Coincidence Measurements of Reaction Branchings in Hot Carbon-Nitrogen Cycles and the Thermonuclear Runaway Process.

Details Coincidence Measurements of Reaction Branchings in Hot Carbon-Nitrogen Cycles and the Thermonuclear Runaway Process. Coincidence Measurements of Reaction Branchings in Hot Carbon-Nitrogen Cycles and the Thermonuclear Runaway Process.

Jan 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994phdt........15r&link_type=abstract

Thesis (PH.D.)--UNIVERSITY OF NOTRE DAME, 1994.Source: Dissertation Abstracts International, Volume: 55-07, Section: B, page: 28

Astronomy and Astrophysics

Astrophysics

1

Scientific paper

Nucleosynthesis and energy production in hot stellar environments such as novae and supernovae may take place via explosive hydrogen burning. In the mass range rm 12 < A < 20 reaction sequences follow "Hot" CNO cycles, while for mass A > 20 a thermonuclear runaway known as the rp process may occur, leading to elements in the Ni-Fe region. The main path taken during nucleosynthesis is highly dependent on temperature and density conditions. The present work discusses experiments performed to better understand reaction branchings which can occur at three nuclei in these mass regions. Proton capture reactions on ^{18 }F can become important at temperatures greater than 1.5 times 10^8 K. A competition exists between ^{18 }F(p,alpha)15O and 18F(p,gamma) 19Ne, the stronger reaction determining the path in the HCNO cycles and leading to a possible breakout to higher masses. Previously, very little was known about levels above the proton threshold in 19Ne, therefore the reactions 16O( ^6Li,t)19Ne and 19F(^3He,t)^{19 }Ne were used to determine the location of proton unbound levels. Since a direct measurement of resonance strengths was not possible, proton unbound levels were populated via 19F( ^3He,t)19Ne and the subsequent particle decays were measured using coincidence techniques to investigate the reaction branching. Resonance parameters were assigned on the basis of the measured branchings and the properties of the assumed analogs in ^ {19}F. The stellar reaction rates for 31P(p,gamma)32S and 35Cl(p, gamma)36Ar have previously been determined in the astrophysically interesting temperature range, but the competing (p,alpha) reactions on 31P and 35Cl have large uncertainties due to an inability to directly measure low-lying resonances. Therefore indirect methods were employed to investigate the (p, alpha) resonance strengths using coincidence techniques. The reactions ^{31 }P(^3He,d)32S and 35Cl(^3He,d) 36Ar were used to populate proton unbound states in 32S and 36Ar, and the subsequent particle and gamma decays were measured to determine the branchings. The results of these experiments have allowed an improved calculation of the stellar reaction rates for hydrogen burning in hot stellar environments. The astrophysical implications of these improved rates are discussed.

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