Super-Heavy elements.

Présentation au sujet: "Super-Heavy elements."— Transcription de la présentation:

1 Super-Heavy elements

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

3 Réaction

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

5 Difficile à distinguer expérimentalement
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
48Ca+238URB=14.14fm RC=11.86fm RLB=9.5fm R V 库仑能 液滴能 RLB RC RB 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
Z = 124 A = 312 Au moins 12 % de noyaux avec temps de vie plus longs que s Z = 120 A = 296 Au moins 10 % de noyaux avec temps de vie plus longs que s Z = 114 A = 282 Pas de noyaux (nombre inférieur au seuil de sensibilité) avec temps de vie plus longs que s Prédictions de stabilité (barrière de fission des noyaux) D’après P. MÖLLER et al., At. Dat. And Nucl. Dat. Tab. 59 (1995) 185 Mesures au GANIL par la technique d’ombre dans les monocristaux 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 Collaboration : RCNP, Osaka: Huzhou Teachers’ College:
Yasuhisa Abe Huzhou Teachers’ College: Caiwan Shen Ankara university: Bülent Yilmaz Université d'Oumelbouaghi Aissaoui Ziar 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)

13 KEWPIE 2 Specificity 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 2 Free 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 2 Results EE/2 Hot fusion (Z=112, 114, 116) Cold fusion
108 110 111 112 113 f 0.3 0.23 0.2 0.15 0.1 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) Minimum time is due to the thermal vibrations of the atoms on reticular plans 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 model Fission 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=124 Only neutrons evaporation
We’ve check with charged particles : it does not change the curves

21 Potentials with structure
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 Bf optimizes fission time: 2barriers≈3x1barrier

22 Toy model with a double well
We solve numerically the Langevin equation including neutron evaporation Ed=∞ We add a Monte-Carlo evaporation scheme using Weisskopf formula Fission time distributions cannot be evaluated with a Langevin code because of the fluctuations…  = 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 Dissipative barriers 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)

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

26 Thank you for your attention
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 ≈ , as detected by INDRA)  Light charged particle multiplicity ≈ , 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 Potentials with structure
Master equations We use Kramers’ rate to jump over the potential barriers T<B, any  fis = r+3.k NLRT formalism Analytical formula: A.N. Malakhov, Chaos 7, 488 (1997) >>2, any T NLRT.k = 3.2 Y. Lallouet, private communication