The first science result with the JENSA gas-jet target: Confirmation and study of a strong subthreshold F18(p,α)

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

“…A Γ p of 7.3±0.6 eV was extracted from proton transfer data [10], however, the population of the 330 keV resonance could have been contaminated by nearby states. 19 Ne studies by Laird et al [17] and Bardayan et al [18], respectively, found discrepancies in the spin and parity assignments for levels close to the proton threshold in 19 Ne. This may have ramifications for the 18 F(p,γ) 19 Ne reaction rate at nova temperatures; the new spin and parity assignments differ from those published previously, leading to changes in the interference effects with stronger, higher lying states, hence altering the capture cross section.…”

“…Hence there is significant interest in radioisotopes thought to be abundantly produced in such events, in the hope that their decay signatures can be directly observed. One radioisotope of interest is 18 F, as its positron annihilation, following β + decay, is thought to be the major source of 511 keV line emission γ-rays after nova outbursts [3]. This is due to the fact that the relatively long half life of 18 F (t 1/2 =110 mins) results in a significant number of positrons being emitted shortly after the expanding nova envelope becomes transparent to γ-rays, whilst its halflife is short enough that its absolute decay rate remains high.…”

“…This is due to the fact that the relatively long half life of 18 F (t 1/2 =110 mins) results in a significant number of positrons being emitted shortly after the expanding nova envelope becomes transparent to γ-rays, whilst its halflife is short enough that its absolute decay rate remains high. If the 18 F production and destruction rates are sufficiently well known then observations of this nuclide's γ-ray emission can provide constraints on physical conditions inside novae, leading to improvements in astrophysical models. Such models require additional experimental data to help address discrepancies in their predictions with observations.…”

“…Understanding both the production and destruction rates of 18 F at nova temperatures is critical for the aforementioned reasons. One of the main uncertainties in this radioisotope's final abundance depends on its destruction rate via the (p,γ) and (p,α) channels.…”

“…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. A sensitivity study by Iliadis et al [6] indicates that, for ONe novae, a factor of 10 increase or decrease in the (p,γ) rate changes the calculated abundance of 18 F by a factor of 2.5 or 0.9, respectively. This significantly affects the potential number of novae whose 511 keV γ-ray line emission is detectable via satellite missions.…”