E. Wilson

First Measurement of Several β-Delayed Neutron Emitting Isotopes Beyond N=126

The β-delayed neutron emission probabilities of neutron rich Hg and Tl nuclei have been measured together with β-decay half-lives for 20 isotopes of Au, Hg, Tl, Pb, and Bi in the mass region N≳126. These are the heaviest species where neutron emission has been observed so far. These measurements provide key information to evaluate the performance of nuclear microscopic and phenomenological models in reproducing the high-energy part of the β-decay strength distribution. This provides important constraints on global theoretical models currently used in r-process nucleosynthesis.

Approaching the precursor nuclei of the third r-process peak with RIBs

The rapid neutron nucleosynthesis process involves an enormous amount of very exotic neutron-rich nuclei, which represent a theoretical and experimental challenge. Two of the main decay properties that affect the final abundance distribution the most are half-lives and neutron branching ratios. Using fragmentation of a primary 238U beam at GSI we were able to measure such properties for several neutron-rich nuclei from 208Hg to 218Pb. This contribution provides a short update on the status of the data analysis of this experiment, together with a compilation of the latest results published in this mass region, both experimental and theoretical. The impact of the uncertainties connected with the eta-decay rates and with beta-delayed neutron emission is illustrated on the basis of r-process network calculations. In order to obtain a reasonable reproduction of the third r-process peak, it is expected that both half-lives and neutron branching ratios are substantially smaller, than those based on FRDM+QRPA, commonly used in r-process model calculations. Further measurements around N 126 are required for a reliable modelling of the underlying nuclear structure, and for performing more realistic r-process abundance calculations.

7Li-induced reactions for fast-timing with LaBr3:Ce detectors

7Li induced-reactions have been used with a 186W target to populate nuclei around A∼180-190 at the National Institute of Physics and Nuclear Engineering in Bucharest, Romania. An array of high-purity germanium (HPGe) and cerium-doped lanthanum bromide (LaBr3:Ce) detectors have been used to measure sub-nanosecond half-lives with fast-timing techniques. The yrast 2+ state in 190Os was measured to be t1/2 = 375(20)ps, in excellent agreement with the literature value. The previously unreported half-life of the 564-keV state in 189Ir has also been measured and a value of t1/2 = 540(100)ps ps obtained.

β -decay half-lives and β -delayed neutron emission probabilities for several isotopes of Au, Hg, Tl, Pb, and Bi, beyond N=126

R. Caballero-Folch, ∗ C. Domingo-Pardo, J. Agramunt, A. Algora, 4 F. Ameil, Y. Ayyad, J. Benlliure, M. Bowry, F. Calviño, D. Cano-Ott, G. Cortès, T. Davinson, I. Dillmann, 5, 10 A. Estrade, 11 A. Evdokimov, 10 T. Faestermann, F. Farinon, D. Galaviz, A.R. Garćıa, H. Geissel, 10 W. Gelletly, R. Gernhäuser, M.B. Gómez-Hornillos, C. Guerrero, 15 M. Heil, C. Hinke, R. Knöbel, I. Kojouharov, J. Kurcewicz, N. Kurz, Yu.A. Litvinov, L. Maier, J. Marganiec, M. Marta, 10 T. Mart́ınez, F. Montes, 18 I. Mukha, D.R. Napoli, C. Nociforo, C. Paradela, S. Pietri, Zs. Podolyák, A. Prochazka, S. Rice, A. Riego, B. Rubio, H. Schaffner, Ch. Scheidenberger, 10 K. Smith, 17, 18, 20, 21 E. Sokol, K. Steiger, B. Sun, J.L. Táın, M. Takechi, D. Testov, 23 H. Weick, E. Wilson, J.S. Winfield, R. Wood, P.J. Woods, and A. Yeremin INTE, DFEN Universitat Politècnica de Catalunya, E-08028 Barcelona, Spain TRIUMF, Vancouver, British Columbia V6T 2A3, Canada IFIC, CSIC Universitat de València, E-46071 València, Spain Institute of Nuclear Research of the Hungarian Academy of Sciences, Debrecen H-4001, Hungary GSI Helmholtzzentrum für Schwerionenforschung GmbH, D-64291 Darmstadt, Germany Universidade de Santiago de Compostela, E-15782 Santiago de Compostela, Spain Department of Physics, University of Surrey, Guildford GU2 7XH, United Kingdom CIEMAT, E-28040 Madrid, Spain University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom Justus-Liebig-Universität Giessen, D-35392 Giessen, Germany St. Mary’s University, Halifax, Nova Scotia B3H 3C3, Canada Physik Department E12, Technische Universität München, D-85748 Garching, Germany Centro de F́ısica Nuclear da Universidade de Lisboa, 169-003 Lisboa, Portugal CERN Physics Department, CH-1211 Geneve, Switzerland Universidad de Sevilla, E-41080 Sevilla, Spain ExtreMe Mater Institute, D-64291 Darmstadt, Germany NSCL, Michigan State University, East Lansing, MI 48824, USA Joint Institute for Nuclear Astrophysics, Notre Dame, IN 46615, USA Instituto Nazionale di Fisica Nucleare, Laboratori Nazionale di Legnaro, I-35020 Legnaro, Italy University of Notre Dame, South Bend, IN 46556, USA University of Tennessee, Knoxville, TN 37996, USA Flerov Laboratory, Joint Institute for Nuclear Research, 141980 Dubna, Russia Institute de Physique Nucléaire d’Orsay, F-91405 Orsay, France

Octupole transitions in the 208Pb region

This work is supported by the Science and Technology Facilities Council (STFC), UK, US Department of Energy, Office of Nuclear Physics, under Contract No. DEAC02-06CH11357 and DE-FG02-94ER40834, NSF grant PHY-1404442.

Study of the neutron rich sulfure isotope 43S through intermediate energy Coulomb excitation

The reduced transition probability B(E2: 3/2− 7/2−2) has been measured in 43S using Coulomb excitation at intermediate energy. The nucleus of interest was produced by fragmentation of a 48Ca beam at GANIL. The reaction products were separated in the LISE spectrometer. After Coulomb-excitation of 43S in a 208Pb target, the γ rays emitted inflight were detected by 64 BaF2 detectors of the Chǎteau de Cristal array. The preliminary value deduced for the reduced transition probability B(E2: 3/2−7/2−2) is in agreement with the predictions of the shell model calculations and supports a prolate-spherical shape coexistence in the 43S nucleus.

Electromagnetic Transition Rate Measurements in the N=80 Isotone, 138Ce

A study of intrinsic state halflife measurements in the N=80 nucleus 138Ce has been made using the 130Te(12C,4n)138Ce fusion evaporation reaction at beam energy of 56 MeV. The fast-timing gamma-ray coincidence method was used with a mixed LaBr3(Ce)-HPGe array to establish the lifetimes of the yrast 6+ state at 2294 keV, the Iπ=5− state at 2218 keV, the Iπ=11+ state at 3943 keV and the 14+ state at that at 5312 keV, all of which are in the sub nanosecond regime. Reduced transition probabilities have been calculated for the electromagnetic decays from these states.

Measurement of

This proposal aims at the measurement of both β-decay half-lives and β-delayed neutron emission probabilities of a number of nuclei near the third r-process peak. Assuming a U beam intensity of 2×10 ions per second we have estimated that the isotopes Tl, 213,214Hg, Au, 208,209Pt and 205,206Ir can be implanted in sufficient intensity for such studies at the final focal plane of the GSI fragment separator. This will allow, for the first time, the measurement of their β-decay halflives and neutron emission branching ratios. The high primary beam energy of 1 GeV/u available at GSI will be crucial for such measurements, in order to avoid contaminations due to incompletely stripped charge states. The β-delayed neutron emission probability of these isotopes is expected to be at least 5%, and their implantation rates between ∼ 10 s and 10 s, which should enable their measurement using a high-efficiency neutron detector. The detection system will also include an state-of-the-art array of DSSSDs for the detection of both implanted ions and β-decays. HPGe-detectors will be used for γ-ray tagging and will help for a precise A/q-calibration by measuring isomers in the neighbouring nuclei. 1 Motivation and introduction The rapid neutron capture process (r process) is characterised by extremely high temperatures of T∼10 K and very large neutron densities of 10 to 10 cm. Although the astrophysical site for this process has not been identified yet, it seems clear that it must be related to explosive scenarios such as type II Supernovae [1], where such cataclysmic conditions are presumably encountered. Many of the uncertainties in our understanding of the r process arise from the vast number of neutron-rich nuclei involved (see Fig. 1), where experimental information is rather scarce, uncertain and in most cases non-existent. Provided that the relevant nuclear physics input parameters could be measured with sufficient accuracy, the observed r-process abundance distribution would reflect the history of the r-process nucleosynthesis, its dynamics and the cosmic site or sites where it takes place. Although the nuclear physics input is rather poorly known it is clear that, the r process shows a prominent influence in the evolution and composition of our Universe, thus it accounts for roughly half of the isotopic abundances observed in the solar system (beyond Iron) and it seems to be the unique mechanism responsible for the existence of the actinide nuclei. Furthermore, nucleochronometry based on the long lived isotopes Th and U has recently attracted great interest since UV spectroscopy observations made with the Hubble Space Telescope [2], SUBARU [3], KeckHIRES [4, 5] and the detailed survey from the Hamburg/ESO HERES project [6] revealed the signature of neutron rich heavy elements in ultra metal poor stars, where one can assume that only one single (or few) nucleosynthesis event has contributed to its composition. The age of these ancient stars represents not only a lower limit for the age of the Milky Way Galaxy and of the Universe, but also provides an important constraint on the time of onset of stellar nucleosynthesis, with further implications for galaxy formation and evolution. Radioactive decay ages can be determined by comparing the observed abundances of the Thorium and Uranium elements (relative to a stable r-process element), to the production ratio expected from theoretical r-process yields. Using