative velocity of PLF to IMF Out-of-plane angular distributions for the “dynamical” (gate 1) and “statistical” (gate 2) components: these last are more concentrated in the reaction plane.

ISOSCALING FROM THE RATIO OF ISOTOPE YIELDS M.B. Tsang et al. Phys. Rev. C64, R 21 = Y 2 (N,Z)/Y 1 (N,Z) = C exp(  N +  Z) For two systems having a different isospin asimmetry, the ratio of isotope yields with Z protons and N neutrons obtained from sistem 2 (neutron rich) and system 1 (neutron poor) has been found to follow a significative scaling (exponential dependence) where  and  are scaling parameters. 112 Sn+ 112 Sn and 124 Sn+ 124 Sn 50 A.MeV (MSU data)

E. Geraci et al., Nucl. Phys. A732 (2004) 173 A signal of phase transition: Isospin distillation Neutron enrichment in the gas phase Isoscaling in central collisions 112 Sn+ 58 Ni and 124 Sn+ 64 Ni at 35 AMeV Central collisions CHIMERA-REVERSE experiment

Gating the reduced plot for light IMFs:

ISOSCALING OF ISOTOPIC DISTRIBUTIONS We have started a study upon isoscaling signal for peripheral collisions and neck fragmentations. Infact also if isoscaling relation can be derived assuming chemical and thermal equilibrium, this is not a necessary condition to observe this signal. For the IMFs sequential emission from projectile-like source a nice fit is observed with  =0.61 and  =-0.61 parameter’s values. exp(-0.61*Z) exp(0.61*N)

For the neck region the isoscaling signal seems to be yet present also if the quality of the exp(N  ) fit is poor, especially for heavier IMFs. This study can be interesting for the future prosecution of data analysis because isoscaling parameters could be sensitive to the density dependence of EOS as shown by dynamical calculations. Preliminary data exp(0.53*N) exp(-0.40*Z)

CHIMERAPS-UPGRADING ( ) Method: rise time measurement for Pulse shape application Present threshold for charge identification  10 A.MeV mass(*) (*) charge for particle stopped in silicon detector is reconstructed by EPAX formula Charge Charge and mass for light Ions IDENTIFICATION IN CHIMERA 124 Sn+ 64 Ni 35 A.MeV CFD90% Stop TAC RiseTime ~ Stop-Start Si PA Amp Split CFD30% Start TAC QDC E T TDC Standard ‘’CHIMERA LINE’’ upgrading Results: charge identification up Z  15 With ~ 4 MeV/A energy threshold for particle stopped in silicon detector a + TOF A, Z 40 Ar+ 12 C 20 A.MeV

We have studied with the forward part of the CHIMERA detector the 124 Sn + 64 Ni and 112 Sn + 58 Ni at 35 A.MeV. Fragments produced in semi-peripheral ternary reactions have been investigated. The analysis method gives the possibility to evaluate the time scale of the process. Comparison, for light IMFs ions, with BNV calculations supports the scenario of dynamical production of IMFs in the overlapping zone (neck) between target and projectile nuclei. Conclusions and Outlook Isospin effects, in particular of isoscaling signal are under study. Sistematic evaluation of isoscaling parameters with proper source selection are important quantities for testing symmetry energy density dependence of EOS in asymmetric nuclear matter. The Chimera detector will be upgrated and the combination of pulse-shape analysis and time-of-flight measurements in Silicon detectors will increase the capability of fragment identification in mass and charge: this is important not only for the prosecution of the isospin physics studies with stable beams but of course also for future planning of experiments with exotic beams.

The REVERSE – ISOSPIN COLLABORATION INFN, Sezione di Catania and Dipartimento di Fisica e Astronomia, Università di Catania, Italy INFN, Sezione di Milano and Instituto di Fisica Cosmica, CNR, Milano,Italy INFN, Laboratori Nazionali del Sud and Dipartimento di Fisica e Astronomia, Università di Catania, Italy INFN, Gruppo Collegato di Messina and Dipartimento di Fisica, Università di Messina, Italy INFN, Sezione di Milano and Dipartimento di Fisica Università di Milano, Italy Institute for Physics and Nuclear Engineering, Bucharest, Romania Institute of Physics, University of Silesia, Katowice, Poland M. Smoluchowski Institute of Physics, Jagellonian University, Cracow, Poland Institute de Physique Nucl´eaire, IN2P3-CNRS and Université Paris-Sud, Orsay, France LPC, ENSI Caen and Université de Caen, France INFN, Sezione di Bologna and Dipartimento di Fisica, Università di Bologna, Italy Saha Institute of Nuclear Physics, Kolkata, India GANIL, CEA, IN2P3-CNRS, Caen, France, H. Niewodniczanski Institute of Nuclear Physics, Cracow, Poland DAPNIA/SPhN,CEA-Saclay, France IPN, IN2P3-CNRS and Université Claude Bernard, Lyon, France Institute of Modern Physics, Lanzhou, China Institute of Experimental Physics, Warsaw University, Warsaw, Poland INFN, Sezione Napoli and Dipartimento di Fisica, Università di Napoli Institute for Nuclear Studies, Swierk/Warsaw, Poland

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IMFs CHARGE DISTRIBUTION The charge distribution of the IMF’s fall down exponentially: exp(aZ). Result of BNV calculations (normalized to Z=6) are compared with the experimental distribution. Sequential decay stage is not present in the calculation. Z IMF

Angular distributions: angle definitions In fission studies it is useful to interpret the data by assuming a proper system of reference: the one associated with the “Fissioning nucleus”: [see: A. Stefanini et al., Z.Phys. A351 (1995) 167] In the neck fragmentation studies we can adopt the same reference frame to study the alignment configuration between PLF-IMF-TLF HFz Strong alignement with the separation direction of the two primary fragments:  =0 beam PLF* TLF* LF x  

SEMI-PERIPHERAL EVENT SELECTION Semi-peripheral collisions, roughly selected by requiring that the multiplicity of charged particles is less than 7. Coincidence between projectile-like fragments (PLF) and remnants of the target nucleus (TLF) amount to about 10% of the selected events. M>12 M6M6 7  M  12