What is a “spin-free” isotope? I’ve encountered this term in a paper:
While some elements, such as O and Ca, are known to exist almost exclusively as nuclear spin-free isotopes, elemental species with at least 50% nuclear spin-free isotopes could likely be isotopically enriched to higher concentrations, as has been achieved in diamond and silicon.
These are isotopes of atoms that have no nuclear spin.
Does that just mean that spins of neutrons and protons cancel out?
Yes, exactly.