Neutron excess ($\eta =1-2Y_e$)

The neutron excess is the number of neutrons minus protons per baryon. A mixture of carbon-12 and oxygen-16 has zero neutron excess, while neon-22 contributes $2/22$ per baryon in that isotope. Thus a neon-22 mass fraction $X_{22}$ gives $\eta =X_{22}/11$. A nonzero excess shifts iron-group ashes from symmetric nickel-56 toward neutron-rich isotopes, even before additional weak reactions occur.