What can we learn about reaction mechanism from Au + Au collisions at 23 MeV/A ? POLITA T. Cap, J. Wilczyński, K. Siwek-Wilczyńska, F. Amorini, L. Auditore, G. Cardella, E. De Filippo, E. Geraci, L. Grassi, A. Grzeszczuk, E. La Guidara, J. Han, T. Kozik, G. Lanzalone, I. Lombardo, R. Najman, N. G. Nicolis, A. Pagano, M. Papa, E. Piasecki, S. Pirrone, R. Płaneta, G. Politi, F. Rizzo, P. Russotto, I. Skwira-Chalot, A. Trifiro, M. Trimarchi, G. Verde, W. Zipper

197Au + 197Au @ 15 and 23 MeV/A No fusion due to the Coulomb instability of the composite system. The whole range of impact parameters corresponding to semi-peripheral and near-central collisions is open to collective rearrangements of nuclear matter. New phenomena. Region of energies to study the transformation from one body dissipation (energies below 10 MeV/A) to two body dissipation (intermediate energy range). .

Multidetector CHIMERA (INFN Catania) Charge Heavy Ion Mass and Energy Resolving Array beam target 1192 Si-CsI(Tl) telescopes arranged in 4 geometry low energy thresholds

Au+Au @ 23 MeV/A complete events i – all charge particles in an event Ai > 7 complete events

exclusive events: multiplicity = 3 p/po > 0.8 A1,A2,A3 > 7 A1+A2+A3 > 350 sequential (two step) process Velocity distributions (V vs. V) of F1 and F2 - Coulomb rings characteristic of two body final state interactions. 2. VREL/VViola~ 1 ( VViola–empirically derived velocity corresponding to the energy of Coulomb repulsion for equilibrated fission fragments F1 and F2). 3. Mass distribution of the reconstructed primary fragment is narrower than the distribution for fragments F1 or F2. primary process 197Au+197Au  PLF*+TLF* secondary PLF*  F1+F2 + (evaporated nucleons) not analyzed secondary TLF*  F1+F2 + (evaporated nucleons)

(FMS) Focused Mowing Source Analysis ▪ for heavy colliding systems - strong Coulomb repulsion. The separation axis is turn away from the beam axis. (FMS) Focused Mowing Source Analysis T. Cap et al. Physica Scripta 89 (2014) 054005 21<AF2<30 21<AF2<30 laboratory reference frame separation axis reference

All three fragments detected splitting of TLF* splitting of PLF* symmetric colliding system (Au+Au) results independent of the studied splitting (PLF* or TLF*) splitting of TLF* splitting of PLF* symmetric system 21<AF2<30 51<AF2<60 V (cm/ns) V(cm/ns) experimental advantage for PLF: ▪ detection at small laboratory angles better granulation of the detector. ▪ large laboratory energies of detected fragments. better ID.

semi-inclusive events PLF two heaviest fragments with Vc.m. -1 cm/ns A1+A2  150 f f = AF2/(AF1+AF2) AF2 < AF1 for each bin of f TKE(f) - Total Kinetic Energy of the final 3 body system TKE(f) = Ekin(PLF) + Ekin(rest) calcutated Arest = 397-APLF Vrest –from the balance of momenta measured

for selected events – FSM projections (velocity (Vvs for selected events – FSM projections (velocity (Vvs..V) into the separation axis) for the lighter fragment

Localization of the primary process (Au+Au PLF+TLF) in impact parameter/(angular momentum) space. solid lines - calculations made with the nuclear dynamics model HICOL of Feldmeier (classical trajectory, one body dissipation). T. Cap et al. Acta Physics Polonica B47(2016) 975 Independently of the asymmetry, the maximum of the reaction yields correspond to a narrow range of impact parameters (0.6< b/bmax< 0.7).

Stefanini et al. Z. Phys. A351 (1995) 167 ● Reaction plane is defined by the center of mass velocity and the PLF velocity vectors. ● The out-of-plane angle  - the angle between the perpendicular to the reaction plane and the break up axis (reaction plane  =900). ● The in-plane angle  is the angle between the separation axis and the projection of the break-up axis into the reaction plane.  positive if the vector of the velocity for the lighter fragment turns toward the beam axis

Projections into the reaction plane (efficiency corrected) The lighter fragment of the PLF* breakup. 0.20f0.25 <A2>=40 0.10f0.15 <A2>=22 0.15f0.20 <A2>=30 0.25f0.30 <A2>=49 Counts  (degree) 0.40f0.45 <A2>=77 0.45f0.50 <A2>=85 0.30f0.35 <A2>=57 0.35f0.40 <A2>=67 Counts  (degree) time-scale of the breakup

2= - Schematic picture f T=Δϕ IPLF/JPLF reaction plane 1= - Au beam L1 Schematic picture f T=Δϕ IPLF/JPLF 0.10<f0.15 prompt<100 fm/c T1 ≈ 300 fm/c T2 ≈ 500 fm/c Au reaction plane 1= -1800 beam direction PLF prompt delayed Au Au L2 0.15<f0.20 separation axis L1 > L2 prompt<100 fm/c T1 ≈ 380 fm/c T2 ≈ 480 fm/c 1= - 2= -  - time measure domination of the anticlockwise rotation.  degree

beam   - time measure Schematic picture L1, 1=   degree L1 L2 PLF Au beam L1 L2 Schematic picture 0.30<f0.35 T1 ≈ 380 fm/c T2 ≈ 440 fm/c L1, 1=  delayed  degree beam direction  prompt separation axis   - time measure PLF delayed L2, 2= 

Summary The breakup of Au+Au system (leading to the formation of three heavy fragments) is a sequential process. The primary reaction Au+Au PLF*+TLF* is highly concentrated in the impact parameter space. The PLF* or TLF* breaks-up with all possible partitions. For asymmetric partitions (f<0.3) the dominating mechanism is the prompt emission of the light fragment from the interacting zone (neck). The delayed breakup due to a rotation of the PLF* or TLF* (spin transfer) is also observed. Possibility of time scale calculations. For partitions f>0.3 – evident influence of the two trajectories. More studies needed. For symmetric partitions f>0.4 in addition to equilibrated fission the prompt aligned breakup (along the separation axis) is observed. The effect is much weaker then observed at 15 MeV/A. A growing contribution of equilibrated fission with increasing f.

What is to be done? Very large amount of experimental data. Studies of the reaction mechanism as a function of the impact parameter and energy loss for all possible partitions. To study partition into four and if possible more fragments. Determination of the time scale for different f. Comparison with model calculations (HICOL, QMD).

If= 2/5 (MF1RF12 + MF2RF22) + [MF1MF2(RF1+RF2)2/(MF1+MF2)] Time for different mass asymmetries of the breakup: T(f)=  If/JPLF (fm/c) JPLF - the intrinsic spin of PLF* generated in the PLF* during the primary dissipative stage of the 197Au + 197Au collision (calculated with the HICOL code), If - the moment of inertia of the breaking up PLF* in the near-saddle configuration for a given breakup asymmetry f approximated by a rigid-body value for two touching spheres If= 2/5 (MF1RF12 + MF2RF22) + [MF1MF2(RF1+RF2)2/(MF1+MF2)] MF1 and MF2 - masses of fragments F1 and F2 (the lighter) RF1 and RF2 - radii. Time for 2 groups of asymmetric partitions f: f = 0.10 − 0.15 prompt<100 fm/c T1≈ 300 fm/c T2≈ 500 fm/c f = 0.30 - 0.35 T1  380 fm/c T2  440 fm/c

Table 1. Characteristics of breakup of Au-like primary fragments as a function of the breakup asymmetry parameter f. Numbers represent average values. The last three columns show the results of HICOL calculations (see text). f NE/E AF2 AF1 APLF TKE APLF* TKE* Ntrans L JPLF Nexch 0.10-0.15 0.87 22 149 171 1150 196 1493 -1 1030 103 29 0.15-0.20 0.74 30 142 172 1050 201 1412 4 1003 98 34 0.20-0.25 0.72 40 135 175 950 207 1329 10 976 118 40 0.25-0.30 0.72 49 127 176 950 208 1330 11 976 118 40 0.30-0.35 0.72 57 118 175 1100 203 1458 6 1017 108 32 0.35-0.40 0.58 67 111 178 1300 199 1622 2 1066 88 25 0.40-0.45 0.58 77 103 180 1400 198 1699 1 1091 78 22 0.45-0.50 0.3 85 94 179 1400 198 1699 1 1091 78 2

f = AF2/(AF1+AF2) AF2 < AF1 APLF = AF1+ AF2 for each bin of f TKE(f) - Total Kinetic Energy of the final 3 body system TKE(f) = Ekin(PLF) + Ekin(rest) calcutated Arest = 397-APLF Vrest –from the balance of momenta measured

What can we learn about reaction mechanism from Au + Au collisions at 23 MeV/A ? POLITA T. Cap, J. Wilczyński, K. Siwek-Wilczyńska, F. Amorini, L. Auditore, G. Cardella, E. De Filippo, E. Geraci, L. Grassi, A. Grzeszczuk, E. La Guidara, J. Han, T. Kozik, G. Lanzalone, I. Lombardo, R. Najman, N. G. Nicolis, A. Pagano, M. Papa, E. Piasecki, S. Pirrone, R. Płaneta, G. Politi, F. Rizzo, P. Russotto, I. Skwira-Chalot, A. Trifiro, M. Trimarchi, G. Verde, W. Zipper

SUMARY The breakup of Au+Au system (leading to the formation of three fragments) is a sequential process. The primary reaction Au+Au PLF*+TLF* is well concentrated in the impact parameter space. The PLF* or TLF* breaks-up with all possible partitions. For asymmetric partitions (f<0.3) the dominating mechanism is the prompt emission of the light fragment from the interacting zone (neck). The delayed breakup due to a rotation of the PLF* or TLF* (spin transfer) is also observed. Possibility of time scale calculations. For partitions 0.3<f<0.4 – wide maximum due to almost the same amount of heavier and lighter fragments emitted forward and backward. More studies needed. For symmetric partitions f>0.4 the prompt aligned breakup (along the separation axis) is observed. The effect is much weaker then observed at 15 MeV/A. A growing contribution of equilibrated fission with increasing f.

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