Opportunities for Nuclear Astrophysics at FRANZ

The "Frankfurter Neutronenquelle am Stern-Gerlach-Zentrum" (FRANZ), which is currently under development, will be the strongest neutron source in the astrophysically interesting energy region in the world. It will be about three orders of magnitude more intense than the well-established neutron source at the Research Center Karlsruhe (FZK)

Neutron activation of natural zinc samples at kT = 25 keV

The neutron-capture cross sections of 64Zn, 68Zn, and 70Zn have been measured with the activation technique in a quasistellar neutron spectrum corresponding to a thermal energy of kT = 25 keV. By a series of repeated irradiations with different experimental conditions, an uncertainty of 3% could be achieved for the 64Zn(n,g)65Zn cross section and for the partial cross section 68Zn(n,g)69Zn-m feeding the isomeric state in 69Zn. For the partial cross sections 70Zn(n,g)71Zn-m and 70Zn(n,g)71Zn-g, which had not been measured so far, uncertainties of only 16% and 6% could be reached because of limited counting statistics and decay intensities. Compared to previous measurements on 64,68Zn, the uncertainties could be significantly improved, while the 70Zn cross section was found to be two times smaller than existing model calculations. From these results Maxwellian average cross sections were determined between 5 and 100 keV. Additionally, the beta-decay half-life of 71Zn-m could be determined with significantly improved accuracy. The consequences of these data have been studied by network calculations for convective core He burning and convective shell C burning in massive stars

Tuning the Re/Os Clock: Stellar-Neutron Cross Sections

The neutron-capture cross sections of 186,187Os have been recently measured at the CERN neutron time-of-flight facility n_TOF for an improved evaluation of the Re/Os cosmo-chronometer. This experimental information was complemented by nuclear model calculations for obtaining the proper astrophysical reaction rates at s-process temperatures. The calculated results and their implications for the determination of the time-duration of nucleosynthesis during galactic chemical evolution is discusse

Post-AGB stars in the Magellanic Clouds and neutron-capture processes in AGB stars

We explore modifications to the current scenario for the slow neutron capture process in asymptotic giant branch (AGB) stars to account for the Pb deficiency observed in post-AGB stars of low metallicity ([Fe/H] ~ -1.2) and low initial mass (~ 1 - 1.5 Msun) in the Large and Small Magellanic Clouds. We calculated the stellar evolution and nucleosynthesis for a 1.3 Msun star with [Fe/H]=-1.3 and tested different amounts and distributions of protons leading to the production of the main neutron source within the 13C-pocket and proton ingestion scenarios. No s-process models can fully reproduce the abundance patterns observed in the post-AGB stars. When the Pb production is lowered the abundances of the elements between Eu and Pb, such as Er, Yb, W, and Hf, are also lowered to below those observed. Neutron-capture processes with neutron densities intermediate between the s and the rapid neutron-capture processes may provide a solution to this problem and be a common occurrence in low-mass, low-metallicity AGB stars.Comment: 6 pages, 4 figures. To be published in Astronomy and Astrophysic

New Stellar (n,γ)(n,\gamma) Cross Sections and The "Karlsruhe Astrophysical Database of Nucleosynthesis in Stars"

Since April 2005 a regularly updated stellar neutron cross section compilation is available online at http://nuclear-astrophysics.fzk.de/kadonis. This online-database is called the "Karlsruhe Astrophysical Database of Nucleosynthesis in Stars" project and is based on the previous Bao et al. compilation from the year 2000. The present version \textsc{KADoNiS} v0.2 (January 2007) includes recommended cross sections for 280 isotopes between $^{1}$H and $^{210}$Po and 75 semi-empirical estimates for isotopes without experimental information. Concerning stellar $(n,\gamma)$ cross sections of the 32 stable, proton-rich isotopes produced by the $p$ process experimental information is only available for 20 isotopes, but 9 of them have rather large uncertainties of $\geq$9%. The first part of a systematic study of stellar $(n,\gamma)$ cross sections of the $p$-process isotopes $^{74}$Se, $^{84}$Sr, $^{102}$Pd, $^{120}$Te, $^{130}$Ba, $^{132}$Ba, $^{156}$Dy, and $^{174}$Hf is presented. In another application \textsc{KADoNiS} v0.2 was used for an modification of a reaction library of Basel university. With this modified library $p$-process network calculations were carried out and compared to previous results.Comment: Proceedings "International Conference on Nuclear Data for Science and Technology 2007", Nice/ Franc

pp-Process simulations with a modified reaction library

We have performed $p$-process simulations with the most recent stellar $(n,\gamma)$ cross sections from the "Karlsruhe Astrophysical Database of Nucleosynthesis in Stars" project (version v0.2, http://nuclear-astrophysics.fzk.de/kadonis). The simulations were carried out with a parametrized supernova type II shock front model (``$\gamma$ process'') of a 25 solar mass star and compared to recently published results. A decrease in the normalized overproduction factor could be attributed to lower cross sections of a significant fraction of seed nuclei located in the Bi and Pb region around the $N$=126 shell closure.Comment: 5 pages, 1 figure Proceedings "Nuclear Physics in Astrophysics NPA-III", Dresden/Germany (2007

(n,γ\gamma) Cross Sections of Light p Nuclei -- Towards an Updated Database for the p Process

The nucleosynthesis of elements beyond iron is dominated by the s and r processes. However, a small amount of stable isotopes on the proton-rich side cannot be made by neutron capture and are thought to be produced by photodisintegration reactions on existing seed nuclei in the so-called "p process". So far most of the p-process reactions are not yet accessible by experimental techniques and have to be inferred from statistical Hauser-Feshbach model calculations. The parametrization of these models has to be constrained by measurements on stable proton-rich nuclei. A series of (n,$\gamma$) activation measurements on p nuclei, related by detailed balance to the respective photodisintegrations, were carried out at the Karlsruhe Van de Graaff accelerator using the $^7$Li(p,n)$^7$Be source for simulating a Maxwellian neutron distribution of kT= 25 keV. We present here preliminary results of our extended measuring program in the mass range between A=74 and A=132, including first experimental (n,$\gamma$) cross sections of $^{74}$Se, $^{84}$Sr, $^{120}$Te and $^{132}$Ba, and an improved value for $^{130}$Ba. In all cases we find perfect agreement with the recommended MACS predictions from the Bao et al. compilation.Comment: 6 pages, 1 figure Proceeding "Nuclear Physics in Astrophysics- NPA-II", Debrecen/ Hungary (2005