1 / 29

Super-Heavy elements

Super-Heavy elements. one end of nuclear chart 2008. 294. 120. 119. 290. 291. 292. 293. 118. 287. 288. 117. 286. 287. 288. 289. 290. 116. 278. 283. 284. 115. SHE. 283. 284. 285. 286. 277. 282. 114. 279. 280. 274. 272. 113. 279. 273. 282. 269. 270. 271. 281.

josiah-burks
Download Presentation

Super-Heavy elements

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Super-Heavy elements

  2. one end of nuclear chart2008 294 120 119 290 291 292 293 118 287 288 117 286 287 288 289 290 116 278 283 284 115 SHE 283 284 285 286 277 282 114 279 280 274 272 113 279 273 282 269 270 271 281 112 266 275 276 Rg 268 270 278 270 271 265 275 264 266 267 269 Ds 264 261 262 271 266 267 265 272 Mt 259 263 271 261 264 260 265 266 262 Hs 263 267 258 261 259 260 262 268 263 Bh 257 262 263 267 268 258 259 260 261 261 262 Sg Db Rf 184 162 a-decay A Courtesy of K. Morita (RIKEN) 2008/10/2 Spontaneous fission A b+ or EC decay A FJS at Paris 2

  3. Réaction

  4. KEWPIE 2Principales caractéristiques • Code statistique pouvant calculer des probabilités inférieures à 10-12 en quelques secondes • Code dynamique pouvant calculer des temps de fission très longs (supérieurs à 10-18 s) -> donne des contraintes fortes sur Eshell A. Marchix, thèse Université de Caen, 2007

  5. Réaction Difficile à distinguer expérimentalement

  6. Experimental fusion hindrance C. Sahm et al., Nucl. Phys. A441 (1985) 316

  7. Shematic view of the fusion process 液滴能 V 库仑能 48Ca+238URB=14.14fm RC=11.86fm RLB=9.5fm RC R RB RLB • Can we probe separately : • The fusion barriers • Dissipation

  8. Modèles Coupled channels • Les codes « Coupled channels » peuvent reproduire de façon très précise les sections efficaces de fusion • Voir N. Rowley et al., Phys. Lett. B632 (2006) 243 • Limitations: • Pour des ions très lourds, il y a des problèmes numériques • Une approche purement quantique ne permet pas de connaître les conditions initiales de la deuxième étape de la fusion. • Projet: • Développer un modèle « coupled channels » semi-classique

  9. Caractérisation de la stabilité des éléments super-lourds par leur temps de vie Prédictions de stabilité (barrière de fission des noyaux) Mesures au GANIL par la technique d’ombre dans les monocristaux D’après P. MÖLLER et al., At. Dat. And Nucl. Dat. Tab. 59 (1995) 185 Z = 124 A = 312 Au moins 12 % de noyaux avec temps de vie plus longs que 10-18 s Z = 120 A = 296 Au moins 10 % de noyaux avec temps de vie plus longs que 10-18 s Z = 114 A = 282 Pas de noyaux (nombre inférieur au seuil de sensibilité) avec temps de vie plus longs que 10-18 s Le maximum de stabilité n’est pas à l’endroit prédit par ce modèle Recherche du maximum de stabilité: noyaux doublement magiques? La technique d’ombre dans les monocristaux ne permet pas cette recherche (nécessite cristaux quasi-parfaits) ⇒ Utilisation de l’horloge atomique pour mesurer les temps de fission

  10. RCNP, Osaka: Yasuhisa Abe Huzhou Teachers’ College: Caiwan Shen Ankara university: Bülent Yilmaz Université d'Oumelbouaghi Aissaoui Ziar Collaboration : • GANIL, Caen • D.B.

  11. Le candidat • Zhao-Qing Feng Institute of Modern Physics, Lanzhou, Chine • 16 publications relatives à la problématique

  12. Theoretical fusion hindrance Swiatecki et al, PRC71 (2005) 014602

  13. KEWPIE 2Specificity • It is not a Monte-Carlo code to calculate very low probabilities • It is based on a discretisation in bins of the energy spectra:

  14. KEWPIE 2Free parameters • Shell correction energy -> correction factor • Damping Energy • Originally, Ed=18.5 MeV • Reduced friction =2.1021s-1 A. Marchix, Y. Abe and D.B., in preparation

  15. KEWPIE 2Results • Hot fusion (Z=112, 114, 116) • EE/2 • Cold fusion • Can we trust the fusion cross section ? A. Marchix, PhD thesis, Caen University 2007

  16. Direct evidence for long times • First set of data (2003) 238U + Ni  Z=120 direct evidence for long fission times • Second set of data (2005) 238U + Ge  Z=124 direct evidence for long fission times 208Pb + Ge  Z=114 No hint for long lifetimes Quasi-elastic (target) M. Morjean et al, Eur. Phys. J. D45 (2007) 27 & PRL (2008)

  17. Toy model Bf=Bn • Fission vs neutron evaporation • Solving Bateman equations: • Bn=6 MeV & Bf constant along the chain • Simple analytical solution Fission time Pre-scission neutrons David Wilgenbus, Master report, GANIL 1998, unpublished

  18. Toy modelFission time distribution Bf=Bn/2 Bf=Bn Bf=Bn/5

  19. Fission time of Uranium • Bateman equations are discretized and solved numerically • Data are obtained from crystal blocking techniques • F. Goldenbaum et al, PRL82 (1999) 5012 • Below 50 MeV, asymmetric fission was observed

  20. Fission time distribution of Z=124Only neutrons evaporation Z=114 Z=120 Z=124

  21. Potentials with structure Bf In order to understand the consequences of the potential structure beyond the saddle point two potential shapes has been considered. single-bump shape Some possible double-bump (isomeric) shapes single-bump shape Single-bump shape optimizes fission time: 2barriers≈3x1barrier

  22. We add a Monte-Carlo evaporation scheme using Weisskopf formula Toy model with a double well • We solve numerically the Langevin equation including neutron evaporation • Ed=∞  = 2x1021s-1, E* = 70 MeV, M = A/4, Bn = 6 MeV

  23. Partial conclusions • The results of the crystal blocking experiments means that there are some elements with a large fission barrier in the chain • What about Møller & Nix’s table for the SHE? • Z=114: no contradiction • Z=120 & Z=124: problems • Isomeric structure could enlarge the fission time: • Important for actinides • Not for SHE ? • We cannot exclude other phenomena such as: • the reduced friction that might not be constant • a fission barrier growing with excitation energy (pairing effect)

  24. The fusion process can be approximated as the diffusion over a simple parabolic barrier Effective barrier to have half of the particles to over pass the saddle Y. Abe, D. B., B.G. Giraud and T. Wada, Phys. Rev. E61, 1125 (2000) D. B., Y. Abe and JD Bao, Eur. Phys. J. A18, 627 (2003) Dissipative barriers

  25. Generalized Langevin Equation Three regimes : small  : like markovian with a reduced friction average : oscillations appear large  : friction vanishes Non-Markovian effects D.B., Y. Lallouet, J. Stat. Phys.125 (2006) 477

  26. Conclusions • Challenge on the fission barriers • How to explain the long fission times observed ? • The fusion hindrance give some constraints on the fusion barriers • The neck is a key parameter • For symmetric systems, ok • For asymmetric systems, under progress • Dissipation remains to assessed by other means Thank you for your attention

  27. Characteristics of fragments detected at 20 deg with 60 ≤ Z1 ≤ 85 (U + Ni)  Reaction time longer than 10-18s for at least 10% of the events Z1 + Z2 = 120  Very low intermediate mass fragment multiplicity (MIMF ≈ 4 10-3,as detected by INDRA)  Light charged particle multiplicity ≈ 7 10-2 ,as detected by INDRA Kinematics in good agreement with expectation for fission fragments  Kinetic energy in agreement with Viola systematics

  28. Fission barrier as a function of temperature FTHFB calculations, Laget’s thesis, Orsay 2007

  29. NLRT formalism Analytical formula: A.N. Malakhov, Chaos 7, 488 (1997) >>2, any T NLRT.k = 3.2 Master equations We use Kramers’ rate to jump over the potential barriers T<B, any  fis = r+3.k Potentials with structure Y. Lallouet, private communication

More Related