Isotopic Yields of Fission Fragments from Transfer- Induced Fission F. Rejmund, M. Caama ñ o, X. Derkx, C. Golabek, J. Frankland, M. Morjean, A. Navin, M. Rejmund GANIL, France M. A ï che, G. Barreau, S. Czajkowski, B. JuradoCENBG, 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. RidikasCEA SPhN L. Gaudefroy, J. TaiebCEA DIF Shell effects in fission-fragment yields Presentation of the project Even-odd effects in fission-fragment yields

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

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

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

Cheifetz et al,, Th( 12 C, 8 Be) 236 U 234 U(t,pf) 235 U(n,f) Multi-nucleon transfer reaction 236 U( 12 C, 8 Be) 240 Pu 238 Pu(t,pf) 239 U(n,f) Large range of transfer Channels 238 U+ 12 C Eje RecQ(MeV)  (mb) 13 C 237 U C 236 U B 239 Np B 238 Np B 237 Np Be 240 Pu Be 241 Pu Be 242 Pu Be 239 Pu Li 243 Am Li 244 Am He 246 Cm He 244 Cm High resolution of the fissioning system

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

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

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

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 !!

Even-odd staggering in fission-fragment yields Local even-odd staggering Global even-odd staggering  z =  Y z e -  Y z o /(  Y z e +  Y z o )  z =40%

Qualitative understanding of the even-odd structure 229 Th+n Pairing gap saddlescission ? Th MeV The amplitude of the e-o effects reflects the probability that no pair is broken at scission Without dissipation there would be no odd-Z fragment E intr +E coll Even-odd structure : a consequence of dissipation in the descent

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

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

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 S. Steinha ü ser, PhD Thesis Evidence for the influence of the fission-fragment phase space

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

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

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

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

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

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

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

Statistical description Estimated dissipated energy for asymmetric split symmetric fission : Common asymptotic energy  ~5% E dis ~ 9 MeV Asymmetric fission 232 Th 236 U 240 Pu X=  MeV

Neutron evaporation and energetic balance Cf Cm U Q=TKE+TXE TXE=E def (F1)+E def (F2)+E intr E intr (Z) = Q(Z) - TKE(Z) - E def (Z) - E def (Z CN -Z)) E def (Z) ~  (  n +B n (Z)) 1, 2

Dissipated energy deduced from neutron evaporation… 236 U 248 Cm 252 Cf 244 Cm And compared to statistical analysis of e-o staggering Q max =max(M CN -M F1 -M F2 )) TKE from experiment

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

Additional diapositives

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

Quantitative description of the even-odd structure A combinatory analysis, H. Nifenecker et al., 1982 N the maximum possible number of broken pairs N = E diss /   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 Bag of broken pairs FF2FF1 E diss =-4ln(  Z )  Z =(1-2pq  ) N

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 ( E diss n =2*E diss p ) the even-odd structures in odd-Z fissionning systems (q=1) S. Steinhauser et al., 1998 M. Davi et al., 1998

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

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

Counting rates Reasonable statistics: 10 4 fission events detected Acceptance of VAMOS&TIARA: 10 5 fission events Thin secondary target : at/cm 2 d Secondary target limited by energy resolution && XS Cd2 <0.5mg/cm2  fis ~5mbarn Total number of actinide: N inc =N fis /(  fis N tar )= Primary target limited by the 2nd beam kin. Energy &alpha acceptance==>1mg/cm2 N inc =  fus *N tar *I inc *time*  q = * * * *0.2 = Primary beam intensity: >x20 Fusion evaporation <x2 Gas secondary target >x30 Impinging energy x2

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

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)