Astrophysical Reaction Rate for the Neutron-Generator ReactionC13(α,n)O16

Abstract

The reaction 13C(alpha,n) is considered to be the main source of neutrons for the s process in asymptotic giant branch stars. At low energies, the cross section is dominated by the 1/2+ 6.356 MeV subthreshold resonance in (17)O whose contribution at stellar temperatures is uncertain by a factor of 10. In this work, we performed the most precise determination of the low-energy astrophysical S factor using the indirect asymptotic normalization (ANC) technique. The alpha-particle ANC for the subthreshold state has been measured using the sub-Coulomb alpha-transfer reaction ((6)Li,d). Using the determined ANC, we calculated S(0), which turns out to be an order of magnitude smaller than in the nuclear astrophysics compilation of reaction rates.

NEW DETERMINATION OF THE13C(α,n)16O REACTION RATE AND ITS INFLUENCE ON THEs-PROCESS NUCLEOSYNTHESIS IN AGB STARS

“…In Table 4, we list the present adopted rate as a function of the temperature together with its upper and lower limits. In Figure 5, we compare the present rate with the previous compilations by CF88 (Caughlan & Fowler 1988) and NACRE (Angulo et al 1999) and with other rates available in the literature (Denker et al 1995;Kubono et al 2003;Johnson et al 2006;Pellegriti et al 2008;Heil et al 2008) at a temperature of 100 MK. Our rate agrees with that of Denker et al (1995) and it is about two times smaller than the NACRE recommended rate, but within its lower limit.…”

“…This indicates that the S α and ANC for the 1/2 + state of 17 O should not significantly depend on the selection of transfer systems and incident energies. On the other hand, theC 2 obtained from the 6 Li( 13 C, d) reaction at sub-Coulomb energies by Johnson et al (2006) is about five times smaller than that obtained in the present work and by Pellegriti et al (2008). The sub-Coulomb ( 6 Li, d) α-transfer cross section was recently remeasured to understand the source of this discrepancy and the data analysis is in progress (G. V. Rogachev 2012, private communication).…”

“…These authors derived larger S α factors of 0.36 and 0.40 via DWBA and CRC calculations, respectively. Johnson et al (2006) measured the 6 Li( 13 C, d) 17 O angular distribution at sub-Coulomb energies (8 and 8.5 MeV) and derivedC 2 = 0.89 ± 0.23 fm −1 . Pellegriti et al (2008) measured the 13 C( 7 Li, t) 17 O angular distribution with two incident energies (28 and 34 MeV).…”

“…The astrophysical S-factor of the 13 C(α, n) 16 O reaction via the resonances can be calculated with the Breit-Wigner formula, Johnson et al (2006) 6 Li( 13 C, d) 8.0 and 8.5 0.89 ± 0.23 Pellegriti et al (2008) 13 C( 7 Li, t) 28 and 34 0.29 ± 0.11 4.5 ± 2.2 Present work 13 C( 11 B, 7 Li) 50 0.37 ± 0.12 4.0 ± 1.1…”

“…Although three indirect measurements via the ( 6 Li, d) or the ( 7 Li, t) system have been performed by Kubono et al (2003), Johnson et al (2006), and Pellegriti et al (2008) to study the S α or the ANC of the 1/2 + state, a significant discrepancy of up to a factor of ∼30 still exists in the derived S α and ANC. Therefore, it is interesting to perform a new measurement of the S α and the ANC via an independent transfer reaction.…”