EVIDENCE FOR TRANSIENT EFFECTS IN FISSION AND IMPORTANCE FOR NUCLIDE PRODUCTION

1. EVIDENCE FOR TRANSIENT EFFECTS IN FISSION ANDIMPORTANCE FOR NUCLIDE PRODUCTION B. Jurado1,2, K.-H. Schmidt1, A. Kelić1, C. Schmitt1, J. Benlliure3, A. Junghans4 1GSI, Darmstadt, Germany 2GANIL, Caen, France 3University Santiago de Compostela, Spain 4RFZ, Rossendorf, Germany

2. Contents • Introduction • What are transient effects in fission? • How to observe experimentally transient effects? • Quantitative results for transient effects • Importance of transient effects for nuclide production • Conclusions

3. Two equivalent views of the process: Single Langevin trajectories in phase space + Evaporation Evaporation code + Γf(t), Fokker-Planck eq. Dynamical description of the deexcitation process of a heavy nucleus: Transport theories Collective deg. freedom Intrinsic deg. freedom Dissipation: (T,q) Langevin/Fokker-Planck eq.

4. Transient effects in fission • To observe transient effects… • Small deformation and high E* • Appropriate observables Evolution of the probability distribution t = 0 s t = 1·10-21 s t = 3·10-21 s Transient time τf(β, A, Z, T)

5. ?? • Small shape distortion • Low angular momentum < 20 ħ • High intrinsic excitation energies E* ~ ∆A • Inverse Kinematics Fusion-fission reactions Peripheral heavy-ion collisions at relativistic energies

7. Experimental set-up for fission studies in inverse kinematics 238U (1 A GeV)

8. Yfiss (Z1 +Z2) E*initial New observables: Partial fission cross sections 238U (1 A GeV) + (CH2)n Z1+Z2 = 92

9. New observables: Partial fission cross sections Z1 + Z2 = 89 Tfiss z2 = Tfiss/Cz Z1+Z2 = 92 E*initial

10. Realistic description of the time-dependent fission-decay width f(t) = f(t)/ħ  = 51021s-1 T= 3 MeV A = 248 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 (B. Jurado, et al., Phys. Lett. B 553 (2003) 186)

11. EVAPORATION / FISSION af/an(Ignatyuk) Bf (Sierk) The model Updated version of GSI code ABRABLA: If T< 5 MeV SIMULTANEOUS BREAK-UP ABRASION If T > 5 MeV Freeze out T = 5 MeV (W. A. Friedman, PRL, 60 (1988) 2125 W. Nörenberg et al. Eur. Phys. J A 9 (2000) 327 K.-H. Schmidt et al., Nucl. Phys. A 710 (2002) 157)

12. The value of β depends on the description for f(t) Total fission cross sections are not sensitive to the shape of f(t) Total fission cross sections fnucl 238U(1 A GeV) + Pb

13. Sensitivity to the shape of f(t) 238U (1 A GeV) + (CH2)n Experimental data f(t) ~1-exp(-t/),  = 41021 s-1 f(t) step,  = 21021 s-1 f(t) FPE,  = 21021 s-1

14. Experimental data Transition-state model Kramers  = 4·1021s-1  = 2·1021s-1  = 0.5·1021s-1  = 5·1021s-1  = 2·1021s-1 f  (1.7±0.4)10-21 s Sensitivity to the dissipation coefficient β 238U (1 A GeV) + (CH2)n

15. Influence on nuclide production… 238U (1 A· GeV) + p Experimental Data No transient effects Calculation INCL-ABLA Transient effects β =1·1021s-1 Calculation INCL-ABLA (Data from J. Taieb et al., Nucl. Phys A 724 (2003) 413-430)

16. 208P (1 A GeV) + p Experimental data Transition state model  = 21021 s-1 (PhD. J. Pereira, Univ. Santiago de Compostela)

17. Conclusions • Experimental observation of transient effects • All observables described by a constant value of  = 21021s-1 f≈ (1.7±0.4)10-21 s • Transient effects in fission strongly influence nuclide production • Very realistic analytical approximation for f (t)