On s-process nucleosynthesis during thermal pulses of helium shell-burning

The neutron capture process during the thermal pulses in intermediate-mass stars is investigated for the case of single irradiation, i.e., for the case when matter is exposed to neutrons only once. It is found that neutrons are produced through the 22Ne(α, n)25Mg reaction in the case of a core mass larger than 1.26M sub(solar mass). Because neutron poisons such as 25mg are initially absent in the case of single irradiation, more neutrons are found to be captured by seed nuclei than in the case of multiple or repeated irradiation by neutrons. However, single irradiation synthesizes s-process elements of lower mass numbers 60 <= A <= 90 only. These results are shown to depend only slightly on the ratio of neutron source nuclei to seed nuclei. The role of neutron-producing reaction 18O(α, n)21Ne is also discussed. (auth.)