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EVIDENCE FOR TRANSIENT EFFECTS IN FISSION AND IMPORTANCE FOR NUCLIDE PRODUCTIONPowerPoint Presentation

EVIDENCE FOR TRANSIENT EFFECTS IN FISSION AND IMPORTANCE FOR NUCLIDE PRODUCTION

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EVIDENCE FOR TRANSIENT EFFECTS IN FISSION AND IMPORTANCE FOR NUCLIDE PRODUCTION

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EVIDENCE FOR TRANSIENT EFFECTS IN FISSION AND IMPORTANCE FOR NUCLIDE PRODUCTION

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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

- 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

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.

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)

??

- 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

Experimental set-up for fission studies in inverse kinematics

238U (1 A GeV)

Yfiss (Z1 +Z2)

E*initial

New observables: Partial fission cross sections

238U (1 A GeV) + (CH2)n

Z1+Z2 = 92

New observables: Partial fission cross sections

Z1 + Z2 = 89

Tfiss

z2 = Tfiss/Cz

Z1+Z2 = 92

E*initial

Realistic description of the time-dependent fission-decay width

f(t) = f(t)/ħ

= 51021s-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)

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)

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

Sensitivity to the shape of f(t)

238U (1 A GeV) + (CH2)n

Experimental data

f(t) ~1-exp(-t/), = 41021 s-1

f(t) step, = 21021 s-1

f(t) FPE, = 21021 s-1

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

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)

208P (1 A GeV) + p

Experimental data

Transition state model

= 21021 s-1

(PhD. J. Pereira, Univ. Santiago de Compostela)

- Conclusions
- Experimental observation of transient effects
- All observables described by a constant value of
= 21021s-1 f≈ (1.7±0.4)10-21 s

- Transient effects in fission strongly influence nuclide production
- Very realistic analytical approximation for f (t)