EXPERIMENTAL APPROACH TO

THE DYNAMICS OF FISSIONG. ISHAK BOUSHAKI University of Sciences and TechnologyAlgiersALGERIA

Pr M. Asghar Insitute of Sciences NucleairesGrenobleFrancePr M. ALLAB

University of Sciences and TechnologyAlgiersALGERIA

Ghardaia south of ALGERIA

Plan

IntroductionDivers low energy fission data 230Th - 250Cf Models of fissionConclusion

Introduction

Introduction

0 20 40 60 80 100

0

20

40

60

80

100

~ 30 MeV

~ 10-19s~ 10-20s> 10-16s élongation

étapes

énergie potentielle

~ 6 MeV

~200 MeV

dernierpoint selle

point de scission

Y A

xis

Title

X axis title

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 we detect fission fragments of odd charge q-p excitation do occur past the saddle point.

2-1 Odd even effect in charge:

100),(),(

),(),()(%)(

Z Zoe

Z Zoe

P EZYEZY

EZYEZYE

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.

80 85 90 95 100 105 110 115 120 125

0

10

20

30

40

50

60 230Th

233U

236U

240Pu

250Cf

P(%

)

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.

)(2

)1()1()( ZE

ZEZEZE

Z

Zeo

ZY

ZEZYE

)(

)()(

Y(Z): yied of charge Z

)(ZE Mean kinetic energy of charge Z.

30 32 34 36 38 40 42 44 46

98

100

102

104

106

108

23 3U* 23 6U*( Nos résultats)

25 0Cf* 24 0Pu*

23 0Th* 23 4U*

23 6U*(Lang et al.)

<E

>(M

eV

)

Z

Variation of mean kinetic energy by charge for different actinides

2.3 Neutronique covariance:

Nifenecker(IAEA 1974) shows : For any fragmentation M1/M2

 

1 , 2 : numbers of neutrons emitted by the

two fragments of masses M1 and M2 .  

0)/:,Cov( 2121 MM

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-SM-174/141 p.179], Kalinin[ISINN 10 Dubna 2002] and Vorobyev[ISINN 9 Dubna 2001]

2221

*2 3020~)/,:( MeVMeVMMEE K

3-Models of fission

3.1 Adiabatic modelNix 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  : 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.

),( 000 kn

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:

contrary to experimental observations

2*2 1),:( MeVmEE K

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.

Pprévu

P mesuré

P

EK<E

K>

(1)(2)100%

0

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.Signarbieux and al.(IAEA 1974): 2*2 20~),:( MeVmEE KCompatible 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.

0 10 20 30 400,0

0,5

1,0

1,5

250Cf

240Pu

236U

233U 230Th

EKo-e(M

eV

)

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.