Progress in half lives of nuclei approaching the r-process path at N=126 José Benlliure Universidad de Santiago de Compostela, Spain INPC 2007
Motivation One of the most relevant questions of physics in the new century is the origin of heavy elements in the Universe (Connecting quarks with the cosmos, USA NRC repport 2003) The r-process is though to be one on the main actors in the production of heavy elements in stellar nucleosynthesis Very little is known about the stellar sites where the r-process occurs, but still less about the nuclei involved in this process Nuclei around the waiting point A=195 are completely unexplored J. BenlliureINPC 2007
Motivation Our present understanding of the r-process around A=195 is far from being complete: - shell quenching at N=126 - nuclear deformation J. BenlliureINPC 2007
Motivation Our present understanding of the r-process around A=195 is far from being complete: - shell quenching at N=126 - nuclear deformation Our goal: - investigate the production of heavy neutron-rich nuclei - measured its -decay half-lives J. BenlliureINPC 2007
Production of heavy neutron-rich nuclei - Mainly abrasion of protons - Low excitation energy Projectile fragmentation in peripheral heavy-ion collisions at relativistic energies: Experimental technique: - large fluctuation in the N/Z of the projectile residue - large fluctuations in the excitation energy induced in the collision - good isotopic identification (charge states rejection) Cold-fragmentation channels: J. BenlliureINPC 2007
Production of heavy neutron-rich nuclei Experiment: 208 Pb(1 A GeV)+Be (10 7 ions/s) FRS at GSI One of the challenges of the experiment was the iden- tification of charge states Beam energy (~ 700 A MeV) FRS+energy degrader J. Benlliure
Production of heavy neutron-rich nuclei Results: 208 Pb(1 A GeV)+Be 25 new heavy neutron-rich nuclei “south” of lead have been produced Their production cross sections allow us to investigate the predictive power of different reaction models EPAX COFRA J. BenlliureINPC 2007
Measurement of half lives Experimental technique and setup: Ion-beta time-position correlations using an active stopper Detectors: ionization chamber (reaction rejection in the stopper) 2 scintillator detectors (veto) 4 double-side Si strip detectors: surface: 5x5 cm2 thickness 1 mm 2x mm strips Position correlation Setting centerd on 198 Ir produced implanted J. BenlliureINPC 2007
Measurement of half lives Time correlations under complex background conditions: pulsed beam + activation due to long-lived neighbouring nuclei Spill length 3 s Cycle 10 s Implantation decay Time Forward-time correlated : Backward-time correlated : J. BenlliureINPC 2007 Forward time Backward time
Measurement of half lives Fitting procedure: - fit of the measured ratio of forward/backward correlations with simulated ratios - Fitting functions are obtained from Monte Carlo simulations including spill sequence, ion implan- tation rate, -decay lifetime, e - background frequency, during spill and pause, and e - efficiency -The only free parameters of the fit are the lifetime ( ) and the detection efficiency ( ) J. BenlliureINPC 2007
Results Results for 195 Re (analysis during beam pause): T 1/2 = s = 9 s = 30% J. BenlliureINPC 2007
Results Results for 198 Ir (benchmarcking of the method): Previously measured value: T 1/2 ( 198 Ir) = s A. Szalay et al., Radiochem. Radioanal. Letters 14/2/136 (1973) This work: T 1/2 ( 198 Ir) = s J. BenlliureINPC 2007
Results Measured half lives: half lives of 8 new neutron-rich nuclei and an upper limit for other 5 nuclei have been determined J. BenlliureINPC 2007
Results Bechmark of model calculations: RPA ( Moller et al.) clearly overestimate the measured half lives close to the shell CQRPA ( I. Borzov et al.) assuming almost spherical nuclei but with a realistic description of first forbidden transitions describes the data close to the shell First forbidden transitions seem to play major role in decay close to the neutron shell N=126 Some half lives may also indicate the influence of deformation J. BenlliureINPC 2007 Nuclei Exp. Data Gross theory [1] RPA(GT)+ f(Gr.Th) [2] DF3+ CQRPA [3] 203 Pt< 11 s41.1 s564.0 s12.7 s 202 Ir s8.5 s68.4 s9.8 s 201 Ir< 42 s18.5 s130.0 s 200 Ir< 30 s13.3 s124.1 s 199 Ir s96.6 s370.6 s46.7 s 198 Ir s377.1 s19.1 s 200 Os s16.0 s187.1 s6.9 s 199 Os s17.2 s106.8 s6.6 s 196 Re s5.1 s3.6 s1.4 s 195 Re s10.3 s3.3 s8.5 s 194 Re s16.1 s70.8 s2.1 s 190 Ta< 6 s8.5 s1.4 s1.5 s 189 Ta< 14 s34.7 s96.8 s10.0 s [1] T.Tachibana, et al. Proc. ENAM95, Arles, 1995, p.763 [2] P. Möller, et al. Phys. Rev. C 67, (2003) [3] I. N. Borzov Private Communication
We have demonstrated that it is possible to explore heavy neutron-rich nuclei Around 25 new neutron-rich nuclei close to N = 126 have been produced using cold-fragmentation reactions It is technically possible to implant those nuclei and performe 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 half lives (or upper limits) of 13 new heavy neutron-rich nuclei near N=126 have been measured. Recent model calculations (CQRPA, I. Borzov) show an improved prediction power compared to standard RPA (Möller et al) and Gross Theory (Tachibana et al). First-forbidden transitions close to the shell and deformation far from the shell seems to strongly influence decay of heavy neutron-rich nuclei New measurements including - coincidences are being performed with RISING New RNB facilities like RIKEN or FAIR will offer a tremendous potential to further investigate the r-process path around the waiting point A = 195 Summary and conclusions INPC 2007
S227-Experiment Collaboration T. Kurtukian, J. Benlliure, E. Casarejos, D. Cortina-Gil, M.Fernandez- Ordoñez, J. Pereira Universidad de Santiago de Compostela, Spain K.-H. Schmidt, F.Becker, D. Henzlova, O. Yordanov Gesellschaft für Schwerionenforschung mbH, Darmstadt, Germany F. Rejmund GANIL, Caen, France B. Blank, J.Giovinazzo, B. Jurado CENBG Bourdeaux-Gradignan, France