Role reversal in proton capture

The synthesis of heavy nuclei in stars and stellar explosions is for the most part well understood. Most isotopes of elements beyond iron are created through a sequence of neutron captures and subsequent radioactive decays. But a few dozen naturally occurring nuclides, all with high proton-to-neutron ratios, defy explanation by that mechanism, and their origin remains mysterious. To try to understand how those proton-rich nuclei can form, nuclear astrophysicists turn to the p process, or proton-capture reaction. The conventional technique for measuring p-process cross sections, necessary for modeling nucleosynthesis, involves accelerating a beam of protons onto a stationary target. Because that method requires a target material that doesn’t decay away over the course of the experiment, many important p-process reactions involving short-lived nuclides have been inaccessible. Now René Reifarth and colleagues , working at the GSI Helmholtz Center for Heavy Ion Research in Darmstadt, Germany, are flipping the script: They circulate the nuclide of interest in GSI’s Experimental Storage Ring and allow it to interact with a stationary hydrogen target. For their proof-of-principle experiment, they looked at proton capture by ruthenium-96, which is stable, but they anticipate that the method can work for nuclei with half-lives as short as a few minutes. The researchers measured the p-process cross section for collision energies between 9 MeV and 11 MeV per nucleon; though low by particle-accelerator standards, that range is still several times higher than the energies most relevant for nucleosynthesis. The next step is to figure out how to decelerate the ions in the storage ring without losing them. (B. Mei et al., Phys. Rev. C 92, 035803, 2015 .)