What environments provide the extreme neutron density required for the rapid neutron capture (r-process) to create the heaviest elements?

Answer

Type II core-collapse supernova or neutron star collision (kilonova).

The r-process requires a massive, rapid influx of neutrons, which occurs during the catastrophic shockwaves of Type II supernovae or the collision of two neutron stars (a kilonova event).

What environments provide the extreme neutron density required for the rapid neutron capture (r-process) to create the heaviest elements?
ElementstarastrophysicsFusionnucleosynthesis