Resonances in19Ne with relevance to the astrophysically important18F(p,

David Mountford

¹

,

A. St. J. Murphy

²

,

N. L. Achouri

³

et al.

Abstract

The most intense gamma-ray line observable from novae is likely to be from positron annihilation associated with the decay of 18 F. The uncertainty in the destruction rate of this nucleus through the 18 F(p,α) 15 O reaction presents a limit to interpretation of any future observed gamma-ray flux. Direct measurements of the cross section of both this reaction and the 18 F(p,p) 18 F reaction have been performed between center of mass energies of 0.5 and 1.9 MeV. Simultaneous fits to both data sets with the R-Matrix formalism reveal several resonances, with the inferred parameters of populated states in 19 Ne in general agreement with previous measurements. Of particular interest, extra strength has been observed above ECM ∼1.3 MeV in the 18 F(p,p) 18 F reaction and between 1.3-1.7 MeV in the 18 F(p,α) 15 O reaction. This is well described by a broad 1/2 + state, consistent with both a recent theoretical prediction and an inelastic scattering measurement. The astrophysical implications of a broad sub-threshold partner to this state are discussed.

Recent results in nuclear astrophysics

“…Seven resonances in total were identified and their parameters were deduced from a χ 2 minimization R-matrix fit of the measured excitation functions of both the 18 F(p,p) 18 F and 18 F(p,α) 15 O reactions [196]. An overall agreement was found between the derived parameters and associated er- [196]. Copyright 2012 by the American Physical Society.]…”

“…14. Several resonant structures are observed and the most important one, at 655 keV, belongs to the well known 7076 keV J π =3/2 + state in 19 Ne. Seven resonances in total were identified and their parameters were deduced from a χ 2 minimization R-matrix fit of the measured excitation functions of both the 18 F(p,p) 18 F and 18 F(p,α) 15 O reactions [196]. An overall agreement was found between the derived parameters and associated er- [196].…”

“…If they exist they would lead to a significant contribution in the relevant energy range, especially the lowest energy one, above the threshold. Indeed, experiments performed at LLN [195] and GANIL [196] support the presence of such a broad state at E r ≈1.3 MeV, motivating the search for its subthreshold predicted counterpart.…”

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

“…One of the main uncertainties in this radioisotope's final abundance depends on its destruction rate via the (p,γ) and (p,α) channels. The 18 F(p,α) 15 O reaction is estimated to be a few thousand times faster [7] and therefore it has been the focus of many previous studies [8][9][10] (and references therein). Little effort has gone into the study of the 18 F(p,γ) 19 Ne reaction however, based on its much lower estimated cross section and the limited availability of sufficiently intense radioactive beams.…”

“…Calculations from an R-matrix fit to the experimental data given in Ref. [8] were used instead. Inspection of the experimental data points and their associated uncertainty revealed a cross section measurement uncertainty on the order of 6.5% in the relevant energy region.…”