The only stable yttrium isotope, Y, will be found with yield somewhat less than 1% in a fission product mixture which has been allowed to age for months or years, as the next-longest lived yttrium isotopes have half-lives of only 107 days (Y) or 59 days (Y). However, a small amount of yttrium-90 will be found in secular equilibrium with its parent strontium-90 unless the two elements are separated from each other.