C. Domingo-Pardo

Advanced nuclear energy systems and the need of accurate nuclear data: the n_TOF project at CERN

To satisfy the worlds constantly increasing demand for energy, a suitable mix of different energy sources has to be devised. In this scenario, an important role could be played by nuclear energy, provided that major safety, waste and proliferation issues affecting current nuclear reactors are satisfactorily addressed. To this purpose, a large effort has been under way for a few years towards the development of advanced nuclear systems with the aim of closing the fuel cycle. Generation IV reactors, with full or partial waste recycling capability, accelerator driven systems, as well as new fuel cycles are the main options being investigated. The design of advanced systems requires improvements in basic nuclear data, such as cross-sections for neutron-induced reactions on actinides. In this paper, the main concepts of advanced reactor systems are described, together with the related needs of new and accurate nuclear data. The present activity in this field at the neutron facility n_TOF at CERN is discussed.

Cross sections for proton-induced reactions on Pd isotopes at energies relevant for the gamma process

Proton-activation reactions on natural and enriched palladium samples were investigated via the activation technique in the energy range of E{sub p}=2.75-9 MeV, close to the upper end of the respective Gamow window of the {gamma} process. We have determined cross sections for {sup 102}Pd(p, {gamma}){sup 103}Ag, {sup 104}Pd(p, {gamma}){sup 105}Ag, and {sup 105}Pd(p, n){sup 105}Ag, as well as partial cross sections of {sup 104}Pd(p, n){sup 104}Ag{sup g}, {sup 105}Pd(p, {gamma}){sup 106}Ag{sup m}, {sup 106}Pd(p, n){sup 106}Ag{sup m}, and {sup 110}Pd(p, n){sup 110}Ag{sup m} with uncertainties between 3% and 15% for constraining theoretical Hauser-Feshbach rates and for direct use in {gamma}-process calculations.

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.

High-energy excited states in 98Cd

In 98Cd a new high-energy isomeric γ-ray transition was identified, which confirms previous spin-parity assignments and enables for the first time the measurement of the E2 and E4 strength for the two decay branches of the isomer. Preliminary results on the 98Cd high-excitation level scheme are presented. A comparison to shell-model calculations as well as implications for the nuclear structure around 100Sn are discussed.

First tests of the applicability of γ-ray imaging for background discrimination in time-of-flight neutron capture measurements

In this work we explore for the first time the applicability of using gamma-ray imaging in neutron capture measurements to identify and suppress spatially localized background. For this aim, a pinhole gamma camera is assembled, tested and characterized in terms of energy and spatial performance. It consists of a monolithic CeBr3 scintillating crystal coupled to a position-sensitive photomultiplier and readout through an integrated circuit AMIC2GR. The pinhole collimator is a massive carven block of lead. A series of dedicated measurements with calibrated sources and with a neutron beam incident on a Au-197 sample have been carried out at n_TOF, achieving an enhancement of a factor of two in the signal-to-background ratio when selecting only those events coming from the direction of the sample

Low-lying level structure of Cu 56 and its implications for the rp process

The low-lying energy levels of proton-rich Cu56 have been extracted using in-beam γ-ray spectroscopy with the state-of-the-art γ-ray tracking array GRETINA in conjunction with the S800 spectrograph at the National Superconducting Cyclotron Laboratory at Michigan State University. Excited states in Cu56 serve as resonances in the Ni55(p,γ)Cu56 reaction, which is a part of the rp process in type-I x-ray bursts. To resolve existing ambiguities in the reaction Q value, a more localized isobaric multiplet mass equation (IMME) fit is used, resulting in Q=639±82 keV. We derive the first experimentally constrained thermonuclear reaction rate for Ni55(p,γ)Cu56. We find that, with this new rate, the rp process may bypass the Ni56 waiting point via the Ni55(p,γ) reaction for typical x-ray burst conditions with a branching of up to ∼40%. We also identify additional nuclear physics uncertainties that need to be addressed before drawing final conclusions about the rp-process reaction flow in the Ni56 region.

Conceptual design of a hybrid neutron-gamma detector for study of β-delayed neutrons at the RIB facility of RIKEN

BRIKEN is a complex detection system to be installed at the RIB-facility of the RIKEN Nishina Center. It is aimed at the detection of heavy-ion implants, s-particles, ?-rays and s-delayed neu- trons. The whole detection setup involves the Advanced Implantation Detection Array (AIDA), two HPGe Clover detectors and a large set of 166 counters of 3He embedded in a high-density polyethy- lene matrix. This article reports on a novel methodology developed for the conceptual design and optimisation of the 3He-tubes array, aiming at the best possible performance in terms of neutron detection. The algorithm is based on a geometric representation of two selected parameters of merit, namely, average neutron detection efficiency and efficiency flatness, as a function of a reduced num- ber of geometric variables. The response of the detection system itself, for each configuration, is obtained from a systematic MC-simulation implemented realistically in Geant4. This approach has been found to be particularly useful. On the one hand, due to the different types and large number of 3He-tubes involved and, on the other hand, due to the additional constraints introduced by the ancillary detectors for charged particles and gamma-rays. Empowered by the robustness of the al- gorithm, we have been able to design a versatile detection system, which can be easily re-arranged into a compact mode in order to maximize the neutron detection performance, at the cost of the gamma-ray sensitivity. In summary, we have designed a system which shows, for neutron energies up to 1(5) MeV, a rather flat and high average efficiency of 68.6%(64%) and 75.7%(71%) for the hybrid and compact modes, respectively. The performance of the BRIKEN system has been also quantified realistically by means of MC-simulations made with different neutron energy distributions.

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.

Kr-96(36)60-Low-Z Boundary of the Island of Deformation at N=60

Prompt γ-ray spectroscopy of the neutron-rich ^{96}Kr, produced in transfer- and fusion-induced fission reactions, has been performed using the combination of the Advanced Gamma Tracking Array and the VAMOS++ spectrometer. A second excited state, assigned to J^{π}=4^{+}, is observed for the first time, and a previously reported level energy of the first 2^{+} excited state is confirmed. The measured energy ratio R_{4/2}=E(4^{+})/E(2^{+})=2.12(1) indicates that this nucleus does not show a well-developed collectivity contrary to that seen in heavier N=60 isotones. This new measurement highlights an abrupt transition of the degree of collectivity as a function of the proton number at Z=36, of similar amplitude to that observed at N=60 at higher Z values. A possible reason for this abrupt transition could be related to the insufficient proton excitations in the g_{9/2}, d_{5/2}, and s_{1/2} orbitals to generate strong quadrupole correlations or to the coexistence of competing different shapes. An unexpected continuous decrease of R_{4/2} as a function of the neutron number up to N=60 is also evidenced. This measurement establishes the Kr isotopic chain as the low-Z boundary of the island of deformation for N=60 isotones. A comparison with available theoretical predictions using different beyond mean-field approaches shows that these models fail to reproduce the abrupt transitions at N=60 and Z=36.

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