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