Y. Ayyad

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

Structural design of an RPC-based time-of-flight wall for ions (iTOF) for the R3B-FAIR experiment

In this work we describe the mechanical design of a time-of-flight detector based on strip RPCs dedicated to measure relativistic heavy ions. The proposed design includes innovative solutions which meet the specific requirements to work with ions. The proposal is based on the results of the previous R&D program to build prototypes to test designs, materials and construction solutions, complemented by tests with relativistic ion beams. The first module of the detector has been built to be studied and characterized under beam conditions.

Performance of timing RPC detectors for relativistic ions and design of a time-of-flight detector (iToF) for the R3B-FAIR experiment for fission and spallation reactions

Resistive-plate-chambers (RPCs) were proposed to be used to build a time-of-flight detector for relativist heavy ions of the R3B-FAIR experiment, as well as other applications. State-of-the-art reaction codes allow for evaluating the requirements of the detector. The specific needs that working with heavy ions impose about material thicknesses are solved with new design concepts. We built prototypes and investigated the behaviour of RPCs tested with relativistic heavy ions. We measured the efficiency and streamer presence for ions with atomic numbers up to 38. Electron beams were used to study the timing capabilities of the prototypes.

Time resolution of small-size RPC prototypes for relativistic ions

Narrow-gap Resistive Plate Chambers (tRPCs) present excellent performances for timing with minimum ionization particles, but their performance with relativistic ions is almost unknown. We have developed small-size RPC prototypes that have been tested with electron and ion beams. The time resolution measured in both cases are below 70 ps σ, close to the challenging requirements for the R3B experiment Time-of-flight Wall at future FAIR facility.

Fission dynamics at high excitation energies investigated in complete kinematics measurements

Light-charged particles emitted in proton-induced fission reactions on 208 Pb have been measured at different kinetic energies: 370A, 500A, and 650A MeV. The experiment was performed by the SOFIA collaboration at the GSI facilities in Darmstadt (Germany). The inverse kinematics technique was combined with a setup especially designed to measure light-charged particles in coincidence with fission fragments. The data were compared with different model calculations to assess the ground-to-saddle dynamics. The results confirm that transient and dissipative effects are required for an accurate description of the fission observables.

New experimental approaches to investigate the fission dynamics

The first ever achieved full identification of both fission fragments, in atomic and mass number, made it possible to define new observables sensitive to the fission dynamics along the fission path up to the scission point. Moreover, proton-induced fission of 208Pb at high energies offers optimal conditions for the investigation of dissipative, and transient effects, because of the high-excitation energy of the fissioning nuclei, its low angular momentum, and limited shape distortion by the reaction. In this work we show that the charge distribution of the final fission fragments can constrain the ground-to-saddle dynamics while the mass distribution is sensitive to the dynamics until the scission point.

Multiplicity of light-charged particles investigated with proton-induced fission of

Spallation reactions produce large quantities of lightcharged particles (hydrogenand helium isotopes) which are a concern in spallation target design. For instance, the pro duction of tritium is a concern for radioprotection, especially in the case of liquid targets from which it can escape easily. Therefore, a reliable prediction of the light-char ged particle yields by high-energy transport codes [1] becomes important for the design of spallation targets. Furthermor e, light-charged particle emission has been well established as a sensitive tool in probing the dynamics of heavy-ioninduced nuclear reactions and it could help to investigate some fundamental questions about fission such as the dissipative and transient effects in the last stages of the fissi on process [2]. In the present work, we report on the first results of a new generation of accurate measurements on multiplicities of light-charged particles in spallation reaction of Pb at different relativistic energies: 370 A, 500A and 650A MeV. The experiment takes advantage of the inverse kinematics technique, in which fission fragments and light particles are emitted in forward direction. Time coincidence with the fission fragments discriminates the particles produced during fission from other reaction channels. The experiment [3] was performed at the ALADINLAND cave at GSI. Measurements were performed with a hydrogen target isolated by two windows consisting of aluminized mylar foils of 35μm. Fission events were selected in a double multi-sampling ionization chamber (Twin MUSIC) [4]. Between the target and the Twin MUSIC a pipe filled with helium gas was placed to transmit the fission fragments. A Time-of-Flight Wall detector (ToF Wall), based on plastic-scintillator paddles and two photomultipliers (PM) per paddle, was placed in front of the Twin MUSIC to detect the light-charged particles. It consists of two orthogonally-oriented planes with six paddles each (60×6×1 cm), which leave a square hole ( 12.5 × 12.5 cm) in the middle to transmit the fission fragments. Because the reaction kinematics, while most of the fission fragments go through the Twin MUSIC, a large fraction of the light-charged particles escape the target to reach th e ToF Wall. The time and charge of the scintillator signals are registered by using a TDC and a QDC, respectively. The particle multiplicity is provided by the number of fired paddles in each plane and the particle identification is obtaine d by using the time and the charge signals as shown in the inset of Fig. 1. Detection efficiency was determined from GEANT4 simulations [5] using INCL4.6-ABLA07 [1, 6] for the particle kinematics and the ToF Wall dimensions. Figure 1 shows the multiplicity for hydrogen-like particles measured at different energies. The multiplicity is strongly correlated with the impact parameter, so that central collisions lead to high multiplicities whereas low mul tiplicities are related to peripherical reactions that are th major contribution [7]. The average value of the distribution increases with the bombarding energy, as expected because the reactions are more violent.

^{208}

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

Pb at different kinetic energies

In this work we report the proton- and deuteron-induced fission of 208Pb at 500A MeV in inverse kinematics. We obtained two observables that allow us to investigate dynamical effects in the fission process: partial fission cross sections and the width of the fission fragment charge distribution as a function of the atomic number of the fissioning system. Results are compared to nuclear reaction model calculations in order to describe the evolution of the system from ground to saddle.

Measurement of

Total fission cross section induced by protons in 181Ta and 208Pb at energies in the range of 300 to 1000 A MeV have been measured at GSI (Germany) using the inverse kinematics technique. A dedicated setup with high efficiency made it possible to determine these cross sections with high accuracy. The new data seed light in the controversial results obtained so far and contribute to the understanding of the fission process at high excitation energies.