Production and beta decay lifetimes of heavy neutron-rich nuclei approaching the r-process path Teresa Kurtukian-Nieto Universidad de Santiago de Compostela (Spain) Hirschegg 2006 Astrophysics and Nuclear Structure January 15 to 21, 2006

Robustness of the closed neutron shell N=126 with the neutron excess Evolution of collective structure and shapes Motivation Understanding the astrophysical r- process close to the waiting point Reproduce the abundance pattern The solar system r-process abundances as determined by Käppeler et al. (1989)

Motivation J. Benlliure et al.,Nucl. Phys. A 660 (1999) 87 Production of heavy neutron-rich nuclei in cold-fragmentation reactions (S227): r-process nucleosynthesis 208 Pb(1000 A MeV)+Be : production cross sections of neutron-rich nuclei around N=126 measurements of  half lives Knowing  half-lives is important: Half-lives determine the r-process time scale at the r-process “ladders” (N=50, 82, 126) where abundances peak But also the others are important because affect abundances between peaks.

Experimental method 194 W 2h 41 min 186 Lu 10 h 45 min A/Z 1. Identification and implantation of heavy neutron-rich nuclei (FRS-GSI): 30 new heavy neutron-rich nuclei

Production cross sections EPAX COFRA J. Benlliure et al.,Nucl. Phys. A 660 (1999) 87

Implantation technique 4 double-side strip detectors: surface: 5x5 cm 2 thickness 1 mm 2x mm strips Detectors: ionization chamber (reaction rejection in the stopper) 2 scintillator detectors (veto) Centered on 198 Ir Produced Implanted Experimental setup for β’s half-lives Position correlation

Implantation-decay correlations with large background Forward-time correlated  : Backward-time correlated  : Spill length 3 s Cycle 10 s Time Implantation Background evaluation: backward-time correlations  Forward time Backward time

Half-live determination: Fit to an analytical function: exponential decay + const/Poisson-distributed background Chi 2 of the ratios in forward/backward time of the measured data with respect to a numerical function Monte Carlo simulation of time correlation distributions with the experimental conditions found at the FRS and two free parameters: lifetime and efficiency

Forward/backward time ratios for 195 Re

Previous experimental data: T 1/2 ( 198 Ir) = 8 ± 3 s A. Szalay et al., Radiochem. Radioanal. Letters 14/2/136 (1973) Our result:  = 12.5 s (T 1/2 = 8.6 ± 1 s ) 2D Chi 2 map:  versus efficiency for 198 Ir 77 Good agreement between our measurement and the previous one

In general theoretical predictions do not reproduce the measured half lives But Gross Theory by Tachibana et al, seems to works better than QRPA(GT) + Gr. Th. (ff) It would be nice to compare with other theoretical models like I. N. Borzov β-decay time correlations QRPA (GT) + Gross Th. (ff) : P. Möller, B. Pfeiffer and K.-L. Kratz. Phys. Rev. C 67, (2003) Gross Theory :T.Tachibana, M.Yamada, Proc. Int. Conf on exotic nuclei and atomic masses, Arles, 1995, p.763

 We have demonstrated that is possible to explore the region of the heavy neutron-rich nuclei  Cold-Fragmentation constitutes a suitable reaction mechanism to produce heavy neutron-rich nuclei. We have a tremendous potential in future at GSI-FAIR  Around 30 new neutron-rich nuclei close to N = 126 have been identified  It is technically possible to implant those nuclei and to make position and time correlations to determine β half-lives  In spite of the pulsed beam structure we have introduced a technique to evaluate the β background.  Preliminary half-lives of some new heavy neutron-rich nuclei near N=126 have been measured.  In general the existing theoretical models in this region (Möller et al and Tachibana et al ) do not reproduce the experimental data.  New measurements coupling with  ’s (RISING) will be performed next months Summary

S227-Experiment Collaborators J. Benlliure, E. Casarejos, D. Cortina-Gil,M.Fernandez- Ordoñez, J. Pereira Universidad de Santiago de Compostela, Spain F.Becker, D. Henzlova, B. Jurado,K.-H. Schmidt,O. Yordanov Gesellschaft für Schwerionenforschung mbH, Darmstadt, Germany L. Audouin F. Rejmund Institut de Physique Nucleaire, Orsay, France B. Blank, J.Giovinazzo CENBG Bourdeaux-Gradignan, France