A similar multi neutron resonance was seen in a study of neutron rich beryllium maybe 10ish years ago, but the details are escaping me. Anyway if you look at a

A similar multi neutron resonance was seen in a study of neutron rich beryllium maybe 10ish years ago, but the details are escaping me. Anyway if you look at a plot of stable nuclei with neutron number on one axis and proton number on the other, the distribution generally doesn't follow the x=y line but unstead tends toward prefering nuclei with more neutrons than protons. A natural question is whether protons are necessary at all but there are many competing factors at play. For instance protons repel each other electrostatically because they are charged, and that falls off as 1/r^2 because the electromagnetic force carrier (the photon) is massless. On the other hand, nucleons also exchange mesons with each other (pions in particular), and these are the carriers of so called "nuclear forces" which can create extremely strong attractive and repulsive forces depending on distance, and fall off much faster than 1/r^2 due to the uncertainty principle limiting the travel distance of massive particles like pions. There are neutral, negative, and positively charged pions, and not all can be exchanged between all pairs of nucleons. For instance two neutrons can't exchange a charged pion with each other as this would violate charge conservation. However, a neutron and a proton can do so, which is part of why they are more tightly bound than neutron-neutron pairings. This may be the main reason why protons seem to be necessary to form stable nuclei.