Developing an "Atomic Clock" for Fission Lifetime Measurements

1. Outline : Introduction: dissipation, friction, viscosity fromtohow fast? The atomic clock: direct K-shell ionization too little K, too slow, too much background? Experimental results: fission-X ray-PLF Conclusions and outlook: viability of method Developing an “Atomic Clock” for Fission Lifetime Measurements H.W. Wilschut and V.L. Kravchuk Kernfysisch Versneller Instituut Groningen, The Netherlands

2.  1 >> 1 fission neutron Do we know ? Fission: Bohr-Wheeler vs. Kramers potential deformation

3. Wall-Window dissipation (H.T.Feldmeier) strongly damped  varies 2.5-10

4. Mean-field  underdamped NN collision  no effect cf. Larionov et al. PRC61(00)064614 Mean-field (BUU)

5. 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 clockscf. Dioszegi PRC61(99)024613(but overdamped) • Direct time measurement needed: • relate to independent process: crystal blocking and X-ray methods

6. 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 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) ? Evidence long lived fission component > 10-18 s in hot nuclei (T  2 MeV) Nuclear methods 10-19 s K-shell hole has K 610-18

7. U E* =120 MeV T  2 MeV J = 20  16O KX-ray K X-Ray Direct Ionization Method K-shell 20Ne 30 MeV/u Th K = 610-18 s PK= 1.7% L-shell

8. Direct K-shell Ionization Probabilities 30 MeV/u 20Ne + 120Sn, 159Tb, 208Pb,232Th 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 + U at 30 MeV/u

9. Characteristic X-Ray Spectra  Fission Lifetime Ain,Astick,Aout Ain,Astick,Aout K K2 K1 K K3 K1 K/2  need better theory……..! 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

10. Experimental Setup Triple Coincidence Experiment

11. The Observed X-ray Spectra Not much left! inclusive spectrum coincidence spectrum with Oxygen • Average count rate 25 kHz • Highly intensive L x-rays were stopped with 2 mm Al • efficiency 1% of 4

12. standard shape for Th modified for U assume f=210-19 s standard characteristic components PK(Th) = 0.0027 PK(U) = 0.00026 PK(Th) = 0.0017 PK(U) = 0.00098 Modified shape Th:f=5.910-19 smarginal U : f=3.510-19 s consistent Shape vs PK is an extra! Current status for Th and U Why Th? channel selection incomplete Oxygen trigger contains 70 % O binary channel 20 % O +  10 % O + H Standard shape Th:f=9.510-19 sconsistent U : f=9.210-20 s inconsistent

13. 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)

14. 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)

15. 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

16. How large is  ? HICOL 12 • 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 BUU

17. FISSION 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)

18. Experimental Setup TRIPLE COINCIDENCE EXPERIMENT

19. FORWARD WALL Ne F O N C B Be Li He H 8Be “PID” H He Li Be B C N O F Ne “Energy” • 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

20. EXPERIMENTAL SETUP TRIPLE COINCIDENCE EXPERIMENT

21. Fission barriers of U isotopes PRL 80(98)2073; NPA590(95)680 Triple humped barrier persists in Th-U region

22. INTRODUCTION • Bohr-Wheeler statistical modelfor nuclear fission • Kramers approach:fission process described as diffusion over the fission barrier • 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

23. NEUTRON MULTIPLICITIES • 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

24. GDR GAMMA-RAY MULTIPLICITIES • Same disadvantages as for neutronmultiplicities

25. SUMMARY OF THE EXPERIMENTAL STATUS NEUTRON MULTIPLICITIES CRYSTAL 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 J.D. Molitoris et al. Phys. Rev. Lett. 70(1993)537 O.A. Yuminov et al. Journ. Phys. Soc. Jap. 70(2001)689 NO CLEAR UNDERSTANDING - OTHER METHODS NEEDED

26. KVI X-RAY METHOD K610-18s PK=2% E*=115 MeV 232Th 236U* 20Ne 30A MeV 16O U x ray Systematics of fragmentation reactions • 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 >0.02K • 20Ne16O 70% transfer (U K x rays)<E*>=115 MeV=35 MeV • 20Ne*16O+ 30% break-up (Th K x rays)<E*>50 MeV

27. SHAPE OF THE K X-RAY SPECTRA RESULTING IN HEAVY-ION REACTIONS 20, 80 MeV/u 4He, 12C, 16O, 20Ne + 181Ta, 208Pb, 232Th V.L. Kravchuk et al. Phys.Rev. A64(2001)062710 • The effect of additional L-shell ionization changes K peak shape • Never more then one additional L-shell hole created • K x rays due to Direct Ionization and Internal Conversion processes • Characteristic fingerprint of each element

28. DATA ANALYSIS TRIVIAL APPROACH IMPORTANT ASSUMPTION:CHARACTERISTIC K X-RAY SHAPE IS NOT AFFECTED BY THE FISSION LIFETIME

29. DATA ANALYSIS NON-TRIVIAL APPROACH SLFC + LLFC ANALYZING METHOD LLFC SLFC

30. NUMERICAL RESULTS O-gated TRIVIAL APPROACH URGENCY FOR HAVING TIME DISTRIBUTION

31. SUMMARY WE DEVELOPED AN ATOMIC CLOCK METHOD FORMESURING THE FISSION LIFETIME DISTRIBUTION 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  610-18 s @ E*  50 MeV FOR HIGHER EXCITATION ENERGY (115  35) MEV SHORT LIVED FISSION COMPONENT (10-19 s) IS DOMINATING

32. OUTLOOK 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

33. ACKNOWLEDGMENTS KVI, THE NETHERLANDS • H.W. Wilschut • H. Löhner • F. Fleurot • M. Hunyadi • A. Rogachevskiy • R.H. Siemssen ATOMKI, HUNGARY • A. Krasznahorkay JYFL, FINLAND • S. Kopecky

34. SCHEMATIC ENERGY LEVEL DIAGRAM OF TRANSITIONS FILLING A K-SHELL VACANCY

35. 30 MeV/u 20Ne+232Th PK = 0.021 • 30 MeV/u 20Ne+232Th236U*+16O • Transitional behavior from the United Atom (UA) to the Separated Atom (SA) approximation for the reduced velocity about 1

37. FIRST RESULTS SLFC+LLFC FIT(NON-TRIVIAL APPROACH) C-gated O-gated • 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

38. FIRST RESULTS CHARACTERISTIC FIT(TRIVIAL APPROACH) O-gated • Trivial approach can be applied only to fit target K x-rays • Transfer channels require SLFC+LLFC procedure • Excess yield in the energy region of interest • Presence of target K x-rays

39. 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?