isotopic yields of fission fragments from transfer- induced fission f. rejmund, m. caama ñ o, x....
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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, FranceK.-H. Schmidt, A. Kelic, GSI, GermanyC. Shmitt IPNL, FranceG. Simpson LPSC,FranceJ. Benlliure, E. Casarejos, USC, SpainL. Audouin, C.-O. Bacri, L. Tassan-Got, IPNO, FranceT. Enqvist, CUPP, FinlandD. Doré, S. Panebianco, D. Ridikas CEA SPhNL. Gaudefroy, J. Taieb CEA DIF
Shell effects in fission-fragment yieldsPresentation of the projectEven-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
PF1E,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. SchmidtExp. data
Wide systematcis on element yields for U fragmentation products
GSI data in inverse kinematics
Af=Zf+Nf
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,,1981
232Th(12C,8Be) 236U234U(t,pf) 235U(n,f)
Multi-nucleon transfer reaction
236U(12C,8Be) 240Pu238Pu(t,pf)239U(n,f)
•Large range of transfer Channels 238U+12CEje Rec Q(MeV) (mb)13C 237U -1.2 2314C 236U 1.8 811B 239Np -10 2512B 238Np -13 513B 237Np -14 0.810Be 240Pu -15 109Be 241Pu -17 58Be 242Pu -12 511Be 239Pu -21 0.87Li 243Am -26 0.56Li 244Am -19 3 4He 246Cm -17 3 6He 244Cm -24 0.5
•High resolution of the fissioning system
Transfer-induced fission reactions: wide range of fissioning systems
• Neutron-rich actinides : 238U beam, 12C Target• Energy range 0-40 MeV
-Inverse kinematics (high Z resolution)-Isotopic identification (spectrometer)-Wide range of actinidesPrecise measure of the excitation energy (particle detection)
Multinucleon induced fission in inverse kinematics@GANIL
238U
12C
recoilheavy FF
light FF
FF
€
A =2Ev2
Bρ =f (x,y, θ,ϕ )
Bρ =AQ
vX,Y,θ,
ToFEE
Identification of fission fragments in VAMOS
€
E ∝Z2
v2
M. Rejmund et al. PRC76(2007)
238U+48Ca
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
€
YG (Z) ≈YG (Z)(1±δZ (Z))
δZ (Z + 32) = 1
8−1( )
Z +1lnY (Z + 3) −lnY (Z) −3 lnY (Z + 2) −lnY (Z + 1)
Global even-odd staggering
δz = Yze- Yz
o/(Yze+Yz
o)δz =40%
Qualitative understanding of the even-odd structure
229Th+n
Pairing gap
saddle scission ?
23090Th
0
5
-25
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
Eintr
+Ecoll
•Even-odd structure : a consequence of dissipation in the descent
Even-odd effect depends on fissility of the system
Global even-odd effect δz = Yz
e- Yzo As the Coulomb repulsion inside
the nucleus increases, the saddle shape becomes more and more compact
Saddle CmSaddle Th
The descent from saddle to scission increases, as Ediss, with fissilityEdiss 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 atsymmetry: Unpaired nucleon chooses both fragments with equal probability
Negative staggering forasymmetry: unpaired nucleon chooses theheaviest fragment
S. Steinhaüser, 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
€
g(Z) ∝ g(A)ZCN
ACN
∝ Z
€
p(Z) ∝g(Z)
g(Z) + g(ZCN −Z)
€
δp
n(Z) =(1−2p(Z))n
δp
n(Z) =(1−2 ZZCN
)n
€
δp(Z) =0.1δp
0 (Z) + 0.9δp
2 (Z)
δp(Z) =0.1δp
1 (Z) + 0 .9δp
3 (Z)
Data reproduced with
Relative statistical weight of 1 nucleon in fragment (Z):
€
p(Z) ∝Z
ZCN
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
€
ρn =gn(E −n)n−1
(n/ 2)!2 (n−1)!Strutinsky 1958
€
P0Z (U) =
ρnZ =0 ,nN (U )nN
∑ρnZ ,nN (U )
nN ,nZ
∑
€
ρn(U) =gn(Ueff )
n−1
(n/ 2)!2 (n−1)!
Ueff =U −14
g( 0 −n) −ΠnIgnatyuk 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 ?
δp globalδp local asy(Z=54)δp 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
€
δp(Z) =0.1δp
0 (Z) + 0.9δp
2 (Z)
GSI data reproduced with
€
δp
n(Z) =(1−2p(Z))n
δp
n(Z) =(1−2 ZZCN
)n
Probability to have nZ proton pairs broken at scission
€
PnZ
Z (U) =
ρnZ ,nN (U )nN
∑ρnZ ,nN (U )
nN ,nZ
∑
€
δp(Z) =P0Zδ
p
0 (Z) + P2Zδ
p
2 (Z) + ..
δp(Z) = PnZ
Z δp
nZ (Z)nZ
∑
nZ=0nZ=2nZ=4nZ=6
E-o staggering:
Statistical description Estimated dissipated energy for asymmetric split
€
δp(Z) = PnZ
Z δp
nZ (Z)nZ
∑
symmetric fission : Common asymptotic energy δ~5% <-> Edis~ 9 MeVAsymmetric fission232Th 236U 240PuX= 34.9 35.7 36.8
δ 0.32 0.25 0.1 5.7 6.2 7.1 MeV
Neutron evaporation and energetic balance
Cf
Cm U
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
Dissipated energy deduced from neutron evaporation…
236U
248Cm252Cf244Cm
And compared to statistical analysis of e-o staggering
Qmax=max(MCN-MF1-MF2))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 fission @GANIL
Additional diapositives
Electromagnetic induced fission of secondary beams
E* distribution<E*> ~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 = Ediss/ the broken pair is a proton pair Zf/Af0.4q break a pair when the required energy is available 0.5p the 2 protons of a given pair to end up into 2 different fragments 0.5
Bag of broken pairs
FF2FF1
Ediss =-4ln(δZ )δZ=(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 δz with Z of the fission fragment (p=0.5)
•the amplitude of δn (Edissn=2*Ediss
p)•the even-odd structures in odd-Z fissionning systems (q=1)
S. Steinhauser et al., 1998M. Davi et al., 1998
Lohengrin (ILL)
-Only the LIGHT fragments are identified=>No experimental evidence ofshell 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: 104 fission events detectedAcceptance of VAMOS&TIARA: 105 fission eventsThin secondary target : 6 1019at/cm2 dSecondary target limited by energy resolution && XS Cd2 <0.5mg/cm2fis ~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: >x20Fusion 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 distributionsAnd the isotopic distributionfrom ISOLDE and GSI
(fissioning system and excitationenergy are model dependent)
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