F. Kaeppeler

The s-process between A = 120 and 124 : signature of neutron density and temperature in red giants

The mass region between tin and tellurium is of relevance for s-process nucleosynthesis, since the s-only isotopes 122 Te, 123 Te, and 124 Te provide for a sensitive test of the neutron density during helium burning in red giant stars. In addition to the very accurate cross sections that are available for these s-only nuclei, complementary measurements are reported on 120 Sn, 121 Sb, 123 Sb, and 128 Te for an improved analysis of the s-process flow. The measurements were carried out via the activation technique in a quasi-stellar neutron spectrum for kT=25 keV, using gold as a cross section standard. The cross sections of the unstable isotopes, 121 Sn and 122 Sb, were determined by statistical model calculations with a carefully evaluated set of input parameters

Stellar (n,{gamma}) cross sections of p-process isotopes. II. {sup 168}Yb, {sup 180}W, {sup 184}Os, {sup 190}Pt, and {sup 196}Hg

The neutron-capture cross sections of {sup 168}Yb, {sup 180}W, {sup 184}Os, {sup 190}Pt, and {sup 196}Hg have been measured by means of the activation technique. The samples were irradiated in a quasistellar neutron spectrum of kT=25 keV, which was produced at the Karlsruhe 3.7-MV Van de Graaff accelerator via the {sup 7}Li(p,n){sup 7}Be reaction. Systematic uncertainties were investigated in repeated activations with different samples and by variation of the experimental parameters, that is, irradiation times, neutron fluxes, and {gamma}-ray counting conditions. The measured data were converted into Maxwellian-averaged cross sections at kT=30 keV, yielding 1214{+-}61, 624{+-}54, 590{+-}43, 511{+-}46, and 201{+-}11 mb for {sup 168}Yb, {sup 180}W, {sup 184}Os, {sup 190}Pt, and {sup 196}Hg, respectively. The present results either represent first experimental data ({sup 168}Yb, {sup 184}Os, and {sup 196}Hg) or could be determined with significantly reduced uncertainties ({sup 180}W and {sup 190}Pt). These measurements are part of a systematic study of stellar (n,{gamma}) cross sections of the stable p isotopes.

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

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