EXPERIMENTAL APPROACH TO THE DYNAMICS OF FISSION G. ISHAK BOUSHAKI University of Sciences and Technology Algiers ALGERIA Pr M. Asghar Insitute of Sciences Nucleaires Grenoble France Pr M. ALLAB University of Sciences and Technology Algiers ALGERIA Ghardaia south of ALGERIA

Plan Introduction Divers low energy fission data 230 Th Cf Models of fission Conclusion

Introduction

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 230 Th to 250 Cf

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: Y e (Z,E) yield of fragmentation of even Z Y o (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.

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. Y(Z): yied of charge Z Mean kinetic energy of charge Z.

Variation of mean kinetic energy by charge for different actinides

2.3 Neutronique covariance: Nifenecker(IAEA 1974) shows : For any fragmentation M 1 /M 2 1, 2 : numbers of neutrons emitted by the two fragments of masses M 1 and M 2.

2.4 Dispersion in excitation energy : The dispersion in excitation energy E* of two fragments of fission of mass M 1 and M 2 of total kinetic energy E K have been determined by: Signarbieux[IAEA-SM- 174/141 p.179], Kalinin[ISINN 10 Dubna 2002] and Vorobyev[ISINN 9 Dubna 2001]

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 : 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: 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.

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 3 rd 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): 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.

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.