The role of the C13(α,n)O16 reaction in the synthesis of heavy elements.

Abstract

view Abstract Citations (4) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The role of the C13(α,n)O16 reaction in the synthesis of heavy elements. Cameron, A. G. W. Abstract The author has suggested that C'3 is made when helium thermonuclear reactions commence in a stellar core and the resulting C'2 mixes with envelope hydrogen, and that subsequent C'3 (a,n) 016 reactions will produce enough neutrons to synthesize large abundances of heavy elements. This has been criticized on the grounds that there would be too much competition for the neutrons by the N'4(n,p)C14 reaction. The situation has been clarified by numerical integrations. It was assumed that the onset of the Be' (a,~) C'2 reaction released a lot of energy in the core, causing expansion and mixing of core and envelope material. The amount of C'2 formed and the amount of hydrogen mixed with it were treated as variable parameters. Changes of composition were followed as (I), carbon-cycle reactions exhausted the admixed hydrogen; (2), the C" formed reacted with helium; (3) the resulting neutrons were absorbed by heavy elements in competition with N'4 and (4), the protons produced by the N'4 (n,p) C'4 reaction regenerated some C'3. The variation of the heavy-element absorption cross section with composition was taken into account. Cases considered were neutron-production time scales long and short compared to the C'4 half- life, a possible thermal resonance in the C'4 (p,~)N" reaction, and a solar and 0.1 times solar (Pop. II) ratio of heavy elements to hydrogen. For a solar composition, if more than 4 to 8 per cent of the core helium (dependent on time scale) is converted to C", then overabundance ratios of heavy elements >~ 100 are produced when 0.05 to 0.2 protons are admixed per C'2 nucleus in the core. The acceptable range of conditions is wider for a Population II composition. Hence, if envelope hydrogen does not mix to the centre of the core, then large overabundance ratios of heavy elements may be produced in a shell of undetermined thickness. A very much wider range of acceptable conditions exists if the admixed hydrogen is consumed at temperatures > 100 X 106 0K, for then the most abundant equilibrium product of the carbon cycle is N'3. Atomic Energy of Canada Limited, Chalk River, Ontario. Publication: The Astronomical Journal Pub Date: July 1957 DOI: 10.1086/107489 Bibcode: 1957AJ.....62..138C full text sources ADS |