The $r$-process in supernovae happens when a high flux of neutrons irradiates the heaviest abundant nuclei (in the region of iron), resulting in endothermic nuclear reactions.

To the extent that at least some of the highest $r$-process elements are now thought to be synthesised in neutron star mergers, is the mechanism the same: that is, is it a question of the (hypothesised) iron crust of the neutron stars being irradiated by a sudden intense flux of neutrons at the moment of the merger? Or is it that the "splashed" neutronium resolves itself into a range of atomic nuclei, especially the heavier $r$-process ones?

I appreciate that this is a question about hypothesis built upon hypothesis, but it would be interesting to get some idea of current thinking.

EDIT (from a comment): This Stack Exchange answer suggests that splashed neutronium will end up as the lightest elements, but tantalisingly raises another interesting question: whether the $r$-process elements are created in the merger or whether they are constantly present at the bottom of the crust, in some sort of equilibrium between the infinite supply of neutrons within the star and the iron in the crust itself. In which case the merger would create nothing, merely liberate what is already there.

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    $\begingroup$ Good question. Rob Jeffries talks a little about it here. He may have more info about it now, but it's still a very new field of study. $\endgroup$
    – PM 2Ring
    Jun 19, 2019 at 16:13
  • $\begingroup$ You are right: I hadn't seen this: it is very relevant. A year and a half ago, perhaps there has been progress... $\endgroup$ Jun 19, 2019 at 17:20
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    $\begingroup$ In the meantime, it's definitely worthwhile browsing Rob's answers about neutronium etc, here & on Physics.SE, eg astronomy.stackexchange.com/a/23173/16685 $\endgroup$
    – PM 2Ring
    Jun 19, 2019 at 18:01
  • $\begingroup$ No, the r-process elements are not already in the crust. Heavy, neutron-rich elements are in the crust. If brought to lower densities, these would decay. $\endgroup$
    – ProfRob
    Jun 20, 2019 at 7:20


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