Noun
English Wikipedia has an article on:Isotopes of neonWikipedia
neon-21 (uncountable)
(physics) A rare stable isotope of neon, 10Ne, having ten protons and eleven neutrons; it amounts to less than 0.3% of the element in nature
USGS.gov Neon-21 may also be produced in a nucleogenic reaction, when 20 Ne absorbs a neutron from various natural terrestrial neutron sources. Source: Internet