New measurements of theF19(n,γ)F20cross section and their implications for the stellar reaction rate

Abstract

The puzzle concerning the nucleosynthetic origin of $^{19}\mathrm{F}$ has been a topic of much interest in astrophysics. After the observation of an overabundance of $^{19}\mathrm{F}$ in thermally pulsing asymptotic giant branch (AGB) stars, attention has been drawn to the He shell flash, characteristic of these stars, as a possible site of fluorine synthesis. As the He intershell of AGBs is also known to undergo periods of high neutron exposure, $^{19}\mathrm{F}$ synthesized in this zone would be in danger of destruction by $^{19}\mathrm{F}$($n,\ensuremath{\gamma}$)$^{20}\mathrm{F}$. The current recommended value of the Maxwellian averaged cross section (MACS) is uncertain by 20% in a temperature region corresponding to the He flash, which is insufficient for accurate stellar modeling. A measurement of the cross section has been performed at the Karlsruhe 3.7 MV Van de Graaff accelerator by cyclic activation of fluorine samples in a quasistellar neutron spectrum with a mean thermal energy of $\mathit{kT}=25$ keV. The new MACS at $\mathit{kT}=25$ keV is 44% lower and six times more accurate than reported previously.