The Astrophysical Reaction Rates For ～(18)F(P, α)～(15)O And ～(18)F(P, γ)～(19)Ne And Reaction Network

Nuclear astrophysics involves study of the synthesis of elements and the evolution of cosmic sites where such syntheses occur. Systems as diverse as the early universe, the interstellar medium, red giant stars, and supernova explosions are currently the focus of intense studies utilizing sophisticated computer models - models which require large quantities of nuclear data as input. These models require, in general, the rates of and energy released in nuclear reactions occurring in astrophysical environments.The reaction rates of 18F(p,α)150 and 18F(p,γ)19 are important in hot astrophysical sites and events, to understand the gamma-ray emission from Nova. Observations of such gamma rays by the coming generation of space-based telescopes would provide a rather direct test of nova models.The rate of the 18F(p,α)15O and 18F(p,γ)19 reactions, however, have been uncertain because of the unknown level structure of 19Ne above the 18F+p threshold. To accurately calculate the rate, one needs to know the excitation energies and the proton- and alpha-decay widths of several important 19Ne states. We have used a 18F beam at the ORNL Holifield Radioactive Ion Beam Facility (HRIBF) to significantlyreduce the uncertainties in the 18F(p,α)15O rate. Specially, we have determined the properties (strength γ, resonance energy Er and spin-parity Jp) of the two most important resonances at Ecm = 330 and 665 keV by measuring the 18F(p,a)15O cross section directly [Bar02, Bar01]. We have additionally searched for missing states in 19Ne via a measurement of the thick-target lH(18F,p)18F excitation function. Finally, we have indirectly studied the 19Ne states of interest by measuring the energy and angular distributions of protons emitted from the 2H(18F,p)19F reaction at 6 MeV/u [Koz03].This work is to combine the information obtained from these studies, and make estimations for unknown quantities, to determine the best reaction rates for 18F(p,α)15O and 18F(p,γ)19Ne. The rates are given in tabulated numerical data with uncertainties. The parameterized analytical expressions as function of temperature are also presented.A reaction network program is created and used to analyze the astrophysical effects of the new reaction rates 18F(p,α)15O, 18F(p,γ)19Ne and 11C(p,γ)12N. The new rate of 18F + p could produce form more production of 18F that expected by the rates of Ref. [Coc00]. The new rate of 11C(p,γ)12N is larger by about a factor of 2 than that of Ref. [Tan02], and could transfer 3He to CNO materials earlier, so that it may act as a seed of the CNO cycle and cause the explosive of the massive star.