Measurement of radiative proton capture onF18and implications for oxygen-neon novae reexamined

Abstract

Background: The rate of the 18 F(p,γ) 19 Ne reaction affects the final abundance of the radioisotope 18 F ejected from novae. This nucleus is important as its abundance is thought to significantly influence the first stage 511 keV and continuum γ-ray emission in the aftermath of novae. No successful measurement of this reaction existed prior to this work, and the rate used in stellar models had been calculated based on incomplete information from contributing resonances. Purpose:Of the two resonances thought to provide a significant contribution to the astrophysical reaction rate, located at Ec.m.=330 and 665 keV, the former has a radiative width estimated from the assumed analogue state in the mirror nucleus, 19 F, while the latter resonance does not have an analogue state assignment at all, resulting in an arbitrary radiative width being assumed. As such, a direct measurement was needed to establish what role this resonance played in the destruction of 18 F at nova temperatures. This paper extends and takes the place of a previous Letter which reported the strength of the Ec.m.=665 keV resonance.Method: The DRAGON recoil separator was used to directly measure the strength of the important 665 keV resonance in this reaction, in inverse kinematics, by observing 19 Ne reaction products. Radioactive 18 F beam was provided by the ISAC facility at TRIUMF. R-matrix calculations were subsequently used to evaluate the significance of the results at astrophysical energies. Results:We report the direct measurement of the 18 F(p,γ) 19 Ne reaction with the re-evaluation of several detector efficiencies and the use of an updated 19 Ne level scheme in the reaction rate analysis. The strength of the 665 keV resonance (Ex=7.076 MeV) is found to be an order of magnitude weaker than currently assumed in nova models. An improved analysis of the previously reported data is presented here, resulting in a slightly different value for the resonance strength. These small changes, however, do not alter the primary conclusions.Conclusions: Reaction rate calculations definitively show that the 665 keV resonance plays no significant role in the destruction of 18 F at novae temperatures.