a supernova are much hotter (more energetic) than the electron neutrinos. Normally neutrino effects are muted because muon and tau neutrinos do not interact so easily with nuclei, while electron neutrinos are not produced so hot. But if oscillations scramble the identities, the hot muon and tau neutrinos can turn into hot electron neutrinos and readily disintegrate nuclei just built by the r-process.

The nuclei built in rapid neutron capture lie at the boundary of nuclear stability, the neutron “drip line.” To trace the path of nucleosynthesis, researchers need to know the masses and lifetimes of nuclei far from the ones that can be reached with existing technology. The binding energy of such exotic nuclei can be calculated well for nuclei nearer the “valley of stability” (the region in the diagram of all possible nuclei described by their numbers of neutrons and protons where the most stable nuclei are found). How well those equations serve in extrapolation to r-process nuclei is completely unknown. In the last few years it has been realized that these nuclei can be produced and measured in a two-stage acceleration, isotope-production, re-acceleration facility. With a suitably designed facility, every r-process nucleus may be accessible for direct measurement.

Finally, there will be in the coming decades the opportunity to observe directly the synthesis of heavy elements where it is believed that synthesis occurs—that is, in the explosions of stars. These explosions create radioactive nuclei, which decay over time, usually with the emission of a gamma ray of specific energy. Future sensitive high-energy x-ray and gamma-ray space experiments will allow these decays to be observed and monitored over time soon after the explosion and the distribution of newly synthesized material in the remnant matter expelled in the explosion to be mapped with high fidelity. These remnants can “glow” for tens of thousands of years in observable radiation. Such observations can be used to constrain the theoretical models for the explosions, directly measure the quantities of synthesized material, and observe how it gets distributed into the space between stars.

Earth is continuously bombarded by relativistic particles called cosmic rays, which are known to originate beyond the solar system. Cosmic rays with energies up to at least 10¹⁴ eV are probably accelerated at the shock fronts associated with supernova explosions, and radio emissions and x rays give direct evidence that electrons are accelerated there to nearly the speed of light. However, the evidence that high-energy cosmic-ray protons and nuclei have a supernova origin is only circumstantial and needs confirma-