EXPERIMENTAL APPROACH
TO
THE DYNAMICS OF FISSION
SHAK BOUSHAKI
Pr M. Asghar
versity of Sciences and Technology
Insitute of Sciences Nucleaires
ers
Grenoble
Pr
M.
ALLAB
GERIA
France
University of Sciences and Technology
Algiers
ALGERIA
Ghardaia south of ALGERIA
Plan
Introduction
Divers low energy fission data
Models of fission
Conclusion
230Th
-
250Cf
Introduction
Introduction
énergie
potentielle
I
étapes
III
II
100
~ 6 MeV
80
Y Axis Title
~ 30 MeV
~200 MeV
60
40
dernier
point selle
20
point de scission
0
0
> 10 -16s
20
~ 10-20s40
~ 10-19s80
60
X axis title
100
élongation
Variation of potential energy with elongation during the process of fission
Energy released during the transition from the saddle point to
scission point ~ 30MeV distribute itself between collective
degree of freedom (elongation, vibration, rotation) and intrinsic
degrees of freedom (break of pairs of nucleons or excitation of
quasi-particles).
Two extreme situations can be envisaged:
- Process adiabatic: the fragments of fission at the scission point
are in their fundamental states but presents a very deformed
configuration.
- Transition of statistical nature: the nascents fragments are very
excited and little deformed.
After the scission point, the fragments convert their
deformation energies in intrinsic excitation energy by
amortization of collective vibrations.
The fragments excited will desexcite by emitting:
prompt neutron

Fragments too rich in neutron, join the line of stability by
successive - decay and by emitting retarded neutron.
Fragments detected have kept only their charge state of
the point of scission.
2- Divers low energy fission data
230Th
to
250Cf
Fission induced by thermal neutron of actinides the
available energy is just sufficient to overcome the
fission barrier.
Fissionning systems of even Z are in a paired
configuration at saddle point.
Experience
charge
point.
we detect fission fragments of odd
q-p excitation do occur past the saddle
2-1 Odd even effect in charge:
P
 Y (Z , E )   Y (Z , E )
( E )(%) 
 Y (Z , E )   Y (Z , E )
e
o
Z
Z
e
Z
 100
o
Z
Ye (Z,E) yield of fragmentation of even Z
Yo (Z,E) yield of fragmentation of odd Z at the kinetic energy E.
For fissioning nuclei of even charge, the measure of proton odd
even effect no null is a measure of the probability of break of pairs
of protons between the last saddle point and the scission point.
60
230Th
233U
50
236U
240Pu
40
30
P
 (%)
250Cf
20
10
0
80
85
90
95
100
105
110
115
120
125
E(MeV)
Odd even effect in charge function of fragments kinetic energy.
2.2 Odd even effect in energy:
Fragments of even charge are more energetic than their
neighbors of odd charge.

E o e 
 Y (Z )  E (Z )
 Y (Z )
Z
Z
Y(Z): yied of charge Z
E ( Z  1)  E ( Z  1)
 E (Z ) 
 E (Z )
2
E (Z ) Mean kinetic energy of charge Z.
108
106
23 3U*
23 6U*(
25 0C f*
24 0Pu *
23 0Th *
<E>(MeV)
23 6U*(Lang
Nos résultats)
23 4U*
et al.)
104
102
100
98
30
32
34
36
38
Z
40
42
44
46
Variation of mean kinetic energy by charge for different actinides
2.3 Neutronique covariance:
Nifenecker(IAEA 1974) shows :
For any fragmentation M1/M2
Cov(1 , 2 : M1 / M 2 )  0
1 , 2 : numbers of neutrons emitted by the two
fragments of masses M1 and M2 .
2.4 Dispersion in excitation energy :
The dispersion in excitation energy E* of two fragments
of fission of mass M1 and M2 of total kinetic energy EK
have been determined by: Signarbieux[IAEA-SM174/141 p.179], Kalinin[ISINN 10 Dubna 2002] and
Vorobyev[ISINN 9 Dubna 2001]
 ( E : EK , M1 / M 2 ) ~ 20MeV  30MeV
2
*
2
2
3-Models of fission
3.1 Adiabatic model
Nix and Moller(Nucl.Phys.(1969),Phys.Lett.(1970)) :
During the transition of the fissioning nuclei from
saddle point to scission point the system deforms but it
remains in its fundamental state :  0  (n0 , k0 )
k : quantique number characterising the vibrationnal
collective excitations.
n : quantique number characterising the intrinsic
excitations .
The excitation energy of the two fragments at infinite
results from their deformation energies acquired during
the descent from saddle point to scission point.
The adiabatic model expect to have a strong correlation between
the excitation energies of the two binary fragments .
The experimental value of neutronique covariance shows absence
of correlation between excitation energies of the two fragments
and then the invalidity of the adiabatic assumption.
3.2 Statistical model:
Fong (Phys.Rev.1956)
The energy released during the transition from saddle
point to scission point is found in form of intrinsic
excitation energy of the two binary fragments.
Statistical model: the odd even effect in charge would be
equal zero (G.Ishak Boushaki PhD thesis Algiers 2003)
Experience: P # 0
Statistical model: 2 *
2
 ( E : EK , m)  1MeV
contrary to experimental observations
3.3 Superfluid model (Ignatyuk and Rejmund
(Nucl.Phys 2000))
Fissioning system: two ``sub-systems`` of protons
and neutron. The energy released is converted in
excitation of quasi – particles of the fissionning
system.
The probability of survival of a configuration
completely apparied in protons is calculated on the
basis of statistical considerations.

P
100%
(1)
(2)
 prévu
P
 mesuré
P
0
<E >
K
E
K
Odd even effect measured and that expected by the superfluid model
The existing low energy fission data can not be explained
in the assumption of statistical dynamics of the process
of fission or adiabatic dynamics.
3.4 Model of band of fission (Norenberg 3rd
Symp on Phys and Chemistry 1974)
The system occupy the band of fission constructed on
the fundamental.
It can not have break of pairs or excitation of quasi –
particles during the transition from saddle point to
scission point.
The energy released during the transition is converted in
excitation of collective vibrationnels states of the
systems.
2
*
2
Signarbieux and al.(IAEA 1974):  ( E : EK , m) ~ 20MeV
Compatible with experimental value
The dynamics of the transition from saddle point to
the scission point is of nature collective: without
break of pairs of protons and without excitation of
quasi-particles.
Detection of fragments of odd charge: the pairs of
proton are breaking during the rupture of the neck
joining the pre fragments at the scission point.
Fragments of even charge more energetic than that
of odd charge: the necessary energy to break a pair
of proton is taken probably from the kinetic energy of
prescission which form part of the kinetic energy.
1,5
230Th
1,0
236U
K
E o-e(MeV)
233U
0,5
240Pu
250Cf
0,0
0
10
20
30
40
 (%)
P
Effect parity in energy function of effect parity in charge
Conclusion
In fission induced by thermal neutron of actinides, fission
dynamics seems to be mostly collective in nature and the
pairs of proton seems to be broken during the rupture of the
neck joining the nascent fragments at the scission point.