  The conditions for the leading r-process site candidate, neutrino-driven
winds, can not be reproduced self-consistently in current supernova models. For
that reason, we investigate an alternate model involving the mass ejected by
fallback in a supernova explosion, through hydrodynamic and nucleosynthesis
calculations. The nucleosynthetic products of this ejected material produces
r-process elements, including those in the vicinity of the elusive 3rd peak at
mass number 195. Trans-iron element production beyond the second peak is made
possible by a rapid (<1ms) freezeout of alpha particles which leaves behind a
large nucleon (including protons!) to r-process seed ratio. This rapid phase is
followed by a relatively long (>15 ms) simmering phase at ~2e9 K, which is the
thermodynamic consequence of the hydrodynamic trajectory of the turbulent flows
in the fallback outburst. During the slow phase high mass elements beyond the
second peak are first made through rapid capture of both protons and neutrons.
The flow stays close to valley of stability during this phase. After freeze-out
of protons the remaining neutrons cause a shift out to short-lived isotopes as
is typical for the r-process. A low electron fraction isn't required in this
model, however, the detailed final distribution is sensitive to the electron
fraction. Our simulations suggest that supernova fallback is a viable
alternative scenario for the r-process.
