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

This Letter reports on the first cross-section measurements for the ^{94}Sr(α,n)^{97}Zr and ^{86}Kr(α,n)^{89}Sr reactions. In particular, our measurement of ^{94}Sr(α,n)^{97}Zr 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 ^{86}Kr(α,n)^{89}Sr reaction, and 9.9 MeV for ^{94}Sr(α,n)^{97}Zr, 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 ^{94}Sr(α,n)^{97}Zr, 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.