Nuclear Structure Studies at ISOLDE and their Impact on the Astrophysical r-Process

The focus of the present review is the production of the heaviest elements in nature via the r-process. A correct understanding and modeling requires the knowledge of nuclear properties far from stability and a detailed prescription of the astrophysical environment. Experiments at CERN/ISOLDE have played a pioneering role in exploring the characteristics of nuclear structure in terms of masses and beta-decay properties. Initial examinations paid attention to far unstable nuclei with magic neutron numbers related to r-process peaks, while present activities are centered on the evolution of shell effects with the distance from the valley of stability. We first show in site-independent applications the effect of both types of nuclear properties on r-process abundances. Then, we explore the results of calculations related to two different `realistic' astrophysical sites, (i) the supernova neutrino wind and (ii) neutron star mergers. We close with a list of remaining theoretical and experimental challenges needed to overcome for a full understanding of the nature of the r-process, and the role CERN/ISOLDE can play in this process.Comment: LATEX, 38 pages, 16 figures, submitted to Hyperfine Interaction

Charged-Particle and Neutron-Capture Processes in the High-Entropy Wind of Core-Collapse Supernovae

The astrophysical site of the r-process is still uncertain, and a full exploration of the systematics of this process in terms of its dependence on nuclear properties from stability to the neutron drip-line within realistic stellar environments has still to be undertaken. Sufficiently high neutron to seed ratios can only be obtained either in very neutron-rich low-entropy environments or moderately neutron-rich high-entropy environments, related to neutron star mergers (or jets of neutron star matter) and the high-entropy wind of core-collapse supernova explosions. As chemical evolution models seem to disfavor neutron star mergers, we focus here on high-entropy environments characterized by entropy $S$, electron abundance $Y_e$ and expansion velocity $V_{exp}$. We investigate the termination point of charged-particle reactions, and we define a maximum entropy $S_{final}$ for a given $V_{exp}$ and $Y_e$, beyond which the seed production of heavy elements fails due to the very small matter density. We then investigate whether an r-process subsequent to the charged-particle freeze-out can in principle be understood on the basis of the classical approach, which assumes a chemical equilibrium between neutron captures and photodisintegrations, possibly followed by a $\beta$-flow equilibrium. In particular, we illustrate how long such a chemical equilibrium approximation holds, how the freeze-out from such conditions affects the abundance pattern, and which role the late capture of neutrons originating from $\beta$-delayed neutron emission can play.Comment: 52 pages, 31 figure

Closed shells at drip-line nuclei

The shell structure of magic nuclei far from stability is discussed in terms of the self-consistent spherical Hartree-Fock-Bogoliubov theory. In particular, the sensitivity of the shell-gap sizes and the two-neutron separation energies to the choice of particle-hole and particle-particle components of the effective interaction is investigated.Comment: 19 pages, LaTeX, 8 uuencoded figures available upon reques

Effects of single and repeated experimental acid pulses on invertebrates in a high altitude Sierra Nevada stream

1. We examined responses of aquatic macroinvertebrates to pulsed acidification experiments in twelve streamside channels located in the Sierra Nevada, California. Experiment 1 consisted of a single 8 h acid addition, and Experiment 2 consisted of two 8 h acid additions administered 2 weeks apart. Replicated treatments (four reps/ treatment) consisted of a control (pH 6.5–6.7) and pH levels of 5.1–5.2 and 4.4–4.6. Invertebrate drift was monitored continuously and benthic densities were determined before and after acid addition. 2. Drift responses to pH reduction were: (i) increased drift during acidification in pH 5.2 and pH 4.6 treatment channels, often with depressed post‐acidification drift in treatment channels relative to controls (exhibited by Baetis only). Depressed post‐acidification drift in treatment channels appeared to be due to low benthic densities because a positive relationship between benthic and drift densities was noted for most common taxa; (ii) increased drift rates during acidification only at pH 4.6 (Epeorus, Drunella, Paraleptophlebia, Zapada, and Simulium); (iii) decreased drift at pH 5.2 and/or pH 4.6 relative to control channels (Rhyacaphila and chironomid larvae); (iv) no significant response to acidification (Ameletus, Amiocentrus, Dixa and Hydroporus). 3. A high proportion (45–100%) of acid‐induced drift in Baetis, Epeorus, and chironomid larvae could be attributed to dead, drifting individuals. 4. Except for chironomids, most common invertebrates (i.e. Baetis and Paraleptophlebia) showed reduced benthic densities in treatment relative to control channels after acidification. 5. For sensitive taxa, drift was enhanced and benthic densities reduced by single (Experiment 1) and initial [Experiment 2(a)] acid pulses. Drift responses to a second acid pulse [Experiment 2(b)] were not as pronounced as those to the single or initial acid pulses [Experiments 1 and 2(a)], and the second acid pulse had no additional effect on benthic density. Copyright © 1994, Wiley Blackwell. All rights reserve

Nucleosynthesis Modes in the High-Entropy-Wind of Type II Supernovae: Comparison of Calculations with Halo-Star Observations

While the high-entropy wind (HEW) of Type II supernovae remains one of the more promising sites for the rapid neutron-capture (r-) process, hydrodynamic simulations have yet to reproduce the astrophysical conditions under which the latter occurs. We have performed large-scale network calculations within an extended parameter range of the HEW, seeking to identify or to constrain the necessary conditions for a full reproduction of all r-process residuals N_{r,\odot}=N_{\odot}-N_{s,\odot} by comparing the results with recent astronomical observations. A superposition of weighted entropy trajectories results in an excellent reproduction of the overall N_{r,\odot}-pattern beyond Sn. For the lighter elements, from the Fe-group via Sr-Y-Zr to Ag, our HEW calculations indicate a transition from the need for clearly different sources (conditions/sites) to a possible co-production with r-process elements, provided that a range of entropies are contributing. This explains recent halo-star observations of a clear non-correlation of Zn and Ge and a weak correlation of Sr - Zr with heavier r-process elements. Moreover, new observational data on Ru and Pd seem to confirm also a partial correlation with Sr as well as the main r-process elements (e.g. Eu).Comment: 15 pages, 1 table, 4 figures; To be published in the Astrophysical Journal Letter

On three topical aspects of the N=28 isotonic chain

The evolution of single-particle orbits along the N=28 isotonic chain is studied within the framework of a relativistic mean-field approximation. We focus on three topical aspects of the N=28 chain: (a) the emergence of a new magic number at Z=14; (b) the possible erosion of the N=28 shell; and (c) the weakening of the spin-orbit splitting among low-j neutron orbits. The present model supports the emergence of a robust Z=14 subshell gap in 48Ca, that persists as one reaches the neutron-rich isotone 42Si. Yet the proton removal from 48Ca results in a significant erosion of the N=28 shell in 42Si. Finally, the removal of s1/2 protons from 48Ca causes a ~50% reduction of the spin-orbit splitting among neutron p-orbitals in 42Si.Comment: 12 pages with 5 color figure