NEW SIGNATURES ON DISSIPATION FROM THE STUDY OF RELATIVISTIC HEAVY-ION COLLISIONS Beatriz Jurado Apruzzese August 2002

Contents Introduction and motivation New experimental approach to investigate dissipation Experimental set-up Experimental observables sensitive to dissipation Results Conclusions

Introduction Deexcitation process of the nucleus: Statistical model Dynamical model Transport theories Two types of degrees of freedom Collective intrinsic Dissipation:  = dEcoll/dt [1/(Eeqcoll – Ecoll)]  rules the relaxation of the coll. degrees of freedom  (T, q) Fission is an appropriate tool for investigating dissipation

Current knowledge on dissipation Theory: Hilscher et al. Phys. Atom. Nucl. 57 (1994) 1187

Standard reaction mechanisms to induce fission Experiment: Standard reaction mechanisms to induce fission Heavy-ion collisions at Eprojectile ≈ 5-10 A MeV (Fusion-Fission, Fast fission, Quasifission)  ?? Dynamical models needed to describe these reactions Antiproton annihilation and spallation reactions ... Simplified theoretical description Difficulty to reach very high E* with large cross sections

Standard experimental observables U Deformation (q) fiss , evap l-distrib. of evap. residues Even-odd effect TKE Pre-scission part. and -multiplicities Ang., mass and charge distrib.

Latest experimental results Deformation dependence Small deformation Large &small deformation [FrG93, fiss, evap, Mp] [VeM99, A-, - distrib.] [ShD00, M] [DiS01, evap, M] [HuS00, Dio01, l-distrib] [JiP01, fiss] [LoG01,Pfss] [BeA02, fiss, 2z] [ChP02, Pre-sadel, Mn] [NaA02, Pfiss, fiss] [SaF02,Pfiss] Here you have to comment the possible reasons for controversy: Fusion-Fission reactions require complicated theoretical tools that have to take into account the side effects Often the excitation energy of the nucleus is too low Additional observables are required that allow for selecting the excitation energy of the fissioning nucleus Temperature dependence ?? Fissility dependence ??

Peripheral heavy-ion collisions at relativistic energies Small shape distortion Low angular momentum High intrinsic excitation energies E* ~ ∆A 238U (1 AGeV) + Pb (Calculation Abrasion Model) P (1.2 GeV) + U (Experimental Data) (Goldenbaum et al., Phys. Rev. Lett. 77 (1996 ) 1230) Inverse kinematics

Experimental set-up for fission studies in inverse kinematics

Total fission cross sections Observables Total fission cross sections Y Z Beam Target IC Fragmentation background Fission events Energy loss in IC Double IC

New observables: Partial fission cross sections & Widths of the charge distributions Tfiss Z1 + Z2 = 89 E*initial z2 = Tfiss/Cz Z1+Z2 = 92 238U (1 A GeV) + (CH2)n Bf E*initial Y fiss (Z1 + Z2)

M.V. Ricciardi PhD. Thesis The model Updated version of GSI code ABRABLA: If T< 5.5 MeV ABRASION EVAPORATION / FISSION af/an (Ignatyuk) Bf (Sierk) If T > 5.5 MeV SIMULTANEOUS BREAK-UP Freeze out T = 5.5 MeV M.V. Ricciardi PhD. Thesis

Model of Grangé & Weidenmüller (1980) Kramers (1940) Numerical solution of the FPE under specific initial conditions f(t) = f(t)/ħ  = 51021s-1 T= 3 MeV A = 248 Transient time f f(t) =Num. Sol. FPE (K.-H. Bhatt, et al., Phys. Rev. C 33 (1986) 954) f(t) = Step Function f(t) ∝(1-exp(-2.3t/f)) f(t) = Analytical approximation

Dependence of  on fiss(t) fnucl 238U(1 A GeV) + Pb Experiment 2.160.14 b Transition-state model 3.33 b f(t) step  = 21021 s-1 2.00 b f(t) ~1-exp(-t/)  = 41021 s-1 2.04 b f(t) FPE 2.09 b The value of  depends on the description for f(t)

Influence of  on f (Z1+Z2) and Z-Width(Z1+Z2) 238U (1 A GeV) + (CH2)n Experimental data Transition-state model  = 2·1021s-1  = 0.5·1021s-1  = 5·1021s-1  = 2·1021s-1 f  (1.7±0.4)10-21 s

Target dependence of ftot 238U (1 A GeV) Experimental data Transition-state model  = 2·1021s-1 The minimum at Ztarget = 6 can only be reproduced if dissipation is included

For fission events produced 238U(1A GeV)+Pb Calculations: For fission events produced 238U(1A GeV)+Pb NO BREAK-UP BREAK-UP Fission is mainly suppressed by dissipation at high E* Fission completely suppressed at E*  350 MeV

Our result! [FrG93] [VeM99] [ShD00] [DiS01] [HuS00, Dio01] [JiP01] [LoG01] [BeA02] [ChP02] [NaA02] Deformation dependence Small deformation Large & small deformation

Conclusions Fission induced by peripheral heavy-ion collisions at relativistic energies, ideal conditions for the investigation of dissipation at small deformations Determination of new observables Total nuclear fission cross sections for different targets Partial fission cross sections Partial widths of the charge distributions of fission fragments Realistic description for f (t) All observables described by a constant value of  = 21021s-1 f ≈ (1.7±0.4)10-21 s (critical damping) No indications for dependence on T or Z2/A Evidence for strong increase of  with deformation

af/an

Fragmentation background

Transient time

Excitation energy vs. Z

Outlook