CNRS Université Bordeaux

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First access to β-decay half-lives of nuclei approaching the r-process waiting point at A = 195

Teresa Kurtukian-Nieto, CENBG Groupe Noyaux Exotiques
Vendredi 30 Mai 2008 à 10h30 - Salle des Séminaires

The astrophysical r-process is responsible for the synthesis of approximately half of the heavy nuclei in the Universe. The abundance distribution of this process clearly shows a dominant feature : large peaks at the nuclear mass numbers A = 80,130,195. The location of these peaks is related to neutron shell closures, which clearly reflects the influence of the nuclear properties in the stellar nucleosynthesis. The present work deals with a measurement on β-decay half-lives of nuclei close to the 126-neutron shell south of 208Pb, which has remained for a long time as a practically unexplored territory. The half-lives of these nuclei determine the relative A = 195 r-process peak abundances as well as the matter flow to even heavier nuclei. The main reason for the progress achieved in the present experiment was the choice of a novel production mechanism : the nuclides of interest were produced by cold fragmentation using relativistic heavy-ion beams. 25 heavy neutron-rich nuclei have been synthesized for the first time and the half-lives of some of them have been determined. The nuclei of interest were identified in flight and implanted into an active catcher. Due to the time structure of the beam from the GSI SIS18 heavy-ion synchrotron, the implantations, the β decays, and background events are influenced by the repetition rate and the length of the beam pulses. As a consequence, conventional analysis tools for determining half-lives, based on analytical time-distribution functions could not be applied. Therefore, a new analysis procedure was developed, which will be explained in details in the talk, as well as the comparison of the measured data with the different theoretical model calculations available in this region of the chart of nuclides. The measured data indicate that the matter flow in the r-process to heavier fissioning nuclei is faster than previously expected.