Sub-threshold states in Ne19 relevant to F18(p,α)

Abstract

Background: Classical novae result from thermonuclear explosions producing several γ-ray emitters which are prime targets for satellites observing in the MeV range. The early ≤ 511 keV gamma-ray emission depends critically on the 18 F(p,α) 15 O reaction rate which, despite many experimental and theoretical efforts, still remains uncertain.Purpose: One of the main uncertainties in the 18 F(p,α) 15 O reaction rate is the contribution in the Gamow window of interference between sub-threshold 19 Ne states and known broad states at higher energies. Therefore the goal of this work is to clarify the existence and the nature of these sub-threshold states.Method: States in the 19 Ne compound nucleus were studied at the Tandem-ALTO facility using the 19 F( 3 He,t) 19 Ne charge exchange reaction. Tritons were detected with an Enge Split-pole spectrometer while decaying protons or α-particles from unbound 19 Ne states were collected, in coincidence, with a double-sided silicon strip detector array. Angular correlations were extracted and constraints on the spin and parity of decaying states established. Results:The coincidence yield at Ex = 6.29 MeV was observed to be high spin, supporting the conclusion that it is indeed a doublet consisting of high spin and low spin components. Evidence for a broad, low spin state was observed around 6 MeV. Branching ratios were extracted for several states above the proton threshold and were found to be consistent with the literature. R-matrix calculations show the relative contribution of sub-threshold states to the astrophysically important energy region above the proton threshold. Conclusions:The levels schemes of 19 Ne and 19 F are still not sufficiently well known and further studies of the analogue assignments are needed. The tentative broad state at 6 MeV may only play a role if the reduced proton width is large.

New measurement of 4He and proton decays from resonant states in 19Ne*

“…The present work focuses on the Ne-resonances above 7.0 MeV, where both α-and p-decays become significant. As indicated in the introduction section, these resonances are directly related to the destruction of F via the F( ) O reaction [6,7,14,16,34,42,43]. Dufour and Descouvemont [13] had predicted a broad 1/2 resonance at about 1.49 MeV above the proton threshold ( 7.9 MeV).…”

“…It occurs not only in α-conjugate nuclei, for example, C, O, Ne , but also in neutronproton asymmetric nuclei-forming molecule-like structures [1−4]. In recent years, cluster-structure studies have also been extended to systems with a hole in one of the cluster cores, such as the α + α + t configuration in B, α + N or t + O configurations in F, and α + O or He + O configurations in Ne ( [5,6] and references therein). Clustering with a hole configuration is useful not only for nuclear structures but also for the synthesis of nuclei in stars during the hydrogen-burning initiated processes [6,7].…”

“…In recent years, cluster-structure studies have also been extended to systems with a hole in one of the cluster cores, such as the α + α + t configuration in B, α + N or t + O configurations in F, and α + O or He + O configurations in Ne ( [5,6] and references therein). Clustering with a hole configuration is useful not only for nuclear structures but also for the synthesis of nuclei in stars during the hydrogen-burning initiated processes [6,7]. For instance, the α-cluster structure in Ne would largely affect the O( ) Ne reaction rate, which is closely related to the breakout of the hot-CNO cycles and rp 18 15 18 β + 18 the ignition of the rapid-proton ( ) process [8−11].…”

“…This is based on the assumption of sufficient knowledge about γ-ray sources, which are believed to be dominated by positron annihilation associated with the decay of F [7,12]. However, after years of studies, there still exist large uncertainties in the reaction F(p, α) O , which destroy the F system, although the production mechanisms for F have been studied wel [6,7]. This is mostly attributed to the limited knowledge about the resonances in Ne around its proton-decay threshold.…”

The impact of metallicity on nova populations

“…Novae are luminous transients that occur when degenerate shells of accreted material burn explosively on the surface of white dwarfs (WDs). As one of the most commonly detected optical transients, novae have the potential to be extremely useful in furthering our understanding of binary evolution (e.g., Starrfield et al 2016;Shara et al 2018;Darnley & Henze 2020;Della Valle & Izzo 2020) and nucleosynthesis (e.g., Wallace & Woosley 1981;Gehrz et al 1998;José & Hernanz 1998;Izzo et al 2015;Tajitsu et al 2015;Starrfield et al 2020;Riley et al 2021).…”