Reaction rate sensitivity of Ti-44 production in massive stars and implications of a thick target yield measurement of Ca-40(alpha, gamma)Ti-44

Abstract

We evaluate two dominant nuclear reaction rates and their uncertainties that affect Ti-44 production in explosive nucleosynthesis. Experimentally we develop thick target yields for the Ca-40(alpha, gamma)Ti-44 reaction at E alpha = 4.13, 4.54, and 5.36 MeV using gamma-ray spectroscopy. At the highest beam energy, we also performed an activation measurement which agrees with the thick target result. From the measured yields a stellar reaction rate was developed that is smaller than current statistical-model calculations and recent experimental results, which would suggest lower Ti-44 production in scenarios for the alpha-rich freezeout. Special attention has been paid to assessing realistic uncertainties of stellar reaction rates produced from a combination of experimental and theoretical cross sections. With such methods, we also develop a re-evaluation of the Ti-44(alpha, p)V-47 reaction rate. Using these two rates we carry out a sensitivity survey of Ti-44 synthesis in eight expansions representing peak temperature and density conditions drawn from a suite of recent supernova explosion models. Our results suggest that the current uncertainty in these two reaction rates could lead to as large an uncertainty in Ti-44 synthesis as that produced by different treatments of stellar physics.