1. www.cea.fr THE SPY MODEL: HOW A MICROSCOPIC DESCRIPTION OF THE NUCLEUS CAN SHED SOME LIGHT ON FISSION | PAGE 1 S. Panebianco, N. Dubray, S. Hilaire, J-F. Lemaître, J-L. Sida CEA - Irfu/SPhN, Saclay, France CEA – DIF, Arpajon, France FUSTIPEN topical meeting October 13 th 2014 GANIL, Caen (FR)

2. INTRODUCTION Why a scission-point model? N-body problem Nuclear structure High spin exotic nuclei Viscosity and friction Non-adiabatic dynamics Intrinsic vs collective DoF Event-odd effects Shell effects Deformations Fission is the ideal nuclear physics laboratory and is still a challenge for theory and experiments

3. INTRODUCTION Why a scission-point model? N-body problem Nuclear structure High spin exotic nuclei Viscosity and friction Non-adiabatic dynamics Intrinsic vs collective DoF Event-odd effects Shell effects Deformations Fission is the ideal nuclear physics laboratory and is still a challenge for theory and experiments Two main approaches are used to model the fission process : Models based on phenomenology Based on experimental data Describe well know properties Low predictive power far from data Low computing cost Models based on microscopy Only few parameters (N-N interaction) Less precise agreement with data High predictive power far from know regions High computing cost

4. INTRODUCTION Why a scission-point model? N-body problem Nuclear structure High spin exotic nuclei Viscosity and friction Non-adiabatic dynamics Intrinsic vs collective DoF Event-odd effects Shell effects Deformations Fission is the ideal nuclear physics laboratory and is still a challenge for theory and experiments Scission-point model B. D. Wilkins et al., Phys. Rev. C 14 (1976) 1832 Strong hypothesis are needed: Static CN formation neglected All fragment properties are freezed Energy balance at scission : LDM+shell corections (Strutinski)+pairing corrections Parameters needed (intrinsic and collective temperature) Very low computing cost

5. INTRODUCTION Why a new scission-point model? N-body problem Nuclear structure High spin exotic nuclei Viscosity and friction Non-adiabatic dynamics Intrinsic vs collective DoF Event-odd effects Shell effects Deformations Fission is the ideal nuclear physics laboratory and is still a challenge for theory and experiments SPY SPY : a new scission-point model based on microscopic ingredients Scission-point model Strong hypothesis are needed: Static CN formation neglected All fragment properties are freezed Very low computing cost Microscopic data Precise treatement of nuclear structure No parameters needed Only way to explore unkown regions Microscopic data are tabulated (fast!)

6. THE SCISSION-POINT DEFINITION

7. - Thermodynamic equilibrium at scission is assumed → statistical equilibrium among system degrees of freedom - Isolated fragments → microcanonical statistical description  all states at scission are equiprobable

8. THE SCISSION-POINT DEFINITION The system configuration is defined by the two fragments DoF : - proton and neutron numbers (Z 1, N 1, Z 2, N 2 ) - quadrupolar deformations (  1,  2 ) - intrinsic excitation energy (E 1 *, E 2 *) Two quantities are needed to calculate average observables : - available energy for each configuration : E avail - state density of the two fragments:  1,  2

9. THE ENERGY BALANCE AT SCISSION 9 - Available energy calculation for each fragmentation (500-1000) 236 U*  132 Sn + 104 Mo → fragments individual energy from HFB calculation with Gogny D1S interaction (Amedee data base) S. Hilaire et al., Eur. Phys. Jour. A 33 (2007) 237 → interaction energy (nuclear + Couloub interactions) J. Blocki et al., Annals of Physics 105 (1977) 427 S. Cohen et al., Annals of Physics 19 (1962) 67

10. WHAT THE MICROSCOPY BRINGS 10 - Available energy calculation for each fragmentation (500-1000)

11. THE STATISTICS AT SCISSION 11 The probability of a given fragmentation is related to the phase space available at scission The phase space is defined by the number of available states of each fragment, i.e. the intrinsic state density The energy partition at scission is supposed to be equiprobable between each state available to the system (microcanonical) Therefore the probability of a configuration is defined as: with x the fraction of energy available to excite fragment 1 Hence, the probability of a fragmentation is easily calculated:

12. THE STATISTICS AT SCISSION 12 For the time being, a Fermi gas level density is used (CT model) A. Koning et al., Nucl. Phys. A 810 (2008) 13 No dependence on deformation

13. SOME EXPECTED RESULTS 13 Fission yields of 235 U(n th,f) and 252 Cf(sf) J. F. Lemaître et al., Proc. «Fission 2013», Caen (France), 28-31/05/2013.

14. SOME EXPECTED RESULTS 14 Kinectic Energy of fragments from 235 U(n th,f) J. F. Lemaître et al., Proc. «Fission 2013», Caen (France), 28-31/05/2013.

15. SOME REMARKABLE RESULTS 15

16. A REMARKABLE POWER OF DESCRIPTION 16 Charge yields from Ac to U isotopic chains SPY vs GSI data (Nucl. Phys. A 665, 221) J. F. Lemaître et al., Proc. «Zakopane 2014», 31/08-07/09/2014

17. A REMARKABLE PREDICTION POWER 17 Peak multiplicity over the whole nuclear chart J. F. Lemaître et al., Proc. «Zakopane 2014», 31/08-07/09/2014

18. WHAT THE MICROSCOPY BROUGHT The integration of microscopic description of the nuclei in a statistical scission point model shows that shell effects drive the mass asymetry However, these effects are energy (temparature) and deformation dependent and still too pronounced (i.e., 132 Sn plays as a strong attractor) J. F. Lemaître et al., paper in preparation for PRC

19. WHAT THE MICROSCOPY CAN STILL BRING But : nuclear structure affects also state density: Include microscopic state density from combinatorial on HFB nucleonic level diagram

20. SOME PRELIMINARY RESULTS Fission yields of 235 U(n th,f) and 252 Cf(sf) Fragment nuclear structure is present on: Individual energy (HFB – Gogny D1S) State density (Fermi gas)

21. SOME PRELIMINARY RESULTS Fission yields of 235 U(n th,f) and 252 Cf(sf) Fragment nuclear structure is present on: Individual energy (HFB – Gogny D1S) State density (Fermi gas)

22. PERSPECTIVES The integration of microscopic description of the nuclei in a statistical scission point model showed that shell effects drive the mass asymetry However, these effects are energy (temparature) and deformation dependent and still too pronounced (i.e., 132 Sn plays as a strong attractor) The ongoing developments consist of: Explore the richness of microscopic state density from HFB Include collectivity on both HFB energy and states density Include HFB data at finite temperature (Gogny D1M)

23. THE LESSON WE’VE LEARNT

25. BACKUP

26. AVAILABLE ENERGY AT SCISSION: SYMMETRIC FRAGMENTATION 90 Zr 180 Hg fission @ E*=10MeV SPY

27. BACKUP AVAILABLE ENERGY AT SCISSION: ASYMMETRIC FRAGMENTATION 76 Se 104 Pd 180 Hg fission @ E*=10MeV SPY

28. SOME UNEXPECTED RESULTS 28 The strange case of 180 Hg  -delayed fission of 180 Tl (@ Isolde) Surprising asymmetric yields of 180 Hg fission fully attributed to the nuclear structure of the fissioning nucleus A. Andreyev et al., Phys. Rev. Lett. 105 (2011) 252502 P. Möller et al., Phys. Rev. C 85 (2012) 024306

29. SOME UNEXPECTED RESULTS 29 The strange case of 180 Hg SPY results for 180 Hg fission @ E*=10MeV S. Panebianco et al., Phys. Rev. C 86 (2012) 064601 Self-consistent HFB of 180 Hg: most probable configuration (q 20 =256.12b ; q 30 =33.28b 3/2 ) d = 5.7 fm SPY

30. BACKUP TWO REFERENCE CASES Itkis et al., Yad. Fiz. 53 (1991) 1225 198 Hg fission @ E*=10 MeV SPY 236 U fission @ E*=8 MeV

31. SOME UNEXPECTED RESULTS 31 Microscopic data open the possibility to explore the whole nuclear chart Mean deformation of heavy fragmentMean deformation of light fragment Y peak /Y valley

32. SOME UNEXPECTED RESULTS 32 Microscopic data open the possibility to explore the whole nuclear chart Mean deformation of heavy fragmentMean deformation of light fragment Y peak /Y valley 132 Sn

33. SOME UNEXPECTED RESULTS 33 Microscopic data open the possibility to explore the whole nuclear chart Mean deformation of heavy fragmentMean deformation of light fragment Y peak /Y valley 132 Sn

34. SOME UNEXPECTED RESULTS 34 Microscopic data open the possibility to explore the whole nuclear chart Mean deformation of heavy fragmentMean deformation of light fragment Y peak /Y valley

35. SOME UNEXPECTED RESULTS 35 White : Solar r-abundance distribution Red : fission yields from SPY Blue : fission yields from empirical formula Impact of SPY prediction on stellar nucleosynthesis (collab. with S. Goriely) S. Goriely, Astron. Astrophys. 342, 881 (1999) T. Kodoma et al., Nucl. Phys. A. 239, 489 (1975)

36. SOME UNEXPECTED RESULTS 36 Impact of SPY prediction on stellar nucleosynthesis (collab. with S. Goriely) Very unexpected four-humped mass distribution from A=278 isobars

37. SOME UNEXPECTED RESULTS 37 Impact of SPY prediction on stellar nucleosynthesis (collab. with S. Goriely) Very unexpected four-humped mass distribution from A=278 isobars Confirmed by full HFB calculation S. Goriely, SP et al., to be published PES of 278 Cf

38. The SPY model is “parameter free” The distance d is fixed at 5 fm The distance is chosen on the exit points selection criteria used on Bruyères microscopic fission calculations BACKUP ON THE SCISSION POINT DEFINITION Elongation Asymmetry Energy Exit Points H. Goutte z (fm) r (fm) Nucleon density at the neck  < 0.01 fm 3 Total binding energy drop (  15 MeV) Hexadecupolar moment drop (  1/3) d=5fm

39. BACKUP Wilkins VS SPY Wilkins modelSPY model Individual energyLiquid drop + Strutinski + pairing HFB (Gogny D1S) http://www-phynu.cea.fr/HFB-Gogny.htm S. Hilaire and M. Girod, EPJ A 33 (2007) 237 Energy balanceRelative : E pot Absolute : E avail = E pot – E CN (if < 0 ; fragmentation is allowed) Temperature parameters Collective (statistics) + Intrinsic (shell effects) No temperature States density for statistics DeformationOnly prolateOblate + prolate Distance d1.2 - 1.4 fm5 fm StatisticsCanonicalMicrocanonical

40. BACKUP On the mean deformation energy The deformation energy is directly related to the number of emitted particles