What is known about the isotope distribution of neutronium decay?

Question

Sometimes it is known to happen. For example, neutron star mergers might result in unstable neutronium droplets which lose the enormous pressure that makes them stable. A "nucleon" of $10^{30}$ neutrons can not remain together, neither by its gravity nor by the strong force.

For example, in the case of other nuclear fission reactions (most importantly, neutron-induced fission of ${}^{235} U$, ${}^{238} U$ and ${}^{239} Pu$), the ratio of the individual daughter isotopes are well known and well measured.

What is known about the isotope distribution of an evaporating (..exploding) neutron droplet?

Answer 1

"Neutronium" - in the sense of the matter that makes up the bulk of the fluid interior of a neutron star - consists of neutrons protons and electrons. If you release the pressure suddenly then the neutrons will beta decay into electrons, protons and anti-neutrinos.

If you release the pressure very gradually then the matter will go through a number of phases - nuclear pasta, exotic ultra-heavy, neutron-rich nuclei surrounded by a free neutron fluid plus relativistic electrons, the ultra-heavy nuclei then become susceptible to fission but there will be a competition between this process and neutron capture trying to build heavier elements back up.

The final stages are characterised by decay back towards the "valley of stability" once the density falls enough that the neutron capture cross-section becomes small. This is the locus of atomic number versus nucleon number that minimises the binding energy per nucleon for a fixed number of nucleons.

Because the neutron capture cross-section falls abruptly for "closed shells" of neutrons at magic numbers of 50, 80, 126, then there tends to be a build up of nuclei at atomic numbers that occur around three peaks at selenium, xenon and platinum respectively, corresponding to the beta decay path onto the valley of stability from these neutron numbers. These three peaks can be identified in the abundances of chemical elements in the Solar System and is one of the reasons that the r-process is implicated in the production of heavy elements - see below from Friedman & Stergioulas (2020).

Any remaining free neutrons just decay into protons, electrons and anti-neutrinos.

Answer 2

The theoretical cascade of nuclear reactions that occurs in environments with high neutron densities, but not enough pressure to stabilize neutron-rich nuclei is known as the r-process. It can start from the bottom up, bombarding light nuclei with neutrons (theoretically in a supernova), or from the top down from neutron matter (theoretically the ejecta from a neutron star merger). As the matter decompresses, it reaches a quasi-steady state rich in heavy radioactive elements.