Isotopic Yields of Fission Fragments from Transfer-Induced Fission

1. Isotopic Yields of Fission Fragments from Transfer-Induced Fission F. Rejmund, M. Caamaño, X. Derkx, C. Golabek, J. Frankland, M. Morjean, A. Navin, M. Rejmund GANIL, France M. Aïche, G. Barreau, S. Czajkowski, B. Jurado CENBG, France K.-H. Schmidt, A. Kelic, GSI, Germany C. Shmitt IPNL, France G. Simpson LPSC,France J. Benlliure, E. Casarejos, USC, Spain L. Audouin, C.-O. Bacri, L. Tassan-Got, IPNO, France T. Enqvist, CUPP, Finland D. Doré, S. Panebianco, D. Ridikas CEA SPhN L. Gaudefroy, J. Taieb CEA DIF Shell effects in fission-fragment yields Presentation of the project Even-odd effects in fission-fragment yields

2. PF1 PF1 Fission fragments from irradiation E,ToF =>M • Mass distribution n • Isotopic distribution • Spectrometer =>light fragments •  Spectroscopy =>branching ratio, unknown isomers • Limitations due to target activity, neutron energy

3. Mass distribution of fission fragments - Stabilisation of heavy fragment when changing mass of the fissioning nucleus -Two fission modes (spherical and deformed ) N~ 88 deformed shell N=82 spherical shell Closed shell at N=86,88,90 ?? Still under debate!!

4. GSI data in inverse kinematics Profi, K.-H. Schmidt Exp. data Wide systematcis on element yields for U fragmentation products Necessity to get isotopic yields in heavy FF!! Af=Zf+Nf Average charge constant =>Influence of moving neutron shell =>Existence of proton closed shell ? J. Benlliure et al, EPJA 13(2002)

5. Multi-nucleon transfer reaction • High resolution of the fissioning system • Large range of transfer • Channels • 238U+12C • Eje Rec Q(MeV) (mb) • 13C 237U -1.2 23 • 14C 236U 1.8 8 • 11B 239Np -10 25 • 12B 238Np -13 5 • 13B 237Np -14 0.8 • 10Be 240Pu -15 10 • 9Be 241Pu -17 5 • 8Be 242Pu -12 5 • 11Be 239Pu -21 0.8 • 7Li 243Am -26 0.5 • 6Li 244Am -19 3 • 4He 246Cm -17 3 • 6He 244Cm -24 0.5 232Th(12C,8Be) 236U 234U(t,pf) 235U(n,f) 236U(12C,8Be) 240Pu 238Pu(t,pf) 239U(n,f) Cheifetz et al,,1981

6. Transfer-induced fission reactions: wide range of fissioning systems • Neutron-rich actinides : 238U beam, 12C Target • Energy range 0-40 MeV

7. Multinucleon induced fission in inverse kinematics@GANIL -Inverse kinematics (high Z resolution) -Isotopic identification (spectrometer) -Wide range of actinides Precise measure of the excitation energy (particle detection) 12C 238U heavy FF FF recoil light FF

8. Identification of fission fragments in VAMOS ToF E E 238U+48Ca X,Y,, M. Rejmund et al. PRC76(2007)

9. Seeking for information.. • We propose to use multi-nucleon transfer induced fission in inverse kinematics in order to • Identify isotopic fission yields in complete fragment distribution • Define the fissioning system in excitation energy, mass, charge • Over a broad range of neutron-rich actinides • Study the structure effects as a function of excitation energy and fissioning nucleus • These data would complement GSI data • Important results on shell effects and pairing effects are expected !!

10. Even-odd staggering in fission-fragment yields Global even-odd staggering z = Yze- Yzo/(Yze+Yzo) z =40% Local even-odd staggering

11. Qualitative understanding of the even-odd structure Without dissipation there would be no odd-Z fragment MeV Pairing gap 229Th+n 5 Eintr +Ecoll ? saddle scission 0 23090Th • Even-odd structure : • a consequence of dissipation in the descent -25 • The amplitude of the e-o effects reflects the probability that no pair is broken at scission

12. Even-odd effect depends on fissility of the system Global even-odd effect z = Yze- Yzo As the Coulomb repulsion inside the nucleus increases, the saddle shape becomes more and more compact Saddle Th Saddle Cm The descent from saddle to scission increases, as Ediss, with fissility Ediss decreases with scission asymmetry

13. Electromagnetic induced fission of secondary beams K.-H. Schmidt et al., NPA665(2000)221

14. Even-odd staggering in odd-Z nuclei Zero staggering at symmetry: Unpaired nucleon chooses both fragments with equal probability Negative staggering for asymmetry: unpaired nucleon chooses the heaviest fragment Evidence for the influence of the fission-fragment phase space S. Steinhaüser, PhD Thesis

15. Statistical analysis of e-o staggering Relative statistical weight of 1 nucleon in fragment (Z): level density at Fermi level in FF E-o staggering produced with n unpaired uncleons Data reproduced with S. Steinhauser et al., NPA634(1998)89

16. Probability for a completely proton paired configuration at scission Level density of only broken neutron pairs Level density of all possible excitations Strutinsky 1958 Ignatyuk 1973

17. Statistical description of the even-odd staggering -Estimation of the dissipated energy -For the first time the difference between proton and neutron number yields is reproduced without further assumption F. Rejmund et al. NPA678 (2000)215

18. Systematics on even-odd staggering Ra,Rn U,Th Constant e-o staggering at symmetry !! Important impact on our understanding Of fission dynamics fissility

19. E-o effect at symmetry: neutron-induced fission Difficult to measure Z yields at symmetry in direct kinematics

20. E-o effect at symmetry in n-induced fission:constant with fissility ? p global p local asy(Z=54) p local reachable sym No conclusion can be drawn due to the lack of data at symmetry

21. Statistical description of the even-odd effect for asymmetric split GSI data reproduced with Probability to have nZ proton pairs broken at scission nZ=0 nZ=2 nZ=4 nZ=6 E-o staggering:

22. Statistical description Estimated dissipated energy for asymmetric split symmetric fission : Common asymptotic energy ~5% <-> Edis~ 9 MeV Asymmetric fission 232Th236U 240Pu X= 34.9 35.7 36.8  0.32 0.25 0.1 5.7 6.2 7.1 MeV

23. Q=TKE+TXE TXE=Edef(F1)+Edef(F2)+Eintr Eintr(Z) = Q(Z) - TKE(Z) - Edef(Z) - Edef(ZCN-Z)) Edef(Z) ~ (n+Bn(Z)) 1,2 Neutron evaporation and energetic balance Cm U Cf

24. Dissipated energy deduced from neutron evaporation… Qmax=max(MCN-MF1-MF2)) TKE from experiment 236U 252Cf 248Cm 244Cm And compared to statistical analysis of e-o staggering

25. E-O staggering : summary • Different sets of data (fission yields in e-m fission and neutron yields) give a coherent picture of a dissipation at symmetry independent on fissility. • This should have important impact on our understanding of the descent dynamics • Statistical analysis of even-odd effect : • description of the even-odd effect at symmetry and asymmetry • dissipated energy at asymmetry taking into account the phase space effect in the final fragments • Improvement can be achieved by using a rigorous description of the level density in the Fission fragments • Importance of systematic measures to point out new properties/ideas • Importance of reverse kinematics to have an access to the complete fission fragment characterization =>Transfer-induced fission @GANIL

26. Additional diapositives

27. Electromagnetic induced fission of secondary beams E* distribution <E*> ~12 MeV for all pre-actinides

28. FF1 FF2 Quantitative description of the even-odd structure A combinatory analysis, H. Nifenecker et al., 1982 Bag of broken pairs Ediss =-4ln(Z ) Z=(1-2pq)N Nthe maximum possible number of broken pairs N = Ediss/  the broken pair is a proton pair Zf/Af0.4 q break a pair when the required energy is available 0.5 p the 2 protons of a given pair to end up into 2 different fragments 0.5

29. Limitations of the combinatory analysis • Model is based on the number of broken pairs and NOT on the available phase space • As a consequence the model cannot reproduce • the variation of z with Z of the fission fragment (p=0.5) • the amplitude of n(Edissn=2*Edissp) • the even-odd structures in odd-Z fissionning systems (q=1) S. Steinhauser et al., 1998 M. Davi et al., 1998

30. Isotopic distribution in direct kinematics Rochman PhD, Lohengrin 2001 Lohengrin (ILL) -Only the LIGHT fragments are identified =>No experimental evidence of shell effects in heavy fragments Exfor data base Radiochemical methods Small part of the distribution : distortions in the neutron yields

31. Advantage of inverse kinematics • High radioactivity : • the production of samples for irradiation is difficult • (=>systematics in direct kinematics is limited) • Combined with a spectrometer • isotopic resolution of the full isotopic distribution • (light and heavy fragments) • in-flight measure of the isotopic distribution • (before beta decay) • Using transfer reaction to induce fission • precise knowledge of the excitation energy

32. Description of fission fragment distribution Liquid drop model : symmetric fission in equally deformed fragments Shell effects: Minima of the potential landscape are modified Deformed shell Spherical shell Closed shell at N=86,88,90 ?? Still under debate!!

33. Counting rates Reasonable statistics: 104 fission events detected Acceptance of VAMOS&TIARA: 105 fission events Thin secondary target : 6 1019at/cm2 d Secondary target limited by energy resolution && XS Cd2 <0.5mg/cm2 fis ~5mbarn Total number of actinide: Ninc=Nfis/(fis Ntar)= 3 1011 Primary target limited by the 2nd beam kin. Energy &alpha acceptance==>1mg/cm2 Ninc= fus *Ntar *Iinc *time*q=5 10-27*7 1019*5 1010*1.3 106*0.2 =3 109 Primary beam intensity: >x20 Fusion evaporation <x2 Gas secondary target >x30 Impinging energy x2

34. Advantages reaction with cross section >mb => sufficient statistics Disadvantage Imprecision on the excitation energy (excitation energy distributed to ejectile) Threshold ??

35. Predictions for SPIRAL2 PROFI code (K.H. Schmidt) reproduces the mass distributions And the isotopic distribution from ISOLDE and GSI (fissioning system and excitation energy are model dependent)