Fission of Atomic Nuclei

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Fusion-Fission Dynamics for Super-Heavy Elements
Bülent Yilmaz1,2 and David Boilley1,3
1 GANIL,Caen,France 2
Ankara U.,Ankara,Turkey 3 Caen U.,Caen,France

• Fission of Atomic Nuclei
• Super-Heavy Elements (SHE)
• Measurement of fission time of SHE _at_ GANIL
• Kewpie2 A cascade code
• Fission time of SHE isomeric structure effects
• Conclusions

Advances in Nuclear Physics, Istanbul, 01/07/2008
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Symmetric fission process
Surface tension vs. Coulomb
repulsion
fission time
saddle
LDM
scission
initial state
Advances in Nuclear Physics, Istanbul, 01/07/2008
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Symmetric fission process
1- Statistical Models
2- Dynamical Models
(q,p) space
diffusion over the fission barrier by Langevin or
Fokker-Planck equations
Advances in Nuclear Physics, Istanbul, 01/07/2008
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Symmetric fission process
sample stochastic events
scission
initial state
saddle
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Deexcitation Scheme of Hot Compound Nucleus
Fission Fragments
Fission Fragments
.
.
.
gamma
neutron
proton
Evaporation Residue
alpha
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Super-Heavy Elements

• Heaviest nuclei
• Synthesis by heavy-ion fusion reactions
• Existance of critical initial center of mass
  energy
• Low production cross section (picobarn)
• No liquid drop potential barrier
• Stability by shell correction energies

Reaction mechanism leading to SHE
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Island of stability predicted due to shell
closure
Z124
?
Z120
Z114
N184
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Measurement of fission time of SHE _at_ GANIL
long fission times
measured with crystal blocking technique
with
238U Ge _at_ 6.1 MeV/u
MeV
10 of the capture events has
M. Morjean et al, EPJD 45, 27 (2007).
with
MeV
238U Ni _at_ 6.6 MeV/u
long fission times observed
D. Jacquet et al, AIP Conf. Proc. 853, 239
(2006).
208Pb Ge _at_ 6.2 MeV/u
no hint of long fission time
A. Drouart et al, AIP Conf. Proc. 1005, 215
(2008).
New probe into SHE stability M. Morjean et al,
accepted to PRL
Advances in Nuclear Physics, Istanbul, 01/07/2008
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Kewpie2 A
Cascade Code
Populations
Bohr-Wheeler fission rate with Strutinsky and
Kramers corrections
Weisskopf neutron rate
Bateman equations
Mean Fission Time
.
.
.
Fission Time Distribution
Kewpie2 A. Marchix, PhD Thesis (2007).
Kewpie B. Bouriquet, Comp. Phys. Com. 159, 1
(2004).
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Kewpie2 A
Cascade Code
How can we reach 10 of long-fission events?
with a constant potential
at
MeV
at
MeV
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Kewpie2 A
Cascade Code
How can we reach 10 of long-fission events?
with a damped potential according to Ignatyuks
presription
Using Moller and Nix shell correction energies,
the statistics of the long fission times
calculated by Kewpie2 are far smaller than what
is observed experimentally for Z120 and Z124
nuclei.
with arbitrarily fixed shell correction energies
along the deexitation chain
MeV
up to 5 isotopes
MeV
up to next 2 isotopes
MeV
MeV
for the others
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Isomeric Structure
Are the observed long fission times due to high
fission barriers or/and isomeric potential
structures?
some possible double-bump (isomeric) shapes
single-bump shape
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Isomeric Structure
In order to understand the consequences of the
potential structure beyond the saddle point two
potential shapes has been considered.
some possible double-bump (isomeric) shapes
single-bump shape
optimizes fission time
no evaporation
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Dynamical Model
Langevin Equations for the deformation coordinate
Monte-Carlo neutron evaporation scheme using
Weisskopf rate formula

MeV
MeV
MeV
s-1
double-bump potential
single-bump potential
saddle
saddle
initial position
initial position
scission
scission
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fission time for Z124-like nuclei

Preliminary
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fission time for Z124-like nuclei
double-bump potential
single-bump potential
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fission time for Z124-like nuclei
Preliminary
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Conclusions

• Very large barriers are still necessary to
  explain the long fission time

• In future, if we have more precise measurement
  of fission time and
• distribution we cannot forget about the
  isomeric states.

• This study will be continued with more realistic
  model...

Thank You
Advances in Nuclear Physics, Istanbul, 01/07/2008