Astronomy and Astrophysics – Astrophysics
Scientific paper
Oct 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998phdt........15b&link_type=abstract
Thesis (PHD). YALE UNIVERSITY , Source DAI-B 59/04, p. 1716, Oct 1998, 97 pages.
Astronomy and Astrophysics
Astrophysics
1
Fluorine-19, Nucleosynthesis
Scientific paper
In extremely hot and dense astrophysical environments, such as in novae events, the energy generation mechanism may shift from cyclic processes involving hydrogen fusion reactions on only low mass (A < 19) nuclei (the HotCNO cycles), to a violent thermonuclear runaway producing heavier mass nuclei up to 68Se and higher (the rp-process). This shift will determine the power output and the timescale of the explosive event and is, thus, clearly important to understand. One of the pathways for this transition to occur is via the 18F(p,/gamma)19Ne reaction which channels material away from the HotCNO cycles to the rp-process. This reaction competes with the 18F(p,α)15O reaction which returns material back to the HotCNO cycles. To determine the temperatures and densities under which such a transition may occur one must know the locations, widths, and spin-parities of resonant states in the compound nucleus 19Ne just above the 18F + p threshold. The purpose of this work was to check on the existence of the astrophysically important 3/2+ resonant state recently reported in 19Ne at E x~ 7.07 MeV, by searching for its isospin mirror state in 19F. This mirror state was, indeed, observed via the 15N(α,γ)19F reaction to lie at E x = 7101 ± 1 keV, with alpha width, Γα≅ Γ tot = 28 ± 1.1 keV, γ-width, Γgamma = 0.38 ± 0.06 eV, and resonance strength, ωgamma = 0.77 ± 0.11 eV. Under the assumption of equivalent alpha structure of the mirror states, the alpha width of the 19Ne state was determined to be ~30 keV. This is in resonable agreement with two previous studies, but in disagreement, by a factor of ~3, with one. The latest information indicates that the 18F(p,/gamma)19Ne reaction rate under astrophysical conditions is too low for it to be an important pathway for breakout from the HotCNO cycles to the rp-process.
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