Transcript Isotopic distribution in direct kinematics - Irfu

Isotopic Yields of Fission Fragments from TransferInduced Fission
F. Rejmund, M. Caamaño, X. Derkx, C. Golabek,
J. Frankland, M. Morjean, A. Navin, M. Rejmund
M. Aïche, G. Barreau, S. Czajkowski, B. Jurado
K.-H. Schmidt, A. Kelic,
C. Shmitt
G. Simpson
J. Benlliure, E. Casarejos,
L. Audouin, C.-O. Bacri, L. Tassan-Got,
T. Enqvist,
D. Doré, S. Panebianco, D. Ridikas
L. Gaudefroy, J. Taieb
GANIL, France
CENBG, France
GSI, Germany
IPNL, France
LPSC,France
USC, Spain
IPNO, France
CUPP, Finland
CEA SPhN
CEA DIF
Shell effects in fission-fragment yields
Presentation of the project
Even-odd effects in fission-fragment yields
Fission fragments from irradiation
•
Mass distribution
PF1
n
PF1
•
Isotopic distribution
– Spectrometer
=>light fragments
–  Spectroscopy
=>branching ratio, unknown isomers
•
Limitations due to target activity, neutron energy
E,ToF =>M
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!!
GSI data in inverse kinematics
Wide systematcis on element yields for U fragmentation products
Profi, K.-H. Schmidt
Exp. data
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!!
Multi-nucleon transfer reaction
•High resolution of the fissioning system
232Th(12C,8Be) 236U
234U(t,pf)
235U(n,f)
236U(12C,8Be) 240Pu
238Pu(t,pf)
239U(n,f)
Cheifetz et al,,1981
•Large range of transfer
Channels
238U+12C
Eje
13C
14C
11B
12B
13B
10Be
9Be
8Be
11Be
7Li
6Li
4He
6He
Rec
237U
236U
239Np
238Np
237Np
240Pu
241Pu
242Pu
239Pu
243Am
244Am
246Cm
244Cm
Q(MeV)
-1.2
1.8
-10
-13
-14
-15
-17
-12
-21
-26
-19
-17
-24
(mb)
23
8
25
5
0.8
10
5
5
0.8
0.5
3
3
0.5
Transfer-induced fission reactions: wide range of
fissioning systems
•
•
Neutron-rich actinides : 238U beam, 12C Target
Energy range 0-40 MeV
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
recoil
heavy FF
light FF
FF
Identification of fission fragments in VAMOS
2E
A  2
v
B  f (x , y ,  ,  )
X,Y,
,
A
B  v
Q

M. Rejmund et al. PRC76(2007)
ToF
E
E

Z2
E  2
v
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
Global even-odd staggering
z = Yze- Yzo/(Yze+Yzo)
z =40%
Local even-odd staggering
YG (Z )  YG (Z )(1 Z (Z ))
3 1 Z 1
Z (Z  )  1 lnY (Z  3)  lnY (Z ) 3lnY (Z  2)  lnY (Z  1)
2 8
Qualitative understanding of the even-odd structure
Without dissipation there would be no odd-Z fragment
MeV
Pairing gap
229Th+n
Eintr
+Ecoll
230
5
saddle
scission
?
0
90Th
•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
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
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
Evidence for the influence of the fission-fragment phase space
S. Steinhaüser, PhD Thesis
Statistical analysis of e-o staggering
Relative statistical weight of 1 nucleon in fragment (Z): p(Z ) 
level density at Fermi level in FF
p(Z ) 
Z
Z CN
g (Z )  g (A)
g (Z )
g (Z )  g (Z CN  Z )
Z CN
Z
ACN

 n unpaired uncleons
E-o staggering produced with
 n (Z )  (1  2p(Z ))n
p
Z n
 p (Z )  (1  2
)
Z CN
n

Data reproduced with

p (Z )  0.1 0 (Z )  0.9 2 (Z )
p
p
p (Z )  0.1 1 (Z )  0 .9 3 (Z )
p
S. Steinhauser et al., NPA634(1998)89
p
Probability for a completely proton paired
configuration at scission
Level density of only
broken neutron pairs

n Z  0 ,n N (U )
P0Z (U) 
nN

n Z ,n N (U )
n N ,n Z
Level density of all
possible excitations

g n (E  n)n 1
n 
(n / 2)!2 (n  1)!Strutinsky 1958
g n (Uef f )n 1
n (U) 
(n / 2)!2 (n  1)!

1
Uef f  U  g ( 0   n )  n
4
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
U,Th
Ra,Rn
Constant e-o staggering at symmetry !!
Important impact on our understanding
Of fission dynamics
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
 n (Z )  (1  2p(Z ))n
p
p (Z )  0.1 0 (Z )  0.92 (Z )
p
Z n
 p (Z )  (1  2
)
Z CN
n
p
Probability to have nZ proton pairs broken at scission

n Z ,n N (U )

P (U) 
Z
nZ
nN

n Z ,n N (U )
n N ,n Z
E-o staggering:
Z 0
Z 2

(Z
)

P

(Z
)

P
0
2  (Z )  ..
 p
p
p (Z ) 
P
Z
nZ
nZ
p
 n (Z )
Z
p

nZ=0
nZ=2
nZ=4
nZ=6
Statistical description
Estimated dissipated energy for asymmetric split
p (Z )   PnZ n (Z )
Z
Z
p
nZ

symmetric fission :
Common asymptotic energy
~5% <-> 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
Neutron evaporation and energetic balance
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
Cm
Cf
U
Dissipated energy deduced from neutron
evaporation…
Qmax=max(MCN-MF1-MF2))
TKE from experiment
236U
248Cm
244Cm
252Cf
And compared
to statistical
analysis of
e-o staggering
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
FF1
FF2
Bag of broken pairs
Z=(1-2pq)N
Ediss =-4ln(Z )
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
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
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
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
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!!
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
Total number of actinide:
Ninc=Nfis/(fis Ntar)=
~5mbarn
3 1011
Primary target limited by the 2nd beam kin. Energy &alpha acceptance==>1mg/cm2
Ninc= fus *Ntar *Iinc *time*q
Primary beam intensity: >x20
Fusion evaporation
<x2
Gas secondary target >x30
Impinging energy
x2
=5 10-27*7 1019*5 1010*1.3 106*0.2
=3 109
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)