1. New technique to determine beta half-lives in complex background conditions T. Kurtukian-Nieto 1, J. Benlliure 1, K.-H. Schmidt 2, L. Audouin 3, F. Becker 2, B. Blank 4, E. Casarejos 1, M. Fernandez-Ordoñez 1, J. Giovinazzo 4, D. Henzlova 2, B. Jurado 4, J. Pereira 1, F. Rejmund 5, and O. Yordanov 2 1 Universidad de Santiago de Compostela, E-15782 Santiago de Compostela, Spain 2 Gesellschaft für Schwerionenforschung mbH (GSI), D-64291 Darmstadt, Germany 3 Institut de Physique Nucléaire, F-91406 Orsay Cedex, France 4 CENBG, F-33175 Gradignan, France 5 GANIL, Bd Henri Becquerel, BP 55027, 14076 Caen Cedex 5, France

2. 2 Layout Introduction Experiment Numerical analysis method Results Conclusion

3. 3 208 Pb(1 A GeV)+Be (10 7 ions/s) Experiment SIS18/FRS at Darmstadt, Germany Centered on 198 Ir Produced Implanted active stopper 4 double-side strip detectors: surface: 5x5 cm 2 thickness 1 mm 2x16 3.125 mm strips 2 scintillator detectors (veto) Monoenergetic degrader

4. 4 Implantation-decay correlations Time Implantation like Spill length ~ 2 s Cycle 10 s

5. 5 Numerical analysis method Forward time Backward time conventional analysis tools, based on analytical time-distribution functions could not be applied A new analysis procedure has been developed to extract the β- decay half-lives by using a numerical function Background evaluation (uncorrelated events): backward-time correlations

6. 6 The time sequence of fragment implantation and β detection are simulated according to the experimental conditions observed in the FRS: spill sequence fragment implantation rate background rate during spill and pause leaving two free parameters: the lifetimes and the efficiency The code produces time-correlation spectra in forward- and backward-time direction. Monte-Carlo simulation

7. 7 Fitting procedure: example spill+pause only pause

8. 8 Fitting procedure: example The ratios typical behaviour: Starts with a value larger than one. It decreases and crosses the value of one at about 2·. At still larger times, the ratio decreases further due to the 'shielding' of background correlations in forward time direction We learn that it is necessary to make time correlations covering several spills in order to distinguish between different lifetimes. only pause spill+pause

9. 9 Results for 195 Re : T 1/2 = s spill+pause only pause T 1/2 = s Fitting procedure: example

10. 10 Applicability of the method N F be the total number of implanted fragments the beta detection efficiency the background rate of beta-like signals Nº. of 'true' beta decays detected during a time T 1/2 after the implantation The N. of beta-like background detected in the same time is From the experience we gained during the analysis we found that we can expect to obtain reliable results with our analysis method if

11. 11 Results NuclideT 1/2 / s (experiment) Gross Theory 1 FRDM + RPA + Gross Theory 2 202 Ir 125 11±38.568.4 199 Ir 122 6 (+3 -5)96.6370.6 198 Ir 121 8±2377.1 200 Os 124 6 (+4 -3)16187.1 199 Os 123 5 (+4 -2)17.2106.8 196 Re 121 3 (+1 -2)5.13.6 195 Re 120 6±110.33.3 194 Re 119 1±0.516.170.8 1 T.Tachibana, et al. Proc. ENAM95, Arles, 1995, p.763 2 P. Möller, et al. Phys. Rev. C 67, 055802 (2003) (incl. deformation and 1. forbidden)

12. 12 Conclusions We have developed a new numerical method for extracting the half- life information from delayed-coincidence experiments in the case of complex background conditions. The method is also applicable for any other kind of complex time- dependent background. With this method, we have provided a tool which considerably extends the possibilities for exploiting the manifold advantages of the in-flight technique in determining half-lives of exotic nuclei. Beta half lives have been measured for 8 new neutron-rich isotopes. Many measured values deviate strongly from the Gross Theory and from the FRDM + RPA + Gross Theory. (T 1/2 values are smaller than expected. r-process is faster.)