430 research outputs found
Stellar neutron capture cross sections of ⁴¹K and ⁴⁵Sc
The neutron capture cross sections of light nuclei (
New Stellar 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
H and Po and 75 semi-empirical estimates for isotopes without
experimental information. Concerning stellar cross sections of the
32 stable, proton-rich isotopes produced by the process experimental
information is only available for 20 isotopes, but 9 of them have rather large
uncertainties of 9%. The first part of a systematic study of stellar
cross sections of the -process isotopes Se, Sr,
Pd, Te, Ba, Ba, Dy, and 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 -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
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
Stellar neutron capture cross sections of ²⁰ ²¹ ²²Ne
The stellar (n,γ) cross sections of the Ne isotopes are important for a number of astrophysical quests, i.e., for the interpretation of abundance patterns in presolar material or with respect to the s-process neutron balance in red giant stars. This paper presents resonance studies of experimental data in the keV range, which had not been fully analyzed before. The analyses were carried out with the R-matrix code sammy. With these results for the resonant part and by adding the components due to direct radiative capture, improved Maxwellian-averaged cross sections (MACS) could be determined. At kT=30keV thermal energy we obtain MACS values of 240±29,1263±160, and 53.2±2.7 μbarn for ²⁰Ne,²¹Ne, and ²²Ne, respectively. In earlier work the stellar rates of ²⁰Ne and ²¹Ne had been grossly overestimated. ²²Ne and ²⁰Ne are significant neutron poisons for the s process in stars because their very small MACS values are compensated by their large abundances
(n,γ) cross-sections of light p nuclei: Towards an updated database for the p process
Abstract.: 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 is 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,γ) activation measurements on p nuclei, related by detailed balance to the respective photodisintegrations, was carried out at the Karlsruhe Van de Graaff accelerator using the 7Li(p, n) 7Be source for simulating a Maxwellian neutron distribution of kT = 25keV. We present here preliminary results of our extended measuring program in the mass range between A = 74 and A = 132, including first experimental (n,γ) cross-sections of 74Se, 84Sr, 120Te and 132Ba, and an improved value for 130Ba. In all cases we find perfect agreement with the recommended MACS predictions from the Bao et al. compilatio
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