Speaker
Description
The efficiency of the weak s-process in low metallicity rotating massive stars depends strongly on the ratio of the reaction rates of the two competing $^{17}$O($\alpha$,n)$^{20}$Ne and $^{17}$O($\alpha$,$\gamma$)$^{21}$Ne reactions, which impacts the poisoning effect of $^{16}$O that consumes the neutrons released by the $^{22}$Ne($\alpha$,n)$^{25}$Mg reaction [1] .
However, the reaction rates of these two competing reactions are poorly known in the astrophysical energy range of interest due to the lack of spectroscopic information (partial widths, spin-parities) on the relevant states in the compound nucleus $^{21}$Ne. Therefore, the $\alpha$-widths of these states were determined experimentaly for the first time by measuring their $\alpha$-spectroscopic factors using the $\alpha$-transfer reaction $^{17}$O($^7$Li,t)$^{21}$Ne. The latter was performed at MLL-Munich using the high-energy resolution magnetic spectrometer Q3D [2].
The measured differential cross sections of the different populated states as well as their analysis using the DWBA formalism will be presented, along with the obtained $\alpha$-spectroscopic factors and $\alpha$-widths of the relevant states in $^{21}$Ne. The new $^{17}$O($\alpha$,n)$^{20}$Ne and $^{17}$O($\alpha,\gamma$)$^{21}$Ne reaction rates calculated using the obtained $\alpha$-widths will be presented and compared with previous evaluations. The new rates favour neutron recycling through the $^{17}$O($\alpha$,n)$^{20}$Ne reaction and suggest an enhancement by more than 1.5 dex of the weak s-elements between zirconium and neodymium in metal-poor rotating massive stars.
[1] A. Choplin, R. Hirschi et al. Astron. Astrophys. 618, A133 (2018)
[2] F. Hammache, P. Adsley, L. Lamia et al., submitted
