Developing an “Atomic Clock” for Fission Lifetime Measurements
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Transcript of Developing an “Atomic Clock” for Fission Lifetime Measurements
Developing an “Atomic Clock” for Fission Lifetime Measurements
H.W. Wilschut and V.L. Kravchuk
Kernfysisch Versneller Instituut
Groningen, The Netherlands
Outline : Introduction: dissipation, friction, viscosityfrom to how fast?
The atomic clock: direct K-shell ionization too little K, too slow, too much background?
Experimental results: fission-X ray-PLFConclusions and outlook: viability of method
1 >> 1
pote
nti
al
deformation
Fission: Bohr-Wheeler vs. Kramers
fission
neutron
BW2
f γ)Γγ1(Γ
Do we know ?
2
5.2102
105
2 21
21
Wall-Window dissipation (H.T.Feldmeier)
disskin t
E
E
1
2222 yxzQzz
strongly damped varies 2.5-10
Mean-field (BUU)
)(sin 02 tteE t
zz
13.0106.02
1015.021
21
Mean-field underdampedNN collision no effectcf. Larionov et al. PRC61(00)064614
How large is ?• Compare with damped heavy ion collisions
– One-body dissipation: window-wall: overdamped (2.5<<10)strongly shape dependent
– One-body dissipation: BUU/BNV: underdamped (0.2)
• Consider damping of Giant Resonances– (hot) isovector GDR …. need isoscaler GQR
• Fission-evaporation competition– Prescission neutrons , GDR , evaporation residues …
strongly model dependent, fixed , slowing ticks of clocks cf. Dioszegi PRC61(99)024613 (but overdamped)
• Direct time measurement needed: – relate to independent process: crystal blocking and
X-ray methods
25.0GQRE
Current Results with Atomic Techniques• 24 MeV/u 238U+28Si• E* determined from <Mn>• Uncertainty in Z of the fission nucleus F. Goldenbaum et al. PRL 82(99)5012
Evidence long lived fission component > 10-18 s in hot nuclei (T 2 MeV)Nuclear methods 10-19 s
K-shell hole has K 610-18
O.A. Yuminov et al. Journ. Phys. Soc. Jap. 70(01)689
J.D. Molitoris et al. PRL 70(93)537
U+U collisions (M.O. X-rays) ?
KFK
KKF NNP
N
20Ne30 MeV/u
16O
Th
L-shellKX-ray
K = 610-18 sPK = 1.7%
K-shell
K X-Ray Direct Ionization Method
UE* =120 MeV T 2 MeV J = 20
Direct K-shell Ionization Probabilities
30 MeV/u 20Ne + 120Sn, 159Tb, 208Pb, 232Th
0.0170.81P
Pz
z
2
1P
2
1O)(NeP
Ne
NeNe
K
2
1/2K
Ne
01/2KK
K-shell hole creation probability obeys scaling for < 1. Checked validity with elastic – KX-ray coincidences.V.L. Kravchuk et al. PRA 67 (03)052709
For Ne + Th O + Uat 30 MeV/u
Characteristic X-Ray Spectra Fission Lifetime
Critical value to observe a characteristic K x-ray line shape is at 20
(I.e. >10-19 s for U as a Compound-Nucleus). Use shape and yield
K
K
K2 K1
K1
K3
K/2
need better theory……..!
Ain,Astick,Aout Ain,Astick,Aout
Experimental Setup
Triple Coincidence Experiment
The Observed X-ray Spectra
• Average count rate 25 kHz• Highly intensive L x-rays were
stopped with 2 mm Al• efficiency 1% of 4
inclusive spectrum coincidence spectrum with Oxygen
Not much left!
standard characteristic components
PK(Th) = 0.0027 PK(U) = 0.00026
Current status for Th and U
standard shape for Th
modified for U assume f=210-19 s
PK(Th) = 0.0017 PK(U) = 0.00098
Why Th?channel selection incomplete
Oxygen trigger contains 70 % O binary channel 20 % O + 10 % O + H
Standard shape Th:f=9.510-19 s consistentU : f=9.210-20 s inconsistentModified shape Th:f=5.910-19 s marginalU : f=3.510-19 s consistent
Shape vs PK is an extra!
Comparison and Possible Pitfalls
• More consistent with nuclear methods
• Are we looking at the same nucleus?
• Single fission lifetime? (isomers)
• shape of background (fission -rays)
• normalization (channel partition)
Viability of the K-hole method
• Consistency shape and time• Lower time threshold (Anholt): 20 ( 10-19 s)
also limited by shape of background• Upper time threshold: none (yield only)
consistency resolution limited: 1keV 10-18
s
• Fold in fission time distribution (other than exp(-t/f))
• Use larger PK (Ne Ar ? )
• Look at L X rays (PL PK)
Conclusions
• Friction in fission: an unresolved problem• Atomic clock based on K-shell holes adds a new
tool to study fission lifetimes > 10-19 s
• High yields in K X ray region (= high PK) are manageable
• The results till now contradict other direct methods, but support indirect (nuclear) methods
• Improvements are possible
V.L. Kravchuk, F. Fleurot, M. Hunyadi, S. Kopecky, A. Krasznahorkay,H. Löhner, A. Rogachevskiy, R.H. Siemssen; 98PR1760
How large is ?
• Compare with damped heavy ion collisions– One-body dissipation: window-wall– One-body dissipation: BUU/BNV
• Consider damping of Giant Resonances– (hot) isovector GDR …. need isoscaler GQR
• Fission competition– Prescission neutrons , GDR , evaporation residues …
strong model dependence
• Direct methods needed crystal blocking X-ray methods
HICOL 1 2
BUU
FISSION DETECTORSFISSION DETECTORS
• 2 multiwire gaseous fission detectors
• Operated with low-pressure (5 Torr) isobutane gas
• Placed inside the vacuum chamber
• Solid angle covered:=22.6% each
• Intrinsic efficiency for the fission fragments:about 100%
• Average count rate:25 kHz for each
E(FD-1) VS E(FD-2)
Experimental Setup
TRIPLE COINCIDENCE EXPERIMENT
FORWARD WALL
• 26 E-E phoswich detectors• 1 mm NE102A scintillator as E• 5 cm NE115 scintillator as E
• Average count rate: 17 kHz• element separation for reaction channels
“Energy”
“PID”
NeFONCB BeLi HeH
8Be
H He Li
Be B C N O F
Ne
EXPERIMENTAL SETUP
TRIPLE COINCIDENCE EXPERIMENT
Fission barriers of U isotopes
PRL 80(98)2073; NPA590(95)680
Triple humped barrier persists in Th-U region
INTRODUCTIONINTRODUCTION
• Bohr-Wheeler statistical modelfor nuclear fission
*2N
dBW
• Kramers approach:fission process described as diffusion over the fission barrier
)1( 2 BW
2 4
• Modern theoretical models (multi-dimensional Langevin approach) shows that fission process is strongly dissipativeMOTIVATION: Fission time scale measurement is the way to determine how viscous is hot nuclear matter
BW
NEUTRON MULTIPLICITIESNEUTRON MULTIPLICITIES
prenpreF M
0
||
||
*
0
)(),*(
)*,(2
)12()*,(
l
lIJ
lIJ
BE
iilii
ii
i
dTJBE
IE
sIE
i ipre
• Highly model dependent• Charged particles emission is not
considered• Last neutron takes longest. Inaccuracy in
fission time scale due to this fact. • The long lived fission component is not
accounted for in the analysis
GDR GAMMA-RAY MULTIPLICITIESGDR GAMMA-RAY MULTIPLICITIES
• Same disadvantages as for neutronmultiplicities
SUMMARY OF THE SUMMARY OF THE
EXPERIMENTAL STATUSEXPERIMENTAL STATUS NEUTRON MULTIPLICITIESNEUTRON MULTIPLICITIES CRYSTAL BLOCKINGCRYSTAL BLOCKING
• K. Siwek-Wilczyńska et al. Phys. Rev. C51 (1995) 2054* D.J. Hinde et al. Phys. Rev. C45 (1992) 1229 V.A. Rubchenya et al. Phys. Rev. C58 (1998) 1587
• I. Gontchar et al. Europhys. Lett. 57 (2002) 355
NO CLEAR UNDERSTANDING - OTHER METHODS NEEDEDNO CLEAR UNDERSTANDING - OTHER METHODS NEEDED
J.D. Molitoris et al. Phys. Rev. Lett. 70(1993)537
O.A. Yuminov et al. Journ. Phys. Soc. Jap. 70(2001)689
20Ne 30A MeV
16O
232Th 236U*
K 610-
18s PK=2% E*=115 MeV
U x ray
• Direct method• Clear separation between atomic
physics of the K-shell hole production and nuclear physics
• Atomic process is quantitatively known • Excitation energy is well defined • Z of the fission nucleus is certain:
unique K x-ray energies for >0.02K
KVI X-RAY METHODKVI X-RAY METHOD
• 20Ne16O 70% transfer (U K x rays)<E*>=115 MeV=35 MeV
• 20Ne*16O+ 30% break-up
(Th K x rays)<E*>50 MeV
Systematics of fragmentation reactionsSystematics of fragmentation reactions
SHAPE OF THE K X-RAY SPECTRA RESULTING IN SHAPE OF THE K X-RAY SPECTRA RESULTING IN HEAVY-ION REACTIONSHEAVY-ION REACTIONS
• K x rays due to Direct Ionization and Internal Conversion processes
• Characteristic fingerprint of each element
• The effect of additional L-shell ionization changes K peak shape
• Never more then one additional L-shell hole created
20, 80 MeV/u 4He, 12C, 16O, 20Ne + 181Ta, 208Pb, 232Th V.L. Kravchuk et al. Phys.Rev. A64(2001)062710
DATA ANALYSISDATA ANALYSIS
TRIVIAL APPROACHTRIVIAL APPROACH
)exp()( ttNPdt
dNKKCNK
K
)/exp()( FFCN tNtN
IMPORTANT ASSUMPTION:IMPORTANT ASSUMPTION: CHARACTERISTIC K X-RAY SHAPE IS NOT CHARACTERISTIC K X-RAY SHAPE IS NOT
AFFECTED BY THE FISSION LIFETIMEAFFECTED BY THE FISSION LIFETIME
)( KFKK
KF NNP
N
DATA ANALYSISDATA ANALYSIS
sss DLS211819 104,106,10
NON-TRIVIAL APPROACHNON-TRIVIAL APPROACH
dTTDTd
dP
D
FF
F 2|),(|)/exp(1),(
)))//(()2/1(
2/12
1
11()
1(
4)//(
1)/,(
2222 FFK
FK
FKFK
FK
K
F EE
PX
SLFC + LLFC ANALYZING SLFC + LLFC ANALYZING METHODMETHOD
)/,()/,()/,( LSF PbPaP SLFC
LLFC
NUMERICAL RESULTSNUMERICAL RESULTSTRIVIAL APPROACHTRIVIAL APPROACH
1501500KN
017.0KP1
18106
1
sK
8103.5 FN
01.0X
sF19104
3.0URGENCY FOR HAVING TIME URGENCY FOR HAVING TIME
DISTRIBUTIONDISTRIBUTION
O-gated
SUMMARYSUMMARY
THE PROBABILITY TO CREATE THE K-SHELL HOLE IS ABOUT 2% WHICH IS SUFFICIENT FOR
PERFORMING THE COINCIDENCE EXPERIMENTS
PRESENCE OF TARGET K X-RAYS INDICATES A LARGE FRACTION OF LONG LIVED FISSION
LIFETIME COMPONENT OF 610-18 s @ E* 50 MeV
FOR HIGHER EXCITATION ENERGY (115 35) MEV SHORT LIVED FISSION COMPONENT (10-19 s) IS DOMINATING
WE DEVELOPED AN ATOMIC CLOCK METHOD FORMESURING THE FISSION LIFETIME DISTRIBUTION
OUTLOOKOUTLOOK
FINAL ANALYSIS NEEDS TO BE DONE
X-RAY METHOD CAN BE USED IN NUCLEAR
REACTION TIME MEASUREMENTS FOR >(>)20
K-SHELL IONIZATION FOR LIFETIME MEASUREMENTS IN TRANSFER REACTIONS WITH
ADVANCED PLF DETECTION SYSTEM
IT MAY BE WORTHWHILE TO USE L-SHELL IONIZATION FOR LIFETIME MEASUREMENTS IN FUSION REACTIONS
ACKNOWLEDGMENTSACKNOWLEDGMENTS
• H.W. Wilschut• H. Löhner• F. Fleurot• M. Hunyadi• A. Rogachevskiy• R.H. Siemssen
KVI, THE NETHERLANDSKVI, THE NETHERLANDS
ATOMKI, HUNGARYATOMKI, HUNGARY
• A. Krasznahorkay
JYFL, FINLANDJYFL, FINLAND
• S. Kopecky
SCHEMATIC ENERGY LEVEL DIAGRAM OF TRANSITIONS SCHEMATIC ENERGY LEVEL DIAGRAM OF TRANSITIONS FILLING A K-SHELL VACANCYFILLING A K-SHELL VACANCY
017.081.02
1
2
12
2/102/1
0
NeNeNeNe KKNe
KK PPz
zPP
• 30 MeV/u 20Ne+232Th PK = 0.021• 30 MeV/u 20Ne+232Th236U*+16O
• Transitional behavior from the United Atom (UA) to the Separated Atom (SA) approximation for the reduced velocity about 1
tM
T
22
FIRST RESULTSFIRST RESULTSSLFC+LLFC FIT(NON-TRIVIAL APPROACH)SLFC+LLFC FIT(NON-TRIVIAL APPROACH)
• SLFC+LLFC procedure gives better overall fit
• For low E* LLFC is found (target K x-rays clearly seen)
• For higher E* SLFC is dominating
O-gated C-gated
FIRST RESULTSFIRST RESULTSCHARACTERISTIC FIT(TRIVIAL APPROACH)CHARACTERISTIC FIT(TRIVIAL APPROACH)
O-gated
• Excess yield in the energy region of interest
• Presence of target K x-rays
• Trivial approach can be applied only to fit target K x-rays
• Transfer channels require SLFC+LLFC procedure
Lifetime of ‘hot’ fissioning nuclei
• Bridges nuclear structure and reaction dynamics• Extreme shapes of nuclei• Large-scale motion in nuclei • Friction and viscosity (zero vs first sound)• Temperature dependence of nuclear dissipation
(phase transition?)• Obstacle: model dependence of time measurements.
Absolute fission time measurement possible?