Experimental determination of the 17O(p,α)14N and 17O(p,γ)18F reaction rates

The O-17(p,alpha)N-14 and O-17(p,gamma)F-18 reactions are of major importance to hydrogen-burning nucleosynthesis in a number of different stellar sites. In particular, O-17 and F-18 nucleosynthesis in classical novae is strongly dependent on the thermonuclear rates of these two reactions. The previously estimated rate for O-17(p,alpha)N-14 carries very large uncertainties in the temperature range of classical novae (T=0.01-0.4 GK), whereas a recent measurement has reduced the uncertainty of the O-17(p,gamma)F-18 rate. We report on the observation of a previously undiscovered resonance at E-c.m.=183.3 keV in the O-17(p,alpha)N-14 reaction, with a measured resonance strength omega gamma(p alpha)=(1.6 +/- 0.2)x10(-3) eV. We studied in the same experiment the O-17(p,gamma)F-18 reaction by an activation method, and the resonance strength was found to amount to omega gamma(p gamma)=(2.2 +/- 0.4)x10(-6) eV. The excitation energy of the corresponding level in F-18 was determined to be 5789.8 +/- 0.3 keV in a Doppler shift attenuation method measurement, which yielded a value of tau < 2.6 fs for the level lifetime. The O-17(p,alpha)N-14 and O-17(p,gamma)F-18 reaction rates were calculated using the measured resonance properties and reconsidering some previous analyses of the contributions of other levels or processes. The O-17(p,alpha)N-14 rate is now well established below T=1.5 GK, with uncertainties reduced by orders of magnitude in the temperature range T=0.1-0.4 GK. The uncertainty in the O-17(p,gamma)F-18 rate is somewhat larger because of remaining obscurities in the knowledge of the direct capture process. These new resonance properties have important consequences for O-17 nucleosynthesis and gamma-ray emission of classical novae.