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

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