1 Decay of 135,136 Sn isolated by use of a laser ion source and evidence for a more harmonic-oscillator-like nuclear potential Jason Shergur a , Michael Hannawald b , Darek Seweryniak a,c , Hans Fynbo d , Ulli Koester d , Andreas Woehr e , Dimitri Fedorov f , Valentin Fedoseyev g , Viatcheslav Mishin g , Per Hoff h , Jo Ressler a , Abigail Bickley a , Bernd Pfeiffer b , Haik Simon d , Thomas Nilsson d , Henryk Mach i , Katarina Wilhelmsen Rolander d,j , Helge Ravn d , Karl-Ludwig Kratz b , William Walters a , and the ISOLDE collaboration a Department of Chemistry, University of Maryland, College Park, MD 20742, USA b Institut für Kernchemie, Universität Mainz, D-55128 Mainz, Germany c Argonne National Laboratory, Argonne, IL 60439, USA d ISOLDE, CERN, CH-1211 Geneva 23, Switzerland e Department of Physics, Oxford University, Oxford OX1 3PU, England f Petersburg Nuclear Physics Institute, RAS 188350, Gatchina, Russia g Institute of Spectroscopy, Russian Academy of Sciences, 142092 Troitzk, Russia h Department of Chemistry, University of Oslo, N-0315 Oslo, Norway i Department of Neutron Research Studsvik SE-611 82 Nyköping, Sweden j Department of Physics, Stockholm University, SE-113 85 Stockholm, Sweden The use of a resonance ionization laser ion source at CERN/ISOLDE has made it possible to study the decay of very neutron-rich 135-137 Sn. The decay of 135 Sn is found to populate low-energy levels in 135 Sb via direct beta decay and the first excited state in 134 Sb by beta-delayed neutron emission. The level structure of 135 Sb will be discussed and a possible signature for a more diffuse nuclear surface considered. 1. INTRODUCTION Recent studies of the structure of very neutron-rich Sn and Sb nuclides have given indication that the structure of these nuclides is somewhat different than might have been expected on the basis of extrapolation of the observed structures of lighter nuclides. Moreover, the structure and decay properties of these nuclides play a critical role in r- process nucleosynthesis calculations as these nuclides lie directly in the path of the r-process under a wide range of astrophysical conditions. 2. EXPERIMENTAL CONSIDERATIONS Extensive data for the heavy Sn nuclides up through 134 Sn and for the heavy Sb nuclides up through 136 Sb are available in the literature. Isotopes of Sn, Sb, Te, I, and Xe can be ionized with varying efficiencies in plasma ion sources. As the neutron number increases, the yield drops for the isobars furthest from stability to the point that they can not be observed with the use of conventional ion sources with limited selectivity.