4,227 research outputs found
Reaction Rate Sensitivity of the gamma-Process Path
The location of the (gamma,p)/(gamma,n) and (gamma,alpha)/(gamma,n) line at
gamma-process temperatures is discussed, using recently published reaction
rates based on global Hauser-Feshbach calculations. The results can directly be
compared to previously published, classic gamma-process discussions. The nuclei
exhibiting the largest sensitivity to uncertainties in nuclear structure and
reaction parameters are specified.Comment: 4 pages, contribution to Nuclei in the Cosmos VIII, to appear in
Nucl. Phys.
Relevant energy ranges for astrophysical reaction rates
Effective energy windows (Gamow windows) of astrophysical reaction rates for
(p,gamma), (p,n), (p,alpha), (alpha,gamma), (alpha,n), (alpha,p), (n,gamma),
(n,p), and (n,alpha) on targets with 10<=Z<=83 from proton- to neutron-dripline
are calculated using theoretical cross sections. It is shown that widely used
approximation formulas for the relevant energy ranges are not valid for a large
number of reactions relevant to hydrostatic and explosive nucleosynthesis. The
influence of the energy dependence of the averaged widths on the location of
the Gamow windows is discussed and the results presented in tabular form (also
at http://download.nucastro.org/astro/gamow/).Comment: 8 pages, 12 figures; ASCII table of results at
http://download.nucastro.org/astro/gamow/ ; slightly revised text, to appear
in Phys. Rev.
Comment on "187Re(gamma,n) cross section close to and above the neutron threshold"
The work of M\"uller et al. [Phys. Rev. C 73, 025804 (2006);
astro-ph/0512603] provides interesting experimental data on neutron emission by
photodisintegration of 187Re. However, the comparison to theory and the
discussed implications for the Re/Os clock require considerable amendment.Comment: 2 pages; accepted for publication in Phys. Rev.
Astrophysical Reaction Rates as a Challenge for Nuclear Reaction Theory
The relevant energy ranges for stellar nuclear reactions are introduced.
Low-energy compound and direct reactions are discussed. Stellar modifications
of the cross sections are presented. Implications for experiments are outlined.Comment: 8 pages, 2 figures; invited talk at OMEG10, March 2010. To appear in
the OMEG10 proceedings published by AI
Nuclear Partition Functions at Temperatures Exceeding 10^10 K
Nuclear partition functions were calculated for a grid of temperatures from
1.2x10^10 K to 2.75x10^11 K (1<=kT<=24 MeV) within a Fermi-gas approach,
including all nuclides from the proton-dripline to the neutron-dripline with
proton number 9<=Z<=85. The calculation is based on a nuclear level density
description published elsewhere, thus extending the previous tables of
partition functions beyond 10^10 K. Additional high temperature corrections had
to be applied.Comment: 12 pages with 2 figures, accepted by Ap. J. Suppl.; additional
material can be downloaded from http://ftp.nucastro.org/astro/fits/partfuncs
Possible solution to the alpha-potential mystery in the gamma-process and the Nd/Sm ratio in meteorites
Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence.The 146Sm/144Sm ratio in the early solar system has been constrained by Nd/Sm isotope ratios in meteoritic material. Predictions of 146Sm and 144Sm production in the gamma-process in massive stars are at odds with these constraints and this is partly due to deficiences in the prediction of the reaction rates involved. The production ratio depends almost exclusively on the (gamma,n)/(gamma,alpha) branching at 148Gd. A measurement of 144Sm(alpha,gamma)148Gd at low energy had discovered considerable discrepancies between cross section predictions and the data. Although this reaction cross section mainly depends on the optical alpha+nucleus potential, no global optical potential has yet been found which can consistently describe the results of this and similar alpha-induced reactions. The untypically large deviation in 144Sm(alpha,gamma) can be explained, however, by low-energy Coulomb excitation which is competing with compound nucleus formation at very low energies. Low-energy (alpha,gamma) and (alpha,n) data on other nuclei can also be consistently explained in this way. Since Coulomb excitation does not affect alpha-emission, the 148Gd(gamma,alpha) rate is much higher than previously assumed. This leads to very small 146Sm/144Sm stellar production ratios, in even more pronounced conflict with the meteorite data
General properties of astrophysical reaction rates in explosive nucleosynthesis
“Published under licence in Journal of Physics: Conference Series by IOP Publishing Ltd. 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.”Fundamental differences in the prediction of reaction rates with intermediate and heavy target nuclei compared to the ones with light nuclei are discussed, with special emphasis on stellar modifications of the rates. Ground and excited state contributions to the stellar rates are quantified, deriving a linear weighting of excited state contributions despite of a Boltzmann population of the nuclear states. A Coulomb suppression effect of the excited state contributions is identified, acting against the usual Q-value rule in some reactions. The proper inclusion of experimental data in revised stellar rates is shown, containing revised uncertainties. An application to the s-process shows that the actual uncertainties in the neutron capture rates are larger than would be expected from the experimental errors alone. Sensitivities of reaction rates and cross sections are defined and their application in reaction studies is discussed. The conclusion provides a guide to experiment as well as theory on how to best improve the rates used in astrophysical simulations and how to assess their uncertainties.Peer reviewe
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