<p>The <span class="math-container">$r$</span>-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.</p>

<p>To the extent that at least some of the highest <span class="math-container">$r$</span>-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 <span class="math-container">$r$</span>-process ones?</p>

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

<p><strong>EDIT (from a comment):</strong> <a href="https://physics.stackexchange.com/a/379404/97235">This Stack Exchange answer</a> suggests that splashed neutronium will end up as the lightest elements, but tantalisingly raises another interesting question: whether the <span class="math-container">$r$</span>-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.</p>

CC BY-SA 4.0
2019-06-19T16:13:10.417Z
Good question. Rob Jeffries talks a little about it [here](https://physics.stackexchange.com/a/379404/123208). He may have more info about it now, but it's still a very new field of study.
PM 2Ring
CC BY-SA 4.0
2019-06-19T17:20:52.547Z
You are right: I hadn't seen this: it is very relevant. A year and a half ago, perhaps there has been progress...
Martin Kochanski
CC BY-SA 4.0
2019-06-19T18:01:36.147Z
In the meantime, it's definitely worthwhile browsing Rob's answers about neutronium etc, here & on Physics.SE, eg https://astronomy.stackexchange.com/a/23173/16685
PM 2Ring
CC BY-SA 4.0
2019-06-20T07:20:16.967Z
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.
ProfRob
<neutron-star><nucleosynthesis>
Neutron star r-process nucleosynthesis
Martin Kochanski
# Neutron star r-process nucleosynthesis

(Asked by: username_0 on 2019-06-19)

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](https://physics.stackexchange.com/a/379404/97235) 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.

**Tags:** neutron-star, nucleosynthesis

> **Comments:**
> **username_1**: Good question. Rob Jeffries talks a little about it [here](https://physics.stackexchange.com/a/379404/123208). He may have more info about it now, but it's still a very new field of study.  
> **username_0**: You are right: I hadn't seen this: it is very relevant. A year and a half ago, perhaps there has been progress...  
> **username_1**: In the meantime, it's definitely worthwhile browsing Rob's answers about neutronium etc, here & on Physics.SE, eg https://astronomy.stackexchange.com/a/23173/16685  
> **username_2**: 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.  

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