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?