First Measurement of a Weak r -Process Reaction on a Radioactive Nucleus

Abstract

This Letter reports on the first cross-section measurements for the Sr94(α,n)Zr97 and Kr86(α,n)Sr89 reactions. In particular, our measurement of Sr94(α,n)Zr97 is the first weak r-process reaction cross section obtained using a radioactive ion beam. This experiment was enabled by the use of novel solid helium targets, comprised of silicon thin films with high helium incorporation obtained via a sputtering technique. Yield measurements were performed at center-of-mass energies of 10.4 and 9.0 MeV for the Kr86(α,n)Sr89 reaction, and 9.9 MeV for Sr94(α,n)Zr97, extending into the respective Gamow energy windows for a temperature of 5 GK. Reactions were uniquely identified by prompt γ rays detected in coincidence with heavy ions selected by a recoil mass spectrometer. The obtained cross sections are smaller than predicted for both reactions. In the case of Sr94(α,n)Zr97, the reaction rate found here is lowered by an order of magnitude at temperatures below 5 GK, which is expected to impact the predicted abundance of ruthenium, a signature weak r-process element.