Experimental determination of theO17(p,α)

Abstract

The 17 O(p, α) 14 N and 17 O(p, γ ) 18 F reactions are of major importance to hydrogen-burning nucleosynthesis in a number of different stellar sites. In particular, 17 O and 18 F nucleosynthesis in classical novae is strongly dependent on the thermonuclear rates of these two reactions. The previously estimated rate for 17 O(p, α) 14 N 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 17 O(p, γ ) 18 F rate. We report on the observation of a previously undiscovered resonance at E c.m. = 183.3 keV in the 17 O(p, α) 14 N reaction, with a measured resonance strength ωγ pα = (1.6 ± 0.2) × 10 −3 eV. We studied in the same experiment the 17 O(p, γ ) 18 F reaction by an activation method, and the resonance strength was found to amount to ωγ pγ = (2.2 ± 0.4) × 10 −6 eV. The excitation energy of the corresponding level in 18 F was determined to be 5789.8 ± 0.3 keV in a Doppler shift attenuation method measurement, which yielded a value of τ < 2.6 fs for the level lifetime. The 17 O(p, α) 14 N and 17 O(p, γ ) 18 F reaction rates were calculated using the measured resonance properties and reconsidering some previous analyses of the contributions of other levels or processes. The 17 O(p, α) 14 N 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 17 O(p, γ ) 18 F rate is somewhat larger because of remaining obscurities in the knowledge of the direct capture process. These new resonance properties have important consequences for 17 O nucleosynthesis and γ -ray emission of classical novae.A. CHAFA et al. PHYSICAL REVIEW C 75, 035810 (2007) 035810-2 E R (keV) a

Effects of nuclear cross sections on19F nucleosynthesis at low metallicities

“…After the pioneering work of Rolfs (1973) in the seventies, several studies (Fox et al 2005;Chafa et al 2007;Newton et al 2010;Hager et al 2012;Kontos et al 2012;Scott et al 2012) have determined the reaction rate with improving precision in the past decade. Nevertheless, at the temperatures relevant for AGB nucleosynthesis, the reaction rate is dominated by the lowest energy resonance, E cm = 65 keV, which is too weak to be directly measured with the current experiment possibilities.…”

“…17 O(p,α) 14 N. Several experiments (Chafa et al 2007;Newton et al 2010;Moazen et al 2007) have determined the magnitude of most of the several resonances that influence the rate of this reaction. As for the 17 O(p,γ) 18 F, the E cm = 65 keV resonance, directly at the astrophysically relevant energy, makes the reaction rate determination difficult, and the uncertainty is correspondingly large.…”

The AGB star nucleosynthesis in the light of the recent 17O(p,α)14N and 18O(p,α)15N reaction rate determinations

“…Panel a) of figure 1 shows the ratio (red middle line) between the reaction rate R extracted including the 65 keV resonance strength measured by THM, and the reaction rate R Cha f a by [20], also reported in the compilation by [23]. The other red lines mark the position of the upper and lower limits as deduced in [11].…”

“…The E c.m. = 65keV resonance has been measured by [11] applying the THM to the quasi-free 2 H( 17 O, 14 Nα)n reaction and by normalizing experimental data to the weighted average of the three values for the 183 keV resonance strength reported in the literature [20,21,22]. This result has been used to calculate the contribution of the 65 keV resonance to the total reaction rate adopting the narrow resonance approximation, whose conditions are satisfied for the resonance under investigation [13,14].…”

“…The other red lines mark the position of the upper and lower limits as deduced in [11]. The blue band represents the Chafa reaction-rate [20] range allowed for by the experimental uncertainties. A small difference (∼ 20%) can be seen in the range T 9 = 0.02 − 0.1, while no significant differences are present for T 9 ≥ 0.2, where the contribution of the 65keV resonance to the reaction rate is negligible.…”