D. Robertson

Experimental cross sections of

Background: Optical potentials are crucial ingredients for the prediction of nuclear reaction rates needed in simulations of the astrophysical

{}^{165}

ensuremath{gamma}

Ho

process. Associated uncertainties are particularly large for reactions involving

(\alpha,

ensuremath{alpha}

n

particles. This includes

){}^{168}

(ensuremath{gamma},ensuremath{alpha})

Tm and

reactions which are of special importance in the

{}^{166}

ensuremath{gamma}

Er

process.

(\alpha,

the interpretation of the direct capture and the contributions of the broad resonances in the lower energy range. This work aims to address these issues. Method: The reaction cross section was measured in a wide proton energy range (Ec:m: = 345 - 1700 keV) and at several angles ( lab = 0 ; 45 ; 90 ; 135 ). The observed primary -transitions were used as input in an R-matrix code in order to obtain the contribution of the direct capture and the two broad resonances to the low-energy region. Results: The extrapolated S-factor from the present data is in good agreement with the existing literature data in the low-energy region. A new reaction rate was calculated from the combined results of this work and literature S-factor determinations. Resonance strengths and branchings are reported for several 18 F states. Conclusions: We were able to extrapolate the astrophysical S-factor of the reaction 17 O(p; ) 18 F at low energies from cross section data taken at higher energies. No signicant changes in the nucleosynthesis are expected from the newly calculated reaction rate.