The purpose of this web page is to provide further details on the r-process at low neutron excess through figures and movies. You will need an appropriate media player to view the movies. If you are using Windows, Solaris, or a Mac operating system, you can download Windows Media Player from here.
The Calculations:
The calculations were performed with the Clemson nuclear network code. Some details are provided in the text of the paper and the references.The network employed included all species from the proton-drip line to the neutron-drip line up to Z=91. The last species (275Pa) was allowed to fission into two fragments.
The nuclear masses used were from Wapstra, Audi, and Hoekstra and from Moeller, Nix, and Swiatecki.Details are provided on the calculations discussed in the paper, namely, the Ye = 0.5, s/k = 150 expansions (at tau = 0.03, 0.003, and 0.0003 seconds), the Ye = 0.4975, s/k = 150 expansions (at tau = 0.0007 ,0.0008 ,0.0009 seconds), and a Big-Bang expansion. In addition, for further illustration of the key points, details are provided on a more standard alpha-rich freezeout expansion (Ye = 0.5, s/k =10, and tau = 0.2 s) and on a Ye = 0.4975, s/k = 150, and tau = 0.0009 s expansion.
The Expansions:Tau=0.2 second, s/k=10, Ye=0.5 expansion:
This expansion models the explosive nucleosynthesis typical in material near the mass cut of core-collapse supernovae. Such matter has been shocked to high temperature and an entropy per nucleon of roughly 10 k. It has nearly equal numbers of neutrons and protons. In the expansion presented here, the material began at high temperature (T9 = 10), so the initial state was NSE. As the matter expanded and cooled, the inefficiency of the three-body reactions led to an underabundance of heavy nuclei relative to the NSE. Nevertheless, the heavy nuclei themselves remained in a QSE such that they were in equilibrium under exchange of light particles. Eventually the network abundances diverged from the QSE and the matter froze out.
This expansion models the synthesis in the
early universe. The entropy per nucleon in the expansion was 2.42 x 1010
k, which gives a current nucleon-to-photon ratio of 3 x 10-10.
The network code integrated the Friedmann equations and the relevant integrals
for the weak interaction rates on free neutrons and protons.
These expansions are those explicitly discussed in the paper. The material begins at T9 near 10 and expands and cools on a density e-folding timescale tau. The expansion converts to a constant velocity expansion at lower temperature, as described in the paper. While the tau=0.03 and 0.003 second expansions result in abundances dominated by alpha particles and 56Ni, as in the tau=0.2 second, s/k=10, Ye = 0.5 expansion above, the tau=0.0003 second expansion surprisingly produces heavy r-process nuclei because of a persistent disequilibrium between the free nucleons and alpha particles.
These expansions are similar to the s/k=150, Ye=0.5 expansions above except that they are for slightly neutron-rich matter (5 excess neutrons for every 1000 total nucleons). The e-folding timescales are 0.0007, 0.0008, and 0.0009 seconds. The 0.0008 and 0.0009 second expansions make second-peak (mass number A=130) nuclei, but few third-peak (mass number A=195) nuclei. The 0.0007 second expansion predominantly makes third-peak nuclei because it has a persistent disequilibrium between nucleons and alpha particles below T9=8, just like in the Big Bang.
Work on this site has been supported by NASA grant NAG5-4703, by NSF grant AST 98-19877, and by a SciDAC grant from the High Energy and Nuclear Physics Division of the Department of Energy (the Terascale Supernova Initiative). Any opinions, findings and conclusions or recommendations are those of the authors and do not necessarily reflect the views of the National Aeronautic and Space Agency (NASA), the National Science Foundation (NSF), or the Department Of Energy (DOE).
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This page has been accessedtimes since December 14th,2001