[show abstract][hide abstract] ABSTRACT: The γ-ray decay of isomeric states in the even-even 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. Large-scale shell-model 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). · 3.72 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 neutron-rich tin isotopes 130,132Sn. The resulting dipole strengths have been implemented in Hauser–Feshbach calculations of astrophysical neutron capture rates relevant for r-process 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.
[show abstract][hide abstract] ABSTRACT: Coincidences between charged particles emitted in the β-decay 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.
[show abstract][hide abstract] ABSTRACT: With currently known input physics and computer simulations in one dimension, a self-consistent treatment of core-collapse supernovae does not yet lead to successful explosions, while two-dimensional 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 low-metallicity 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 neutrino-scattering cross sections in order to mimic in one dimension the possible increase of neutrino luminosities caused by uncertainties in proto-neutron 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 weak-interaction timescale and a negligible electron degeneracy, unveiling the proton-neutron mass difference. We include all weak interactions (electron and positron capture, β-decay, 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 proton-rich environment results in enhanced abundances of 45Sc, 49Ti, and 64Zn as required by chemical evolution studies and observations of low-metallicity stars, as well as appreciable production of nuclei in the mass range up to A = 80.
The Astrophysical Journal 12/2008; 637(1):415. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: The r-process 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 r-process elements and that the r-process responsible for the production of elements heavier than Z=56 operates quite robustly producing always the same relative abundances. From the nuclear-physics point of view the r-process requires the knowledge of a large number of reaction rates involving exotic nuclei. These include neutron capture rates, beta-decays and fission rates, the latter for the heavier nuclei produced in the r-process. We have developed for the first time a complete database of reaction rates that in addition to neutron-capture rates and beta-decay half-lives includes all possible reactions that can induce fission (neutron-capture, beta-decay 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 r-process calculations for the neutrino-driven wind scenario to explore whether or not fission can contribute to provide a robust r-process pattern.
[show abstract][hide abstract] ABSTRACT: The masses of very neutron-deficient nuclides close to the astrophysical r p- and ν p-process 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 heavy-ion fusion-evaporation reactions. In total, 21 nuclides were studied, and almost half of the mass values were experimentally determined for the first time: 88Tc, 90-92Ru, 92-94Rh, and 94,95Pd. For the 95Pdm, (21/2+) high-spin state, a first direct mass determination was performed. Relative mass uncertainties of typically δm/m=5×10-8 were obtained. The impact of the new mass values has been studied in ν p-process 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). · 3.72 Impact Factor
[show abstract][hide abstract] ABSTRACT: We investigate the effects of single-particle structure and pairing on the equilibration of positive and negative-parity level densities for the even-even nuclei 58,62,66Fe and 58Ni and the odd-A nuclei 59Ni and 65Fe. Calculations are performed using the shell model Monte Carlo method in the complete fp−gds shell-model space using a pairing+quadrupole type residual interaction. We find for the even-even nuclei that the positive-parity states dominate at low excitation energies due to strong pairing correlations. At excitation energies at which pairs are broken, single-particle structure of these nuclei is seen to play the decisive role for the energy dependence of the ratio of negative-to-positive parity level densities. We also find that equilibration energies are noticeably lower for the odd-A nuclei 59Ni and 65Fe than for the neighboring even-even nuclei 58Ni and 66Fe.
[show abstract][hide abstract] ABSTRACT: Systematic mean-field calculations of the β-decay rates for the nuclei with charge numbers Z=42–49 approaching the possible r-process paths in vicinity of the spherical neutron shell at N=82 are performed within the DF+CQRPA framework employing a self-consistent approach to the nuclear ground states based on the local energy-density functional (DF) theory. The calculated ground state properties are compared with the available experimental data. The beta-strength-functions of the Gamow–Teller and first-forbidden decays and the total β-decay half-lives are calculated within the continuum QRPA approach. If available, data and half-lives obtained on the basis of the Finite Range Droplet Model and the shell model are compared to our results. The effects of our calculated half-lives on the r-process abundances are explored in local r-process calculations.
[show abstract][hide abstract] ABSTRACT: Baryonic outflows from proto-neutron stars formed in core-collapse supernova explosions are one of the possible scenarios for the production of heavy elements via the r-process. If the ejected mat-ter reaches supersonic velocities the outflow is known as neutrino-driven wind. We have studied the long-time evolution of proto-neutron stars with one-and two-dimensional hydrodynamical simulations and performed nucleosynthesis calculations with the resulting wind trajectories. We find that the present wind models are much closer than previous studies to provide suitable con-ditions for r-process nucleosynthesis. Moreover, we explored the effect of the wind terminations shock, nuclear physics input, and composition of the outer layers of the neutron star on the nucle-osynthesis production.
[show abstract][hide abstract] ABSTRACT: The (d,2He) charge-exchange reaction on the double-beta decay (ββ) nucleus 64Zn has been studied at an incident energy of 183 MeV. The two protons in the 1S0 state (indicated as 2He) were both momentum analyzed and detected simultaneously by the BBS magnetic spectrometer and its position-sensitive detector. 2He spectra with a resolution of about 115 keV (FWHM) have been obtained allowing identification of many levels in the residual nucleus 64Cu with high precision. 64Zn is one of the rare cases undergoing a ββ decay in β+ direction. In the experiment presented here, Gamow-Teller (GT+ ) transition strengths have been extracted. Together with the GT− transition strengths from 64 Ni(3 He,t) data to the same intermediate nucleus 64Cu, the nuclear matrix elements of the ββ decay of 64Zn have been evaluated. Finally, the GT± distributions are compared with shell-model calculations and a critical assessment is given of the various residual interactions presently employed for the pf shell.
Physical Review C 06/2008; 77(6):064303. · 3.72 Impact Factor
[show abstract][hide abstract] ABSTRACT: Nuclear astrophysics combines inputs from different fields with the objective of explaining the abundances and evolution of chemical elements in the Universe. Future radioactive ion beam facilities will provide access to many of the nuclei that participate in explosive nucleosynthesis scenarios. To fully exploit the potential of these facilities progress in theoretical nuclear physics will be required. This article reviews different many-body methods currently used for the description of processes relevant for nuclear astrophysics.
[show abstract][hide abstract] ABSTRACT: Microscopic calculations show a strong parity dependence of the nuclear level density at low excitation energy of a nucleus. Previously, this dependence has either been neglected or only implemented in the initial and final channels of Hauser–Feshbach calculations. We present an indirect way to account for a full parity dependence in all steps of a reaction, including the one of the compound nucleus formed in a reaction. To illustrate the impact on astrophysical reaction rates, we present rates for neutron captures in isotopic chains of Ni and Sn. Comparing with the standard assumption of equipartition of both parities, we find noticeable differences in the energy regime of astrophysical interest caused by the parity dependence of the nuclear level density found in the compound nucleus even at sizeable excitation energies.
[show abstract][hide abstract] ABSTRACT: Based on the shell model for Gamow-Teller and the random phase approximation for forbidden transitions, we calculate cross sections for inelastic neutrino-nucleus scattering (INNS) under supernova (SN) conditions, assuming a matter composition given by nuclear statistical equilibrium. The cross sections are incorporated into state-of-the-art stellar core-collapse simulations with detailed energy-dependent neutrino transport. While no significant effect on the SN dynamics is observed, INNS increases the neutrino opacities noticeably and strongly reduces the high-energy tail of the neutrino spectrum emitted in the neutrino burst at shock breakout. Relatedly the expected event rates for the observation of such neutrinos by earthbound detectors are reduced by up to about 60%.
[show abstract][hide abstract] ABSTRACT: The occurrence of a pygmy dipole resonance in proton rich Ar-32 and Ar-34 is studied using the unitary correlator operator method interaction Vucom, based on Argonne V18. Predictions from the random phase approximation (RPA) and the shell model in a no-core basis are compared. It is found that the inclusion of configuration mixing up to two-particle--two-holes broadens the pygmy strength slightly and reduces sensibly its strength, as compared to the RPA predictions. For Ar-32 a clear peak associated with a pygmy resonance is found. For Ar-34, the pygmy states are obtained close to the giant dipole resonance and mix with it.
[show abstract][hide abstract] ABSTRACT: Core-collapse and the launch of a supernova explosion form a very short episode of few seconds in the evolution of a massive star, during which an enormous gravitational energy of several times 1053 erg is transformed into observable neutrino-, kinetic-, and electromagnetic radiation energy. We emphasize the wide range of matter conditions that prevail in a supernova event and sort the conditions into distinct regimes in the density and entropy phase diagram to briefly discuss their different impact on the neutrino signal, gravitational wave emission, and ejecta.
New Astronomy Reviews 01/2008; 52:373-376. · 1.82 Impact Factor
[show abstract][hide abstract] ABSTRACT: We study the composition of the outer layers of a protoneutron star and show that light nuclei are present in substantial amounts. The composition is dominated by nucleons, deuterons, tritons and alpha particles; 3He is present in smaller amounts. This composition can be studied in laboratory experiments with new neutron-rich radioactive beams that can reproduce similar densities and temperatures. After including the corresponding neutrino interactions, we demonstrate that light nuclei have a small impact on the average energy of the emitted electron neutrinos, but are significant for the average energy of antineutrinos. During the early post-explosion phase, the average energy of electron antineutrinos is slightly increased, while at later times during the protoneutron star cooling it is reduced by about 1 MeV. The consequences of these changes for nucleosynthesis in neutrino-driven supernova outflows are discussed.
[show abstract][hide abstract] ABSTRACT: This manuscript deals with the influence of parity-dependent nuclear level densities on astro- physical reaction rates calculated in the Hauser-Feshbach framework. The inclusion of a parity non-equipartition within the compound nucleus in these reactions has not been examined before. We show that our approximative treatment - that accounts for such non-uniformly distributed parities in the compound nucleus - can have an influence on reaction rates. For a more detailed examination we refer to the corresponding paper (1). Moreover, we shortly discuss the influence of microscopic calculated E1-strength functions on reaction rates and show that for specific nuclei, additional low-lying E1 strength can increase the reaction rate.
[show abstract][hide abstract] ABSTRACT: Level densities of J pi=2+ and 2- states extracted from high-resolution studies of E2 and M2 giant resonances in 58Ni and 90Zr are used to test recent predictions of a possible parity dependence. The experimental results are compared to a combinatorial approach based on the Hartree-Fock-Bogoliubov model and to shell-model Monte Carlo calculations including both spin and parity projection. No parity dependence is observed experimentally, which is in agreement for 90Zr but in contrast with the model predictions for 58Ni.