Neutron excess
= Neutron excess
{title2=$\eta=1-2Y_e$}
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.