Measurement of (α,n) reaction cross sections of erbium isotopes for testing astrophysical rate predictions

Date of Acceptance: 30/01/2015The γ-process in core-collapse and/or type Ia supernova explosions is thought to explain the origin of the majority of the so-called p nuclei (the 35 proton-rich isotopes between Se and Hg). Reaction rates for γ-process reaction network studies have to be predicted using Hauser-Feshbach statistical model calculations. Recent investigations have shown problems in the prediction of α-widths at astrophysical energies which are an essential input for the statistical model. It has an impact on the reliability of abundance predictions in the upper mass range of the p nuclei. With the measurement of the 164,166Er(α,n)167,169Yb reaction cross sections at energies close to the astrophysically relevant energy range we tested the recently suggested low energy modification of the α+nucleus optical potential in a mass region where γ-process calculations exhibit an underproduction of the p nuclei. Using the same optical potential for the α-width which was derived from combined 162Er(α,n) and 162Er(α,γ) measurement makes it plausible that a low-energy modification of the optical α+nucleus potential is needed.Peer reviewedFinal Accepted Versio

Parity-Dependence in the Nuclear Level Density

Astrophysical reaction rates are sensitive to the parity distribution at low excitation energies. We combine a formula for the energy-dependent parity distribution with a microscopic-macroscopic nuclear level density. This approach describes well the transition from low excitation energies, where a single parity dominates, to high excitations where the two densities are equal.Comment: 4 pages, 3 figures; contribution to Nuclei In The Cosmos VIII, to appear in Nucl. Phys.

Uncertainties In Direct Neutron Capture Calculations Due To Nuclear Structure Models

The prediction of cross sections for nuclei far off stability is crucial in the field of nuclear astrophysics. For spherical nuclei close to the dripline the statistical model (Hauser-Feshbach) approach is not applicable and direct contributions may dominate the cross sections. For neutron-rich, even-even Sn targets, we compare the resulting neutron capture cross sections when consistently taking the input for the direct capture calculations from three different microscopic models. The results underline the sensitivity of cross sections calculated in the direct model to nuclear structure models which can lead to high uncertainties when lacking experimental information.Comment: 4 pages, using espcrc1.sty, Proc. Intl. Conf. "Nuclei in the Cosmos IV", Univ. Notre Dame 1996, Nucl. Phys. A, in press. A postscript version can also be obtained from http://quasar.physik.unibas.ch/research.htm

Revision of the derivation of stellar rates from experiment and impact on Eu s-process contributions

The final, definitive version of this paper has been published in Journal of Physics: Conference Series, 665(1), January 5, 2016, and is available on line at doi: 10.1088/1742-6596/665/1/012024 Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing LtdA new, general formalism to include experimental data in revised stellar rates is discussed, containing revised uncertainties. Application to the s-process shows that the actual uncertainties in the neutron capture rates can be larger than would be expected from the experimental errors alone. As a specific example for how astrophysical conclusions can depend on the approach selected to derive stellar rates, the 151Eu/(151Eu+153 Eu) abundance ratio from AGB star models is presented. Finally, a recommended workflow for the derivation of stellar rates from experiment is laid out.Peer reviewe

Thermal noise influences fluid flow in thin films during spinodal dewetting

Experiments on dewetting thin polymer films confirm the theoretical prediction that thermal noise can strongly influence characteristic time-scales of fluid flow and cause coarsening of typical length scales. Comparing the experiments with deterministic simulations, we show that the Navier-Stokes equation has to be extended by a conserved bulk noise term to accomplish the observed spectrum of capillary waves. Due to thermal fluctuations the spectrum changes from an exponential to a power law decay for large wavevectors. Also the time evolution of the typical wavevector of unstable perturbations exhibits noise induced coarsening that is absent in deterministic hydrodynamic flow.Comment: 4 pages, 3 figure

Nucleosynthesis Basics and Applications to Supernovae

This review concentrates on nucleosynthesis processes in general and their applications to massive stars and supernovae. A brief initial introduction is given to the physics in astrophysical plasmas which governs composition changes. We present the basic equations for thermonuclear reaction rates and nuclear reaction networks. The required nuclear physics input for reaction rates is discussed, i.e. cross sections for nuclear reactions, photodisintegrations, electron and positron captures, neutrino captures, inelastic neutrino scattering, and beta-decay half-lives. We examine especially the present state of uncertainties in predicting thermonuclear reaction rates, while the status of experiments is discussed by others in this volume (see M. Wiescher). It follows a brief review of hydrostatic burning stages in stellar evolution before discussing the fate of massive stars, i.e. the nucleosynthesis in type II supernova explosions (SNe II). Except for SNe Ia, which are explained by exploding white dwarfs in binary stellar systems (which will not be discussed here), all other supernova types seem to be linked to the gravitational collapse of massive stars (M$>$8M$_\odot$) at the end of their hydrostatic evolution. SN1987A, the first type II supernova for which the progenitor star was known, is used as an example for nucleosynthesis calculations. Finally, we discuss the production of heavy elements in the r-process up to Th and U and its possible connection to supernovae.Comment: 52 pages, 20 figures, uses cupconf.sty (included); to appear in "Nuclear and Particle Astrophysics", eds. J. Hirsch., D. Page, Cambridge University Pres

Sibling Genes as Environment: Sibling Dopamine Genotypes and Adolescent Health Support Frequency Dependent Selection

While research consistently suggests siblings matter for individual outcomes, it remains unclear why. At the same time, studies of genetic effects on health typically correlate variants of a gene with the average level of behavioral or health measures, ignoring more complicated genetic dynamics. Using National Longitudinal Study of Adolescent Health data, we investigate whether sibling genes moderate individual genetic expression. We compare twin variation in health-related absences and self-rated health by genetic differences at three locations related to dopamine regulation and transport to test sibship-level cross-person gene–gene interactions. Results suggest effects of variation at these genetic locations are moderated by sibling genes. Although the mechanism remains unclear, this evidence is consistent with frequency dependent selection and suggests much genetic research may violate the stable unit treatment value assumption

The Path to Improved Reaction Rates for Astrophysics

This review focuses on nuclear reactions in astrophysics and, more specifically, on reactions with light ions (nucleons and alpha particles) proceeding via the strong interaction. It is intended to present the basic definitions essential for studies in nuclear astrophysics, to point out the differences between nuclear reactions taking place in stars and in a terrestrial laboratory, and to illustrate some of the challenges to be faced in theoretical and experimental studies of those reactions. The discussion revolves around the relevant quantities for astrophysics, which are the astrophysical reaction rates. The sensitivity of the reaction rates to the uncertainties in the prediction of various nuclear properties is explored and some guidelines for experimentalists are also provided.Comment: 100 pages, 33 figures, 1 table; accepted for publication in Int. J. Mod. Phys. E (scheduled for February 2011 issue); the formatting here differs in that it includes a table of contents and numbered paragraphs 5.4.2.1-5.4.2.10; v2: updated references; v3: typos fixed; v4: final typo fix, content similar to published version

Colloids dragged through a polymer solution: experiment, theory and simulation

We present micro-rheological measurments of the drag force on colloids pulled through a solution of lambda-DNA (used here as a monodisperse model polymer) with an optical tweezer. The experiments show a violation of the Stokes-Einstein relation based on the independently measured viscosity of the DNA solution: the drag force is larger than expected. We attribute this to the accumulation of DNA infront of the colloid and the reduced DNA density behind the colloid. This hypothesis is corroborated by a simple drift-diffusion model for the DNA molecules, which reproduces the experimental data surprisingly well, as well as by corresponding Brownian dynamics simulations.Comment: 9 pages, 13 figures, 3 table