New measurements of the ¹⁹F(n,γ)²⁰F cross section and their implications for the stellar reaction rate

The puzzle concerning the nucleosynthetic origin of ¹⁹F has been a topic of much interest in astrophysics. After the observation of an overabundance of ¹⁹F 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, ¹⁹F synthesized in this zone would be in danger of destruction by ¹⁹F(n,γ)²⁰F. 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 kT=25 keV. The new MACS at kT=25 keV is 44% lower and six times more accurate than reported previously.