Investigating Early Nucleosynthesis of the Lighter Heavy Elements

The \textit{r}-process produced half of all heavy elements in the universe today. While models successfully reproduce the abundance distributions of the heaviest elements observed in Ultra-Metal Poor stars, the lighter heavy elements ($36<Z<47$) are found to be more abundant than predicted. An additional nucleosynthesis process operating at early times in the universe, preferentially producing lighter heavy elements, has been proposed. This project consisted of experimental investigations into two candidates for this process: the weak \textit{r}-process in the neutrino-driven winds of core-collapse supernovae and the \textit{s}-process in rotating massive stars. The $^{20}$Ne(d,p)$^{21}$Ne reaction was studied using the HELIOS spectrometer at Argonne National Laboratory; angular distributions of energy levels in the $^{21}$Ne nucleus were measured to determine their neutron widths and spin-parities. These parameters are important for determining the neutron poisoning effects of $^{16}$O on the \textit{s}-process in rotating massive stars. Results for several levels are reported. $J^{\pi}=\frac{3}{2}^-$ and $\Gamma_n=7600\pm2100$\,eV was found for the 7820\,keV state disagreeing with the literature assignment of $2J^{\pi}=(3,5)^+$ and a neutron width limit of $\Gamma_n<7200$\,eV was determined for the first time for the 7749\,keV state. Resonance strengths calculated with these results are compared to literature. Both the 7749\,keV and 77820\,keV energy levels are of astrophysical interest. The reaction $^{86}$Kr($\alpha$,n)$^{89}$Sr was studied at TRIUMF using the EMMA recoil mass spectrometer. Partial cross sections of $1.0^{+0.6}_{-0.8}$\,mb and $0.8^{+0.7}_{-0.8}$\,mb were measured for transitions in the recoiling $^{89}$Sr nucleus from the 1032\,keV excited state to the ground state and from the 1473\,keV excited state to the ground state respectively; both measurements are in agreement with predictions. These results will help constrain uncertainty in model predictions of nucleosynthesis for each site and inform future experiments. Ultimately, these results contribute to determining the contribution of these two processes to the abundances of the lighter heavy elements at early times in the universe.