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
Fission fragments from irradiation

• Mass distribution
• n
• Isotopic distribution
• Spectrometer
• gtlight fragments
• ? Spectroscopy
• gtbranching ratio, unknown isomers
• Limitations due to target activity, neutron
  energy

E,ToF gtM
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
N82 spherical shell
Closed shell at N86,88,90 ?? Still under
debate!!
4
GSI data in inverse kinematics
Wide systematcis on element yields for U
fragmentation products
Necessity to get isotopic yields in heavy FF!!
AfZfNf Average charge constant gtInfluence of
moving neutron shell gtExistence of proton closed
shell ?
J. Benlliure et al, EPJA 13(2002)
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Multi-nucleon transfer reaction

• High resolution of the fissioning system

• Large range of transfer
• Channels
• 238U12C
• 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
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Transfer-induced fission reactions wide range of
fissioning systems

• Neutron-rich actinides 238U beam, 12C Target
• Energy range 0-40 MeV

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Multinucleon induced fission in inverse
kinematics_at_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
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Identification of fission fragments in VAMOS
ToF E ?E
238U48Ca
X,Y,?,?
M. Rejmund et al. PRC76(2007)
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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 !!

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Even-odd staggering in fission-fragment yields
Global even-odd staggering
?z ?Yze- ?Yzo/(?Yze?Yzo) ?z 40
Local even-odd staggering
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Qualitative understanding of the even-odd
structure
Without dissipation there would be no odd-Z
fragment
MeV
Pairing gap
229Thn
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

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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
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Electromagnetic induced fission of secondary beams
K.-H. Schmidt et al., NPA665(2000)221
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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
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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
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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
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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
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Systematics on even-odd staggering
Ra,Rn
U,Th
Constant e-o staggering at symmetry !! Important
impact on our understanding Of fission dynamics
fissility
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E-o effect at symmetry neutron-induced fission
Difficult to measure Z yields at symmetry in
direct kinematics
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E-o effect at symmetry in n-induced
fissionconstant with fissility ?
?p global ?p local asy(Z54) ?p local reachable
sym
No conclusion can be drawn due to the lack of
data at symmetry
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Statistical description of the even-odd effect
for asymmetric split
GSI data reproduced with
Probability to have nZ proton pairs broken at
scission
nZ0 nZ2 nZ4 nZ6
E-o staggering
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Statistical description Estimated dissipated
energy for asymmetric split
symmetric fission Common asymptotic energy
?5 lt-gt Edis 9 MeV Asymmetric fission 232Th
236U 240Pu X 34.9 35.7 36.8 ? 0.32 0.25
0.1 5.7 6.2 7.1 MeV
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Neutron evaporation and energetic balance
Cm
U
Cf
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Dissipated energy deduced from neutron
evaporation
Qmaxmax(MCN-MF1-MF2)) TKE from experiment
236U
252Cf
248Cm
244Cm
And compared to statistical analysis of e-o
staggering
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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 gtTransfer-induced fission
  _at_GANIL

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Additional diapositives
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Electromagnetic induced fission of secondary beams
E distribution ltEgt 12 MeV for all pre-actinides
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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

• N the 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

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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 (p0.5)
• the amplitude of ?n (Edissn2Edissp)
• the even-odd structures in odd-Z fissionning
  systems (q1)

S. Steinhauser et al., 1998 M. Davi et al., 1998
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Isotopic distribution in direct kinematics
Rochman PhD, Lohengrin 2001
Lohengrin (ILL) -Only the LIGHT fragments are
identified gtNo experimental evidence of shell
effects in heavy fragments
Exfor data base
Radiochemical methods Small part of the
distribution distortions in the neutron yields
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Advantage of inverse kinematics

• High radioactivity
• the production of samples for irradiation
  is difficult
• (gtsystematics 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

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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 N86,88,90 ?? Still under
debate!!
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Counting rates
Reasonable statistics 104 fission
events detected Acceptance of VAMOSTIARA 105
fission events Thin secondary target
6 1019at/cm2 d Secondary target limited by
energy resolution XS Cd2 lt0.5mg/cm2 ?fis
5mbarn
Total number of actinide NincNfis/(?fis
Ntar) 3 1011 Primary
target limited by the 2nd beam kin. Energy alpha
acceptancegt1mg/cm2 Ninc ?fus Ntar Iinc
time?q 5 10-277 10195 10101.3
1060.2
3 109 Primary beam intensity
gtx20 Fusion evaporation ltx2 Gas
secondary target gtx30 Impinging energy
x2
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Advantages
reaction with cross section gtmb gt sufficient
statistics
Disadvantage
Imprecision on the excitation energy
(excitation energy distributed to
ejectile) Threshold ??
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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)