Publications (241)763.1 Total impact
 01/2010;
 Nuclear Physics A 01/2010; · 2.50 Impact Factor
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ABSTRACT: One of the most important discoveries made with Swift is that long and shortduration gammaray bursts (GRBs) originate from distinct stellar progenitors. While long GRBs track ongoing star formation and result from the deaths of massive stars, short GRBs have been localized to both early and latetype galaxies, suggesting a more evolved progenitor population. Although the origin of short GRBs remains a mystery, the most popular and wellstudied model is accretion following the merger of neutron star binaries. This model is qualitatively consistent with both the demographics of short GRBs and the lack of a bright associated supernova in some cases. Despite these successes, this picture has grown complex with the discovery that short GRBs are often followed by a "tail" of emission (usually soft Xrays) lasting 100 seconds after the burst. Such energetic, latetime emission from the central engine is difficult to explain in standard merger pictures. One proposed explanation is latetime "fallback" onto the black hole of material that was ejected during the merger into highly eccentric, marginallybound orbits. As this matter decompresses from nuclear densities, however, it undergoes rapidneutron capture (rprocess) nucleosynthesis, which can release energy comparable to the orbital binding energy. This implies that the rprocess (normally thought unimportant dynamically in astrophysical contexts) has important implications for the quantity and timedependence of fallback and, ultimately, the source of flaring and identity of the central engine.01/2010;  01/2010;
 01/2010;
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ABSTRACT: Large scale shellmodel calculations with an effective interaction derived from the realistic Gmatrices were performed for N=80 isotones for which socalled mixedsymmetry states were recently observed experimentally. Calculated spectra are shown to be in good agreement with data. The calculated transition rates reveal the necessity of modifying the strength of the pairing interaction. The structure of mixedsymmetry 2+ states is analyzed in terms of seniority components and by decomposition into the Qphonon scheme.Physical Review C 11/2009; 80(5). · 3.88 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Neutroninduced reaction rates, including fission, are calculated in the temperature range 1.d8 <T (K) < 1.d10 within the framework of the statistical model for targets with atomic number 83 < Z < 119 (from Po to Uuo) from the neutron to the proton dripline. Four sets of rates have been calculated, utilizing  where possible  consistent nuclear data for neutron separation energies and fission barriers from ThomasFermi (TF), Extended ThomasFermi plus Strutinsky Integral (ETFSI), FiniteRange Droplet Model (FRDM) and HartreeFockBogolyubov (HFB) predictions. Tables of calculated values as well as analytic seven parameter fits in the standard REACLIB format are supplied. We also discuss the sensitivity of the rates to the input, aiming at a better understanding of the uncertainties introduced by the nuclear input. Comment: 14 pages, 10 figures, 2 tables in paper, 2 in Annex and online tables examplesAstronomy and Astrophysics 11/2009; · 4.48 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We propose a new method to calculate stellar weakinteraction rates. It is based on the ThermoFieldDynamics formalism and allows the calculation of the weakinteraction response of nuclei at finite temperatures. The thermal evolution of the GT$_+$ distributions is presented for the sample nuclei $^{54, 56}$Fe and ~$^{76,78,80}$Ge. For Ge we also calculate the strength distribution of firstforbidden transitions. We show that thermal effects shift the GT$_+$ centroid to lower excitation energies and make possible negative and lowenergy transitions. In our model we demonstrate that the unblocking effect for GT$_+$ transitions in neutronrich nuclei is sensitive to increasing temperature. The results are used to calculate electron capture rates and are compared to those obtained from the shell model. Comment: 16 pages, 9 figuresPhysical Review C 11/2009; · 3.88 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We have performed hydrodynamical simulations of the longtime evolution of protoneutron stars to study the nucleosynthesis using the resulting wind trajectories. Although the conditions found in the present wind models are not favourable for the production of heavy elements, a small enhancement of the entropy results in the production of rprocess elements with A $\approx$ 195. This allows us to explore the sensitivity of their production to the hydrodynamical evolution (wind termination shock) and nuclear physics input used. Comment: Conference proceedings: Nuclear Physics in Astrophysics IVJournal of Physics Conference Series 09/2009;  [Show abstract] [Hide abstract]
ABSTRACT: Electron captures on nuclei play an important role in the dynamics of the collapsing core of a massive star that leads to a supernova explosion. Recent calculations of these capture rates were based on microscopic models which account for relevant degrees of freedom. Due to computational restrictions such calculations were limited to a modest number of nuclei, mainly in the mass range A=45–110. Recent supernova simulations show that this pool of nuclei, however, omits the very neutronrich and heavy nuclei which dominate the nuclear composition during the last phase of the collapse before neutrino trapping. Assuming that the composition is given by Nuclear Statistical Equilibrium we present here electron capture rates for collapse conditions derived from individual rates for roughly 2700 individual nuclei. For those nuclei which dominate in the early stage of the collapse, the individual rates are derived within the framework of microscopic models, while for the nuclei which dominate at high densities we have derived the rates based on the Random Phase Approximation with a global parametrization of the single particle occupation numbers. In addition, we have improved previous rate evaluations by properly including screening corrections to the reaction rates into account.Nuclear Physics A 09/2009; · 2.50 Impact Factor  Physical Review C 07/2009; 80(1). · 3.88 Impact Factor
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ABSTRACT: We calculate the lowlying spectra and several highspin states of zirconium isotopes (Z=40) with neutron numbers from N=50 to N=58 using a large valence space with the 78Ni inert core, which a priori allows one to study the interplay between spherical and deformed configurations, necessary for the description of nuclides in this part of the nuclear chart. The effective interaction is derived by monopole corrections of the realistic G matrix. We reproduce essential nuclear properties, such as subshell closures in 96Zr and 98Zr. The sphericaltodeformed shape transition in 100Zr is addressed as well.Physical Review C 06/2009; 79(6). · 3.88 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Coincidences between charged particles emitted in the βdecay of 11Li were observed using highly segmented detectors. The breakup channels involving three particles were studied in full kinematics allowing for the reconstruction of the excitation energy of the 11Be states participating in the decay. In particular, the contribution of a previously unobserved state at 16.3 MeV in 11Be has been identified selecting the channel. The angular correlations between the α particle and the center of mass of the 6He + n system favors spin and parity assignment of 3/2− for this state as well as for the previously known state at 18 MeV.Physics Letters B 06/2009; · 6.02 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The γray decay of isomeric states in the eveneven nucleus 128Cd has been observed. The nucleus of interest was produced both by the fragmentation of 136Xe and the fission of 238U primary beams. The level scheme was unambiguously constructed based on γγ coincidence relations in conjunction with detailed lifetime analysis employed for the first time on this nucleus. Largescale shellmodel calculations, without consideration of excitations across the N=82 shell closure, were performed and provide a consistent description of the experimental level scheme. The structure of the isomeric states and their decays exhibit coexistence of proton, neutron, and strongly mixed configurations due to πν interaction in overlapping orbitals for both proton and neutron holes.Physical Review C 05/2009; 79(1):011301(R). · 3.88 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We have computed dipole strength distributions for nickel and tin isotopes within the Relativistic Quasiparticle Time Blocking Approximation (RQTBA). These calculations provide a good description of data, including the neutronrich tin isotopes 130,132Sn. The resulting dipole strengths have been implemented in Hauser–Feshbach calculations of astrophysical neutron capture rates relevant for rprocess nucleosynthesis studies. The RQTBA calculations show the presence of enhanced dipole strength at energies around the neutron threshold for neutron rich nuclei. The computed neutron capture rates are sensitive to the fine structure of the low lying dipole strength, which emphasizes the importance of a reliable knowledge of this excitation mode.Nuclear Physics A 03/2009; · 2.50 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: With currently known input physics and computer simulations in one dimension, a selfconsistent treatment of corecollapse supernovae does not yet lead to successful explosions, while twodimensional models show some promise. Thus, there are strong indications that the delayed neutrino mechanism works combined with a multidimensional convection treatment for unstable layers (possibly with the aid of rotation, magnetic fields and/or still existent uncertainties in neutrino opacities). On the other hand, there is a need to provide correct nucleosynthesis abundances for the progressing field of galactic evolution and observations of lowmetallicity stars. The innermost ejecta is directly affected by the explosion mechanism, i.e., most strongly, the yields of Fe group nuclei for which an induced piston or thermal bomb treatment will not provide the correct yields because the effect of neutrino interactions is not included. We apply parameterized variations to the neutrinoscattering cross sections in order to mimic in one dimension the possible increase of neutrino luminosities caused by uncertainties in protoneutron star convection. Alternatively, parameterized variations are applied to the neutrino absorption cross sections on nucleons in the "gain region" to mimic the increase in neutrino energy deposition enabled by convective turnover. We find that both measures lead to similar results, causing explosions and a Ye > 0.5 in the innermost ejected layers, due to the combined effect of a short weakinteraction timescale and a negligible electron degeneracy, unveiling the protonneutron mass difference. We include all weak interactions (electron and positron capture, βdecay, neutrino and antineutrino capture on nuclei, and neutrino and antineutrino capture on nucleons) and present first nucleosynthesis results for these innermost ejected layers to discuss how they improve predictions for Fe group nuclei. The protonrich environment results in enhanced abundances of 45Sc, 49Ti, and 64Zn as required by chemical evolution studies and observations of lowmetallicity stars, as well as appreciable production of nuclei in the mass range up to A = 80.The Astrophysical Journal 12/2008; 637(1):415. · 6.28 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The rprocess constitutes one of the major challenges in nuclear astrophysics. Its astrophysical site has not yet been identified but there is observational evidence suggesting that at least two possible sites should contribute to the solar system abundance of rprocess elements and that the rprocess responsible for the production of elements heavier than Z=56 operates quite robustly producing always the same relative abundances. From the nuclearphysics point of view the rprocess requires the knowledge of a large number of reaction rates involving exotic nuclei. These include neutron capture rates, betadecays and fission rates, the latter for the heavier nuclei produced in the rprocess. We have developed for the first time a complete database of reaction rates that in addition to neutroncapture rates and betadecay halflives includes all possible reactions that can induce fission (neutroncapture, betadecay and spontaneous fission) and the corresponding fission yields. In addition, we have implemented these reaction rates in a fully implicit reaction network. We have performed rprocess calculations for the neutrinodriven wind scenario to explore whether or not fission can contribute to provide a robust rprocess pattern.12/2008;  [Show abstract] [Hide abstract]
ABSTRACT: The masses of very neutrondeficient nuclides close to the astrophysical r p and ν pprocess paths have been determined with the Penning trap facilities JYFLTRAP at JYFL/Jyväskylä and SHIPTRAP at GSI/Darmstadt. Isotopes from yttrium (Z=39) to palladium (Z=46) have been produced in heavyion fusionevaporation reactions. In total, 21 nuclides were studied, and almost half of the mass values were experimentally determined for the first time: 88Tc, 9092Ru, 9294Rh, and 94,95Pd. For the 95Pdm, (21/2+) highspin state, a first direct mass determination was performed. Relative mass uncertainties of typically δm/m=5×108 were obtained. The impact of the new mass values has been studied in ν pprocess nucleosynthesis calculations. The resulting reaction flow and the final abundances are compared with those obtained with the data of the Atomic Mass Evaluation 2003.Physical Review C 11/2008; 78(5). · 3.88 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the effects of singleparticle structure and pairing on the equilibration of positive and negativeparity level densities for the eveneven nuclei 58,62,66Fe and 58Ni and the oddA nuclei 59Ni and 65Fe. Calculations are performed using the shell model Monte Carlo method in the complete fp−gds shellmodel space using a pairing+quadrupole type residual interaction. We find for the eveneven nuclei that the positiveparity states dominate at low excitation energies due to strong pairing correlations. At excitation energies at which pairs are broken, singleparticle structure of these nuclei is seen to play the decisive role for the energy dependence of the ratio of negativetopositive parity level densities. We also find that equilibration energies are noticeably lower for the oddA nuclei 59Ni and 65Fe than for the neighboring eveneven nuclei 58Ni and 66Fe.AIP Conference Proceedings. 11/2008; 1072(1).  [Show abstract] [Hide abstract]
ABSTRACT: We respond to a Comment on our recent paper (Phys.Rev.C77:024304,2008) by Paar (arXiv:0803.0274).Physical Review C 10/2008; · 3.88 Impact Factor
Publication Stats
5k  Citations  
763.10  Total Impact Points  
Top Journals
Institutions

2007–2014

Technical University Darmstadt
 Institute of Nuclear Physics
Darmstadt, Hesse, Germany 
GSI Helmholtzzentrum für Schwerionenforschung
 ExtreMe Matter Institute EMMI and Research Division
Darmstadt, Hesse, Germany


2004–2008

Autonomous University of Barcelona
Cerdanyola del Vallès, Catalonia, Spain 
University of Münster
Muenster, North RhineWestphalia, Germany


2000–2008

Universität Basel
 Department of Physics
Basel, BS, Switzerland


1998–2007

Aarhus University
 Department of Physics and Astronomy
Aars, Region North Jutland, Denmark


1993–2007

Universidad Autónoma de Madrid
 Departamento de Física Teórica
Madrid, Madrid, Spain


2003–2006

IEEC Institute of Space Studies of Catalonia
Barcino, Catalonia, Spain 
University of Tennessee
 Department of Physics & Astronomy
Knoxville, TN, United States


2003–2004

Catalan Institution for Research and Advanced Studies
Barcino, Catalonia, Spain


2001

University of California, Santa Cruz
 Department of Astronomy and Astrophysics
Santa Cruz, CA, United States


1998–1999

California Institute of Technology
 Department of Physics
Pasadena, CA, United States
